While the primary properties of thermoelectric devices, directly related to the conversion efficiency, are considered in design efforts, the secondary (thermomechanical) properties are often ignored or overlooked even though they can lead to failure. Here, thermomechanical properties of thermoelectric ZrNiSn in the amorphous and crystalline state (space group F-43m), comprising thermal conductivity, thermal expansion, elastic (Young’s) modulus, and thermal shock, are studied using density functional theory and two phonon models. Thermal conductivity is also a key primary property for thermoelectric applications. Amorphous ZrNiSn exhibits a fourfold lower thermal conductivity than the crystalline counterpart due to high phonon–phonon scattering, which is conducive to thermoelectric performance. However, this is conflicting since a high thermal conductivity value is required to attain high resistance to thermal shock. Due to stronger bonds in the crystalline counterpart, facilitated by the stronger Zr 3d – Ni 3d and Sn 5p – Ni 3d hybridization and higher coordination than in the amorphous state, the linear coefficient of thermal expansion is lower, and the elastic modulus is higher. Hence, the crystalline state yields higher resistance to thermal shock. It is suggested that samples entailing both amorphous and crystalline regions can concurrently satisfy the primary and secondary requirements for enhanced efficiency and durability.

In the present work, we have used classical molecular dynamics and quantum mechanical density functional theory modeling to investigate the grain size-dependent thermal expansion coefficient (CTE) of nanocrystalline Cu. We find that the CTE increases by up to 20% with a gradually decreasing grain size. This behavior emerges as a result of the increased population of occupied anti-bonding states and bond order variation in the grain boundary regions, which contribute to thereduced resistance against thermally-induced bond stretching and dictate the thermal expansion behavior in the small grain size limit. As a part of the present work, we have established a procedure to produce ab initio thermal expansion maps that can be used for the prediction of the grain size dependent CTE. This can serve as a modeling tool, e.g., to explore the impact of grain boundary impurity segregation on the CTE.

Refractory high/medium entropy nitrides (HENs/MENs) exhibit comprehensive application prospects as protective films on mechanical parts, particularly those subjected to sliding contacts at elevated temperatures. In this study, a new MEN system TiNbWN, forming a single fcc solution, is designed and its wear performance at temperatures ranging from 25 to 750 °C is explored. The wear mechanisms can be rationalized by examining the subsurface microstructural evolutions using the transmission electron microscopy as well as calculating the phase diagrams and interfacial adhesion behavior employing calculation of phase diagram (CALPHAD) and density functional theory (DFT). To be specific, increased wear losses occur in a temperature range of 25–600 °C, being predominantly caused by the thermally-induced hardness degradation; whereas at the ultimate temperature (750 °C), the wear loss is refrained due to the formation of nanocrystalline oxides (WnO3n−2, TiO2, and γTiOx), as synergistically revealed by microscopy and CALPHAD, which not only enhance the mechanical properties of the pristine nitride film, but also act as solid lubricants, reducing the interfacial adhesion. Thus, our work delineates the role of the in situ formed nanocrystalline oxides in the wear mechanism transition of TiNbWN thin films, which could shed light on the high-temperature wear behavior of refractory HEN/MEN films.

To increase the understanding of the role of carbide precipitates on the hydrogen embrittlement of martensitic steels, we have performed a density functional theory study on the solution energies and energy barriers for hydrogen diffusion in orthorhombic M₇C₃ (M = Cr, Mn, Fe). Hydrogen can easily diffuse into the lattice and cause internal stresses or bond weakening, which may promote reduced ductility. Solution energies of hydrogen at different lattice positions have systematically been explored, and the lowest values are -0.28, 0.00, and 0.03 eV/H-atom for Cr₇C₃, Mn₇C₃, and Fe₇C₃, respectively. Energy barriers for the diffusion of hydrogen atoms have been probed with the nudged elastic band method, which shows comparably low barriers for transport via interstitial octahedral sites for all three systems. Analysis of the atomic volume reveals a correlation between low solution energies and energy barriers and atoms with large atomic volumes. Furthermore, it shows that the presence of carbon tends to increase the energy barrier. Our results can explain previous experimental findings of hydrogen located in the bulk of CrC precipitates and provide a solid basis for future design efforts of steels with high strength and commensurable ductility.

