Vacancy Clusters Research Articles

Effect of radiation defects on grain boundary evolution under shock loading

Grain boundary (GB) failure in tungsten under shock loading after irradiation is a key factor to estimate the performance of tungsten as a plasma facing material in nuclear fusion reactors and as a target in spallation neutron sources. In this work, GB evolution after absorption of different radiation defect clusters under shock loading has been investigated at atomic scale through non-equilibrium molecular dynamics (NEMD) method. Different to the cases without radiation defects, after absorption of interstitial dislocation loops and voids, two mechanisms have been identified for the decrease of critical shock velocity (vc) and the related GB failure. The direct void growth accompanied by appearance of disordered structure around void, from the remaining vacancy cluster which could not be annihilated by compressive stress wave, is the 1st mechanism. In the 2nd mechanism, activation of slip systems, dislocation formation, motion and multiplication, void nucleation and growth dominate the failure process. The higher potential energy and local stress concentration around a defect absorption region in GB are recognized for atomic motion deviations from shock loading direction, resulting in formation of disordered structure, activation of slip system and dislocation multiplication with lower vc under coupling effects of radiation damage and shock loading. These results indicate that under a high dose of radiation damage, GB failure induced by shock loading should be considered seriously in order to estimate the lifetime of tungsten-based plasma facing materials and spallation target materials.

Ordering of oxygen vacancies in hydroxyapatite under electron irradiation

Understanding the mechanisms of radiation induced defect generation is essential for assessment of materials to be used in nuclear environments. In the current work, we investigate the effect of 10 MeV electron irradiation on the generation and possible ordering of defects/oxygen vacancies in hydroxyapatite (HAP) powders. Comprehensive analyses of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Electron Paramagnetic Resonance (EPR) and Raman spectra establish that (a) HAP has a high level of structural stability even after exposure to electron irradiation; (b) irradiation leads to the generation of surface defects/oxygen vacancy predominantly in the PO43− polyanion site, particularly in part of asymmetric bonding within the P–O bonds and (c) there is noticeable evidence for an ordered arrangement of oxygen vacancies within the HAP irradiated at 1 MGy dose. These radiation-induced defects, such as oxygen vacancies, can migrate within the HAP lattice, leading to formation of defect/oxygen vacancies clusters which ultimately results in the ordering tendency of oxygen vacancies within the lattice. The observed ordering, as evidenced by the photoluminescence intensities, aligns with the sequence HAP-1MGy > HAP-20MGy > HAP, providing further support for the presence of ordered oxygen vacancies.

Defect dynamics studies during heat treatments in plastically deformed metals predicted for nuclear applications

We report on defects dynamics during heat treatment in plastically deformed metallic materials using positron annihilation lifetime spectroscopy carried out on the intense pulsed positron beam. The conducted experiment allowed us to observe the changes in the concentration and sizes of vacancy-like defects observed during in situ annealing. We monitored heat treatments up to 300 °C in hydrostatic extruded Ti and cavitation peened V–4Cr–4Ti alloy. We were able to track the recovery processes in Ti and redistribution of large voids at the surface of cavitation peened V–4Cr–4Ti alloy. The relaxation time during recovery was about 20 min. Performed experiments show that in cold-worked metallic materials significant changes in vacancy clusters concentrations occur at mildly elevated temperatures. The presented results give opportunity to the application of in situ observation of defects dynamic to similar problems related to thermomechanical processing of metallic materials.Graphical abstract

Characterization of Vacancy Defects Using TEM in Heavy-Ion-Irradiated Tungsten Foils

The nature and type of defects formed due to heavy-ion irradiation in tungsten foils are analyzed using transmission electron microscopy. The recrystallized tungsten foils were irradiated by 80 MeV gold ions at room temperature for a fluence of 1.3 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 1014\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{14}$$\\end{document} ions/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^2$$\\end{document} that amounts to a net displacement per atom (dpa) of 0.22. The defect structures were analyzed using bright-field and weak-beam dark field imaging at two different depths to understand the depth profile of the defects. It is found that the defect clusters formed during the irradiation, both at the near-surface and at 2 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu $$\\end{document}m depth are of vacancy type that is confirmed by the local strain analysis and supports the findings of vacancy clusters in positron lifetime measurements. The analysis also shows that the dislocation-lines were of pure edge, pure screw and mixed. The fraction of mixed dislocation is found to increase during irradiation at the expense of pure edge and screw dislocations.

