- Ruhr-Universität Bochum

The magnetic order for several compositions of CaK(Fe 1-x Mn x )4As4 has been studied by nuclear magnetic resonance (NMR), Mössbauer spectroscopy, and neutron diffraction. Our observations for the Mn-doped 1144 compound are consistent with the hedgehog spin vortex crystal (hSVC) order which has previously been found for Ni-doped CaKFe4As4 . The hSVC state is characterized by the stripe-type propagation vectors (π0) and (0π) just as in the doped 122 compounds. The hSVC state preserves tetragonal symmetry at the Fe site, and only this SVC motif with simple antiferromagnetic (AFM) stacking along c is consistent with all our observations using NMR Mössbauer spectroscopy, and neutron diffraction. We find that the hSVC state in the Mn-doped 1144 compound coexists with superconductivity, and by combining the neutron scattering and Mössbauer spectroscopy data we can infer a quantum phase transition, hidden under the superconducting dome, associated with the suppression of the AFM transition temperature (T N) to zero for x ≈ 0.01. In addition, unlike several 122 compounds and Ni-doped 1144, the ordered magnetic moment is not observed to decrease at temperatures below the superconducting transition temperature (T c). © 2023 The Author(s). Published by IOP Publishing Ltd.

We present capacitance-voltage [C(V)] measurements of self-assembled wurtzite-GaN quantum dots (QDs). The QDs are embedded in a charge-tunable diode structure and were grown by molecular beam epitaxy in the Stranski-Krastanov growth method. The internal electric fields present in GaN and its alloys together with its wide bandgap make this material system an ideal candidate for high-temperature quantum applications. Charges and the internal electric fields influence the energy spacing in the QDs. We correlate photoluminescence measurements with C(V) measurements and show single-electron charging of the QDs and a Coulomb blockade energy of around 60 meV at room temperature. This finding demonstrates the possibility of quantum applications at room temperature. © 2022 Author(s).

Chirality switching of self-assembled molecular structures is of potential interest for designing functional materials but is restricted by the strong interaction between the embedded molecules. Here, we report on an unusual approach based on reversible chirality changes of self-assembled oligomers using variable-temperature scanning tunneling microscopy supported by quantum mechanical calculations. Six functionalized diazomethanes each self-assemble into chiral wheel-shaped oligomers on Ag(111). At 130 K, a temperature far lower than expected, the oligomers change their chirality even though the molecules reside in an embedded self-assembled structure. Each chirality change is accompanied by a slight center-of-mass shift. We show how the identical activation energies of the two processes result from the interplay of the chirality change with surface diffusion, findings that open the possibility of implementing various functional materials from self-assembled supramolecular structures. © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.

One of the key factors, which hampers the application of metallic glasses as structural components, is the localization of deformation in narrow bands of a few tens up to one hundred nanometers thickness, the so-called shear bands. Processes, which occur inside shear bands are of central importance for the question whether a catastrophic failure of the material is unavoidable or can be circumvented or, at least, delayed. Via molecular dynamics simulations, this study addresses one of these processes, namely the local temperature rise due to viscous heat generation. The major contribution to energy dissipation is traced back to the plastic work performed by shear stress during steady deformation. Zones of largest strain contribute the most to this process and coincide with high-temperature domains (hottest spots) inside the sample. Magnitude of temperature rise can reach a few percent of the sample’s glass transition temperature. Consequences of these observations are discussed in the context of the current research in the field. © 2022 by the authors.

Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m2, far above the SFE regime for twinning (<~50 mJ/m2) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties. © 2022, The Author(s).

Aluminum oxides, oxyhydroxides, and hydroxides are important in different fields of application due to their many attractive properties. However, among these materials, tohdite (5Al2O3·H2O) is probably the least known because of the harsh conditions required for its synthesis. Herein, we report a straightforward methodology to synthesize tohdite nanopowders (particle diameter ∼13 nm, specific surface area ∼102 m2g-1) via the mechanochemically induced dehydration of boehmite (γ-AlOOH). High tohdite content (about 80%) is achieved upon mild ball milling (400 rpm for 48 h in a planetary ball mill) without process control agents. The addition of AlF3can promote the crystallization of tohdite by preventing the formation of the most stable α-Al2O3, resulting in the formation of almost phase-pure tohdite. The availability of easily accessible tohdite samples allowed comprehensive characterization by powder X-ray diffraction, total scattering analysis, solid-state NMR (1H and 27Al), N2-sorption, electron microscopy, and simultaneous thermal analysis (TG-DSC). Thermal stability evaluation of the samples combined with structural characterization evidenced a low-temperature transformation sequence: 5Al2O3·H2O → κ-Al2O3→ α-Al2O3. Surface characterization via DRIFTS, ATR-FTIR, D/H exchange experiments, pyridine-FTIR, and NH3-TPD provided further insights into the material properties. © 2022 American Chemical Society. All rights reserved.

Low temperature (1023 K) high stress (800 MPa) tensile creep behavior of the superalloy single crystal ERBO-1 (CMSX-4 type) is investigated. Three loading directions are compared: precise [001] and 15 ° deviations from [001] towards [111] and [011]. It is found that creep rates ε˙ scale as ε˙[001]→[111]>ε˙[001]>ε˙[001]→[011]already in the early stages of creep (ε≤1%), where dislocation network formation and planar fault intersections cannot rationalize the observed rate effects. An analysis based on Peach-Köhler force calculations suggests, that fast creep rates are observed, when dislocations from two octahedral systems, which are required to react and form the leading part of a planar fault ribbon in the γ’-phase, experience similar driving forces. Creep data, micromechanical calculations and TEM results are in good qualitative agreement. From a technological point of view, the results show that while 15 ° deviations from [001] towards [011] can be tolerated, deviations towards [111] must be avoided. © 2021

Deformation-induced martensitic transformation (DIMT) has been used for designing high-performance alloys to prevent structural failure under static loads. Its effectiveness against fatigue, however, is unclear. This limits the application of DIMT for parts that are exposed to variable loads, although such scenarios are the rule and not the exception for structural failure. Here we reveal the dual role of DIMT in fatigue crack growth through in situ observations. Two antagonistic fatigue mechanisms mediated by DIMT are identified, namely, transformation-mediated crack arresting, which prevents crack growth, and transformation-mediated crack coalescence, which promotes crack growth. Both mechanisms are due to the hardness and brittleness of martensite as a transformation product, rather than to the actual transformation process itself. In fatigue crack growth, the prevalence of one mechanism over the other critically depends on the crack size and the mechanical stability of the parent austenite phase. Elucidating the two mechanisms and their interplay allows for the microstructure design and safe use of metastable alloys that experience fatigue loads. The findings also generally reveal how metastable alloy microstructures must be designed for materials to be fatigue-resistant. © 2022 National Academy of Sciences. All rights reserved.

The deformation-induced phase transition from fcc to hcp causes local embrittlement of the metal matrix in Cobalt-base alloys, facilitating subcritical crack growth under cyclic loading and reducing fatigue resistance. Our approach to increasing the fatigue life of Co-based hard alloys is to suppress the phase transition from fcc to hcp by an alloy modification that increases the stacking fault energy (SFE) of the metal matrix. Therefore, we substitute various contents (15, 25, and 35 mass pct) of Co by Fe and analyze the effect on the fatigue life and resistance against subcritical crack growth. Subcritical crack growth in the specimens takes place in a cyclic load test. The proceeding crack growth and the occurrence of phase transformations are monitored by scanning electron microscope (SEM) investigations and electron backscatter diffraction (EBSD). We determined an SFE of 35 mJ/m2 at an iron content of 35 mass pct, which leads to a change of the main deformation mechanism from deformation-induced martensitic transformation to deformation twinning. Analysis of cyclically loaded specimens revealed that the resistance against subcritical crack growth in the metal matrix is facilitated with increasing Fe content, leading to a significant increase in fatigue life. © 2022, The Author(s).

We investigate the effect of low temperature (cryogenic) thermal cycling on dynamics of a generic model glass via molecular dynamics simulations. By calculating mean squared displacements after a varying number of cycles, a pronounced enhancement of dynamics is observed. This rejuvenation effect is visible already after the first cycle and accumulates upon further cycling in an intermittent way. Our data reveal an overall deformation (buckling of the slab-shaped system) modulated by a heterogeneous deformation field due to deep cryogenic thermal cycling. It is shown via strain maps that deformation localizes in the form of shear-bands, which gradually fill the entire sample in a random and intermittent manner, very much similar to the accumulation effect observed in dynamics. While spatial organization of local strain may be connected to the specific geometry, we argue that the heterogeneity of the structure is the main cause behind rejuvenation effects observed in the present study. © 2022 Author(s).

Halogen bonding has recently been recognized as an interaction whose relevance is on par with hydrogen bonding. While observed frequently in solution chemistry, the significance of halogen bonds in forming extended supramolecular structures on surfaces is less explored. Herein, we report on the self-assembly of chlorobenzene molecules adsorbed on the Cu(111) surface into nanosized clusters at submonolayer coverages, where the molecular planes are close to parallel to the surface. A comprehensive study of the role of intermolecular interactions through both halogen and weak hydrogen bonds on nanocluster formation is presented, gained by combining the results of temperature-programmed desorption, reflection-absorption infrared spectroscopy, scanning tunneling microscopy, and density functional theory calculations. Based on an unprecedented precise determination of the molecules’ orientation within the clusters, the binding motifs that lead to the formation and stability of nanoclusters with magic sizes are identified and explained. A complex and delicate interplay of halogen bonds with weak hydrogen bonds, van-der-Waals forces, and surface–adsorbate interactions leads to a preference for hexamers and tetramers with an observable propensity for halogen bonding over weak hydrogen bonding when adsorbed to the Cu(111) surface. © 2021 American Chemical Society

Unlike other thermal spraying methods, it is difficult to determine the temperature of the particles during cold gas spraying due to the relatively low radiation. In the present study, the velocities and in-flight temperatures of metal particles were measured during cold gas spraying. A state-of-the-art high-speed infrared camera was used to study the behavior of two different base materials, In718 and TiAlCrNb, both used as structural materials in gas turbine engines. The experiments aimed to improve the fundamental understanding of the process, in particular the heating of the particles, and to compare the experimental results with theoretical calculations of the particle temperatures. © 2022, The Author(s).

When computing finite-temperature properties of materials with atomistic simulations, nuclear quantum effects are often neglected or approximated at the quasiharmonic level. The inclusion of these effects beyond this level using approaches like the path integral method is often not feasible due to their large computational effort. We discuss and evaluate the performance of a temperature-remapping approach that links the finite-temperature quantum system to its best classical surrogate via a temperature map. This map, which is constructed using the internal energies of classical and quantum harmonic oscillators, is shown to accurately capture the impact of quantum effects on thermodynamic properties at an additional cost that is negligible compared to classical molecular dynamics simulations. Results from this approach show excellent agreement with previously reported path integral Monte Carlo simulation results for diamond cubic carbon and silicon. The approach is also shown to work well for obtaining thermodynamic properties of light metals and for the prediction of the fcc to bcc phase transition in calcium. © 2022 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Ni-Mn-based Heusler alloys have a high technical potential related to a large change of magnetization at the structural phase transition. These alloys show a subtle dependence of magnetic properties and structural phase stability on composition and substitution by 3d elements and although they have been extensively investigated, there are still ambiguities in the published results and their interpretation. To shed light on the large spread of reported properties, we perform a comprehensive study by means of density functional theory calculations. We focus on Cr and Co co-substitution whose benefit has been predicted previously for the expensive Ni-Mn-In-based alloy and study the more abundant iso-electronic counterpart Ni-Mn-Ga. We observe that substituting Ni partially by Co and/or Cr enhances the magnetization of the Heusler alloy and at the same time reduces the structural transition temperature. Thereby, Cr turns out to be more efficient to stabilize the ferromagnetic alignment of the Mn spins by strong antiferromagnetic interactions between Mn and Cr atoms. In a second step, we study Cr on the other sublattices and observe that an increase in the structural transition temperature is possible, but depends critically on the short-range order of Mn and Cr atoms. Based on our results, we are able to estimate composition dependent magnetic phase diagrams. In particular, we demonstrate that neither the atomic configuration with the lowest energy nor the results based on the coherent potential approximation are representative for materials with a homogeneous distribution of atoms and we also predict a simple method for fast screening of different concentrations which can be viewed as a blueprint for the study of high entropy alloys. Our results help to explain the large variation of experimentally found materials properties. © 2021 The Author(s). Published by IOP Publishing Ltd.

The detailed catalytic influence of minerals on solid biomass in oxy-fuel combustion is yet to be fully understood. The catalytic influence of metal sulfates on a mineral-free, cellulose-based model biomass was investigated during slow and high heating in air and oxy-fuel combustion. Measurements were performed in a thermogravimetric setup in air with slow heating rates and in a flat-flame burner in oxy-fuel combustion atmosphere with high heating rates. Temperature-programmed experiments identified the catalytic activity scale of Fe > K > Na > Mg ∼ Ca in synthetic air (20% O2/He) for the sulfates. The highly active metals Fe and K were chosen for more detailed investigations in oxy-fuel combustion experiments using an additional loading of Mg as less-volatile mineral tracer. Samples doped with Fe and Mg (FeMg-MH) exhibited lower thermal stability and higher particle combustion temperatures in the flat-flame burner compared with the undoped model fuel, while the combination of K and Mg (KMg-MH) decreased the particle combustion temperature drastically during oxy-fuel combustion. X-ray diffraction patterns acquired between 25 and 800 °C showed that in FeMg-MH the mineral phases FeSO4 and MgSO4 were still separated and independently active, while the addition of MgSO4 to K2SO4 formed the stable mineral phase Langbeinite inhibiting the K mobility. The influence of metal chlorides and nitrates was also investigated by slow heating rate TGA experiments showing an overlapping of metal salts decomposition and carbon devolatilization and oxidation. © 2022 Elsevier Ltd

The TriplexPro™-210 plasma spray torch (Oerlikon Metco) is a three-cathode plasma generator. It became a kind of workhorse for the wide range of tasks handled at the Jülich Thermal Spray Center (JTSC). Compared to conventional single-cathode torches, the cascaded design of the nozzle suggests low fluctuations of the arc and thus high stability. However, after a certain time, degradation sets in even with such a torch, impairing the reliability of the process. It is therefore important to detect indications of performance loss in time and not only during the inspection of the deposited layer. In this study, standard samples of YSZ thermal barrier coatings were sprayed regularly over a period of two years. Operational data and feedstock characteristics were collected and correlated with the area-specific mass deposition. It turned out that the measured substrate surface temperature showed a distinct correlation. Searching for the reasons for the temperature variations, several process parameters could be ruled out as they are monitored by calibrated sensors, controlled, and their time course is recorded by the control unit. Moreover, there are other parameters, which can have a considerable impact such as the robot alignment or the substrate cooling conditions. However, the purposeful experimental variation of such variables resulted in a variability of the mass deposition being considerably smaller than observed over the two years. Thus, it can be concluded that torch degradation had a pronounced effect, too. The substrate surface temperature can be used as indicator for the torch status and the reliability of the spray process. © 2022 by the authors.

While age-hardened austenitic high-Mn and high-Al lightweight steels exhibit excellent strength-ductility combinations, their properties are strongly degraded when mechanically loaded under harsh environments, e.g. with the presence of hydrogen (H). The H embrittlement in this type of materials, especially pertaining to the effect of κ-carbide precipitation, has been scarcely studied. Here we focus on this subject, using a Fe-28.4Mn-8.3Al-1.3C (wt%) steel in different microstructure conditions, namely, solute solution treated and age-hardened. Contrary to the reports that grain boundary (GB) κ-carbides precipitate only during overaging, site-specific atom probe tomography and scanning transmission electron microscopy (STEM) reveal the existence of nanosized GB κ-carbides at early stages of aging. We correlate this observation with the deterioration of H embrittlement resistance in aged samples. While H pre-charged solution-treated samples fail by intergranular fracture at depths consistent with the H ingress depth (∼20 µm), age-hardened samples show intergranular fracture features at a much larger depth of above 500 µm, despite similar amount of H introduced into the material. This difference is explained in terms of the facile H-induced decohesion of GB κ-carbides/matrix interfaces where H can be continuously supplied through internal short-distance diffusion to the propagating crack tips. The H-associated decohesion mechanisms are supported by a comparison with the fracture behavior in samples loaded under the cryogenic temperature and can be explained based on dislocation pileups and elastic misfit at the GB κ-carbide/matrix interfaces. The roles of other plasticity-associated H embrittlement mechanisms are also discussed in this work based on careful investigations of the dislocation activities near the H-induced cracks. Possible alloying and microstructure design strategies for the enhancement of the H embrittlement resistance in this alloy family are also suggested. © 2022

The tensors of thermal expansion of monoclinic RCa4O(BO3)3 with R = Er, Y, Dy, Gd, Sm, Nd, La were studied in the temperature range from 100 K to 1373 K using high-resolution dilatometry. Reproducible anomalies, characterized by an excess strain at high temperatures, occur at different temperatures depending on the type of the R3+ cation. Additional single-crystal diffraction experiments on quenched samples and heat capacity measurements indicate that non-convergent cation ordering processes involving Ca2+ and R3+ play an essential role here. The cation distribution on the specific structural sites and the evolution of disorder with temperature are mainly influenced by the size of the trivalent cation, with the minimization of internal stresses being the driving force. The onset temperatures and the specific anisotropy of the anomalies in the thermal expansion are directly related to these processes. © 2022 The Authors

We present a first-principles approach for the computation of the magnetic Gibbs free energy of materials using magnetically constrained supercell calculations. Our approach is based on an adiabatic approximation of slowly varying local moment orientations, the so-called finite-temperature disordered local moment picture. It describes magnetic phase transitions and how electronic and/or magnetostructural mechanisms generate a discontinuous (first-order) character. We demonstrate that the statistical mechanics of the local moment orientations can be described by an affordable number of supercell calculations containing noncollinear magnetic configurations. The applicability of our approach is illustrated by firstly studying the ferromagnetic state in bcc Fe. We then investigate the temperature-dependent properties of a triangular antiferromagnetic state stabilizing in two antiperovskite systems Mn3AN (A=Ga, Ni). Our calculations provide the negative thermal expansion of these materials as well as the ab initio origin of the discontinuous character of the phase transitions, electronic and/or magnetostructural, in good agreement with experiment. © 2022 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Spokes are long wavelength oscillations observed in the magnetized region of direct current magnetron sputtering (DCMS), high power impulse magnetron sputtering (HiPIMS), as well as other E → × B → discharges. Spokes rotate in front of the cathode with velocities between about 2 km s−1 and 15 km s−1, making it difficult to perform quantitative measurements. This is overcome by synchronizing Langmuir probe measurements to the movement of spokes in DCMS to obtain the probe current-voltage (I-V) characteristic without averaging out the spoke influence. The I-V curves are then evaluated using magnetized probe theory, revealing the strong plasma parameter modulations, caused by the spokes. The plasma density was found to oscillate between 2.5 × 1016 m−3 and 1.7 × 1017 m−3, which corresponds to a modulation strength of more than 70% or an almost seven times increase of density. In good agreement with previous work, a plasma potential minimum of −55 V is found ahead of the spoke followed by a sudden increase to about 2 V inside the spoke. The electron temperature was found to oscillate between 3 eV and 7 eV. On top of that oscillation, electrons experience a sudden energy increase as they move inside the spoke, crossing the potential jump at the leading edge for the spoke. On basis of these observations a model is presented to explain spokes in DCMS. These results are then compared to HiPIMS spokes under otherwise similar conditions. The plasma parameter modulation found for HiPIMS is much weaker than for DCMS, which is explained by the higher collision frequency for electrons in HiPIMS plasmas. © 2022 The Author(s). Published by IOP Publishing Ltd.

The gradient microstructures of surface layer in single crystal nickel-based superalloy were produced by creep-feed grinding. The mechanical properties (i.e., hardness, elastic modulus) and room-temperature (RT) creep behavior of such structures were evaluated using a nano-indentation technique. Results show that the gradient structures along depth from ground surface consisted of nanograins, submicron grains and lamellar-shape structures, and dislocation structures. Furthermore, it was found that the hardness and elastic modulus of gradient structures were higher by 8–10% than that of bulk material on average. However, the regions containing nanograins showed a remarkable increase in creep depth compared to bulk material, implying that the creep behavior of ground layer was changed unfavorably. The obtained stress exponents of gradient structures suggested that dislocation activities were the main mechanism for indentation creep deformation. © 2021 Elsevier B.V.

Physics-based crystal plasticity models rely on certain statistical assumptions about the collective behavior of dislocation populations on one slip system and their interactions with the dislocations on the other slip systems. One main advantage of using such physics-based constitutive dislocation models in crystal plasticity kinematic frameworks is their suitability for predicting the mechanical behavior of polycrystals over a wide range of deformation temperatures and strain rates with the same physics-based parameter set. In this study, the ability of a widely used temperature-dependent dislocation-density-based crystal plasticity formulation to reproduce experimental results, with a main focus on the yield stress behavior, is investigated. First, the material parameters are identified from experimental macroscopic stress–strain curves using a computationally efficient optimization methodology that uses a genetic algorithm along with the response surface methodology. For this purpose, a systematic set of compression tests on interstitial free (IF) steel samples is performed at various temperatures and strain rates. Next, the influence of the individual parameters on the observed behavior is analyzed. Based on mutual interactions between various parameters, the ability to find a unique parameter set is discussed. This allows identifying shortcomings of the constitutive law and sketch ideas for possible improvements. Particular attention is directed toward identifying possibly redundant material parameters, narrowing the acceptable range of material parameters based on physical criteria, and modifying the crystal plasticity formulation numerically for high-temperature use. © 2021 The Author(s)

The direct sulfonation of methane to methanesulfonic acid was achieved in an electrochemical reactor without adding peroxide initiators. The synthesis proceeds only from oleum and methane. This is possible due to in situ formation of an initiating species from the electrolyte at a boron-doped diamond anode. Elevated pressure, moderate temperature and suitable current density are beneficial to reach high concentration at outstanding selectivity. The highest concentration of 3.7 M (approximately 62 % yield) at 97 % selectivity was reached with a stepped electric current program at 6.25–12.5 mA cm−2, 70 °C and 90 bar methane pressure in 22 hours. We present a novel, electrochemical method to produce methanesulfonic acid, propose a reaction mechanism and show general dependencies between parameters and yields for methanesulfonic acid. © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.

This study presents an investigation on the specific electrical conductivity of the cathode materials used in an aluminium electrolysis cell over a temperature range between room temperature and 950 °C. Those materials are subjected to a diffusion related aging process due to the high operating temperature of the cell, leading to a change in chemical composition and microstructure. The materials were investigated both in the initial state before use in an aluminium electrolysis cell and after an operating period of 5 years. It is shown that the changes in chemical composition and thus also in microstructure over the service life at elevated operating temperature exert an effect on the electrical conductivity. In addition, calculations based on thermodynamic data were used to relate phase transformations to the changes in electrical conductivity. On the one hand, the electrical conductivity of the collector bar at 950 °C is reduced by about 11% after 5 years of service. On the other hand, the ageing process has a positive influence on the cast iron with an increased conductivity by about 41% at 950 °C. The results provide an understanding how diffusion related processes in the cathode materials affect energy efficiency of the aluminium electrolysis cell. © 2022, The Author(s).

Physical properties of ten single-phase FCC CrxMn20Fe20Co20Ni40-x high-entropy alloys (HEAs) were investigated for 0 ≤ x ≤ 26 at%. The lattice parameters of these alloys were nearly independent of composition while solidus temperatures increased linearly by ∼30 K as x increased from 0 to 26 at.%. For x ≥ 10 at.%, the alloys are not ferromagnetic between 100 and 673 K and the temperature dependencies of their coefficients of thermal expansion and elastic moduli are independent of composition. Magnetic transitions and associated magnetostriction were detected below ∼200 K and ∼440 K in Cr5Mn20Fe20Co20Ni35 and Mn20Fe20Co20Ni40, respectively. These composition and temperature dependencies could be qualitatively reproduced by ab initio simulations that took into account a ferrimagnetic ↔ paramagnetic transition. Transmission electron microscopy revealed that plastic deformation occurs initially by the glide of perfect dislocations dissociated into Shockley partials on {111} planes. From their separations, the stacking fault energy (SFE) was determined, which decreases linearly from 69 to 23 mJ·m−2 as x increases from 14 to 26 at.%. Ab initio simulations were performed to calculate stable and unstable SFEs and estimate the partial separation distances using the Peierls-Nabarro model. While the compositional trends were reasonably well reproduced, the calculated intrinsic SFEs were systematically lower than the experimental ones. Our ab initio simulations show that, individually, atomic relaxations, finite temperatures, and magnetism strongly increase the intrinsic SFE. If these factors can be simultaneously included in future computations, calculated SFEs will likely better match experimentally determined SFEs. © 2022

We investigate the effect of low temperature (cryogenic) thermal cycling on a generic model glass and observe signature of rejuvenation in terms of per-particle potential energy distributions. Most importantly, these distributions become broader and its average values successively increase when applying consecutive thermal cycles. We show that linear dimension plays a key role for these effects to become visible, since we do only observe a weak effect for a cubic system of roughly one hundred particle diameter but observe strong changes for a rule-type geometry with the longest length being two thousand particle diameters. A consistent interpretation of this new finding is provided in terms of a competition between relaxation processes, which are inherent to glassy systems, and excitation due to thermal treatment. In line with our previous report (Bruns et al., PRR 3, 013234 (2021)), it is shown that, depending on the parameters of thermal cycling, rejuvenation can be either too weak to be detected or strong enough for a clear observation. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.

Water diffusion across the surfaces of materials is of importance to disparate processes such as water purification, ice formation, and more. Despite reports of rapid water diffusion on surfaces the molecular level, details of such processes remain unclear. Here, with scanning tunneling microscopy, we observe structural rearrangements and diffusion of water trimers at unexpectedly low temperatures (<10 K) on a copper surface, temperatures at which water monomers or other clusters do not diffuse. Density functional theory calculations reveal a facile trimer diffusion process involving transformations between elongated and almost cyclic conformers in an inchworm-like manner. These subtle intermolecular reorientations maintain an optimal balance of hydrogen-bonding and water–surface interactions throughout the process. This work shows that the diffusion of hydrogen-bonded clusters can occur at exceedingly low temperatures without the need for hydrogen bond breakage or exchange; findings that will influence Ostwald ripening of ice nanoclusters and hydrogen bonded clusters in general. © 2021 American Chemical Society

To better understand why amines catalyze the reactivity of SiOH with silanes, we examined the adsorption of trimethylamine under a low pressure (10-9-10-8 mbar) and a low temperature (105-160 K) on water-terminated (H,OH)-Si(001), which is both a model surface for adsorption studies and a promising starting substrate for atomic layer deposition. Trimethylamine bonding configurations were determined by combining real-time synchrotron radiation X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) with density functional theory (DFT) calculations of core-level ionization energies and vibrational spectra. Both spectroscopies showed that the majority of species are trimethylamine molecules making acceptor H bonds with surface hydroxyls. Moreover, HREELS indicated that the hydrogen-bonding modes (single and double hydrogen acceptor bonds) depend on temperature and/or coverage, which may in turn affect the weakening of the O-H bond, and hence the catalytic effects of trimethylamine. XPS also clearly detected a minority species, trimethylamine, datively bonded to the isolated silicon dangling bonds (a few 1/100th of a monolayer). This species is prone to breaking, and a detailed analysis of the reaction products was made. The reactivity of the electrically active isolated silicon dangling bonds with the amine may impact the Fermi-level position in the gap. © 2022 American Chemical Society

The full-cell entropy and its temperature dependency were measured for automotive lithium-ion cells with a graphite anode in dependence of the state of charge. Resulting entropy curves can be related to certain characteristic conditions of the graphite anode. Those characteristics are induced by a certain lithium-ion concentrations within the graphite. Comparing the entropy curves of fresh to aged cells shows a shift in the characteristics of these curves at a similar charge input. Those shifts were assigned to a change in the anodic net capacity, enabling an entropy based state of health estimation of the anode. The execution of the differential voltage analysis leads to similar results. © 2022 Elsevier Ltd

Co-Ni-Ga shape memory alloys attracted scientific attention as promising candidate materials for damping applications at elevated temperatures, owing to excellent superelastic properties featuring a fully reversible stress-strain response up to temperatures as high as 500 °C. In the present work, the effect of aging treatments conducted in a wide range of aging temperatures and times, i.e. at 300–400 °C for 0.25–8.5 h, was investigated. It is shown that critical features of the martensitic transformation are strongly affected by the heat treatments. In particular, the formation of densely dispersed γ’-nanoparticles has a strong influence on the martensite variant selection and the morphology of martensite during stress-induced martensitic transformation. Relatively large, elongated particles promote irreversibility. In contrast, small spheroidal particles are associated with excellent functional stability during cyclic compression loading of 〈001〉-oriented single crystals. In addition to mechanical experiments, a detailed microstructural analysis was performed using in situ optical microscopy and neutron diffraction. Fundamental differences in microstructural evolution between various material states are documented and the relations between thermal treatment, microstructure and functional properties are explored and rationalized. © 2022 Acta Materialia Inc.

Dislocation-enhanced electrical conductivity is an emerging topic for ceramic oxides. In contrast to the majority of present studies which focus on large-scale crystal deformation or thin film fabrication to introduce dislocations, we use a nanoindentation “pop-in stop” method to locally generate 〈011〉 edge-type dislocations at room temperature, without crack formation, on the (100) surface of a rutile TiO2 single-crystal. Ion beam assisted deposition of microcontacts allowed for both deformed and non-deformed zones to be locally probed by impedance spectroscopy. Compared to the dislocation-free region, a local enhancement of the electrical conductivity by 50% in the dislocation-rich regions is found. The study paves the way for local “mechanical-doping” of ceramics and oxide materials, allowing for the use of dislocations to tune the local conductivity with high spatial resolution. © 2022 The Author(s)

La2Ni7 is an intermetallic compound that is thought to have itinerant magnetism with small moment (∼0.15μB/Ni) ordering below 65 K. A recent study of single crystal samples by Ribeiro et al. [Phys. Rev. B 105, 014412 (2022)2469-995010.1103/PhysRevB.105.014412] determined detailed anisotropic H-T phase diagrams and revealed three zero-field magnetic phase transitions at T1∼61.0 K, T2∼56.5 K, and T3∼42 K. In that study only the highest temperature phase is shown to have a clear ferromagnetic component. Here we present a single crystal neutron diffraction study determining the propagation vector and magnetic moment direction of the three magnetically ordered phases, two incommensurate and one commensurate, as a function of temperature. The higher temperature phases have similar, incommensurate propagation vectors, but with different ordered moment directions. At lower temperatures, the magnetic order becomes commensurate with magnetic moments along the c direction as part of a first-order magnetic phase transition. We find that the low-temperature commensurate magnetic order is consistent with a proposal from earlier DFT calculations. © 2022 American Physical Society.

The mechanical behavior and deformation mechanisms of a body-centered cubic (BCC) Ti29Zr24Nb23Hf24 (at%) high entropy alloy (HEA) was investigated in temperatures and strain rates from 700° to 1100 °C and 10−3 to 10 s−1, respectively. The HEA exhibits a substantial increase in yield stress with increasing strain rate. The strain rate sensitivity (SRS) coefficient is ~3 times that of BCC alloy Nb-1Zr and even ~3.5 times that of pure Nb. This high SRS is attributed to the high Peierls stress of the HEA, which is about twice the Peierls stress of pure Nb. On the other hand, the flow stress exhibits a tendency from strain softening to strain hardening with the increase of strain rate especially at the relatively low temperatures. This behavior is explained by a change in dislocation motion from climbing to multiple slip with the increase of strain rate. Taking the specimen subjected to 800 ºC for example, dislocation walls formed at the early stage of deformation and at low strain rate of 10−3 s−1. In this case there is sufficient time to activate dislocations climb, which results in discontinuous dynamic recrystallization, and strain softening. However, when the strain rate amounts to 1 s−1, thermally activated processes such as dislocation climb are too sluggish. As a consequence, multiple slip systems are activated, and the dislocation interactions lead to the evolution of deformation bands, leading to strain hardening. © 2021 Elsevier B.V.

We report the cyclic deformation behavior of CoCrNi at 550 °C under a strain amplitude of ± 0.5% and compare it to that of CoCrFeMnNi. CoCrNi manifests cyclic hardening followed by minor softening and a near-steady state until failure. Transmission electron microscopy investigations of CoCrNi revealed that increasing the number of cycles from 10 to 2500/5000 leads to a transition of dislocation arrangements from slip bands to tangles. Compared to CoCrFeMnNi, CoCrNi exhibits higher strength, longer lifetime and persistent serrated flow. Owing to its lower stacking fault energy (even at 550 °C), planar slip is more pronounced in CoCrNi than CoCrFeMnNi, which additionally shows wavy slip. © 2022 Acta Materialia Inc.

In the present study, the CPD model originally developed based on predictions from heated grid (HGR) and entrained flow (EFR) experiments, has been adapted to analyze pyrolysis kinetics in a small-scale fluidized bed reactor. Impacts of particle feed, particle heat up as well as tar cracking reactions in the gas phase are considered. Furthermore, an optimized solver structure allows a time step independent solution and enables the use of implicit methods. A comparison with experimental results is undertaken for pulverized Rhenish lignite fuel particles in the temperature range from 673 to 973 K in N2 atmosphere. The comparison between simulated and experimentally derived volatile release rates reveals a good agreement, indicating that the high temperature derived kinetic parameters from HRG and EFR experiments can be extrapolated to lower temperatures. Nevertheless, discrepancies in the tar to light gas ratio occur with the proposed model implementation. © 2022 Taylor & Francis Group, LLC.

Titanium alloys can suffer from halide-associated stress corrosion cracking at elevated temperatures e.g., in jet engines, where chlorides and Ti-oxide promote the cracking of water vapour in the gas stream, depositing embrittling species at the crack tip. Here we report, using isotopically-labelled experiments, that crack tips in an industrial Ti-6Al-2Sn-4Zr-6Mo alloy are strongly enriched (>5 at.%) in oxygen from the water vapour, far greater than the amounts (0.25 at.%) required to embrittle the material. Surprisingly, relatively little hydrogen (deuterium) is measured, despite careful preparation and analysis. Therefore, we suggest that a combined effect of O and H leads to cracking, with O playing a vital role, since it is well-known to cause embrittlement of the alloy. In contrast it appears that in α + β Ti alloys, it may be that H may drain away into the bulk owing to its high solubility in β-Ti, rather than being retained in the stress field of the crack tip. Therefore, whilst hydrides may form on the fracture surface, hydrogen ingress might not be the only plausible mechanism of embrittlement of the underlying matrix. This possibility challenges decades of understanding of stress-corrosion cracking as being related solely to the hydrogen enhanced localised plasticity (HELP) mechanism, which explains why H-doped Ti alloys are embrittled. This would change the perspective on stress corrosion embrittlement away from a focus purely on hydrogen to also consider the ingress of O originating from the water vapour, insights critical for designing corrosion resistant materials. © 2022 Acta Materialia Inc.

The purpose of this study is to find a direct and quantitative correlation of the structure of Co3-xFexO4nanoparticles with catalytic performance in 2-propanol oxidation. Eight nanocrystalline samples with varying iron contents are synthesized, and quantitative information regarding their structure is obtained from nitrogen physisorption, X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) analyzed by reverse Monte Carlo simulations. The catalytic performance is tested in 2-propanol oxidation in the gas phase. Overall, catalytic conversion data as a function of temperature are deconvoluted to obtain conversion and half-conversion temperatures as quantitative parameters for the different catalytic reaction channels. The crystal structure is described by a spinel structure with interstitial cation defects. These defects result in a reduced electronic state of the nanoparticles. The defect density depends on the cationic composition. We also observe a complex cationic distribution on tetrahedral and octahedral sites, which is strongly influenced by the overall cationic composition. In the catalytic tests, the samples exhibit a low-temperature pathway, which is deactivated in subsequent runs but can be recovered by an oxidative treatment of the catalyst. We find that the frequency of cation pairs CoO-CoOand CoO-CoTof the individual samples correlates directly to their catalytic activity and selectivity. © 2022 American Chemical Society. All rights reserved.

Iron- and steelmaking cause ∼7% of the global CO2 emissions, due to the use of carbon for the reduction of iron ores. Replacing carbon by hydrogen as the reductant offers a pathway to massively reduce these emissions. However, the production of hydrogen using renewable energy will remain as one of the bottlenecks at least during the next two decades, because making the gigantic annual crude steel production of 1.8 billion tons sustainable requires a minimum stoichiometric amount of ∼97 million tons of green hydrogen per year. Another fundamental aspect to render the ironmaking sector more sustainable lies in an optimal utilization of green hydrogen and energy, thus reducing efforts for costly in-process hydrogen recycling. We therefore demonstrate here how the efficiency in hydrogen and energy consumption during iron ore reduction can be dramatically improved by the knowledge-based combination of two technologies: partially reducing the ore at low temperature via solid-state direct reduction (DR) to a kinetically defined degree, and subsequently melting and completely transforming it to iron under a reducing plasma (i.e. via hydrogen plasma reduction, HPR). Results suggest that an optimal transition point between these two technologies occurs where their efficiency in hydrogen utilization is equal. We found that the reduction of hematite through magnetite into wüstite via DR is clean and efficient, but it gets sluggish and inefficient when iron forms at the outermost layers of the iron ore pellets. Conversely, HPR starts violent and unstable with arc delocalization, but proceeds smoothly and efficiently when processing semi-reduced oxides, an effect which might be related to the material's high electrical conductivity. We performed hybrid reduction experiments by partially reducing hematite pellets via DR at 700 °C to 38% global reduction (using a standard thermogravimetry system) and subsequently transferring them to HPR, conducted with a lean gas mixture of Ar-10%H2 in an arc-melting furnace, to achieve full conversion into liquid iron. This hybrid approach allows to exploit the specific characteristics and kinetically favourable regimes of both technologies, while simultaneously showing the potential to keep the consumption of energy and hydrogen low and improve both, process stability and furnace longevity by limiting its overexposure to plasma radiation. © 2022 Elsevier Ltd

Soft magnetic materials (SMMs) serve in electrical applications and sustainable energy supply, allowing magnetic flux variation in response to changes in applied magnetic field, at low energy loss1. The electrification of transport, households and manufacturing leads to an increase in energy consumption owing to hysteresis losses2. Therefore, minimizing coercivity, which scales these losses, is crucial3. Yet meeting this target alone is not enough: SMMs in electrical engines must withstand severe mechanical loads; that is, the alloys need high strength and ductility4. This is a fundamental design challenge, as most methods that enhance strength introduce stress fields that can pin magnetic domains, thus increasing coercivity and hysteresis losses5. Here we introduce an approach to overcome this dilemma. We have designed a Fe–Co–Ni–Ta–Al multicomponent alloy (MCA) with ferromagnetic matrix and paramagnetic coherent nanoparticles (about 91 nm in size and around 55% volume fraction). They impede dislocation motion, enhancing strength and ductility. Their small size, low coherency stress and small magnetostatic energy create an interaction volume below the magnetic domain wall width, leading to minimal domain wall pinning, thus maintaining the soft magnetic properties. The alloy has a tensile strength of 1,336 MPa at 54% tensile elongation, extremely low coercivity of 78 A m−1 (less than 1 Oe), moderate saturation magnetization of 100 A m2 kg−1 and high electrical resistivity of 103 μΩ cm. © 2022, The Author(s).

The distribution of water on metal supported oxides is an important step in understanding heterogeneous catalysis such as in the water gas shift reaction. Here, we study water structures on Ag(111)-supported graphitic zinc oxide islands by variable temperature scanning tunneling microscopy around 150 K and ab initio calculations. Water clusters, accumulating on the ZnO islands, are confined to the hcp regions of the ZnO moiré pattern. A significantly higher cluster density at the island border is related to the dimensions of its capture zone. This suggests an upward mass transport of the water from the supporting metal to the ultrathin oxide film, increasing the water density at the active metal-oxide border. © 2022 American Chemical Society. All rights reserved.

The entropy coefficient is an important quantity to describe thermodynamic processes of battery cells and to model the temperature dependency of the open-circuit voltage. Determining the entropy via potentiometric measurements is often time-consuming. Therefore, several methods were developed to quickly estimate the entropy coefficient. This paper presents a new method, which is relatively simple in its execution and evaluation. The method is based on relating a dynamic temperature change to the cell's voltage change at an induced exponential temperature progression. Due to the dynamic of this process, the measurement time will be reduced compared to the potentiometric method by factor 13 for the executed experiments under its stated conditions. © 2022 Elsevier Ltd

Nickel-based superalloys have a wide range of applications in high temperature and stress domains due to their unique mechanical properties. Under mechanical loading at high temperatures, rafting occurs, which reduces the service life of these materials. Rafting is heavily affected by the loading conditions associated with plastic strain; therefore, understanding plastic strain evolution can help understand these material's service life. This research classifies nickel-based superalloys with respect to creep strain with deep learning techniques, a technique that eliminates the need for manual feature extraction of complex microstructures. Phase-field simulation data that displayed similar results to experiments were used to build a model with pre-trained neural networks with several convolutional neural network architectures and hyper-parameters. The optimized hyper-parameters were transferred to scanning electron microscopy images of nickel-based superalloys to build a new model. This fine-tuning process helped mitigate the effect of a small experimental dataset. The built models achieved a classification accuracy of 97.74% on phase-field data and 100% accuracy on experimental data after fine-tuning. © 2022 The Author(s). Published by IOP Publishing Ltd.

CO2 methanation using nickel-based catalysts has attracted large interest as a promising power-to-gas route. Ni nanoparticles supported on nitrogen-doped CNTs with Ni loadings in the range from 10 wt% to 50 wt% were synthesized by impregnation, calcination and reduction and characterized by elemental analysis, X-ray powder diffraction, H2 temperature-programmed reduction, CO pulse chemisorption and transmission electron microscopy. The Ni/NCNT catalysts were highly active in CO2 methanation at atmospheric pressure, reaching over 50% CO2 conversion and over 95% CH4 selectivity at 340 °C and a GHSV of 50,000 mL g−1 h−1 under kinetically controlled conditions. The small Ni particle sizes below 10 nm despite the high Ni loading is ascribed to the efficient anchoring on the N-doped CNTs. The optimum loading of 30 wt%–40 wt% Ni was found to result in the highest Ni surface area, the highest degree of conversion and the highest selectivity to methane. A constant TOF of 0.3 s−1 was obtained indicating similar catalytic properties of the Ni nanoparticles in the range from 10 wt% to 50 wt% Ni loading. Long-term experiments showed that the Ni/NCNT catalyst with 30 wt% Ni was highly stable for 100 h time on stream. © 2020 Science Press

A step in the direction of the real-life application of fuel cells (FCs) has been realized through the fabrication of a promising proton conductive membrane comprising a perfluorosulfonic-acid ionomer and nitrogen-rich poly[2,2′-(4,4′-bipyridine)-5,5′-bibenzimidazole] (BiPyPBI). The BiPyPBI-perfluorosulfonic acid membranes displayed remarkable oxidative and mechanical stabilities with significant proton conduction over wide ranges of temperatures (40 to 140 °C) and humidities (30 to 90% RH). A 0.5 molar BiPyPBI feed ratio increased the proton conduction of perfluorosulfonic acid by 2.6- and 1.5-fold at 40 and 80 °C, respectively, due to the enhancement in the ion-exchange capacity (1.9 mmol/g, which was twofold higher than that of bare Nafion). The protonic conductivity reached 0.171 S/cm at 140 °C. Using a BiPyPBI feed increased the stability of the Nafion membrane, corresponding to a 3.5-fold increase in the mechanical stress (9.6 MPa) and a 2.2-fold decrease in the elongation at break. In addition, the oxidation stability of the Nafion membrane increased by 26%. The measured activation energy suggested that the presence of BiPyPBI created an easier proton transport pathway (by the Grotthuss mechanism) because of a stronger hydrogen-bonding network than in bare Nafion. Compared to the power density of a perfluorosulfonic-based MEA, the power density of the BiPyPBI-perfluorosulfonic-based membrane electrode assembly (MEA) at 140 °C increased by approximately 20-fold to 175 mW cm-1 at 30% RH and by approximately 5-fold to 201 mW cm-1 at 90% RH. Impedance spectra confirmed the improvement of the FC performance of the BiPyPBI-perfluorosulfonic-based MEA, indicating enhanced charge transfer. After 10,000 cycles of relative humidity stress testing, the BiPyPBI-perfluorosulfonic-based MEA showed a power density of 146 mW cm-1 (corresponding to a 16% loss in the initial power density measured at 30% RH). The MEA lost only 26% of its initial power density upon relative humidity stress cycling. © 2021 American Chemical Society.

In this issue of Matter, Yao et al. report on advanced non-equilibrium high-temperature entropy-controlled synthesis of polyelemental nanoparticles. They achieve extreme mixing of 15 metals, some of them previously immiscible, in the form of a single phase solid solution. The compositionally tunable properties of such atomic scale mixtures within a simple crystal structure makes them highly interesting for the design of new materials, e.g., electrocatalysts. © 2021 Elsevier Inc.

The unique structural and electronic properties of transition metal dichalcogenides (TMDs) and in particular tungsten disulphide (WS2) make them interesting for a variety of applications such as the electrocatalytic hydrogen evolution reaction (HER) for water splitting devices and chemiresistive gas sensors. The key parameter for the realisation of these devices is the controlled large-area growth of WS2 combined with tuning the surface morphology and electronic properties which is achieved by bottom-up fabrication methods such as chemical vapour deposition (CVD). In this study, 2H-WS2 films are fabricated by a new metal-organic CVD (MOCVD) process resulting in the growth of crystalline, pure, and stoichiometric films which was accomplished at temperatures as low as 600 °C as confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS)/nuclear reaction analysis (NRA), and Raman spectroscopy. The surface morphology of WS2 layers was investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). Following successful process development, the WS2 layers were deposited on conducting FTO/glass substrates and their catalytic activity for the HER was evaluated in a linear sweep voltammetry (LSV) experiment. Furthermore, the temperature-dependent sensor response towards NO2, CO, and NH3 was investigated for WS2 films deposited on special sensor chips, revealing a p-type response towards NO2 and NH3 and sensitivities of around 20% for NO2 and NH3 concentrations of 1.5 ppm and 7.6 ppm, respectively. These promising results demonstrate the effectiveness of scalable CVD-grown WS2 and pave the way for practical applications by modulating the properties of materials to achieve enhanced electrocatalytic and sensing performances employing WS2 layers. © 2021 The Royal Society of Chemistry.

The plastic deformation behavior of single crystals of the quinary, equiatomic Cr−Mn−Fe−Co−Ni high-entropy alloy (HEA) with the face-centered cubic structure has been investigated in tension and compression as a function of crystal orientation and temperature from 10 K to 1373 K. The critical resolved shear stress (CRSS) for {111}<110> slip at room temperature is 42−45 MPa. It does not depend much on crystal orientation (i.e., the Schmid law holds true) and the sense (tension vs. compression) of the applied load. The CRSS for {111}<110> slip increases with the decrease in temperature, without showing any significant inertia effects at cryogenic temperatures below 77 K. Extrapolation from the measured yield stresses down to 10 K yields a CRSS value at 0 K of 168 MPa. At cryogenic temperatures, the measured strain-rate sensitivity of flow stress is consistent with a very small activation volume. The concept of stress equivalence holds true both for the temperature dependence of CRSS and the stress dependence of activation volume, indicating that solid-solution hardening is the major strengthening mechanism. Deformation twinning occurs at 77 K but not at room temperature, resulting in higher tensile elongation to failure at 77 K than at room temperature. Deformation twinning at 77 K occurs at a shear stress of 378 MPa on conjugate (1¯1¯1) planes in the form of Lüders deformation after large plastic strain (about 85%) achieved by the stage I (easy glide) and stage II (linear work-hardening) deformation. © 2020

Employing atomistic simulations, we investigated the void collapse mechanisms in single crystal Ni during hydrostatic compression and explored how the atomistic mechanisms of void collapse are influenced by temperature. Our results suggest that the emission and associated mutual interactions of dislocation loops around the void is the primary mechanism of void collapse, irrespective of the temperature. The rate of void collapse is almost insensitive to the temperature, and the process is not thermally activated until a high temperature (1200-1500 K) is reached. Our simulations reveal that, at elevated temperatures, dislocation motion is assisted by vacancy diffusion and consequently the void is observed to collapse continuously without showing appreciable strain hardening around it. In contrast, at low and ambient temperatures (1 and 300 K), void collapse is delayed after an initial stage of closure due to significant strain hardening around the void. Furthermore, we observe that the dislocation network produced during void collapse remains the sample even after complete void collapse, as was observed in a recent experiment of nickel-base superalloy after hot isostatic pressing. © 2021 by the authors.

The severe sliding abrasion of single‐phase metallic materials is a complex issue with a gaining importance in industrial applications. Different materials with different lattice structures react distinctly to stresses, as the material reaction to wear of counter and base body is mainly de-termined by the deformation behavior of the base body. For this reason, fcc materials in particular are investigated in this work because, as shown in previous studies, they exhibit better hot wear behavior than bcc materials. In particular, three austenitic steels are investigated, with pure Ni as well as Ni20Cr also being studied as benchmark materials. This allows correlations to be worked out between the hot wear of the material and their microstructural parameters. For this reason, wear tests are carried out, which are analyzed on the basis of the wear characteristics and scratch marks using Electron Backscatter Diffraction. X‐Ray experiments at elevated temperatures were also carried out to determine the microstructural parameters. It was found that the stacking fault energy, which influences the strain hardening potential, governs the hot wear behavior at elevated temper-atures. These correlations can be underlined by analysis of the wear affected cross section, where the investigated materials have shown clear differences. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Silica fibers with a dimension of 0.3 μm ∙ 3.2 μm2 nm were prepared by a modified Stöber synthesis as model particles. The particles were characterized by scanning electron microscopy, elemental analysis, thermogravimetry and X-ray powder diffraction. Their uptake by macrophages (THP-1 cells and NR8383 cells) was studied by confocal laser scanning microscopy and scanning electron microscopy. The uptake by cells was very high, but the silica fibers were not harmful to NR8383 cells in concentrations up to 100 μg mL−1. Only above 100 μg mL−1, significant cell toxic effects were observed, probably induced by a high dose of particles that had sedimented on the cells and led to the adverse effects. The chemotactic response as assessed by the particle-induced migration assay (PICMA) was weak in comparison to a control of agglomerated silica particles. The as-prepared fibers were fully X-ray amorphous but crystallized to β-cristobalite after heating to 1000 °C and converted to α-cristobalite upon cooling to ambient temperature. The fibers had sintered to larger aggregates but retained their elongated primary shape. The particle cytotoxicity towards THP-1 cells was not significantly enhanced by the crystallization. © 2021 Elsevier B.V.

Nanosecond plasmas in liquids are being used for water treatment, electrolysis, or biomedical applications. The exact nature of these very dynamic plasmas and, most importantly, their ignition physics are strongly debated. The ignition itself may be explained by two competing hypotheses: ignition in water may occur (i) via field effects at the tip of the electrode followed by tunneling of electrons in between water molecules causing field ionization or (ii) via gaseous processes of electron multiplication in nanovoids that are created from liquid ruptures due to the strong electric field gradients. Both hypotheses are supported by theory, but experimental data are very sparse due to the difficulty in monitoring the very fast processes in space and time. In this paper, we analyze nanosecond plasmas in water that are created by applying a positive and a negative polarity to a sharp tungsten electrode. The main diagnostics are fast camera measurements and fast emission spectroscopy. It is shown that plasma ignition is dominated by field effects at the electrode-liquid interface either as field ionization for positive polarity or as field emission for negative polarity. This leads to a hot tungsten surface at a temperature of 7000 K for positive polarity, whereas the surface temperature is much lower for negative polarity. At ignition, the electron density reaches 4 × 10 25 m - 3 for the positive and 2 × 10 25 m - 3 for the negative polarity. At the same time, the emission of the H α light for the positive polarity is four times higher than that for the negative polarity. During plasma propagation, the electron densities are almost identical of the order of 1- 2 × 10 25 m - 3 followed by a decay after the end of the pulse over 15 ns. It is concluded that plasma propagation is governed by field effects in a low density region that is created either by nanovoids or by density fluctuations in supercritical water surrounding the electrode that is created by the pressure and temperature at the moment of plasma ignition. © 2021 Author(s).

In the present article, we present a data compilation reflecting recrystallized microstructures and the corresponding mechanical properties of an equiatomic, single-phase face-centered cubic (FCC) CrFeNi medium-entropy alloy (MEA). For the analysis, interpretation, and discussion of the data, the reader can refer to the original research article entitled “Effects of temperature on mechanical properties and deformation mechanisms of the equiatomic CrFeNi medium-entropy alloy”, see Ref. (Schneider and Laplanche, Acta Mater. 204, 2020). The data related to recrystallized microstructures comprise raw backscatter electron (BSE) micrographs (tif-files) obtained using a scanning electron microscope (SEM) for six grain sizes in the range [10–160 µm], optical micrographs of the alloy with the largest grain size (d = 327 µm), pdf-reports and tables presenting the corresponding grain-size distributions (d, accounting for grain boundaries only) and crystallite-size distributions (c, which accounts for both grain and annealing twin boundaries), the annealing twin thicknesses (t), the average number of annealing twin boundaries per grain (n), and the average Taylor factor (M) of each recrystallized microstructure. These are benchmark datasets that may serve to develop new algorithms for the automated evaluation of microstructural parameters. Such algorithms would help to speed up the analyses of microstructures and improve their reliability. Furthermore, several groups pointed out that in addition to the mean grain size, other microstructural parameters such as the grain size distribution (Raeisinia et al., Model. Simul. Mater. Sc. 16, 2008) and the average number of twins per grain (Schneider et al., Int. J. Plasticity, 124, 2020) may affect some material properties (e.g. Hall-Petch strengthening). Therefore, an effort was made here to determine and report almost all the microstructural parameters describing recrystallized microstructures of FCC alloys. The mechanical-properties data are provided as excel-sheets in which the raw stress-strain curves can be found. Compression tests for alloys with different grain sizes were performed at room temperature. Additional compression tests and tensile tests for the grain size d = 160 µm were performed at temperatures between 77 K and 873 K. Characteristic mechanical properties, such as yield stresses at 0.2% plastic strain (σ0.2%) and Hall-Petch parameters (σ0 and ky) are given for all temperatures in the tables below. Moreover, the Hall-Petch parameters as well as the mechanical data reported in the present study could be used for data mining and implemented in programs used for alloy design. © 2021 The Author(s)

Controlling the transport of thermal energy is key to most applications of metal–organic frameworks (MOFs). Analyzing the evolution of the effective local temperature, the interfaces between the metal nodes and the organic linkers are identified as the primary bottlenecks for heat conduction. Consequently, changing the bonding strength at that node–linker interface and the mass of the metal atoms can be exploited to tune the thermal conductivity. This insight is generated employing molecular dynamics simulations in conjunction with advanced, ab initio parameterized force fields. The focus of the present study is on MOF-5 as a prototypical example of an isoreticular MOF. However, the key findings prevail for different node structures and node–linker bonding chemistries. The presented results lay the foundation for developing detailed structure-to-property relationships for thermal transport in MOFs with the goal of devising strategies for the application-specific optimization of heat conduction. © 2020 The Authors. Advanced Theory and Simulations published by Wiley-VCH GmbH

During the operation of turbines in jet engines or in power plants, high thermal and intermittent mechanical loads appear, which can lead to high-temperature fatigue failure if thermal and mechanical loads vary at the same time. Since fatigue testing is a time-consuming process, it is important to develop realistic material models with predictive capabilities that are able to extrapolate the limited experimental results for cyclic plasticity within a wide range of temperatures. To accomplish this, an approach based on a representative volume element (RVE), mimicking the typical γ/γ′ microstructure of a Ni-based single crystal superalloy, is adopted for cyclic loading conditions. With the help of this RVE, the temperature- and deformation-dependent internal stresses in the microstructure can be taken into account in a realistic manner, which proves to be essential in understanding the fatigue behavior of this material. The material behavior in the elastic regime is described by temperature-dependent anisotropic elastic constants. The flow rule for plastic deformation is governed by the thermal activation of various slip systems in the γ matrix, the γ′ precipitate and also by cube slip along the γ/γ′ microstructure. This phenomenological crystal plasticity/creep model takes different mechanisms into account, including thermally activated dislocation slip, the internal stresses due to inhomogeneous strains in different regions of γ matrix channels and in γ′ precipitates, the softening effect due to dislocation climb, the formation of 〈112〉 dislocation ribbons for precipitate shearing, and Kear-Wilsdorf locks. This constitutive law is parameterized based on experimental data for the CMSX-4 single-crystal superalloy by applying an inverse analysis to identify the material parameters based on many low cycle fatigue tests in the intermediate temperature and high stress regime. The identified material parameters could predict cyclic plasticity and low cycle fatigue behavior at different temperatures. The model does not only reliably reproduce the experimental results along different crystallographic loading directions, but it also reveals the relative importance of the different deformation mechanisms for the fatigue behavior under various conditions. © 2021 Elsevier Ltd

In this work, silicon–hafnia (Si-HfO2, 80/20 mol. %) feedstock was plasma sprayed for Environmental Barrier Coating bond coat application. In the as-sprayed coating, hafnium disilicide (HfSi2), HfO2 tetragonal (t), and cubic (c) phases with a total volume of ~20 % were detected together with Si and HfO2 monoclinic (m). The temperature-dependent evolution of these phases was analyzed and paired with microstructural observations. It was found that above 700 °C, HfSi2 oxidizes and HfO2 (t) and (c) transforms into (m) polymorph. Up to this temperature, as-sprayed coating showed a non-linear expansion behavior. Estimated volume expansion at ~750 °C was 3.6 % based on dilatometry measurement. The primary and secondary mechanisms leading to the expansion in the coating were identified as oxidation of HfSi2 and polymorphic phase transitions in HfO2, respectively. As a consequence of the volume expansion, the coating was extensively cracked during cyclic oxidation and hence not protective anymore. After 100 h at 1300 °C, the volume fraction of oxidation product SiO2 was significant in the coating (0.34), while HfO2 was largely consumed (0.1) in the formation of HfSiO4 (0.56). This result suggested that reversible α↔β phase transitions in SiO2-cristobalite could be another factor contributing to the cracking in the coating during cyclic oxidation. © 2021 Acta Materialia Inc.

The global demand for lightweight design is increasing to provide sustainable solutions to counteract climate change. We developed a novel Ti-bearing lightweight steel (8% lower mass density than general steels), which exhibits an excellent combination of strength (491 MPa ultimate tensile strength) and tensile ductility (31%) at elevated temperature (600 °C). The developed steel is suitable for parts subjected to high temperature at reduced dynamical load. The composition of the developed steel (Fe–20Mn–6Ti–3Al–0.06C–NbNi (wt%)) lends the alloy a multiphase structure with austenite matrix, partially ordered ferrite, Fe2Ti Laves phase, and fine MC carbides. At elevated temperature (600 °C), the ductility of the new material is at least 2.5 times higher than that of conventional lightweight steels based on the Fe–Mn–Al system, which become brittle at elevated temperatures due to the inter/intragranular precipitation of κ-carbides. This is achieved by the high thermal stability of its microstructure and the avoidance of brittle κ-carbides in this temperature range. © 2021 Elsevier B.V.

Using both novel high-throughput screening via combinatorial thin film deposition and conventional bulk alloy synthesis techniques, a large region of the AlCrFeNiTi composition space has been probed for alloys that could serve as low cost alternatives to nickel-base superalloys for medium-to-high temperature structural applications. Phase formation trends in this highly complex alloying system have been determined using characterisation techniques that span multiple length scales—from bulk X-ray diffraction and differential scanning calorimetry to atomically resolved scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. A large region of stability for both disordered A2 and ordered B2/L21 type phases is observed, with several compositions exhibiting fine-scaled precipitation structures of these two phases. For alloys with ≥20 at.% Al, the precipitation structure was further refined to a nano-scale lamellar arrangement of A2 and B2/L21 phases. Formation of C14 Laves phase, especially for compositions with >10 at.% Ti, has consistently been observed. We include a screening of the mechanical properties based on nanoindentation and macroscopic hardness test data correlated with scanning electron microscope (SEM) observations of the hardness indents. The phase formation trends observed by both combinatorial thin film deposition and bulk alloy synthesis are discussed in detail for samples in the as-deposited and as-cast conditions, respectively. © 2020 Elsevier Ltd

We present a first-principles assessment of the finite-temperature thermodynamic properties of the intermetallic Al3Sc phase including the complete spectrum of excitations and compare the theoretical findings with our dilatometric and calorimetric measurements. While significant electronic contributions to the heat capacity and thermal expansion are observed near the melting temperature, anharmonic contributions, and electron–phonon coupling effects are found to be relatively small. On the one hand, these accurate methods are used to demonstrate shortcomings of empirical predictions of phase stabilities such as the Neumann–Kopp rule. On the other hand, their combination with elasticity theory was found to provide an upper limit for the size of Al3Sc nanoprecipitates needed to maintain coherency with the host matrix. The chemo-mechanical coupling being responsible for the coherency loss of strengthening precipitates is revealed by a combination of state-of-the-art simulations and dedicated experiments. These findings can be exploited to fine-tune the microstructure of Al-Sc-based alloys to approach optimum mechanical properties. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Modeling the geothermal energy production cycle of a deep geothermal system at laboratory scale is challenging because of high-temperature and pressure conditions. In this work, a high-pressure high-temperature column to simulate production, heat transfer, and reinjection of a geothermal fluid in a fractured rock system is presented. The column includes two independently heated pressure vessels, a heat exchanger, and sensors for temperatures, pressures, flow rate, electric conductivity, and pH value of the circulating fluid at different locations. The presented column enables the quantitative analysis of coupled hydro-thermo-chemical processes in fractured rock cores close to in situ geothermal conditions. Heat extraction and reinjection of geothermal fluids into fractured reservoirs can be reproduced because of the possibility of heating and cooling of the circulating fluid. Further, it is possible to inject a second fluid phase into the column to investigate additional processes, such as mineral precipitation during reinjection. In this work, we present the experimental setup of the column and first results showing the capability of the system. © 2021 ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.