Ion-irradiation-induced changes in structure, elastic properties, and thermal stability of metastable c-(Ti,Al)N thin films synthesized by high-power pulsed magnetron sputtering (HPPMS) and cathodic arc deposition (CAD) are systematically investigated by experiments and density functional theory (DFT) simulations. While films deposited by HPPMS show a random orientation at ion kinetic energies (E_k)>105 eV, an evolution towards (111) orientation is observed in CAD films for E_k>144 eV. The measured ion energy flux at the growing film surface is 3.3 times larger for CAD compared to HPPMS. Hence, it is inferred that formation of the strong (111) texture in CAD films is caused by the ion flux- and ion energy-induced strain energy minimization in defective c-(Ti,Al)N. The ion energy-dependent elastic modulus can be rationalized by considering the ion energy- and orientation-dependent formation of point defects from DFT predictions: The balancing effects of bombardment-induced Frenkel defects formation and the concurrent evolution of compressive intrinsic stress result in the apparent independence of the elastic modulus from E_k for HPPMS films without preferential orientation. However, an ion energy-dependent elastic modulus reduction of ∼18% for the CAD films can be understood by considering the 34% higher Frenkel pair concentration formed at E_k=182 eV upon irradiation of the experimentally observed (111)-oriented (Ti,Al)N in comparison to the (200)-configuration at similar E_k. Moreover, the effect of Frenkel pair concentration on the thermal stability of metastable c-(Ti,Al)N is investigated by differential scanning calorimetry: Ion-irradiation-induced increase in Frenkel pairs concentration retards the wurtzite formation temperature by up to 206 °C.

Mg3Sb2-based thermoelectric materials attract attention for applications near room temperature. Here, Mg-Bi films were synthesized using magnetron sputtering at deposition temperatures from room temperature to 400 °C. Single-phase Mg3Bi2 thin films were grown on c-plane-oriented sapphire and Si(100) substrates at a low deposition temperature of 200 °C. The Mg3Bi2 films grew epitaxially on c-sapphire and fiber-textured on Si(100). The orientation relationships for the Mg3Bi2 film with respect to the c-sapphire substrate are (0001) Mg3Bi2‖(0001) Al2O3 and [112⎯⎯2¯0] Mg3Bi2‖[112⎯⎯2¯0] Al2O3. The observed epitaxy is consistent with the relatively high work of separation, calculated by the density functional theory, of 6.92 J m−2 for the Mg3Bi2 (0001)/Al2O3 (0001) interface. Mg3Bi2 films exhibited an in-plane electrical resistivity of 34 μΩ m and a Seebeck coefficient of +82.5 μV K−1, yielding a thermoelectric power factor of 200 μW m−1 K−2 near room temperature.This work was supported financially by the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971), the Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program (No. KAW-2020.0196), the Swedish Research Council (VR) under Project Nos. 2016-03365 and 2021-03826, the National Key Research and Development Program of China under Grant No. 2018YFB0703600, the National Natural Science Foundation of China under Grant No. 51872133, the Guangdong Innovative and Entrepreneurial Research Team Program under Grant No. 2016ZT06G587, and the Tencent Foundation through the XPLORER PRIZE, Guangdong Provincial Key Laboratory Program (No. 2021B1212040001) from the Department of Science and Technology of Guangdong Province. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at National Supercomputer Centre (NSC) partially funded by the Swedish Research Council through Grant Agreement No. 2018-05973.