Mechanical Properties and Hydrogen Embrittlement Resistance of the High‐Entropy Alloy CrFeMnNiCo and Its Subsystems

The effects of hydrogen on the mechanical properties of CrNiCo and CrFeNiCo medium‐entropy alloys (MEAs) and CrFeMnNiCo high‐entropy alloys (HEAs) are investigated. Although their total elongation is less than that of the commonly used stainless steel (SS) 316L (SS316L), the tensile strengths of HEAs and MEAs are 150–350 MPa higher than that of SS316L. Hydrogen charging up to 1400 appm (nominal concentration) does not affect the tensile strength of SS316L; however, it decreases the elongation by less than 20%. In contrast, hydrogen increases the tensile strength of MEAs and HEA, but has little effect on elongation. Among the MEAs and HEAs, CrNiCo exhibits the highest tensile strength and total elongation. No brittle fracture due to hydrogen is observed on the fracture surfaces of the H‐charged samples. However, nanotwin structures are more common in H‐charged MEAs and HEAs than in H‐uncharged MEAs and HEA. Additionally, the calculation results based on the first‐principles reveal for the first time that single vacancies or tiny vacancy clusters do not trap H in MEAs compared to HEAs, such that cracks due to H are unlikely to occur. Thus, the hydrogen embrittlement resistance of MEAs may be improved.

Unveiling the magnetic and optical properties of barium bismuthate thin films with distinct Ba/Bi ratios

This research aims to investigate the optical and magnetic properties of BaBiO3 (80:20), BaBiO3 (50:50), and BaBiO3 (30:70) thin films obtained by the polymeric precursor method. For better sample designations, the following codes were employed: (80:20) (BBO82), (50:50) (BBO55), and (30:70) (BBO37). The observed properties for barium bismuthates thin films were elucidated in terms of bismuth and oxygen vacancies as well as grain morphology, which can be controlled by stoichiometric changes (Ba/Bi ratios). According to the x-ray results the films crystallize in a rhombohedral BaBiO3 structure with an R-3R space group without any secondary phase as confirmed by Rietveld analyses. According to the photoluminescence results, x-ray photoelectron spectroscopy measurements as well as the law of approach to saturation (LAS), there are a significant presence of oxygen vacancies and vacancy clusters [BiO5.Vo.] in the films with high proportion of bismuth. The optical and magnetic properties developed by these films are originated from the following factors: tilting of BiO6 clusters which changes the length and angle of Bi–O–Bi bonds, creating a magnetic field responsible for the distinct Mr/Ms ratios, the formation of bipolarons caused by charge transfer between [BiO6] and [BiO5. Vo⋅] due to the oxygen vacancies formation, plate-like morphology which makes the domains easier to be switched under an applied external field resulting in a superparamagnetic behavior, charge disproportionation of various valence states (Bi+3 and Bi+5). Based on these results, new potential devices can emerge as possible tools to be applied in switches, actuators, spintronics, magnetic field sensors, etc.

Physical properties of MnFeAl-based alloys affected by Mn content and annealing

The present study is devoted to the Mn2.4Fe0.8Al0.8 and Mn2FeAl alloys prepared by induction melting and studied in their original and subsequently annealed states. The annealing was carried out at 773 K/100 h and 1073 K/100 h in the argon atmosphere. The microstructure, phase composition, magnetic properties, and atom arrangement are followed with regard to Mn content and annealing conditions.The scanning electron microscopy completed by the energy dispersive X-ray spectroscopy and neutron activation analysis has detected single-phase alloys with compositions close to the nominal ones. Their structure, analyzed by X-ray diffraction, was found to be primitive cubic β-Mn with the lattice parameters of 0.6359(2) nm for Mn2.4Fe0.8Al0.8 and 0.6339(1) nm for Mn2FeAl. The coherent potential approximation calculations and positron annihilation spectroscopy have allowed obtaining an overview to the arrangement of Mn, Fe, and Al atoms in the β-Mn structure and formation of the open volume defects. It is shown that Mn atoms occupy predominantly 8c Wyckoff sites and remaining Mn, Fe and Al atoms occupy 12d sites in equal 1/3 proportion. The open volume defects, vacancies and vacancy clusters, occur in both alloys and both annealed states in a very low concentration. From the magnetic viewpoint, both alloys in the as-prepared state and after annealing at 773 K/100 h are paramagnetic at room temperature with transition to antiferromagnetic state at about 30-40 K. The ac susceptibility measurements have indicated spin glass nature of the Mn2FeAl alloys. The annealing at a higher temperature, 1073 K/100 h, has affected mainly Mn2.4Fe0.8Al0.8 alloy manifesting a weak ferro-/ferrimagnetic contribution at room temperature contributing to a strong magnetic ordering below 42 K.