The wet-chemical synthesis of hollow graphitic spheres, a highly defined model catalyst support for electrocatalytic processes, is laborious and not scalable, which hampers potential applications. Here, we present insights into the chemical vapor deposition (CVD) of ferrocene as a simple, scalable method to synthesize hollow graphitic spheres (HGScvd). During the CVD process, iron and carbon are embedded in the pores of a mesoporous silica template. In a subsequent annealing step, iron facilitates the synthesis of highly ordered graphite structures. We found that the applied temperature treatment allows for controlling of the degree of graphitization and the textural properties of HGScvd. Further, we demonstrate that platinum loaded on HGScvd is significantly more stable during electrochemical degradation protocols than catalysts based on commercial high surface area carbons. The established CVD process allows the scalable synthesis of highly defined HGS and therefore removes one obstacle for a broader application. © 2021 The Authors. Published by American Chemical Society.

Yttria-stabilized zirconia (YSZ) has been for several decades the state of the art material for thermal barrier coating (TBC) applications in gas turbines. Although the material has unique properties, further efficiency improvement by increasing the temperature is limited due to its maximum temperature capability of about 1200°C. Above this temperature the deposited metastable tetragonal (t´) phase undergoes a detrimental phase transformation as well as enhanced sintering. Both processes promote the failure of the coatings at elevated temperatures and this early failure has been frequently observed in gradient tests. In this paper, we now experimentally shown for the first time that under typical cycling conditions not the time at elevated temperatures leads to the reduced lifetime but the transient cooling rates. If cooling rates were reduced to 10K/s, TBC systems could be operated in a burner rig at a surface temperature well above 1500°C without showing a lifetime reduction. The explanation of these astonishing findings is given by the evaluation of energy release rate peaks during fast transient cooling in combination with the phase evolution during cooling with the used cooling rates. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)

We report the results of our in-depth analysis of spectroscopic and thermodynamic properties of a multiorbital metal, like FeSe, which first develops a nematic order and then undergoes a transition into a superconducting state, which coexists with nematicity. We analyze the angular dependence of the gap function and specific heat of such a nematic superconductor. We specifically address three issues: (i) the angular dependence of the gap in light of the competition between the nematicity-induced mixture and the orbital transmutation of low-energy excitations in the nematic state, (ii) the effect of nematicity on the magnitude of the jump of the specific heat at and the temperature dependence of below , and (iii) a potential transition at from an state to an state that breaks time-reversal symmetry. We consider two scenarios for a nematic order: scenario A, in which this order develops between and orbitals on hole and electron pockets, and scenario B, in which there is an additional component of the nematic order for fermions on the two electron pockets. ©2021 American Physical Society

The phase stability of a bcc AlNbTiV high-entropy alloy at elevated temperatures is studied using a combination of machine-learning interatomic potentials, first-principles calculations, and Monte Carlo simulations. The simulations reveal a B2 ordering below about 1700 K, mainly caused by a strong site preference of Al and Ti. A much weaker site preference for V and Nb is observed, strongly affecting the alloys total configurational entropy. The underlying mechanisms of the B2 phase stability as opposed to the random solid solution are discussed in terms of a high persisting configurational entropy of the B2 phase due to strong sublattice site disorder. © 2021 authors.

CoxFe3-xO4nanoparticles (x= 0.4 tox= 2.5) and thin films (x= 0.9 tox= 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co2+and Fe3+cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, CoxFe3-xO4nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperature-resolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles’ surface. © 2021 The Authors. Published by American Chemical Society

CoxFe3-xO4 nanoparticles (x = 0.4 to x = 2.5) and thin films (x = 0.9 to x = 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co2+ and Fe3+ cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, CoxFe3-xO4 nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperature-resolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles' surface. © 2021 The Authors. Published by American Chemical Society.

Flights in rotary drums are used to improve the mixing and the heat transfer. They lift particles out of the solid bed to shower them as curtains through the gas phase of the drum. The number of particles and their distribution are influenced by several operational parameters. An indirectly heated flighted rotary drum was designed and constructed to conduct experiments relating transverse particle motion to heat transfer. Batch experiments divided into heating and cooling periods were performed with glass beads as reference material. The axial, circumferential and radial temperature distributions were measured. It was found that increasing the rotational speed as well as the volumetric airflow rate leads to quick temperature changes, while an increase in the filling degree results in a gradual decrease of the temperature drop during cooling. © 2020 Elsevier Ltd

A single Cr-rich σ-phase alloy with a composition of Co17Cr46Fe16.3Mn15.2Ni5.5 (at.%) and a tetragonal lattice structure was produced. The tracer diffusion coefficients of Ni and Fe were measured by secondary electron mass spectroscopy using the highly enriched 64Ni and 58Fe natural isotopes. On the homologous temperature scale, Ni and Fe diffuse in the σ phase faster as compared to the corresponding diffusion rates in the equiatomic and face-centered cubic CoCrFeMnNi alloy. In contrast, on the absolute temperature scale, these elements diffuse roughly at the same rates in both materials. Factors influencing element diffusion and phase stability of the σ phase compared to the equiatomic alloy are discussed. © 2020 Acta Materialia Inc.

Tempered martensite ferritic steels (TMFSs) have been and are being used for critical components in high temperature plant operating in the 600 °C range. They are exposed to creep conditions for long time periods, exceeding 100,000 h. In the present study we investigate a 12% Cr TMFS, after creep-testing at 550 °C at 120 MPa for 139,000 h. We had previously investigated this material in the TEM using thin foils. We now use an extraction replica technique to analyze four particle families: M23C6, MX, Laves-phase and Z-phase, considering statistically relevant numbers of particles (between 120 and 720). We show how EELS mapping can help in identifying Z-phase particles and use Cr-V-maps to differentiate between the four particle families. The chemical evolution of particles is investigated. The experimental results are discussed in the light of previous thin foil data and with respect to predictions from computational thermodynamics. The strength and weakness of thin foil and replica procedures are compared. Improvements for thermodynamic databases are suggested. © 2021

Short-term heat treatments of steels are used for tools and cutlery but also for the surface treatment of a variety of other workpieces. If corrosion resistance is required, martensitic stainless steels like AISI 420L or AISI 420MoV are typically used. The influence of short-term heat treatment on the different metastable states of the AISI 420L steel was examined and reported in this article. Starting from a defined microstructural state, the influence of a short-term heat treatment is investigated experimentally with the help of a quenching dilatometer and computer assisted simulations are carried out. With the results obtained, a simulation model is built up which allows to compute the microstructural changes during a short-term heat treatment to be evaluated without the need for an experiment. As an indicator, the value of the martensite start temperature is calculated as a function of different holding times at austenitizing temperature. The martensite start temperature is measured by dilatometry and compared to calculated values. Validation of simulated results reveals the potential of optimizing steel heat treatment processes and provides a reliable approach to save time, resources and energy. © 2021, The Author(s).

Flow dynamics and temperatures of solid fuel particles strongly influence flame stabilization, local heat release and fuel conversion inside pulverized solid fuel combustors. To investigate these phenomena, experiments are carried out under well-controlled inflow and boundary conditions inside a gas-assisted, swirled oxy-fuel combustion chamber. Flow fields of small particles that represent the gas phase velocity are determined in the near-burner region by PIV using a particle separation algorithm. Trajectories of large solid fuel particles are evaluated in a two-dimensional plane using a combined high-speed PIV/PTV approach. Particle temperatures and particles sizes are measured at different levels downstream the burner exit to reveal different stages of combustion. Therefore, a two-color pyrometer is used that dissolve single particles to achieve local particle temperature and particle size distributions. Two oxy-fuel operation conditions with an oxygen fraction of 33%V and a reference operation point in air are investigated within this study. In the flow fields of the gas phase the impact of the atmosphere is clearly visible in the spatial expansion of the internal recirculation area. Regions of high slip velocities and high heat release could be identified by analyzing particle trajectories in terms of direction, velocity and acceleration. Residence times of small and large particles are estimated from the flow fields. Significantly larger residence times are observed for large particles which leads to higher burn out rates in the near-burner region. Furthermore, particle temperature measurements reveal similar particle temperatures for the investigated oxy-fuel and air operation conditions. © 2020 Elsevier Ltd

In this work, the nanometer-scale mass flow coupled to dislocation processes near the γ/γ′-interface during high temperature and low stress creep of a model Ni-base single crystal superalloy is investigated. In the early creep stages, the dislocation networks in the γ-phase at γ/γ′-interfaces attract γ-stabilizing elements like Cr, Co and in particular Re, resulting in compositional gradients close to the interface. At larger strains, where dislocations frequently cut into the γ′-phase, this local interfacial enrichment in these elements is no longer observed. The cutting dislocations take part of the segregated atoms away, whilst the remaining atoms are released and diffuse back into the γ-channels. © 2020

Metastable Cr0.8Cu0.2 alloy thin films with nominal thickness of 360 nm have been deposited on Si(100) substrate by co-evaporation of Cu and Cr using molecular beam epitaxy (MBE). Phase evolution, microstructure, stress development, and crystallographic texture in Cr0.8Cu0.2 thin films have been investigated by X-ray diffraction (XRD), atom probe tomography (APT) and transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDS) during annealing of the films in the temperature range 200–450 °C. X-ray diffraction of the as-deposited thin film shows single phase bcc crystal structure of the film whereas APT observation of fine precipitates in the film matrix due to inherent compositional fluctuation indicates onset of phase separation via spinodal decomposition regime. XRD (in-situ) and APT investigation of 300 °C annealed film reveals that the early stage of phase separation involves localized formation of metastable intermediate bcc precipitate phase having 60 at% Cr and 40 at% Cu approximately (~Cr0.6Cu0.4). For longer duration of annealing at temperature ≥350 °C, such metastable bcc precipitates act as heterogeneous nucleation sites for the onset of precipitation of Cu rich fcc Cu(Cr) phase which indicates a change of phase separation mechanism from ‘spinodal decomposition’ to ‘nucleation and growth’. Annealing of the film at temperature ≥400 °C for longer duration leads to the formation of a two phase structure with Cu rich fcc precipitate phase in a Cr rich bcc matrix. Observed phase decomposition is accompanied by significant changes in the microstructure, residual stress and crystallographic texture in the Cr rich bcc film matrix which leads to the minimization of both surface and strain energies and thereby a reduction of total Gibbs free energy of the thin film. Thermodynamic model calculation has been presented in order to understand the nucleation pathway of Cu rich stable fcc Cu(Cr) precipitates via non-classical nucleation of metastable intermediate bcc Cr0.6Cu0.4 phase. © 2021 Elsevier B.V.

Capacitance-voltage measurements on self-assembled quantum dot layers exposed to strong electric fields and with large distances to the reservoirs show a marked hysteretic behavior. It is shown that at low temperatures this hysteresis can be explained quantitatively in terms of state-dependent capture and emission rates that are obtained by a rate equation model, applied to the measured capacitance transients. The occupation dynamics and the steady-state configuration can be extracted from these data via a Markov chain model. © 2021 American Physical Society.

Ti75Ta25 high-temperature shape memory alloys exhibit a number of features which make it difficult to use them as spring actuators. These include the high melting point of Ta (close to 3000 °C), the affinity of Ti to oxygen which leads to the formation of brittle α-case layers and the tendency to precipitate the ω-phase, which suppresses the martensitic transformation. The present work represents a case study which shows how one can overcome these issues and manufacture high quality Ti75Ta25 tensile spring actuators. The work focusses on processing (arc melting, arc welding, wire drawing, surface treatments and actuator spring geometry setting) and on cyclic actuator testing. It is shown how one can minimize the detrimental effect of ω-phase formation and ensure stable high-temperature actuation by fast heating and cooling and by intermediate rejuvenation anneals. The results are discussed on the basis of fundamental Ti–Ta metallurgy and in the light of Ni–Ti spring actuator performance. © 2021, The Author(s).

Multiple-principal element alloys hold great promise for multifunctional material discovery (e.g., for novel electrocatalysts based on complex solid solutions) in a virtually unlimited compositional space. Here, the phase constitution of the noble metal system Ag-Ir-Pd-Pt-Ru was investigated over a large compositional range in the quinary composition space and for different annealing temperatures from 600 to 900 °C using thin-film materials libraries. Composition-dependent X-ray diffraction mapping of the as-deposited thin-film materials library indicates different phases being present across the composition space (face-centered cubic (fcc), hexagonal close packed (hcp) and mixed fcc + hcp), which are strongly dependent on the Ru content. In general, low Ru contents promote the fcc phase, whereas high Ru contents favor the formation of an hcp solid-solution phase. Furthermore, a temperature-induced phase transformation study was carried out for a selected measurement area of fcc-Ag5Ir8Pd56Pt8Ru23. With increasing temperature, the initial fcc phase transforms to an intermediate C14-type Laves phase at 360 °C, and then to hcp when the temperature reaches 510 °C. The formation and disappearance of the hexagonal Laves phase, which covers a wide temperature range, plays a crucial role of bridging the fcc to hcp phase transition. The obtained composition, phase and temperature data are transformed into phase maps which could be used to guide theoretical studies and lay a basis for tuning the functional properties of these materials. [Figure not available: see fulltext.] © 2021, The Author(s).

Motivated by the recent experiments suggesting the importance of nematicity in the phase diagrams of iron-based and cuprate high-Tc superconductors, we study the influence of nematicity on the collective modes inside the superconducting state in a nonequilibrium. In particular, we consider the signatures of collective modes in short-time dynamics of a system with competing nematic and s- and d-wave superconducting orders. In the rotationally symmetric state, we show that the Bardasis-Schrieffer mode, corresponding to the subdominant pairing, hybridizes with the nematic collective mode and merges into a single in-gap mode, with the mixing vanishing only close to the phase boundaries. For the d-wave ground state, we find that nematic interaction suppresses the damping of the collective oscillations in the short-time dynamics. Additionally, we find that even inside the nematic s+d-wave superconducting state, a Bardasis-Schrieffer-like mode leads to order parameter oscillations that strongly depend on the competition between the two pairing symmetries. We discuss the connection of our results to the recent pump-probe experiments on high-Tc superconductors. © 2021 American Physical Society.

Even for processes with only a few gas species involved the detailed description of plasma-assisted conversion processes in gas mixtures requires a large amount of processes to be taken into account and a large number of neutral and charged particles must be considered. In addition, setting up the corresponding reaction kinetics model needs the knowledge of the rate coefficients and their temperature dependence for all possible reactions between those species. Reduced reaction networks offer a simplified and pragmatic way to obtain an overall reaction kinetics model, already useful for the analysis of experimental data even if not all details of chemistry can be covered. In this paper we present a derivation of a data driven reduced model for plasma-assisted conversion of methane in an helium environment. By consideration of a small number of elementary reactions, a simple model is set up. Experimental data are analyzed by a genetic algorithm that provides best-fit approximations for the open parameters of the model. In a further step non-relevant parameters of the model are identified and a further model reduction is achieved. The data driven analysis of methane conversion serves as an illustrative example of the proposed method. The parameters and reaction channels found are compared with known results from the literature. The method is described in detail. The main goal of this work is to present the potential of this data driven method for a simplified and pragmatic modeling in the increasingly important field of plasma-assisted catalytic processes. © 2021, The Author(s).

1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT CN) was synthesized mechanochemically at room temperature. The coupling of hexaketocyclohexane and diaminomaleonitrile was conducted in 10 min by vibratory ball milling. The effects of milling parameters, acids, dehydrating agents, and liquid-assisted grinding were rationalized. With 67%, the yield of this mechanochemical approach exceeds that of state-of-the-art wet-chemical syntheses while being superior with respect to time-, resource-, and energy-efficiency as quantified via green metrics. © 2021 The Authors. Published by American Chemical Society.

We devise automated workflows for the calculation of Helmholtz and Gibbs free energies and their temperature and pressure dependence and provide the corresponding computational tools. We employ nonequilibrium thermodynamics for evaluating the free energy of solid and liquid phases at a given temperature and reversible scaling for computing free energies over a wide range of temperatures, including the direct integration of P-T coexistence lines. By changing the chemistry and the interatomic potential, alchemical and upscaling free energy calculations are possible. Several examples illustrate the accuracy and efficiency of our implementation. ©2021 American Physical Society.

Membrane separation processes including reverse osmosis are now considered essential techniques for water and wastewater treatment, especially in water-scarce areas where desalination and water reuse can augment the water supply. However, biofouling remains a significant challenge for these processes and in general for marine biological fouling, which results in increased energy consumption and high operational costs. Especially in flat sheet membrane modules, intense biofilm growth occurs on the feed spacer at points of contact to the membrane surface. Here, we developed an ultrastable superhydrophobic antibiofouling feed spacer that resists biofilm growth. A commercial polypropylene feed spacer was coated with poly(dimethylsiloxane) (PDMS), and then, candle soot nanoparticles (CSNPs) were embedded into the ultrathin layer of PDMS, which resulted in a superhydrophobic nanostructured surface with a contact angle >150°. The CSNP-coated spacer was examined for inhibition of biofilm growth by a cross-flow membrane channel using Pseudomonas aeruginosa (PA01), and the coating was examined for effectiveness in marine fouling by testing the adhesion of marine bacterium Cobetia marina and diatom Navicula perminuta in a dynamic accumulation assay. In all cases, the CSNP coatings showed almost complete elimination of biofilm growth under the conditions tested. Confocal laser scanning microscopy and scanning electron microscopy indicated a 99% reduction in biofilm growth on the modified spacers compared to the uncoated controls. This effect was attributed to the superhydrophobic nanostructured surface, where trapped gasses formed a plastron on the coating. This plastron was observed to be extremely stable over time and could even be replenished at elevated temperatures. Development of similar antibiofouling coatings on feed spacers or other marine applications might lead to improvements in many industrial processes including membrane filtration where increased membrane life span and reduced energy consumption are key to implementation. © 2021 American Chemical Society.

The low-temperature plasticity of body-centered cubic (bcc) metals is governed by [Formula presented] screw dislocations due to their compact, non-planar core. It has been proposed that 70.5° mixed (M111) dislocations may also exhibit special core structures and comparably large Peierls stresses, but the theoretical and experimental evidence is still incomplete. In this work, we present a detailed comparative study of the M111 dislocation in five bcc transition metals on the basis of atomistic simulations. We employ density functional theory and semi-empirical interatomic potentials to investigate both the core structure and the Peierls barrier of the M111 dislocation. Our calculations demonstrate that reliable prediction of M111 properties presents not only a very stringent test for the reliability of interatomic potentials but is also challenging for first-principles calculations for which careful convergence studies are required. Our study reveals that the Peierls barrier and stress vary significantly for different bcc transition metals. Sizable barriers are found for W and Mo while for Nb, Ta and Fe the barrier is comparably small. Our predictions are consistent with internal friction measurements and provide new insights into the plasticity of bcc metals. © 2021 Acta Materialia Inc.

In human-made malleable materials, microdamage such as cracking usually limits material lifetime. Some biological composites, such as bone, have hierarchical microstructures that tolerate cracks but cannot withstand high elongation. We demonstrate a directionally solidified eutectic high-entropy alloy (EHEA) that successfully reconciles crack tolerance and high elongation. The solidified alloy has a hierarchically organized herringbone structure that enables bionic-inspired hierarchical crack buffering. This effect guides stable, persistent crystallographic nucleation and growth of multiple microcracks in abundant poor-deformability microstructures. Hierarchical buffering by adjacent dynamic strain–hardened features helps the cracks to avoid catastrophic growth and percolation. Our self-buffering herringbone material yields an ultrahigh uniform tensile elongation (~50%), three times that of conventional nonbuffering EHEAs, without sacrificing strength. © 2021 American Association for the Advancement of Science. All rights reserved.

The development of accurate kinetic databases by parametrizing the composition and temperature dependence of elemental atomic mobilities, is essential for correct multicomponent calculations and simulations. In this work the automated assessment procedure for the establishment of CALPHAD-type kinetic databases is proposed, including the storage of raw data and assessment results, automatic weighting of data, parameter selection and automated reassessments. This allows the establishment of reproducible up-to-date databases. The proposed software, written in python, is applied to the assessment of a kinetic database for the fcc Co-Cr-Fe-Mn-Ni high entropy alloy using only tracer diffusion data for a sharp separation of thermodynamic and kinetic data. The established database is valid for the whole composition range of the five-component high entropy alloy. © 2021 The Author(s). Published by IOP Publishing Ltd Printed in the UK

Diffusion of hydrogen bearing species in glasses plays a significant role in numerous applications in commercial as well as scientific domains. The investigation of diffusion of water in glasses at low temperatures led to experimental and analytical difficulties in the past. We present a new approach that lets us overcome these complications. Diffusion couples of An50Di50 glass (mol %, NBO/T = 0.67) were produced by coating anhydrous glass substrates with thin films of hydrated glass (~200 nm, ~2 wt% H2O) using pulsed laser deposition (PLD). Bonding the diffusant to the glass matrix of the thin film instead of using free water at the interface during experiments precludes other glass altering processes such as dissolution and precipitation. This allows us to confidently interpret the measured profiles to be a result of diffusion only. Nanoscale concentration profiles that result from diffusion at low temperatures on experimentally feasible time scales were measured with the Nuclear Resonance Reaction Analysis (NRRA, 1H(15N,αγ)12C). The non-destructive nature of NRRA enables us to observe and better understand the evolution of diffusion profiles with time within one sample. Evaluation of the sample quality by EPMA, SEM, optical microscopy, Rutherford backscattering spectroscopy (RBS), and NRRA was performed and confirmed the suitability of the samples for diffusion studies. Experiments at 1 atm in a box furnace and at 2 kbar in a CSPV (pressure medium = water) and an IHPV (pressure medium = Argon) prove that the diffusion couples can be used under various experimental conditions. We present diffusion profiles that were measured in experiments carried out in these devices and discuss the distinct features of each that result from different boundary conditions in the experiments. © 2020 Elsevier B.V.

A twin surface dielectric barrier discharge is used for the catalyst-enhanced plasma oxidation of 300 ppm n-butane in synthetic air. Plasma-only operation results in the conversion of n-butane into CO and CO2. Conversion is improved by increasing the temperature of the feed gas, but selectivity shifts to undesired CO. α-MnO2 is used as a catalyst deposited on the electrodes by spray coating with a distance of 1.5 mm between the uncoated grid lines and the square catalyst patches to prevent the inhibition of plasma ignition. The catalyst strongly influences selectivity, reaching 40% conversion and 73% selectivity to CO2 at a specific energy density of 390 J·L−1 and 140°C, which is far below the onset temperature of thermocatalytic n-butane conversion. © 2021 The Authors. Plasma Processes and Polymers published by Wiley-VCH GmbH.

Magnesium silicide stannide solid solutions, Mg2(Si,Sn), are prominent materials in the development of devices for thermoelectric energy conversion for intermediate operating temperatures, owing to the high values of their thermoelectric figure of meritzT, elemental abundance, and non-toxicity. The manufacturing of thermoelectric generators, however, relies also upon long-term stable contacts with low thermal and electrical resistivity and good bonding of the metallic contact bridge (electrode) to the thermoelectric legs of Mg2(Si,Sn) with a similar thermal expansion coefficient. In the assembly of thermoelectric generators, the thermoelectric legs have to be bonded to metallic electrodes to establish an electrical circuit. In this work, contacts between Mg2(Si0.3Sn0.7) and Cu were made at 600 °C and investigated using thermodynamic equilibrium calculations to gain understanding on the phase transformations occurring in the bonding process. Cu is selected as a metallic electrode as it is a highly conductive element with a thermal expansion coefficient similar to that of the thermoelectric material. Contacting methods usually deviate from equilibrium conditions; nevertheless, we use this contact couple to illustrate that equilibrium thermodynamic considerations are an efficient support to anticipate and identify the reaction products forming the final microstructure of the bonded region, and ultimately, for improving the contact design. A thermodynamic database of Gibbs energies for quaternary Cu-Mg-Si-Sn was built up and made available in this work. With this database, thermodynamic calculations were done in order to complement the experimental observations on the microstructure and thermochemistry of the Mg2(Si0.3Sn0.7)/Cu interconnections. The approach developed in this work is general and therefore applicable to the investigations of different thermoelectric materials and/or metallic electrodes, by enlarging the thermodynamic description, providing an effective guide to the experimental settings of the contacting process. © The Royal Society of Chemistry 2021.

Ammonia synthesis via the high-temperature and high-pressure Haber-Bosch process is one of the most important chemical processes in the world. In spite of numerous attempts over the last 100 years, continuous Haber-Bosch type ammonia synthesis at room-temperature had not been possible, yet. We report the development of a mechanocatalytic system operating continuously at room-temperature and at pressures down to 1 bar. With optimized experimental conditions, a cesium-promoted iron catalyst was shown to produce ammonia at concentrations of more than 0.2 vol. % for over 50 hours. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH

Science-driven design of future thermoelectric materials requires a deep understanding of the fundamental relationships between microstructure and transport properties. Grain boundaries in polycrystalline materials influence the thermoelectric performance through the scattering of phonons or the trapping of electrons due to space-charge effects. Yet, the current lack of careful investigations on grain boundary-associated features hinders further optimization of properties. Here, we study n-type NbCo1-xPtxSn half-Heusler alloys, which were synthesized by ball milling and spark plasma sintering (SPS). Post-SPS annealing was performed on one sample, leading to improved low-temperature electrical conductivity. The microstructure of both samples was examined by electron microscopy and atom probe tomography. The grain size increases from ~230 nm to ~2.38 μm upon annealing. Pt is found within grains and at grain boundaries, where it locally reduces the resistivity, as assessed by in situ four-point-probe electrical conductivity measurement. Our work showcases the correlation between microstructure and electrical conductivity, providing opportunities for future microstructural optimization by tuning the chemical composition at grain boundaries. © 2021 The Authors

The interface method is a well established approach for predicting melting points of materials using interatomic potentials. However, applying the interface method is tedious and involves significant human intervention. The whole procedure involves several successive tasks: estimate a rough melting point, set up the interface structure, run molecular dynamic calculations and analyze the data. Loop calculations are necessary if the predicted melting point is different from the estimated one by more than a certain convergence criterion, or if full melting/solidification occurs. In this case monitoring the solid–liquid phase transition in the interface structure becomes critical. As different initial random seeds for the molecular dynamic simulations within the interface method induce slightly different melting points, a few ten or hundred interface method calculations with different random seeds are necessary for performing a statistical analysis on these melting points. Considering all these technical details, the work load for manually executing and combining the various involved scripts and programs quickly becomes prohibitive. To simplify and automatize the whole procedure, we have implemented the interface method into pyiron (http://pyiron.org). Our fully automatized procedure allows to efficiently and precisely predict melting points of stable unaries represented by arbitrary potentials with only two user-specified parameters (interatomic potential file and element). For metastable or dynamically unstable unary phases, the crystal structure needs to be provided as an additional parameter. We have applied our automatized approach on fcc Al, Ni, dynamically unstable bcc Ti and hcp Mg and employed a large set of available interatomic potentials. Melting points for classical interatomic potentials of these metals have been obtained with a numerical precision well below 1 K. © 2020 The Authors

Abstract: Surface degradation phenomena of two model equiatomic alloys from the CrMnFeCoNi alloy system were investigated in 2% O2 and 10% H2O (pO2 = 0.02 and 10−7 atm, respectively) at 800 °C for times up to 96 h. The crystallographic structures, morphologies, and chemical compositions of the corrosion layers developing on CrMnFeCoNi and CrCoNi were comparatively analyzed by mass gain analysis, X-ray diffraction, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The oxidation resistance of CrMnFeCoNi is relatively poor due to the fast growth of porous Mn-oxide(s). CrCoNi forms an external chromia layer that is dense and continuous in a dry 2% O2 atmosphere. This layer buckles and spalls off after exposure to 10% H2O atmosphere. Beneath the chromia layer, a Cr-depleted zone forms in the CrCoNi alloy in both environments. As the oxide scale spalls off in the H2O-containing atmosphere, a secondary chromia layer was observed and correspondingly enlarges the Cr-depleted zone. In contrast, as the chromia layer remains without significant spallation when CrCoNi is exposed to a dry oxidizing atmosphere, the region depleted in Cr is narrower. Graphic Abstract: [Figure not available: see fulltext.]. © 2020, The Author(s).

Sintering-induced aggregation of active metals is a major cause of catalyst deactivation. Catalysts that can operate above 800°C are rare. Here, we report an unusual noble metal catalyst with sintering-induced activation at temperatures up to 1,000°C. The catalyst consists of atomically dispersed palladium embedded in a reducible SnO2 support designated for lean methane combustion. High temperature reaction simultaneously causes favorable changes of palladium ensemble state combining synergistically with lattice oxygen activation. Such changes lead to at least one order of magnitude improvement of the intrinsic reactivity, which compensates the surface area loss. Extensive characterizations such as atom probe tomography, X-ray absorption spectroscopy, and isotope tracking together with theoretical calculations illustrate the structure and surface chemistry changes and their impacts on the reaction mechanism. The catalyst also shows notable long-term stability and facile regeneration after poisoning. Our work may provide new insights into designing active and thermally stable catalysts. © 2020 The Author(s) Yang et al. report that atomic Pd catalysts on SnO2 undergo sintering of both Pd and support during high-temperature catalytic oxidation process. The sintered catalyst not only shows significant increase in activity for methane combustion under harsh reaction conditions, but also retains catalytic stability with high-temperature tolerance up to 1,000°C. © 2020 The Author(s)

An equiatomic CrFeNi medium-entropy alloy (MEA) that constitutes a cornerstone of austenitic stainless steels and Fe-based superalloys is investigated. Anneals at various temperatures revealed that CrFeNi forms a stable face-centered cubic (FCC) solid solution above ~1223 K. Based on this result, this alloy was cold-worked and recrystallized between 1273 K and 1473 K to produce different grain sizes. Compression tests were carried out at 293 K to investigate grain boundary strengthening (Hall-Petch slope: 966 MPa µm1/2) and this contribution was then subtracted from the overall strength to reveal the intrinsic uniaxial lattice strength (80 MPa). Additional compression and tensile tests were performed between 77 K and 873 K to study the effect of temperature on mechanical properties and deformation mechanisms. Ductility, yield and ultimate tensile strengths increased with decreasing temperature. To reveal the active deformation mechanisms in CrFeNi with the coarsest grain size (160 µm), tensile tests at 77 K and 293 K were interrupted at different strains followed by transmission electron microscopy analyses. In all cases, the deformation was accommodated by dislocation glide at low strains, while twinning additionally occurred above a critical resolved shear stress of 165 MPa, which was roughly temperature independent. This value compares well with predictions (180 MPa) based on the Kibey's model for twin nucleation. Moreover, the fact that this value is roughly temperature-independent is also consistent with the Kibey's model since the twin nucleation barrier (unstable twin stacking fault energy) of FCC metals and alloys does not vary significantly with temperature. © 2020 Acta Materialia Inc.

The cation distribution at room temperature, as well as elastic properties and thermal expansion of single crystal ZnGa2O4 (ZGO) and Zn1−xMgxGa2O4 (x ≈ 0.33; ZMGO) with spinel-type structure were studied in a wide temperature range using single crystal X-ray diffraction, neutron powder diffraction, inductive gauge dilatometry and resonant ultrasound spectroscopy. ZGO adopts an almost normal spinel structure, whereas ZMGO is significantly disordered. At room temperature, the elastic properties of ZMGO mostly fall between those of ZGO and MgGa2O4 (MGO). The temperature dependences of the thermoelastic properties of ZGO and ZMGO, as well as thermal expansion of ZGO reveal distinct signatures of glass-like transitions, which separate states in which the cation dynamics are fast enough to relax the cation order in response to temperature change in laboratory timescales from those in which they are not. In equilibrium, thermal expansion is increased in ZMGO, whereas the thermoelastic coefficients are decreased in both ZGO and ZMGO. The temperature range of the transition is significantly larger in ZGO compared to ZMGO and MGO. Trends within the elastic properties, thermoelastic properties, thermal expansion and the glass-like transition in the (Zn,Mg)Ga2O4 solid solution series are discussed based on the impact of inversion, structural disorder, bond character and in comparison to other spinels. © 2021 Elsevier B.V.

Reduced activation ferritic/martensitic (RAFM) steels are among the most competitive candidates of structural materials for nuclear fusion reactors, due to their superior comprehensive properties. Friction stir welding (FSW) was investigated in joining RAFM steel, considering its potential advantages in obtaining an optimal microstructure and mechanical properties of welded joint. To evaluate the feasibility of FSW in joining RAFM steel, an in-depth understanding of the microstructure-property relationships for friction stir welded joints of RAFM steel is necessary. In this research, the quantitative relationships between microstructural evolution and tensile properties in the stir zone (SZ) of friction stir welded RAFM steel after post-weld tempering treatment (PWTT) were systematically studied. Three different post-weld tempering temperatures namely 720 °C, 760 °C, and 800 °C were adopted. Then the uniaxial tensile properties were tested at room temperature and 550 °C, respectively. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and the Thermo-Calc Calphad software were adopted to systematically investigate the microstructural evolution. Martensite lath width, precipitate number density, equilibrium solid solubility of alloying elements in the matrix, and geometrically necessary dislocation (GND) density were analyzed quantitatively. With the results obtained, we assessed the contribution of each strengthening mechanism to the 0.2% offset yield strength. According to the effective inter-barrier spacing theory, a microstructure-sensitive yield strength model was obtained to well predict the change in yield strength at different conditions. Finally, the results calculated by equivalent strengthening effect indicated that the crucial microstructure determining the yield strength of the SZ for RAFM steel after PWTT is the high density of dislocation substructures. © 2021 Elsevier B.V.

A big challenge of the 21st century is to cope with the huge amounts of plastic waste on Earth. Especially the oceans are heavily polluted with plastics. To counteract this issue, biological (enzymatic) plastic decomposition is increasingly gaining attention. Recently it was shown that polyethylene terephthalate (PET) can be degraded in a saltwater-based environment using bacterial PETase produced by a marine diatom. At moderate temperatures, plastic biodegradation is slow and requires sensitive methods for detection, at least at initial stages. However, conventional methods for verifying the plastic degradation are either complex, expensive, time-consuming or they interfere with the degradation process. Here, we adapt lensless digital holographic microscopy (LDHM) as a new application for efficiently monitoring enzymatic degradation of a PET glycol copolymer (PETG). LDHM is a cost-effective, compact and sensitive optical method. We demonstrate enzymatic PETG degradation over a time course of 43 days employing numerical analysis of LDHM images. © 2020 Elsevier Ltd

Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys. © 2021 American Physical Society.

We introduce a novel approach to sample the canonical ensemble at constant temperature and applied electric potential. Our approach can be straightforwardly implemented into any density-functional theory code. Using thermopotentiostat molecular dynamics simulations allows us to compute the dielectric constant of nanoconfined water without any assumptions for the dielectric volume. Compared to the commonly used approach of calculating dielectric properties from polarization fluctuations, our thermopotentiostat technique reduces the required computational time by 2 orders of magnitude. © 2021 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

A thermogravimetric analyzer (TGA), a fluidized bed reactor (FBR) and a drop tube reactor (DTR) are used to study the effect of reactor type, heating rate and temperature on the pyrolysis of pulverized walnut shell particles in N2 and in CO2. These setups cover a temperature range of 400–1300 K with heating rates of 10−1 to 105 K s−1. The single first-order model in combination with an Arrhenius approach is used to describe the pyrolysis reaction. Derived activation energies for all setups show similar values (Ea,TGA = 71.96 kJ mol−1, Ea,FBR = 68.60 kJ mol−1 and Ea,DTR = 60.83 kJ mol−1), while an increase in the reactor temperature tend to lower the activation energy. Pyrolysis gas compositions in FBR and DTR reveal consistent trends towards lower H2O and higher CO contents with increasing reactor temperature. To evaluate the impact of CO2 on the solid conversion, TGA measurements in CO2 are used to determine gasification kinetics (Ea,g = 214.1 kJ mol−1, Ag = 71.96 s−1). CFD simulations using these kinetics in CO2 drop tube experiments let assume that the changed thermophysical properties of the gas and not the gasification reaction lead to the observed stronger conversion in CO2 compared to N2. © 2020 Elsevier Ltd

Introducing regions of looser atomic packing in bulk metallic glasses (BMGs) was reported to facilitate plastic deformation, rendering BMGs more ductile at room temperature. Here, we present a different alloy design approach, namely, doping the nonmetallic elements to form densely packed motifs. The enhanced structural fluctuations in Ti-, Zr- and Cu-based BMG systems leads to improved strength and renders these solutes’ atomic neighborhoods more prone to plastic deformation at an increased critical stress. As a result, we simultaneously increased the compressive plasticity (from ∼8% to unfractured), strength (from ∼1725 to 1925 MPa) and toughness (from 87 ± 10 to 165 ± 15 MPa√m), as exemplarily demonstrated for the Zr20Cu20Hf20Ti20Ni20 BMG. Our study advances the understanding of the atomic-scale origin of structure-property relationships in amorphous solids and provides a new strategy for ductilizing BMG without sacrificing strength. © 2021, The Author(s).

The open circuit voltage of galvanic cells is temperature dependent and the effect responsible for this dependency is its entropy. While it is well known that the Nernst equation plays an important role in describing this temperature dependency of the open-circuit voltage, this paper displays another effect. Measurements of the entropy for lithium-ion batteries show a significant temperature dependency, which cannot be explained by the linear Nernst equation. But this temperature dependency can be described by the free electron potential adapting via Fermi–Dirac statistics. This approach results in a quadratic temperature dependence of the measured potentials, which in the here shown cases for commercial lithium ion cells, could explain the measured effect. © 2021 Elsevier B.V.

The present paper focuses on the analysis of functional fatigue properties in 〈001〉-oriented single crystalline Fe-Ni-Co-Al-Ta shape memory alloys. Superelastic cycling experiments up to 4.5% at different temperatures were conducted and revealed excellent cyclic stability at lower testing temperatures. Transmission electron microscopy observations shed light on the influence of precipitation and dislocation activity on functional stability. © 2021

By considering the valence-electron concentration of 3d transition-metal alloys and compounds, we develop 3d high-entropy alloy Mn12.1Fe34.2Co33.5Ni12.3Cu7.9 with 8.7 electrons per atom, which is identical to that of Fe65Ni35 Invar. We carry out X-ray diffraction, scanning electron microscopy, magnetization, thermal expansion, and elastic modulus measurements, by which we show that the HEA alloy indeed carries Invar properties. This is evidenced particularly by the observed spontaneous volume magnetostriction and the lattice softening covering a broad temperature-range around the ferromagnetic Curie temperature. © 2021 American Physical Society.

Persistent photoconductivity of GaAs/AlGaAs heterostructures has hampered the measurement of charge- and spin-related quantum effects in gate-defined quantum devices and integrated charge sensors due to Si-dopant-related deep donor levels (DX centers). In this study, this effect is overcome by using an undoped GaAs/AlGaAs heterostructure for photonic purposes. We also measure the electron transport before and after LED illumination at low temperatures. In addition to a regular rapid saturation showing the increased carrier density, a slow accumulation of illumination effects appeared when different top-gate voltages were applied during illumination, which indicated the redistribution of charge at the oxide-GaAs interface. This study provides interesting insights into the development of optically stable devices for efficient semiconductor quantum interfaces. © 2021 Author(s).

The NMR spectrum of FeSe shows a dramatic broadening on cooling towards the bulk nematic phase at Ts=90 K, due to the formation of a quasistatic, short-range-ordered nematic domain structure. However, a quantitative understanding of the NMR broadening and its relationship to the nematic susceptibility is still lacking. Here, we show that the temperature and pressure dependence of the broadening is in quantitative agreement with the mean-field Edwards-Anderson parameter of an Ising-nematic model in the presence of random-field disorder introduced by nonmagnetic impurities. Furthermore, these results reconcile the interpretation of NMR and Raman spectroscopy data in FeSe under pressure. © 2021 American Physical Society.

A calibration routine is presented for an array of retarding field energy analyzer (RFEA) sensors distributed across a planar electrode surface with a diameter of 450 mm that is exposed to a low temperature plasma. Such an array is used to measure the ion velocity distribution function at the electrode with radial and azimuthal resolutions as a basis for knowledge-based plasma process development. The presented calibration procedure is tested by exposing such an RFEA array to a large-area capacitively coupled argon plasma driven by two frequencies (13.56 and 27.12 MHz) at a gas pressure of 0.5 Pa. Up to 12 sensors are calibrated with respect to the 13th sensor, called the global reference sensor, by systematically varying the sensor positions across the array. The results show that the uncalibrated radial and azimuthal ion flux profiles are incorrect. The obtained profiles are different depending on the sensor arrangement and exhibit different radial and azimuthal behaviors. Based on the proposed calibration routine, the ion flux profiles can be corrected and a meaningful interpretation of the measured data is possible. The calibration factors are almost independent of the external process parameters, namely, input power, gas pressure, and gas mixture, investigated under large-area single-frequency capacitively coupled plasma conditions (27.12 MHz). Thus, mean calibration factors are determined based on 45 different process conditions and can be used independent of the plasma conditions. The temporal stability of the calibration factors is found to be limited, i.e., the calibration must be repeated periodically. © 2021 Author(s).

During the last decade a new probe design for active plasma resonance spectroscopy, the multipole resonance probe (MRP), was proposed, analyzed, developed, and characterized in two different designs: the spherical MRP (sMRP) and the planar MRP (pMRP). The advantage of the latter is that it can be integrated into the chamber wall and can minimize the perturbation of the plasma. Both designs can be applied for monitoring and control purposes of plasma processes for industrial applications. As usual for this measurement technique, a mathematical model is required to determine plasma parameter (electron density, electron temperature, and collision frequency of electrons with neutral atoms) from the measured resonances. Based on the cold plasma model a simple relationship between the resonance frequency and the electron density can be derived and leads to excellent measurement results. However, a simultaneous measurement of the electron temperature in low-pressure plasmas requires a kinetic model, because the half-width of the resonance peak is broadened by kinetic effects. Such a model has been derived and first results show the broadening of the spectra as expected. Deriving a relation between the half-width and the electron temperature will allow the simultaneous measurement and an improvement of monitoring and control concepts. IEEE

A thin-film materials library in the system V-Bi-O was fabricated by reactive co-sputtering. The composition of Bi relative to V was determined by Rutherford backscattering spectroscopy, ranging from 0.06 to 0.84 at% along the library. The VO2phase M1 was detected by X-ray diffraction over the whole library, however a second phase was observed in the microstructure of films with Bi contents > 0.29 at%. The second phase was determined by electron diffraction to be BiVO4, which suggests that the solubility limit of Bi in VO2is only ∼0.29 at%. For Bi contents from 0.08 to 0.29 at%, the phase transformation temperatures of VO2:Bi increase from 74.7 to 76.4 °C by 8 K per at% Bi. With X-ray photoemission spectroscopy, the oxidation state of Bi was determined to be 3+. The V5+/V4+ratio increases with increasing Bi content from 0.10 to 0.84 at%. The similarly increasing tendency of the V5+/V4+ratio andTcwith Bi content suggests that although the ionic radius of Bi3+is much larger than that of V4+, the charge doping effect and the resulting V5+are more prominent in regulating the phase transformation behavior of Bi-doped VO2 © The Royal Society of Chemistry 2021.

The emergence of novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern with increased transmission dynamics has raised questions regarding stability and disinfection of these viruses. We analyzed surface stability and disinfection of the currently circulating SARS-CoV-2 variants B.1.1.7 and B.1.351 compared to wild type. Treatment with heat, soap, and ethanol revealed similar inactivation profiles indicative of a comparable susceptibility towards disinfection. Furthermore, we observed comparable surface stability on steel, silver, copper, and face masks. Overall, our data support the application of currently recommended hygiene measures to minimize the risk of B.1.1.7 and B.1.351 transmission. © 2021 The Author(s) 2021.

In this work, we report a new and promising approach toward the atomic layer deposition (ALD) of metallic Co thin films. Utilizing the simple and known CoCl2(TMEDA) (TMEDA = N,N,N′,N′-tetramethylethylenediamine) precursor in combination with the intramolecularly stabilized Zn aminoalkyl compound Zn(DMP)2 (DMP = dimethylaminopropyl) as an auxiliary reducing agent, a thermal ALD process is developed that enables the deposition of Zn-free Co thin films. ALD studies demonstrate the saturation behavior of both precursors and linearity depending on the applied number of cycles as well as temperature dependency of film growth in a regime of 140-215 °C. While the process optimization is carried out on Si with native oxide, additional growth studies are conducted on Au and Pt substrates. This study is complemented by initial reactivity and suitability tests of several potential Zn alkyl-reducing agents. For the CoCl2(TMEDA)-Zn(DMP)2 combination, these findings allow us to propose a series of elemental reaction steps hypothetically leading to pure Co film formation in the ALD process whose feasibility is probed by a set of density functional theory (DFT) calculations. The DFT results show that for reactions of the precursors in the gas phase and on Co(111) substrate surfaces, a pathway involving C-C coupling and diamine formation through reductive elimination of an intermediate Co(II) alkyl species is preferred. Co thin films with an average thickness of 10-25 nm obtained from the process are subjected to thorough analysis comprising atomic force microscopy, scanning electron microscopy, and Rutherford backscattering spectrometry/nuclear reaction analysis as well as depth profiling X-ray photoemission spectroscopy (XPS). From XPS analysis, it was found that graphitic and carbidic carbon coexist in the Co metal film bulk. Despite carbon concentrations of ∼20 at. % in the Co thin film bulk, resistivity measurements for ∼22 nm thick films grown on a defined SiO2 insulator layer yield highly promising values in a range of 15-20 μω cm without any postgrowth treatment. © 2021 American Chemical Society.

Pinning-type magnets with high coercivity at high temperatures are at the core of thriving clean-energy technologies. Among these, Sm2Co17-based magnets are excellent candidates owing to their high-temperature stability. However, despite intensive efforts to optimize the intragranular microstructure, the coercivity currently only reaches 20–30% of the theoretical limits. Here, the roles of the grain-interior nanostructure and the grain boundaries in controlling coercivity are disentangled by an emerging magnetoelectric approach. Through hydrogen charging/discharging by applying voltages of only ≈1 V, the coercivity is reversibly tuned by an unprecedented value of ≈1.3 T. In situ magneto-structural characterization and atomic-scale tracking of hydrogen atoms reveal that the segregation of hydrogen atoms at the grain boundaries, rather than the change of the crystal structure, dominates the reversible and substantial change of coercivity. Hydrogen reduces the local magnetocrystalline anisotropy and facilitates the magnetization reversal starting from the grain boundaries. This study opens a way to achieve the giant magnetoelectric effect in permanent magnets by engineering grain boundaries with hydrogen atoms. Furthermore, it reveals the so far neglected critical role of grain boundaries in the conventional magnetization-switching paradigm of pinning-type magnets, suggesting a critical reconsideration of engineering strategies to overcome the coercivity limits. © 2020 The Authors. Advanced Materials published by Wiley-VCH GmbH

Photosynthesis in deserts is challenging since it requires fast adaptation to rapid night-to-day changes, that is, from dawn’s low light (LL) to extreme high light (HL) intensities during the daytime. To understand these adaptation mechanisms, we purified photosystem I (PSI) from Chlorella ohadii, a green alga that was isolated from a desert soil crust, and identified the essential functional and structural changes that enable the photosystem to perform photosynthesis under extreme high light conditions. The cryo-electron microscopy structures of PSI from cells grown under low light (PSILL) and high light (PSIHL), obtained at 2.70 and 2.71 Å, respectively, show that part of light-harvesting antenna complex I (LHCI) and the core complex subunit (PsaO) are eliminated from PSIHL to minimize the photodamage. An additional change is in the pigment composition and their number in LHCIHL; about 50% of chlorophyll b is replaced by chlorophyll a. This leads to higher electron transfer rates in PSIHL and might enable C. ohadii PSI to act as a natural photosynthesiser in photobiocatalytic systems. PSIHL or PSILL were attached to an electrode and their induced photocurrent was determined. To obtain photocurrents comparable with PSIHL, 25 times the amount of PSILL was required, demonstrating the high efficiency of PSIHL. Hence, we suggest that C. ohadii PSIHL is an ideal candidate for the design of desert artificial photobiocatalytic systems. © 2021, The Author(s), under exclusive licence to Springer Nature Limited.

This study investigates the high strain-rate tensile properties of a cold-rolled medium-Mn steel (Fe–12Mn–3Al-0.05C % in mass fraction) designed to have a multi-phase microstructure and positive strain-rate sensitivity. At the intercritical annealing temperature of 585 °C, increasing the annealing time from 0.5 h to 8 h increased the phase volume fraction of ultrafine-grained (UFG) austenite from 2% to 35% by reversion. The remainder of the microstructure was composed of UFG ferrite and recovered α′-martensite (the latter resembles the cold-rolled state). Servo hydraulic tension testing and Kolsky-bar tension testing were used to measure the tensile properties from quasi-static strain rates to dynamic strain rates (ε˙ = 10-4 s-1 to ε˙ = 103 s-1). The strain-rate sensitivities of the yield strength (YS) and ultimate tensile strength (UTS) were positive for both annealing times. Tensile properties and all non-contact imaging modalities (infrared imaging and digital image correlation) indicated an advantageous suppression of Lüders bands and Portevin Le Chatelier (PLC) bands (a critical challenge in multi-phase medium-Mn steel design) due to the unique combination of microstructural constituents and overall composition. Fracture surfaces of specimens annealed for 0.5 h showed some instances of localized cleavage fracture (approximately 30 μm wide areas and lath-like ridges). Specimens annealed for 8 h maintained a greater product of strength and elongation by at least 2.5 GPa % (on average for each strain rate). The relevant processing-structure-property relationships are discussed in the context of recommendations for design strategies concerning multi-phase steels such that homogeneous deformation behavior and positive strain-rate sensitivities can be achieved. © 2020

Abstract: The present work shows that thermal expansion experiments can be used to measure the γʼ-solvus temperatures of four Ni-base single-crystal superalloys (SX), one with Re and three Re-free variants. In the case of CMSX-4, experimental results are in good agreement with numerical thermodynamic results obtained using ThermoCalc. For three experimental Re-free alloys, the experimental and calculated results are close. Transmission electron microscopy shows that the chemical compositions of the γ- and the γʼ-phases can be reasonably well predicted. We also use resonant ultrasound spectroscopy (RUS) to show how elastic coefficients depend on chemical composition and temperature. The results are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. Graphical abstract: [Figure not available: see fulltext.] © 2021, The Author(s).

The production of thin-wall ductile iron (TWDI) by high-pressure die-casting (HPDC) is complex because of several metallurgical and microstructural challenges. The present work aims to evaluate the austemperability of components (4 mm thickness) produced by HPDC process. The graphitization kinetics, the pearlite formation during continuous cooling, and the effect of austempering on the evolution of the ausferritic microstructure were investigated using dilatometric tests, microstructural analysis as well as Vickers hardness tests and tensile tests. Results show that components exhibit a brittle behavior because of white structures, small shrinkage cavities, and microporosity in the as-cast condition. Graphitization at 1100 °C allows rapid formation of small graphite particles within a short time (40 s). The critical cooling time (t8/5) to avoid the formation of pearlite upon cooling was found to be 5 s at a martensite start temperature of 193 ± 14 °C. Austempering at 360 °C for 40 min results in an ausferritic microstructure with stable carbon-enriched austenite which provides a high hardness (355 ± 4 HV10) and tensile strength (Rm = 709 ± 65 MPa). The results represent main criteria regarding the producibility of die-casted TWDI, which are helpful for future alloy and heat treatment design. © 2021, The Author(s).

An accurate prediction of atomic diffusion in Fe alloys is challenging due to thermal magnetic excitations and magnetic transitions. We investigate the diffusion of Mn in bcc Fe using an effective interaction model and first-principles based spin-space averaged relaxations in magnetically disordered systems. The theoretical results are compared with the dedicated radiotracer measurements of Mn54 diffusion in a wide temperature range of 773 to 1173 K, performed by combining the precision grinding (higher temperatures) and ion-beam sputtering (low temperatures) sectioning techniques. The temperature evolution of Mn diffusion coefficients in bcc iron in theory and experiment agree very well and consistently reveal a reduced acceleration of Mn solute diffusion around the Curie point. By analyzing the temperature dependencies of the ratio of Mn diffusion coefficients to self-diffusion coefficients we observe a dominance of magnetic disorder over chemical effects on high-temperature diffusion. Therefore, the missing acceleration mainly reflects an anomalous behavior of the Mn solute in the magnetically ordered low-temperature state of the Fe host, as compared to other transition metals. © 2021 authors. Published by the American Physical Society.

We investigate the polymorphism of complexes formed by the hydration of a functionalized azobenzene molecule by low-temperature scanning tunneling microscopy. Under conditions at which the water-less azobenzene molecules remain as monomers on Au(111), co-adsorption of water leads to water-azobenzene complexes. These complexes prefer to adopt linear arrangements of the azobenzene mediated by its functionalized end groups. Such structures may serve as model systems for investigating the influence of a solvent on a surface reaction. © 2021 Author(s).

The aluminum alloy 2618A is applied for engine components such as radial compressor wheels which operate for long time at elevated temperatures. This results in coarsening of the hardening precipitates and degradation in mechanical properties during the long-term operation, which is not taken into account in the current lifetime prediction models due to the lack of quantitative microstructural and mechanical data. To address this issue, a quantitative investigation on the evolution of precipitates during long-term aging at 190 °C for up to 25,000 h was conducted. Detailed transmission electron microscopy (TEM) was combined with Brinell hardness measurements and thorough differential scanning calorimetry (DSC) experiments. The results show that GPB zones and S-phase Al2CuMg grow up to < 1,000 h during which the GPB zones dissolve and S-phase precipitates form. For longer aging times, only S-phase precipitates coarsen, which can be well described using the Lifshitz–Slyozov–Wagner theory of ripening. A thorough understanding of the underlying microstructural processes is a prerequisite to enable the integration of aging behavior into the established lifetime models for components manufactured from alloy 2618A. © 2021, The Author(s).

An active learning approach to train machine-learning interatomic potentials (moment tensor potentials) for multicomponent alloys to ab initio data is presented. Employing this approach, the disordered body-centered cubic (bcc) TiZrHfTax system with varying Ta concentration is investigated via molecular dynamics simulations. Our results show a strong interplay between elastic properties and the structural ω phase stability, strongly affecting the mechanical properties. Based on these insights we systematically screen composition space for regimes where elastic constants show little or no temperature dependence (elinvar effect). © 2021 American Physical Society.

We derive an effective field theory model for magnetic topological insulators and predict that a magnetic electronic gap persists on the surface for temperatures above the ordering temperature of the bulk. Our analysis also applies to interfaces of heterostructures consisting of a ferromagnetic and a topological insulator. In order to make quantitative predictions for  and for EuS- heterostructures, we combine the effective field theory method with density functional theory and Monte Carlo simulations. For we predict an upwards Néel temperature shift at the surface up to , while the EuS- interface exhibits a smaller relative shift. The effective theory also predicts induced Dzyaloshinskii-Moriya interactions and a topological magnetoelectric effect, both of which feature a finite temperature and chemical potential dependence. © 2021 Published by the American Physical Society

Highly selective one-step hydrogenation of phenol to cyclohexanone, an important intermediate in the production of nylon 6 and nylon 66, is desirable but remains a challenge. Pd nanoparticles supported on nitrogen- and oxygen-functionalized carbon nanotubes (NCNTs, OCNTs) were prepared, characterized, and applied in the hydrogenation of phenol to cyclohexanone to study the effect of N-doping. Almost full conversion of phenol with high selectivity to cyclohexanone was achieved over Pd/NCNT under mild reaction conditions using either H2 or formic acid (FA) as a hydrogen source. The effects of reaction temperature and FA/phenol ratio and the reusability were investigated. Separate FA decomposition experiments without and with the addition of phenol were performed to investigate the reaction mechanism, especially the deactivation behavior. Deactivation was observed for both catalysts during the FA decomposition, while only Pd/OCNT rather than Pd/NCNT was deactivated in the transfer hydrogenation with FA and the FA decomposition in the presence of phenol, indicating the unique role of N-doping. Therefore, we assume that deactivation is caused by the strongly bound formates on the active Pd sites, suppressing further FA decomposition and/or transfer hydrogenation on Pd. The nonplanar adsorption of phenol on NCNTs via weak O−H⋅⋅⋅N interactions enables the occurrence of the subsequent hydrogenation by adsorbed formate on Pd. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH

The analysis of the current literature on the subject of detailed single-particle measurements revealed that, despite the fact that numerous experimental investigations are reported, the database for the calibration of predictive burnout models is insufficient. To close this gap, a test rig for the optical investigation of the burning behavior of pulverized fuel particles was put into operation and introduced in the first part of this two-article series. In the second part, the results of a measurement campaign, on a high-volatile bituminous coal, and torrefied Miscanthus in eight oxy-fuel atmospheres are presented. The experimental data contains profiles of particle temperature, size, shape, burnout progress over residence time and, thus, provides a sound basis for the calibration of char burnout models. The combination of chosen optical techniques enables distinguishing between burning and cold particles, the latter being non-ignited or already burnt out. This information is important for the analysis of particle size distributions. In addition to optical measurements, partially reacted solid samples were extracted from the reactor. Besides the proximate and elemental analysis, also the porosity of the samples was determined at several burnout levels. Interestingly, the evolution of particle porosity shows atmosphere-independent trends. © 2020 Taylor & Francis Group, LLC.

Submicron Ti2AlC MAX phase powder was synthesized by molten salt shielded synthesis (MS3) using a Ti:Al:C molar ratio of 2:1:0.9 at a process temperature of 1000°C for 5 hours. The synthesized powder presented a mean particle size of ~0.9 µm and a purity of 91 wt. % Ti2AlC, containing 6 wt. % Ti3AlC2. The Ti2AlC powder was sintered by pressureless sintering, achieving a maximal relative density of 90%, hence field-assisted sintering technology/spark plasma sintering was used to enhance densification. The fine-grained microstructure was preserved, and phase purity of Ti2AlC was unaltered in the latter case, with a relative density of 98.5%. Oxidation was performed at 1200°C for 50 hours in static air of dense monolithic Ti2AlC with different surface finish, (polished, ground and sandblasted) which resulted in the formation of an approx. 8 µm thin aluminum oxide (Al2O3) layer decorated with titanium dioxide (rutile, TiO2) colonies. Surface quality had no influence on Al2O3 scale thickness, but the amount and size of TiO2 crystals increased with surface roughness. A phenomenon of rumpling of the thermally grown oxide (TGO) was observed and a model to estimate the extent of deformation is proposed. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American

In order to enable the application of atomic probe tomography combinatorial processing platforms for atomic-scale investigations of phase evolution at elevated temperatures, the pre-sharpened Si tip of 10–20 nm in diameter must be protected against interdiffusion and reaction of the reactive Si with a film of interest by a conformal coating on the Si tip. It is shown that unwanted reactions can be suppressed by introducing a 20-nm-thick intermediate Al2O3 layer grown by atomic layer deposition (ALD). As a representative case, Pt is chosen as a film of interest, as it easily forms silicides. Whereas without the ALD coating diffusion/reactions occur, with the protective film, this is prevented for temperatures up to at least 600°C. The effectiveness of the Al2O3 layer serving as a diffusion barrier is not limited to a sharpened Si tip but works generally for all cases where a Si substrate is used. © 2021 The Authors. Surface and Interface Analysis published by John Wiley & Sons Ltd.

We report the application of tris(N,N′-diisopropyl-formamidinato)yttrium(iii) [Y(DPfAMD)3] as a promising precursor in a water-assisted thermal atomic layer deposition (ALD) process for the fabrication of high quality Y2O3 thin films in a wide temperature range of 150 °C to 325 °C. This precursor exhibits distinct advantages such as improved chemical and thermal stability over the existing Y2O3 ALD precursors including the homoleptic and closely related yttrium tris-amidinate [Y(DPAMD)3] and tris-guanidinate [Y(DPDMG)3], leading to excellent thin film characteristics. Smooth, homogeneous, and polycrystalline (fcc) Y2O3 thin films were deposited at 300 °C with a growth rate of 1.36 Å per cycle. At this temperature, contamination levels of C and N were under the detectable limits of nuclear reaction analysis (NRA), while X-ray photoelectron spectroscopy (XPS) measurements confirmed the high purity and stoichiometry of the thin films. From the electrical characterization of metal-insulator-semiconductor (MIS) devices, a permittivity of 13.9 at 1 MHz could be obtained, while the electric breakdown field is in the range of 4.2 and 6.1 MV cm-1. Furthermore, an interface trap density of 1.25 × 1011 cm-2 and low leakage current density around 10-7 A cm-2 at 2 MV cm-1 are determined, which satisfies the requirements of gate oxides for complementary metal-oxide-semiconductor (CMOS) based applications. © 2021 The Royal Society of Chemistry.