The influence of changes induced by ion irradiation on structure and thermal stability of metastable cubic (Ti,Al)N coatings deposited by cathodic arc evaporation is systematically investigated by correlating experiments and theory. Decreasing the nitrogen deposition pressure from 5.0 to 0.5 Pa results in an ion flux-enhancement by a factor of three and an increase of the average ion energy from 15 to 30 eV, causing the stress-free lattice parameter to expand from 4.170 to 4.206 Å, while the chemical composition of Ti0.27Al0.21N0.52 remains unchanged. The 0.9% lattice parameter increase is a consequence of formation of Frenkel pairs induced by ion bombardment, as revealed by density functional theory (DFT) simulations. The influence of the presence of Frenkel pairs on the thermal stability of metastable Ti0.27Al0.21N0.52 is investigated by scanning transmission electron microscopy, differential scanning calorimetry, atom probe tomography and in-situ synchrotron X-ray powder diffraction. It is demonstrated that the ion flux and ion energy induced formation of Frenkel pairs increases the thermal stability as the Al diffusion enabled crystallization of the wurtzite solid solution is retarded. This can be rationalized by DFT predictions since the presence of Frenkel pairs increases the activation energy for Al diffusion by up to 142%. Hence, the thermal stability enhancement is caused by a hitherto unreported mechanism - the Frenkel pair impeded Al mobility and thereby retarded formation of wurtzite solid solution.

Crystalline-to-amorphous transitions induced by chemical modulation have been attracting great research interests and an in-depth understanding of such transitions is always demanding. Here we design the Nb-Ti-Zr-(Si) alloy systems employing the empirical criteria and the related phase diagrams for enhancing mechanical properties accompanied by Si-induced microstructure evolutions. (NbTiZr)100-xSix (x = 0, 3.0, 10.2, 22.2, and 25.0 at.%) refractory medium-entropy alloy (RMEA) coatings are synthesized by magnetron co-sputtering. With increasing Si content, the pristine body-centered cubic (bcc) single phase transforms into a nanocomposite structure consisting of bcc nanocrystals embedded in an amorphous matrix and eventually into an entirely amorphous structure. This is well rationalized with a thermodynamic database of the Nb-Ti-Zr-Si system constructed using the CALPHAD (CALculation of PHAse Diagrams) approach, which suggests the bcc structure to be thermodynamically stable at low Si contents and the formation of amorphous RMEA to be preferred at higher Si contents. The superior mechanical property of the amorphous (NbTiZr)75.0Si25.0 (at.%) coating compared to the Si-free counterpart is achieved, i.e., the hardness (H) increases by 115% and the elastic modulus (E) increases by 70%. The Si-induced crystalline-to-amorphous transition in RMEA which leads to a consistently impressive strengthening effect was rarely found in other alloys or coatings.

Thermal stability of protective coatings is one of the performance-defining properties for advanced cutting and forming applications as well as for energy conversion. To investigate the effect of oxygen incorporation on the high-temperature behavior of (Ti,Al)N, metastable cubic (Ti,Al)N and (Ti,Al)(OxN1-x) coatings are synthesized using reactive arc evaporation. X-ray diffraction of (Ti,Al)N and (Ti,Al)(OxN1-x) coatings reveals that spinodal decomposition is initiated at approximately 800 degrees C, while the subsequent formation of wurtzite solid solution is clearly delayed from 1000 degrees C to 1300 degrees C for (Ti,Al)(OxN1-x) compared to (Ti,Al)N. This thermal stability enhancement can be rationalized based on calculated vacancy formation energies in combination with spatially-resolved composition analysis and calorimetric data: Energy dispersive X-ray spectroscopy and atom probe tomography data indicate a lower O solubility in wurtzite solid solution compared to cubic (Ti,Al)(O,N). Hence, it is evident that for the growth of the wurtzite, AlN-rich phase in (Ti,Al)N, only mobility of Ti and Al is required, while for (Ti,Al)(O,N), in addition to mobile metal atoms, also non-metal mobility is required. Prerequisite for mobility on the non-metal sublattice is the formation of non-metal vacancies which require larger temperatures than for the metal sublattice due to significantly larger magnitudes of formation energies for the non-metal vacancies compared to the metal vacancies. This notion is consistent with calorimetry data which indicate that the combined energy necessary to form and grow the wurtzite phase is larger by a factor of approximately two in (Ti,Al)(O,N) than in (Ti,Al)N, causing the here reported thermal stability increase. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A phenomenon appears in a few examples of the chalcopyrites (space group I-42 d) where heavier atoms do not necessarily lead to lower lattice thermal conductivity, in contradiction with Keyes expression that formulates an inverse relation of thermal conductivity with mean atomic mass. Herewith, the thermal conductivity of CuInSe2, CuInTe2, AgInSe2, and AgInTe2 was calculated and compared at room temperature from the linearized Boltzmann transport equation using ab initio density functional theory. CuInSe2 and AgInSe2 solids exhibit lower lattice thermal conductivity than that of CuInTe2 and AgInTe2, respectively, despite the fact that Te atoms are significantly heavier than Se. A comparison between dispersion relation, the Grüneisen parameter, and projected density of states leads to the conclusion that anharmonic transverse acoustic modes in the form of anomalous vibrations of Cu and Ag cause the lower values of the thermal conductivity. By analyzing the electronic structure, the compounds under study fit perfectly into a recently defined region of the metavalent bonding well known for its pronounced anharmonicity. The insight gained from the current results deepens our understanding of the unusual heat transfer phenomenon related to the metavalent bonding and sheds light on design and discovery of thermally functional materials that break the prediction by the conventional theory.