High-quality copper metal thin films are demanded for a number of advanced technologies. Herein, a facile ALD (atomic layer deposition) process for the fabrication of Cu metal films directly from two solid readily usable precursors, copper acetylacetonate as the source of copper and hydroquinone as the reductant is reported. This process yields highly crystalline, dense, specularly reflecting, and electrically conductive Cu films with an appreciably high growth rate of 1.8 Å/cycle at deposition temperatures as low as 160 to 240 °C. © 2021 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH

Plasma catalysis, the combination of plasma and catalysis, is used to achieve efficient molecule conversion, supporting the flexibility of operating parameters and feed gases. By combining plasmas with conventional thermal catalysis, the temperature windows may be changed and the process may be made insensitive to catalyst poisoning. However, understanding plasma catalysis mechanisms is extremely difficult, due to the strong coupling between plasma, gas-phase chemistry and surface. A multitude of reaction pathways may be enhanced or reduced by the presence of a plasma that provides excited species as reaction partners. We developed a robust setup to analyse those processes, based on a parallel-plate atmospheric-pressure plasma jet that allows a plug flow design. The plasma chemistry is analysed by Fourier transform infrared absorption spectroscopy and mass spectrometry. The electrodes in contact with the plasma are temperature controlled and can easily be replaced to apply a catalyst on top of them. The basic characteristics of the setup are discussed and three examples for its application are given: (a) the analysis of methane oxidation using the plug flow scheme; (b) the plasma catalytic conversion of CO2, and (c) the plasma catalytic conversion of methane in methane–oxygen mixtures. © 2021 IOP Publishing Ltd.

The influence of small plastic pre-strains on the elevated-temperature stability and microstructure of the equiatomic CrMnFeCoNi FCC solid solution is investigated. Particular attention is given to whether any of the alloy elements segregate to individual dislocations. To that end, CrMnFeCoNi samples were first deformed in tension at room temperature to plastic strains of 0.2 and 2.3%, and subsequently annealed at 973 K for 800 hours. The pre-strains activated planar slip of 1/2<110>-type dislocations on {111}-type glide planes. Interactions of this planar slip with special Σ3 grain boundaries formed a large number of dislocation segments with a <110>-type crystallographic orientation suitable for a credible end-on analysis of dislocation cores in HR-STEM. The cores of the 1/2<110> dislocations pushed up against the investigated grain boundaries were found to be close to the compact configuration. Within the sensitivity of the Super-X EDS mapping, no concentration gradient was detected near dislocations that would indicate enrichment at dislocation cores of any of the elemental constituents of the alloy after the pre-deformation and annealing. However, a Cr-rich tetragonal sigma phase nucleated and grew at grain boundary triple junctions during this anneal, processes that were not accelerated by the enhanced dislocation density present after pre-strain. A clear chromium gradient was observed in the Cr-depleted zones near grain boundaries suggesting that Cr transport occurred by relatively slow diffusion from the bulk to the grain boundaries and then by relatively fast diffusion along the grain boundaries to the precipitates. Accompanying the Cr depletion near grain boundaries is a simultaneous Ni and Mn enrichment, which promotes formation of the L10 NiMn phase that is observed on the grain boundaries after prolonged annealing. © 2021

It is know from literature that small additions (<1 wt%) of Ca, Al and Zn significantly improve the intrinsic ductility of Mg. The exact role of each element, both qualitatively and quantitatively, and their combined effects, however, are poorly understood. Here we achieved a much clearer view on the quantitative role of each element with respect to ductility improvement and on the collaborative effect, particularly of Ca and Zn in Mg. Some of our findings and conclusions are in disagreement with data and interpretation found in literature. Four different alloys, namely, Mg-0.1 Ca, Mg-0.1 Ca-1 Al, Mg-0.05 Ca-1 Al, Mg-0.1 Ca-2 Al-1 Zn (all are in wt%) were selected for this investigation. All alloys were treated such that approx. similar grain sizes and textures were obtained. This largely excludes the effect of extrinsic factors on ductility. EBSD-guided slip trace analyses reveal that the addition of Ca eases activation of prismatic and pyramidal II slip systems. Using in-situ deformation experiments in SEM and atom probe tomography observations of grain boundaries direct evidence is given for the individual and synergetic effects of Ca and Zn on grain boundary cohesion as an important contribution to improve the ductility of these alloys. We conclude that Ca reduces the slip anisotropy and ameliorates ductility, however, the weak grain boundary cohesion in the Mg-0.1 wt% Ca alloy limits the material's tensile ductility. The addition of Zn alters the Ca segregation at the grain boundaries and helps to retain their cohesive strength, an effect which thus enables higher ductility and strength. The further addition of Al primarily improves the strength. The results show that the balanced influence of reduced slip anisotropy on the one hand and increased grain boundary cohesion on the other hand allow to design a high strength high ductility rare-earth free Mg alloy. © 2021

The formation of regions of solid coating, where agglomerated clinker material adheres to the refractory lining of the kiln wall, is very common during cement clinker production. While a thin coating layer protects the refractory lining, strong deposit formation can impair the material flow through the kiln. In this study, the impact of these coating layers on the clinker production process within a rotary kiln is investigated with CFD simulations. The fuel injected at the main burner is a mixture of pulverized coal and refuse derived fuel (RDF). Advanced models were developed to accurately describe the trajectories and thermal conversion of non-spherical RDF particles in the gas phase. These models are based on a detailed fuel analysis of major RDF fractions. A blocked-off region approach is used to consider different coating profiles within the simulation domain. The thermochemical processes in the clinker bed of the kiln are approximated with a one-dimensional model that calculates heat and mass exchange with the gas phase, the incorporation of fuel ashes into the bed and the chemical-mineralogical reactions of the clinker. The blocked-off region approach is also employed to account for the clinker bed geometry in the kiln, which greatly depends on the considered coating profile. Two cases, one with a thin and evenly distributed coating profile and one with a thick and locally concentrated coating, are simulated. The resulting impact on RDF conversion, gas phase properties and clinker phase formation are assessed and compared to a reference case without any coating. Results show that the insulation effect of a thin coating profile increases the gas phase temperature in the kiln and helps to reduce the free lime content of the final clinker product. In the case of heavy coating, a temperature shift towards the solid material inlet of the kiln occurs, which outweighs the beneficial insulation effect of the coating in the sintering zone and leads to lower local gas phase temperatures. In combination with reduced clinker residence times, this results in a slight increase of the free lime content in the clinker. © 2020 Elsevier Ltd

One major hindrance that alloy design for additive manufacturing (AM) faces nowadays is hot tearing. Contrary to the previous works which either try to reduce solidification range or introduce grain refinement, the current work presents a new approach of employing segregation engineering to alter the residual stress states at the interdendritic and grain boundary regions and consequently prevent hot tearing. Here, in situ Al alloying is introduced into an existing hot-cracking susceptible high-entropy alloy CoCrFeNi. It is found that within a certain range of compositions, such as Al0.5CoCrFeNi, the hot crack density was drastically decreased. During the solidification of this specific alloy composition, Al is firstly ejected from the primary dendritic face-centred cubic (FCC) phase and segregates into the interdendritic regions. Spinodal decomposition then occurs in these Al-enriched regions to form the ordered B2 NiAl and disordered body-centred cubic (BCC) Cr phases. Due to the higher molar volume and lower homologous temperatures of these B2/BCC phases, the inherent residual strain is accommodated and transformed from a maximum 0.006 tensile strain in CoCrFeNi to a compressive strain of ~0.001 in Al0.5CoCrFeNi. It is believed that this grain boundary segregation engineering method could provide a new pathway to systematically counteract the hot tearing problem in additive manufacturing of metals and alloys, using available thermodynamic and kinetic database information. © 2020

Carbon black was functionalized by gas-phase oxidation using nitric acid vapor at 150 °C, and temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR) experiments were performed in a plug-flow reactor to analyze the decomposition mechanisms of oxygen-containing surface groups by monitoring evolved H2O, CO2, and CO quantitatively. Subsequent TPD measurements detected an enrichment of acidic surface groups with increasing duration of the HNO3 functionalization from 2 h to 24 h. A significant amount of H2O was released during the TPD experiments, yielding H2O evolution profiles which were deconvoluted into two Gaussian peaks at 162 °C and 228 °C. The combined analysis of the CO2 and H2O profiles indicates that desorbed H2O originates from chemisorbed water bound to carboxylic acid groups and from condensation reactions of carboxylic acids and phenols. Phenols and carbonyls were found to be reduced selectively by H2 during TPR, generating a pronounced H2O peak at 650 °C. A new peak in the CO2 evolution profile appeared at 575 °C in reducing atmosphere, which is assigned to the hydrolysis of anhydrides and lactones with subsequent decomposition. Thus, taking H2O into account is mandatory for a complete quantitative analysis of the decomposition mechanisms occurring during TPD and TPR experiments. © 2021 Elsevier B.V.

An experimental parameter study in an indirectly heated rotary drum (L = 1.76 m, D = 0.5 m) with L-shaped flights was conducted. The influence of the flight design parameters as the flight length ratio (0.375–2.0) and the number of flights (6–18) on the temperature-time profile and the temperature drop are analyzed. Therefore, the rotary drum is filled with glass beads as reference material and heated in batch operation while contact heat transfer is the main heat transfer mechanism. After reaching an upper temperature of 330 °C, the system is cooled by forced convection as ambient air is sucked through the drum. Type k-thermocouples in three axial and six radial positions are used to measure the time evolution of the temperature. The thermocouples are mounted to a flight, such that they rotate with the drum and also capture the circumferential temperature distribution. © 2021 Elsevier Ltd

Co3O4 nanospheres with a mean diameter of 19 nm were applied in the selective oxidation of 2-propanol to acetone in the gas phase. Compared with 9 nm spheres, the 19 nm spheres exhibited superior catalytic activity and stability with 100% selectivity to acetone up to 500 K. Transmission electron microscopy, N2 physisorption, 2-propanol and O2 temperature-programmed desorption, and 2-propanol temperature-programmed surface reaction in O2 were applied to characterize the bulk and surface properties. Despite the smaller specific surface area (35 m2 g-1), an increased 2-propanol adsorption capacity was observed for the larger nanospheres ascribed to a preferential (110) surface orientation. Temperature-programmed oxidation experiments after reaction showed multilayer coke deposition and severe reduction of the Co3O4 surface, but excellent stability was maintained at 430 K using the 19 nm spheres in a steady-state oxidation experiment for 100 h with only 10% loss of the initial activity. The good agreement of the 2-propanol decomposition profiles indicates that the superior activity is caused by the enhanced interaction of the larger nanospheres with O2. A Mars-van Krevelen mechanism on the (110) surface was identified by density functional theory calculations with a Hubbard U term, favoring faster reoxidation compared with the (100) surface predominantly exposed by the 9 nm spheres. © The Royal Society of Chemistry.

Advanced methods for the repair of single-crystalline (SX) Ni-based superalloys are of special interest for the gas turbine industry. Polycrystalline repair approaches show promising results, while the repair of SX materials is still challenging. Directional annealing experiments resulted in large columnar grains by imposing thermal gradients at the abnormal grain growth temperature of a specific Ni-based superalloy. A numerical model of the Bridgman process is applied to provide an insight into the temperature evolution during zone annealing of the Vacuum-Plasma-Spray (VPS) repair coatings with the aim of promoting grain growth from the SX substrate. The results presented here suggest that this is a promising approach to repair or manufacture SX turbine blades. © 2020 Elsevier B.V.

Differential scanning calorimetry measurements on different bulk metallic glasses show no measurable rejuvenation upon deeply cooled (cryogenic) thermal cycling. This applies both to as-quenched and well-annealed samples. Extensive molecular dynamics simulations of a generic model glass former corroborate these observations. We disentangle the effects of aging from those of thermal treatment and show that aging is slowed down but not stopped - neither reversed - during thermal cycling. These observations are corroborated further by a survey of energy distribution, which continues narrowing, albeit with a smaller rate. © 2021 authors.

In the present work, we study the role of programming strain (50% and 100%), end loads (0, 0.5, 1.0, and 1.5 MPa), and chemical environments (acetone, ethanol, and water) on the exploitable stroke of linear shape memory polymer (SMP) actuators made from ESTANE ETE 75DT3 (SMP-E). Dynamic mechanical thermal analysis (DMTA) shows how the uptake of solvents results in a decrease in the glass temperature of the molecular switch component of SMP-E. A novel in situ technique allows chemically studying triggered shape recovery as a function of time. It is found that the velocity of actuation decreases in the order acetone > ethanol > water, while the exploitable strokes shows the inverse tendency and increases in the order water > ethanol > acetone. The results are interpreted on the basis of the underlying chemical (how solvents affect thermophysical properties) and micromechanical processes (the phenomenological spring dashpot model of Lethersich type rationalizes the behavior). The study provides initial data which can be used for micromechanical modeling of chemically triggered actuation of SMPs. The results are discussed in the light of underlying chemical and mechanical elementary processes, and areas in need of further work are highlighted. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

The restriction imposed by the lattice structure of different surfaces is used to investigate the influence of the distance between two monomers on their ability to bind to each other. We compare the interaction of ammonia monomers at two distinct distances imposed by the surface structure of a Cu(511) high-index surface to that of a Cu(110) low-index surface using low-temperature scanning tunneling microscopy, inelastic tunneling spectroscopy, and density functional theory. Frustrated translational and rotational modes, the Mulliken and Bader charge analyses, and electrostatic potential mapping indicate chemisorption of ammonia monomers on both surfaces, with their dipoles oriented perpendicular to the surface plane. At a larger intermolecular distance of around 0.51 nm on step edges of Cu(511), the monomers slightly repel each other due to electrostatic repulsion. At a shorter distance of around 0.36 nm perpendicular to the close-packed rows on Cu(110), a noticeable charge transfer between adjacent monomers indicates binding, that is, dimer formation in parallel orientation. This binding energy of the molecules compensates for the electrostatic repulsion. Our results outline how the choice of the surface structure may be utilized to alter the intermolecular interaction of solvent molecules and to enforce or suppress dimer formation. ©

The synthesis temperature dependent phase formation of Ni10Ti10Al25Fe35Cr20 thin films is compared to a bulk processed sample of identical composition. The as-cast alloy exhibits a dual-phase microstructure which is composed of a disordered BCC phase and AlNiTi-based B2- and/or L21-ordered phase(s). Formation of the BCC phase as well as an ordered AlNi-based B2 phase is observed for a thin film synthesised at 500 °C (ratio of synthesis temperature of thin film to melting temperature of bulk alloy: T/Tm = 0.49), which is attributed to both surface and bulk diffusion mediated growth. Post deposition annealing at 900 °C (T/Tm = 0.75) of a thin film deposited without intentional heating results in the formation of NiAlTi-based B2 and/or L21-phase(s) similar to the bulk sample, which is attributed to bulk diffusion. Depositions conducted at room temperature without intentional substrate heating (T/Tm = 0.20) resulted in the formation of an X-ray amorphous phase, while a substrate temperature increase to 175 °C (T/Tm = 0.28) causes the formation of a BCC phase. Atom probe tomography of the thin films deposited without intentional substrate heating and at 175 °C indicates the formation of ∼5 nm and ∼10 nm FeAl-rich domains, respectively. This can be rationalized based on the activation energy for surface diffusion, as Ti and Ni exhibt 2.5 to 4 times larger activation energy barriers than Al, Fe and Cr. It is evident from the homologous temperature that the phase formation observed at 500 °C (T/Tm = 0.49) is a result of both surface and bulk diffusion. As the temperature is reduced, the formation of FeAl-rich domains can be understood based on the differences in activation energy for surface diffusion and is consistent with kinetically limited thin film growth. © 2020 Elsevier B.V.

We report the magnetic ordering and structural distortion in PrFeAsO crystals, the basis compound for one of the oxypnictide superconductors, using high-resolution X-ray diffraction, neutron diffraction, and X-ray resonant magnetic scattering (XRMS). We find the structural tetragonal-to-orthorhombic phase transition at TS=147K, the AFM phase transition of the Fe moments at TFe=72K, and the Pr AFM phase transition at TPr=21K. Combined high-resolution neutron diffraction and XRMS show unambiguously that the Pr moments point parallel to the longer orthorhombic a axis and order antiferromagnetically along the a axis but ferromagnetically along the b and c directions in the stripelike AFM order. The temperature-dependent magnetic order parameter of the Pr moments shows no evidence for a reorientation of moments. © 2021 American Physical Society.

Canfieldite, Ag8SnS6, is a semiconducting mineral notable for its high ionic conductivity, photosensitivity, and low thermal conductivity. We report the solution growth of large single crystals of Ag8SnS6 of mass up to 1 g from a ternary Ag–Sn–S melt. On cooling from high temperature, Ag8SnS6 undergoes a known cubic (F4̅3m) to orthorhombic (Pna21) phase transition at ≈460 K. By studying the magnetization and thermal expansion between 5–300 K, we discover a second structural transition at ≈120 K. Single crystal X-ray diffraction reveals the low-temperature phase adopts a different orthorhombic structure with space group Pmn21 (a = 7.662 9(5) Å, b = 7.539 6(5) Å, c = 10.630 0(5) Å, Z = 2 at 90 K) that is isostructural to the room-temperature forms of the related Se-based compounds Ag8SnSe6 and Ag8GeSe6. The 120 K transition is first-order and has a large thermal hysteresis. On the basis of the magnetization and thermal expansion data, the room-temperature polymorph can be kinetically arrested into a metastable state by rapidly cooling to temperatures below 40 K. We last compare the room- and low-temperature forms of Ag8SnS6 with its argyrodite analogues, Ag8TQ6 (T = Si, Ge, Sn; Q = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller phase volume favors the Pna21 arrangement. We support this picture by showing that the transition to the Pmn21 phase is avoided in Ge alloyed Ag8Sn1–xGexS6 samples as well as in pure Ag8GeS6 © 2021 American Chemical Society

A combination of complementary high-energy X-ray diffraction, containerless solidification during electromagnetic levitation and transmission electron microscopy is used to map in situ the phase evolution in a prototype Cu-Zr-Al glass during flash-annealing imposed at a rate ranging from 102 to 103 K s−1 and during cooling from the liquid state. Such a combination of experimental techniques provides hitherto inaccessible insight into the phase-transformation mechanism and its kinetics with high temporal resolution over the entire temperature range of the existence of the supercooled liquid. On flash-annealing, most of the formed phases represent transient (metastable) states – they crystallographically conform to their equilibrium phases but the compositions, revealed by atom probe tomography, are different. It is only the B2 CuZr phase which is represented by its equilibrium composition, and its growth is facilitated by a kinetic mechanism of Al partitioning; Al-rich precipitates of less than 10 nm in a diameter are revealed. In this work, the kinetic and chemical conditions of the high propensity of the glass for the B2 phase formation are formulated, and the multi-technique approach can be applied to map phase transformations in other metallic-glass-forming systems. © 2021, The Author(s).

Most oxide ceramics are known to be brittle macroscopically at room temperature with little or no dislocation-based plasticity prior to crack propagation. Here, we demonstrate the size-dependent brittle to ductile transition in SrTiO3 at room temperature using nanoindentation pop-in events visible as a sudden increase in displacement at nominally constant load. We identify that the indentation pop-in event in SrTiO3 at room temperature, below a critical indenter tip radius, is dominated by dislocation-mediated plasticity. When the tip radius increases to a critical size, concurrent dislocation activation and crack formation, with the latter being the dominating process, occur during the pop-in event. Beyond the experimental examination and theoretical justification presented on SrTiO3 as a model system, further validation on α-Al2O3, BaTiO3, and TiO2 are briefly presented and discussed. A new indentation size effect, mainly for brittle ceramics, is suggested by the competition between the dislocation-based plasticity and crack formation at small scale. Our finding complements the deformation mechanism in the nano-/microscale deformation regime involving plasticity and cracking in ceramics at room temperature to pave the road for dislocation-based mechanics and functionalities study in these materials. © 2021 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)

A new wrought CrCoNi-based medium-entropy superalloy (MESA) was designed by changing the composition of a commercial superalloy of type C-263, which is used for stationary components in gas turbines. ∼5 at.% Cr and 0.85 at.% Ti + Al were added at the expense of Ni while the Ti/Al ratio was decreased. Owing to these modifications, the brittle η phase, which is stable in C-263 below 900 °C is no longer observed in C-264. Besides, the solvus temperature and volume fraction of the γ′ phase in the peak-aged state are larger in C-264 (∼935 °C, 13.5%) compared to C-263 (∼890 °C, 12.8%), resulting in superior tensile and creep properties. The stress and temperature dependencies of the creep rates were described by power-law and Arrhenius relationships. The stress exponents were between 4 and 5, while the apparent activation energies were 550 and 400 kJ/mol for C-264 and C-263, respectively. During creep at 880 °C in air, internal nitridation in both MESAs resulted in the formation of TiN precipitates, with C-264 being slightly more affected due to its higher nitrogen solubility. Due to its superior creep resistance, good malleability and machinability, the C-264 MESA is currently commercially available from VDM Metals International. © 2021 The Authors

The γ′ volume fraction is a key parameter in precipitation-strengthened Co- and Ni-base superalloys and mainly determines the alloys’ properties. However, systematic studies with varying γ′ volume fractions are rare and the influence on thermal expansion has not been studied in detail. Therefore, a series of six Ta-containing Co-based alloys was designed with compositions on a γ–γ′ tie-line, where the γ′ volume fraction changes systematically. During solidification, Laves (C14-type) and µ (D85-type) phases formed in alloys with high levels of W and Ta. Single-phase γ or two-phase γ/γ′ microstructures were obtained in four experimental alloys after heat treatment as designed, whereas secondary precipitates, such as χ (D019-type), Laves, and μ, existed in alloys containing high levels of γ′-forming elements. However, long-term heat treatments for 1000 hours revealed the formation of the χ phase also in the former χ-free alloys. The investigation of the thermal expansion behavior revealed a significant anomaly related to the dissolution of γ′, which can be used to determine the γ′ solvus temperature with high accuracy. Compared to thermodynamic calculations, differential scanning calorimetry (DSC) and thermal expansion analysis revealed a larger increase of the γ′ solvus temperatures and a lesser decline of the solidus temperatures when the alloy composition approached the composition of the pure γ′ phase. © 2021, The Author(s).

We present a first principles-quality potential energy surface (PES) describing the inter-atomic forces for hydrogen atoms interacting with free-standing graphene. The PES is a high-dimensional neural network potential that has been parameterized to 75 945 data points computed with density-functional theory employing the PBE-D2 functional. Improving over a previously published PES [Jiang et al., Science, 2019, 364, 379], this neural network exhibits a realistic physisorption well and achieves a 10-fold reduction in the RMS fitting error, which is 0.6 meV per atom. The chemisorption barrier is 172 meV, which is lower than that of the REBO-EMFT PES (260 meV). We used this PES to calculate about 1.5 million classical trajectories with carefully selected initial conditions to allow for direct comparison to results of H- and D-atom scattering experiments performed at incidence translational energy of 1.9 eV and a surface temperature of 300 K. The theoretically predicted scattering angular and energy loss distributions are in good agreement with experiment, despite the fact that the experiments employed graphene grown on Pt(111). Compared to previous calculations, the agreement with experiments is improved. The remaining discrepancies between experiment and theory are likely due to the influence of the Pt substrate only present in the experiment. This journal is © the Owner Societies.

Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001) α-Al2O3 (c-sapphire). Phase and morphological stability of 200 to 670 nm thick films were investigated between 973 K and 1423 K. The fcc-phase persists while the original <111> texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 µm wide grains upon annealing. Only the metallic M grains having two specific orientation relationships (ORs) to the c-sapphire grow. These ORs are OR1 (M(111)[11¯0]//α-Al2O3(0001)[11¯00]) and OR2 (M(111)[11¯0]//α-Al2O3(0001)[112¯0])and their twin-related variants (OR1t and OR2t). They are identical to those reported for several pure fcc metal (M) films. Thus, the ORs in these fcc/c-sapphire systems appear not to be controlled by the fcc phase chemistry or its lattice parameter as usually assumed in literature. Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. Below 1000 K, the film microstructure stabilizes into 10 µm wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures. © 2020 Acta Materialia Inc.

Understanding deformation mechanisms in silicon is critical for reliable design of miniaturized devices operating at high temperatures. Bulk silicon is brittle, but it becomes ductile at about 540 °C. It creeps (deforms plastically with time) at high temperatures (∼800 °C). However, the effect of small size on ductility and creep of silicon remains elusive. Here, we report that silicon at small scales may deform plastically with time at lower temperatures (400 °C) above a threshold stress. We achieve this stress by bending single-crystal silicon microbeams using an in situ thermomechanical testing stage. Small size, together with bending, localize high stress near the surface of the beam close to the anchor. This localization offers flaw tolerance, allowing ductility to win over fracture. Our combined scanning, transmission electron microscopy, and atomic force microscopy analysis reveals that as the threshold stress is approached, multiple dislocation nucleation sites appear simultaneously from the high-stressed surface of the beam with a uniform spacing of about 200 nm between them. Dislocations then emanate from these sites with time, lowering the stress while bending the beam plastically. This process continues until the effective shear stress drops and dislocation activities stop. A simple mechanistic model is presented to relate dislocation nucleation with plasticity in silicon. © 2020 National Academy of Sciences. All rights reserved.

We investigate the shape of monatomic high Cu islands on a Cu(111) surface by variable-temperature scanning tunneling microscopy between 110 K and 240 K. Low temperature dendrites evolve towards more compact shapes at increasing temperature; finally reaching the equilibrium shape of a hexagon with rounded corners. Time-lapsed imaging at increasing temperature reveals the onset of shape change to be at ≈170 K, corresponding to the onset of edge and corner diffusion of atoms along the island's borders. Despite a substantial variation for individual islands at each temperature, the mean fractal dimension increases monotonously between 170 K up to 240 K, from the smallest to the largest values feasible for islands grown on surfaces. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.

Noise and mechanical vibrations not only cause damage to devices, but also present major public health hazards. High-damping alloys that eliminate noise and mechanical vibrations are therefore required. Yet, low operating temperatures and insufficient strength/ductility ratios in currently available high-damping alloys limit their applicability. Using the concept of high-entropy alloy (HEA), we present a class of high-damping materials. The design is based on refractory HEAs, solid-solutions doped with either 2.0 atomic % oxygen or nitrogen, (Ta0.5Nb0.5HfZrTi)98O2 and (Ta0.5Nb0.5HfZrTi)98N2. Via Snoek relaxation and ordered interstitial complexes mediated strain hardening, the damping capacity of these HEAs is as high as 0.030, and the damping peak reaches up to 800 K. The model HEAs also exhibit a high tensile yield strength of ~1400 MPa combined with a large ductility of ~20%. The high-temperature damping properties, together with superb mechanical properties make these HEAs attractive for applications where noise and vibrations must be reduced. © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

The properties of a cobalt-free cast superaustenitic stainless steel (SASS) is investigated comparatively to the commercial high-cobalt alloyed GX15CrNiCo21-20-20 (1.4957, N-155) steel regarding its global hardness and wear resistance at elevated temperature by means of in situ hot hardness tests and cyclic abrasive sliding wear tests against an Al2O3 (corundum) counter-body at 600◦C. In the aged condition, results show that the 1.4957 steel suffers a higher material loss due to brittle failure initiated by coarse eutectic Cr-rich carbides which are incorporated into a mechanically mixed layer during abrasive loading. In contrast, within the Co-free steel eutectic M6(C,N) carbonitrides are distributed more homogeneously showing less tendency to form network structures. Due to the combination of primary Nb-rich globular-blocky MX-type carbonitrides and eutectic M6(C,N) carbonitrides dispersed within an Laves phase strengthened austenitic matrix, this steel provides comparable hardness and significantly improved wear resistance at elevated temperature. Thus, it may be an adequate alternative material to commercial SASS and offers the possibility to save cobalt for future applications. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

A comprehensive study of sillenite Bi12SiO20 single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c 11 and c 12 are found to increase continuously with pressure while c 44 increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi12 MO20 crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c 44 and the piezoelectric stress coefficient e 123 are almost unaffected by this dissipation. © 2020 The Author(s). Published by IOP Publishing Ltd.

The application of nanocrystalline (NC) materials at high temperatures is challenging due to their poor thermal stability or low creep resistance. Here we report that a thermally stable NC Fe-5 at.% Zr steel produced by High-Pressure-Thermal-Compression sintering exhibits an excellent creep resistance (with a creep rate of 3.92 × 10−8 s−1 at 923 K and under the applied stress of 250 MPa). The excellent creep resistance is ascribed to its highly stable NC structure stabilized by nano-sized precipitates. Mechanical testing suggests that the creep of the NC Fe-5 at.% Zr steel is controlled by dislocation activities rather than diffusion dominated mechanisms. © 2019

High temperature ferromagnetic shape memory alloys (HT-FSMAs) in form of a thin film materials library of Ni–Mn–Ga, alloyed with Fe, Co and Cu has been fabricated by co-sputtering and characterized by high-throughput screening techniques. The weak dependence of martensitic transformation temperature and tetragonal ratio of the martensitic lattice on the doping composition and their irregular variation are ascribed to the competitive influence of the alloying elements Fe, Co and Cu. © 2019 Acta Materialia Inc.

Ni–Ti–based shape memory alloys (SMAs) have found widespread use in the last 70 years, but improving their functional stability remains a key quest for more robust and advanced applications. Named for their ability to retain their processed shape as a result of a reversible martensitic transformation, SMAs are highly sensitive to compositional variations. Alloying with ternary and quaternary elements to fine-tune the lattice parameters and the thermal hysteresis of an SMA, therefore, becomes a challenge in materials exploration. Combinatorial materials science allows streamlining of the synthesis process and data management from multiple characterization techniques. In this study, a composition spread of Ni–Ti–Cu–V thin-film library was synthesized by magnetron co-sputtering on a thermally oxidized Si wafer. Composition-dependent phase transformation temperature and microstructure were investigated and determined using high-throughput wavelength dispersive spectroscopy, synchrotron X-ray diffraction, and temperature-dependent resistance measurements. Of the 177 compositions in the materials library, 32 were observed to have shape memory effect, of which five had zero or near-zero thermal hysteresis. These compositions provide flexibility in the operating temperature regimes that they can be used in. A phase map for the quaternary system and correlations of functional properties are discussed with respect to the local microstructure and composition of the thin-film library. © 2020 THE AUTHORS

Alkaline electrolyte solutions are important components in rechargeable batteries and alkaline fuel cells. As the ionic conductivity is thought to be a limiting factor in the performance of these devices, which are often operated at elevated temperatures, its temperature dependence is of significant interest. Here we use NaOH as a prototypical example of alkaline electrolytes, and for this system we have carried out reactive molecular dynamics simulations with an experimentally verified high-dimensional neural network potential derived from density-functional theory calculations. It is found that in concentrated NaOH solutions elevated temperatures enhance both the contributions of proton transfer to the ionic conductivity and deviations from the Nernst-Einstein relation. These findings are expected to be of practical relevance for electrochemical devices based on alkaline electrolyte solutions. This journal is © the Owner Societies.

This data article presents a compilation of microstructural and mechanical data regarding the ternary single-phase FCC MnFeNi medium-entropy alloy (MEA). For the analysis, interpretation, and comparison of the data to literature values, the reader can refer to the original related research article entitled “Effect of Temperature and Texture on Hall-Petch Strengthening by Grain and Annealing Twin Boundaries in the MnFeNi Medium-Entropy Alloy”, see Schneider et al. (Metals 9, 2019, 84). The microstructural data reported here include: (i) raw backscatter electron (BSE) micrographs (tif-files) obtained using a scanning electron microscope (SEM) for nine different grain sizes with four images for each grain size and (ii) pdf reports and tables shown below presenting the distributions of the grain- (d, accounting for grain boundaries only) and crystallite- (c, which accounts for both grain and annealing twin boundaries) sizes and of the annealing twin thicknesses (t). These datasets may be useful to develop new algorithms for the automated evaluation of microstructural parameters in recrystallized alloys, i.e. with these benchmark data, an algorithm for image analysis could be trained to assess the above mentioned microstructural parameters. This would help to speed up the analysis of microstructures and improve its reliability. Additional tables describing the recrystallized microstructures and texture include the average number of annealing twin boundaries per grain (n), and the average Taylor factors (M). Raeisinia et al. (Model. Simul. Mater. Sc. 16, 2008, 025001) recently used a viscoplastic model to show that differences in the distribution of microstructural parameters affect the Hall-Petch parameters, but no attempt has been carried out so far to experimentally investigate this possibility since grain size distributions are rarely reported. Here, our benchmark data (e.g. distribution in grain/crystallite sizes, annealing twins per grain, distribution of annealing twin thicknesses) could be used to address these issues. The data describing the mechanical properties reported here are excel-sheets of raw stress-strain curves for temperatures ranging from 77 K to 873 K and different grain sizes. The yield stress (σ0.2%) and the normalized Hall-Petch parameters (σ0/G and ky/Gb2) are given for all temperatures. The normalized Hall-Petch parameters are reported here since they allow to better compare the strength and the magnitude of grain boundary strengthening of different alloys with the same crystallographic structure, see Cordero et al. (Int. Mater. Rev. 61, 2016, 495–512). Moreover, the Hall-Petch parameters as well as the mechanical data reported here could be used for data mining and implemented in programs used for alloy design. © 2019 The Author(s)

Strong and ductile materials that have high resistance to corrosion and hydrogen embrittlement are rare and yet essential for realizing safety-critical energy infrastructures, hydrogen-based industries, and transportation solutions. Here we report how we reconcile these constraints in the form of a strong and ductile CoNiV medium-entropy alloy with face-centered cubic structure. It shows high resistance to hydrogen embrittlement at ambient temperature at a strain rate of 10−4 s−1, due to its low hydrogen diffusivity and the deformation twinning that impedes crack propagation. Moreover, a dense oxide film formed on the alloy’s surface reduces the hydrogen uptake rate, and provides high corrosion resistance in dilute sulfuric acid with a corrosion current density below 7 μA cm−2. The combination of load carrying capacity and resistance to harsh environmental conditions may qualify this multi-component alloy as a potential candidate material for sustainable and safe infrastructures and devices. © 2020, The Author(s).

We present a double-layer design for two-dimensional lateral surface superlattice systems in GaAs-AlGaAs heterostructures. Unlike previous studies, our device (1) uses an in situ gate, which allows a very short period superlattice in high mobility, shallow heterostructures and (2) enables independent control of the carrier density and the superlattice modulation potential amplitude over a wide range. We characterize this device design using low-temperature magneto-transport measurements and show that the fabrication process caused minimal damage to the system. We demonstrate the tuning of potential modulation from weak (much smaller than Fermi energy) to strong (larger than the Fermi energy) regimes. © 2020 Author(s).

Cr–Mo-alloyed cast martensitic stainless steels are suitable tool materials for a wide field of applications. Local inhomogeneities in the chemical composition, however, affect their local and global properties such as the hardenability and the corrosion resistance. Herein, the influence of microsegregations on phase stabilities and properties is investigated by means of property distribution maps (PDM) which are determined via thermodynamic and empirical calculations based on measured local chemical composition data. The results show that the enrichment of Cr and Mo in interdendritic regions benefits the local corrosion resistance but increases the solvus temperature of M23C6 carbides from 1040 to 1150 °C and depresses the martensite start temperature (Ms) to temperatures below 50 °C locally. As predicted from the PDM, high-temperature austenitization at 1150 °C combined with a cryogenic treatment at −80 °C ensures a martensitic microstructure with relatively high hardness (592 ± 12 HV10) and significantly higher critical pitting potential compared with specimens austenitizized at 1050 °C, which proves PDM to be a powerful tool for the optimization of heat treatment parameters. However, local transformation of austenite into δ-ferrite during austenitization at 1150 °C must be considered. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The excitation and dissociation of CO2 admixed to argon and helium atmospheric pressure radio frequency plasmas is analyzed. The absorbed plasma power is determined by voltage and current probe measurements and the excitation and dissociation of CO2 and CO by transmission mode Fourier-transform infrared spectroscopy (FTIR). It is shown, that the vibrational temperatures of CO2 and CO are significantly higher in an argon compared to a helium plasma. The rotational temperatures remain in both cases close to room temperature. The conversion efficiency, expressed as a critical plasma power to reach almost complete depletion, is four times higher in the argon case. This is explained by the lower threshold for the generation of energetic particles (electrons or metastables) in argon as the main reactive collision partner, promoting excitation and dissociation of CO2, by the less efficient quenching of vibrational excited states of CO and CO2 by argon compared to helium and by a possible contribution of more energetic electrons in an argon plasma compared to helium. © 2020 IOP Publishing Ltd.

The photoluminescence of quasi two-dimensional (2D) electrons in modulation doped Al x Ga1 - x As-GaAs single heterostructure recombining with bound holes was studied at liquid helium temperatures in magnetic fields up to 9 T. Luminescence contributions from the ground and a weakly populated first excited 2D subband were observed as well as bulk-like contributions from the GaAs buffer. The 2D subband and the bulk signals are linked and oscillate with the magnetic field strength. It is not the filling factor that rules but the alignment of the two subband Landau-ladders paired with intersubband scattering that drives an oscillatory behavior of the subband populations. Asymmetric Landau-level transition line shapes indicate a perturbed 2D electron system. We performed a detailed analyses of Landau-level dependent line strengths, positions and half widths and compared the experiment with calculations of subband energies and transition matrix elements. The total electron density is not fixed in our experiment but decreases with magnetic field strength. This might indicate that the 2D system exchanges electrons with a reservoir and/or that the correlation between radiative and nonradiative recombination channels vary strongly with the magnetic field strength. © 2020 IOP Publishing Ltd.

Alloys based on titanium aluminides received a lot of attention because of their capability to substitute Ni-based superalloys in high-temperature applications. However, the phase equilibria between the main microstructure constituents (αTi), (βTi), γ (TiAl) and α2(Ti3Al) can be shifted significantly by impurities such as oxygen especially at high temperatures. This behavior is investigated on the tie-triangle (αTi) + (βTi) + γ (TiAl) in the ternary Ti-Al-Nb system at 1300 °C. An explanation for this behavior could be the occupation of octahedral voids by impurities in certain phases. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

Cross-correlated noise measurements are performed in etched Al x Ga 1 - xAs/GaAs based quantum rings in equilibrium at a bath temperature of T bath = 4.2 K. The measured white noise exceeds the thermal (Johnson-Nyquist) noise expected from the measured electron temperature T e and the electrical resistance R. This excess part of the white noise decreases as T bath increases and vanishes for T bath ≥ 12 K. Excess noise is neither observed if one arm of a quantum ring is depleted of electrons nor in one-dimensional-constrictions that have a length and width comparable to the quantum rings. A model is presented that suggests that the excess noise originates from the correlation of noise sources, mediated by phase-coherent propagation of electrons. © 2020 Author(s).

Recently, a single particle pyrolysis-combustion fragmentation model has been developed (Senneca et al., 2013, 2017) [1,2] to predict the propensity of coal particles to fragment under a wide range of heating conditions as a consequence of mechanical failure of the particle. Stress inside the particle arises from thermal shock, associated to particles’ heat up, as well as from overpressure generated by volatiles release upon devolatilization. The model is now used to calculate the propensity of coal particles to undergo fragmentation in the early stages of oxy-combustion, with gaseous atmospheres of 5–30% O2 in CO2 in entrained flow and fluidized beds reactors. Accordingly particles size of 0.1–10 mm are assumed, temperatures of 1123 and 2073 K, heating rates of 100 and 10,000 K/s. Results show that under entrained flow reactor conditions the particles break in the first 20–30 ms, producing a bimodal particle-size distribution. Under fluidized bed conditions, the particles undergo explosive fragmentation after 1–2 s, before pyrolysis is complete, generating broad particle size distribution. In both cases fragmentation occurs over short timescales compared to char combustion and gasification. Operative conditions where fragmentation occurs before or in parallel with char combustion or gasification are inferred by comparing on an Arrhenius plot the timescale of fragmentation and heterogeneous reactions for a larger array of operating conditions. The figure reveals that for high reaction temperatures, more reactive coals, larger particles size, gasification reactions can have an important role and maybe enhance porosity and percolative fragmentation. © 2020 Elsevier Ltd

This review presents an overview of the developments in small-scale shape memory materials: from alloys to oxides and ceramics. Shape memory oxides such as zirconia, different ferroelectric perovskites and VO2-based materials have favorable characteristics of high strength, high operating temperature and chemical resistance, which make this class of shape memory materials interesting for special applications, e.g., in harsh environments or at the nanoscale. Because of the constraint and mismatch stress from neighboring grains in polycrystalline/bulk oxides, the transformation strain of shape memory oxides is relatively small, and micro-cracks can appear after some cycles. However, recent progress in shape memory oxide research related to small-scale approaches such as decreasing the amounts of grain boundaries, strain-engineering, and application in the form of nanoscale thin films shows that some oxides are capable to exhibit excellent shape memory effects and superelasticity at nano/micro-scales. The materials systems ZrO2, BiFO3, and VO2 are discussed with respect to their shape memory performance in bulk and small-scale. © 2020, The Author(s).

The equiatomic CrCoNi alloy is regarded as a model single-phase face-centered cubic medium-entropy alloy. A CrCoNi single crystal was grown by a Bridgman technique using a Ni-base superalloy seed. The elastic stiffnesses and thermal expansion coefficient were determined between 100 K and 673 K employing resonant ultrasound spectroscopy and dilatometry, respectively. All data were found to be in excellent agreement with those reported for polycrystalline CrCoNi. A comparison of the normalized Cauchy pressure of CrCoNi with those of other alloys indicates that interatomic bonds become more directional with increasing Cr-concentration while Co and Ni promote a metallic character. © 2019 Acta Materialia Inc.

The contribution comprises the investigation of the microstructure and residual stresses in thermally sprayed Mn1.0Co1.9Fe0.1O4.0 (MCF) protective coatings for interconnectors of SOFC stacks, deposited on ferritic steel Crofer 22 APU via atmospheric plasma spraying (APS). The coatings are designated to prevent Cr evaporation during high operation temperature of the SOFCs. The local microstructure, pore distributions and pore shapes, phase fractions, micro-hardness, Youngs’ modulus and residual stresses through the coating thickness were characterized in as-sprayed state and compared with longtime (10-100 h) heat-treated samples (700 and 850 °C). The results show that the long-term thermal aging treatment causes a successive high sintering of the coatings characterized by a reduction in pore density, by phase transformation from the metastable rock salt structure that gradually transformed to a spinel structure and by a slight relaxation of the process-induced tensile residual stresses in the coating. For SOFC application of the MCF coating, this indicates an improvement in the coatings integrity. During operation, a self-repair proceeds leading to dense and gas-proof coatings, while the mechanical properties are mainly retained. © 2020, The Author(s).

Herein we report the mechanochemical Scholl polymerization of 1,3,5-triphenylbenzene in a high speed ball mill. The reaction is conducted solvent-free, solely using solid FeCl3. The resulting porous polymer was obtained in >99% yield after very short reaction times of only 5 minutes and exhibits a high specific surface area of 658 m2 g-1, which could be further enhanced up to 990 m2 g-1 by liquid assisted grinding. Within this study we illuminate the origin of porosity by investigating the impact of various milling parameters and milling materials, temperature and pressure, and different liquids for LAG as well as post polymer milling. Finally we expand the procedure to different monomers and mills, to present the mechanochemical Scholl reaction as a versatile synthesis tool for porous polymers. © The Royal Society of Chemistry.

An advanced infrared emitter, consisting of a non-periodic silicium-microstructure and a platinium-nano-composition, which enables extraordinary highly emission intensities is presented. A spectral broadband emission coefficient ϵ of nearly 1 is achieved. The foundation of the emitter is a MEMS hot plate design containing a high temperature stable molybdenum silicide resistance heater layer embedded in a multilayer membrane consisting of silicon nitride and silicon oxide. The temperature resistance of the silicon-platinum micro-nanostructure up to 800 °C is secured by a SiO2 protection layer. The long-term stability of the spectral behavior at 750 °C has been demonstrated over 10,000 h by FTIR measurements. The low thermal mass of the multilayer MEMS membrane leads to a time constant of 28 ms which enables high chopper frequencies. A precondition for long term stability under rough conditions is a real hermetic housing. High temperature stable packaging technologies for infrared MEMS components were developed. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.

Semiconductor quantum dots in photonic integrated circuits enable scaling quantum-information processing to many single photons and quantum-optical gates. Obtaining high-purity and coherent single photons from quantum dots requires spectral filtering to select individual excitonic transitions. Here, an on-chip wavelength-tunable filter integrated with a single-photon source, which preserves the optical properties of the emitter, is demonstrated. Nanomechanical motion is used for tuning the resonant wavelength over 10 nm, enabling operation at cryogenic temperatures, and single-photon emission from a quantum dot under non-resonant excitation is demonstrated without resorting to free-space optical filters. These results are key for the development of fully integrated de-multiplexing, multi-path photon encoding schemes, and multi-emitter circuits. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In the present study, X-ray diffraction was applied to measure stacking fault energy of Cr–Ni austenitic steels containing different amounts of alloying elements. The results in austenitic steels show that the Ni content and Cr/Ni ratio have a strong effect on SFE. Cu, Si and N increase SFE, being the effect of nitrogen more pronounced; Mo has the opposite effect. In situ XRD experiments up to 300 °C were employed to determine experimentally the SFE and its temperature dependence in Ni and AISI 304. The microstructural parameters required to determine SFE, obtained by Rietveld refinement, made possible to determine experimentally an increase in the SFE with the temperature, related to a decrease in the accumulated deformation, a lower tendency to form stacking faults and a thermal expansion as the temperature increases. The accuracy was determined based on SFE measurements of Au, Cu and Ni pure metals, where the error of the applied method was carefully evaluated. © 2020, Springer Science+Business Media, LLC, part of Springer Nature.

A simple Peltier cooled cold trap aperture is presented to minimize the flux of hydrocarbons on optics in vacuum UV systems. The system can be cooled down to -40 °C under vacuum. To test the effect of the cold trap, the aperture is placed in front of a high-intensity D2 lamp used for calibration in the range of 116 nm to 300 nm which is flanged to a VUV spectrometer. The influence of the aperture temperature is monitored by measuring the intensity loss rate of the Lyman-alpha emission line at 121.6 nm due to the formation of carbon contamination on the MgF2 window of the lamp depending on the Peltier temperature. The application of the aperture significantly reduced the intensity loss from approx. 20% h-1 to less than 2% h-1 and enables the reliable use of a D2 lamp for the relative intensity calibration of the spectrometer. © 2020 IOP Publishing Ltd.

The internal structure of the superconducting state in Sr2RuO4 remains elusive at present, and exhibits evidence for time-reversal symmetry breaking. Recent muon spin relaxation measurements under uniaxial strain have revealed an increasing splitting between the superconducting critical temperature Tc and the onset of time-reversal symmetry breaking TTRSB with applied strain (Grinenko et al., arXiv:2001.08152). In addition, static magnetic order is induced by the uniaxial strain beyond ∼1 GPa, indicating that unstrained Sr2RuO4 is close to a magnetic quantum critical point. Here we perform a theoretical study of the magnetic susceptibility and the associated pairing structure as a function of uniaxial strain. It is found that the recent muon relaxation data can be qualitatively explained from the perspective of spin-fluctuation mediated pairing and the associated strain dependence of accidentally degenerate pair states in unstrained Sr2RuO4. In addition, while unstrained Sr2RuO4 features mainly (2π/3,2π/3) magnetic fluctuations, uniaxial strain promotes (π,±π/2) magnetic order. © 2020 American Physical Society.

We investigate the adsorption geometry of a three-dimensional organic molecule, anilino-nitro azobenzene, within hydrogen-bonded supramolecular structures on Au(111) by low temperature scanning tunneling microscopy. Therein, three conformational isomers exist, completely planar trans and cis-isomers and a non-planar, but surface adapted cis-isomer. The anilino-end of the molecule is planar for all isomers. In contrast, the nitro-end of the cis-isomer is only planar, if the nitro-end of the molecule forms hydrogen bonds. Our study pinpoints the subtle balance between molecule-substrate and molecule-molecule interaction in adsorption-induced bond-angle distortion that drive partial or full planarization of the molecule. © 2019 Elsevier B.V.

High entropy shape memory alloys (HESMAs) represent a relatively young class of functional materials. They show a reversible martensitic phase transformation which allows to exploit shape memory effects at relatively high temperatures. HESMAs represent ordered complex solid-solutions. Their high temperature phase is of B2 type, and various elements (e.g. Ni, Cu, Ti, Zr, Hf) occupy sites in specific sub-lattices. In the present work, we study the processing and the functional properties of HESMAs. We study effects of chemical complexity on solidification microstructures and martensitic transformations. Binary, ternary, quaternary, quinary and senary model alloys were investigated using advanced microstructural and thermal characterization methods. The results show that element partitioning during solidification results in a redistribution of individual alloy elements in dendritic/interdendritic regions. Surprisingly, the atomic ratios of the two groups of elements which occupy the Ni- (first group: Ni, Cu and Pd) and Ti-sub-lattice (second group: Ti, Zr, Hf) are maintained. This allows the material to form martensite throughout its heterogeneous microstructure. The effect of chemical complexity/composition on martensite start temperatures, MS, is discussed on the basis of valence electron concentrations, cV. Some of the alloys fall into MS(cV)-regimes which are uncommon for classical Ni-Ti-based shape memory alloys. In the present work, a new HESMA of type NiCuPdTiZrHf was identified which has the potential to provide maximum shape memory strains close to 15%. © 2020

For decades, grain boundary engineering has proven to be one of the most effective approaches for tailoring the mechanical properties of metallic materials, although there are limits to the fineness and types of microstructures achievable, due to the rapid increase in grain size once being exposed to thermal loads (low thermal stability of crystallographic boundaries). Here, we deploy a unique chemical boundary engineering (CBE) approach, augmenting the variety in available alloy design strategies, which enables us to create a material with an ultrafine hierarchically heterogeneous microstructure even after heating to high temperatures. When applied to plain steels with carbon content of only up to 0.2 weight %, this approach yields ultimate strength levels beyond 2.0 GPa in combination with good ductility (>20%). Although demonstrated here for plain carbon steels, the CBE design approach is, in principle, applicable also to other alloys. Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Bis(pyrazolyl)bipyridinylmethane iron(II) complexes show a versatile spin state switching behavior in different solvents. In the solid, the magnetic properties of the compounds have been characterized by X-ray diffraction, Mößbauer spectroscopy, and SQUID magnetometry and point toward a high spin state. For nitrilic solvents, the solvation of the complexes leads to a change of the coordination environment from {N5O} to {N6} and results in a temperature-dependent SCO behavior. Thermodynamic properties of this transformation are obtained via UV/vis spectroscopy, SQUID measurements, and the Evans NMR method. Moreover, a coordination-induced spin state switch (CISSS) to low spin is observed by using methanol as solvent, triggered through a rearrangement of the coordination sphere. The same behavior can be observed by changing the stoichiometry of the ligand-to-metal ratio in MeCN, where the process is reversible. This transformation is monitored via UV/vis spectroscopy, and the resulting new bis-meridional coordination motif, first described for bis(pyrazolyl)methanes, is characterized in the solid state via X-ray diffraction, Mößbauer spectroscopy, and SQUID measurements. The sophisticated correlation of these switchable properties in dependence on different types of solvents reveals that the influence of the solvent on the coordination environment and magnetic properties should not be underestimated. Furthermore, careful investigation is necessary to differentiate between a thermally-induced spin crossover and a coordination-induced spin state switch. © 2020 American Chemical Society.

Topological spin configurations in proximity to a superconductor have recently attracted great interest due to the potential application of the former in spintronics and also as another platform for realizing nontrivial topological superconductors. Their application in these areas requires precise knowledge of the existing exchange fields and/or the stray fields, which are therefore essential for the study of these systems. Here, we determine the effective stray field Hstr and the Meissner currents jS in a superconductor/ferromagnet/superconductor (S/F/S) junction produced by various nonhomogenous magnetic textures M(r) in the F. The inhomogeneity arises either due to a periodic structure with flat domain walls (DW) or is caused by an isolated chiral magnetic skyrmion (Sk). We consider both Bloch- and Néel-type Sk and also analyze in detail the periodic structures of different types of DW's, that is, Bloch-type DW (BDW) and Néel-type DW (NDW) of finite width with in- and out-of-plane magnetization vector M(x). The spatial dependence of the fields Hstr(r) and Meissner currents jS(r) are shown to be qualitatively different for the case of Bloch- and Néel-type magnetic textures. While the spatial distributions in the upper and lower S are identical for Bloch-type Sk and DW's they are asymmetric for the case of Néel-type magnetic textures. The depairing factor, which determines the critical temperature Tc and which is related to the vector potential of the stray field, can have its maximum at the center of a magnetic domain but also, as we show, above the DW. For Sk's, the maximum is located at a finite distance within the Sk radius rSk. Based on this, we study the nucleation of superconductivity in the presence of DW's. Because of the asymmetry for Néel-type structures, the critical temperature Tc in the upper and lower S is expected to be different. The obtained results can also be applied to S/F bilayers. © 2020 American Physical Society.

Ni-base single-crystal superalloys exhibit a dynamic evolution of their microstructure during operation at elevated temperatures. The rafting of γ′ precipitates changes the mechanical behavior in a way that was understood insufficiently. In this work, we combine a phase-field method with a discrete dislocation dynamics model to clarify the influence of different rafted microstructures with the same initial dislocation density and configuration on creep behavior. The unrafted and rafted microstructures of Ni-base single crystal superalloys are simulated by a phase-field crystal plasticity method. By introducing these microstructures into a 3D discrete dislocation dynamics (DDD) model, the creep behavior under uniaxial loads of 350 and 250 MPa along [100] direction at 950 °C is studied. Due to the negative lattice mismatch of Ni-base superalloys, the N-type rafting with the formation of plate-like γ′ precipitates occurs under uniaxial tensile loads along {100} direction at high temperatures, while the P-type rafting with the formation of rod-like γ′ precipitates occurs under compressive loads. Taking the cuboidal, N-type rafted and P-type rafted microstructures as the initial and fixed microstructures for the same loading conditions, it is found from DDD simulations that the rafted microstructures result in smaller creep deformation than the cuboidal microstructure. The reason for this is that the coalescence of γ′ precipitates during the rafting diminishes the width of some γ channels, so as to increase the local Orowan stresses which retard the dislocation glide. For tensile loads, the N-type rafted microstructure has the best creep resistance. For a low compressive load, the P-type rafting shows a better creep resistance than N-type rafting. © 2019 IOP Publishing Ltd.

In low temperature plasmas, the interaction of the electrons with the electric field is an important current research topic that is relevant for many applications. Particularly, in the low pressure regime (≤ 10 Pa), electrons can traverse a distance that may be comparable to the reactor dimensions without any collisions. This causes "nonlocal,"dynamics which results in a complicated space- and time-dependence and a strong anisotropy of the distribution function. Capacitively coupled radio frequency (CCRF) discharges, which operate in this regime, exhibit extremely complex electron dynamics. This is because the electrons interact with the space- and time-dependent electric field, which arises in the plasma boundary sheaths and oscillates at the applied radio frequency. In this tutorial paper, the fundamental physics of electron dynamics in a low pressure electropositive argon discharge is investigated by means of particle-in-cell/Monte Carlo collisions simulations. The interplay between the fundamental plasma parameters (densities, fields, currents, and temperatures) is explained by analysis (aided by animations) with respect to the spatial and temporal dynamics. Finally, the rendered picture provides an overview of how electrons gain and lose their energy in CCRF discharges. © 2020 Author(s).

In high power impulse magnetron sputtering (HiPIMS) bright plasma spots are observed during the discharge pulses that rotate with velocities in the order of 10 km s-1 in front of the target surface. It has proven very difficult to perform any quantitative measurements on these so-called spokes, which emerge stochastically during the build-up of each plasma pulse. In this paper, we propose a new time shift averaging method to perform measurements integrating over many discharge pulses, but without phase averaging of the spoke location, thus preserving the information of the spoke structure. This method is then applied to perform Langmuir probe measurements, employing magnetized probe theory to determine the plasma parameters inside the magnetic trap region of the discharge. Spokes are found to have a higher plasma density, electron temperature and plasma potential than the surrounding plasma. The electron density slowly rises at the leading edge of the spoke to a maximum value of about 1 1020 m-3 and then drops sharply at the trailing edge to 4 1019 m-3. The electron temperature rises from 2.1 eV outside the spoke to 3.4 eV at the trailing end of the spoke. A reversal of the plasma potential from about -7 V outside the spoke to values just above 0 V in a spoke is observed, as has been proposed in the literature. © 2020 IOP Publishing Ltd.

In the present work, we investigate the stress and temperature dependence of low-temperature (750 ± 20 °C) and high-stress (300 ± 20 MPa) shear creep of a Ni-base single crystal superalloy. From continuous isothermal experiments and stress and temperature change tests the stress exponent n and the apparent activation energy Qapp of the phenomenological Sherby-Dorn equation were determined for the two macroscopic crystallographic shear systems (MCSS) [011¯](111) and [112¯](111). The activation parameters of creep, the stress exponents and the apparent activation energies were identified as 16 and 620 kJ/mol (MCSS: [011¯](111)) and 14 and 460 kJ/mol (MCSS: [112¯](111)). We show that during shear creep testing these phenomenological parameters do not change between the early (0.5–1% strain) and later stages of creep (4.5–5% strain), in contrast to what was observed for uniaxial tensile testing. The results are discussed in the light of what is known about stress and temperature dependencies of deformation rates in the creep literature and in view of the recent work by Bürger et al., 2020, who combined shear creep testing with analytical transmission electron microscopy to identify the elementary deformation mechanism, which governs low temperature and high stress creep. © 2020 The Authors

Aerosol deposition (AD) is a novel deposition process for the fabrication of dense and rather thick oxide films at room temperature. The bonding of the deposited ceramic particles is based on a shock-loading consolidation, resulting from the impact of the ceramic particles on the substrate. However, the deposition mechanism is not fully understood. In addition, many technical challenges have been observed for achieving a successful deposition of the oxides with higher efficiency. In this work, the influence of different processing parameters on the properties of the deposited layer is studied. Proof of concept was done using 8 mol.% yttria-stabilized zirconia (8YSZ) powder as starting material. The window of deposition with respect to carrier gas flows for successful deposition was identified. The influence of this carrier gas flow, the substrate materials and the carrier gas species on the coating thickness, interface quality and coating microstructure was systematically investigated. The derived mechanical characteristics revealed an unexpected behavior related to a gradient microstructure. This study supports understanding of the mechanism of room-temperature impact consolidation and its effect on the mechanical properties of the deposited layer. © 2020, ASM International.

In high-temperature combustion atmospheres, well-adhering environmental barrier coatings (EBCs) are required to protect the underlying ceramic matrix composites (CMCs) from corrosion. Herein the adhesion mechanisms of three different coatings produced by atmospheric plasma spraying (APS) on an Al2O3/Al2O3-CMC are investigated. In particular, the influence of surface structuring by laser ablation prior to coating production is investigated. Y2O3, yttria-stabilized zirconia (YSZ), and Gd2Zr2O7 are chosen as potential EBCs. The coating adhesion on CMC-substrates with and without surface structuring is analyzed by furnace cycling, pull-adhesion tests, and burner-rig tests with gradient. Special interest is paid to the interactions at the coating–substrate interface before and after heat treatment and their effect on the coating adhesion and lifetime. Two different adhesion mechanisms are found: adhesion promoted by chemical reaction and adhesion promoted by mechanical interlocking. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The kinetic modeling of photocatalytic reactions is a powerful tool for process optimization. We applied a holistic kinetic model for the gas-phase photocatalytic oxidation of HCl to Cl2 to identify suitable operation conditions and further optimization potential. We used a flat-plate photoreactor with UV LEDs and iodometric titration as online analytics and performed a comprehensive parameter variation. High O2 and moderate HCl partial pressures resulted in the highest reaction rates, indicating a favorable reactant ratio of 4:1. An Arrhenius dependence of the reaction rate with an apparent activation energy of 25.7 kJ mol-1 identifies a suitable reaction temperature of ∼120 °C. This temperature combines high reaction rates with high apparent quantum yields up to 8.4%, showing a logarithmic dependence of reaction rates on light intensity. The well-fitting kinetic model predicts that improving the intrinsic activity of the photocatalyst is the key for further enhancing the efficiency of photocatalytic HCl recycling. Copyright © 2020 American Chemical Society.

The identification of bis-3-(N,N-dimethylamino)propyl zinc ([Zn(DMP)2], BDMPZ) as a safe and potential alternative to the highly pyrophoric diethyl zinc (DEZ) as atomic layer deposition (ALD) precursor for ZnO thin films is reported. Owing to the intramolecular stabilization, BDMPZ is a thermally stable, volatile, nonpyrophoric solid compound, however, it possesses a high reactivity due to the presence of Zn-C and Zn-N bonds in this complex. Employing this precursor, a new oxygen plasma enhanced (PE)ALD process in the deposition temperature range of 60 and 160 °C is developed. The resulting ZnO thin films are uniform, smooth, stoichiometric, and highly transparent. The deposition on polyethylene terephthalate (PET) at 60 °C results in dense and compact ZnO layers for a thickness as low as 7.5 nm with encouraging oxygen transmission rates (OTR) compared to the bare PET substrates. As a representative application of the ZnO layers, the gas sensing properties are investigated. A high response toward NO2 is observed without cross-sensitivities against NH3 and CO. Thus, the new PEALD process employing BDMPZ has the potential to be a safe substitute to the commonly used DEZ processes. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The current work compares the deformation behavior of CoCrFeMnNi and CoCrNi in the temperature interval between 295 K and 8 K through a series of quasi-static tensile tests. Temperature-dependent yield stress variation was found to be similarly high in these two alloys. Previous investigations only extended down to 77 K and showed that a small amount of ε-martensite was formed in CoCrNi while this phase was not observed in CoCrFeMnNi. The present study extends these investigations down to 8 K where similar low levels of ε-martensite were presently detected. Based on this result, a rough assessment has been made estimating the importance of deformation twinning to the strength. The relative work hardening rates of CoCrFeMnNi and CoCrNi were comparable in value despite the differences in ε-martensite formation during deformation. CoCrFeMnNi deforms by dislocation slip and deformation twinning while deformation in CoCrNi is also accommodated by the formation of ε-martensite at cryogenic temperatures. Additionally, CoNi, a solid solution from the Co–Cr–Fe–Mn–Ni system with low strength, was used for comparison, showing contrasting deformation behavior at cryogenic temperatures. © 2020 Elsevier B.V.

Spray-flame synthesis was used to produce high-surface-area perovskite electrocatalysts with high phase purity, minimum surface contamination, and high electrochemical stability. In this study, as-prepared LaCo1–xFexO3 perovskite nanoparticles (x=0.2, 0.3, and 0.4) were found to contain a high degree of combustion residuals, and mostly consist of both, stoichiometric and oxygen-deficient perovskite phases. Heating them at moderate temperature (250 °C) in oxygen could remove combustion residuals and increases the content of stoichiometric perovskite while preventing particle growth. A higher surface crystallinity was observed with increasing iron content coming along with a rise in oxygen deficient phases. With heat treatment, OER activity and stability of perovskites improved at 30 and 40 at.% Fe while deteriorating at 20 at.% Fe. This study highlights spray-flame synthesis as a promising technique to synthesize highly active nanoscale perovskite catalysts with improved OER activity. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

We study the elastoresistance of the highly correlated material CsFe2As2 in all symmetry channels. Neutralizing its thermal expansion by means of a piezoelectric-based strain cell is demonstrated to be essential. The elastoresistance response in the in-plane symmetric channel is found to be large, while the response in the symmetry-breaking channels is weaker and provides no evidence for a divergent nematic susceptibility. Rather, our results can be interpreted naturally within the framework of a coherence-incoherence crossover, where the low-temperature coherent state is sensitively tuned by the in-plane atomic distances. © 2020 American Physical Society.

The degradation of moulds, dies and tools employed in plastic, food and chemical processing industries has necessitated the development of suitable wear and corrosion-resistant materials. As improving the wear and corrosion resistance of iron base alloys tend to have opposing demands regarding chemical composition and heat treatment, optimisation of both parameters has to be kept in mind. One alloying element that is known to improve both corrosion and wear resistance of steels is nitrogen. Hence, an investigation into the densification of high chromium X190CrVMo20-4-1 cold work tool steel in a vacuum and under a nitrogen atmosphere at different pressures via supersolidus liquid-phase sintering (SLPS) process is reported in this paper. The investigation aimed to elucidate the influence of different atmospheres and nitrogen partial pressures employed during densification on the microstructure, optimal heat treatment parameters and micromechanical properties of the steel. Experimental findings were supplemented by computational thermodynamics calculations. The results revealed that increasing nitrogen pressure promoted the diffusion of vanadium from Cr-rich carbides (M7C3) to form V-rich carbonitrides, M(C,N). Optimum quench-hardening temperature was strongly influenced by the matrix chemistry. Upon tempering, the nitrogen-sintered samples had higher secondary hardening potential than the vacuum-sintered at a higher temperature, but a low-temperature tempering is beneficial to the corrosion resistance of the steel. The mechanical properties of the carbides in the densified steels in different atmospheres were influenced by their chemical composition. Experimental observations are in good agreement with computational thermodynamic evaluations. © 2020 Elsevier B.V.

Conventionally produced tungsten (W) sheets are brittle at room temperature. In contrast to that, severe deformation by cold rolling transforms W into a material exhibiting room-temperature ductility with a brittle-to-ductile transition (BDT) temperature far below room temperature. For such ultrafine-grained (UFG) and dislocation-rich materials, the mechanism controlling the BDT is still the subject of ongoing debates. In order to identify the mechanism controlling the BDT in room-temperature ductile W sheets with UFG microstructure, we conducted campaigns of fracture toughness tests accompanied by a thermodynamic analysis deducing Arrhenius BDT activation energies. Here, we show that plastic deformation induced by rolling reduces the BDT temperature and also the BDT activation energy. A comparison of BDT activation energies with the trend of Gibbs energy of kink-pair formation revealed a strong correlation between both quantities. This demonstrates that out of the three basic processes, nucleation, glide, and annihilation, crack tip plasticity in UFG W is still controlled by the glide of dislocations. The glide is dictated by the mobility of the screw segments and therefore by the underlying process of kink-pair formation. Reflecting this result, a change of the rate-limiting mechanism for plasticity of UFG W seems unlikely, even at deformation temperatures well below room temperature. As a result, kink-pair formation controls the BDT in W over a wide range of microstructural length scales, from single crystals and coarse-grained specimens down to UFG microstructures. © 2020, The Author(s).

Photocatalytic conversion of carbon dioxide (CO2) is a promising strategy for both renewable solar energy storage and carbon emission reduction. Forming multicarbon products in CO2 photocatalytic reduction remains very difficult, due to the kinetic barriers of C–C coupling. In this study, we introduce surface pentavalent tungstic (W5+) species in nonstoichiometric tungsten oxides via carbothermal reduction of commercial WO3 powder, achieving photocatalytic CO2 conversion with an aldehyde selectivity of 35% at a total normal CO2 conversion rate of 1.8 µmol·gcat−1·h−1. The as-prepared nonstoichiometric tungsten oxides exhibit improved band structures and electron transport properties, and the W5+ surface species with oxygen vacancies play a pivotal role in facilitating C–C coupling of key intermediates. In-depth carbothermal reduction at an elevated temperature (880 °C) delivers more tetravalent W4+ species, decreasing the CO and aldehyde production rates. This work provides fundamental information to facilitate C–C coupling in CO2 photoreduction via the introduction of surface active species. © 2020 Elsevier B.V.

Alloying nanocrystalline copper (Cu) with immiscible elements, such as tantalum (Ta) and zirconium (Zr), is a promising technique to manipulate grain boundary properties and by this suppress grain growth at elevated temperatures. However, insights on the atomistic origins on the influence of impurity elements on grain boundaries are lacking. In this study, the atomistic effects of Ta and Zr on [111] tilt grain boundaries in Cu are investigated by high resolution scanning transmission electron microscopy techniques. In case of Ta, the formation of spherical, nano-scale precipitates in close vicinity to the grain boundaries is observed, but no sign of segregation. The particles induce a repelling force to migrating boundaries and act as local pinning points. The segregation of Zr is observed to occur either at confined grain boundary steps or homogeneously along the boundaries without steps. In both cases a strong disordering of the defect or grain boundary structure is revealed. Furthermore, at low Zr concentrations it induces structural grain boundary transitions and partial atomic reordering of the grain boundary structural units. © 2020 Acta Materialia Inc.

The theory of grain boundary (the interface between crystallites, GB) structure has a long history1 and the concept of GBs undergoing phase transformations was proposed 50 years ago2,3. The underlying assumption was that multiple stable and metastable states exist for different GB orientations4–6. The terminology ‘complexion’ was recently proposed to distinguish between interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexion and transitions between complexions have been characterized, mostly in binary or multicomponent systems8–19. Simulations have provided insight into the phase behaviour of interfaces and shown that GB transitions can occur in many material systems20–24. However, the direct experimental observation and transformation kinetics of GBs in an elemental metal have remained elusive. Here we demonstrate atomic-scale GB phase coexistence and transformations at symmetric and asymmetric [11 1 ¯] tilt GBs in elemental copper. Atomic-resolution imaging reveals the coexistence of two different structures at Σ19b GBs (where Σ19 is the density of coincident sites and b is a GB variant), in agreement with evolutionary GB structure search and clustering analysis21,25,26. We also use finite-temperature molecular dynamics simulations to explore the coexistence and transformation kinetics of these GB phases. Our results demonstrate how GB phases can be kinetically trapped, enabling atomic-scale room-temperature observations. Our work paves the way for atomic-scale in situ studies of metallic GB phase transformations, which were previously detected only indirectly9,15,27–29, through their influence on abnormal grain growth, non-Arrhenius-type diffusion or liquid metal embrittlement. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

An electrosurgical argon plasma with a 5% admixture of molecular hydrogen is studied in order to investigate time averaged plasma parameters by optical emission spectroscopy (OES). Electron densities in the range of 1015-1016 cm-3 are determined from the Stark broadening of the time averaged line profiles of the Balmer-α and -β emission lines of hydrogen. A two-profile fit corresponding to regions of different electron densities is found to provide a better representation of the line broadening than a single profile fit. This is consistent with time resolved ICCD imaging, acquired with 150 ns time resolution, that shows strong radial gradients in the plasma emission and the asymmetry produced by the discharge arrangement. Gas temperatures are determined using two different methods. Firstly, simulated spectra for different rotational temperatures are fitted to the measured N2(C-B, 0-1) emission band originating from ambient air diffusion into the argon/hydrogen gas flow. From the best fit, rotational temperatures between 1500 K and 1800 K are inferred. These measurements are in good agreement with those inferred by the second method, which is based on the collisional broadening of the emission lines of neutral argon at 750 nm and 751 nm. This latter method may be useful for the measurement of gas temperatures when the device is used inside hollow organs during endoscopic or laparoscopic interventions, where air mixing will be limited. Therefore, the results of this study are highly relevant to applications of these devices, e.g. for controlling tissue effects and the avoidance of excessive heating. © 2020 IOP Publishing Ltd.

When a magnetic impurity exists in a metal, conduction electrons form a spin cloud that screens the impurity spin. This basic phenomenon is called the Kondo effect1,2. Unlike electric-charge screening, the spin-screening cloud3–6 occurs quantum coherently, forming spin-singlet entanglement with the impurity. Although the spins interact locally around the impurity, the Kondo cloud can theoretically spread out over several micrometres. The cloud has not so far been detected, and so its physical existence—a fundamental aspect of the Kondo effect—remains controversial7,8. Here we present experimental evidence of a Kondo cloud extending over a length of micrometres, comparable to the theoretical length ξK. In our device, a Kondo impurity is formed in a quantum dot2,9–11, coupling on one side to a quasi-one-dimensional channel12 that houses a Fabry–Pérot interferometer of various gate-defined lengths L exceeding one micrometre. When we sweep a voltage on the interferometer end gate—separated by L from the quantum dot—to induce Fabry–Pérot oscillations in conductance we observe oscillations in the measured Kondo temperature TK, which is a signature of the Kondo cloud at distance L. When L is less than ξK the TK oscillation amplitude becomes larger as L becomes smaller, obeying a scaling function of a single parameter L/ξK, whereas when L is greater than ξK the oscillation is much weaker. Our results reveal that ξK is the only length parameter associated with the Kondo effect, and that the cloud lies mostly within a length of ξK. Our experimental method offers a way of detecting the spatial distribution of exotic non-Fermi liquids formed by multiple magnetic impurities or multiple screening channels13–16 and of studying spin-correlated systems. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

The role of coherency loss on rafting of superalloys under high temperature low stress creep conditions is investigated by phase-field crystal plasticity simulations. It is demonstrated that coalescence, critically depending on the state of coherency between precipitate and matrix is crucial to understand the rafting behavior of superalloys. An explicit mechanisms is developed predicting coherency loss based on the plastic activity in the matrix. The simulations are verified using experimental creep test results. © 2019 Elsevier B.V.

Laser additive manufacturing is attractive for the production of complex, three-dimensional parts from metallic powder using a computer-aided design model1–3. The approach enables the digital control of the processing parameters and thus the resulting alloy’s microstructure, for example, by using high cooling rates and cyclic re-heating4–10. We recently showed that this cyclic re-heating, the so-called intrinsic heat treatment, can trigger nickel-aluminium precipitation in an iron–nickel–aluminium alloy in situ during laser additive manufacturing9. Here we report a Fe19Ni5Ti (weight per cent) steel tailor-designed for laser additive manufacturing. This steel is hardened in situ by nickel-titanium nanoprecipitation, and martensite is also formed in situ, starting at a readily accessible temperature of 200 degrees Celsius. Local control of both the nanoprecipitation and the martensitic transformation during the fabrication leads to complex microstructure hierarchies across multiple length scales, from approximately 100-micrometre-thick layers down to nanoscale precipitates. Inspired by ancient Damascus steels11–14—which have hard and soft layers, originally introduced via the folding and forging techniques of skilled blacksmiths—we produced a material consisting of alternating soft and hard layers. Our material has a tensile strength of 1,300 megapascals and 10 per cent elongation, showing superior mechanical properties to those of ancient Damascus steel12. The principles of in situ precipitation strengthening and local microstructure control used here can be applied to a wide range of precipitation-hardened alloys and different additive manufacturing processes. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

CoFe2O4 nanoparticles (NPs) were synthesized by using a colloidal one-pot synthesis method based on the decomposition of metal acetylacetonates in the presence of oleyl amine. The characterization by X-ray diffraction, transmission electron microscopy and N2 physisorption revealed non-porous spinel phase CoFe2O4 NPs with an average particle size of 4 nm. The unsupported metal oxide NPs were applied in the selective oxidation of 2-propanol in a continuously operated fixed-bed reactor under quasi steady-state conditions using a heating rate of 0.5 k min−1. 2-Propanol was found to be oxidatively dehydrogenated over CoFe2O4 yielding acetone and H2O with high selectivity. Only to a minor extent dehydration to propene and total oxidation to CO2 was observed at higher temperatures. The detected low-temperature reaction pathway with maxima at 430 and 510 K was inhibited after the initial 2-propanol oxidation up to 573 K, but an oxidative treatment in O2 or N2O atmosphere led to full regeneration. No correlation between the desorbing amount or the surface oxygen species investigated by O2 temperature-programmed desorption experiments and the low-temperature activity was observed. The amounts of evolving CO2 during the TPO experiments indicate deactivation due to formation of carbonaceous species. Inhibition experiments with pre-adsorbed reaction intermediates and infrared spectroscopy identified acetate species as reversible poison, whereas carbonates are rather spectators. In addition, carbon deposition was detected by X-ray photoelectron spectroscopy, which also revealed a minor influence of cobalt reduction during the deactivation process as confirmed by X-ray absorption spectroscopy studies. © 2019 Elsevier Inc.

Quantum dots are both excellent single-photon sources and hosts for single spins. This combination enables the deterministic generation of Raman-photons—bandwidth-matched to an atomic quantum-memory—and the generation of photon cluster states, a resource in quantum communication and measurement-based quantum computing. GaAs quantum dots in AlGaAs can be matched in frequency to a rubidium-based photon memory, and have potentially improved electron spin coherence compared to the widely used InGaAs quantum dots. However, their charge stability and optical linewidths are typically much worse than for their InGaAs counterparts. Here, we embed GaAs quantum dots into an n-i-p-diode specially designed for low-temperature operation. We demonstrate ultra-low noise behaviour: charge control via Coulomb blockade, close-to lifetime-limited linewidths, and no blinking. We observe high-fidelity optical electron-spin initialisation and long electron-spin lifetimes for these quantum dots. Our work establishes a materials platform for low-noise quantum photonics close to the red part of the spectrum. © 2020, The Author(s).

Several applications of perovskite solar cells (PSCs) demand a semitransparent top electrode to afford top-illumination or see-through devices. Transparent conductive oxides, such as indium tin oxide (ITO), typically require postdeposition annealing at elevated temperatures, which would thermally decompose the perovskite. In contrast, silver nanowires (AgNWs) in dispersions of water would be a very attractive alternative that can be deposited at ambient conditions. Water is environmentally friendly without safety concerns associated with alcohols, such as flammability. Due to the notorious moisture sensitivity of lead-halide perovskites, aqueous processing of functional layers, such as electrodes, on top of a perovskite device stack is elusive. Here, impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of the perovskite cell without damage. The polyvinylpyrrolidone (PVP) capping agent of the AgNWs is found to cause a work–function shift and an energy barrier between the AgNWs and the adjacent ETL. Thus, a high carrier density (≈1018 cm−3) in the ETL is required to achieve well-behaved J/V characteristics free of s-shapes. Ultimately, semitransparent PSCs are demonstrated that provide an efficiency of 17.4%, which is the highest efficiency of semitransparent p-i-n perovskite solar cells with an AgNW top electrode. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The high demands on wear resistant tools have led to the development of wear resistant claddings on a substrate, which can be a low alloyed steel with higher ductility than the cladding to improve the resistance of the tool against fracture. In this study, the post heat treatment of sinter-cladded X245VCrMo9-4 steel coating on X120Mn12 steel substrate was investigated, as it is expected that the substrate remained austenitic while the coating possessed a tough martensitic matrix with uniform dispersion of carbide precipitates. Samples were prepared by sintering at 1250 °C in a vacuum furnace under a nitrogen atmosphere at 80 kPa and a heating rate of 10 K/min, and was allowed to cool in the furnace after a dwell of 30 min at sintering temperature. These samples were subjected to heat treatment by austenitisation, oil quenching and tempering. The effect of heat treatment procedures deployed on the samples was examined using optical microscopy, scanning electron microscopy, X-ray diffraction and hardness. Experimental results were supported by computational thermodynamic calculations. The results indicated that the optimised heat treatment, through which the hardness of the steel coating is significantly enhanced while the substrate microstructure remained austenitic, is by austenitising at 950 °C, quenching and low temperature tempering at 150 °C. Quenching temperature was significant to the hardness of the steel coating, as quenching from higher temperature led to a lower hardness of the matrix when compared to quenching at lower austenitisation temperature owing to a high fraction of retained austenite. © 2020 Carl Hanser Verlag. All rights reserved.

Double layer thermal barrier coatings (TBCs) consisting of a Gd2Zr2O7 (GZO) top and an ytrria stabilized zirconia (YSZ) interlayer have been tested in a burner rig facility and the results compared to the ones of conventional YSZ single layers. In order to gain insight in the high temperature capability of the alternative TBC material, high surface temperatures of up to 1550 °C have been chosen while keeping the bond coat temperature similar. It turned out that the performance of all systems is largely depending on the microstructure of the coatings especially reduced porosity levels of GZO being detrimental. In addition, it was more difficult in GZO than in YSZ coatings to obtain highly porous and still properly bonded microstructures. Another finding was the reduced lifetime with increasing surface temperatures, the amount of reduction is depending on the investigated system. The reasons for this behavior are analyzed and discussed in detail. © 2019 Elsevier Ltd

Aiming to devise suitable materials for sliding wear protection at high temperature, aluminium oxide-dispersion strengthened (ODS) CoNiCrAlY coatings were manufactured by vacuum plasma spraying (VPS). Feedstock materials were ball-milled powders with 2, 10 and 30 wt% Al2O3 content. The ball-on-disc sliding wear behaviour of the coatings was tested at 750 °C against an Al2O3 counterpart, and compared to a pure CoNiCrAlY coating (obtained from a commercial feedstock not subjected to ball milling) and to an uncoated Ni-base superalloy. Sliding wear rates decrease from the uncoated superalloy (≈3 × 10−5 mm3/(N·m)) to the pure CoNiCrAlY coating (≈2 × 10−5 mm3/(N·m)) and to the ODS ones, with the notable exception of the 10 wt% Al2O3-containing sample. Analyses of worn samples indicate that pure CoNiCrAlY is subject to severe adhesive wear, mitigated by the formation of a thick (>1 μm) “glaze” layer via compaction and (probable) sintering of tribo-oxidized debris particles. Addition of Al2O3 particles to the CoNiCrAlY matrix can either enhance or worsen the “glaze” stability. Specifically, a coating strengthened with 30 wt% Al2O3 provides an especially good mechanical support to the “glaze”. This produces beneficial effects resulting in a particularly low wear rate of ≈3 × 10−6 mm3/(N·m). © 2019 Elsevier B.V.

Attempted preparation of a chelated CoII β-silylamide resulted in the unprecedented disproportionation to Co0 and a spirocyclic cobalt(IV) bis(β-silyldiamide): [Co[(NtBu)2SiMe2]2] (1). Compound 1 exhibited a room-temperature magnetic moment of 1.8 B.M. and a solid-state axial EPR spectrum diagnostic of a rare S=1/2 configuration for tetrahedral CoIV. Ab initio semicanonical coupled-cluster calculations (DLPNO-CCSD(T)) revealed the doublet state was clearly preferred (−27 kcal mol−1) over higher spin configurations only for the bulky tert-butyl-substituted analogue. Unlike other CoIV complexes, 1 had remarkable thermal stability, and was demonstrated to form a stable self-limiting monolayer in preliminary atomic layer deposition (ALD) surface saturation experiments. The ease of synthesis and high stability make 1 an attractive starting point to investigate otherwise inaccessible CoIV intermediates and for synthesizing new materials. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

The interfacial fracture toughness of sintered hybrid silver nanoparticles (AgNPs) on both Au and Cu substrates is studied as a function of sintering temperature. Interfacial microstructure and porosity evolution of Au/AgNPs and Cu/AgNPs are observed to impact the fracture toughness. An Au–Ag interfacial diffusion layer is resolved at the interface of Au/AgNPs interconnects, while an oxide layer is found at the interface of Cu/AgNPs interconnects. Both porosity and pore sizes of the sintered silver interconnects are analyzed across the micro- and macro-length scales and related to the interfacial fracture toughness. The experimental observations can be theoretically described, which permits to predict the fracture toughness of the sintered silver interconnects. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.

The compressive creep of silicon carbide fiber reinforced Ti3SiC2 MAX phase with both fine and coarse microstructure was investigated in the temperature range of 1000-1300°C. Comparison of only steady-state creep was done to understand the response of fabricated composite materials toward creep deformation. It was demonstrated that the fibers are more effective in reducing the creep rates for the coarse microstructure by an increase in activation energy compared to the variant with a finer microstructure, being partly a result of the enhanced creep rates for the microstructure with larger grain size. Grain boundary sliding along with fiber fracture appears to be the main creep mechanism for most of the tested temperature range. However, there are indications for a changed creep mechanism for the fine microstructure for the lowest testing temperature. Local pores are formed to accommodate differences in strain related to creeping matrix and predominantly elastically deformed fibers during creep. Microstructural analysis was done on the material before and after creep to understand the deformation mechanics. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)

We investigated the flow behavior of γ/γʹ-strengthened Co-12Ti and Co-12Ti-4Mo (at.%) alloys at room and elevated temperatures (up to 900°C) by electron microscopy and density functional theory. The Mo-added alloy exhibited an enhanced compressive yield strength and strain hardening behavior as compared to the reference binary alloy. This behavior could be attributed to a ~25% larger γʹ volume fraction and ~7% higher planar fault energies in Co-12Ti-4Mo. Using electron channeling contrast imaging, we observed interrupted slip bands in the Co-12Ti-4Mo alloy deformed to a strain of 6%, which led to enhanced strain hardening, in contrast to extended slip bands along {111} planes in the Co-12Ti alloy. Interrupted slip band formation in Co-12Ti-4Mo could be explained by rapid exhaustion of dislocation sources and a higher energy barrier required to cut the γʹ precipitates. These effects are due to a reduced γ channel width and substantial hardening effect of γʹ-Co3(Ti,Mo) in the ternary alloy as well as due to the large shear modulus difference between γʹ and γ. © 2020

The fabrication of SnOx in thin film form via chemical solution deposition (CSD) processes is favored over vacuum based techniques as it is cost effective and simpler. The precursor employed plays a central role in defining the process conditions for CSD. Particularly for processing SnO2 layers that are appealing for sensor or electronic applications, there are limited precursors available for CSD. Thus the focus of this work was to develop metalorganic precursors for tin, based on the ketoiminate ligand class. By systematic molecular engineering of the ligand periphery, a series of new homoleptic Sn(ii) β-ketoiminate complexes was synthesized, namely bis[4-(2-methoxyethylimino)-3-pentanonato] tin, [Sn(MEKI)2] (1), bis[4-(2-ethoxyethylimino)-2-pentanonato] tin, [Sn(EEKI)2] (2), bis[4-(3-methoxypropylimino)-2-pentanonato] tin, [Sn(MPKI)2] (3), bis[4-(3-ethoxypropylimino)-2-pentanonato] tin, [Sn(EPKI)2] (4) and bis[4-(3-isopropoxypropylimino)-2-pentanonato] tin, [Sn(iPPKI)2] (5). All these N-side-chain ether functionalized compounds were analyzed by nuclear magnetic resonance (NMR) spectroscopy, electron impact mass spectrometry (EI-MS), elemental analysis (EA) and thermogravimetric analysis (TGA). The solid state molecular structure of [Sn(MPKI)2] (3) was eludicated by means of single crystal X-ray diffraction (SCXRD). Interestingly, this class of compounds features excellent solubility and stability in common organic solvents alongside good reactivity towards H2O and low decomposition temperatures, thus fulfilling the desired requirements for CSD of tin oxides. With compound 3 as a representative example, we have demonstrated the possibility to directly deposit SnOx layers via hydrolysis upon exposure to air followed by heat treatment under oxygen at moderate temperatures and most importantly without the need for any additive that is generally used in CSD. A range of complementary analytical methods were employed, namely X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to analyse the structure, morphology and composition of the SnOx layers. This journal is © The Royal Society of Chemistry.

According to experimental observations, the temperature dependence of self-diffusion coefficient in most body-centered cubic metals (bcc) exhibits non-Arrhenius behavior. The origin of this behavior is likely related to anharmonic vibrational effects at elevated temperatures. However, it is still debated whether anharmonicity affects more the formation or migration of monovacancies, which are known to govern the self-diffusion. In this extensive atomistic simulation study we investigated thermodynamic properties of monovacancies in bcc molybdenum, here taken as a representative model system, from zero temperature to the melting point. We combined first-principles calculations and classical simulations based on three widely used interatomic potentials for Mo. In our analysis we employ static and dynamic atomistic calculations as well as statistical sampling techniques and thermodynamic integration to achieve thorough information about temperature variations of vacancy formation and migration free energies and diffusivities. In addition, we carry out large-scale molecular dynamics simulations that enable direct observation of high-temperature self-diffusion at the atomic scale. By scrutinizing the results obtained by different models and methods, we conclude that the peculiar self-diffusion behavior is likely caused by strong temperature dependence of the vacancy formation energy. © 2020 American Physical Society.

A possible application of the dispersed optical microresonator based sensor in aerospace-related tasks is discussed. Configuration of the proposed sensor as a set of spherical glass microparticles together with its working principle is discussed. Sensing capabilities have been verified on examples of detection of temperature variations together with monitoring of the small microparticles adsorption. Thermal sensitivity is determined on the level of 8.6 pm/°C. The possibility for detection of the adsorption of polesterene microparticles (0.9 and 3.27 µ m size) in frequency sweeping and spectrally unresolved illumination schemes have been shown. © 2020 IEEE.

In situ high-temperature powder X-ray diffraction analysis (HT-XRD) was carried out in the temperature range from 25°C-1430°C to investigate the crystal structure of double perovskites, La2(Al1/2MgTa1/2)O6 (LAMT) and its phase transitions. This complex perovskite is a promising candidate for application in thermal barrier coating systems. Rietveld analysis shows a rock-salt type ordering of the B-site cations in the monoclinic space group symmetry, P21/n at room temperature. Upon heating, a structural phase transition occurs at ~855°C, and the crystal structure becomes rhombohedral with the space group symmetry (Formula presented.). On further heating, LAMT transforms to the ideal cubic phase at ~1390°C with the space group symmetry (Formula presented.). Both of the structural phase transitions are completely reversible, and were confirmed through complementary differential scanning calorimetry and thermogravimetry measurements. With increasing temperature, the degree of the octahedral tilting decreases and the variance of the different B–O bond lengths is reduced, until in the cubic phase, no tilting is present, and almost equal B–O bond lengths are obtained. © 2019 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals, Inc. on behalf of American Ceramic Society (ACERS)

CrCoNi-based high-entropy alloys have demonstrated outstanding mechanical properties, particularly at cryogenic temperatures. Here we investigate the fatigue-crack propagation properties of the equiatomic, single-phase, face-centered cubic, medium-entropy alloy (MEA), CrCoNi, that displays exceptional strength, ductility and toughness, all of which are enhanced at cryogenic temperatures. Fatigue-crack growth is examined, at a load ratio of 0.1 over a wide range of growth rates, from ~10−11 to >10−7 m/cycle, at room (293 K) and cryogenic (198 K, 77 K) temperatures for two grain sizes (~7 and 68 µm), with emphasis on near-threshold behavior. We find that the ΔKth fatigue thresholds are increased with decreasing temperature and increasing grain size: from 5.7 MPa√m at 293 K to 8 MPa√m at 77 K in the fine-grained alloy, and from 9.4 MPa√m at 293 K to 13.7 MPa√m at 77 K in the coarse-grained alloy. Mechanistically, transgranular cracking at 293 K transitions to a mixture of intergranular and transgranular at cryogenic temperatures, where the increased propensity of nano-twins appears to inhibit growth rates by deflecting the crack path. However, the main factor affecting near-threshold behavior is roughness-induced crack closure from interference between the crack flanks, which is enhanced by the rougher fracture surfaces at low temperatures, particularly in the coarser-grained microstructure. Fatigue-crack propagation behavior in CrCoNi is comparable to nickel-based superalloys but is superior to that of the high-entropy CrMnFeCoNi (Cantor) alloy and many high-strength steels, making the CrCoNi alloy an excellent candidate material for safety-critical applications, particularly involving low temperatures. © 2020

In recent years, metal-rich sulfides of the pentlandite type (M9S8) have attracted considerable attention for energy storage applications. However, common synthetic routes towards pentlandites either involve energy intensive high temperature procedures or solvothermal methods with specialized precursors and non-sustainable organic solvents. Herein, we demonstrate that ball milling is a simple and efficient method to synthesize nanosized bimetallic pentlandite particles (Fe4.5Ni4.5S8, Pn) with an average size of ca. 250 nm in a single synthetic step from elemental- or sulfidic mixtures. We herein highlight the effects of the milling ball quantity, precursor types and milling time on the product quality. Along this line, Raman spectroscopy as well as temperature/pressure monitoring during the milling processes provide valuable insights into mechanistic differences between the mechanochemical Pn-formation. By employing the obtained Pn-nanosized particles as cathodic electrocatalysts for water splitting in a zero-gap PEM electrolyzer we provide a comprehensive path for a potential sustainable future process involving non-noble metal catalysts. © 2020 The Royal Society of Chemistry.

Single crystal thermal expansion and elastic stiffness of NdGaO3 and NdScO3 were investigated by inductive gauge dilatometry and resonant ultrasound spectroscopy between 103K and 1673K, as they are used extensively as perovskite-type substrates for epitaxial crystal growth. Thermal expansion of NdGaO3 is in agreement with literature data and has very similar magnitude and anisotropy compared to NdScO3. The anisotropy of the elastic stiffness of NdGaO3 is more pronounced and qualitatively different from what is found for NdScO3. It is explained in terms of structural instabilities, which lead to known phase transitions in other perovskites. The anisotropy of the elastic stiffness of NdGaO3 is compatible with what is found for other orthorhombic perovskites that undergo a transition to a rhombohedral structure at high temperatures. The elastic properties of NdScO3 directly follow from the properties of other REScO3. The samples were characterized with regards to their compositions and lattice parameters using electron probe microanalysis and X-ray powder diffraction. © 2020 Elsevier B.V.

A major technical challenge for modern aero engines is the development of designs which reduce noise and emission whilst increasing aerodynamic efficiency and ensuring aeroelastic stability of low-temperature engine components such as fans and low-pressure compressors. Composites are used in aviation due to their excellent stiffness and strength properties, which also enable additional flexibility in the design process. The weight reduction of the turbomachine components, due to composite materials and lighter engines, is especially relevant for the design and developments of hybrid-electric or distributed propulsion systems [1]. To accomplish this, a representative volume element (RVE) of a glass-fiber reinforced polymer is created, describing the geometrical arrangement of the textile reinforcement structure within the polymer matrix. For both phases, realistic linear elastic properties are assumed. This RVE will be investigated with the finite element method under various loading conditions to assess its anisotropic elastic properties and also its damping behaviour for elastic waves. To study the influence of delamination on the mechanical properties, small defects will be introduced into the model at the interface between reinforcement and matrix. Based on this micromechanical approach, a constitutive model for the composite will be formulated that describes the anisotropic properties as well as the damping behaviour. This constitutive model is then used to describe the material response in a macro-mechanical model, which serves as the basis for an aeroelastic analysis of a 1/3-scaled high-speed fan using a conventional (Ti-6Al -4V) and fiber composite material. Copyright © 2020 ASME

In order to identify possible optimizations regarding the electrical energy efficiency of an aluminium electrolysis cell, the impact of service temperature on microstructure and electrical properties of the cell cathode was investigated. The investigations include experiments regarding the chemical composition, especially the content of carbon, the electrical conductivity and the microstructure at selected positions. Thermodynamic calculations were used to estimate local service temperatures and explain phase transformations and formations. It was found that due to the increased service temperature diffusion processes of carbon took place to a particular extent between cast iron and collector bar. As a result, the carbon content in the collector bar changed from 0.06 to 1.05–1.4 wt%, while in the cast iron a reduction from 3.47 to < 1.50 wt% took place. These processes led to isothermal phase transformations and formations, that changed the matrix of the collector bar from austenitic with low content of ferrite to an austenitic matrix accompanied by precipitation of secondary, predominantly allotriomorphic cementite at service temperature. It was then shown that this has a negative effect on collector bar and decreases the electrical conductivity by up to 26 %. It was also discovered that graphite spheroidization within the grey cast iron has a positive effect on its electrical conductivity, which has increased by 52 %. The results provide the basis to gain an understanding of the carbon diffusion related processes within the cathode of an electrolysis cell and reveal further potential to increase the energy efficiency of primary aluminium production. © 2020, The Author(s).

Nanosecond plasmas in liquids are an important method to trigger the water chemistry for electrolysis or for biomedical applications in plasma medicine. The understanding of these chemical processes relies on knowing the variation of the temperatures in these dynamic plasmas. This is analyzed by monitoring nanosecond pulsed plasmas that are generated by high voltages at 20 kV and pulse lengths of 15 ns applied to a tungsten tip with 50 μm diameter immersed in water. Plasma emission is analyzed by optical emission spectroscopy ranging from UV wavelengths of 250 nm to visible wavelengths of 850 nm at a high temporal resolution of 2 ns. The spectra are dominated by the black body continuum from the hot tungsten surface and line emissions from the hydrogen Balmer series. Typical temperatures from 6000 K up to 8000 K are reached for the tungsten surface corresponding to the boiling temperature of tungsten at varying tungsten vapor pressures. The analysis of the ignition process and the concurrent spectral features indicate that the plasma is initiated by field ionization of water molecules at the electrode surface. At the end of the pulse, field emission of electrons can occur. During the plasma pulse, it is postulated that the plasma contracts locally at the electrode surface forming a hot spot. This causes a characteristic contribution to the continuum emission at small wavelengths. © 2020 The Author(s). Published by IOP Publishing Ltd.

In recent years, quantum-mechanically guided materials design has been used to identify candidate hard magnetic materials with a reduced content of rare earth elements. The focus of these studies was on optimal magnetic properties. In the present work we address the issue of thermodynamic stability of such materials. As prototype system we consider CeFe11Ti and focus on the impact of magnetism on the free energy. To this end, we use the magnetic model suggested by Gerhard Inden as a reference. The performance of this model is compared to Monte Carlo simulations for the magnetic entropy contribution. We conclude that despite the empirical nature of the Inden model, it provides a surprisingly accurate description of the magnetic contribution. Based on this approach we are able to faithfully predict the critical temperature for the decomposition of CeFe11Ti into competing Laves phases. We further show that the Inden model can be improved if the reduction of the magnetic moment at finite temperatures is taken into account. This is demonstrated for the hard magnetic phase Nd2Fe14B. In addition, the impact of magnetism on the lattice vibrations of relevant phases in the Ce–Fe–Ti system is analyzed. © 2019 Elsevier Ltd

The realization of a highly efficient optical spot-size converter for the end-face coupling of single photons from GaAs-based nanophotonic waveguides with embedded quantum dots is reported. The converter is realized using an inverted taper and an epoxy polymer overlay providing a 1.3 µm output mode field diameter. The collection of single photons from a quantum dot into a lensed fiber with a rate of 5.84 ± 0.01 MHz is demonstrated and a chip-to-fiber coupling efficiency of ≈48% is estimated. The stability and compatibility with cryogenic temperatures make the epoxy waveguides a promising material to realize efficient and scalable interconnects between heterogeneous quantum photonic integrated circuits. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In the course of developing functionally graded tungsten/steel-layer systems as protective coatings for the first wall (FW) of future fusion reactors, an overview of the results attained so far is given. This includes the determined parameters for creating such systems by vacuum plasma spraying on a laboratory scale and the achieved material properties determined in previous works. To realize the coating of future full scale FWs as well, the coating process is adapted to larger coating areas in the form of mock-ups. For such components, special attention needs to be paid to the challenges of the limited temperature window during coating to achieve good coating adhesion, whilst avoiding exceeding the tempering temperature of the steel. One successfully coated mock-up is also exposed to fusion-relevant heat loads in HELOKA (Helium Loop Karlsruhe) to evaluate the coating system behavior and verify its durability. Finally, for even larger components the coating design and process are further optimized, supported by finite element simulations. © 2020 EURATOM.

The self-organised plasma patterns, known as spokes or ionisation zones in magnetron sputtering discharges, were observed in a wide range of power densities, from low power direct current magnetron sputtering (dcMS) discharge to high power impulse magnetron sputtering (HiPIMS) discharge. For some target materials and non-reactive gases, it was observed that at very high power densities (>3 kW cm-2) the plasma exhibits a transition from a regime where spokes are observed to a homogeneous plasma regime. In this contribution, we present a comparison of plasma properties: plasma emission (optical emission spectroscopy) and flux of argon and chromium ions (mass spectrometry), measured both in the spoke regime and in the homogeneous plasma regime, aimed to expand the understanding of the plasma transition between the two modes. A simple biased flat probe was used to distinguish between the spoke regime and the homogeneous plasma regime. It was found that the flux of multiply charged ions (Ar2+, Cr2+, Cr3+, Cr4+) increases abruptly at the transition between the spoke regime and the homogeneous plasma regime. Similarly, the emission from Cr+ ions exhibits a strong increase of about 50% when the plasma torus becomes homogeneous. These observations are interpreted as an increase in electron temperature and a change in the electron heating mode, from a combination of secondary electron heating and Ohmic heating towards pure Ohmic heating. The transition to the homogeneous plasma regime and pure Ohmic heating is only observed in non-reactive HiPIMS discharges for target atoms with the second ionisation potential higher than the first ionisation potential of Ar (15.76 eV), and a self-sputter yield larger than 1. © 2020 The Author(s). Published by IOP Publishing Ltd.

The compressive creep of a SiC whisker (SiCw) reinforced Ti3SiC2 MAX phase-based ceramic matrix composites (CMCs) was studied in the temperature range 1100-1300°C in air for a stress range 20-120 MPa. Ti3SiC2 containing 0, 10, and 20 vol% of SiCw was sintered by spark plasma sintering (SPS) for subsequent creep tests. The creep rate of Ti3SiC2 decreased by around two orders of magnitude with every additional 10 vol% of SiCw. The main creep mechanisms of monolithic Ti3SiC2 and the 10% CMCs appeared to be the same, whereas for the 20% material, a different mechanism is indicated by changes in stress exponents. The creep rates of 20% composites tend to converge to that of 10% at higher stress. Viscoplastic and viscoelastic creep is believed to be the deformation mechanism for the CMCs, whereas monolithic Ti3SiC2 might have undergone only dislocation-based deformation. The rate controlling creep is believed to be dislocation based for all the materials which is also supported by similar activation energies in the range 650-700 kJ/mol. © 2020 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society (ACERS)

VO2-based thin-film libraries with a continuous composition spread of Cr were obtained by reactive cosputtering. Gradual changes in the crystalline structures of VO2 were observed in the thin-film libraries at room temperature as the M1 phase exists for Cr < 1.2 at. %, the M2 phase for Cr > 4.2 at. %, and the T phase in between. Although X-ray diffraction indicates that only VO2 phases exist in the library, X-ray photoelectron spectroscopy reveals an increased V5+/V4+ ratio with increasing Cr content along the V-Cr-O library. A V-Cr-O phase diagram was assessed based on the results of temperature-dependent X-ray diffraction of the libraries. Microstructures of the V-Cr-O libraries were studied by scanning electron microscopy and atomic force microscopy. High-throughput temperature-dependent electrical resistance [R(T)] and stress [σ(T)] measurements were performed on the V-Cr-O libraries to systematically study the influence of Cr on the transformation properties. The transformation temperature Tc was increased by 4.9 K/at. % in the composition range 2.8 at. % < Cr < 7.3 at. % and by 1.2 K/at. % for Cr > 7.3 at. %. The resistance change across the phase transformation was decreased from 3 to 1 order of magnitude with Cr content increasing from 1.1 at. % up to 12.6 at. %, and the R(T) curves became less abrupt. The addition of Cr increased the stress change across the phase transformation up to 1.3 GPa for a Cr content of 3.3 at. %. However, for increased Cr contents from 3.3 to 9 at. %, the stress change decreased to 380 MPa. This could be because of the increased fraction of an O-rich VOx phase in the films and a changed crystallographic orientation for Cr-rich V-Cr-O. Copyright © 2020 American Chemical Society.

We investigate and compare the intensity and polarization dynamics in a vertical-cavity surface-emitting laser (VCSEL) with a monolithically integrated, electrically controlled birefringence tuning mechanism. The influence of the bias current on the polarization dynamics is investigated over a large range of birefringence values. Bias current tuning toward low values and simultaneous maximization of the resonance frequency is an important strategy to optimize the spin-VCSEL toward energy-efficient operation. A polarization dynamics resonance tuning range from a few GHz up to the maximum frequency of 36 GHz was achieved, and polarization dynamics at maximum frequency are demonstrated at minimum bias current and at high temperatures of approximately 70 °C. We propose a strategy for data communication with low energy consumption and low cooling effort. © 2020 Author(s).

Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic “signature” in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic – ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the “signature” thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars. © 2020

A new medium-entropy superalloy was produced based on the compositions of equiatomic CrCoNi and Ni-base superalloy Inconel 740H. Initial alloy design was performed using Thermo-Calc. The aging response and microstructural stability were assessed following heat treatment at temperatures between 600 and 900 °C and durations up to 100 h. Aging from a fully recrystallized state resulted in negligible grain growth and produced γ’ and σ phases. The same phases were present after aging from a cold-rolled state, but partially recrystallized microstructures resulted in multi-modal size distributions and heterogeneous spatial arrangements. Room temperature hardness measurements were used to correlate aging conditions with quantitative precipitate measurements and mechanical properties. © 2019 Elsevier B.V.

In the present work, three Ni-based single-crystal superalloys (SXs) were investigated, a Re-containing alloy ERBO/1 (CMSX-4 type) and two Re-free SXs referred to as ERBO/15 and ERBO/15-W, which differ in W content. The microstructural evolution of the three alloys during heat treatment and their creep behavior is investigated. When one applies one heat treatment to all three alloys, one obtains different γ/γ′-microstructures. Subjecting these three alloys to creep in the high-temperature low-stress creep regime, ERBO/15 outperforms ERBO/1. In order to separate the effects of alloy chemistry and microstructure, the kinetics of the microstructural evolution of the three alloys was measured. The results were used to establish similar microstructures in all three alloys. Comparing ERBO/15 with ERBO/15-W, it was found that in ERBO/15-W particles grow faster during the first precipitation heat treatment and that ERBO/15-W creeps significantly faster. At constant microstructures, ERBO/15 and ERBO/1 show similar creep behavior. In the high-temperature and low-stress creep regime, ERBO/15 shows lower minimum creep rates but ERBO/1 features a slower increase of creep rate in the tertiary creep regime. It was also found that in the high-temperature low-stress creep regime, ERBO/1 shows a double minimum creep behavior when particles are small. © 2020, The Minerals, Metals & Materials Society.

In order for biomass drying processes to be efficient, it is crucial to achieve the target residual water content within a close margin, since more conservative drying would result in a waste of energy. A method for a reliable estimation of the water content is therefore of obvious importance. Ideally, such a method does not require any expensive sensors. We show reduced order models and extended Kalman filters can be combined to reliably determine the water content and temperature of wood particles based on only surface temperature measurements. The proposed observer works reliably if measurements are only available for parts of a particle face. It can therefore still be applied if particle surfaces are partially obstructed, which is a prerequisite for use in industrial processes and units, such as rotary dryers. The extended Kalman filter uses a reduced order model that is obtained by applying proper orthogonal decomposition and Galerkin projection to coupled PDEs that model heat conduction and water diffusion in anisotropic particles. In contrast to the original PDE simulation model, the reduced model and the filter based on it are suitable for real time computations and monitoring. © 2020 The Authors

Nickel Titanium (NiTi) alloys have been used for many years based on their unique ability to exhibit the shape-memory and pseudoelastic effects. The indentation-induced two-way shape memory effect (TWSME) is a specific sub-capability of this alloy such that a repeatably switchable surface can be created by “training” the material through mechanical indentation and activated through temperature transitions between the austenitic and martensitic phases. This study sought to observe the effect Ti3Ni4 precipitate aging would have on the indentation-induced TWSME. Ti3Ni4 has previously been shown as an effective method to alter NiTi transformation temperatures, yet it was unclear what effect localized stress fields around precipitates would have on the TWSME. The results presented here indicate that growth of precipitates in the alloy before training suppresses the resultant indentation-induced TWSME, and small precipitates, which cause minimal lattice mismatch to the matrix (i.e. highest coherency), have the strongest role in suppressing the effect. It is suggest that lattice coherency acts to inhibit plastic deformation, suppressing the creation of the preferred microstructure under the indent required to guide the TWSME. Therefore, precipitate aging is not a recommended alternative to precise alloying in order to alter transformation temperatures with the goal of maximizing the indentation-induced TWSME effect within a targeted temperate transformation regime. © 2020 Elsevier B.V.

We investigate superstructures formed by CO2 on Ag(100) and Cu(111) from small clusters forming at 21 K up to multilayers grown at 43 K by low temperature scanning tunneling microscopy. On both surfaces, CO2 nucleates only at defects, here at co-adsorbed CO. At the lower adsorption temperature, superstructures of different symmetry coexist on both surfaces at submonolayer coverage, while the superstructures formed at the higher adsorption temperature differ largely for the two surfaces. On Ag(100), the CO2 monolayer exhibits a long-range order interrupted by antiphase domain boundaries. On Cu(111), a random distribution of domain structures of different symmetry leads to a monolayer without long-range order. Surprisingly, the degree of ordering is inverted for the 2nd layer of CO2. On Ag(100), the coexistence of different superstructures in the 2nd layer leads to reduced long-range order. On Cu(111), a hexagonal 2nd layer exhibits long-range order. A layer of a similar superstructure, hexagonal with long-range order, exists as the 3rd layer of Ag(100). Despite the different substrates, a multitude of common structural features of CO2 exist. Hexagonal layers grow with a long-range order on less ordered layers on both surfaces. Our results suggest that the preferred structure of a CO2 layer is hexagonal. © 2020 the Owner Societies.

Key mechanisms and elementary diffusion processes that control the growth kinetics of σ precipitates in high-entropy alloys were investigated in the present study. For this purpose, an off-equiatomic Cr26Mn20Fe20Co20Ni14 alloy with an initially single-phase FCC structure was subjected to isothermal heat treatments, which are known to promote the formation of σ phase, i.e., aging between 600 °C and 1000 °C for times ranging from 0.1 h to 1000 h. The growth kinetics of σ precipitates at grain boundaries of the FCC matrix and those located within the interior of the grains were analyzed separately. The latter precipitates are found to grow through direct substitutional diffusion of Cr-solutes towards and Mn, Fe, Co, and Ni away from them and the growth rate of the allotriomorphs can be rationalized by the collector plate mechanism of interfacial diffusion-aided growth. From the growth-kinetics data obtained in the present study, lattice interdiffusion coefficients as well as diffusivities along crystalline defects were obtained. Above 800 °C, the growth kinetics are dominated by lattice interdiffusion of Cr in the FCC matrix described by DL = 9.8 × 10-4 exp[(-300 kJ/mol)/(RT)] m2/s. At lower temperatures, the growth kinetics are enhanced by fast interdiffusion along dislocation pipes, which temperature dependence is given by DD = 5.0 × 10-3 exp[(-205 kJ/mol)/(RT)] m2/s. The Cr-diffusivity along σ/FCC interphase boundaries deduced from the thickening kinetics of grain boundary precipitates can be represented by the Arrhenius relationship DI = 0.5 × 10-4 exp[(-145 kJ/mol)/(RT)] m2/s, which is similar to that found for grain boundary interdiffusion in metals and alloys. © 2020 Acta Materialia Inc.

The martensitic start temperature (M s) is a technologically fundamental characteristic of high-temperature shape memory alloys. We have recently shown [Chakraborty et al 2016 Phys. Rev. B 94 224104] that the two key features in describing the composition dependence of M s are the T = 0 K phase stability and the difference in vibrational entropy which, within the Debye model, is directly linked to the elastic properties. Here, we use density functional theory together with special quasi-random structures to study the elastic properties of disordered martensite and austenite Ti-Ta alloys as a function of composition. We observe a softening in the tetragonal shear elastic constant of the austenite phase at low Ta content and a non-linear behavior in the shear elastic constant of the martensite. A minimum of 12.5% Ta is required to stabilize the austenite phase at T = 0 K. Further, the shear elastic constants and Young's modulus of martensite exhibit a maximum for Ta concentrations close to 30%. Phenomenological, elastic-constant-based criteria suggest that the addition of Ta enhances the strength, but reduces the ductile character of the alloys. In addition, the directional elastic stiffness, calculated for both martensite and austenite, becomes more isotropic with increasing Ta content. The reported trends in elastic properties as a function of composition may serve as a guide in the design of alloys with optimized properties in this interesting class of materials. © 2020 IOP Publishing Ltd.

Cr2AlC layers with thickness up to 100 µm were deposited by high-velocity-atmospheric plasma spray (HV-APS) on Inconel 738 substrates to analyze the potential of MAX phases as bond coat in thermal barrier coating systems (TBCs). The deposited Cr2AlC layers showed high purity with theoretical densities up to 93%, although some secondary phases were detected after the deposition process. On top of this MAX phase layer, a porous yttria-stabilized zirconia (YSZ) was deposited by atmospheric plasma spraying. The system was tested under realistic thermal loading conditions using a burner rig facility, achieving surface and substrate temperatures of 1400°C and 1050°C, respectively. The system failed after 745 cycles mainly for three reasons: (i) open porosity of the bond coat layer, (ii) oxidation of secondary phases, and (iii) inter-diffusion. Nevertheless, these results show a high potential of Cr2AlC and other Al-based MAX phases as bond coat material for high-temperature applications. Furthermore, future challenges to transfer MAX phases as eventual bond coat or protective layer are discussed. © 2019 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals, Inc. on behalf of American Ceramic Society (ACERS)

Electron attachment and solvation at ice structures are well-known phenomena. The energy liberated in such events is commonly understood to cause temporary changes at such ice structures, but it may also trigger permanent modifications to a yet unknown extent. We determine the impact of electron solvation on D2O structures adsorbed on Cu(111) with low-Temperature scanning tunneling microscopy, two-photon photoemission, and ab initio theory. Solvated electrons, generated by ultraviolet photons, lead not only to transient but also to permanent structural changes through the rearrangement of individual molecules. The persistent changes occur near sites with a high density of dangling OD groups that facilitate electron solvation. We conclude that energy dissipation during solvation triggers permanent molecular rearrangement via vibrational excitation. Copyright © 2020 American Chemical Society.

This work presents a novel method of obtaining in situ strain measurements at high temperature by simultaneous digital image correlation (DIC), which provides the total strain on the specimen surface, and synchrotron x-ray diffraction (XRD), which provides lattice strains of crystalline materials. DIC at high temperature requires specialized techniques to overcome the effects of increased blackbody radiation that would otherwise overexpose the images. The technique presented herein is unique in that it can be used with a sample enclosed in an infrared heater, remotely and simultaneously with synchrotron XRD measurements. The heater included a window for camera access, and the light of the heater lamps is used as illumination. High-temperature paint is used to apply a random speckle pattern to the sample to allow the tracking of displacements and the calculation of the DIC strains. An inexpensive blue theatrical gel filter is used to block interfering visible and infrared light at high temperatures. This technique successfully produces properly exposed images at 870 °C and is expected to perform similarly at higher temperatures. The average strains measured by DIC were validated by an analytical calculation of the theoretical strain. Simultaneous DIC and XRD strain measurements of Inconel 718 (IN718) tensile test specimens were performed under thermal and mechanical loads and evaluated. This approach uses the fact that with DIC, the total strain is measured, including plastic strain, while with XRD, only elastic strain is captured. The observed differences were discussed with respect to the effective deformation mechanisms. © 2020 Author(s).

In this study CFD simulations of an industrial scale rotary kiln for cement clinker production are conducted. A solid layer of agglomerated clinker material, which adheres to the kiln wall and forms a stable coating of variable thickness during kiln operation, is considered in the simulations of the furnace. During operation of the kiln the thickness of the coating layer is unknown, but the routinely measured temperature profile along the kiln shell is an indicator for the local layer thickness. Therefore, a first estimate of the initially unknown thickness of the coating layer is calculated by a one-dimensional heat transfer model, based on the temperature profile along the kiln shell, and introduced into the CFD simulations. As the process conditions within the furnace and, thus, the heat transfer to the kiln wall and the resulting coating thickness change over time, an adaptive modelling approach is required to describe the solid coating region in the CFD simulations. A method to represent the geometry of the coating layer as a blocked-off region for momentum is presented and extended for radiation. The results are compared to the conventional approach where the coating layer is represented by a highly resolved CFD mesh following the wall contour. Two generic temperature profiles of the kiln shell are assumed and used to simulate the corresponding coating profile in the furnace. The effect on the process conditions within the kiln are assessed and compared to a reference case without coating regions. Results show that the blocked-off approach employed is suitable to model the additional solid boundary profile in the furnace. The coating regions are found to have a significant impact on the process conditions within the kiln, especially if the coating layers are located towards the burner end of the kiln. © 2019 Elsevier Ltd

Since todays Internet traffic is more and more concentrated in hyperscale datacenters,1 new concepts for shortrange optical communication systems with high modulation bandwidth, high temperature stability, and low energy consumption are urgently needed. Birefringent spin-lasers, in particular spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs), are a novel type of ultrafast laser devices which promise to serve as ultrafast transmitters for the next generation of optical communication systems. While current-driven intensitymodulated VCSELs are state-of-the-art laser devices for short-range communication, their modulation bandwidth is limited to values below 40 GHz.2, 3 Recently, we were able to demonstrate that modulating carrier spin and light polarization in spin-VCSELs instead of carrier density and light intensity in conventional devices enables ultrafast polarization dynamics and a modulation bandwidth of more than 200 GHz.4 This high modulation bandwidth was achieved by increasing the resonance frequency of the coupled carrier spin-photon system by implementing high values of birefringence to the cavity of 850 nm GaAs/GaAlAs VCSELs. Here, we show experimental results for the intensity and polarization dynamics in highly birefringent spin-VCSELs as a function of bias current, birefringence, and temperature and demonstrate the capability of spin-VCSELs for ultralow energy consumption and high temperature stability. Furthermore, we present first results on polarization dynamics in 1.3 μm VCSELs for potential long-range communication systems and discuss novel concepts for future integrated and electrically pumped devices. © 2020 SPIE.

Low-temperature phase transitions in the EZ33A Mg-cast alloy have been investigated. Based on the structure assessment of the alloy after annealing at 150◦ C (1826 h) and at 200◦ C (2371 h) a grain boundary wetting transition by a second solid phase was documented. Within a 50◦ C temperature range, substantial differences in the α(Mg) grain boundary fraction wetted by the (Mg,Zn)12 RE intermetallic were observed. In contrast to what was reported in the literature, two different types of precipitates were found within α(Mg) grains. With increasing annealing temperatures, both types of precipitates dissolved. © 2020 by the authors.

To enable the development of novel Fe–C–B–Cr and Fe–C–B–Cr–Mo cold work tool steels, the microstructures and hardness-tempering behaviors of hypoeutectic laboratory melts are investigated. The results show that increasing Cr content enhances the thermodynamic stability of the ultrahard M2B borides. The formation of carboborides is suppressed by adjusting the B/(C + B) ratio, Cr content, and austenitization temperature. A secondary hardenability at 500 °C is achieved by Mo addition. In addition, Mo stabilizes the M23(C,B)6 phase and at higher contents the M3B2 boride. Based on these investigations, Fe0.4C1B–Cr alloys are designed which, inspired by the microstructure of the steel X153CrMoV12-1, feature a α′-Fe hardenable matrix but 15 vol% of eutectic M2B borides instead of M7C3 for wear protection. The Fe0.4C1B–Cr steels are produced by casting and hot rolling as well as powder metallurgy and hot isostatic pressing. The (tribo-) mechanical properties are investigated and compared with X153CrMoV12-1. Fracture toughness, bending strength, wear resistance, and hardness of the novel Fe0.4C1B–Cr alloys are found to be similar or superior to the steel X153CrMoV12-1, at decreased material cost. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Polycrystalline MoS2 from (NH4)2MoS4 thermolysis was activated in dilute H2 at 523 K < TR < 873 K and studied by XRD, total scattering analysis, XPS, HRTEM, and chemisorption to explain, why coordinative unsaturation decreases with growing TR contrary to expectations from the MoS2 structure. Hydrogenation rates were measured for identifying active sites. With increasing TR, activity and chemisorption peaked at different TR,peak. Below TR,peak, increasing activity was not paralleled by changes in MoS2 microstructure. Decreasing chemisorption above TR,peak was assigned to saturation of vacancies by sulfide from internal defects and to inclusion of vacancies in the interior of aggregates. Upon high-temperature reduction, stacks grew anisotropically (basal planes extended), retaining defects like bending, turbostratic disorder. Preferential exposure of stack bases in aggregate surfaces resulted in enhanced decrease of chemisorption. Correlations between activity, edge area and (b)rim length estimated from a morphological model localized active sites in the (b)rim region. © 2020 Elsevier B.V.

We mapped the metal sequences within crystals of metal-oxide rods in multivariate metal-organic framework-74 containing mixed combinations of cobalt (Co), cadmium (Cd), lead (Pb), and manganese (Mn). Atom probe tomography of these crystals revealed the presence of heterogeneous spatial sequences of metal ions that we describe, depending on the metal and synthesis temperature used, as random (Co, Cd, 120°C), short duplicates (Co, Cd, 85°C), long duplicates (Co, Pb, 85°C), and insertions (Co, Mn, 85°C). Three crystals were examined for each sequence type, and the molar fraction of Co among all 12 samples was observed to vary from 0.4 to 0.9, without changing the sequence type. Compared with metal oxides, metal-organic frameworks have high tolerance for coexistence of different metal sizes in their rods and therefore assume various metal sequences. © 2020 American Association for the Advancement of Science. All rights reserved.

Applying atmospherically plasma-sprayed (APS) Mn1.0Co1.9Fe0.1O4 (MCF) protective coatings on interconnector steels minimized the chromium-related degradation within solid oxide fuel cell stack-tests successfully. Post-test characterization of the coatings disclosed a severe microstructural and phase evolution. A self-healing of micro-cracks, the formation and agglomeration of small pores, the occurrence of a dense spinel layer at the surface and a strong elemental de-mixing were reported in ex situ experiments. In the present publication, we prove for the first time these mechanisms by tracking the microstructure in situ at a single APS coating using synchrotron X-ray nano-tomography at the European Synchrotron Radiation Facility. Therefore, a 100-µm-long cylindrical sample with a diameter of 123 µm was cut from an APS-MCF free-standing layer and measured within a high-temperature furnace. All microstructural changes mentioned above could be verified. Porosity measurements reveal a decrease in the porosity from 9 to 3% during the annealing, which is in good accordance with the literature. Additionally, a partial detachment of an approximately 5-µm-thick layer at the sample surface is observed. The layer is dense and does not exhibit any cracks which are penetrating the layer. This kind of shell is assumed to be gastight and thus protecting the bulk from further oxidation. © 2020, The Author(s).