Multiscale modelling, involving thermodynamic assessment and molecular dynamics based on density functionaltheory, was employed to unravel oxidation mechanisms pertinent to half-Heusler TiNiSn (space group F-43m), inparticular counterintuitive Ni inertness. O₂ molecules dissociate and chemisorb onto TiNiSn(001) and TiNiSn(110), which is followed by ingress of O. Both Ti and Sn egress, while Ni is less mobile. Such diffusion processesyield point defects (vacancies and interstitials) and give rise to Ti and Sn binary oxide formation, while Ni isinert, which may be corelated to its relatively low mobility. Based on the Mulliken analysis and thermodynamicsat 900 K, the Ti oxide formation sequence is suggested to be Ti₂O₃ → Ti₃O₅ → TiO₂ → TiO. These data explain theexperimental observations on the Ni inertness during oxidation of TiNiSn.

The present research front of NbO2 based memory, energy generation, and storage thin film devices is reviewed. Sputtering plasmas contain NbO, NbO2, and NbO3 clusters, affecting nucleation and growth of NbO2, often leading to a formation of nanorods and nanoslices. NbO2 (I41/a) undergoes the Mott topological transition at 1081 K to rutile (P42/mnm), yielding changes in the electronic structure, which is primarily utilized in memristors. The Seebeck coefficient is a key physical parameter governing the performance of thermoelectric devices, but its temperature behavior is still controversial. Nonetheless, they perform efficiently above 900 K. There is a great potential to improve NbO2 batteries since the theoretical capacity has not been reached, which may be addressed by future diffusion studies. Thermal management of functional materials, comprising thermal stress, thermal fatigue, and thermal shock, is often overlooked even though it can lead to failure. NbO2 exhibits relatively low thermal expansion and high elastic modulus. The future for NbO2 thin film devices looks promising, but there are issues that need to be tackled, such as dependence of properties on strain and grain size, multiple interfaces with point and extended defects, and interaction with various natural and artificial environments, enabling multifunctional applications and durable performance.

Ion irradiation-induced changes in the structure and mechanical properties of metastable cubic (V,Al)N deposited by reactive high power pulsed magnetron sputtering are systematically investigated by correlating experiments and theory in the ion kinetic energy (E_k) range from 4 to 154 eV. Increasing E_k results in film densification and the evolution from a columnar (111) oriented structure at E_k ≤ 24 eV to a fine-grained structure with (100) preferred orientation for E_k ≥ 104 eV. Furthermore, the compressive intrinsic stress increases by 336 % to -4.8 GPa as E_k is increased from 4 to 104 eV. Higher ion kinetic energy causes stress relaxation to -2.7 GPa at 154 eV. These ion irradiation-induced changes in the thin film stress state are in good agreement with density functional theory simulations. Furthermore, the measured elastic moduli of (V,Al)N thin films exhibit no significant dependence on E_k. The apparent independence of the elastic modulus on E_k can be rationalized by considering the concurrent and balancing effects of bombardment-induced formation of Frenkel pairs (causing a decrease in elastic modulus) and evolution of compressive intrinsic stress (causing an increase in elastic modulus). Hence, the evolution of the film stresses and mechanical properties can be understood based on the complex interplay of ion irradiation-induced defect generation and annihilation.