The synthesis of porous electrode materials is often linked with the generation of waste that results from extensive purification steps and low mass yield. In contrast to porous carbons, covalent triazine frameworks (CTFs) display modular properties on a molecular basis through appropriate choice of the monomer. Herein, the synthesis of a new pyridine-based CTF material is showcased. The porosity and nitrogen-doping are tuned by a careful choice of the reaction temperature. An in-depth structural characterization by using Ar physisorption, X-ray photoelectron spectroscopy, and Raman spectroscopy was conducted to give a rational explanation of the material properties. Without any purification, the samples were applied as symmetrical supercapacitors and showed a specific capacitance of 141 F g−1. Residual ZnCl2, which acted formerly as the porogen, was used directly as the electrolyte salt. Upon the addition of water, ZnCl2 was dissolved to form the aqueous electrolyte in situ. Thereby, extensive and time-consuming washing steps could be circumvented. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

Today, a large number of different steels are being processed by Additive Manufacturing (AM) methods. The different matrix microstructure components and phases (austenite, ferrite, martensite) and the various precipitation phases (intermetallic precipitates, carbides) lend a huge variability in microstructure and properties to this class of alloys. This is true for AM-produced steels just as it is for conventionally-produced steels. However, steels are subjected during AM processing to time-temperature profiles which are very different from the ones encountered in conventional process routes, and hence the resulting microstructures differ strongly as well. This includes a very fine and highly morphologically and crystallographically textured microstructure as a result of high solidification rates as well as non-equilibrium phases in the as-processed state. Such a microstructure, in turn, necessitates additional or adapted post-AM heat treatments and alloy design adjustments. In this review, we give an overview over the different kinds of steels in use in fusion-based AM processes and present their microstructures, their mechanical and corrosion properties, their heat treatments and their intended applications. This includes austenitic, duplex, martensitic and precipitation-hardening stainless steels, TRIP/TWIP steels, maraging and carbon-bearing tool steels and ODS steels. We identify areas with missing information in the literature and assess which properties of AM steels exceed those of conventionally-produced ones, or, conversely, which properties fall behind. We close our review with a short summary of iron-base alloys with functional properties and their application perspectives in Additive Manufacturing. © 2019 Elsevier B.V.

Amorphous silica-aluminas (ASAs) are widely used in acid-catalyzed C-H activation reactions and biomass conversions in large scale, which can be promoted by increasing the strength of surface Brønsted acid sites (BAS). Here, we demonstrate the first observation on a synergistic effect caused by two neighboring Al centers interacting with the same silanol group in flame-made ASAs with high Al content. The two close Al centers decrease the electron density on the silanol oxygen and thereby enhance its acidity, which is comparable to that of dealuminated zeolites, while ASAs with small or moderate Al contents provide mainly moderate acidity, much lower than that of zeolites. The ASAs with enhanced acidity exhibit outstanding performances in C–H bond activation of benzene and glucose dehydration to 5-hydroxymethylfurfural, simultaneously with an excellent calcination stability and resistance to leaching, and they offer an interesting potential for a wide range of acid and multifunctional catalysis. © 2020, The Author(s).

Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called “Re effect” have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO2 emissions in air-traffic. © 2020, The Author(s).

We combined experimental investigations and theoretical calculations to unveil an abnormal magnetic behavior caused by addition of the nonmagnetic element Cu in face-centered-cubic FeNiCoMn-based high-entropy alloys (HEAs). Upon Cu addition, the probed HEAs show an increase of both Curie temperature and saturation magnetization in as-cast and homogenized states. Specifically, the saturation magnetization of the as-cast HEAs at room temperature increases by 77% and 177% at a Cu content of 11 and 20 at. %, respectively, compared to the as-cast equiatomic FeNiCoMn HEA without Cu. The increase in saturation magnetization of the as-cast HEAs is associated with the formation of an Fe-Co rich phase in the dendritic regions. For the homogenized HEAs, the magnetic state at room temperature transforms from paramagnetism to ferromagnetism after 20 at. % Cu addition. The increase of the saturation magnetization and Curie temperature cannot be adequately explained by the formation of Cu enriched zones according to atom probe tomography analysis. Ab initio calculations suggest Cu plays a pivotal role in the stabilization of a ferromagnetic ordering of Fe, and reveal an increase of the Curie temperature caused by Cu addition which agrees well with the experimental results. The underlying mechanism behind this phenomenon lies in a combined change in unit-cell volume and chemical composition and the related energetic stabilization of the magnetic ordering upon Cu alloying as revealed by theoretical calculations. Thus, the work unveils the mechanisms responsible for the Cu effect on the magnetic properties of FeNiCoMn HEAs, and suggests that nonmagnetic elements are also crucial to tune and improve magnetic properties of HEAs. © 2020 American Physical Society.

Recycling strategies for waste products from grinding processes have become an essential concern for the industrial sector, as up to 250,000 tons of grinding sludge is generated annually in Germany alone. In this paper, a suitable recycling strategy for the recovery of the metallic component of industrial sludge generated from cold work tool steel grinding is investigated and reported. Possible reuse of the recovered metallic material as a precursor for supersolidus liquid phase sintering (SLPS), a powder metallurgy process, is assessed for the first time. Using a novel technique, including washing, dry screening and magnetic separation, 50 wt% metallic swarf and 50 wt% abrasives can be recovered from dried, industrial grinding sludge. These metallic swarf are a mixture of discontinuous and continuous microchips with a particle size of up to 250 μm. The metallic swarf tend to agglomerate, resulting in larger accumulations and a correspondingly larger overall size with a particle size of over 1000 μm. Using SLPS, the densification of the metallic swarf was considered promising, as density increases with increasing sintering temperature. A maximum density of 76 vol% was achieved, owing to the morphology of the swarf. Occasionally, a few abrasives were observed in the microstructure. Hence, a new sustainable recycling strategy for the recovery of the metallic swarf in industrial grinding sludge has been proposed and the reuse of swarf as precursor for SLPS has demonstrated promising results. Further work is being undertaken to improve the densification of the metallic swarf by SLPS. © 2020 Elsevier Ltd

Ferroelectric materials exhibit the largest dielectric permittivities and piezoelectric responses in nature, making them invaluable in applications from supercapacitors or sensors to actuators or electromechanical transducers. The origin of this behavior is their proximity to phase transitions. However, the largest possible responses are most often not utilized due to the impracticality of using temperature as a control parameter and to operate at phase transitions. This has motivated the design of solid solutions with morphotropic phase boundaries between different polar phases that are tuned by composition and that are weakly dependent on temperature. Thus far, the best piezoelectrics have been achieved in materials with intermediate (bridging or adaptive) phases. But so far, complex chemistry or an intricate microstructure has been required to achieve temperature-independent phase-transition boundaries. Here, we report such a temperature-independent bridging state in thin films of chemically simple BaTiO3. A coexistence among tetragonal, orthorhombic, and their bridging low-symmetry phases are shown to induce continuous vertical polarization rotation, which recreates a smear in-transition state and leads to a giant temperature-independent dielectric response. The current material contains a ferroelectric state that is distinct from those at morphotropic phase boundaries and cannot be considered as ferroelectric crystals. We believe that other materials can be engineered in a similar way to contain a ferroelectric state with gradual change of structure, forming a class of transitional ferroelectrics. Similar mechanisms could be utilized in other materials to design low-power ferroelectrics, piezoelectrics, dielectrics, or shape-memory alloys, as well as efficient electro- and magnetocalorics. © 2020 Author(s).

The present work investigates γ-channel dislocation reactions, which govern low-temperature (T = 750◦C) and high-stress (resolved shear stress: 300 MPa) creep of Ni-base single crystal superalloys (SX). It is well known that two dislocation families with different b-vectors are required to form planar faults, which can shear the ordered γ’-phase. However, so far, no direct mechanical and microstructural evidence has been presented which clearly proves the importance of these reactions. In the mechanical part of the present work, we perform shear creep tests and we compare the deformation behavior of two macroscopic crystallographic shear systems [011](111) and [112](111). These two shear systems share the same glide plane but differ in loading direction. The [112](111) shear system, where the two dislocation families required to form a planar fault ribbon experience the same resolved shear stresses, deforms significantly faster than the [011](111) shear system, where only one of the two required dislocation families is strongly promoted. Diffraction contrast transmission electron microscopy (TEM) analysis identifies the dislocation reactions, which rationalize this macroscopic behavior. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

The paper explores the primary products from fast pyrolysis of biomass components: Lignin, Cellulose and Hemicellulose (Xylan). A heated strip reactor is employed at temperatures of 1573 K and 2073 K with N2 and CO2 atmospheres. Volatiles quench immediately after volatilization on a cold pyrex bridge, while char remains on the heated strip for 3 s. Tar, soot and char are collected and subject to chemical treatments and analyses, including gas chromatography-mass spectrometry and Size Exclusion Chromatography, Thermogravimetric analysis, Raman spectroscopy and Scanning Electron Microscopy. Fast pyrolysis of Lignin produces “Light tar” (soluble in acetone) and “Heavy tar” (soluble in NMP), char, a minor fraction of soot. The “Light tar” contains Vanillin, which can be considered the main primary depolymerization product, but also aliphatics and PAHs. Higher temperature enhances “Heavy tar” and graphitization of the char. Cellulose at 1573 K produces only “Light tar”, largely made of Levoglucosan, as the result of depolymerization. At higher temperature the tar becomes heavier. Hemicellulose has a peculiar behavior: it produces a “Light tar” which is chemically similar to that of Cellulose and, at high temperature also “Heavy tar”. Hemicellulose pyrolysis results also in the production of an atypical solid residue: swollen ad spongy at lower temperature, bright and glassy at higher temperature. CO2 affects the pyrolysis products, particularly those of Lignin, promoting tar cracking and oxygenation already at the stage of primary pyrolysis and hindering thermal annealing and structural ordering of the solid carbonaceous structure. © 2020 Elsevier Ltd

If complex systems are to be studied in molecular simulation, one usually attempts to combine existing interaction models in order to describe the new system. This is, however, not always feasible. We thus propose here a new pairwise-additive interaction model for liquid methanol and solvated Cl− to be used to study the immersion of Metal-Organic Frameworks (MOFs) in methanol. Practically, it entails that all interactions must be written to be compatible with the family of MOF-FF models, which have been specifically developed and then widely employed in molecular simulations of such MOFs, in particular flexible ones. The new model for liquid methanol has been mostly tailored to provide densities and dielectric constants as close to experiment as possible in a large temperature domain. This is important since the flexible MOFs modify their shapes according to their loading with guest molecules of various types, and also according to the thermodynamic conditions. The model yields excellent agreement for the density-temperature, dielectric constant-temperature, and self-diffusion-temperature relationships, properties. Other properties such as e.g. the compressibilities or thermal expansion coefficients are of the correct order of magnitude. Since some MOF frameworks are electrically charged, counterions will be present in these cases. The interactions of Cl− with the liquid are thus also considered here. The solvation of this ion is also found to be satisfactory when compared to other MD studies. © 2019 Elsevier B.V.

If a material grows on another material with a largely different lattice constant, which of the two adapts for an energetically favorable growth? To tackle this question, we investigate the growth of Ag on Cu(111) by variable temperature scanning tunneling microscopy. The structures grown between 120 and 170 K are remarkably different from those grown between 200 and 340 K. The low-temperature structure is rectangular-like and consists of stacked rods, 7 to 8 Ag atoms long, which form a superstructure without long-range order. This structure covers the whole surface prior to nucleation of further layers. The high-temperature structure is hexagonal and consists of misfit dislocations forming 8 × 8 to 10 × 10 superstructures. For this structure, second layer nucleation sets in far before the closure of the first monolayer. While both structures are driven by the large lattice misfit between the two materials, the growing Ag layer adapts to the Cu surface at low temperature, while the Cu surface adapts to the growing Ag layer at higher temperature. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.

The paper explores changes in reactivity and chemico-physical characteristics of char and tar produced by severe heat treatment of lignite in both inert atmospheres and CO2 rich atmospheres. The role of mineral matter, in particular metal oxides, in catalysing chemical and physical transformations is also addressed. A Rhenish Lignite from the Garzweiler mine was studied and compared with: a) mineral-free synthetic carbon (HTC), obtained from cellulose; b) a synthetic carbon doped with iron oxide (Fe2O3). A heated strip reactor (HSR) was employed at temperatures of 1300 and 1800 °C in N2 and CO2 atmospheres. Liquid and solid products (tar and char) were analysed and compared. Tar composition was evaluated by extraction and gas chromatography-mass spectrometry, whereas the solid carbonaceous material produced by pyrolysis, mainly composed of char, was characterized regarding its thermal behaviour by thermogravimetric analysis and its structure by Raman spectroscopy and scanning electron microscopy. Results show that iron oxide exerts a catalytic influence on both pyrolysis and char oxidation. Upon severe heat treatment, it reduces char reactivity promoting graphitization and structural ordering. The overall effect on char reactivity is therefore not easy to predict. © 2018 Elsevier Ltd

A Co-Ni-Ga-Cr thin film materials library with a Cr concentration gradient ranging from 1.5 at.% to 5 at.% was magnetron-sputtered to determine the influence of Cr additions on the martensite transformation temperature of non-stoichiometric Co2NiGa-based Heusler alloys. An increase of the phase transformation temperature from 30 °C for 5 at.% Cr up to 100 °C for 1.5 at.% Cr was determined by a peak fitting analysis of temperature-dependent X-ray diffraction data. © 2019, ASM International.

Whereas magnetic frustration is typically associated with local-moment magnets in special geometric arrangements, here we show that SrCo2As2 is a candidate for frustrated itinerant magnetism. Using inelastic neutron scattering (INS), we find that antiferromagnetic (AF) spin fluctuations develop in the square Co layers of SrCo2As2 below T≈100 K centered at the stripe-type AF propagation vector of (12,12), and that their development is concomitant with a suppression of the uniform magnetic susceptibility determined via magnetization measurements. We interpret this switch in spectral weight as signaling a temperature-induced crossover from an instability toward ferromagnetism ordering to an instability toward stripe-type AF ordering on cooling, and show results from Monte-Carlo simulations for a J1-J2 Heisenberg model that illustrates how the crossover develops as a function of the frustration ratio -J1/(2J2). By putting our INS data on an absolute scale, we quantitatively compare them and our magnetization data to exact-diagonalization calculations for the J1-J2 model [N. Shannon, Eur. Phys. J. B 38, 599 (2004).EPJBFY1434-602810.1140/epjb/e2004-00156-3], and show that the calculations predict a lower level of magnetic frustration than indicated by experiment. We trace this discrepancy to the large energy scale of the fluctuations (Javg 75 meV), which, in addition to the steep dispersion, is more characteristic of itinerant magnetism. © 2019 American Physical Society.

The study of mechanical properties of materials at high temperatures at the microstructural length regime requires dedicated setups for testing. Despite the advances in the instrumentation in these setups over the last decade, further optimization is required in order to extend the temperature range well-beyond 600 °C. Particularly, an improvement of the contact temperature measurement is required. A design with a novel approach of temperature measurement with independent tip and sample heating is developed to characterize materials at high temperatures. This design is realized by modifying a displacement controlled room temperature microstraining rig with the addition of two miniature hot stages, one each carrying the sample and indenter tip. The sample reaches temperatures of >600 °C with a 50 W diode laser system. The stages have slots for the working sample as well as a reference sample on both ends for precise temperature measurements, relying on the symmetry of the stage toward the ends. The whole setup is placed inside a custom-made steel chamber, capable of attaining a vacuum of 10-4 Pa. Alternatively, the apparatus can be operated under environmental conditions by applying various gases. Here, the unique design and its high temperature capabilities will be presented together with the first results of microtension experiments on freestanding copper thin films at 400 °C. © 2019 Author(s).

Ti–Ta alloys represent candidate materials for high-temperature shape-memory alloys (HTSMAs). They outperform several other types of HTSMAs in terms of cost, ductility, and cold workability. However, Ti–Ta alloys are characterized by a relatively fast microstructural degradation during exposure to elevated temperatures, which gives rise to functional fatigue. In the present study, we investigate how isothermal aging affects the martensitic transformation behavior and microstructures in Ti70Ta30 HTSMAs. Ti–Ta sheets with fully recrystallized grain structures were obtained from a processing route involving arc melting, heat treatments, and rolling. The final Ti–Ta sheets were subjected to an extensive aging heat treatment program. Differential scanning calorimetry and various microstructural characterization techniques such as scanning electron microscopy, transmission electron microscopy, conventional X-ray, and synchrotron diffraction were used for the characterization of resulting material states. We identify different types of microstructural evolution processes and their effects on the martensitic and reverse transformation. Based on these results, an isothermal time temperature transformation (TTT) diagram for Ti70Ta30 was established. This TTT plot rationalizes the dominating microstructural evolution processes and related kinetics. In the present work, we also discuss possible options to slow down microstructural and functional degradation in Ti–Ta HTSMAs. © 2018, ASM International.

FeSe is argued as a superconductor in the Bardeen-Cooper-Schrieffer Bose-Einstein condensation crossover regime where the superconducting gap size and the superconducting transition temperature Tc are comparable to the Fermi energy. In this regime, vortex bound states should be well quantized and the preformed pairs above Tc may yield a pseudogap in the quasiparticle-excitation spectrum. We performed spectroscopic-imaging scanning tunneling microscopy to search for these features. We found Friedel-like oscillations near the vortex, which manifest the quantized levels, whereas the pseudogap was not detected. These apparently conflicting observations may be related to the multiband nature of FeSe. © 2019 American Physical Society.

The hypothesis that strain has a permeating influence on ferroelastic, magnetic and superconducting transitions in 122 iron pnictides has been tested by investigating variations of the elastic and anelastic properties of a single crystal of Ba(Fe0.957Co0.043)2As2 by resonant ultrasound spectroscopy as a function of temperature and externally applied magnetic field. Non-linear softening and stiffening of C 66 in the stability fields of both the tetragonal and orthorhombic structures has been found to conform quantitatively to the Landau expansion for a pseudoproper ferroelastic transition which is second order in character. The only exception is that the transition occurs at a temperature (T S ≈ 69 K) ∼10 K above the temperature at which C 66 would extrapolate to zero ( ≈ 59 K). An absence of anomalies associated with antiferromagnetic ordering below T N ≈ 60 K implies that coupling of the magnetic order parameter with shear strain is weak. It is concluded that linear-quadratic coupling between the structural/electronic and antiferromagnetic order parameters is suppressed due to the effects of local heterogeneous strain fields arising from the substitution of Fe by Co. An acoustic loss peak at ∼50-55 K is attributed to the influence of mobile ferroelastic twin walls that become pinned by a thermally activated process involving polaronic defects. Softening of C 66 by up to ∼6% below the normal - superconducting transition at T c ≈ 13 K demonstrates an effective coupling of the shear strain with the order parameter for the superconducting transition which arises indirectly as a consequence of unfavourable coupling of the superconducting order parameter with the ferroelastic order parameter. Ba(Fe0.957Co0.043)2As2 is representative of 122 pnictides as forming a class of multiferroic superconductors in which elastic strain relaxations underpin almost all aspects of coupling between the structural, magnetic and superconducting order parameters and of dynamic properties of the transformation microstructures they contain. © 2019 IOP Publishing Ltd.

Most conventional materials display expansion in response to heating, so there is considerable interest in identifying materials that display the opposite behavior, negative thermal expansion (NTE). The current study investigated the temperature-induced contraction of seven mesoporous metal-organic frameworks (MOFs) of varying topology and composition, which exhibit outstanding porosity, using molecular simulation. We found exceptional NTE for the most porous MOFs, as well as a correlation between the coefficient of NTE and porosity. The large molecular subunits of the MOFs were further studied to find they intrinsically display NTE, corresponding to terahertz vibrational modes. As a result, NTE has a considerable effect on the mechanical properties of these MOFs and is an important consideration for understanding the mechanical stability of new extremely porous materials. © 2019 The Royal Society of Chemistry.

We present a ballistic rectification effect in an orthogonal four-terminal cross junction where the symmetry is broken by local magnetic fields. The input current is injected between opposing branches and the current-free branches serve as potential probes. The local magnetic field is induced by two permalloy (Py) stripes with a magnetic single-domain structure, where one end of each stripe is positioned close to the junction center. The Py stripes are oriented such that an external in-plane magnetic field can magnetize them into two different main configurations having either equally or oppositely magnetized ends. Equal magnetic ends are expected to result in a Hall-effect device, while for opposite magnetic ends, the stray field should deflect the electrons into the same output lead for both current polarities, leading to a rectifying behavior. Here, we present the proof of concept for stray-field controlled transfer characteristics. First, we show by magnetic force microscopy that both configurations are stable and the Py stripes exhibit a remanent magnetic single-domain structure. Second, we demonstrate the influence of the remanent magnetization on the low-temperature dc characteristics which are superimposed by a parasitic background. Third, we present the extracted Hall and the rectified voltage which are, respectively, linearly and parabolically dependent on the input current up to ±55 μA. © 2019 Author(s).

The temperature dependence of vibrational spectra can provide information about structural changes of a system and also serve as a probe to identify different vibrational mode couplings. Fully anharmonic temperature-dependent calculations of these quantities are challenging due to the cost associated with statistically converging trajectory-based methods, especially when accounting for nuclear quantum effects. Here, we train a high-dimensional neural network potential energy surface for the porphycene molecule based on data generated with DFT-B3LYP, including pairwise van der Waals interactions. In addition, we fit a kernel ridge regression model for the molecular dipole moment surface. The combination of this machinery with thermostatted path integral molecular dynamics (TRPMD) allows us to obtain well-converged, full-dimensional, fully-anharmonic vibrational spectra including nuclear quantum effects, without sacrificing the first-principles quality of the potential-energy surface or the dipole surface. Within this framework, we investigate the temperature and isotopologue dependence of the high-frequency vibrational fingerprints of porphycene. While classical-nuclei dynamics predicts a red shift of the vibrations encompassing the NH and CH stretches, TRPMD predicts a strong blue shift in the NH-stretch region and a smaller one in the CH-stretch region. We explain this behavior by analyzing the modulation of the effective potential with temperature, which arises from vibrational coupling between quasi-classical thermally activated modes and high-frequency quantized modes. © 2019 Royal Society of Chemistry. All rights reserved.

A novel advanced design of the planar multipole resonance probe (pMRP) using LTCC-technology is investigated in this contribution. Integrated into the reactor wall, the planar sensor structure enables a minimally invasive in-situ plasma monitoring. Due to the ceramic substrate material, a substantial improved resistance against high temperatures can be achieved, extending the potential fields of application. The used multilayer structure with vertically stacked components ensures a high level of integration and further enhances the mechanical robustness leading to an industry compatible plasma sensor design. The probe is investigated within 3D electromagnetic simulations and its applicability is demonstrated by measurements in a double inductively coupled plasma (DICP). © 2018 IEICE

Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale. © 2019, The Author(s).

High-pressure metamorphism at relatively low to moderate temperature is considered to reflect the presence of plate margin processes. However, evidence of such metamorphism is scarce in the preserved archive of the Archean-early Proterozoic rock record. Extant geochronological studies show that parts of the Southern Granulite Terrain (SGT) of India experienced regional metamorphism from ∼2·49 to 2·44 Ga and thus provide an excellent natural laboratory to study the geodynamics that was prevalent within the first ∼50 Myr from the Archean-Proterozoic boundary. Here, we have constrained the pressureerature-time (P-T-t) evolution of a suite of mafic granulites from the Cauvery Shear System - a part of the SGT - that underwent the early Proterozoic regional metamorphism. Integrated results from mineral reaction histories, kinetically constrained thermobarometry and pseudosection analyses suggest that the studied mafic granulites, with a peak assemblage of garnet + aluminous clinopyroxene + plagioclase + rutile + quartz, were metamorphosed under high-P granulite (HPG) facies conditions of ∼800°C, 12-14 kbar. Subsequently, the rocks underwent simultaneous cooling and decompression, which is recorded by the formation of clinopyroxene + plagioclase coronae at ∼710°C, ∼10-11 kbar, and stabilization of amphibole in various modes at >580-620°C, ∼6-8 kbar. The constrained peak P-T values suggest that the studied rocks were buried significantly deeply below the Earth's surface at moderate temperatures and, at present, such metamorphic conditions are attained in orogenic plate margins. Consequently, the studied rocks suggest the presence of plate tectonics at the Archean-Proterozoic boundary. Furthermore, diffusion modeling of the preserved major element compositional zonations within garnet-clinopyroxene pairs shows that the mafic granulites cooled continuously, but relatively slowly, from peak-T to ∼650°C at rates varying between 5 and 30°C Ma -1. Such a cooling history (during exhumation) at high temperatures indicates that high, perturbed crustal temperatures >600°C were sustained for tens of millions of years, which is uncommon in modern collisional settings. However, the recently proposed model of peeling-off (planar delamination) orogenesis involving hotter mantle during the Archean is consistent with the maintenance of high temperatures for longer durations. Therefore, we propose that the studied rocks preserve evidence for plate motions at the Archean-Proterozoic transition, but possibly with a style that is different from that operating at modern orogens. © 2019 The Author(s).

Atomistic simulations of the thermodynamic properties of magnetic materials rely on an accurate modeling of magnetic interactions and an efficient sampling of the high-dimensional spin space. Recent years have seen significant progress with a clear trend from model systems to material-specific simulations that are usually based on electronic-structure methods. Here we develop a Hamiltonian Monte Carlo framework that makes use of auxiliary spin dynamics and an auxiliary effective model, the temperature-dependent spin-cluster expansion, in order to efficiently sample the spin space. Our method does not require a specific form of the model and is suitable for simulations based on electronic-structure methods. We demonstrate fast warm-up and a reasonably small dynamical critical exponent of our sampler for the classical Heisenberg model. We further present an application of our method to the magnetic phase transition in bcc iron using magnetic bond-order potentials. © 2019 American Physical Society.

Titanium is the base material for a number of technologically important alloys for energy conversion and structural applications. Atomic-scale studies of Ti-based metals employing first-principles methods, such as density functional theory, are limited to ensembles of a few hundred atoms. To perform large-scale and/or finite temperature simulations, computationally more efficient interatomic potentials are required. In this work, we coarse grain the tight-binding (TB) approximation to the electronic structure and develop an analytic bond-order potential (BOP) for Ti by fitting to the energies and forces of elementary deformations of simple structures. The BOP predicts the structural properties of the stable and defective phases of Ti with a quality comparable to previous TB parameterizations at a much lower computational cost. The predictive power of the model is demonstrated for simulations of martensitic transformations. © 2019 IOP Publishing Ltd.

To prevent spontaneous oxidation during the high-temperature synthesis of non-oxide ceramics, an inert atmosphere is conventionally required 1,2 . This, however, results in high energy demand and high production costs. Here, we present a process for the synthesis and consolidation of oxidation-prone materials, the ‘molten salt shielded synthesis/sintering’ process (MS 3 ), which uses molten salts as a reaction medium and also to protect the ceramic powders from oxidation during high-temperature processing in air. Synthesis temperatures are also reduced, and the final product is a highly pure, fine and loose powder that does not require additional milling steps. MS 3 has been used for the synthesis of different ternary transition metal compounds (MAX phases, such as Ti 3 SiC 2 3 , Ti 2 AlN 4 , MoAlB 5 ), binary carbides (TiC) and for the sintering of titanium. The availability of high-quality powders, combined with energy and cost savings, may remove one of the bottlenecks for the industrial application of these materials. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.

The equiatomic HfNbTaTiZr solid solution is currently regarded as a model disordered body-centered cubic high-entropy alloy. Therefore, the temperature dependence of its elastic moduli is of prime importance to improve our understanding of the mechanical properties of this refractory alloy. In this study, the alloy was found to be single phase, fully recrystallized with a slight texture along the normal direction after thermomechanical processing at room temperature. Elastic moduli were determined over the temperature range [293 K-1100 K]. © 2019 The Authors

An indentation creep testing approach has been developed which allows measuring creep properties at high temperatures. In contrast to existing indentation or impression creep experiments, the approach described here allows to achieve a quite high spatial resolution, as flat punch indenters with a diameter of only 20 μm are used. First indentation creep tests have been performed on single crystalline nickel and nickel binary solid solution alloys with Re, Ta or W as alloying elements, respectively. The indentation creep tests have been carried out at a temperature of 650 °C and stress levels in the range of 85 to 400 MPa. Using crystal plasticity finite element modeling, the indentation creep response is converted into equivalent uniaxial creep properties. It is shown that the conversion parameters, evaluated for differently oriented single crystals, can be chosen independently of the creep rate exponent in the power law creep regime. It is found that the indentation creep results agree well with conventional uniaxial creep tests. Furthermore, the results show that Ta is the most effective solid solution strengthener of all tested solid-solution strengtheners at 650 °C because of the large atomic size mismatch, followed by W and Re. © 2019 The Authors

Multiple-principal element alloys known as high-entropy alloys have rapidly been gaining attention for the vast variety of compositions and potential combinations of properties that remain to be explored. Of these alloys, one of the earliest, the ‘Cantor alloy’ CrMnFeCoNi, displays excellent damage-tolerance with tensile strengths of ∼1 GPa and fracture toughness values in excess of 200 MPa√m; moreover, these mechanical properties tend to further improve at cryogenic temperatures. However, few studies have explored its corresponding fatigue properties. Here we expand on our previous study to examine the mechanics and mechanisms of fatigue-crack propagation in the CrMnFeCoNi alloy (∼7 μm grain size), with emphasis on long-life, near-threshold fatigue behavior, specifically as a function of load ratio at temperatures between ambient and liquid-nitrogen temperatures (293 K–77 K). We find that ΔKth fatigue thresholds are decreased with increasing positive load ratios, R between 0.1 and 0.7, but are increased at decreasing temperature. These effects can be attributed to the role of roughness-induced crack closure, which was estimated using compliance measurements. Evidence of deformation twinning at the crack tip during fatigue-crack advance was not apparent at ambient temperatures but seen at higher stress intensities (ΔK ∼ 20 MPa√m) at 77 K by post mortem microstructural analysis for tests at R = 0.1 and particularly at 0.7. Overall, the fatigue behavior of this alloy was found to be superior, or at least comparable, to conventional cryogenic and TWIP steels such as 304 L or 316 L steels and Fe-Mn steels; these results coupled with the remarkable strength and fracture toughness of the Cantor alloy at low temperatures indicate significant promise for the utility of this material for applications at cryogenic environments. © 2019

We present anomalous behavior of temperature-dependent photoluminescence (PL) measurements on InAs quantum dot ensembles coupled to an electron reservoir in an n-i-p diode structure. When negative gate voltages are applied to the sample, an anomalous initial increase of the integrated PL signal with rising temperature is observed for the ground-state and first-excited-state emission peaks. In contrast, measurements at positive gate voltages show no such anomaly and are well described by the commonly used Arrhenius model. Unlike previous studies on uncoupled quantum dot ensembles, we show that in quantum dot diode structures the anomalous temperature dependence and its dependence on the applied bias voltage is dominated by electrons tunneling from the electron reservoir to the quantum dots. Tunneling electrons enhance the PL signal by recombining with holes stored in the quantum dots and the tunneling rate depends on temperature via the Fermi distribution in the electron reservoir. With the implementation of a rate-based tunnel coupling, we develop a modified Arrhenius model that takes the observed anomalies excellently into account. Gate voltage dependent PL measurements at 77 K are further compared to capacitance-voltage spectroscopy measurements on the same sample, supporting the proposed interpretation. The PL peak width shows a characteristic evolution as a function of temperature, which is discussed qualitatively in terms of our model. © 2019 American Physical Society.

Sputter deposition is one of the most important techniques for the fabrication of memristive devices. It allows us to adjust the concentration of defects within the fabricated metal-oxide thin film layers. The defect concentration is important for those memristive devices whose resistance changes during device operation due to the drift of ions within the active layer while an electric field is applied. Reversible change of the resistance is an important property for devices used in neuromorphic circuits to emulate synaptic behavior. These novel bioinspired hardware architectures are ascertained in terms of advantageous features such as lower power dissipation and improved cognitive capabilities compared to state-of-the-art digital electronics. Thus, memristive devices are intensively studied with regard to neuromorphic analog systems. Double-barrier memristive devices with the layer sequence Nb/Al/Al2O3/NbOx/Au are promising candidates to emulate analog synaptic behavior in hardware. Here, the niobium oxide acts as the active layer, in which charged defects can drift due to an applied electric field causing analog resistive switching. In this publication, crucial parameters of the process plasma for thin film deposition, such as floating potential, electron temperature, and the energy flux to the substrate, are correlated with the I-V characteristics of the individual memristive devices. The results from plasma diagnostics are combined with microscopic and simulation methods. Strong differences in the oxidation state of the niobium oxide layers were found by transmission electron microscopy. Furthermore, kinetic Monte Carlo simulations indicate the impact of the defect concentration within the NbOx layer on the I-V hysteresis. The findings may enable a new pathway for the development of plasma-engineered memristive devices tailored for specific application. © 2019 Author(s).

Austenite (γ) reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and in-situ magnetic measurements during slow continuous annealing. A splitting of the γ-reversion into two stages was observed to be a result of strong elemental partitioning between γ and α′-martensite during the low temperature stage between 390 and 575 °C. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed-γ and the Mn-depleted remaining α′-martensite. Because of its lower Mn content, the reversion of the remaining α′-martensite into austenite takes place at a higher temperature range between 600 and 685 °C. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for γ-nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed γ-growth during isothermal annealing at 500 °C and 600 °C. Si partitioning to γ was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during γ-growth and homogenizes over time. We used the chemical composition of the remaining α′-martensite from APT data to calculate its Curie temperature (TCurie) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only γ-reversion but also the TCurie of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties. © 2018 Acta Materialia Inc.

Thermodynamic properties of liquid water as well as hexagonal (Ih) and cubic (Ic) ice are predicted based on density functional theory at the hybrid-functional level, rigorously taking into account quantum nuclear motion, anharmonic fluctuations, and proton disorder. This is made possible by combining advanced free-energy methods and state-of-the-art machine-learning techniques. The ab initio description leads to structural properties in excellent agreement with experiments and reliable estimates of the melting points of light and heavy water. We observe that nuclear-quantum effects contribute a crucial 0.2 meV/H 2 O to the stability of ice Ih, making it more stable than ice Ic. Our computational approach is general and transferable, providing a comprehensive framework for quantitative predictions of ab initio thermodynamic properties using machine-learning potentials as an intermediate step. © 2019 National Academy of Sciences. All rights reserved.

We report the influence of lithium ions on binding and structure of water nanoislands on Au(111) by temperature-programmed desorption and variable-temperature scanning tunneling microscopy. Water coverages between a fraction and full bilayer and two lithium coverages (<0.15% ML) are explored. Lithium enhances selectively the binding of some of the water molecules on precovered Au(111) as compared to water on pristine Au(111), which is revealed by an increase of the water desorption temperature by approx. 10 K. Surprisingly, the effect of lithium on the structure of water is much more extended than expected from these desorption experiments. A small amount of lithium changes the structure of water nanoislands drastically compared to those on pristine Au(111). On pristine Au(111), water ice grows in the form of crystalline islands that are two or three bilayers high. On Li precovered Au(111), the islands are more corrugated, at a 5 times broader apparent height distribution and much smaller, at a 4 times smaller area distribution. These changes reflect the influence of lithium as a structure maker, or kosmotrope, on water. Our study provides unprecedented real-space information of the influence of a kosmotrope on the water structure at the nanoscale. We utilize its kosmotropic behavior to provide real-space images of desorption. © Copyright © 2019 American Chemical Society.

The electron spin dynamics in spontaneously formed GaN nanowires (NWs) on Si(111) is investigated by time-resolved magneto-optical Kerr-rotation spectroscopy for temperatures from 15 to 260 K. A strong increase in the electron spin relaxation time by more than an order of magnitude is found as compared to bulk GaN. The temperature dependence of spin relaxation is characterized by two regimes, which are explained by a model taking into account the coexistence of two different mechanisms. As a result, the spin lifetime is limited by hyperfine interaction of localized electron spins with nuclear spins at low temperatures. The mesoscopic electron confinement in the NWs leads to a dominance of Dyakonov-Perel spin relaxation driven by interface-induced contributions at high temperatures, resulting in a slow-down, but not complete suppression of spin relaxation as compared to bulk GaN. These findings underline the important role of the high surface-to-volume ratio in NWs. © 2019 Author(s).

Severely deformed cold-rolled tungsten is a promising structural material for future fusion reactor applications due to high melting temperature and excellent mechanical properties. However, the fine-grained microstructure after deformation is not stable at temperatures above 800 °C, leading to brittle material behaviour. In this study, we utilize potassium-doping to inhibit recrystallization of tungsten sheets, a mechanism well known from incandescent lamp wires. We produced K-doped tungsten sheets by warm-rolling and subsequent cold-rolling with five different logarithmic strains up to 4.6, and equivalently rolled pure tungsten sheets. Both sets of materials are compared using EBSD and microhardness testing. In both materials, the hardness increases and the grain size along normal direction decreases with strain; the densities of low and high angle boundaries increase in particular during cold-rolling. The K-doped W sheet reaches the highest hardness with 772 ± 8 HV0.1, compared to the pure W sheet with 711 ± 14 HV0.1. All boundaries taken into account, a Hall-Petch relation describes the hardness evolution nicely, except a deviation of the K-doped tungsten sheet rolled to highest strain with its much higher hardness. The similar structural and mechanical properties of both materials in the as-rolled condition allow further studies of recrystallization behaviour of the new K-doped material with a benchmark against the equivalent pure tungsten sheets. Isochronal annealing for 1 h was performed at different temperatures between 700 °C and 2200 °C. A sharp decrease in hardness to intermediate values is observed at around 900 °C for both materials, presumably reflecting extended recovery. A second decrease is observed at 1400 °C for pure tungsten, approaching the hardness of a single crystal and indicating recrystallization and severe growth of grains. For K-doped tungsten, however, the occurrence of the second decrease is shifted to higher temperatures from 1400 °C to 1800 °C with increasing strain and an intermediate hardness is maintained up to 1800 °C. We refer this dependence of the recrystallization resistance on strain in the K-doped material to the dispersion of K-bubbles, resulting in increased Zener pinning forces retarding boundary motion. © 2019

Pyrolysis experiments on a high volatile bituminous Columbian coal were performed in a laminar drop tube reactor at T = 1300 K and 1475 K. Measurements in CO2 were carried out at different residence times up to 150 ms, and the data were complemented by end-point measurements in N2 at approximately 165 ms. These low residence times are typical for the duration of pyrolysis in pulverized coal flames. Mass loss has been determined by solid sampling based on the ash tracer method, and the evolution of porosity was evaluated. Pyrolysis mass loss kinetics were determined based on a single first order reaction and a competing two-step reaction model with distributed activation energies. The particle temperature and residence time needed for the determination of the kinetics were derived by CFD simulations. Results indicate that, despite the low residence time selected, the influence of the Boudouard reaction on mass loss and, hence, evolution of porosity cannot be neglected. In general, porosity increases with increasing residence time and progressing mass loss and porosity is influenced by the both, the release of volatiles and the contribution of gasification. © 2019 Elsevier Ltd

The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol-1 for Cu/ZrO2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.

The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO 2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H 2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO 2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol -1 for Cu/ZrO 2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.

Advanced thermal barrier coatings are essential to further increase the efficiency of gas turbine engines. One limiting factor of the TBC lifetime is the temperature dependent formation of the thermally grown oxide (TGO) during thermal exposure resulting in critical stress levels at the top coat-bond coat interface. Oxide dispersion strengthened (ODS) bond coats demonstrated slower oxygen scale growth during thermal exposure in comparison to standard bond coats. In this study for the first time TBC samples on single-crystal substrates (comparable to CMSX4) with thin ODS Co-based flash coats on the same Co-based bond coat (Amdry 995) and a porous atmospherically plasma sprayed (APS) yttria stabilized zirconia (YSZ) topcoat were manufactured by thermal spray techniques and evaluated with respect to their thermal cyclic behavior. As the major performance test cyclic burner rigs, which can establish thermal conditions similar to those in gas turbines, were applied. TBC samples with the new material combination show superior performance compared to previous samples. Cross sections of the samples were analyzed by scanning electron and laser scanning microscopy. Lifetime data and failure mode of the samples are discussed with respect to material properties such as thermal expansion coefficients, microstructural changes and TGO growth. © 2019 Elsevier B.V.

Recent theoretical investigations [A. B. Belonoshko et al. Nat. Geosci. 10, 312 (2017)1752-089410.1038/ngeo2892] revealed the occurrence of the concerted migration of several atoms in bcc Fe at inner-core temperatures and pressures. Here, we combine first-principles and semiempirical atomistic simulations to show that a diffusion mechanism analogous to the one predicted for bcc iron at extreme conditions is also operative and of relevance for the high-temperature bcc phase of pure Ti at ambient pressure. The mechanism entails a rapid collective movement of numerous (from two to dozens) neighbors along tangled closed-loop paths in defect-free crystal regions. We argue that this phenomenon closely resembles the diffusion behavior of superionics and liquid metals. Furthermore, we suggest that concerted migration is the atomistic manifestation of vanishingly small ω-mode phonon frequencies previously detected via neutron scattering and the mechanism underlying anomalously large and markedly non-Arrhenius self-diffusivities characteristic of bcc Ti. © 2019 American Physical Society.

Neodymium sulphide (Nd 2 S 3 ) belongs to the exciting class of rare earth sulphides (RES) and is projected to have a serious potential in a wide spectrum of application either in pure form or as dopant. We demonstrate a facile and first growth of Nd 2 S 3 thin films via metal-organic chemical vapour deposition (MOCVD) at moderate process conditions using two new Nd precursors, namely tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)Nd(iii) and tris(N,N′-diisopropyl-acetamidinato)Nd(iii). The promising thermal properties and suitable reactivity of both Nd precursors towards elemental sulphur enabled the formation of high purity γ-Nd 2 S 3 . While the process temperature for film growth ranged from 400 °C to 600 °C, the films were crystalline above 500 °C. We also demonstrate that the as-deposited γ-Nd 2 S 3 are luminescent, with the optical bandgap ranging from 2.3 eV to 2.5 eV. The process circumvents post-deposition treatments such as sulfurisation to fabricate the desired Nd 2 S 3 , which paves the way for large scale synthesis and also opens up new avenues for exploring the potential of this class of materials with properties for functional applications. © 2019 The Royal Society of Chemistry.

Technologically important mechanical properties of engineering materials often degrade at low temperatures. One class of materials that defy this trend are CrCoNi-based medium- and high-entropy alloys, as they display enhanced strength, ductility, and toughness with decreasing temperature. Here we show, using in situ straining in the transmission electron microscope at 93 K (−180 °C)that their exceptional damage tolerance involves a synergy of deformation mechanisms, including twinning, glide of partials and full dislocations, extensive cross-slip, and multiple slip activated by dislocation and grain-boundary interactions. In particular, massive cross-slip occurs at the early stages of plastic deformation, thereby promoting multiple slip and dislocation interactions. These results indicate that the reduced intensity of thermal activation of defects at low temperatures and the required increase of applied stress for continued plastic flow, together with high lattice resistance, play a pivotal role in promoting the concurrent operation of multiple deformation mechanisms, which collectively enable the outstanding mechanical properties of these alloys. © 2019 Elsevier Ltd

We report the one-pot mechanochemical synthesis of N-doped porous carbons at room temperature using a planetary ball mill. The fast reaction (5 minutes) between calcium carbide and cyanuric chloride proceeds in absence of any solvent and displays a facile bottom-up strategy that completely avoids typical thermal carbonization steps and directly yields a N-doped porous carbon containing 16 wt% of nitrogen and exhibiting a surface area of 1080 m2 g-1. © 2019 The Royal Society of Chemistry.

Large magnetocrystalline anisotropy (MCA) is of high technical relevance, in particular for magnetic actuators, permanent magnets, and memory devices with high density. Large MCAs have been reported for the low temperature L10 phase of Ni2MnGa. Both, Mn and Pt substitution can stabilize this phase at and above room temperature. Despite the larger spin-orbit coupling in the heavy 5d-element Pt, it has been reported that Pt substitution may result in degeneration of the MCA. In this paper, we study the MCA for a combination of epitaxial strain and Mn and Pt substitution based on density functional theory methods. We show that one can stabilize both large uniaxial and easy-plane anisotropies depending on the value of strain. In particular, small changes of the applied strain may allow to switch between low- and high-anisotropy states or even switch the direction of the easy-axis magnetization direction. © 1965-2012 IEEE.

We introduce a novel alloy design concept for density reduced ultra-high strength steels. It is based on the effects of C to increase martensite strength and the Al-solubility in austenite, in conjunction with Al to increase the martensite start temperature and to reduce density. This alloy combination results in inherently strong but light martensitic microstructures, whose mechanical properties (i.e. strength and ductility) can be readily adjusted over a wide range by applying straightforward and established heat treatments. The concept is validated on an Fe – 8 Al – 1.1C (wt.%) alloy subjected to quench and tempering treatments. The steel exhibits relatively low yield strength (~600 MPa) and reasonable ductility (~ 15% elongation) after hot rolling and soft annealing, but can be brought to a maximum hardness of 62 HRC after quenching and tempering at 250 °C. These attractive mechanical properties are coupled with a low density (6.95 g cm–3) and high elastic stiffness (Young's modulus 199 GPa). These lightweight martensitic steels are cost-effective structural materials for weight-critical applications, promising unprecedented specific strength. Advanced characterisation presented herein allows us to derive the fundamental underpinnings of carbide precipitation and phase transformation, and outline and discuss perspectives for refining the alloy composition and processing parameters. © 2019 Elsevier B.V.

The high temperature low cycle fatigue behavior of specimens manufactured from a single crystalline nickel base superalloy processed by selective electron beam melting (SEBM) has been investigated with respect to the effect of different heat treatments. The fatigue lifetime of heat treated material was significantly higher than that of as-built material. Applying hot isostatic pressing (HIP) with an integrated heat treatment resulted in even longer fatigue life. Lifetime limiting crack initiation occurred at interfaces of melting layers, at micro-porosity generated during solidification or, in HIP treated samples, at precipitates which formed at the location of collapsed pores. © 2019 Elsevier Ltd

The catalytic influence of iron oxide on the oxidation of synthetic chars as a function of the phase composition was investigated by temperature-programmed measurements in a thermobalance and isothermal oxidation experiments in a fixed-bed reactor. The synthetic solid fuels originated from hydrothermal carbonization of cellulose and subsequent pyrolysis of the obtained hydrochars. Incorporation of iron oxide was either achieved by in situ doping during the hydrothermal carbonization or by tight contact mixing of the chars with iron oxide particles. Temperature-programmed oxidation of the synthetic char doped by tight contact resulted only in a slight decrease of the oxidation temperature. Pyrolysis of the in situ doped chars at 800 °C led to the carbothermal reduction of iron oxide to catalytically inactive iron carbide, and it was not possible to re-oxidize iron carbide by means of an additional pretreatment in 20 % O2 at 350 °C. When pyrolysis of the in situ doped hydrochar was performed at 500 °C, iron oxide was not reduced, and the oxidation of the corresponding char occurred much faster due to the catalytic effect of the iron oxide particles, which had a high degree of contact with the embedding carbon matrix. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.

Turbine blades in aviation engines and land based gas-turbines are exposed to extreme environments. They suffer damage accumulation associated with creep, oxidation and fatigue loading. Therefore, advanced repair methods are of special interest for the gas-turbine industry. In this study, CMSX-4 powder is sprayed by Vacuum Plasma Spray (VPS) on single-crystalline substrates with similar compositions. The influence of the substrate temperature is investigated altering the temperature of the heating stage between 850 °C to 1000 °C. Different spray parameters were explored to identify their influence on the microstructure. Hot isostatic pressing (HIP) featuring fast quenching rates was used to minimize porosity and to allow for well-defined heat-treatments of the coatings. The microstructure was analysed by orientation imaging scanning electron microscopy (SEM), using electron backscatter diffraction (EBSD). The effects of different processing parameters were analysed regarding their influence on porosity and grain size. The results show that optimized HIP heat-treatments can lead to dense coatings with optimum γ/γ′ microstructure. The interface between the coating and the substrate is oxide free and shows good mechanical integrity. The formation of fine crystalline regions as a result of fast cooling was observed at the single-crystal surface, which resulted in grain growth during heat-treatment in orientations determined by the crystallography of the substrate. © 2019

Plasma jets belong to the category remote plasma. This means that the discharge conditions and the chemical effect on samples can be tuned separately, this being a big advantage compared to standard low-pressure reactors. The inductive coupling brings the advantage of a pure and dense plasma. The microwave excitation allows furthermore miniaturization and generation of low temperature plasmas. The present paper shows the state of the art of the research on such sources, demonstrating their work up to atmospheric pressure. © 2019 IEEE.

A bottom-up process from precursor development for tin to plasma-enhanced atomic layer deposition (PEALD) for tin(IV) oxide and its successful implementation in a working thin-film transistor device is reported. PEALD of tin(IV) oxide thin films at low temperatures down to 60 °C employing tetrakis-(dimethylamino)propyl tin(IV) [Sn(DMP)4] and oxygen plasma is demonstrated. The liquid precursor has been synthesized and thoroughly characterized with thermogravimetric analyses, revealing sufficient volatility and long-term thermal stability. [Sn(DMP)4] demonstrates typical saturation behavior and constant growth rates of 0.27 or 0.42 Å cycle-1 at 150 and 60 °C, respectively, in PEALD experiments. Within the ALD regime, the films are smooth, uniform, and of high purity. On the basis of these promising features, the PEALD process was optimized wherein a 6 nm thick tin oxide channel material layer deposited at 60 °C was applied in bottom-contact bottom-gate thin-film transistors, showing a remarkable on/off ratio of 107 and field-effect mobility of μFE ≈ 12 cm2 V-1 s-1 for the as-deposited thin films deposited at such low temperatures. © 2019 American Chemical Society.

Recently the possibility of using electric fields as a further stimulus to trigger structural changes in metal-organic frameworks (MOFs) has been investigated. In general, rotatable groups or other types of mechanical motion can be driven by electric fields. In this study we demonstrate how the electric response of MOFs can be tuned by adding rotatable dipolar linkers, generating a material that exhibits paraelectric behavior in two dimensions and dielectric behavior in one dimension. The suitability of four different methods to compute the relative permittivity κ by means of molecular dynamics simulations was validated. The dependency of the permittivity on temperature T and dipole strength μ was determined. It was found that the herein investigated systems exhibit a high degree of tunability and substantially larger dielectric constants as expected for MOFs in general. The temperature dependency of κ obeys the Curie-Weiss law. In addition, the influence of dipolar linkers on the electric field induced breathing behavior was investigated. With increasing dipole moment, lower field strengths are required to trigger the contraction. These investigations set the stage for an application of such systems as dielectric sensors, order-disorder ferroelectrics, or any scenario where movable dipolar fragments respond to external electric fields. Copyright © 2019 American Chemical Society.

The displacive phase transformation of metal-organic frameworks (MOFs), referred to as “breathing,” is computationally investigated intensively within periodic boundary conditions (PBC). In contrast, the first-principles parameterized force field MOF-FF is used to investigate the thermal- and pressure-induced transformations for non-periodic nanocrystallites of DMOF-1 (Zn2(bdc)2(dabco); bdc: 1,4-benzenedicarboxylate; dabco: 1,4-diazabicyclo[2.2.2]octane) as a model system to investigate the effect of the PBC approximation on the systems' kinetics and thermodynamics and to assess whether size effects can be captured by this kind of simulation. By the heating of differently sized closed pore nanocrystallites, a spontaneous opening is observed with an interface between the closed and open pore phase moving rapidly through the system. The nucleation temperature for the opening transition rises with size. By enforcing the phase transition with a distance restraint, the free energy can be quantified via umbrella sampling. The apparent barrier is substantially lower than for a concerted process under PBC. Interestingly, the barrier reduces with the size of the nanocrystallite, indicating a hindering surface effect. The results demonstrate that the actual free energy barriers and the importance of surface effects for the transformation under real conditions can only be studied beyond PBC. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Enzymes can perform complex multistep cascade reactions by linking multiple distinct catalytic sites via substrate channeling. We mimic this feature in a generalized approach with an electrocatalytic nanoparticle for the carbon dioxide reduction reaction comprising a Ag core surrounded by a porous Cu shell, providing different active sites in nanoconfined volumes. The architecture of the nanozyme provides the basis for a cascade reaction, which promotes C-C coupling reactions. The first step occurs on the Ag core, and the subsequent steps on the porous copper shell, where a sufficiently high CO concentration due to the nanoconfinement facilitates C-C bond formation. The architecture yields the formation of n-propanol and propionaldehyde at potentials as low as-0.6 V vs RHE. Copyright © 2019 American Chemical Society.

Recent developments in electrical transportation and renewable energies have significantly increased the demand of hard magnetic materials with a reduced critical rare-earth content, but with properties comparable to (Nd,Dy)-Fe-B permanent magnets. Though promising alternative compositions have been identified in high-throughput screenings, the thermodynamic stability of these phases against decomposition into structures with much less favorable magnetic properties is often unclear. In the case of Ce-Fe-Ti alloys, we have used finite temperature ab initio methods to provide this missing information. Employing state-of-the-art approaches for vibrational, electronic, and magnetic entropy contributions, the Helmholtz free energy, F(T,V), is calculated for the desired hard magnetic CeFe11Ti phase and all relevant competing phases. The latter have been confirmed experimentally by employing reactive crucible melting (RCM) and energy-dispersive x-ray spectroscopy (EDS). Our ab initio based free energy calculations reveal that the presence of the CeFe2 Laves phase suppresses the formation of CeFe11Ti up to 700 K. The result is in agreement with RCM, in which CeFe11Ti is only observed above 1000 K, while the CeFe2 and Ce2Fe17 phases are stable at lower temperatures. © 2019 American Physical Society.

We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions. © 2019, The Author(s).

We investigate the induced growth of a Ag layer on a Cu(111) surface by variable low-temperature scanning tunneling microscopy between 100 and 140 K at submonolayer coverage. Without any interference by the scanning process, the Ag atoms form a two-dimensional gas on the Cu(111) surface. Imaging the surface at elevated voltage induces nucleation and growth of one-dimensional Ag stripes of monolayer height, eventually filling the surface of the imaged area completely. The stripes consist of rods of atoms with a preferential length of (1.88 ± 0.10) nm, corresponding to approx. seven or eight Ag atoms on eight to nine Cu hollow sites. At a ratio of approximately 1:3, rods of double length are the second most observed species. The rods stack in the 112 directions at the √3 distance of Cu(111). Although all equivalent three surface directions are observed, their abundance is not equally distributed, such that the rod direction aligned with the fast scanning direction predominates. At slow growth rates, it is possible to create a striped pattern with one surface direction only. Copyright © 2019 American Chemical Society.

The aim of this paper is to elucidate the mechanisms controlling the brittle-to-ductile transition (BDT) in pre-deformed, textured, polycrystalline body-centred cubic (bcc) metals by the example of cold rolled tungsten (W). For this purpose, five sheets were rolled out from one and the same sintered ingot, by various levels, representing degrees of deformation of 1.8, 2.5, 3.0, 3.4, and 4.1 (this refers to 83.5%, 91.8%, 95.0%, 96.7%, and 98.3% in the technical notation). Toughness tests show that the BDT temperature decreases with increasing degree of deformation from 115 °C ± 15 °C (388 K ± 15 K) down to −65 °C ± 15 °C (208 K ± 15 K). This is an improvement of >600 K compared with a sintered ingot. In this paper we perform an in-depth analysis of the microstructure of the five sheets mentioned above. This analysis includes the assessment of (i) crystallographic texture, (ii) grain size and (iii) dislocation density. A comparison between microstructural features and experimental data confirms our working hypothesis which states that the BDT is controlled by the glide of screw dislocations and that the transition temperature decreases with decreasing spacing, λ of dislocation sources along the crack front. Sources for dislocations may be the intersection points of grain boundaries with the crack front (BDT-temperature-grain-size-relation) or dislocation multiplication processes such as e.g., the expansion of open and closed loops (impact of dislocation density). © 2018 Elsevier Ltd

We study the effect of gallium and yttrium doping on both the electrical performance and the stability of indium based metal-oxide thin-film transistors (MOTFTs) at varied concentrations. As the Ga (Y) content in the In1.0GaxOy (In1.0YxOy) channel material was increased to x = 0.1 the mobility of the MOTFTs degrades by a factor of 4. Thereby the temperature stress stability is clearly enhanced by increasing doping concentration: the onset voltage shift is reduced by a factor of 3 for both In1.0Ga0.1Oy and In1.0Y0.1Oy films compared to that in indium-oxide TFTs. Also the stability during negative bias stress (NBS) is improved since the strong oxygen binders Ga and Y prevent the desorption of oxygen at the surface. In contrast, the onset voltage shift during positive bias stress (PBS) of doped metal oxide TFTs is higher ΔVon = 12 V for InGaO (100:10) TFTs and ΔVon = 15 V for InYO ((100:10) TFTs) compared to that of intrinsic indium oxide TFTs (ΔVon = 6 V), which could be attributed to the generation of flat trap states at the dielectric/semiconductor interface. Doping with Ga and Y significantly enhances the temperature and NBS stability of TFTs and simultaneously degrades the performance. © 2019 The Royal Society of Chemistry.

The chapter describes the different aspects of ceramic materials in gas turbines. The operation conditions such as high-pressure ratio and high temperatures result in improved efficiencies and make necessary the use of materials with high-temperature capability. In addition to the often used single-crystal alloys ceramic materials are discussed. Different bulk ceramics, for example, based on silicon nitride are described. A special focus is laid on ceramic matric composites, both oxide and nonoxide-based materials, which are of increasing interest for gas-turbine applications. In addition to the structural applications ceramics are also often used as coating material. Standard coating processes for protective coatings in gas turbines are described. Furthermore, thermal barrier coatings, a widely used coating system in gas turbines, and environmental barrier coatings as protective coatings for ceramic matrix composites are discussed in detail. Finally, also degradation and failure modes for the different high-temperature coating systems are the topics of this chapter. © 2020 Elsevier Ltd. All rights reserved.

Recent nuclear magnetic resonance studies [A. Pustogow et al., Nature 574, 72 (2019)] have challenged the prevalent chiral triplet pairing scenario proposed for Sr2RuO4. To provide guidance from microscopic theory as to which other pair states might be compatible with the new data, we perform a detailed theoretical study of spin fluctuation mediated pairing for this compound. We map out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that even-parity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spin-orbit coupling favors a near-nodal odd-parity triplet superconducting state, which is either helical or chiral depending on the proximity of the γ band to the van Hove points. A surprising near degeneracy of the nodal s′ and dx2-y2 wave solutions leads to the possibility of a near-nodal time-reversal symmetry broken s′+idx2-y2 pair state. Predictions for the temperature dependence of the Knight shift for fields in and out of plane are presented for all states. © 2019 American Physical Society.

New precursor chemistries for the atomic layer deposition (ALD) of aluminium oxide are reported as potential alternatives to the pyrophoric trimethylaluminium (TMA) which is to date a widely used Al precursor. Combining the high reactivity of aluminium alkyls employing the 3-(dimethylamino)propyl (DMP) ligand with thermally stable amide ligands yielded three new heteroleptic, non-pyrophoric compounds [Al(NMe2)2(DMP)] (2), [Al(NEt2)2(DMP)] (3, BDEADA) and [Al(NiPr2)2(DMP)] (4), which combine the properties of both ligand systems. The compounds were synthesized and thoroughly chemically characterized, showing the intramolecular stabilization of the DMP ligand as well as only reactive Al−C and Al−N bonds, which are the key factors for the thermal stability accompanied by a sufficient reactivity, both being crucial for ALD precursors. Upon rational variation of the amide alkyl chains, tunable and high evaporation rates accompanied by thermal stability were found, as revealed by thermal evaluation. In addition, a new and promising plasma enhanced (PE)ALD process using BDEADA and oxygen plasma in a wide temperature range from 60 to 220 °C is reported and compared to that of a modified variation of the TMA, namely [AlMe2(DMP)] (DMAD). The resulting Al2O3 layers are of high density, smooth, uniform, and of high purity. The applicability of the Al2O3 films as effective gas barrier layers (GBLs) was successfully demonstrated, considering that coating on polyethylene terephthalate (PET) substrates yielded very good oxygen transmission rates (OTR) with an improvement factor of 86 for a 15 nm film by using DMAD and a factor of 25 for a film thickness of just 5 nm by using BDEDA compared to bare PET substrates. All these film attributes are of the same quality as those obtained for the industrial precursor TMA, rendering the new precursors safe and potential alternatives to TMA. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Phase decomposition is commonly observed experimentally in single-phase high entropy alloys (HEAs). Hence, it is essential for the consideration of HEAs for structural applications to study and understand the nature of phase decomposition in HEAs, particularly the influence it has on mechanical behavior. This paper describes the phase decomposition in the equiatomic CoCuFeMnNi HEA and how the reported secondary phases influence mechanical behavior. Thermomechanical processing, followed by systematic post deformation annealing treatments, revealed the formation of two distinct secondary phases within the equiatomic face-centered cubic (FCC) matrix phase. Low temperature annealing treatments at 600 °C and below led to the nucleation of Fe-Co rich ordered B2 precipitates that contributed precipitation hardening while sufficiently small in size, on the order of 140 nm in diameter. At temperatures <800 °C Cu segregation, due to its immiscibility with the other constituents, eventually forms a Cu-rich disordered FCC phase that is determined to increase the yield strength of the alloy while reducing the ductility, likely attributable to the presence of additional interfaces. The thermal stability and chemistry of these phases are compared to those predicted on the basis of calculated phase diagram (CALPHAD) analyses. © 2019 Acta Materialia Inc.

In its nanoparticulate form, corundum (a-Al2O3) could lead to several applications. However, its production into nanoparticles (NPs) is greatly hampered by the high activation energy barrier for its formation from cubic close-packed oxides and the sporadic nature of its nucleation. We report a simple synthesis of nanometer-sized a-Al2O3 (particle diameter ~13 nm, surface areas ~140 m2 g-1) by the mechanochemical dehydration of boehmite (g-AlOOH) at room temperature. This transformation is accompanied by severe microstructural rearrangements and might involve the formation of rare mineral phases, diaspore and tohdite, as intermediates. Thermodynamic calculations indicate that this transformation is driven by the shift in stability from boehmite to a-Al2O3 caused by milling impacts on the surface energy. Structural water in boehmite plays a crucial role in generating and stabilizing a-Al2O3 NPs. © 2019 American Association for the Advancement of Science. All rights reserved.

Crystalline Co3O4 nanoparticles with a uniform size of 9 nm as shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM) were synthesized by thermal decomposition of cobalt acetylacetonate in oleylamine and applied in the oxidation of 2-propanol after calcination. The catalytic properties were derived under continuous flow conditions as a function of temperature up to 573 K in a fixed-bed reactor at atmospheric pressure. Temperature-programmed oxidation, desorption (TPD), surface reaction (TPSR), and 2-propanol decomposition experiments were performed to study the interaction of 2-propanol and O2 with the exposed spinel surfaces. Co3O4 selectively catalyzes the oxidative dehydrogenation of 2-propanol, yielding acetone and H2O and only to a minor extent the total oxidation to CO2 and H2O at higher temperatures. The high catalytic activity of Co3O4 reaching nearly full conversion with 100% selectivity to acetone at 430 K is attributed to the high amount of active Co3+ species at the catalyst surface as well as surface-bound reactive oxygen species observed in the O2 TPD, 2-propanol TPD, TPSR, and 2-propanol decomposition experiments. Density functional theory calculations with a Hubbard U term support the identification of the 5-fold-coordinated octahedral surface Co5c3+ as the active site, and oxidative dehydrogenation involving adsorbed atomic oxygen was found to be the energetically most favored pathway. The consumption of surface oxygen and reduction of Co3+ to Co2+ during 2-propanol oxidation derived from X-ray absorption spectroscopy and X-ray photoelectron spectroscopy measurements before and after reaction and poisoning by strongly bound carbonaceous species result in the loss of the low-temperature activity, while the high-temperature reaction pathway remained unaffected. © 2019 American Chemical Society.

We report on atom probe tomography studies of variable chemical decorations at extended defects in Cu-poor and Zn-rich Cu2ZnSnSe4 thin films. For a precursor film, which was co-evaporated at 320C, grain boundaries and dislocations are found enriched with Cu. Furthermore, Na out-diffusion from the soda-lime glass substrate occurs even at such a low temperature, resulting in Na segregation at defects. In contrast, stacking faults in the precursor film show clear Zn enrichment as well as Cu and Sn depletion. After an annealing step at 500C, we detect changes in the chemical composition of grain boundaries as compared to the precursor. Moreover, we measure an increase in the grain boundary excess of Na by one order of magnitude. We show that grain boundaries and dislocations in the annealed Cu2ZnSnSe4 film exhibit no or only slight variations in composition of the matrix elements. Thus, the effect of annealing is a homogenization of the chemical composition. © 2019 American Physical Society.

We present an approach that enables an efficient and accurate study of dynamically unstable crystals over the full temperature range. The approach is based on an interatomic potential fitted to ab initio molecular dynamics energies for both the high- and low-temperature stable phases. We verify by comparison to explicit ab initio simulations that such a bespoke potential, for which we use here the functional form of the embedded atom method, provides accurate transformation temperatures and atomistic features of the transformation. The accuracy of the potential makes it an ideal tool to study the important impact of finite size and finite time effects. We apply our approach to the dynamically unstable β (bcc) titanium phase and study in detail the transformation to the low-temperature stable hexagonal ω phase. We find a large set of previously unreported linear-chain disordered (LCD) structures made up of three types of [111]β linear-chain defects that exhibit randomly disordered arrangements in the (111)β plane. © 2019 American Physical Society.

We have used neutron powder diffraction to demonstrate the existence of long-range antiferromagnetic order of Ising-like Dy moments in the DyCd6 quasicrystal approximant phase. This cubic compound undergoes a slight distortion to a monoclinic cell at low temperatures. The Néel temperature is 18.0(2) K and the magnetic order of the Dy sublattice may be described in the parent cubic Im3 structure using a combination of two propagation vectors, k1 = [0 0 0] and k2 = [12 0 12], yielding 'anti-I' order. Alternatively, when referred to the monoclinic C2/c cell, the magnetic structure may be described by a single propagation vector: k = [1 0 0]. © 2019 Author(s).

The microstructural evolution in the single crystal Ni-base superalloy ERBO/1 (CMSX 4 type) is investigated after load controlled low cycle fatigue (LCF) at 950 °C (load-ratio: 0.6, tensile stress range: 420–740 MPa, test frequency: 0.25 Hz, fatigue rupture life: about 1000 - 3000 cycles). Bulk topologically close packed (TCP) phase particles precipitated and were analyzed by three-dimensional focus ion beam slice and view imaging and analytical transmission electron microscopy. The particles did not precipitate homogenously but at locations with enhanced levels of local stresses/strains, such as isolated γ-channels subjected to cross channel stresses, shear bands and in front of micro cracks. The influence of stress/strain is furthermore apparent in the spatial arrangement and the shape of the TCP phase particles. Only μ-phase TCP particles were found by electron diffraction. Results of a structure-map analysis suggest that most of these TCP particles observed after LCF testing would not precipitate in thermodynamic equilibrium. In order to rationalize this effect, the atomic volume was analyzed that transition-metal (TM) elements take in unary fcc and in unary μ-phase crystal structures and found that all TM elements except Zr and V take a larger volume in a unary μ phase than in a unary fcc phase. This trend is in line with the observed localized precipitation of TCP phases that are rich in Ni and other late TM elements. The experimental and theoretical findings suggest consistently that formation of TCP particles in LCF tests is considerably influenced by the local tensile stress/strain states. © 2019 Acta Materialia Inc.

In the present work, the influence of oxidation on the martensitic transformation in Ti–Ta high-temperature shape memory alloys is investigated. Thermogravimetric analysis in combination with microstructural investigations by scanning electron microscopy and transmission electron microscopy were performed after oxidation at 850 °C and at temperatures in the application regime of 450 °C and 330 °C for 100 h, respectively. At 850 °C, internal oxidation results in the formation of a mixed layered scale of TiO2 and β-Ta2O5, associated with decomposition into Ta-rich bcc β-phase and Ti-rich hexagonal α-phase in the alloy. This leads to a suppression of the martensitic phase transformation. In addition, energy dispersive X-ray analysis suggests an oxygen stabilization of the α-phase. At 450 °C, a slow decomposition into Ta-rich β-phase and Ti-rich α-phase is observed. After oxidation at 330 °C, the austenitic matrix shows strong precipitation of the ω-phase that suppresses the martensitic transformation on cooling. © 2019, ASM International.

With the aim to evaluate the performance of functionally graded (FG) tungsten/EUROFER multilayer under fusion relevant transient heat loads, this study summarizes the experimental results on type-I edge localized mode (ELM)-like thermal shock exposures of a FG tungsten/EUROFER multilayer. Absorbed power densities of 0.19 and 0.38 GW/m 2 with a pulse duration of 1 ms, and base temperatures of room temperature and 550 °C were chosen. We assessed the effect of variable FG-layers that consisted of three designed thicknesses and two kinds of layer numbers. The conclusion is that the introduction of FG-layers improves the thermal shock resistance. The thermal shock crack thresholds at RT and 550 °C of five multilayers lie between 0.19 and 0.38 GW/m 2 . © 2019

Thin layers of polypropylene (PP) have been treated by argon low-temperature plasmas in an inductively coupled plasma setup. The etched thickness of PP was monitored in situ by means of single-wavelength ellipsometry. The ellipsometric model of the polymer surface exposed to plasma consists of a UV-modified layer, a dense amorphous carbon layer because of ion bombardment, and an effective medium approximation layer, which accounts for moderate surface roughness. The etching behavior has been compared to a model based on argon ion beam irradiation experiments. In this approach, surface processes are described in terms of etching yields and crosslinking probabilities as a function of incident fluxes and energies of Ar ions and UV photons. The ion beam model fits well with the plasma etching results. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Despite the large number of experiments demonstrating that grains in a metallic material can grow at room temperature due to applied mechanical load, the mechanisms and the driving forces responsible for mechanically induced grain coarsening are still not understood. Here we present a systematic study of room temperature grain coarsening induced by cyclic strain in thin polymer-supported gold films. By means of detailed electron backscatter diffraction analysis we were able to capture both the growth of individual grains and the evolution of the whole microstructure on the basis of statistical data over thousands of grains. The experimental data are reported for three film thicknesses with slightly different microstructures and three different amplitudes of cyclic mechanical loading. Although different kinds of grain size evolution with increasing cycle number are observed depending on film thickness and strain amplitude, a single model based on a thermodynamic driving force is shown to be capable to explain initiation and stagnation of grain coarsening in all cases. The main implication of the model is that the grains having lower individual yield stress are coarsening preferentially. Besides, it is demonstrated that the existence of local shear stresses imposed on a grain boundary is not a necessary requirement for room-temperature grain coarsening. © 2019 Acta Materialia Inc.

Tin(IV) oxide is a promising semiconductor material with leading-edge properties toward chemical sensing and other applications. For the growth of its thin films, metal–organic chemical vapor deposition (MOCVD) routes are advantageous due to their excellent scalability and potential to tune processing temperatures by careful choice of the reactants. Herein, a new and highly efficient MOCVD process for the deposition of tin(IV) oxide thin films employing a terminally amino alkyl substituted tin(IV) tetra-alkyl compound is reported for the first time. The liquid precursor, tetrakis-[3-(N,N-dimethylamino)propyl] tin(IV), [Sn(DMP)4], is thermally characterized in terms of stability and vapor pressure, yielding highly pure, polycrystalline tin(IV) oxide thin films with tunable structural and morphological features in the presence of oxygen. Detailed X-ray photoelectron spectroscopy (XPS) analysis reveals the presence of oxygen vacancies and high amounts of chemisorbed oxygen species. Based on these promising features, the MOCVD process is optimized toward downscaling the thickness of tin(IV) oxide films from 25 to 50 nm to study the impact of incipient surface morphological changes occurring after initial thin-film formation on the electrical properties as investigated by van der Pauw (vdP) resistivity measurements. Optical bandgaps of thin films with varying thicknesses are estimated using ultraviolet–visible (UV–vis) spectroscopy. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We investigate the decay of Ag islands on Cu(111) by variable low temperature scanning tunneling microscopy between 195 and 250 K. Such islands exhibit a misfit dislocation pattern forming (8×8) to (10×10) superstructures because of a major lattice mismatch between silver and copper. The decay of islands smaller than 200nm2 alternates between a slower and a faster decay. It is slower for specific island sizes, in particular those with magic numbers of superstructure unit cells. We relate these changes to the complexity of the heteroepitaxial decay, involving a deconstruction of the misfit dislocation pattern and a simultaneous diffusion of several adspecies during decay. © 2019 American Physical Society.

Reversible martensitic transformations (MTs) are the origin of many fascinating phenomena, including the famous shape memory effect. In this work, we present a fully ab initio procedure to characterize MTs in alloys and to assess their reversibility. Specifically, we employ ab initio molecular dynamics data to parametrize a Landau expansion for the free energy of the MT. This analytical expansion makes it possible to determine the stability of the high- and low-temperature phases, to obtain the Ehrenfest order of the MT, and to quantify its free energy barrier and latent heat. We apply our model to the high-temperature shape memory alloy Ti-Ta, for which we observe remarkably small values for the metastability region (the interval of temperatures in which the high- and low-temperature phases are metastable) and for the barrier: these small values are necessary conditions for the reversibility of MTs and distinguish shape memory alloys from other materials. © 2019 American Physical Society.

We present a microscopic study of nematicity and magnetism in FeSe over a wide temperature and pressure range using high-energy x-ray diffraction and time-domain Mössbauer spectroscopy. The low-temperature magnetic hyperfine field increases monotonically up to ∼6 GPa. The orthorhombic distortion initially decreases under increasing pressure but is stabilized at intermediate pressures by cooperative coupling to the pressure-induced magnetic order. Close to the reported maximum of the superconducting critical temperature at p=6.8GPa, the orthorhombic distortion suddenly disappears and a new tetragonal magnetic phase occurs. The pressure and temperature evolution of the structural and magnetic order parameters suggests that they have distinct origins. © 2019 American Physical Society.

Deformation-induced nanoscale twinning is one of the mechanisms responsible for the excellent combination of strength and fracture toughness of the single-phase, face-centered cubic CrMnFeCoNi (Cantor)alloy, especially at cryogenic temperatures. Here, we use a novel, modified dogbone geometry that permits the sampling of varying stress and strain regions within a single tensile specimen to characterize the onset of twinning in CrMnFeCoNi at 293 K, 198 K and 77 K. Electron backscatter diffraction (EBSD)and backscattered electron (BSE)imaging revealed the presence of deformation nano-twins in regions of the samples that had experienced plastic strains of ∼25% at 293 K, ∼16% at 198 K, and ∼8% at 77 K, which are similar to the threshold strains described by Laplanche et al. (Acta Mater. 118, 2016, 152–163). From these strains we estimate that the critical tensile stress for the onset of twinning in this alloy is on the order of 750 MPa. © 2019 Elsevier Ltd

Strategies to reduce the incubation time for crystal nucleation and thus the stochasticity of the set process are of relevance for the operation of phase change memories in ultra-scaled geometries. With these premises, in this work we investigate the crystallization kinetics of the phase change compound GeTe. We have performed large scale molecular dynamics simulations using an interatomic potential, generated previously from a neural network fitting of a database of ab initio energies. We have addressed the crystallization of models of amorphous GeTe annealed at different temperatures above the glass transition. The results on the distribution of subcritical nuclei and on the crystal growth velocity of postcritical ones are compared with our previous simulations of the supercooled liquid quenched from the melt. We find that a large population of subcritical nuclei can form at the lower temperatures where the nucleation rate is large. This population partially survives upon fast annealing, which leads to a dramatic reduction of the incubation time at high temperatures where the crystal growth velocity is maximal. This priming effect could be exploited to enhance the speed of the set process in phase change memories. © 2019 The Royal Society of Chemistry.

The equiatomic CrMnFeCoNi high-entropy alloy (HEA) exhibits outstanding toughness and excellent strength-ductility combination at cryogenic temperatures. However, its strength is relatively low at room temperature. In order to strengthen this HEA, microalloying additions of 0.8 at.% Nb and C were made and its properties and microstructure evaluated. It was found that the microalloying resulted in the formation of carbide precipitates and a reduction of the grain size to ∼2.6 μm. As a result, the room-temperature tensile yield strength (732 MPa) of the microalloyed HEA is roughly double that of the base HEA (with a concomitant increase in the ultimate strength) while its ductility is maintained at a relatively high level (elongation to fracture of ∼32%). The strengthening is due to precipitation hardening from the nanoscale carbide particles and grain refinement. © 2019

Hydrogen embrittlement (HE) of several low-nickel austenitic stainless steels (AISI 300 series) was studied with special attention to the impact of strain induced α′-martensite. The susceptibility of the steels to HE is judged with respect to the relative reduction of area (RRA): The HE susceptibility is lower for larger RRA-values. Strain-induced martensite formation was evaluated within in the framework of the Olson-Cohen model, revealing a linear relationship between RRA and the probability β of martensite nucleus formation in the steels. In order to widen the scope of data evaluation to literature data, the consideration of a parameter alternative to β is required. It is demonstrated that among other parameters the Md30 temperature (Nohara), which assesses the stability against martensitic transformation, can serve as an indicator to predict HE of AISI 300 series steels. Regarding the Md30 temperature (Nohara), a trend-line with respect to the RRA-values is found. Thereby, the RRA-values of low-nickel austenitic stainless steels group into three distinct regimes; (1) for Md30 > −80 °C, where RRA-values decrease with increasing Md30 temperature, (2) at Md30 ≈ −80 °C, where RRA-values show a large variation (‘threshold band’), and (3) for Md30 < −80 °C, showing constant RRA-values of nearly 100%. Some RRA data points that deviate from the trend line can be explained by the special microstructure of the investigated samples. © 2019 Hydrogen Energy Publications LLC

We present experimental data on the diffusivity of Pb in CaTiO3 perovskite, which is commonly used for dating kimberlites and carbonatites. Experiments were performed on oriented synthetic and natural CaTiO3 single crystals. The Pb-source was either a laser deposited (Ca0.83Pb0.07)Ti1.05O3 thin film or a (Ca0.9Pb0.1)TiO3 powder reservoir. The crystals were annealed in a high-temperature furnace between 736 and 1135 °C and for durations from 2 to 283 h. The diffusion profiles were measured with Rutherford backscattering and time-of-flight secondary ion mass spectrometry in the depth-profiling mode. The concentration profiles measured on the same samples with the two analytical methods are in agreement. The measured concentration profiles show two regions - a steep gradient at the diffusion interface that transitions sharply (at ~50 to 150 nm from the surface) to a low concentration tail that penetrates deeper into the crystal. This diffusion behavior could be modeled best using diffusion coefficients that are a function of the Pb concentration, with a different set of diffusion coefficient for the high and the low concentration region of the profile, respectively. The diffusion coefficients extracted from the thin film and powder source experiments are similar within uncertainties. Pb diffuses slower at concentrations between 8.5 and 0.6 wt% and 1.6 to 2.6 log units faster below ~0.5 wt% Pb. Temperature dependency for each region is discussed in the text, and the Arrhenius relation for the fast diffusion regime that is most relevant for natural samples is DPbfast=2.5×10-13×exp(-158(24)kJ/mol/RT)m2/s. $$begin array D- text Pb- text fast=2.5 times 10- -13times exp Big(-158big(24big)text kJ/mol/RTBig)m-2/s. end array $$ We found a distinct change in the structure of CaTiO3 in the surface region of the single crystal that is coincidental with the change in diffusivity. This initial region is dominated by planar defects. We propose that Pb is trapped in planar defects that have formed due to the high strain introduced into the perovskite structure caused by the mismatch in the ionic radius between Ca2+ and Pb2+. The activation energies obtained here yield closure temperature for Pb in CaTiO3 between 300 and 400 °C for a range of different cooling scenarios if diffusive resetting of Pb in CaTiO3 occurs at all. At typical cooling rates of hours to days for ascending kimberlite, the age of crystal growth is preserved, with closure temperatures similar to the magma temperature. © 2019 Walter de Gruyter GmbH, Berlin/Boston 2019.

The relatively low hydrothermal stability of Al-rich Cu-SSZ-13 catalysts hinders their practical application in ammonia selective catalytic reduction (NH 3 -SCR) reaction. Rare-earth ions were introduced into the Al-rich SSZ-13 zeolite using an organotemplate-free synthesis prior to the exchange of Cu 2+ ions. Among the rare-earth ions tested (Ce, La, Sm, Y, Yb), Y shows significant enhancement of the hydrothermal stability and NH 3 -SCR activities after severe hydrothermal aging at 800 °C for 16 h when compared with Cu-SSZ-13 without Y. Cu-Y-SSZ-13 catalysts with various amounts of Y were prepared, and it is found that with increasing Y content, the low temperature NO conversions can be improved even after hydrothermal aging. SEM-EDX analysis together with two-dimensional multiple quantum magic-angle-spinning nuclear magnetic resonance ( 23 Na MQ MAS NMR) confirms that the Y ions are successfully incorporated into the ion-exchange sites of the SSZ-13 zeolite. Results from 27 Al MAS, 29 Si MAS NMR, temperature-programmed desorption of ammonia (NH 3 -TPD) and quantitative 1 H MAS NMR indicate that Y can stabilize the framework Al and also preserve the Brønsted acid sites in the Al-rich SSZ-13 zeolite. The hydrogen temperature programmed reduction (H 2 -TPR), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of nitric oxide (NO) or NH 3 adsorption demonstrate that introduction of Y ions causes Cu 2+ ions to preferentially occupy the 6-MR, which has high hydrothermal stability. However, too much of Y may lead to activity loss at both low and high temperatures. The optimized Al-rich Cu-Y-SSZ-13 with 2.8 wt% of copper (Cu) and 1.3 wt% of Y displays almost the same deNO x activities as the conventional organotemplated high silica Cu-SSZ-13 catalyst in a wide reaction temperature window of 150-650 °C after severe hydrothermal treatment. Rare-earth ions could be an effective additive for Cu-SSZ-13 catalysts to further improve their hydrothermal stability for practical applications. © 2019 The Royal Society of Chemistry.

The microstructure-mechanical property relationships of a non-equiatomic FeMnCoCr high entropy alloy (HEA), which shows a single face-centered cubic (fcc) structure in the undeformed state, have been systematically investigated at room and cryogenic temperatures. Both strength and ductility increase significantly when reducing the probing temperature from 293 K to 77 K. During tensile deformation at 293 K, dislocation slip and mechanical twinning prevail. At 173 K deformation-driven athermal transformation from the fcc phase to the hexagonal close-packed (hcp) martensite is the dominant mechanism while mechanical twinning occurs in grains with high Schmid factors. At 77 K athermal martensitic transformation continues to prevail in addition to dislocation slip and twinning. The reduction in the mean free path for dislocation slip through the fine martensite bundles and deformation twins leads to the further increased strength. The joint activation of transformation and twinning under cryogenic conditions is attributed to the decreased stacking fault energy and the enhanced flow stress of the fcc matrix with decreasing temperature. These mechanisms lead to an elevated strain hardening capacity and an enhanced strength-ductility combination. The temperature-dependent synergy effects of martensite formation, twinning and dislocation plasticity originate from the metastability alloy design concept. This is realized by relaxing the equiatomic HEA constraints towards reduced Ni and increased Mn contents, enabling a non-equiatomic material with low stacking fault energy. These insights are important for designing strong and ductile Ni-saving alloys for cryogenic applications. © 2019 Elsevier B.V.

The elastic properties of rare-earth scandates were only reported at room temperature based on simulations and experimental measurements with poor agreement thus far. Using resonant ultrasound spectroscopy and inductive gauge dilatometry, we determined the elastic stiffnesses, their temperature dependence, and thermal expansion coefficients of SmScO 3, TbScO 3, and DyScO 3 between 103 K and 1673 K. Our set of elastic stiffnesses shows high internal consistency, e.g., the relations c 11 > - > c 33 > - > c 22, c 66 > - > c 44 > - > c 55, and c 13 ≥ c 12 > - > c 23 hold for all crystal species at room temperature. The structures become overall stiffer with decreasing R E-radius and increased charge density. The behavior of c 44 at low temperatures indicates in all R EScO 3 a structural instability that might lead to an orthorhombic →monoclinic transition involving shear of the (100)-plane upon increasing pressure. The transition seems to be promoted by a decreasing R E-radius. Anomalies in two mixed resistances of TbScO 3 below room temperature are indicative of at least one more structural instability that may also cause a phase transition where the structure is sheared. So far, only magnetic phase transitions at about 3 K have been observed in R EScO 3 in literature. The thermoelastic properties in [100] and [001] directions of all materials become increasingly isotropic at high temperatures, suggesting decreased structural tilt. (100) or (010) crystal cuts should be chosen for applications of a R EScO 3 as a substrate material, when mostly isotropic thermal expansion or longitudinal stiffness in-plane is desired, respectively. © 2019 Author(s).

We follow the decay of two-dimensional Ag nanoclusters, called islands, on Cu-Ag core-shell supports by variable low temperature scanning tunneling microscopy in the temperature range between 160 and 260 K. We reveal two qualitatively different types of decay mechanisms, either linear in time, indicative of an interface-limited decay, or non-linear in time, indicative of diffusion-limited decay. In contrast to conventional decay on monometallic supports, the decay exponent of the diffusion-limited decay depends on temperature; it varies by one order of magnitude. Moreover, the decay rate decreases with increasing temperature. This unusual behaviour is traced back to the temperature-dependent shell of the core-shell support. © 2019 The Royal Society of Chemistry.

MAX phases have great potential under demands of both high-temperature and high-stress performance, with their mixed atomic bonding producing the temperature and oxidation resistance of ceramics with the mechanical resilience of metals. Here, we measure the mechanical properties up to 980C by nanoindentation on highly dense and pure Cr2AlC, as well as after oxidation with a burner rig at 1200 °C for more than 29 h. Only modest reductions in both hardness and modulus up to 980 °C were observed, implying no change in deformation mechanism. Furthermore, micro-cantilever fracture tests were carried out at the Cr2AlC/Cr7C3 and Cr7C3/Al2O3 interfaces after the oxidation of the Cr2AlC substrates with said burner rig. The values are typical of ceramic-ceramic interfaces, below 4 MPam, leading to the hypothesis that the excellent macroscopic behaviour is due to a combination of low internal strain due to the match in thermal expansion coefficient as well as the convoluted interface. © 2019 Elsevier Ltd

Nanosecond plasmas in liquids play an important role in the field of decontamination, electrolysis or plasma medicine. The understanding of these very dynamic plasmas requires information about the temporal variation of species densities and temperatures. This is analyzed by monitoring nanosecond pulsed plasmas that are generated by high voltages (HVs) between 14 and 26 kV and pulse lengths of 10 ns applied to a tungsten tip with 50 μm diameter immersed in water. Ignition of the plasma causes the formation of a cavitation bubble that is monitored by shadowgraphy to measure the dynamic of the created bubble and the sound speed of the emitted acoustic waves surrounding this tungsten tip. The temporal evolution of the bubble size is compared with cavitation theory yielding good agreement for an initial bubble radius of 25 μm with an initial pressure of 5 ×108 Pa at a temperature of 1200 K for a HV of 20 kV. This yields an initial energy in the range of a few 10-5 J that varies with the applied HV. The dissipated energy by the plasma drives the adiabatic expansion of water vapor inside the bubble from its initial supercritical state to a low pressure, low temperature state at maximum bubble expansion reaching values of 103 Pa and 50 K, respectively. These predictions from cavitation theory are corroborated by optical emission spectroscopy. After igniting the nanosecond plasma, the electrical power oscillates in the feed line between HV pulser and plasma chamber with a ring down time of the order of 60 ns. These reflected pulses re-ignite a plasma inside the expanding bubble periodically. Broadband emission due to recombination and Bremsstrahlung becomes visible within the first 30 ns. At later times, line emission dominates. Stark broadening of the spectral lines of Hα (656 nm) and OI (777 nm) is evaluated to determine both the electron density and the electron temperature in these re-ignited plasmas. © 2019 IOP Publishing Ltd.

A newly developed miniature specimen and respective fixture for high temperature low cycle fatigue testing of nickel based single crystal superalloys is presented. Miniaturization allows the preparation of test specimens in all main crystallographic orientations of the cubic nickel crystal using laboratory sized material samples and enables excellent utilization of the costly material. The specimen geometry is optimized by means of parameter studies employing numerical calculations such that for the main crystallographic orientations the stress concentration at the fillet between gauge length and specimen head is minimized, and failure is likely to occur within the gauge length. The designed fixture allows easy specimen mounting and provides sufficient support for applying an extensometer for strain measurement. Protective metallic coatings against oxidation can be applied on the specimen by plasma spraying for studying the effect of coatings on the fatigue lifetime. The functionality of the specimen geometry and fixture design for low cycle fatigue testing is demonstrated for temperatures up to 950 °C. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A high density of grain boundaries can potentially increase structural materials' strength, but at the expense of losing the materials' strain hardening ability at high flow stress levels. However, endowing materials with grain size gradients and a high density of internal interfaces can simultaneously increase the strength and strain hardening ability. This applies particularly for through-thickness gradients of nanoscale interface structures. Here we apply a machining method that produces metals with nanoscale interface gradients. Conventional bulk plastic deformation such as rolling, a process applied annually to about 2 billion tons of material, aims to reduce the metal thickness. We have modified this process by introducing severe strain path changes, realized by leading the sheet through a U-turn while preserving its shape, an approach known as ‘hard turning’. We applied this process at both room temperature and 77 K to a NiCrCo medium entropy alloy. Micropillar compression was conducted to evaluate the mechanical response. After hard turning at room temperature, the surface microstructure obtained a ∼50% increase in yield stress (0.9 GPa) over the original state with homogeneous grain size (0.4 GPa), but the initial strain hardening rate did not show significant improvement. However, after hard turning at 77 k, the gradient nanolaminate structure tripled in yield stress and more than doubled its initial strain hardening rate. The improvements were achieved by introducing a specific microstructure that consists of gradient nanolaminates in the form of nanospaced twins and martensite in the face center cubic (fcc) phase. This microstructure was formed only at cryogenic temperature. It was found after turning at room temperature that only nanospaced twins were present in the fcc phase inside nanolaminates that had formed at the surface. The origin of the enhanced strain hardening mechanism was studied. Joint density functional theory (DFT) and axial next nearest neighbor Ising (ANNNI) models were used to explain the temperature-dependent phase formation of the NiCrCo nanolaminate at the surface of the hard-turned material. © 2019 Acta Materialia Inc.

The present work compares the microstructures and the creep properties of two types of single crystal Ni-base superalloy CMSX-4 materials (SXs). One was produced by conventional directional solidification Bridgman processing. The other was manufactured by selective electron beam melting (SEBM). The microstructures of the two types of materials are compared with emphasis placed on the large (dendritic/interdendritic regions) and small scale (γ-matrix/γ′-precipitates) microstructural heterogeneities, which characterize SX microstructures and their evolution during processing, heat treatment and creep. It is shown that heat treated SEBM materials have creep properties, which match or even outperform those of conventionally processed SX materials. Creep properties were assessed using a miniature creep test technique where [001] miniature tensile creep specimens were tested in the high temperature/low stress (1050 °C, 160 MPa) and in the low temperature/high stress (850 °C, 600 MPa) creep regimes. The creep behavior is interpreted based on microstructural results, which were obtained using analytical scanning and transmission electron microscopy (SEM and TEM). © 2019 The Authors

Due to their ability to provide a shape memory effect at elevated temperatures, high-temperature shape memory alloys (HT-SMAs) came into focus of academia and industry in the last decades. Ternary and quaternary Ni–Ti-based HT-SMAs have been in focus of a large number of studies so far. Ti–Ta HT-SMAs feature attractive shape memory properties along with significantly higher ductility and lower costs for alloying elements compared to conventional Ni–Ti-based HT-SMAs, which qualifies them as promising candidate alloys for high-temperature applications. Unfortunately, precipitation of undesired phases, e.g., the ω-phase, leads to significant functional degradation upon cyclic loading in binary Ti–Ta. Therefore, additions of ternary elements, such as Al, which suppress the ω-phase formation, are important. In the present study, the influence of different heating–cooling rates on the cyclic functional properties of a Ti–20Ta–5Al HT-SMA is investigated. Transmission electron microscopy as well as in situ synchrotron analysis revealed unexpected degradation mechanisms in the novel alloy studied. Elementary microstructural mechanisms leading to a degradation of the functional properties were identified, and the ramifications with respect to application of Ti–Ta–Al HT-SMAs are discussed. © 2019, ASM International.

A bottom-up approach starting with the development of new Hf precursors for plasma-enhanced atomic layer deposition (PEALD) processes for HfO2 followed by in situ thin-film surface characterization of HfO2 upon exposure to reactive gases via near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is reported. The stability of thin films under simulated operational conditions is assessed, and the successful implementation of HfO2 dielectric layers in metal-insulator-semiconductor (MIS) capacitors is demonstrated. Among the series of newly synthesized mono-guanidinato-tris-dialkyl-amido class of Hf precursors, one of them, namely, [Hf{2-(iPrN)2CNEtMe}(NEtMe)3], was representatively utilized with oxygen plasma, resulting in a highly promising low-temperature PEALD process at 60 °C. The new precursors were synthesized in the multigram scale and thoroughly characterized by thermogravimetric analyses, revealing high and tunable volatility reflected by appreciable vapor pressures and accompanied by thermal stability. Typical ALD growth characteristics in terms of linearity, saturation, and a broad ALD window with constant growth of 1.06 Å cycle-1 in the temperature range of 60-240 °C render this process very promising for fabricating high-purity smooth HfO2 layers. For the first time, NAP-XPS surface studies on selected HfO2 layers are reported upon exposure to reactive H2, O2, and H2O atmospheres at temperatures of up to 500 °C revealing remarkable stability against degradation. This can be attributed to the absence of surface defects and vacancies. On the basis of these promising results, PEALD-grown HfO2 films were used as dielectric layers in the MIS capacitor device fabrication exhibiting leakage current densities less than 10-7 A cm-2 at 2 MV cm-1 and permittivities of up to 13.9 without postannealing. © 2019 American Chemical Society.

Liquid air energy storage (LAES) is a large-scale storage technology, which is using liquefied air as storage medium. Comparable to pumped hydro (PHES) and compressed air energy storage (CAES), LAES is charged with excess electricity from the grid and discharged, when the electricity demand is high. Working as a buffer for the electric grid, the availability and integrability of fluctuating renewable energy sources can be improved by LAES. In the charging process, ambient air is liquefied with an adopted Claude respectively Kapitza process. Compression heat is stored in a hot thermal energy storage device (HTES); a cold thermal energy storage device (CTES) is used as heat sink at cryogenic temperature to significantly improve the efficiency of the liquefaction. In the discharging process, liquid air is pressurized, heated up to ambient temperature by the CTES, superheated by the HTES, and expanded in an air expander for electricity generation. The CTES is used to recycle an exergy flow at cryogenic temperature from the discharging to the charging process. Since the round trip efficiency of the LAES strongly depends on this exergy flow, two different types of CTES are compared within this work. The liquid cold thermal energy storage device (LCTES) is based on a multi-tank storage system using propane and methanol, the direct cold thermal energy storage device (DCTES) is a packed bed storage system with direct contact between the fluid and the solid storage material. In this work, a comparison and an exergetic investigation of both systems is presented. The significant influence of the exergetic efficiency of the CTES and other technical aspects are worked out. Additionally, the influence of the pressure on the liquefaction and discharging process, and on the round trip efficiency is investigated. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 - The 10th International Conference on Applied Energy.

A molecular-scale description of water and ice is important in fields as diverse as atmospheric chemistry, astrophysics, and biology. Despite a detailed understanding of water and ice structures on a multitude of surfaces, relatively little is known about the kinetics of water motion on surfaces. Here, we report a detailed study on the diffusion of water monomers and the formation and diffusion of water dimers through a combination of time-lapse low-temperature scanning tunnelling microscopy experiments and first-principles electronic structure calculations on the atomically flat Cu(111) surface. On the basis of an unprecedented long-time study of individual water monomers and dimers over days, we establish rates and mechanisms of water monomer and dimer diffusion. Interestingly, we find that the monomer and the dimer diffusion barriers are similar, despite the significantly larger adsorption energy of the dimer. This is thus a violation of the rule of thumb that relates diffusion barriers to adsorption energies, an effect that arises because of the directional and flexible hydrogen bond within the dimer. This flexibility during diffusion should also be relevant for larger water clusters and other hydrogen-bonded adsorbates. Our study stresses that a molecular-scale understanding of the initial stages of ice nanocluster formation is not possible on the basis of static structure investigations alone. © 2019 American Chemical Society.

Nanostructured carbons with different pore geometries are prepared with a liquid-free nanocasting method. The method uses gases instead of liquid to disperse carbon precursors, leach templates, and remove impurities, minimizing synthetic procedures and the use of chemicals. The method is universal and demonstrated by the synthesis of 12 different porous carbons with various template sources. The effects of pore geometries in catalysis can be isolated and investigated. Two of the resulted materials with different pore geometries are studied as supports for Ru clusters in the hydrogenolysis of 5-hydroxymethylfurfural (HMF) and electrochemical hydrogen evolution (HER). The porous carbon-supported Ru catalysts outperform commercial ones in both reactions. It was found that Ru on bottleneck pore carbon shows a highest yield in hydrogenolysis of HMF to 2,5-dimethylfuran (DMF) due to a better confinement effect. A wide temperature operation window from 110 to 140 °C, with over 75% yield and 98% selectivity of DMF, has been achieved. Tubular pores enable fast charge transfer in electrochemical HER, requiring only 16 mV overpotential to reach current density of 10 mA·cm-2. © 2019 American Chemical Society.

The magnetic structure of Nd in NdFeAsO compound has been investigated by X-ray resonant magnetic scattering at the Nd L2 edge (E=6.725 keV) at 1.7≤T≤15 K. At T=1.7 K we find that the Nd moments are aligned along the crystallographic c direction with the (1, 0, 0) propagation vector, and are arranged antiferromagnetically along the a direction and ferromagnetically along the b and c directions. At 1.7

Testing of the corrosion resistance of environmental barrier coating (EBC) systems is necessary for developing reliable coatings. Unfortunately tests under realistic gas turbine conditions are difficult and expensive. The materials under investigation as well as parts of the test setup have to withstand high temperatures (≥1200°C), high pressure (up to 30 bar) as well as the corrosive atmosphere (H2O, O2, NOx). Therefore most lab scale test-rigs focus on simplified test conditions. In this work water vapor corrosion testing of EBCs with a high velocity oxy fuel (HVOF) facility is introduced which combines high temperatures and high gas velocities. It leads to quite high recession rates in short periods of time, which are comparable to results from literature. It was found that high flow velocities can easily compensate low gas pressures. HVOF-testing is a simple and fast way to measure the recession rate of an EBC-system. As proof of concept the recession rates of an oxide/oxide CMC with and without EBC were measured. © 2019 The American Ceramic Society

The Co-Ta system has been reviewed and the thermodynamic description was re-assessed in the present work. DFT (density functional theory) calculations considering spin polarization were performed to obtain the energies for all end-member configurations of the C14, C15, C36 and μ phases for the evaluation of the Gibbs energies of these phases. The phase diagram calculated with the present description agrees well with the experimental and theoretical data. Considering the DFT results was essential for giving a better description of the μ phase at lower temperatures. © 2018 Elsevier Ltd

Al2O3/Al2O3 ceramic matrix composites are candidate materials for high-temperature applications such as gas turbines. As water vapor corrosion of Al2O3/Al2O3-CMC (ceramic matrix composite) is a major issue, the application of suitable environmental barrier coatings is inevitable. An important factor for coating adhesion, especially in thermal spraying, is mechanical interlocking. Therefore, a rough substrate surface is needed. Although it has been proven that laser ablation is a suitable method for surface preparation of metallic substrates, no studies on Al2O3/Al2O3-CMCs are available. Therefore, the suitability of surface preparation of an Al2O3/Al2O3-CMC by laser ablation for use prior to atmospheric plasma spraying was examined. The laser ablation threshold fluence for Al2O3/Al2O3-CMC was determined. The effects of different processing parameters on the surface were studied. Various surface morphologies were obtained, such as cauliflower and honeycomb structures. The samples were characterized by white light interferometry, laser microscopy, and scanning electron microscopy. The obtained surface structures were coated with Gd2Zr2O7. It was found that the adhesion strength of coatings on laser treated samples was drastically increased. © 2019 Laser Institute of America.

The iron-based superconductor FeSe offers a unique possibility to study the interplay of superconductivity with purely nematic as well magnetic-nematic order by pressure (p) tuning. By measuring specific heat under p up to 2.36 GPa, we study the multiple phases in FeSe using a thermodynamic probe. We conclude that superconductivity is bulk across the entire p range and competes with magnetism. In addition, whenever magnetism is present, fluctuations exist over a wide temperature range above both the bulk superconducting and the magnetic transitions. Whereas the magnetic fluctuations are likely temporal, the superconducting fluctuations may be either temporal or spatial. These observations highlight similarities between FeSe and underdoped cuprate superconductors. © 2019 American Physical Society.

The rapid degradation of the functional properties of many Ti-based alloys is due to the precipitation of the ω phase. In the conventional high-temperature shape memory alloy Ti-Ta, the formation of this phase compromises completely the shape memory effect, and high (>100°C) transformation temperatures cannot be maintained during cycling. A solution to this problem is the addition of other elements to form Ti-Ta-X alloys, which often modifies the transformation temperatures; due to the largely unexplored space of possible compositions, very few elements are known to stabilize the shape memory effect without decreasing the transformation temperatures below 100°C. In this study, we use transparent descriptors derived from first-principles calculations to search for new ternary Ti-Ta-X alloys that combine stability and high temperatures. We suggest four alloys with these properties, namely Ti-Ta-Sb, Ti-Ta-Bi, Ti-Ta-In, and Ti-Ta-Sc. Our predictions for the most promising of these alloys, Ti-Ta-Sc, are subsequently fully validated by experimental investigations, the alloy Ti-Ta-Sc showing no traces of ω phase after cycling. Our computational strategy is transferable to other materials and may contribute to suppress ω phase formation in a large class of alloys. ©2019 American Physical Society.

The radiative behaviour of coal and char particles is an important input parameter for simulations of coal combustion and gasification processes, as those typically feature elevated reactor temperatures. For consideration of radiative heat transfer, accurate knowledge of particle emissivity is a pre-requisite. Combining theoretical considerations and experimental data from literature, a temperature dependent relation for char emissivity is provided. © 2019 Elsevier Ltd

Titanium silicon carbide (Ti3SiC2) powder was synthesized by molten salt shielded synthesis route of elemental reactants. Potassium bromide (KBr) was used for gas-tight encapsulation of the consolidated reaction mixture for further high temperature processing. The synthesis of Ti3SiC2 powder was carried out in air, the salt cladding and molten salt pool provided for the protection of the material against oxidation both at low and high temperature. The process yielded free flowing Ti3SiC2 powders without the need of a milling step. Al addition to the reaction mixture resulted in a high purity (96 wt. %) of Ti3SiC2 at a synthesis temperature of 1250 °C. The synthesized micro-metric Ti3SiC2 can be milled to nano-metric powders. © 2019 Elsevier Ltd

To improve the photocatalytic oxidation of α-C−H bonds in unsaturated hydrocarbons, N-hydroxyphthalimide (NHPI) was used as a molecular cocatalyst with CdS as the photoabsorber. Compared with previously reported photocatalysts involving solid cocatalysts, metal-free NHPI offers better sustainability in addition to the significantly enhanced performance as cocatalyst. The photogenerated holes were transferred into the more active phthalimide-N-oxyl radical (PINO) by reacting with NHPI. In this way, α-C−H bond oxidation was significantly improved through the activation by PINO; even for the sluggish toluene oxidation, the apparent quantum efficiency was as high as 36.5 %. The effects of substrates/NHPI concentration ratio, reaction temperature, and time as well as the reaction intermediates were comprehensively studied. It was possible to identify ketones/aldehydes as the primary products, and overoxidation was controlled by adjusting the substrates/NHPI concentration ratio and reaction time. Thus, the radical path induced by the NHPI–PINO redox pair is an efficient alternative to boost the sluggish photocatalytic oxidation of α-C−H bonds. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

The formation of dislocation substructures in up to 10 µm deep subsurface regions of two aluminium alloys, Al-5Mg and Al-11Zn, was investigated under conditions of high homologous temperature reciprocal sliding wear (HT-RSW). Under creep conditions, Al-5Mg shows a solid solution type of inverse primary creep. In contrast, Al-11Zn creeps obstacle controlled and exhibits normal primary creep. These two materials were subjected to reciprocal sliding wear at 200 and 300 °C for 100 and 1000 cycles. Flat polished disks were exposed to the 1 mm reciprocal movements of a spherical aluminium oxide counterbody under normal forces of 5 and 10 N at an oscillation frequency of 1 Hz. Using focused ion beam (FIB) micromachining thin electron transparent foils were prepared from the surface regions of the as received and worn material states. Transmission electron microscopy (TEM) was used to study the evolution of nano and micro grain sizes in the surface regions. Despite the different creep behavior, the two materials behave similar under conditions of reciprocal sliding wear. The results obtained in the present work show that subgrain sizes decrease with increasing numbers of wear cycles and increasing normal forces. Subgrain sizes also increase with increasing temperature. At 300 °C, dynamic recrystallization was observed in both Al-alloys. The results of the present work are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. © 2018 Elsevier B.V.

Synthesis of elusive K4O6 has disclosed implications of crucial relevance for new solid materials discovery. K4O6 forms in equilibrium from K2O2 and KO2, in an all-solid state, endothermic reaction at elevated temperature, undergoing back reaction upon cooling to ambient conditions. This tells that the compound is stabilized by entropy alone. Analyzing possible entropic contributions reveals that the configurational entropy of “localized” electrons, i.e., of polaronic quasi-particles, provides the essential contribution to the stabilization. We corroborate this assumption by measuring the relevant heats of transformation and tracking the origin of entropy of formation computationally. These findings challenge current experimental and computational approaches towards exploring chemical systems for new materials by searching the potential energy landscape: one would fail in detecting candidates that are crucially stabilized by the configurational entropy of localized polarons. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

In developing the next generation of Calphad databases, new models are used in which each term contributing to the Gibbs energy has a physical meaning. To continue the development, finite temperature density-functional-theory (DFT) results are used in the present work to discuss and suggest the most applicable and physically based model for Calphad assessments of solid phases above the melting point (the breakpoint for modeling the solid phase in previous assessments). These results are applied to investigate the properties of a solid in the superheated temperature region and to replace the melting temperature as the breakpoint with a more physically based temperature, i.e., where the superheated solid collapses into the liquid. The advantages and limitations of such an approach are presented in terms of a new assessment for unary aluminum. © 2019 Elsevier Ltd

The influence of the optical excitation density on the electron spin dynamics is experimentally investigated in bulk cubic GaN by time-resolved magneto-optical Kerr-rotation spectroscopy. The nanosecond spin relaxation times in moderately n-doped β-GaN decrease with increasing excitation density, though the effective lifetimes of the optically excited carriers are almost two orders of magnitude shorter than the spin relaxation times. This counterintuitive finding is explained by the heating of the electron system due to the excitation process. The spin relaxation times in degenerately n-doped β-GaN are found to be independent of excitation density as the very high electron Fermi temperature completely dominates over carrier heating processes in this case. © 2019 Author(s).

Strain-induced martensite (SIM) formation was evaluated upon cold-rolling of a 17.6 wt.%Mn-TRIP steel by means of magnetic measurements, X-ray diffraction, and high-resolution electron backscatter diffraction (EBSD). α′-martensite formation was observed to be dependent on the presence of prior ε-martensite. Upon deformation, the coercivity of the ferromagnetic α′-martensite is characterized by strong magnetic shape anisotropy. Austenite (γ) reversion was evaluated by means of in situ magnetic measurements during continuous annealing. The experimental results were compared to thermodynamic simulations. It turned out that γ-reversion was not completed in the regime where a γ-single phase field is expected, which suggests the splitting of α′ → γ transformation into two stages. The Curie temperature of remaining α′-martensite was determined as being ∼620 °C. Magnetic properties presented an annealing time-dependence within the temperature range of 500–600 °C, suggesting long-range diffusional α′ → γ transformation. With the aid of electron channeling contrast image technique (ECCI), we noticed that the formation of γ-nanograins in the early stages of reversion is sufficient to induce strong magnetic shape anisotropy in this steel. After full austenitization at 800 °C, further in situ magnetic measurements were also used to track the magnetic response of the material upon controlled cooling. Athermal formation of α′-martensite within the prior athermal ε-phase was clearly observed for temperatures lower than 100 °C. Using thermodynamic modeling we also calculated the start temperature for ε-formation (Ms ε). Results showed that ε-martensite is indeed expected to form before α′ which was confirmed in all cases by means of EBSD. © 2018 Elsevier B.V.

The degradation of mechanical properties due to sintering is one of the major issues during high temperature service of thermal barrier coating system for advanced gas turbines. In this study, a constitutive model was developed by the variational principle, based on the experimentally observed microstructure features of suspension plasma-sprayed thermal barrier coatings. The constitutive model was further implemented in finite element analysis software, in order to investigate the effect of vertical cracks. The evolution of microstructure during sintering, coating shrinkage and mechanical degradation were predicted. The numerical predictions of Young's modulus were generally in agreement with experimental results. Furthermore, the effect of vertical cracks on the strain tolerance and sintering resistance were discussed. It was confirmed that the introduction of vertical cracks contributed to the improvement of both properties. © 2019 The American Ceramic Society

Among equiatomic alloys of the Cr-Mn-Fe-Co-Ni system, MnFeNi was shown to exhibit a strong anti-invar behavior but little is known regarding its mechanical properties. The objective of the present study is to investigate Hall–Petch strengthening by grain and annealing twin boundaries in MnFeNi. For this purpose, seven different grain sizes between 17 and 216 µm were produced. Mean grain sizes (excluding annealing twin boundaries) and crystallite sizes (including them) were determined using the linear intercept method. Overall, 25% of the boundaries were found to be annealing twin boundaries regardless of the grain size. In some cases, two twin boundaries can be present in one grain forming an annealing twin, which thickness represents one quarter of the mean grain size. Based on a comparison of the mean twin thickness of different alloys with different stacking fault energy (SFE), we estimated an SFE of 80 ± 20 mJ/m 2 for MnFeNi. Compression tests of MnFeNi with different grain sizes were performed between 77 and 873 K and revealed a parallel shift of the Hall–Petch lines with temperature. The interaction between dislocations and boundaries was investigated by scanning transmission electron microscopy (STEM) in a deformed specimen. It was found that a large number of dislocations are piling up against grain boundaries while the pile-ups at annealing twin boundaries contain much fewer dislocations. This indicates that annealing twin boundaries in this alloy are less effective obstacles to dislocation motion than grain boundaries. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.

Development of metal-free carbon-based electrocatalysts for reducing oxygen to water (ORR), preferentially following a 4 electron transfer pathway, is of high importance. We present a two-step synthesis of N-doped carbon-based ORR electrocatalysts by using an efficient thermal treatment of hydrothermally carbonized cellulose in ammonia combining devolatilization, reduction and nitrogen doping. The influence of the synthesis temperature as well as of the ammonia concentration used during the synthesis on the electrocatalytic ORR activity was analyzed using bulk- and surface-sensitive techniques. Correlation of electrocatalytic activity with structural features of the catalysts provided deeper mechanistic understanding and enabled us to optimize the synthesis conditions. The nitrogen-doped metal-free catalyst originating from the treatment in 100 % NH3 at 800 °C achieved a current density of −1 mA cm−2 at 0.83 V vs. RHE positioning it among the most active noble-metal free and biomass-based ORR catalysts reported so far. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

The iron aluminides discussed here are Fe-Al-based alloys, in which the matrix consists of the disordered bcc (Fe,Al) solid solution (A2) or the ordered intermetallic phases FeAl (B2) and Fe3Al (D03). These alloys possess outstanding corrosion resistance and high wear resistance and are lightweight materials relative to steels and nickel-based superalloys. These materials are evoking new interest for industrial applications because they are an economic alternative to other materials, and substantial progress in strengthening these alloys at high temperatures has recently been achieved by applying new alloy concepts. Research on iron aluminides started more than a century ago and has led to many fundamental findings. This article summarizes the current knowledge of this field in continuation of previous reviews. © 2019 by Annual Reviews. All rights reserved.

We combine a phase-field model with a crystal plasticity model to simulate the microstructural evolution during creep in the Co-based superalloy ERBOCo-2Ta. Three-dimensional simulations of tensile and compressive creep tests in [100] direction were performed to study the rafting behavior in Co-based superalloys. The loss of coherency between γ matrix and γ′ precipitate, which is essential for the understanding of rafted structures, is modeled in relation to the dislocation activity in the γ-channels. Special attention is given to the interplay between creep deformation and microstructure stability. Appropriate constitutive modeling is applied to simulate realistic microstructure evolution under creep conditions. Thus, with the removal of the misfit stress, γ′ precipitates lose their cuboidal shape and form rafts. During N-type rafting more γ′ precipitates coalesce than during P-type rafting. The γ′ volume fraction during rafting increases under tensile stress but decreases under compressive stress. The morphological evolution of γ′ precipitates under tensile and compressive stresses in Co-based superalloy is consistent with the rafting characteristics in experimental observations. © 2019 Acta Materialia Inc.

The etching of sub micrometer high-aspect-ratio (HAR) features into dielectric materials in low pressure radio frequency technological plasmas is limited by the accumulation of positive surface charges inside etch trenches. These are, at least partially, caused by highly energetic positive ions that are accelerated by the sheath electric field to high velocities perpendicular to the wafer. In contrast to these anisotropic ions, thermal electrons typically reach the electrode only during the sheath collapse and cannot penetrate deeply into HAR features to compensate the positive surface charges. This problem causes significant reductions of the etch rate and leads to deformations of the features due to ion deflection, i.e. the aspect ratio is limited. Here, we demonstrate that voltage waveform tailoring can be used to generate electric field reversals adjacent to the wafer during sheath collapse to accelerate electrons towards the electrode to allow them to penetrate deeply into HAR etch features to compensate positive surface charges and to overcome this process limitation. Based on 1D3V particle-in-cell/Monte Carlo collision simulations of a capacitively coupled plasma operated in argon at 1 Pa, we study the effects of changing the shape, peak-to-peak voltage, and harmonics' frequencies of the driving voltage waveform on this electric field reversal as well as on the electron velocity and angular distribution function at the wafer. We find that the angle of incidence of electrons relative to the surface normal at the wafer can be strongly reduced and the electron velocity perpendicular to the wafer can be significantly increased by choosing the driving voltage waveform in a way that ensures a fast and short sheath collapse. This is caused by the requirement of flux compensation of electrons and ions at the electrode on time average in the presence of a short and steep sheath collapse. © 2019 IOP Publishing Ltd.

Phonon measurements in the A15-type superconductors were complicated in the past because of the unavailability of large single crystals for inelastic neutron scattering, e.g., in the case of Nb3Sn, or unfavorable neutron scattering properties in the case of V3Si. Hence, only few studies of the lattice dynamical properties with momentum resolved methods were published, in particular below the superconducting transition temperature Tc. Here, we overcome these problems by employing inelastic x-ray scattering and report a combined experimental and theoretical investigation of lattice dynamics in V3Si with the focus on the temperature-dependent properties of low-energy acoustic phonon modes in several high-symmetry directions. We paid particular attention to the evolution of the soft phonon mode of the structural phase transition observed in our sample at Ts=18.9K, i.e., just above the measured superconducting phase transition at Tc=16.8K. Theoretically, we predict lattice dynamics including electron-phonon coupling based on density-functional-perturbation theory and discuss the relevance of the soft phonon mode with regard to the value of Tc. Furthermore, we explain superconductivity-induced anomalies in the lineshape of several acoustic phonon modes using a model proposed by Allen et al, [Phys. Rev. B 56, 5552 (1997)10.1103/PhysRevB.56.5552]. © 2019 American Physical Society.

Cu(In,Ga)S2-based solar cells are interesting tandem partners for Si or chalcopyrite solar cells, but suffer from a low open-circuit voltage. Recently, record efficiencies have been achieved by using higher growth temperatures for the absorber. To understand the effect of higher growth temperatures, we investigate the structural and electronic properties of CuInS2 absorbers. By investigating the absorber alone as opposed to complete solar cells, we can separate changes in the absorber from effects of the interface properties. We show that the quasi-Fermi-level splitting, which indicates the maximum open-circuit voltage an absorber is capable of, increases with higher growth temperature. The quasi-Fermi-level splitting is limited by a deep defect, the concentration of which decreases with higher growth temperature and is less prominent in Cu-rich films. Thus, we demonstrate that the open-circuit voltage of CuInS2-based solar cells is limited to below 850 mV by the absorber itself, independent of the interface. In contrast to the changes in the electronic properties, the structural properties are rather independent of temperature within the range investigated but are significantly influenced by the composition. © 2019 authors. Published by the American Physical Society.

We obtain phonon lifetimes in aluminium by inelastic neutron scattering experiments, by ab initio molecular dynamics, and by perturbation theory. At elevated temperatures significant discrepancies are found between experiment and perturbation theory, which disappear when using molecular dynamics due to the inclusion of full anharmonicity and the correct treatment of the multiphonon background. We show that multiple-site interactions are small and that local pairwise anharmonicity dominates phonon-phonon interactions, which permits an efficient computation of phonon lifetimes.

To facilitate the understanding of Invar effects and design of FeNiCo-base Invar alloys characterized by low thermal expansion coefficient (TEC), we investigated the magnetic and thermal expansion behavior of an equiatomic prototype medium entropy alloy FeNiCo and a non-equiatomic (super Invar)Fe63Ni32Co5 (at. %)reference alloy by means of experiments and ab initio calculations. Both alloys consist of a single face-centered cubic phase with fully recrystallized microstructure in the homogenized state. Large spontaneous volume magnetostriction is observed in both alloys below their respective Curie temperatures. The Invar effect in the non-equiatomic Fe63Ni32Co5 alloy is of step-type with nearly zero TEC over a wide temperature range (from room temperature to 120 °C)below its Curie temperature. The equiatomic FeNiCo alloy shows a peak-type Invar effect in a very narrow temperature range (from ∼675 °C to ∼730 °C)with relatively low TECs. The equiatomic FeNiCo alloy shows both higher saturation magnetization and Curie temperature than the non-equiatomic Fe63Ni32Co5 alloy. The relationships among magnetic behavior, spontaneous volume magnetostriction and Invar effects for a wider array of metallic alloys are discussed mainly based on Masumoto's rule combined with Wohlfarth's itinerant electron theory. An Invar alloy search map is constructed based on the present results and available literature data to visualize the relationships among saturation magnetization, Curie temperature and thermal expansion coefficient for a wide range of Invar alloys. Based on this treasure map a design route for further developments of new Invar alloys by tuning their magnetic properties is discussed. © 2019 Elsevier Ltd

We investigate the influence of the annealing temperature on the evolution of the ice nanoclusters’ geometry by means of low-temperature scanning tunneling microscopy. The clusters, grown at 110 K on Ag(100), gradually increase in height and their shape becomes more compact during annealing at 120 K, 125 K, and 130 K. The increase in height indicates an upward mass transport and reflects a stronger water-water than water-surface bonding. The change in shape, quantitatively expressed as an increase in fractal dimension, is driven by a reduction of the total energy of the step edge. The significant changes in geometry induced by a relatively mild temperature increase underline the importance of temperature for the shape and all properties influenced by this shape of hydrogen-bonded clusters of water ice. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A first-principles-based computational tool for simulating phonons of magnetic random solid solutions including thermal magnetic fluctuations is developed. The method takes fluctuations of force constants due to magnetic excitations as well as due to chemical disorder into account. The developed approach correctly predicts the experimentally observed unusual phonon hardening of a transverse acoustic mode in Fe-Pd an Fe-Pt Invar alloys with increasing temperature. This peculiar behavior, which cannot be explained within a conventional harmonic picture, turns out to be a consequence of thermal magnetic fluctuations. The proposed methodology can be straightforwardly applied to a wide range of materials to reveal new insights into physical behaviors and to design materials through computation, which were not accessible so far. © 2018 The Author(s).

Ti-Ta-X (X = Al, Sn, Zr) compounds are emerging candidates as high-temperature shape memory alloys (HTSMAs). The stability of the one-way shape memory effect (1WE), the exploitable pseudoelastic (PE) strain intervals, as well as the transformation temperature in these alloys depend strongly on composition, resulting in a trade-off between a stable shape memory effect and a high transformation temperature. In this work, experimental measurements and first-principles calculations are combined to rationalize the effect of alloying a third component to Ti-Ta-based HTSMAs. Most notably, an increase in the transformation temperature with increasing Al content is detected experimentally in Ti-Ta-Al for low Ta concentrations, in contrast to the generally observed dependence of the transformation temperature on composition in Ti-Ta-X. This inversion of trend is confirmed by the ab initio calculations. Furthermore, a simple analytical model based on the ab initio data is derived. The model can not only explain the unusual composition dependence of the transformation temperature in Ti-Ta-Al but also provide a fast and elegant tool for a qualitative evaluation of other ternary systems. This is exemplified by predicting the trend of the transformation temperature of Ti-Ta-Sn and Ti-Ta-Zr alloys, yielding a remarkable agreement with available experimental data. © 2018 American Physical Society.

Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air. Although this contamination can change the morphology and properties of the consolidated materials, there is a lack of detailed information about the behavior of oxygen in nanocrystalline alloys. In this study, aberration-corrected high-resolution transmission electron microscopy and associated techniques are used to investigate the behavior of oxygen during in situ heating of highly strained Cu-Fe alloys. Contrary to expectations, oxide formation occurs prior to the decomposition of the metastable Cu-Fe solid solution. This oxide formation commences at relatively low temperatures, generating nanosized clusters of firstly CuO and later Fe2O3. The orientation relationship between these clusters and the matrix differs from that observed in conventional steels. These findings provide a direct observation of oxide formation in single-phase Cu-Fe composites and offer a pathway for the design of nanocrystalline materials strengthened by oxide dispersions. © 2018 The Author(s).

A fast and cost efficient approach to fabricate multiple NIL templates with inverse image tone, modified topography and tunable feature sizes is presented. The nanopatterns from the negative master is inverted to positive structures in the produced NIL templates using a UV-curable bi-layer lift-off process which excludes high temperature baking. The bi-layer consisting of a 150 nm sacrificial layer of pure organic resist and a 200 nm patterning resist of inorganic/organic composite is employed. The topography on the new NIL templates are in square layout which is generated from a single master with circular nanoholes. The feature sizes on the master are shrunken down to sub-200 nm (120/200/250/300 nm) in a preciously controllable manner. Nanostructures up to 150 mm wafer scale are transferred by soft UV-NIL using an ambient center-to-edge scheme. The feature sizes of openings in the patterning layer can be precisely and controllably tuned taking advantage of an intermediate template with nanopyramids that is produced from the master. The organic sacrificial layer is descummed and underetched by oxygen plasma. Furthermore, 40 nm Chromium is evaporated and the sacrificial layer along with the patterning layer is lifted off by wet chemical stripper TechniStrip P1316. Silicon etching using the Chromium etch-mask is engaged in for smooth and vertical sidewalls for NIL templates. © 2018 Elsevier B.V.

We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 °C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations, we predicted the growth of M23C6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 °C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable γ-M23C6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth. © 2018 Acta Materialia Inc.

The multipole resonance probe (MRP) is a powerful and economical diagnostic tool for the determination of process-relevant plasma parameters. Due to its in-situ measurement concept even spatially resolved information of the plasma under investigation is provided. In order to minimize the influence of the sensor on the process, the planar multipole resonance probe (pMRP) was introduced as a minimally invasive monitoring tool, mounted into the reactor wall. For an effective application of these sensors in a wide field of real processes, industry compatible implementations are required. In this paper, advanced realizations of the MRP and pMRP based on LTCC-technology are presented, which are applicable for the supervision and control of plasma processes at high temperatures: the mathbf{MRP}-{mathbf{LTCC}} and the stacked mathbf{pMRP}-{mathbf{LTCC}}. The latter represents a novel compact design of the pMRP using an LTCC multilayer structure with vertically stacked components. Both sensors are investigated within 3D electromagnetic simulations and compared to measurements performed in a double inductively coupled plasma (DICP). © 2018 European Microwave Association.

NiFe layered double hydroxide (LDH) is inarguably the most active contemporary catalyst for the oxygen evolution reaction under alkaline conditions. However, the ability to sustain unattenuated performance under challenging industrial conditions entailing high corrosivity of the electrolyte (≈30 wt. % KOH), high temperature (>80 °C) and high current densities (>500 mA cm−2) is the ultimate criterion for practical viability. This work evaluates the chemical and structural stability of NiFe LDH at conditions akin to practical electrolysis, in 30 % KOH at 80 °C, however, without electrochemical polarization, and the resulting impact on the OER performance of the catalyst. Post-analysis of the catalyst by means of XRD, TEM, FT-IR, and Raman spectroscopy after its immersion into 7.5 m KOH at 80 °C for 60 h revealed a transformation of the structure from NiFe LDH to a mixture of crystalline β-Ni(OH)2 and discrete predominantly amorphous FeOOH containing minor non-homogeneously distributed crystalline domains. These structural and compositional changes led to a drastic loss of the OER activity. It is therefore recommended to study catalyst stability at industrially relevant conditions. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

This work investigates the separate influence of porosity and γ/γ′-eutectics on the low-cycle fatigue life of a single-crystal Ni-base superalloy at high temperatures. A conventional vacuum furnace heat-treatment but also integrated heat-treatments in a hot isostatic press are applied to produce different material variants of the same alloy. High-resolution electron microscopy revealed that both pores and γ/γ′-eutectics act as crack starters, thus initiating early failure. Moreover, the results indicate that remaining γ/γ′-eutectics can weaken the fatigue resistance even more than pores. Furthermore, the results confirm the beneficial effect of proper integrated hot isostatic pressing heat-treatments on the fatigue performance. © 2018

We report an angle-resolved photoemission spectroscopy study of the iron-based superconductor family, Ba1-xNaxFe2As2. This system harbors the recently discovered double-Q magnetic order appearing in a reentrant C4 phase deep within the underdoped regime of the phase diagram that is otherwise dominated by the coupled nematic phase and collinear antiferromagnetic order. From a detailed temperature-dependence study, we identify the electronic response to the nematic phase in an orbital-dependent band shift that strictly follows the rotational symmetry of the lattice and disappears when the system restores C4 symmetry in the low temperature phase. In addition, we report the observation of a distinct electronic reconstruction that cannot be explained by the known electronic orders in the system. © 2018 American Physical Society.

Currently, additive manufacturing (AM) experiences significant attention in nearly all industrial sectors. AM is already well established in fields such as medicine or spare part production. Nevertheless, processing of high-performance nickel-based superalloys and especially single crystalline alloys such as CMSX-4® is challenging due to the difficulty of intense crack formation. Selective electron beam melting (SEBM) takes place at high process temperatures (~ 1000 °C) and under vacuum conditions. Current work has demonstrated processing of CMSX-4® without crack formation. In addition, by using appropriate AM scan strategies, even single crystals (SX SEBM CMSX-4®) develop directly from the powder bed. In this contribution, we investigate the mechanical properties of SX SEBM CMSX-4® prepared by SEBM in the as-built condition and after heat treatment. The focus is on hardness, strength, low cycle fatigue, and creep properties. These properties are compared with conventional cast and heat-treated material. © 2018 The Author(s)

External matching networks are crucial and necessary for operating capacitively coupled plasmas in order to maximize the absorbed power. Experiments show that external circuits in general heavily interact with the plasma in a nonlinear way. This interaction has to be taken into account in order to be able to design suitable networks, e.g., for plasma processing systems. For a complete understanding of the underlying physics of this coupling, a nonlinear simulation approach which considers both the plasma and the circuit dynamics can provide useful insights. In this work, the coupling of an equivalent circuit plasma model and an external electric circuit composed of lumped elements is discussed. The plasma model itself is self-consistent in the sense that the plasma density and the electron temperature is calculated from the absorbed power based on a global plasma chemistry model. The approach encompasses all elements present in plasma systems, i.e., the discharge itself, the matching network, the power generator as well as stray loss elements. While the main result of this work is the conceptual approach itself, at the example of a single-frequency capacitively coupled discharge its applicability is demonstrated. It is shown that it provides an effective and efficient way to analyze and understand the nonlinear dynamics of plasma systems including the external circuit and, furthermore, may be applied to synthesize optimal matching networks. © 2018 IOP Publishing Ltd.

Thermal barrier coatings (TBCs) based on yttria-stabilized zirconia (YSZ) were deposited by Atmospheric Plasma Spray (APS) on highly dense and pure Cr2AlC substrates. The Cr2AlC/YSZ systems were tested under thermal cycling conditions at temperatures between 1100 and 1300 °C testing up to 500 h. The response of the system was excellent due to the strong adhesion between the substrate and the coating, and the formation of an outer and protective layer based on α-Al2O3. The oxide scale is formed due to the diffusion of Al atoms from the crystal structure of the Cr2AlC, followed by the reaction with oxygen in the air. The thickness of the oxide scale was 8.9, 17.6 and 39.7 μm at 1100, 1200 and 1300 °C, respectively, which is rather thick in comparison with the classical superalloy/TBC systems. Cr2AlC/YSZ systems survived without any damage under the severe cycling conditions at 1100 and 1200 °C due to the protective oxide scale layer and the sufficient thermal expansion match between the Cr2AlC, YSZ and α-Al2O3. At 1300 °C and after 268 h of cycling conditions, the system failed due to the formation of a porous carbide layer underneath of the oxide scale. The results are rather promising and confirm the potential of the MAX phases to operate under long term applications of high temperature and oxidizing environments. © 2018 Elsevier B.V.

The temperature-dependent intrinsic stacking fault Gibbs energy is computed based on highly converged density-functional-theory (DFT) calculations for the three prototype face-centered cubic metals Al, Cu, and Ni. All relevant temperature-dependent contributions are considered including electronic, vibrational, magnetic, and explicit anharmonic Gibbs energy contributions as well as coupling terms employing state-of-the-art statistical sampling techniques. Particular emphasis is put on a careful comparison of different theoretical concepts to derive the stacking fault energy such as the axial-next-nearest-neighbor-Ising (ANNNI) model or the vacuum-slab approach. Our theoretical results are compared with an extensive set of previous theoretical and experimental data. Large uncertainties in the experimental data highlight the necessity of complementary parameter-free calculations. Specifically, the temperature dependence is experimentally unknown and poorly described by thermodynamic databases. Whereas calphad derived data shows an increase of the stacking fault energy with temperature for two of the systems (Cu and Ni), our results predict a decrease for all studied systems. For Ni, the temperature induced change is in fact so strong that in the temperature interval relevant for super-alloy applications the stacking fault energy falls below one third of the low temperature value. Such large changes clearly call for a revision of the stacking fault energy when modeling or designing alloys based on such elements. © 2018 authors.

Pretreating raw biomass via torrefaction changes fuel specific properties like grindability, volatile content, energy density and biochemical stability and thus enables an enhanced fuel replacement for pulverized fossil fuel fired furnaces. In this study, the influence of torrefaction temperature on devolatilization behavior is investigated in a small-scale fluidized bed reactor approximating flame-equivalent conditions. Therefore the pyrolysis products of two different biofuels with varying degree of torrefaction are determined via ex-situ FTIR gas analysis in an N2 atmosphere in the temperature range from 873 to 1473 K. Furthermore, the mass fraction of residual char particles is determined by adding O2 to the fluidizing gas and analyzing the burnout products. Char fraction and volatile composition are used to estimate the energy release distribution between homogeneous volatile combustion and heterogeneous char burnout. The experiments revealed enlarging char yields at the expense of volatile yields with increasing degree of torrfaction at all investigated pyrolysis temperatures. Furthermore, torrefaction favors higher fractions of CO2 and lower fractions of CO and C2Hx in the light gas. Further on, no significant impact of torrefaction conditions on the tar composition could be identified. The calculation of higher heating value (HHV) based on char yield and gas composition reveals an overall increase of HHV, while the relative contribution from the volatile fraction decreases with increasing degree of torrefaction. Following this, an increase of torrefaction degree will shift combustion from a high intense volatile combustion in the near burner region towards a less intense but prolonged char conversion in the far burner region. © 2018 Elsevier Ltd

Quantum-mechanical calculations are used to determine the temperature dependence of the Gibbs energy of vacancy formation in nickel. Existing data reveal a discrepancy between the high-temperature estimates from experiments and low-temperature approximations from density functional theory. Our finite-temperature calculations - which include the effects of magnetism and fully interacting phonon vibrations - demonstrate that this discrepancy is mostly caused by the previously neglected explicit anharmonic contribution. © 2018 authors. Published by the American Physical Society.

Nanoscale solute segregation to or near lattice defects is a coupled diffusion and trapping phenomenon that occurs in superalloys at high temperatures during service. Understanding the mechanisms underpinning this crucial process will open pathways to tuning the alloy composition for improving the high-temperature performance and lifetime. Here, we introduce an approach combining atom probe tomography with high-end scanning electron microscopy techniques, in transmission and backscattering modes, to enable direct investigation of solute segregation to defects generated during high-temperature deformation such as dislocations in a heat-treated Ni-based superalloy and planar faults in a CoNi-based superalloy. Three protocols were elaborated to capture the complete structural and compositional nature of the targeted defect in the alloy. © 2018 The Author(s)

Shear testing can contribute to a better understanding of the plastic deformation of Ni-base superalloy single crystals. In the present study, shear testing is discussed with special emphasis placed on its strengths and weaknesses. Key mechanical and microstructural results which were obtained for the high-temperature (T ≈ 1000 °C) and low-stress (τ ≈ 200 MPa) creep regime are briefly reviewed. New 3D stereo STEM images of dislocation substructures which form during shear creep deformation in this regime are presented. It is then shown which new aspects need to be considered when performing double shear creep testing at lower temperatures (T < 800 °C) and higher stresses (τ > 600 MPa). In this creep regime, the macroscopic crystallographic [11−2](111) shear system deforms significantly faster than the [01−1](111) system. This represents direct mechanical evidence for a new planar fault nucleation scenario, which was recently suggested (Wu et al. in Acta Mater 144:642–655, 2018). The double shear creep specimen geometry inspired a micro-mechanical in-situ shear test specimen. Moreover, the in-situ SEM shear specimen can be FIB micro-machined from prior dendritic and interdendritic regions. Dendritic regions, which have a lower γ′ volume fraction, show a lower critical resolved shear stress. © 2018 The Author(s)

The segregation of solutes at dislocations in a polycrystalline and a single crystal nickel-based superalloy is studied. Our observations confirm the often assumed but yet unproven diffusion along dislocations via pipe diffusion. Direct observation and quantitative, near-atomic scale segregation of chromium and cobalt at dislocations within γ' precipitates and at interfacial dislocations leading to the partial or complete dissolution of γ' precipitates at elevated temperatures is presented. Our results allow us to elucidate the physical mechanism by which pipe diffusion initiates the undesirable dissolution of γ' precipitates. © 2017 Acta Materialia Inc.

Mo-Si-B alloys are potential creep resistant materials for accessing harsh loading scenarios beyond Ni-based superalloys due to their excellent mechanical performance at ultra-high temperatures (> 1200 °C). Here, we report on the fabrication through laser additive manufacturing of a Mo rich Mo-Si-B alloy with and without dispersion of oxide (La2O3) particles. The major phase in the solidified material is dendritic α-Mo. The inter-dendritic regions contain a mixture of the Mo5Si3 (T1) + Mo5SiB2 (T2) phases, and not the expected equilibrium Mo3Si + Mo5SiB2 (T2) phases. This combination of phases is shown to yield improved high temperature creep resistance but was only accessible through by addition of Nb, W or Ti that substitute Mo in the intermetallic phases. Whereas here it is attributed to the large undercooling in the small melt pool produced during laser processing. We show that this phase mixture, upon annealing, is stable at 1200 °C for 200 h. We also demonstrate successful dispersion of oxide particles mainly in the inter-dendritic regions leading to a high indentation fracture toughness of ∼18 MPa√m at room temperature. Toughening originates from crack trapping in the ductile α-Mo and the formation of micro-cracks and crack deflection in the vicinity of oxide particles. © 2018 Acta Materialia Inc.

An ex situ approach combining fast quenching experiments in a dilatometer and postmortem microstructural observation in transmission electron microscopy (TEM) has been used to observe the dynamic microstructure change during differential scanning calorimetry (DSC) ramping of an Fe–10Cr–0.15C (wt%) alloy fabricated from high-purity components. The DSC measurements reveal two exothermic events at temperatures about 270 and 600 °C in a heating process. The two events were discerned by TEM investigations on specimens interrupted during thermal ramping in a dilatometer. It is found that precipitation and growth of M3C carbide occurred first in a temperature range between 200 and 400 °C, following the Bagaryatskii orientation relationship. Subsequently, M7C3 carbides precipitate on prior martensitic laths boundaries in a temperature range between 500 and 700 °C at the expense of M3C. M23C6 carbides were found precipitating on the interface between M7C3 and matrix at approximately the same time with the precipitation of M7C3. The obtained results are also compared with an in situ TEM heating experiment, and differences between the two approaches are discussed. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.

Although the phase stability of high-entropy alloys in the Cr-Mn-Fe-Co-Ni system has received considerable attention recently, knowledge of their thermodynamic equilibrium states and precipitation kinetics during high-temperature exposure is limited. In the present study, an off-equiatomic Cr26Mn20Fe20Co20Ni14 high-entropy alloy was solutionized and isothermally aged at temperatures between 600 °C and 1000 °C for times to 1000 h. In the original single-phase fcc matrix, an intermetallic σ phase was found to form at all investigated temperatures. Its morphology and composition were determined and the precipitation kinetics analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law. From these analyses, a time-temperature-transformation diagram (TTT diagram) is constructed for this off-equiatomic alloy. We combine our findings with theories of precipitation kinetics developed for traditional polycrystalline fcc alloys to calculate a TTT diagram for the equiatomic CrMnFeCoNi HEA. The results of our investigation may serve as a guide to predict precipitation kinetics in other complex alloys in the Cr-Mn-Fe-Co-Ni system. © 2018 Acta Materialia Inc.

Abstract: A continuously operated flow setup with fixed-bed reactor and online gas analysis enabled kinetic investigations of catalysts for the carbon dioxide reforming of methane under industrially relevant conditions at temperatures up to 1000 °C and at pressures up to 20 bar. A coaxial reactor design consisting of an inner- and an outer highly alloyed steel tube allowed obtaining axial temperature profiles by means of a moveable thermocouple. A NiAl2O4-based catalyst was tested at 820 °C and pressures of 1, 10 or 20 bar and compared to a conventional Ni catalyst used for steam reforming of methane. A significant cold spot was detected even when using only 10 mg of catalysts diluted in 1 g of silicon carbide. The specifically designed NiAl2O4/Al2O3 dry reforming catalyst with a high dispersion of the active Ni0 phase was found to be far superior to the conventional steam reforming catalyst. Graphical Abstract: [Figure not available: see fulltext.] © 2018, Springer Science+Business Media, LLC, part of Springer Nature.

Background Microcalcifications within the fibrous cap of the arteriosclerotic plaques lead to the accrual of plaque-destabilizing mechanical stress. New techniques for plaque screening with small detectors and the ability to differentiate between the smooth and hard elements of plaque formation are necessary. Method Vascular plaque formations are characterized as calcium phosphate containing structures organized as hydroxylapatite resembling the mineral whitlockite. In transmission and reflexion studies with a simple millimeter wave (mm-wave)-demonstrator, we found that there is a narrow window for plaque detection in arterial vessels because of the tissue water content, the differentiation to fatty tissue, and the dielectric property of air or water, respectively. Result The new sensor is based on a sensing oscillator working around 27 GHz. The open-stub capacitance determines the operating frequency of the sensor oscillator. The capacitance depends on the dielectric properties of the surrounding material. The sensor components were completely built up in surface mount technique. Conclusion Completed with a catheter, the sensor based on microwave technology appears as a robust tool ready for further clinical use. © 2018 Georg Thieme Verlag KG Stuttgart, New York.

Co-Ni-Ga high-temperature shape memory alloys (HT-SMAs) are well-known candidate materials for damping applications at elevated temperatures. Recent studies showed that upon heat treatment in stress-induced martensite under compressive loads transformation temperatures can be increased significantly, qualifying Co-Ni-Ga for HT-actuation. The increase in transformation temperatures is related to a change in chemical order recently validated via neutron diffraction experiments. Since SMAs show distinct tension-compression asymmetry in terms of theoretical transformation strains and bearable stresses, understanding the impact of martensite aging in tension is crucial for future applications. The current results indicate that martensite aging in tension provides for a further improvement in functional properties. © 2018 The Author(s).

The coefficient of thermal expansion and elastic stiffnesses of spinel structure MgGa2O4 were determined from 103 K to 1673 K using dilatometry and resonant ultrasound spectroscopy. The state of cation order was investigated on specimens quenched from temperatures up to 1473 K via single-crystal X-ray diffraction. Even at room-temperature, the material is stiffer than what was expected from DFT simulations at 0 K, however, the stiffness falls within the predicted range based on the stiffness of the constituent oxides of MgGa2O4. The anisotropy of its longitudinal elastic stiffness is low, whereas there is a high anisotropy of the shear resistance compared to other cubic materials. At about 820 K-860 K, the temperature dependences of both thermal expansion and elastic properties change rapidly. Cation reordering also starts in this temperature range; the state of order is static at lower temperatures. Thus, MgGa2O4 undergoes a glass-like transition when heated above 820 K-860 K, where the state of cation order starts relaxing towards equilibrium in laboratory timescales. Landau-theory for nonconvergent cation ordering can describe the observed cation order at elevated temperatures well. © 2018 Author(s).

The present article describes aspects of the cold gas spray processability of the intermetallic Ti-48Al-2Cr-2Nb (at. %) alloy, which is employed as a structural material in gas turbine engines. The effects of processing parameters, namely, gas pressure, gas temperature, spray distance, as well as the gas atomized feedstock particle size (d50 = 30 and 42 μm, respectively) and phase composition on deposition, were investigated. The results showed that when the highest available gas pressure (40 bar) and temperature (950 °C) were combined with a short spray distance (20 mm), well-adhering coatings could be deposited regardless of the investigated particle size. However, the maximum coating thickness could be achieved was about 30 μm with a deposition efficiency of 1%. Phase composition of the gas atomized feedstock was investigated with HT-XRD and according to the findings, heat treatment of the feedstock under vacuum was carried out. With this treatment, non-equilibrium, disordered α phase of the atomized powder was transformed into an α α2 and γ phase mixture. At the same time, an increase in the hardness and oxygen content of the powder was detected. Swipe test performed with the heat treated powder revealed no improvement in terms of deposition, in fact, the number of adhering particles on the substrate was decreased in comparison with that of the untreated powder. © 2018 Elsevier B.V.

Carbon nanostructures with a well-developed turbostratic sp2 structure and high porosity are synthesized at room temperature inside a planetary ball mill. The obtained carbons were analyzed in-depth by means of gas adsorption, wide-angle X-ray scattering (WAXS), Raman spectroscopy, and transmission electron microscopy (TEM). Our approach involves the solvent-free reaction between calcium carbide (CaC2) and hexachlorobenzene (C6Cl6) conducted under mechanochemical conditions. After certain mechanical activation time, the exothermic nature of the reaction (−492 kcal) provokes a combustion-like event that results in innocuous salt (CaCl2) and a carbonaceous material. Carbon with a high degree of structural order in the constituting graphene and the graphene stacks, possessing almost no internal surface, can be obtained after 5 min of milling time with a mass ratio CaC2/C6Cl6 of 0.9, while carbon exhibiting a surface area as high as 915 m2/g can be obtained after 2 h of milling time with a mass ratio CaC2/C6Cl6 of 5.1. WAXS results and TEM observations reveal a mixture of amorphous carbon and non-graphitic phases. Among the last one, spherical-shaped carbons and curved nanosized strips can be easily distinguished. © 2018 Elsevier Ltd

In this work, a reactive argon-nitrogen dc magnetron plasma for sputtering of a chromium (Cr) target is characterized with high spatial resolution by optical emission spectroscopy (OES) using molecular nitrogen emission bands at 0.5 Pa and 100 W. Beside the global gas temperature T g, the electron temperature T e, electron density n e, and the steady-state Cr density n Cr are also determined spatially resolved using a movable OES setup inside the chamber and Abel inversion. n e and T e are found to be consistent with the values measured by a Langmuir probe (LP) within the non-magnetized region along the magnetron axis in a pure Ar plasma for the same process parameters. Finally, a nitrogen content c N of 4% in the target surface is found for the reactive plasma by matching the mean steady-state chromium density measured by OES and calculated from TRIDYN simulations. © 2018 IOP Publishing Ltd.

Iron-based superconductors are well known for their complex interplay between structure, magnetism and superconductivity. FeSe offers a particularly fascinating example. This material has been intensely discussed because of its extended nematic phase, whose relationship with magnetism is not obvious. Superconductivity in FeSe is highly tunable, with the superconducting transition temperature, T c, ranging from 8 K in bulk single crystals at ambient pressure to almost 40 K under pressure or in intercalated systems, and to even higher temperatures in thin films. In this topical review, we present an overview of nematicity, magnetism and superconductivity, and discuss the interplay of these phases in FeSe. We focus on bulk FeSe and the effects of physical pressure and chemical substitutions as tuning parameters. The experimental results are discussed in the context of the well-studied iron-pnictide superconductors and interpretations from theoretical approaches are presented. © 2017 IOP Publishing Ltd.

The ultrasound attenuation in langasite crystals increases rapidly at about 800 K with increasing temperature for reasons that are not well understood. In this paper, the attenuation quantified as Q-1 of the langasite-type materials La3Ga5SiO14 (LGS) and La3Ta0.5Ga5.5O14 (LGT) was studied from room temperature to 1653 and 1608 K, respectively, using resonant ultrasound spectroscopy. Two to three attenuation peaks can be seen. A change of the magnitudes of the largest two attenuation peaks in LGT was correlated with the changing color of an LGT sample, which is related to its oxygen vacancy concentration. Thus, the attenuation likely involves oxygen vacancies. The observed Q-1 can be explained well by a model based on the anelastic relaxation of two to three noninteracting point defects causing Debye peak-like attenuation maxima. The activation energies of the largest two relaxation peaks match the activation energies for different conductivity mechanisms in LGS and LGT. Thus, the oxygen movement-based conductivity and the relaxation processes seem to involve the exchange of ions and vacancies on the same positions. The largest two attenuation peaks are probably caused by the movement of ions induced by two different phenomena, the deformation of the lattice (point-defect relaxation) on the one hand and the electric field via the piezoelectric effect (piezoelectric/carrier relaxation) on the other hand. © 1986-2012 IEEE.

The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu4.39Al1.61 to its cubic 2/1 crystalline approximant (CA) Ca13Au56.31(3)Al21.69 (CaAu4.33(1)Al1.67, Pa3 (No. 205); Pearson symbol: cP728; a = 23.8934(4)), starting at ∼570 °C and complete by ∼650 °C, is discovered from in situ, high-energy, variable-temperature powder X-ray diffraction (PXRD), thereby providing direct experimental evidence for the relationship between QCs and their associated CAs. The new cubic phase crystallizes in a Tsai-type approximant structure under the broader classification of polar intermetallic compounds, in which atoms of different electronegativities, viz., electronegative Au + Al vs electropositive Ca, are arranged in concentric shells. From a structural chemical perspective, the outermost shell of this cubic approximant may be described as interpenetrating and edge-sharing icosahedra, a perspective that is obtained by splitting the traditional structural description of this shell as a 92-atom rhombic triacontahedron into an 80-vertex cage of primarily Au [Au59.86(2)Al17.14□3.00] and an icosahedral shell of only Al [Al10.5□1.5]. Following the proposal that the cubic 2/1 CA approximates the structure of the i-QC and on the basis of the observed transformation, an atomic site analysis of the 2/1 CA, which shows a preference to maximize the number of heteroatomic Au-Al nearest neighbor contacts over homoatomic Al-Al contacts, implies a similar outcome for the i-QC structure. Analysis of the most intense reflections in the diffraction pattern of the cubic 2/1 CA that changed during the phase transformation shows correlations with icosahedral symmetry, and the stability of this cubic phase is assessed using valence electron counts. According to electronic structure calculations, a cubic 1/1 CA, "Ca24Au88Al64" (CaAu3.67Al2.67) is proposed. © 2017 American Chemical Society.

Results of inelastic neutron scattering measurements are reported for two annealed compositions of Ca(Fe1-xCox)2As2,x=0.026 and 0.030, which possess stripe-type antiferromagnetically ordered and superconducting ground states, respectively. In the AFM ground state, well-defined and gapped spin waves are observed for x=0.026, similar to the parent CaFe2As2 compound. We conclude that the well-defined spin waves are likely to be present for all x corresponding to the AFM state. This behavior is in contrast to the smooth evolution to overdamped spin dynamics observed in Ba(Fe1-xCox)2As2, wherein the crossover corresponds to microscopically coexisting AFM order and SC at low temperature. The smooth evolution is likely absent in Ca(Fe1-xCox)2As2 due to the mutual exclusion of AFM ordered and SC states. Overdamped spin dynamics characterize paramagnetism of the x=0.030 sample and high-temperature x=0.026 sample. A sizable loss of magnetic intensity is observed over a wide energy range upon cooling the x=0.030 sample, at temperatures just above and within the superconducting phase. This phenomenon is unique amongst the iron-based superconductors and is consistent with a temperature-dependent reduction in the fluctuating moment. One possible scenario ascribes this loss of moment to a sensitivity to the c-axis lattice parameter in proximity to the nonmagnetic collapsed tetragonal phase and another scenario ascribes the loss to a formation of a pseudogap. © 2018 American Physical Society.

In this study, a novel approach for the determination of the solid mass yield from slow pyrolysis based on a comparison of the volatile matter contents of feedstock and char is presented. The approach was tested with experimental data from the literature and our own measurements. For these experiments, gravimetric data are available to determine the mass yield. The proposed method was compared with conventional ash-based calculations and the gravimetric determination of the yield. It was shown that the new approach not only does perform significantly better than ash-based methods but also approximates the real mass yield of slow pyrolysis under atmospheric pressure quite accurately. These findings may indicate that secondary char formation does not contribute significantly to the mass yield of biomass pyrolysis under conditions found in practical production processes (low heating rates, atmospheric pressure, and medium temperatures). © 2018 American Chemical Society.

Using molecular dynamics simulations based on ab initio trained high-dimensional neural network potentials, we study the equation of state of liquid water at negative pressures. From density isobars computed for various pressures down to p = -230 MPa we determine the line of density maxima for two potentials based on the BLYP and the RPBE functionals, respectively. In both cases, dispersion corrections are included to account for non-local long-range correlations that give rise to van der Waals forces. We have followed the density maximum down to negative pressures close to the spinodal instability. For both functionals, the temperature of maximum density increases with decreasing pressure under moderate stretching, but changes slope at P ≈ -200 MPa and p ≈ -20 MPa for BLYP and RPBE, respectively. Our calculations confirm qualitatively the retracing shape of the line of density maxima found for empirical water models, indicating that the spinodal line maintains a positive slope even at strongly negative pressures. © 2018 IOP Publishing Ltd.

We use scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations to investigate the electron-induced dissociation of halogenated benzene molecules adsorbed on ice. Dissociation of halobenzene is triggered by delocalized excess electrons attaching to the π∗ orbitals of the halobenzenes from where they are transferred to σ∗ orbitals. The latter orbitals provide a dissociative potential surface. Adsorption on ice sufficiently lowers the energy barrier for the transfer between the orbitals to facilitate dissociation of bromo- and chloro- but not of flourobenzene at cryogenic temperatures. Our results shed light on the influence of environmentally important ice particles on the reactivity of halogenated aromatic molecules. © 2018 American Physical Society.

Low-temperature scanning tunneling microscopy was used to follow the formation of a solvation shell around an adsorbed functionalized azo dye from the attachment of the first water molecule to a fully solvated molecule. Specific functional groups bind initially one water molecule each, which act as anchor points for additional water molecules. Further water attachment occurs in areas close to these functional groups even when the functional groups themselves are already saturated. In contrast, water molecules surround the hydrophobic parts of the molecule only when the two-dimensional solvation shell closes around them. This study thus traces hydrophilic and hydrophobic properties of an organic molecule down to a sub-molecular length scale. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ion implantation is one of the best methods to manufacture silicon-vacancy (SiV) centers in diamond, which can be used as qubits. In this work, molecular dynamics (MD) simulation was conducted to analyze the damage evolution and distribution during the process of silicon ion implantation into bulk diamond and subsequent annealing. Tersoff-ZBL (Ziegler-Biersack-Littmark) potential was used to describe the atomic interaction. Identify Diamond Structure (IDS) and Wigner-Seitz defect analysis methods were used to calculate damages and vacancies. After 2393 K annealing, about 42.5% of ion induced IDS damages were recovered. During the temperature cooling down from 2393 K to 293 K, the movements of silicon atoms along the implantation direction were sensitive to the temperature variation, while vacancies were almost insensitive. MD simulation is helpful to illustrate the ion implant induced damages’ dynamic evolution and Si-V related defects, which can assist a deeper understanding of SiV center's manufacturing. © 2018 Elsevier B.V.

Although the wetting of hydrophilic surfaces on the macroscale is a well-known phenomenon, we here develop a microscopic understanding of the more recently discovered complete covering of hydrophobic surfaces by a uniform water layer. For this aim, we deposit D2O on Ag(111) at two different temperatures, 20 and 96 K, and investigate the geometry of the layers at coverages up to four bilayers (BL) by low-temperature scanning tunneling microscopy. In the coverage range up to 0.5 BL, the ice grows in the form of islands that differ largely in size, shape, and density, but surprisingly not in their height. Moreover, the water fills the layer with islands of the same thickness of three to four bilayers at both temperatures. The different island shapes and densities in the coverage range before coalescence are attributed to details in the interaction between water nanoclusters and activated cluster diffusion at the higher growth temperature of 96 K, as visualized in time-lapsed series of scanning tunneling microscopy images. © Copyright 2018 American Chemical Society.

Cr2AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90-180 μm, 180-250 μm, and 250-400 μm. Sacrificial template technique was used as the processing method, utilizing NH4HCO3 as a temporary pore former. Cr2AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in-situ formation of an outer thin continuous protective layer of α-Al2O3. The in-situ α-Al2O3 protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter- to transgranular fracture mode. According to the excellent high-temperature response, heat exchangers and catalyst supports are the potential application of these foams. © 2017 The American Ceramic Society

In this contribution the development of an advanced, high-temperature stable plasma sensor based on the multipole resonance probe (MRP) is presented. Using low temperature co-fired ceramics (LTCC) as substrate material, together with a multilayer structure, provides resistance against high temperatures as well as a sufficient mechanical stability. Therefore, the sensor is applicable as a robust measurement tool in a wide field of industrial plasma processes. The ability of the probe to determine the electron density of the plasma as well as the collision frequency of the electrons is investigated by extensive 3D electromagnetic simulations. Measurements in a double inductively coupled plasma (DICP) reactor using different gas compositions with neutral gas temperatures exceeding 500° C confirm the suitability of the probe for a precise plasma monitoring at high temperatures. © 2018 IMA.

Low temperature powder inelastic neutron scattering measurements were performed on three different powder samples; parent BaMn2As2,12.5% K-doped Ba0.875K0.125Mn2As2 and 25% K-doped Ba(0.75)K0.25Mn2As2. The Heisenberg Model involving J1‐J2‐Jz coupling constants were compared to the data by a powder integration routine using Monte Carlo integration methods. The best magnetic parameters were selected using a chi-square test where model intensities were compared to the full (q,E) dependence of magnetic scattering. A key step to this analysis is the characterization of the background which is formed mostly by phonon scattering intensities along with other sources including the magnetic impurity scattering events. The calculated powder magnetic intensities added to the estimated background obtained from the non-magnetic high momentum transfer region. The agreement between the simulated and the raw data enabled us to perform quantitative analysis of the unreacted MnO impurities. Overall, this is another confirmation along with earlier studies using this technique, that magnetic exchange constants can be calculated within an acceptable range with a very quick inelastic neutron powder experiment without need for a single crystal sample. © 2017 Elsevier B.V.

We report inelastic light scattering results on the stoichiometric and fully ordered superconductor CaKFe4As4 as a function of temperature and light polarization. In the energy range between 10 and 315 cm-1 (1.24 and 39.1 meV) we observe the particle-hole continuum above and below the superconducting transition temperature Tc and seven of the eight Raman active phonons. The main focus is placed on the analysis of the electronic excitations. Below Tc all three symmetries projected with in-plane polarizations display a redistribution of spectral weight characteristic for superconductivity. The energies of the pair-breaking peaks in A1g and B2g symmetry are in approximate agreement with the results from photoemission studies. In B1g symmetry the difference between the normal and superconducting state is most pronounced, and the feature is shifted downwards with respect to those in A1g and B2g symmetry. The maximum peaking at 134 cm-1 (16.6 meV) has a substructure on the high-energy side. We interpret the peak at 134 cm-1 in terms of a collective Bardasis-Schrieffer (BS) mode and the substructure as a remainder of the pair-breaking feature on the electron bands. There is a very weak peak at 50 cm-1 (6.2 meV) which is tentatively assigned to another BS mode. © 2018 American Physical Society.

We present a novel non-contact method for the direct measurement of caloric effects in low volume samples. The adiabatic temperature change ΔT of a magnetocaloric sample is very sensitively determined from thermal radiation. Rapid modulation of ΔT is induced by an oscillating external magnetic field. Detection of thermal radiation with a mercury-cadmium-telluride detector allows for measurements at field frequencies exceeding 1 kHz. In contrast to thermoacoustic methods, our method can be employed in vacuum which enhances adiabatic conditions especially in the case of small volume samples. Systematic measurements of the magnetocaloric effect as a function of temperature, magnetic field amplitude, and modulation frequency give a detailed picture of the thermal behavior of the sample. Highly sensitive measurements of the magnetocaloric effect are demonstrated on a 2 mm thick sample of gadolinium and a 60 μm thick Fe80B12Nb8 ribbon. © 2018 Author(s).

Rotary drums are used in many industrial applications for heat and mass transfer between gas and solid particles. Additional flights are installed to improve the mixing of adhesive products and especially to increase the exchange surface between gas and solids. The flights are mounted on the inside of the drum and lift particles out of the solid bed to shower them as curtains in the passing gas phase of the drum, where heat and mass transfer is very intense. The amount of particles and their distribution are influenced by operating parameters (rotational speed, filling degree, air volume flow), which were researched in a previous study about transverse particle motion. In order to couple the transverse particle motion to the heat transfer, heat transfer experiments were carried out. Therefore, an indirectly heated flighted rotary drum was designed and constructed. Batch experiments, which were divided into heating and cooling process, with glass beads as reference material were performed. The temperature distribution inside the drum was measured using thermocouples at different axial and radial positions in the cylindrical drum. The operating parameters rotational speed, filling degree and air volume flow were varied to investigate their influence on the temperature distribution inside the drum. It was found that increasing the rotational speed as well as the air volume flow leads to faster changing temperatures. Meanwhile, increasing the filling degree results in slower temperature drop rates. © 2018 Begell House Inc.. All rights reserved.

A set of advanced single crystalline γ′ strengthened Co-base superalloys with at least nine alloying elements (Co, Ni, Al, W, Ti, Ta, Cr, Si, Hf, Re) has been developed and investigated. The objective was to generate multinary Co-base superalloys with significantly improved properties compared to the original Co-Al-W-based alloys. All alloys show the typical γ/γ′ two-phase microstructure. A γ′ solvus temperature up to 1174 °C and γ′ volume fractions between 40 and 60 pct at 1050 °C could be achieved, which is significantly higher compared to most other Co-Al-W-based superalloys. However, higher contents of Ti, Ta, and the addition of Re decrease the long-term stability. Atom probe tomography revealed that Re does not partition to the γ phase as strongly as in Ni-base superalloys. Compression creep properties were investigated at 1050 °C and 125 MPa in 〈001〉 direction. The creep resistance is close to that of first generation Ni-base superalloys. The creep mechanisms of the Re-containing alloy was further investigated and it was found that the deformation is located preferentially in the γ channels although some precipitates are sheared during early stages of creep. The addition of Re did not improve the mechanical properties and is therefore not considered as a crucial element in the design of future Co-base superalloys for high temperature applications. Thermodynamic calculations describe well how the alloying elements influence the transformation temperatures although there is still an offset in the actual values. Furthermore, a full set of elastic constants of one of the multinary alloys is presented, showing increased elastic stiffness leading to a higher Young’s modulus for the investigated alloy, compared to conventional Ni-base superalloys. The oxidation resistance is significantly improved compared to the ternary Co-Al-W compound. A complete thermal barrier coating system was applied successfully. © 2018, The Minerals, Metals & Materials Society and ASM International.

The oxidation and stability of multi-walled carbon nanotubes (CNTs) have been investigated by exposing CNTs in 30% w/v H2O2 solution at room temperature (RT) for up to 8 weeks and at 80 °C for up to 8 h. H2O2 oxidation not only generated surface oxygen-containing groups, but also created surface defects, as disclosed by results of temperature-programmed desorption and X-ray Photoelectron Spectroscopy. The total surface oxygen content was found to be correlated to the final H2O2 concentration. The higher the total surface oxygen content on CNTs, the lower the final H2O2 concentration. Meanwhile, the carbon oxidation and simultaneous H2O2 decomposition were observed and confirmed by an online analysis of evolved gases during the oxidation stepwise heated from room temperature to 80 °C. Raman study showed that the D/G and D'/G ratios of the CNTs oxidized at RT first decreased with an oxidation time of 4 weeks and then increased when prolonging the oxidation time up to 8 weeks. Similar trend was also observed on the CNTs oxidized at 80 °C. The size of CNTs was gradually reduced with increasing oxidation time as shown by SEM studies. Our work reveals the critical changes in the surface oxygen groups as well as the changes in morphology at two distinct stages of hydrogen peroxide treatment, purification and then functionalization. CNTs can withstand 30% w/v H2O2 oxidation for only a certain time, while they may be damaged or consumed eventually in long-term applications. Our study contributes to filling in the knowledge gap about CNT surface oxidation and structural changes with H2O2 treatment under industrial conditions. © 2018 Elsevier B.V.

Surface segregation, restructuring, and sintering phenomena in size-selected copper-nickel nanoparticles (NPs) supported on silicon dioxide substrates were systematically investigated as a function of temperature, chemical state, and reactive gas environment. Using near-ambient pressure (NAP-XPS) and ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), we showed that nickel tends to segregate to the surface of the NPs at elevated temperatures in oxygen- or hydrogen-containing atmospheres. It was found that the NP pretreatment, gaseous environment, and oxide formation free energy are the main driving forces of the restructuring and segregation trends observed, overshadowing the role of the surface free energy. The depth profile of the elemental composition of the particles was determined under operando CO2 hydrogenation conditions by varying the energy of the X-ray beam. The temperature dependence of the chemical state of the two metals was systematically studied, revealing the high stability of nickel oxides on the NPs and the important role of high valence oxidation states in the segregation behavior. Atomic force microscopy (AFM) studies revealed a remarkable stability of the NPs against sintering at temperatures as high as 700 °C. The results provide new insights into the complex interplay of the various factors which affect alloy formation and segregation phenomena in bimetallic NP systems, often in ways different from those previously known for their bulk counterparts. This leads to new routes for tuning the surface composition of nanocatalysts, for example, through plasma and annealing pretreatments. © 2017 American Chemical Society.

Micro- and nanomechanical testing has seen a rapid development over the last decade with miniaturized test rigs and MEMS-based devices providing access to the mechanical properties and performance of materials from the micrometer down to the tenths of nanometer length scale. In this overview, we summarize firstly the different testing concepts with excursions into recent imaging and diffraction developments, which turn micro- and nanomechanical testing into “quantitative mechanical microscopy” by resolving the underlying material physics and simultaneously providing mechanical properties. A special focus is laid on the pitfalls of micro-compression testing with its stringent boundary conditions often hampering reliable experiments. Additionally, the challenges of instrumented micro- and nanomechanical testing at elevated temperature are summarized. From the wide variety of research topics employing micro- and nanomechanical testing of materials we focus here on miniaturized samples and test rigs and provide three examples to elucidate the state-of-the-art of the field: (i) probing the “strength” of individual grain boundaries in metals, (ii) temperature dependent deformation mechanisms in metallic nanolayered and -alloyed structures, and (iii) the prospects and challenges of fracture studies employing micro- and nanomechanical testing of brittle and ductile monolithic materials, and materials containing interfaces. Proven concepts and new endeavors are reported for the topics discussed in this overview. © 2017 Acta Materialia Inc.

We studied the role of VC precipitation in improving the shape memory effect (SME) of the as-solution treated Fe–Mn–Si-based shape memory alloys by examining the microstructures developed during aging and deformation using transmission electron microscopy and electron channeling contrast imaging. Our results suggest that VC particles are not the only product of aging. Upon aging at 650 °C, the precipitation of VC particles is accompanied by the formation of profuse dislocations (2.26 ± 0.098 × 1013 m−2). In this case, the SME is not improved compared to the as-solution treated reference state. Upon aging at high temperatures (700–900 °C), a number of stacking faults are formed accompanying the VC precipitation and the SME is significantly improved, where the recovery ratios reach almost twice that of the as-solution treated state (<50%). For these high-temperature aged states, in situ straining experiments reveal that the stacking faults rather than the VC particles play an important role in the stress-induced martensitic transformation, leading to the formation of very thin (<3 nm) martensite plates with a single crystallographic variant within each grain. These martensite plates are in contrast to the very thick (from tens to hundreds of nanometers) and multi-variant martensite plates that prevail in the as-solution treated state. By comparing the characteristics of the martensite plates between the as-solution treated and the high-temperature aged states, the reasons for the improvement of SME by precipitating VC were discussed. © 2018 Acta Materialia Inc.

A reduction in synthesis temperature is favorable for hard coatings, which are designed for industrial applications, as manufacturing costs can be saved and technologically relevant substrate materials are often temperature-sensitive. In this study, we analyzed Mo2BC hard coatings deposited by direct current magnetron sputtering at different substrate temperatures, ranging from 380 °C to 630 °C. Transmission electron microscopy investigations revealed that a dense structure of columnar grains, which formed at a substrate temperature of 630 °C, continuously diminishes with decreasing substrate temperature. It almost vanishes in the coating deposited at 380 °C, which shows nanocrystals of ~1 nm in diameter embedded in an amorphous matrix. Moreover, Argon from the deposition process is incorporated in the film and its amount increases with decreasing substrate temperature. Nanoindentation experiments provided evidence that hardness and Young's modulus are modified by the nanostructure of the analyzed Mo2BC coatings. A substrate temperature rise from 380 °C to 630 °C resulted in an increase in hardness (21 GPa to 28 GPa) and Young's modulus (259 GPa to 462 GPa). We conclude that the substrate temperature determines the nanostructure and the associated changes in bond strength and stiffness and thus, influences hardness and Young's modulus of the coatings. © 2018 The Authors

The high temperatures and heating rates typical of PF are known to induce thermal annealing of char and loss of its reactivity. Several authors investigated this effect for coals in inert atmospheres, while little is known about the effects of CO2-rich atmospheres, typical of oxy-combustion and gasification, on the course of thermal annealing. Thermal annealing of biomass has been scarcely investigated in the literature; however, available studies reported that also biomass can suffer from thermo-deactivation. The present study aims to provide further insight on thermal annealing of biomass in the context of gasification and oxy-combustion. A lignin-rich biomass (walnut shells) has been heat-treated in a heated strip reactor at temperatures of 1573-2073 K with a holding time of 3 s using atmospheres of either N2 or CO2. Similar experiments have been performed with a high volatile bituminous coal (Colombian coal) used as reference. The char samples have been analyzed by thermogravimetric analysis and Raman spectroscopy. Results have been further compared with those reported in previous studies where heat treatment of the same fuels were performed in fixed bed, fluidized bed, and drop tube reactors at lower temperature or shorter holding time. Two remarkable results have been obtained: (1) Loss of reactivity by thermal annealing and structural reorganization follow similar pathways for coal and biomass. (2) The effect of CO2 on pyrolysis and thermal annealing is non-monotonic along with heat treatment: in the early instances of heat treatment (T = 1573 K, t < 0.1 s), CO2 fosters pyrolysis and thermal annealing, increasing structural ordering. At longer holding times (T > 1573 K, t > 1 s), instead, CO2 somewhat hampers thermal annealing. © 2018 American Chemical Society.

By a combination of electron paramagnetic resonance spectroscopy, finite-temperature ab initio simulations, and electronic structure analyses, the activation of molecular dioxygen at the interface of gold nanoparticles and titania in Au/TiO2 catalysts is explained at the atomic scale by tracing processes down to the molecular orbital picture. Direct evidence is provided that excess electrons in TiO2, for example created by photoexcitation of the semiconductor, migrate to the gold particles and from there to oxygen molecules adsorbed at gold/titania perimeter sites. Superoxide species are formed more efficiently in this way than on the bare TiO2 surface. This catalytic effect of the gold nanoparticles is attributed to a weakening of the internal O-O bond, leading to a preferential splitting of the molecule at shorter bond lengths together with a 70% decrease of the dissociation free energy barrier compared to the non-catalyzed case on bare TiO2. The findings are an important step forward in the clarification of the role of gold in (photo)catalytic processes. © 2018 American Chemical Society.

The present work studies the nucleation of planar faults in the early stages of low temperature (750 °C) and high stress (800 MPa) creep of a Ni-base single crystal superalloy (SX). Two families of 60° dislocations with different Burgers vectors were detected in the transmission electron microscope (TEM). These can react and form a planar fault in the γ′ phase. A 2D discrete dislocation model helps to rationalize a sequence of events which lead to the nucleation of a planar fault. First, one 60° channel dislocation approaches another 60° interface dislocation with a different Burgers vector. At a distance of 5 nm, it splits up into two Shockley partials. The interface dislocation is pushed into the γ′-phase where it creates a small antiphase boundary. It can only move on when the leading Shockley partial joins it and creates an overall 1/3<112> superdislocation. This process is fast and therefore is difficult to observe. The results obtained in the present work contribute to a better understanding of the processes which govern the early stages of low temperature and high stress primary creep of SX. © 2017 Acta Materialia Inc.

Spillover hydrogen species were generated by dissociative H2 adsorption on Pt nanoparticles supported on nitrogen-doped mesoporous carbon. The spillover hydrogen species on the support can migrate back to the Pt nanoparticles and hydrogenate subsequently adsorbed nitrobenzene to aniline at 80 °C, which was detected during temperature-programmed desorption experiments from 80 to 300 °C in pure He. The amount of spillover hydrogen can be tuned mainly by the pre-reduction temperature rather than by other parameters. The absence of aniline formation during nitrobenzene desorption experiments in the presence of CO indicates that hydrogenation occurs exclusively on Pt and that the spillover hydrogen species are present on the carbon support in a chemically inactive state. Most likely, spillover hydrogen is reversibly stored on the carbon support as adsorbed protons on the surface and as electrons in the bulk. These findings provide a new perspective on Pt/C-based hydrogen storage materials and fuel cell catalysts. © 2018

In the atmospheric plasma spray (APS) process, micro-sized ceramic powder is injected into a thermal plasma where it is rapidly heated and propelled toward the substrate. The coating formation is characterized by the subsequent impingement of a large number of more or less molten particles forming the so-called splats and eventually the coating. In this study, a systematic investigation on the influence of selected spray parameters on the coating microstructure and the coating properties was conducted. The investigation thereby comprised the coating porosity, the elastic modulus, and the residual stress evolution within the coating. The melting status of the particles at the impingement on the substrate in combination with the substrate surface condition is crucial for the coating formation. Single splats were collected on mirror-polished substrates for selected spray conditions and evaluated by identifying different types of splats (ideal, distorted, weakly bonded, and partially molten) and their relative fractions. In a previous study, these splat types were evaluated in terms of their effect on the above-mentioned coating properties. The particle melting status, which serves as a measure for the particle spreading behavior, was determined by in-flight particle temperature measurements and correlated to the coating properties. It was found that the gun power and the spray distance have a strong effect on the investigated coating properties, whereas the feed rate and the cooling show minor influence. © 2018 ASM International

Pseudoelastic NiTi-based shape-memory alloys (SMAs) have recently received attention as candidate materials for solid-state refrigeration. The elastocaloric effect in SMAs exploits stress-induced martensitic transformation, which is associated with large latent heat. Most importantly, cyclic mechanical loading/unloading provides large adiabatic temperature drops exceeding 25 K at high process efficiencies. This article summarizes the underlying principles, important material parameters and process requirements, and reviews recent progress in the development of pseudoelastic SMAs with large coefficients of performance, as well as very good functional fatigue resistance. The application potential of SMA film and bulk materials is demonstrated for the case of cyclic tensile loading/unloading in prototypes ranging from miniature-scale devices to large-scale cooling units. Copyright © Materials Research Society 2018.

In this study, the ultrahigh-temperature tensile creep behaviour of a TiC-reinforced Mo-Si-B-based alloy was investigated in the temperature range of 1400-1600 °C at constant true stress. The tests were performed in a stress range of 100-300 MPa for 400 h under vacuum, and creep rupture data were rationalized with Larson-Miller and Monkman-Grant plots. Interestingly, the MoSiBTiC alloy displayed excellent creep strength with relatively reasonable creep parameters in the ultrahigh-temperature range: a rupture time of ~400 h at 1400 °C under 137 MPa with a stress exponent (n) of 3 and an apparent activation energy of creep (Q app ) of 550 kJ/mol. The increasing rupture strains with decreasing stresses (up to 70%) and moderate strain-rate oscillations in the creep curves suggest that two mechanisms contribute to the creep: phase boundary sliding between the hard T2 and (Ti,Mo)C phases and the Moss phase, and dynamic recovery and recrystallization in Moss, observed with orientation imaging scanning electron microscopy. The results presented here represent the first full analysis of creep for the MoSiBTiC alloy in an ultrahigh-temperature range. They indicate that the high-temperature mechanical properties of this material under vacuum are promising. © 2018 The Author(s).

The fracture behaviour and microstructure evolution of sputtered Mo2BC films as a function of their deposition temperature is studied. Bipolar pulsed direct current magnetron sputtering was used to deposit Mo2BC thin films onto Si (100) wafers at substrate temperatures ranging from 380 to 630 °C. Microstructural characterization by transmission electron microscopy revealed that increasing the deposition temperature induces larger and more elongated grains, and a higher degree of crystallinity, transitioning from a partially amorphous to a fully crystalline film. The intrinsic fracture toughness of the Mo2BC films was studied by focussed ion beam milled micro-cantilever bending tests. A mild dependency of the intrinsic fracture toughness on the substrate deposition temperature was found. Fractograph analysis showed that the fracture behaviour was dominated by intergranular fracture or by fracture within the amorphous regions. Additionally, nanoindentation based fracture toughness measurements were used to probe the fracture behaviour of the Mo2BC/Si system, where residual stresses define the ‘apparent’ fracture toughness of the system. Depending on the substrate deposition temperature either compressive or tensile residual stresses developed in the films. This causes a relative change in the system toughness by up to one order of magnitude. The fracture experiments clearly reveal that notched cantilevers provide intrinsic toughness values of a material, while nanoindentation probes the toughness of the entire coating-substrate system. The combination of both techniques provides valuable design information for enhancing fracture resistance of Mo2BC films. © 2018 Elsevier Ltd

We report first-principles calculations for determining the phase relationships in multi-component cathode materials. We investigate the effect of delithiation on the phase stability, chemical potential, and open circuit voltage for a selection of cathode materials based on Li-Mn-Ni oxides at various temperatures. Entropic contributions are included by calculating the phonon frequencies in the harmonic approximation. The open circuit voltage in multi-component systems is estimated by a convex hull approach. We confirm that spinel-like phases are predominant during the charging process of layered Li-Mn-O cathode materials and that the addition of Ni reduces the spinel content. The analysis of phase stability upon delithiation suggests that the Li2MnO3 component in the Li2MnO3·Li(Mn,Ni)O2 electrode material should not exceed 60% and that the amount of Ni in the LiMnO2 component should be above 40 at% for minimizing spinel-type phase formation and minimizing oxygen formation. Using the computed structural stability at room temperature, we derive property maps for the design of Li-Mn-Ni-O cathode materials. © The Royal Society of Chemistry 2018.

The occurrence of the inverse (or negative) electrocaloric effect, where the isothermal application of an electric field leads to an increase in entropy and the removal of the field decreases the entropy of the system under consideration, is discussed and analyzed. Inverse electrocaloric effects have been reported to occur in several cases, for example, at transitions between ferroelectric phases with different polarization directions, in materials with certain polar defect configurations, and in antiferroelectrics. This counterintuitive relationship between entropy and applied field is intriguing and thus of general scientific interest. The combined application of normal and inverse effects has also been suggested as a means to achieve larger temperature differences between hot and cold reservoirs in future cooling devices. A good general understanding and the possibility to engineer inverse caloric effects in terms of temperature spans, required fields, and operating temperatures are thus of fundamental as well as technological importance. Here, the known cases of inverse electrocaloric effects are reviewed, their physical origins are discussed, and the different cases are compared to identify common aspects as well as potential differences. In all cases the inverse electrocaloric effect is related to the presence of competing phases or states that are close in energy and can easily be transformed with the applied field. © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

The energy efficient excitation of CO2 in atmospheric pressure plasmas may be a method to generate solar fuels from renewable energies. This energy efficiency can be very high, if only specific states of the molecules in the plasma are populated creating a strong non-equilibrium. This requires a specific design of the plasma source, method of plasma excitation and choice of gases and admixtures. In this paper, non-equilibrium excitation and dissociation of CO2 in an atmospheric pressure helium RF plasma jet is analysed for varying absorbed plasma power and admixture levels of CO2. The concentrations of CO2 and of CO, as well as the vibrational and rotational temperatures of the possible degrees of freedom of the molecules are evaluated by Fourier transform infrared spectroscopy (FTIR). The molecular rotational vibrational spectra are modelled based on Maxwell-Boltzmann state populations using individual temperatures for each degree of freedom. A strong non-equilibrium excitation of CO2 and CO has been found. Whereas the rotational temperatures are 400 K or below, the vibrational temperature for CO reaches values up to 1600 K and that of the asymmetric vibration of CO2 of 700 K. The dependence of these excitation temperatures on plasma power and admixture level is rather weak. The mass balance, the energy and conversion efficiency are consistent with a very simple chemistry model that is dominated by CO2 dissociation via Penning collisions with helium metastables. A conversion efficiency up to 30% and an energy efficiency up to 10% is observed in the parameter range of the experiment. © 2018 IOP Publishing Ltd.

We observe the transformation of fractal ice islands grown at 96 K to compact ones annealed at 118 K and compare those to compact islands grown directly at 118 K. The low-temperature grown islands form a four bilayer high wetting layer. The annealing causes a crystallization and reshaping of the islands and a substantial increase in height and roughness in particular at higher coverage. Moreover, it leads to a dewetting of the ice film. The islands grown at the higher temperature show qualitative similarities to the annealed ones at smaller nucleation density. However, their orientation with respect to the surface differs by 30° as compared to the annealed islands. © 2017 American Chemical Society.

The formation and lifetime of point defects is governed by an interplay of kinetics and thermodynamic stability. To evaluate the stability under process conditions, empirical potentials and ab initio calculations at T=0K are often not sufficient. Therefore, various concepts to determine the full temperature dependence of the free energy of point defects with ab initio accuracy are reviewed. Examples for the importance of accurately describing defect properties include the stabilization of vacancies by impurities and the non-Arrhenius behaviour of vacancy formation energies due to anharmonic lattice vibrations. © 2018

The interplay between superconductivity and charge-density wave (CDW) in 2H-NbSe2 is not fully understood despite decades of study. Artificially introduced disorder can tip the delicate balance between two competing long-range orders, and reveal the underlying interactions that give rise to them. Here we introduce disorder by electron irradiation and measure in-plane resistivity, Hall resistivity, X-ray scattering, and London penetration depth. With increasing disorder, the superconducting transition temperature, T c, varies non-monotonically, whereas the CDW transition temperature, T CDW, monotonically decreases and becomes unresolvable above a critical irradiation dose where T c drops sharply. Our results imply that the CDW order initially competes with superconductivity, but eventually assists it. We argue that at the transition where the long-range CDW order disappears, the cooperation with superconductivity is dramatically suppressed. X-ray scattering and Hall resistivity measurements reveal that the short-range CDW survives above the transition. Superconductivity persists to much higher dose levels, consistent with fully gapped superconductivity and moderate interband pairing. © 2018 The Author(s).

Heteroleptic bis(tert-butylimido)bis(N,N′-diisopropylacetamidinato) compounds of molybdenum and tungsten are introduced as precursors for atomic layer deposition of tungsten and molybdenum oxide thin films using ozone as the oxygen source. Both precursors have similar thermal properties but exhibit different growth behaviors. With the molybdenum precursor, high growth rates up to 2 Å/cycle at 300 °C and extremely uniform films are obtained, although the surface reactions are not completely saturative. The corresponding tungsten precursor enables saturative film growth with a lower growth rate of 0.45 Å/cycle at 300 °C. Highly pure films of both metal oxides are deposited, and their phase as well as stoichiometry can be tuned by changing the deposition conditions. The WOx films crystallize as γ-WO3 at 300 °C and above, whereas the films deposited at lower temperatures are amorphous. Molybdenum oxide can be deposited as either amorphous (≤250 °C), crystalline suboxide (275 °C), a mixture of suboxide and α-MoO3 (300 °C), or pure α-MoO3 (≥325 °C) films. MoOx films are further characterized by synchrotron photoemission spectroscopy and temperature-dependent resistivity measurements. A suboxide MoOx film deposited at 275 °C is demonstrated to serve as an efficient hydrogen gas sensor at a low operating temperature of 120 °C. © 2018 American Chemical Society.

The mechanical properties of face-centered cubic (fcc) metals are influenced by physical parameters of the material, such as the stacking fault energy (SFE). It is known that a low SFE improves the strain hardening, thus increasing the abrasive wear resistance over a wide temperature range. Therefore, investigating the SFE is highly important for the characterization of the physical properties of materials at elevated temperatures. In the present study, the SFE of several austenitic stainless steels was determined by using a calculation model based on Calphad data for investigating the SFE depending on temperature. It can be shown that the lowest SFE value was calculated for the system Fe-27Cr-22Ni including interstitial elements (C+N < 0.1 mass%). This constitution was found by increasing the Cr content to a maximum considering the thermal austenite stability. In this context, the influence on the SFE and austenitic stability of the main alloying elements (Cr, Ni) were examined in detail. To determine the SFE values experimentally, alloys were produced on a laboratory scale and analyzed using X-ray diffraction line-profile analysis (XRD-LP). The results show good match between the calculated and measured SFE values. The calculations show that an increase of the Cr/Ni ratio decreases the SFE in FeCrNi alloys. Moreover, the represented calculation model is suitable for estimating the SFE over a wide temperature range, avoiding costly and time-consuming experiments. © 2018 Acta Materialia Inc.

The millisecond-dynamics of the magnetocaloric effect in Gd and La-Fe-Si-Mn, which exhibit first- and second-order phase-transitions, respectively, are investigated. Direct measurements of the adiabatic temperature change ΔT are obtained from modulation infrared thermometry with field-cycling frequencies exceeding 1 kHz at amplitudes of up to 45 mT. The peak amplitude of ΔT(T) shows a dependence on sample thickness and decreases with increasing modulation frequency for both materials despite a frequency independent susceptibility of Gd. The adiabatic ΔT depends quadratically on the external field for Gd while La−Fe−Si−Mn shows a peculiar bucket-shaped curve for temperatures below the peak maximum. A comparative study of non-caloric samples shows that dissipative heating by eddy currents or magnetic hysteresis does not explain the observed behavior. The transient ΔT(t) instead suggests a mechanism involving strong temperature gradients at the ferromagnetic–paramagnetic boundaries and underlines the importance of further dynamical studies for a fundamental understanding of the magnetocaloric effect in first-order materials. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

The etching of glass is still much more challenging than the deep etching of silicon. But in contrast to pure silica, most glasses are complex alloys of serval oxides including aluminium oxide. For this reason, it is quite difficult to find suitable high-rate deep dry etching processes and related masking materials. For extremely temperature-insensitive micromechanical systems it is of interest to use zero-expansion glass ceramics such as Zerodur. But the microstructure of Zerodur consists of crystalline and amorphous phases and shows a high percentage of Al2O3-bonds. This makes plasma etching challenging. Here, deep etching of Zerodur only in a fluorine-based plasma for micro-technical applications is investigated. Different process parameters such as the physical power and gas mixtures of the ICP-RIE-process have been varied. Etch rates of about 250 nm/min and sidewall angles of approximately 71° were reached with a nickel mask and the etch gas SF6. The achieved total etching depth is as large as 150 μm resulting in a release of microelements such as springs and gears from a Zerodur wafer. © 2017 Elsevier B.V.

For the purpose of good reproduction and prediction of creep deformation of nickel-base single crystal superalloys at intermediate temperatures, a phenomenological creep model is developed, which accounts for the typical γ/γ′ microstructure and the individual thermally activated elementary deformation processes in different phases. The internal stresses from γ/γ′ lattice mismatch and deformation heterogeneity are introduced through an efficient method. The strain hardening, the Orowan stress, the softening effect due to dislocation climb along γ/γ′ interfaces and the formation of dislocation ribbons, and the Kear-Wilsdorf-lock effect as key factors in the main flow rules are formulated properly. By taking the cube slip in slip systems and twinning mechanisms into account, the creep behavior for [110] and [111] loading directions are well captured. Without specific interaction and evolution of dislocations, the simulations of this model achieve a good agreement with experimental creep results and reproduce temperature, stress and crystallographic orientation dependences. It can also be used as the constitutive relation at material points in finite element calculations with complex boundary conditions in various components of superalloys to predict creep behavior and local stress distributions. © 2018 IOP Publishing Ltd.

We study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio molecular dynamics. The two degrees of freedom are dynamically coupled, leading to nonadiabatic effects. Those effects suppress the phonon lifetimes at low temperature compared to an adiabatic approach. The dynamic coupling identified here provides an explanation for the experimentally observed unexpected temperature dependence of the thermal conductivity of magnetic semiconductors above the magnetic ordering temperature. © 2018 American Physical Society.

This work presents a novel method of obtaining in situ strain measurements at high temperature by simultaneous digital image correlation (DIC), which provides global strain, and synchrotron x-ray diffraction (XRD), which provides lattice strains. Digital image correlation at high temperature requires specialized techniques to overcome the effects of increased black body radiation that would otherwise overexpose the images. The technique presented herein is unique in that it can be used with a sample enclosed in an infrared heater that cannot be illuminated with additional lighting. A small hole was drilled into the heater to serve as a window for the camera, and the light of the heater lamps is used as illumination. High-temperature paint is used to apply a speckle pattern to the sample to allow the tracking of displacements and the calculation of strains. An inexpensive blue theatrical gel filter is used to block the orange, red, and infrared light at high temperatures. This technique successfully produces properly exposed sample images at 870 ◦C; this temperature was determined by the requirements of the experiment, not a limitation of the technique. Another feature of this method is that the camera is controlled remotely, allowing focusing and image capture during synchrotron XRD measurements. The results were validated by an analytical calculation of the theoretical strain. Simultaneous DIC and XRD measurements of Inconel 718 (IN718) were taken under thermal and mechanical loads. The combination of global and lattice strains can provide important information on the anisotropy of the material. © 2018 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

The modification of polypropylene (PP) by an argon plasma is emulated in a particle beam experiment. An ion beam deflector, used to steer argon ions from an electron-cyclotron-resonance (ECR) plasma source towards the sample, suppresses the UV and VUV photons generated in the plasma volume. The modification of PP surface by 200 and 500eV ions is monitored by in situ Fourier transform infrared spectroscopy (FTIR). One observes a transition from an initial region of fast etching to a steady state without chemical modification and lower etching rate. This behavior is attributed to the progressive graphitization at the surface due to ion bombardment. An anti-synergism arises by adding UV photons because of cross-linking of the polymer at the subsurface region, which renders the etch rate much smaller compared to the etch rate by ion only impact. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Room temperature ionic liquids (RTILs) have been shown to exhibit spatial heterogeneity or structural heterogeneity in the sense that they form hydrophobic and ionic domains. Yet studies of the relationship between this structural heterogeneity and the ∼picosecond motion of the molecular constituents remain limited. In order to obtain insight into the time scales relevant to this structural heterogeneity, we perform molecular dynamics simulations of a series of RTILs. To investigate the relationship between the structures, i.e., the presence of hydrophobic and ionic domains, and the dynamics, we gradually increase the size of the hydrophobic part of the cation from ethylammonium nitrate (EAN), via propylammonium nitrate (PAN), to butylammonium nitrate (BAN). The two ends of the organic cation, namely, the charged Nhead-H group and the hydrophobic Ctail-H group, exhibit rotational dynamics on different time scales, evidencing dynamical heterogeneity. The dynamics of the Nhead-H group is slower because of the strong coulombic interaction with the nitrate counter-ionic anions, while the dynamics of the Ctail-H group is faster because of the weaker van der Waals interaction with the surrounding atoms. In particular, the rotation of the Nhead-H group slows down with increasing cationic chain length, while the rotation of the Ctail-H group shows little dependence on the cationic chain length, manifesting that the dynamical heterogeneity is enhanced with a longer cationic chain. The slowdown of the Nhead-H group with increasing cationic chain length is associated with a lower number of nitrate anions near the Nhead-H group, which presumably results in the increase of the energy barrier for the rotation. The sensitivity of the Nhead-H rotation to the number of surrounding nitrate anions, in conjunction with the varying number of nitrate anions, gives rise to a broad distribution of Nhead-H reorientation times. Our results suggest that the asymmetry of the cations and the larger excluded volume for longer cationic chain are important for both the structural heterogeneity and the dynamical heterogeneities. The observed dynamical heterogeneities may affect the rates of chemical reactions depending on where the reactants are solvated in ionic liquids and provide an additional guideline for the design of RTILs as solvents. © 2018 Author(s).

One of the main obstacles to the industrial application of laser micro-spot welding of copper is the poor process repeatability at common laser wavelengths. The inhomogeneities in the oxide layer in combination with the high reflectivity of copper itself are often stated as the main reason for the poor repeatability of welding results. However, the typical local variations of the reflectivity alone are not sufficient to describe the instability of the welding process completely. Through numerical FEM simulations, the temperature dependence of the absorptivity could be identified as the main reason for the narrowing of the process window. The effect of the temperature-dependent absorptivity is enhanced by the temperature-dependence of the heat conductivity and slightly reduced by the enthalpy of fusion. The results of the simulation show a good agreement with experimental data. © 2018 IOP Publishing Ltd.

Dense Cr2AlC materials were tested under a gradient loading for the first time using a burner rig. The severe thermal cycling conditions consist of 500 short cycles at 1200°C, with an accumulative time at the maximal temperature of more than 29 hours. The samples showed no visible damage under these conditions due to the formation of an outer protective α-Al2O3 layer, which shows a strong adhesion with the Cr2AlC substrate. No cracks, delamination or damage were observed at the interface between the different layers. This excellent response under cyclic loading shows the excellent potential of Cr2AlC compounds for high-temperature applications. © 2018 American Ceramic Society.

We have observed the inversion of the solvation environment of a one-dimensional solid by low-temperature scanning tunneling microscopy. Adsorption of 3-methoxy-9-diazofluorene on Ag(111) yields highly oriented supramolecular chains, which are then exposed to water molecules. The annealing of dry and water-decorated chains results in diametrically opposed outcomes. While the former simply leads to an increase in chain length and number, the latter results in a complete loss of order and produces water clusters decorated with the organic molecule. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Temperature-induced, stress-induced martensitic phase transitions and martensite reorientation process in Ni rich (50.9 at.%) NiTi pseudoelastic NiTi wires with ultra-fine grained (UFG) microstructure were studied by electrical resistance measurements. Measurements of the electrical resistance as a function of temperature at different constant mechanical loads accompanied by complementary experiments with variable loads at constant temperature were performed. Results show that the transformation mechanisms in UFG microstructures exhibit a higher level of complexity when compared with those characterizing the behavior of other microstructures (e.g., recrystallized or larger grains size). It was found that a threshold stress level below 150 MPa delimits the transition from a homogeneous (low stress) to localized but reversible Lüders type transformation (high stress) when the transformations are induced under constant applied stress and that reorientation processes require stresses of 100 MPa in the present UFG wires. Even though the strain evolutions do not always show two distinct yielding events during cooling or heating, electrical resistance measurements proved that a two-step transformation involving R-phase and B19′ martensite was always present in the extended range of temperatures and stresses investigated here. © 2017 Elsevier B.V.

Inspired by our recent finding that Fe4.5Ni4.5S8 rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe:Ni ratio on the performance of the catalyst. We herein describe the synthesis of (FexNi1-x)9S8 (x = 0-1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry, we show that the increase in the iron or nickel content, respectively, lowers the activity of the electrocatalyst toward HER. Electrochemical surface area analysis (ECSA) clearly indicates the highest amount of active sites for a Fe:Ni ratio of 1:1 on the electrode surface pointing at an altered surface composition of iron and nickel for the other materials. Specific metal-metal interactions seem to be of key importance for the high electrocatalytic HER activity, which is supported by DFT calculations of several surface structures using the surface energy as a descriptor of catalytic activity. In addition, we show that a temperature increase leads to a significant decrease of the overpotential and gain in HER activity. Thus, we showcase the necessity to investigate the material structure, composition and reaction conditions when evaluating electrocatalysts. © 2017 American Chemical Society.

Diffusion is often the rate-limiting factor of reactions in condensed phase. Thus, knowing the diffusion coefficient is key in numerous aspects ranging from drug release to steering of reactions in both homogeneous liquid phase and electrochemical reactions. Cyclic voltammetry at macro electrodes and chronoamperometry at micro electrodes are well-established methods to determine the diffusion coefficients of redox-active species dissolved in a solution. However, if the formal potentials of the redox species are outside of the potential window of the solvent, then these methods cannot be readily applied. Here we demonstrate a new concept to determine the diffusion coefficient of ions to overcome this limitation. We use their reaction with a well-defined amount of a redox-active indicator substance, which is confined in a nanoparticle suspended in a solution containing the species of interest. Employing transformative nanoparticle impact analysis, the diffusion-limited reaction of an indicator nanoparticle with the species of interest is initiated and followed by chronoamperometry. Measuring the time it takes to fully convert the indicator particle enables the determination of the diffusion coefficient of interest. This concept is demonstrated for variety of (pseudo-)halides in aqueous solution using Ag nanoparticles as redox indicator. Using chloride as an example, is further shown that this new methodology can be applied to study effects of temperature and viscosity on the diffusion coefficients. Given the multitude of nanoparticles that may serve as electrochemical redox indicator, this approach can be used to determine the diffusion coefficients for a large variety of species in different liquid environments. © 2018 Elsevier Ltd

Strong electronic correlations, emerging from the parent Mott insulator phase, are key to copper-based high-temperature superconductivity. By contrast, the parent phase of an iron-based high-temperature superconductor is never a correlated insulator. However, this distinction may be deceptive because Fe has five actived d orbitals while Cu has only one. In theory, such orbital multiplicity can generate a Hund’s metal state, in which alignment of the Fe spins suppresses inter-orbital fluctuations, producing orbitally selective strong correlations. The spectral weights Zm of quasiparticles associated with different Fe orbitals m should then be radically different. Here we use quasiparticle scattering interference resolved by orbital content to explore these predictions in FeSe. Signatures of strong, orbitally selective differences of quasiparticle Zm appear on all detectable bands over a wide energy range. Further, the quasiparticle interference amplitudes reveal that Zx y< Zx z≪ Zy z, consistent with earlier orbital-selective Cooper pairing studies. Thus, orbital-selective strong correlations dominate the parent state of iron-based high-temperature superconductivity in FeSe. © 2018, The Author(s).

Scanning tunneling microscopy is sensitive to surface adsorbates to a much lower impurity level than most other surface science techniques. Even under the best vacuum and preparation conditions, a very low concentration of depressions of unknown origin is often observed in STM images of the coin metal surfaces. We outline a procedure to identify impurities by apparent height spectroscopy; a technique that can be easily performed by standard scanning tunneling microscopes. Apparent height spectroscopy, performed with a low-temperature scanning tunneling microscope, records the apparent height of an adsorbate with respect to the surface level over an extended voltage range at distinct voltage intervals. The spectra show characteristic features that can be used to identify adsorbates. We exemplify our method for two common impurities on Cu(111), oxygen atoms and carbon monoxide molecules. We reveal three characteristic differences in the apparent height spectroscopy of the two adsorbates: the dark region, the voltage of contrast reversal, and the onset of the lowest unoccupied molecular orbital. Each of these features is characteristic for the specific adsorbate/substrate system; giving three possibilities to identify the two species. The procedure can easily be extended to other impurities. © 2018 Author(s).

Direct current magnetron sputtering of Al by Ar and Ar/N2 low pressure plasmas was characterized by experimental and theoretical means in a unified consideration. Experimentally, the plasmas were analyzed by optical emission spectroscopy, while the film deposition rate was determined by weight measurements and laser optical microscopy, and the film composition by energy dispersive x-ray spectroscopy. Theoretically, a global particle and power balance model was used to estimate the electron temperature T e and the electron density n e of the plasma at constant discharge power. In addition, the sputtering process and the transport of the sputtered atoms were described using Monte Carlo models - TRIDYN and dsmcFoam, respectively. Initially, the non-reactive situation is characterized based on deposition experiment results, which are in agreement with predictions from simulations. Subsequently, a similar study is presented for the reactive case. The influence of the N2 addition is found to be twofold, in terms of (i) the target and substrate surface conditions (e.g., sputtering, secondary electron emission, particle sticking) and (ii) the volumetric changes of the plasma density n e governing the ion flux to the surfaces (e.g., due to additional energy conversion channels). It is shown that a combined experimental/simulation approach reveals a physically coherent and, in particular, quantitative understanding of the properties (e.g., electron density and temperature, target surface nitrogen content, sputtered Al density, deposited mass) involved in the deposition process. © 2018 IOP Publishing Ltd.

For several decades, the formation of carbon(C)-rich domains upon room temperature aging of supersaturated martensite has been a matter of debate. C-rich tweed-like patterns are observed to form after short aging times at room temperature and coarsen upon further aging. Here, we present a systematic atomic-scale investigation of carbide formation in Fe-15Ni-1C (wt.%) martensite after two to three years of isothermal room temperature aging by a combination of atom probe tomography and transmission electron microscopy. Owing to the sub-zero martensite start temperature of −25 °C, a fully austenitic microstructure is maintained at room temperature and the martensitic phase transformation is initiated during quenching in liquid nitrogen. In this way, any diffusion and redistribution of C in martensite is suppressed until heating up the specimen and holding it at room temperature. The microstructural changes that accompany the rearrangement of C atoms have been systematically investigated under controlled isothermal conditions. Our results show that after prolonged room temperature aging nanometer-sized, plate-shaped η-Fe2C carbides form with a macroscopic martensite habit plane close to {521}. The orientation relationship between the η-Fe2C carbides and the parent martensite grain (α′) follows [001]α’//[001]η, (1¯10) α’//(020)η. The observation of η-Fe2C–carbide formation at room temperature is particularly interesting, as transition carbides have so far only been reported to form above 100 °C. After three years of room temperature aging a depletion of Fe is observed in the η carbide while Ni remains distributed homogenously. This implies that the substitutional element Fe can diffuse several nanometers in martensite at room temperature within three years. © 2018

A torrefaction test rig was designed to investigate large single biomass particles up to characteristic sizes of 25 mm, typical for industrial reactors. Time-resolved mass loss for such particles is measured with a magnetic suspension balance at well-defined torrefaction conditions (temperature, residence time, gas atmosphere). This paper comprises the results of woody and non-woody biomass: pine, a coniferous, and beech, a deciduous, wood, palm kernel shells and miscanthus. Influence of process temperature (240 to 320 °C), residence time (up to 1 h) and type of solid biomass on time-resolved mass loss is presented. Additional tests with oxygen in the process gas (0–15 vol%), typical for industrial torrefaction systems, are carried out for selected samples of beech wood. The differences in torrefaction behaviour of bark, sap- and heartwood of pine are evaluated. Finally, it is shown that the torrefaction reactor developed allows to derive kinetic parameters for mass loss. At temperatures up to 300 °C the mass loss for palm kern shells is highest followed by miscanthus, and pine. By examining pine, as an example, it is shown that heartwood is significantly more reactive than sapwood and bark. Finally, it is demonstrated, that for the particle sizes considered here heat and mass transfer limitations can be neglected for the determination of torrefaction kinetics. Kinetic data agree well with data from literature. © 2018 Elsevier Ltd

In the flourishing fields of quantum technology gallium nitride (GaN) quantum dots (QDs) have great appeal by providing high stability and room-temperature operation. Here, we report on the ion implantation of surface GaN QDs grown in the hexagonal crystal structure. An uncapped sample (S1) and two samples capped by 8 ML (S2) and 16 ML (S3) of AlN are subjected to a 100 keV gallium (S1, S2) and a 210 keV erbium (S3) ion beam. The fluence ranged from 5×1010 cm−2 to 1×1015 cm−2 (S1, S2) and from 5×1010 cm−2 to 5×1013 cm−2 (S3). QD characterization is performed by cathodoluminescence measurements at 77 K and atomic force microscopy and scanning electron microscopy. Strong interdiffusion processes upon ion impact at the interfaces are evidenced leading besides other effects to a quenching of the quantum confined Stark effect. Moreover, a model for the QD morphology based on a fluence-dependent diffusion coefficient is developed. © 2017 Elsevier B.V.

We report As75 nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, CaKFe4As4, with Tc=35 K. As75 NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies νQ of the As(1) and As(2) sites show an opposite temperature T dependence. Nearly T independent behavior of the Knight shifts K is observed in the normal state, and a sudden decrease in K in the superconducting (SC) state suggests spin-singlet Cooper pairs. As75 spin-lattice relaxation rates 1/T1 show a power-law T dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wave vector q=(π,0) or (0,π) in the single-iron Brillouin zone notation are revealed by 1/T1T and K measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the As75 quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, 1/T1 shows a rapid decrease below Tc without a Hebel-Slichter peak and decreases exponentially at low T, consistent with an s± nodeless two-gap superconductor. © 2017 American Physical Society.

A martensitic transformation (MT) is a typical first-order diffusionless crystal structural change with strong autocatalysis like avalanche at a speed of sound propagation. This unique characteristic, however, is undetectable in some multifunctional titanium alloys. Recently, a nano-scale elastically confined MT mechanism was proposed because a nano-scale Nb modulation in a Ti-Nb based alloy was observed. Here we analyze the elastic confinement in details and its induced novel properties in a wide temperature range. The statistical analyses of atom probe tomography (APT) data confirm the existence of the nano-scale Nb concentration modulation. The synchrotron X-ray diffraction (SXRD) profiles demonstrate that the nano-scale Nb modulation causes weak diffuse scattering, as evidenced by the extreme broad diffraction bands. The tensile tests find a critical temperature of ∼150 K, where the critical stress to induce the MT and Young's modulus reach the minimum and the superelastic strain reaches the maximum (∼4.5%) and keeps constant as the temperature decreases further to <4.2 K. To reveal these abnormal behaviors of the MT, the Born criterion governing the elastic stability of cubic crystal is modified by introducing an elastic confinement term and a new Clausius-Clapeyron relationship is established to evaluate the elastically confined MT. The results are consistent with the experimental findings, including the solely stress-induced (no thermally induced) reversibility. © 2017 Acta Materialia Inc.

Covalent triazine framework (CTF-1) materials with hierarchical pore structures have been synthesised using a salt templating approach. As salt templates binary mixtures of ZnCl2 with various alkali chlorides (instead of only ZnCl2) in combination with a modified temperature protocol were utilised. The porosity of salt templated CTF-1 materials was analysed by means of argon physisorption at 87 K. In addition to microporosity, typical for CTF-1, the resulting materials show enhanced mesoporosity and have high total pore volumes of up to 2.1 cm3 g−1. The presented synthetic protocol provides an access to materials combining high nitrogen content, hierarchical pore structure, and high total pore volume, while established CTF syntheses at elevated temperatures used to increase the pore volume in general cause nitrogen loss. These new hierarchical CTFs are very promising cathode materials for lithium-sulphur batteries, where both characteristics (nitrogen content and mesoporosity) are crucial. © 2016 Elsevier Inc.

Free energies of bulk materials are nowadays routinely computed by density functional theory. In particular for metals, electronic excitations can significantly contribute to the free energy. For an ideal static lattice, this contribution can be obtained at low computational cost, e.g., from the electronic density of states derived at T=0 K or by utilizing the Sommerfeld approximation. The error introduced by these approximations at elevated temperatures is rarely known. The error arising from the ideal lattice approximation is likewise unexplored but computationally much more challenging to overcome. In order to shed light on these issues we have computed the electronic free energies for all 3d,4d, and 5d transition elements on the ideal lattices of the bcc, fcc, and hcp structures using finite-temperature density-functional theory. For a subset of elements we have explored the impact of explicit thermal vibrations on the electronic free energies by using ab initio molecular dynamics simulations. We provide an analysis of the observed chemical trends in terms of the electronic density of states and the canonical d band model and quantify the errors in the approximate methods. The electronic contribution to the heat capacities and the corresponding errors due to the different approximations are studied as well. © 2017 authors. Published by the American Physical Society.

Atomistic simulations of thermal desorption spectra for effusion from bulk materials to characterize binding or trapping sites are a challenging task as large system sizes as well as extended time scales are required. Here, we introduce an approach where we combine kinetic Monte Carlo with an analytic approximation of the superbasins within the framework of absorbing Markov chains. We apply our approach to the effusion of hydrogen from BCC iron, where the diffusion within bulk grains is coarse grained using absorbingMarkov chains, which provide an exact solution of the dynamics within a superbasin. Our analytic approximation to the superbasin is transferable with respect to grain size and elliptical shapes and can be applied in simulations with constant temperature as well as constant heating rate. The resulting thermal desorption spectra are in close agreement with direct kinetic Monte Carlo simulations, but the calculations are computationally much more efficient. Our approach is thus applicable to much larger system sizes and provides a first step towards an atomistic understanding of the influence of structural features on the position and shape of peaks in thermal desorption spectra. © 2017 The Author(s) Published by the Royal Society. All rights reserved.

Press hardening is used to produce automotive blanks with high tensile strength. In order to gain optimal blank properties it is necessary to rapidly decrease the blank temperature during press hardening. The thermal conductivity of the hot work tool steel used for die material is an important factor that defines the process time and the resulting blank properties. This study investigates the influence of temperature, alloying composition and heat treatment on the thermal conductivity of hot work tool steels used for press hardening dies. The dynamic method is used to determine the thermal conductivity of the hot work tool steels via an indirect measurement. The thermal conductivity decreases with increasing amount of alloying elements. In the temperature range between 295 and 473 K, which is the relevant temperature range for press hardening applications, the thermal conductivity of the hot work tool steels increases with temperature independent of the thermal treatment. With further increase of the temperature above 473 K the thermal conductivity of the hot work tool steels decreases. Copyright © 2017 Carl Hanser Verlag GmbH & Co. KG.

Due to their large latent heats, pseudoelastic Ni–Ti-based shape memory alloys (SMAs) are attractive candidate materials for ferroic cooling, where elementary solid-state processes like martensitic transformations yield the required heat effects. The present work aims for a chemical and microstructural optimization of Ni–Ti for ferroic cooling. A large number of Ni–Ti-based alloy compositions were evaluated in terms of phase transformation temperatures, latent heats, mechanical hysteresis widths and functional stability. The aim was to identify material states with superior properties for ferroic cooling. Different material states were prepared by arc melting, various heat treatments and thermo-mechanical processing. The cooling performance of selected materials was assessed by differential scanning calorimetry, uniaxial tensile loading/unloading, and by using a specially designed ferroic cooling demonstrator setup. A Ni(Formula presented.)Ti(Formula presented.)Cu5V(Formula presented.) SMA was identified as a potential candidate material for ferroic cooling. This material combines extremely stable pseudoelasticity at room temperature and a very low hysteresis width. The ferroic cooling efficiency of this material is four times higher than in the case of binary Ni–Ti. © 2017 World Scientific Publishing Company

Plasma spray-physical vapor deposition (PS-PVD) is a promising technology to produce columnar structured thermal barrier coatings with excellent cyclic lifetime. The characteristics of plasma jets generated by standard plasma gases in the PS-PVD process, argon and helium, have been studied by optical emission spectroscopy. Abel inversion was introduced to reconstruct the spatial characteristics. In the central area of the plasma jet, the ionization of argon was found to be one of the reasons for low emission of atomic argon. Another reason could be the demixing so that helium prevails around the central axis of the plasma jet. The excitation temperature of argon was calculated by the Boltzmann plot method. Its values decreased from the center to the edge of the plasma jet. Applying the same method, a spurious high excitation temperature of helium was obtained, which could be caused by the strong deviation from local thermal equilibrium of helium. The addition of hydrogen into plasma gases leads to a lower excitation temperature, however a higher substrate temperature due to the high thermal conductivity induced by the dissociation of hydrogen. A loading effect is exerted by the feedstock powder on the plasma jet, which was found to reduce the average excitation temperature considerably by more than 700 K in the Ar/He jet. © 2017, Springer Science+Business Media, LLC.

The catalytic hydrogenation of ethyl acetate to ethanol was studied at ambient pressure in the temperature range from 463 K to 513 K using Cu/ZrO2 catalysts obtained by co-precipitation as a function of the Cu loading. The hydrogenation was established as a reproducible probe reaction by determining optimal reaction parameters without deactivation or thermodynamic limitations. Power-law kinetics were determined yielding an apparent activation energy of 74 kJ mol−1 and reaction orders of 0.1–0.3 for H2 and −0.4 to 0.1 for ethyl acetate in the temperature range from 473 K to 503 K. Metallic Cu was found to be essential for the hydrogenation, but the catalytic activity was not proportional to the Cu surface area derived from N2O decomposition and temperature-programmed H2 desorption experiments identifying Cu/ZrO2 as bifunctional catalyst. The acidic sites of the ZrO2 matrix were probed by temperature-programmed experiments with ethyl acetate and NH3. Cu0 is assumed to provide atomic hydrogen by dissociative adsorption and spillover, but the reaction rate is more affected by the tight contact between the embedded Cu nanoparticles and the X-ray amorphous ZrO2 matrix. © 2017 Elsevier Inc.

The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered. © 2016 Elsevier B.V.

A Colombian hard coal was stepwise pyrolyzed from 200 to 800 °C, and the resulting changes in surface and material properties were investigated by thermogravimetric analysis and volumetric adsorption techniques as well as by density, surface area, ATR-IR and GC/MS measurements. It was observed that a loss of volatile compounds occurred up to a pyrolysis temperature of 600 °C. These compounds were identified as CO, CO2 and H2O and mainly large substituted aromatic compounds and long-chain hydrocarbons. The loss of functional groups was also monitored by a decrease of related IR bands. The devolatilization was found to cause an increase in density and surface area; the adsorbed amount of CO2 and O2 increased in this temperature region as well. The char pyrolyzed at 600 °C was the only sample with a hydrophobic surface. Increasing the temperature to 800 °C led to no further mass loss, but to a structural reorganization of the char indicated by the reappearance of aromatic IR bands. This high-temperature restructuring resulted in a decrease of density, surface area and adsorbed gas amount. © 2017 Elsevier B.V.

The difference in quantitative analysis performance between the voltage-mode and laser-mode of a local electrode atom probe (LEAP3000X HR) was investigated using a Fe-Cu binary model alloy. Solute copper atoms in ferritic iron preferentially field evaporate because of their significantly lower evaporation field than the matrix iron, and thus, the apparent concentration of solute copper tends to be lower than the actual concentration. However, in voltage-mode, the apparent concentration was higher than the actual concentration at 40 K or less due to a detection loss of matrix iron, and the concentration decreased with increasing specimen temperature due to the preferential evaporation of solute copper. On the other hand, in laser-mode, the apparent concentration never exceeded the actual concentration, even at lower temperatures (20 K), and this mode showed better quantitative performance over a wide range of specimen temperatures. These results indicate that the pulsed laser atom probe prevents both detection loss and preferential evaporation under a wide range of measurement conditions. © 2017 Elsevier B.V.

Ge was deposited as thickness gradient films at temperatures up to 800°C by direct current (DC) and high power pulsed magnetron sputtering (HPPMS). Structural characterization shows increased crystallization with increasing substrate temperature and film thickness. Thermal conductivity was measured by a novel high-throughput time-domain thermo-reflectance method. Thermo-electrical properties correlate to the degree of crystallization. Conductivities increase with increasing substrate temperature up to 500°C. For higher temperatures the trend reverses. A room temperature deposited/annealed film displays smaller crystallites (10nm) and lower thermal conductivity (5Wm-1K-1) compared to 25Wm-1K-1 for hot DC deposition. Compared to DC, HPPMS films show higher thermal conductivities up to 45Wm-1K-1. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Correlatively employing density functional theory and experiments congregated around high power pulsed magnetron sputtering, a plasma-surface model for metastable Cr0.8Al0.2N (space group Fm 3 m) is developed. This plasma-surface model relates plasma energetics with film composition, crystal structure, mass density, stress state, and elastic properties. It is predicted that N Frenkel pairs form during Cr0.8Al0.2N growth due to high-energy ion irradiation, yielding a mass density of 5.69 g cm-3 at room temperature and Young's modulus of 358-130 GPa in the temperature range of 50-700 K for the stress-free state and about 150 GPa larger values for the compressive stress of 4 GPa. Our measurements are consistent with the quantum mechanical predictions within 5% for the mass density and 3% for Young's modulus. The hypothesis of a stress-induced Young's modulus change may at least in part explain the spread in the reported elasticity data ranging from 250 to 420 GPa. © 2017 Author(s).

The transversal thermal bed mixing was experimentally investigated in a batch rotary drum with a diameter of 0.6 m and a length of 0.45 m. The drum was filled with two fractions of granular material with different thermal conditions and the mixing temperature in the solid bed was measured with thermocouples located at different bed height. Quartz sand with a mean particle diameter of dP=0.2 mm was used as test material. The operating parameters, rotational speed and filling degree of the drum were varied in the range of n=1–6 rpm and F=10–20% respectively, whereas the influence on thermal mixing time was evaluated. The thermal mixing behavior was shown in terms of time constant, number of bed rotation, peak time and mixing number. Thermal mixing time decreases with higher rotational speed and lower filling degree. Comparison between experimental data and penetration model shows good agreement for low rotational speeds. © 2016 Elsevier Ltd

Due to the layer-by-layer build-up of additively manufactured parts, the deposited material experiences a cyclic re-heating in the form of a sequence of temperature pulses. In the current work, this “intrinsic heat treatment (IHT)” was exploited to induce the precipitation of NiAl nanoparticles in an Fe-19Ni-xAl (at%) model maraging steel, a system known for rapid clustering. We used Laser Metal Deposition (LMD) to synthesize compositionally graded specimens. This allowed for the efficient screening of effects associated with varying Al contents ranging from 0 to 25 at% and for identifying promising concentrations for further studies. Based on the existence of the desired martensitic matrix, an upper bound for the Al concentration of 15 at% was defined. Owing to the presence of NiAl precipitates as observed by Atom Probe Tomography (APT), a lower bound of 3–5 at% Al was established. Within this concentration window, increasing the Al concentration gave rise to an increase in hardness by 225 HV due to an exceptionally high number density of 1025 NiAl precipitates per m3, as measured by APT. This work demonstrates the possibility of exploiting the IHT of the LMD process for the production of samples that are precipitation strengthened during the additive manufacturing process without need for any further heat treatment. © 2017

In solid oxide fuel cells (SOFC) for operating temperatures of 800 °C or below, the use of ferritic stainless steel can lead to degradation in cell performance due to chromium migration into the cells at the cathode side [1]. Application of a coating on the ferritic stainless steel interconnect is one option to prevent Cr outward migration through the coating. MnCo1.9Fe0.1O4 (in the following designated as MCF) spinels act as a diffusion barrier and retain high conductivity during operation [2]. Knowledge about the residual stress depth distribution throughout the complete APS coating system is important and can help to optimize the coating process. This implicitly requires reliable residual stress analysis in the coating, the interface region and in the substrate. For residual stress analysis on these specific layered systems diffraction based analysis methods (XRD) using laboratory X-ray sources can only by applied at the very surface. For larger depths sublayer removal is necessary to gain reliable residual stress data. The established method for sublayer removal is electrochemical etching, which fails, since the spinel layer is inert. However, a mechanical layer removal will affect the local residual stress distribution. As an alternative, mechanical residual stress analyses techniques can be applied. Recently, we established an approach to analyse residual stress depth distributions in thick film systems by means of the incremental hole drilling method [5, 6]. In this project, we refined our approach for the application on MCF coatings with a layer thickness between 60 – 125 µm. © 2017 Trans Tech Publications, Switzerland.

Solid state dewetting can be used for targeted patterning, but also causes degradation or failure of thin film devices. In this work the temperature-induced changes of a tetracrystalline model system with inhibited surface diffusion are studied. This is accomplished by growing Al thin films by molecular beam epitaxy on single crystalline (0001) oriented sapphire substrates. The as-deposited Al films form two orientation relationships (OR I and OR II) both subdivided in two twin-related growth variants leading to a tetracrystalline microstructure. Two processes evolve during annealing at 600 °C. Grain growth and texture evolution towards OR II occur in addition to the formation of drum-like voids in the Al film covered by a thin membrane. The surface oxide suppresses Al surface diffusion and in contrast to classical solid state dewetting interface and grain boundary diffusion dominate. High energy grain boundaries were identified as initial points of the void formation. © 2017 Acta Materialia Inc.

Au/Mg(OH)2 catalysts have been reported to be far more active in the catalytic low-temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO2 catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)2 with CO2 formed during the low-temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO2 also catalyzes CO2 formation, but does not allow for CO2 desorption, which results in self-poisoning. At higher temperatures (above 0 °C), however, CO2 formation is rate-limiting, which results in a much higher activity for Au/TiO2 under these reaction conditions. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Titanium–tantalum based alloys can demonstrate a martensitic transformation well above 100 °C, which makes them attractive for shape memory applications at elevated temperatures. In addition, they provide for good workability and contain only reasonably priced constituents. The current study presents results from functional fatigue experiments on a binary Ti–25Ta high-temperature shape memory alloy. This material shows a martensitic transformation at about 350 °C along with a transformation strain of 2 pct at a bias stress of 100 MPa. The success of most of the envisaged applications will, however, hinge on the microstructural stability under thermomechanical loading. Thus, light and electron optical microscopy as well X-ray diffraction were used to uncover the mechanisms that dominate functional degradation in different temperature regimes. It is demonstrated the maximum test temperature is the key parameter that governs functional degradation in the thermomechanical fatigue tests. Specifically, ω-phase formation and local decomposition in Ti-rich and Ta-rich areas dominate when T max does not exceed ≈430 °C. As T max is increased, the detrimental phases start to dissolve and functional fatigue can be suppressed. However, when T max reaches ≈620 °C, structural fatigue sets in, and fatigue life is again deteriorated by oxygen-induced crack formation. Copyright © Materials Research Society 2017

Reduced-activation ferritic-martensitic (RAFM) Eurofer-97 steel is a candidate material for structural applications in future fusion reactors. Depending on the amount of prior cold rolling strain and annealing temperature, important solid-state softening reactions such as recovery, recrystallization, and grain growth occur. Eurofer-97 steel was cold rolled up to 70, 80 and 90% reductions in thickness and annealed in the ferrite phase field (below ≈ 800 °C). Changes in microstructure, micro-, and mesotexture were followed by orientation mappings provided by electron backscatter diffraction (EBSD). Eurofer-97 steel undergoes abnormal grain growth above 650 °C and this solid-state reaction seems to be closely related to the high mobility of a few special grain boundaries that overcome pinning effects caused by fine particles. This solid-state reaction promotes important changes in the microstructure and microtexture of this steel. Abnormal grain growth kinetics for each condition was determined by means of quantitative metallography. © 2016 Elsevier B.V.

Depending on the detailed geometry, gate voltage, and circuitry, nanoscale Si/SiGe cross junctions at low temperatures exhibit full-wave rectification arising from different mechanisms like change in the number of current-carrying modes, stationary ballistic charging of a current-free voltage lead, and hot-electron thermopower. We study the rectifier structures on high-mobility Si/SiGe heterostructures consisting of a straight voltage stem and oblique current-injecting leads. Local gate electrodes are used to control the electron density in the voltage or current channel. Compared to three-terminal Y-branch junctions, the four-terminal cross junction eliminates the mode effect. A gradual increase of output voltage as gate-voltage is reduced until threshold voltage is identified as contribution of hot-electron thermopower. Heating the initially cold reservoir from a second orthogonal cross junction eliminates the electron temperature gradient and suppresses the thermopower. Even if the operation as six-terminal device re-induces a mode-controlled contribution, we demonstrate that it is negligible. As expected, the ballistic signal can be reliably separated from other mechanisms by measurements under positive gate voltage. The ballistic voltage can be described by a parabolic function of the injected current and is proportional to the cosine of the injection angle. © 2017 Author(s).

Iron pnictides and related materials have been a topic of intense research for understanding the complex interplay between magnetism and superconductivity. Here we report on the magnetic structure of SrMn2As2 that crystallizes in a trigonal structure (P3m1) and undergoes an antiferromagnetic (AFM) transition at TN=118(2) K. The magnetic susceptibility remains nearly constant at temperatures T≤TN with H∥c whereas it decreases significantly with H≈ab. This shows that the ordered Mn moments lie in the ab plane instead of aligning along the -axis as in tetragonal BaMn2As2. Single-crystal neutron diffraction measurements on SrMn2As2 demonstrate that the Mn moments are ordered in a collinear Néel AFM phase with 180° AFM alignment between a moment and all nearest neighbor moments in the basal plane and also perpendicular to it. Moreover, quasi-two-dimensional AFM order is manifested in SrMn2As2 as evident from the temperature dependence of the order parameter. © 2016 IOP Publishing Ltd.

A CrMnFeCoNi high-entropy alloy was investigated by nanoindentation from room temperature to 400 °C in the nanocrystalline state and cast plus homogenized coarse-grained state. In the latter case a âŒ100)-orientated grain was selected by electron back scatter diffraction for nanoindentation. It was found that hardness decreases more strongly with increasing temperature than Young's modulus, especially for the coarse-grained state. The modulus of the nanocrystalline state was slightly higher than that of the coarse-grained one. For the coarse-grained sample a strong thermally activated deformation behavior was found up to 100-150 °C, followed by a diminishing thermally activated contribution at higher testing temperatures. For the nanocrystalline state, different temperature dependent deformation mechanisms are proposed. At low temperatures, the governing processes appear to be similar to those in the coarse-grained sample, but with increasing temperature, dislocation-grain boundary interactions likely become more dominant. Finally, at 400 °C, decomposition of the nanocrystalline alloy causes a further reduction in thermal activation. This is rationalized by a reduction of the deformation controlling internal length scale by precipitate formation in conjunction with a diffusional contribution. © 2017 Materials Research Society.

We investigated the deformation mechanisms of long period stacking ordered (LPSO) structures in an extruded Mg97Y2Zn1 (at%) alloy. Tensile deformation was performed in such a way that basal slip and kink band formation were inhibited. Slip trace analysis and transmission electron microscopy reveal a predominant activity of non-basal < a> slip. © 2016 Elsevier B.V.

Aiming at a better understanding of the unusual low-temperature CO oxidation reaction behavior on Au/Mg(OH)2 catalysts, we investigated this reaction mainly by combined kinetic and in situ IR spectroscopy measurements over a wide range of temperatures, from -90 °C to 200 °C. Catalysts with a very narrow Au particle size distribution were prepared by colloidal deposition. Kinetic measurements, performed under differential, dry reaction conditions at different constant temperatures, enabled the separation of thermal and deactivation effects. They revealed that the distinct reaction behavior, with an exceptionally high activity at temperatures below 0 °C and decreasing CO oxidation rates in the range between -50 °C and 30 °C, equivalent to a negative apparent activation energy, does not result from either deactivation effects or H2O trace impurities, but is an intrinsic feature of the reaction. An unusual temperature dependence was also observed for the tendency for deactivation, with a pronounced maximum at -20 °C, which mainly results from an accumulation of surface carbonate species blocking active reaction sites or access of adsorbed reactants to them. Similar measurements on Au/TiO2 catalysts revealed that the high activity of Au/Mg(OH)2 in the low-temperature range compared to Au/TiO2 is first of all due to the weaker interactions of Mg(OH)2 with CO2 compared to TiO2. This leads to an increasing tendency of CO2 product molecules to adsorb on the latter catalyst at reaction temperatures below 0 °C and hence to rapid 'self-poisoning' with CO2 desorption as the rate-limiting step. For Au/Mg(OH)2, CO2 desorption is much faster, allowing much higher rates in the continuous CO oxidation. Based on temporal analysis of products (TAP) reactor measurements, the decay of the reaction rates in the range -50 °C to +50 °C is tentatively attributed to a decreasing steady-state coverage of weakly bound molecularly adsorbed O2 with increasing temperature, while stable adsorbed active surface oxygen is negligible over the entire range of reaction temperatures investigated. The implications of these and earlier findings for the mechanistic understanding of the low-temperature CO oxidation on Au/Mg(OH)2 and support effects therein are discussed. © The Royal Society of Chemistry 2017.

High-throughput methods were used to investigate a Ni-Co-Al thin film materials library, which is of interest for structural and functional applications (superalloys, shape memory alloys). X-ray diffraction (XRD) measurements were performed to identify the phase regions of the Ni-Co-Al system in its state after annealing at 600 °C. Optical, electrical, and magneto-optical measurements were performed to map functional properties and confirm XRD results. All results and literature data were used to propose a ternary thin film phase diagram of the Ni-Co-Al thin film system. © 2017 American Chemical Society.

Identification and synthesis of intramolecularly donor-stabilized aluminium(III) complexes, which contain a 3-(dimethylamino)propyl (DMP) ligand, as novel atomic layer deposition (ALD) precursors has enabled the development of new and promising ALD processes for Al2O3 thin films at low temperatures. Key for this promising outcome is the nature of the ligand combination that leads to heteroleptic Al complexes encompassing optimal volatility, thermal stability and reactivity. The first ever example of the application of this family of Al precursors for ALD is reported here. The process shows typical ALD like growth characteristics yielding homogeneous, smooth and high purity Al2O3 thin films that are comparable to Al2O3 layers grown by well-established, but highly pyrophoric, trimethylaluminium (TMA)-based ALD processes. This is a significant development based on the fact that these compounds are non-pyrophoric in nature and therefore should be considered as an alternative to the industrial TMA-based Al2O3 ALD process used in many technological fields of application. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

The tensile properties of CrCoNi, a medium-entropy alloy, have been shown to be significantly better than those of CrMnFeCoNi, a high-entropy alloy. To understand the deformation mechanisms responsible for its superiority, tensile tests were performed on CrCoNi at liquid nitrogen temperature (77 K) and room temperature (293 K) and interrupted at different strains. Microstructural analyses by transmission electron microscopy showed that, during the early stage of plasticity, deformation occurs by the glide of 1/2<110> dislocations dissociated into 1/6<112> Shockley partials on {111} planes, similar to the behavior of CrMnFeCoNi. Measurements of the partial separations yielded a stacking fault energy of 22 ± 4 mJ m−2, which is ∼25% lower than that of CrMnFeCoNi. With increasing strain, nanotwinning appears as an additional deformation mechanism in CrCoNi. The critical resolved shear stress for twinning in CrCoNi with 16 μm grain size is 260 ± 30 MPa, roughly independent of temperature, and comparable to that of CrMnFeCoNi having similar grain size. However, the yield strength and work hardening rate of CrCoNi are higher than those of CrMnFeCoNi. Consequently, the twinning stress is reached earlier (at lower strains) in CrCoNi. This in turn results in an extended strain range where nanotwinning can provide high, steady work hardening, leading to the superior mechanical properties (ultimate strength, ductility, and toughness) of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi. © 2017 Acta Materialia Inc.

We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 °C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomography (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 °C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions. © 2016 Acta Materialia Inc.

A binary nanocrystalline Cu67Cr33 thin film alloy consisting of columnar grains was synthesized via co-evaporation of the constituent elements under non-equilibrium ultra-high vacuum conditions using molecular beam epitaxy. In the as-deposited state, the alloy film is a supersaturated solid solution with a single-phase body-centered cubic structure. In order to study the thermal stability of the microstructure and phase separation behavior towards the two phase equilibrium structure, isothermal annealing experiments in a temperature range of 150 °C – 500 °C were conducted inside a transmission electron microscope and compared to data obtained by X-ray diffraction under protective N2 atmosphere. It is shown that the single-phase nature of the alloy film is maintained for annealing temperatures of ≤ 300 °C, whereas heat treatment at temperatures of ≥ 400 °C results in the formation of a second phase, i.e. the equilibrium face-centered cubic phase of Cu. Phase separation proceeds predominantly by a spinodal-type decomposition process but a simultaneous diffusion of Cr along the columnar grain boundaries to the surface of the alloy film is observed as well. Temperature dependent diffusion coefficients for volume and grain boundary diffusion along with the activation energy for volume diffusion of Cr within the crystal lattice of the alloy film in a temperature range between 400 °C – 500 °C are determined from analytical in situ transmission electron microscopy experiments. Moreover, grain boundary diffusion of Cr leads to the growth of an external Cr-rich oxide scale. It is found that the growth kinetics of this oxide scale exhibits a transition from a linear to a nearly parabolic growth rate. © 2016 Elsevier B.V.

For the first time, synthesis of two new amidinate-ligand comprising heteroleptic indium complexes, namely [InCl(amd)2] (1) and [InMe(amd)2] (2), via salt-metathesis and their detailed characterization is reported. For comparison, the earlier reported homoleptic tris-amidinate [In(amd)3] (3) was also synthesized and analyzed in detail especially with respect to the thermal properties and molecular crystal structure analysis which are reported here for the first time. From nuclear magnetic resonance spectroscopy (NMR) and single-crystal X-ray diffraction (XRD), all three compounds were found to be monomeric with C2 (compound 1 and 2) and C3 symmetry (compound 3). Both halide-free compounds 2 and 3 were evaluated regarding their thermal properties using temperature-dependent 1H-NMR, thermogravimetric analysis (TGA) and iso-TGA, revealing suitable volatility and thermal stability for their application as potential precursors for chemical vapor phase thin film deposition methods. Indeed, metalorganic chemical vapor deposition (MOCVD) experiments over a broad temperature range (400 °C-700 °C) revealed the suitability of these two compounds to fabricate In2O3 thin films in the presence of oxygen on Si, thermally grown SiO2 and fused silica substrates. The as-deposited thin films were characterized in terms of their crystallinity via X-ray diffraction (XRD), morphology by scanning electron microscopy (SEM) and composition through complementary techniques such as Rutherford-backscattering spectrometry (RBS) in combination with nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS). From UV/Vis spectroscopy, the deposited In2O3 thin films on fused silica substrates were found to be highly transparent (T > 95% at 560 nm, compound 3). In addition, Hall measurements revealed high charge carrier densities of 1.8 × 1020 cm-3 (2) and 6.5 × 1019 cm-3 (3) with a Hall-mobility of 48 cm2 V-1 s-1 (2) and 74 cm2 V-1 s-1 (3) for the respective thin films, rendering the obtained thin films applicable as a transparent conducting oxide that could be suitable for optoelectronic applications. © 2017 The Royal Society of Chemistry.

Many synthesis parameters can influence zeolite crystallization, which include the molar ratio of reagents, water content, temperature, the selection of extraframework cations (organic or inorganic template) and so on. In this paper, two new materials, BUCT-1 with cuboid morphology and BUCT-2 with plate-like morphology were obtained based on the synthesis conditions of ITQ-17 by adjusting the GeO2/SiO2 ratio, H2O/TO2 ratio and temperature. The influence of the three factors on crystal size, crystallinity and phase selection and transformation was carefully discussed. Therein, phase selection and transformation is determined by their synergistic effects; while the influence on crystal size and crystallinity is different for different materials, which is caused by the differences in their structure, or in other words, the building unit, and chemical composition. In addition, the structure of BUCT-2 has already been solved as the stacking of sti layers, and it was identified as pure germanate molecular sieves. Meanwhile, through the characterization of XRD, ICP-AES, IR and STA, some topological information on BUCT-1, such as unit cell parameters, pore size and connectivity, was predicted and the work to present its final structure is still going on. © The Royal Society of Chemistry.

The application of high temperature ceramic coatings has enabled aircraft and power generation turbines to run at higher inlet temperatures for greater efficiency. Their use extends the lifetime of the superalloy blades that bear thermal gradients and mechanical loads during operation. In this work, ex-situ photo-luminescence spectroscopy was conducted to investigate the stresses within the thermally grown oxide of a thermal barrier coated tubular sample following complex realistic conditions, such as induced thermal gradients, and long duration aging. The resulting high spatial resolution stress contour maps highlight the development of the thermally grown oxide in response to the complex conditions. The outcomes highlight both the role of the aging process and the oxide growth's influence on the stress profile which varies spatially across the specimen. The results further provide early detection of micro-damaged zones in the oxide layer nondestructively. Improving the understanding of the coating system's response to loading conditions will allow for more accurate system modeling and early detection and monitoring of damage zones, which is critical for improving efficiency and longevity of aircraft and power generation turbines. © 2016 Elsevier B.V.

Yttria-stabilized zirconia (YSZ) is the state-of-the-art material for the top coat of thermal barrier coatings. To increase the efficiency and lifetime of gas turbines, the integration of MoSi2 as a healing material was proposed. A new method of manufacture was explored in order to enable the spraying of a homogeneous mixed layer of YSZ and MoSi2. As the chemical and physical properties of these powders are very different, they require contrasting process conditions. Due to the evaporation of Si from MoSi2 at spraying conditions suitable for YSZ, more moderate conditions and a shorter time of flight are required for depositing MoSi2. At the same time, the spraying conditions still need to be sufficient for melting the YSZ particles in order to produce a coating. To obtain a homogeneous mixture, both conditions can be matched using an injection system that allows powder injection at two different locations of the plasma jet. Two-color pyrometry during flight (DPV-2000, Tecnar) was used to monitor the actual particle temperature. By optimizing the injection point for the MoSi2, a mixed coating was obtained without decomposition of the MoSi2, which has been analyzed by means of XRD and SEM. © 2017, ASM International.

This study elucidates an innovative mechanochemical approach applying Friedel–Crafts alkylation to synthesize porous covalent triazine frameworks (CTFs). Herein, we pursue a counterintuitive approach by utilizing a rather destructive method to synthesize well-defined materials with intrinsic porosity. Investigating a model system including carbazole as monomer and cyanuric chloride as triazine node, ball milling is shown to successfully yield porous polymers almost quantitatively. We verified the successful structure formation by an in-depth investigation applying XPS, solid-state NMR and FT-IR spectroscopy. An in situ study of pressure and temperature developments inside the milling chamber in combination with two-dimensional liquid-state NMR spectroscopy reveals insights into the polymerization mechanism. The versatility of this mechanochemical approach is showcased by application of other monomers with different size and geometry. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

The spin-flip Raman scattering efficiency of the resident electron is thermally enhanced in singly charged (In,Ga)As/GaAs quantum dots, for probing the s- or p-shell trions. The Raman shift, polarization characteristics, and spectral position of the resonant scattering profile are insensitive to the sample temperature up to 50 K. This indicates a thermally robust mechanism of the coherent electron spin-flip based on exchange interaction. The background of the scattering spectra, whose intensity increases also by about one order of magnitude with temperature, is associated with acoustic phonon scattering. We propose that acoustic phonons enhance the spin-flip probability of the resident electron with growing temperature. The coherent spin-flip Raman scattering is ultimately suppressed at temperatures, which are a few times lower than that required for thermal trion dissociation. © 2017 American Physical Society.

Effect of small molecules, as they penetrate into a polymer system, is investigated via molecular dynamics simulations. It is found that small spherical particles reduce the glass transition temperature and thus introduce a softening of the material. Results are compared to experimental findings for the effect of different types of small molecules such as water, acetone and ethanol on the glass transition temperature of a polyurethane-based shape memory polymer. Despite the simplicity of the simulated model, MD results are found to be in good qualitative agreement with experimental data. © 2017 The Authors. Published by Elsevier B.V.

In modern gas turbines, highly loaded components are internally cooled and furthermore covered with thermal barrier coatings (TBCs) to withstand the harsh operating conditions with temperatures exceeding the application limit of such coatings. Under realistic operating conditions, siliceous minerals, of a calcium-magnesium-aluminum-silicate (CMAS) composition, are ingested into the turbine and deposited on the TBCs. Besides erosion, this also leads to degradation by chemical interaction. The plasma spray-physical vapor deposition (PS-PVD) process is an advanced method for manufacturing TBCs, which fills the gap between traditional thermal spray processes and electron beam physical vapor deposition (EB-PVD). Due to the unique plasma conditions, coatings with columnar microstructures exhibiting high strain tolerance can be created. However, because of the high amount of open porosity the resistance of such structures to CMAS and erosion attack was expected to be low. In the present work, PS-PVD TBCs were investigated in a burner rig facility under thermal gradient cycling conditions and simultaneous CMAS attack. The interactions of the PS-PVD-deposited YSZ and the CMAS melt were studied by means of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS) and compared to EB-PVD coatings. Additionally, the resistance of PS-PVD TBCs to erosion is compared to APS TBCs by means of room temperature tests according to ASTM G76-13. © 2017

We explore the competition and coupling of vibrational and electronic contributions to the heat capacity of Al and Al3Sc at temperatures below 50 K, combining experimental calorimetry with highly converged finite-temperature density functional theory calculations. We find that semilocal exchange-correlation functionals accurately describe the rich feature set observed for these temperatures, including electron-phonon coupling. Using different representations of the heat capacity, we are therefore able to identify and explain deviations from the Debye behavior in the low-temperature limit and in the temperature regime 30-50 K as well as the reduction of these features due to the addition of Sc. © 2017 authors. Published by the American Physical Society.

Oxyfuel ashes are supposed to form more sulfates than ashes from air-fired systems. This can be caused by the increased SO2 concentrations due to intensive flue gas recirculation in oxyfuel systems. Therefore, we investigated the spectral emittance characteristics of typical mineral sulfates in coal ashes, namely Mg and Ca sulfates. The samples were prepared in powder form. Two particle size fractions were examined (x < 32 μm and 125 < x < 160 μm). The powders were investigated concerning their temperature-dependent normal emittance in a radiation test rig. Spectral measurements by a Fourier transform infrared spectrometer in the temperature range from 500 to 1000 °C were carried out. The results reveal that Ca and Mg sulfates show characteristic S-O absorption bands in the wavelength regions from 3 to 4 μm, from 4.5 to 6 μm, and from 8 to 9.5 μm. MgSO4 transforms to MgO at around 930 °C. The total emittance of the oxide is significantly reduced by Δε = 0.15 compared to the sulfate. The small size fractions MgSO4 and CaSO4 undergo sintering when being heated, which influences emittance. An increase of total emittance up to a value of Δε = 0.08 is detected for CaSO4. Finally, it is shown that emittance increases with particle size (Δε in total emittance approximately = 0.1). © 2017 American Chemical Society.

Due to the complex influence of elevated temperatures on the characteristics of a tribological system, severe high temperature sliding abrasion of single phase metals is a unique type of wear. The mechanisms of high temperature sliding abrasion (indentation and grooving of metallic surfaces) are strongly governed by the temperature-dependent interaction between the bulk metal and the abrasive during the wear process. This interaction can be correlated with the metal physical and microstructural parameters of the worn metal, which consequently greatly influence abrasive wear processes. In this context, the present study deals with the influence of microstructural aspects of single phase steels on the mechanisms of high temperature abrasion. Investigations focus on the aspects of abrasion by performing high temperature hardness and sliding wear experiments (two-body, ceramic counter body) on bcc and fcc steels. Results confirm a clear lattice-structure dependence of the abrasion behavior of steels. Major differences exist in the stability of the mechanical and tribological properties of the bcc and fcc materials investigated. Hardness and work hardening of bcc steels decrease above 500 °C, leading to non-stationary wear. In contrast, fcc steels show a steady decrease of mechanical properties, avoiding instabilities. Accordingly, wear experiments and investigations of the wear scars (surface and subsurface regions) show a higher wear resistance and more favorable mechanisms of high temperature abrasion of fcc steels (e.g. pronounced micro-ploughing). Further, the microstructural elements of fcc steels high temperature abrasion resistance are investigated in more detail using X-ray diffraction. Microstructural analysis using diffraction-line broadening (Rietveld analysis) is used to determine the degree of plastic deformation (microstrain) and the phase fraction of α′-martensite of the austenitic wear scars. These parameters are related to the present mechanisms of abrasion, explaining the high temperature wear properties of fcc steels. © 2017 Elsevier B.V.

Tandem catalysis is an attractive strategy to intensify chemical technologies. However, simultaneous control over the individual and concerted catalyst performances poses a challenge. We demonstrate that enhanced pore transport within a Co/Al2O3 Fischer–Tropsch (FT) catalyst with hierarchical porosity enables its tandem integration with a Pt/ZSM-5 zeolitic hydrotreating catalyst in a spatially distant fashion that allows for catalyst-specific temperature adjustment. Nevertheless, this system resembles the case of close active-site proximity by mitigating secondary reactions of primary FT α-olefin products. This approach enables the combination of in situ dewaxing with a minimum production of gaseous hydrocarbons (18 wt %) and an up to twofold higher (50 wt %) selectivity to middle distillates compared to tandem pairs based on benchmark mesoporous FT catalysts. An overall 80 % selectivity to liquid hydrocarbons from syngas is attained in one step, attesting to the potential of this strategy for increasing the carbon efficiency in intensified gas-to-liquid technologies. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

We have developed a low-temperature atomic layer deposition (ALD) process for depositing crystalline and phase pure spinel cobalt oxide (Co3O4) films at 120 °C using [Co(tBu2DAD)2] and ozone as coreagent. X-ray diffraction, UV-vis spectroscopy, atomic force microscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis were performed to characterize the structure and properties of the films. The as-deposited Co3O4 films are crystalline with a low amount of impurities (<2% C and <5% H) despite low deposition temperatures. Deposition of Co3O4 onto thin TiO2 photoanodes (100 nm) for water oxidation resulted in 30% improvement of photocurrent (after 10 ALD cycles yielding small Co3O4 particles) as compared to pristine TiO2 films), and exhibited no detrimental effects on photocurrent response up to 300 deposition cycles (approximately 35 nm thick films), demonstrating the applicability of the developed ALD process for deposition of effective catalyst particles and layers in photoelectrochemical water-splitting devices. © 2017 American Chemical Society.

Spherical microresonators are high sensitivity sensors for the measurement of important physical quantities e.g. temperature or pressure. Measuring methods based on single optical resonators need expensive and delicate laser systems or spectral devices. The aim of this paper is to present a novel multi-purpose sensing technology based on whispering gallery modes in spherical microparticle arrays. Examples for different applications are given to prove the flexibility and usability of the method.

Co-based layered double hydroxide (LDH) catalysts with Fe and Al contents in the range of 15 to 45 at % were synthesized by an efficient coprecipitation method. In these catalysts, Fe3+ or Al3+ ions play an essential role as trivalent species to stabilize the LDH structure. The obtained catalysts were characterized by a comprehensive combination of surface- and bulk-sensitive techniques and were evaluated for the oxygen evolution reaction (OER) on rotating disk electrodes. The OER activity decreased upon increasing the Al content for the Co- and Al-based LDH catalysts, whereas a synergistic effect in Co- and Fe-based LDHs was observed, which resulted in an optimal Fe content of 35 at %. This catalyst was spray-coated on Ni foam electrodes and showed very good stability in a flow-through cell with a potential of approximately 1.53 V at 10 mA cm−2 in 1 m KOH for at least 48 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

We systematically studied the transformation temperatures, type and growth kinetics of phases formed during annealing of Fe – 10.10 Ti – 3.86 B alloys, initially amorphous after rapid solidification. With increasing the temperature, four distinct crystallisation steps could be determined. At 530 °C α-Fe (partly supersaturated with Ti) appeared, and at 600 °C small amounts of not identified phases formed in the amorphous matrix. The matrix fully crystallised to α-Fe and contained (Fe,Ti)-B rich particles at 730 °C, which transformed to the equilibrium TiB2 phase at 755 °C. During annealing at 1000 °C both α-Fe and TiB2 exhibited parabolic, apparently diffusion controlled growth at rates of about 2.333 and 0.466 nm s−1/2, respectively. Factors influencing the amorphisation as well as strategies for the fabrication of nano-structured high stiffness low density steels via crystallisation from an amorphous precursor state are outlined and discussed. © 2017 Elsevier B.V.