Revealing the critical role of vanadium in radiation damage of tungsten-based alloys

Refractory tungsten-based medium- and high-entropy alloys form a promising class of strong, heat- and radiation-resistant materials for nuclear applications. Here, we systematically explore the radiation tolerance of Mo–Nb–Ta–V–W alloys using molecular dynamics simulations and a machine-learned interatomic potential. Going from pure W to W-based equiatomic binaries, ternaries, quaternary, and the quinary high-entropy alloy, we simulate the radiation damage accumulation up to 0.4–0.8 dpa through cumulative collision cascades. We find that the radiation damage and evolution are controlled by a combination of cascade-induced clustering, the balance in migration of vacancies and interstitials, and defect cluster binding energies. These mechanisms are strongly influenced by atom size, among which vanadium as the smallest atom is decisive. In pure W and alloys without V, the microstructure is characterised by large and growing interstitial dislocation loops. In stark contrast, in alloys containing V, vacancy dislocation loops are formed directly in collision cascades and the balanced migration rate of vacancies and interstitials promotes recombination and prevents growth of interstitial loops. The atom size differences also lead to clear segregation, with small atoms (V) in interstitial clusters and larger atoms (Ta, Nb) segregating around vacancy clusters. Furthermore, the small cluster sizes, formation of vacancy loops, segregation, and balanced migration in V-containing alloys lead to low swelling and an exceptionally efficient defect annealing compared to pure W and V-free alloys. Our results highlight WTaV as a promising low-activation material, where almost 90% of defects are annealed at 2000 K during 5 ns compared to <20% in pure W.

Defectivity of Al:ZnO thin films with different crystalline order probed by Positron Annihilation Spectroscopy

Three Positron Annihilation Spectroscopy (PAS) techniques have been employed to investigate the point defects of Al-doped Zinc Oxide (AZO) thin films grown by Radio Frequency (RF) magnetron sputtering with different substrates and deposition parameters. The films were grown with thickness varying from 100 to 300 nm, and their crystalline quality ranged from single crystalline epitaxial to partially amorphous. We found that the main defect in the crystalline samples is the 3VZn−VO four vacancy complex, with a concentration around 1018−1019 cm−3. In polycrystalline films larger vacancy clusters, within 10%−20% of the total concentration, were detected. These vacancy clusters are inferred to be most likely located at the grain boundaries. In partially amorphous films the concentration of these larger vacancy clusters, located either at grain boundaries or in the amorphous regions of the film, approached even the 40%, and also some sub-nano voids have been observed.

Impacts of vacancy complexes on the room-temperature photoluminescence lifetimes of state-of-the-art GaN substrates, epitaxial layers, and Mg-implanted layers

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τPLRT) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τPLRT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τPLRT in n-GaN samples are compared with those of p-GaN, in which τPLRT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

Spatially-resolved cluster dynamics modeling of irradiation growth

We develop here a spatially resolved, three-dimensional continuum model coupling cluster dynamics (SR-CD) and crystal plasticity to investigate irradiation growth in zirconium. The model uses scale separation to divide the population of the irradiation cluster into mobile and immobile families. Small interstitial and vacancy clusters are modeled using anisotropic reaction–diffusion equations. Among the immobile clusters, an atomistically-informed vacancy cluster to vacancy loop transition is taken into account. The coupling between the evolution equation of CD and the plastic deformation of the material is two-fold, with stress-informed bias factors and local inelastic strains computed from the evolution of the evolving cluster population. The numerical implementation of the model utilizes the finite element method to analyze both single-crystal and polycrystalline samples. The growth strains that are computed align well with the experimental data provided by Carpenter for single-crystal Zr. Furthermore, the transformation of a vacancy cluster into a complete vacancy loop, occurring at a size of 14 nm, is in agreement with experimental observations and atomistic simulations. The density, size, and growth rate of the dislocation loops, denoted as 〈c〉 and 〈a〉, also exhibit good agreement with transmission electron microscopy (TEM) analysis of irradiated Zr and its alloys. Our findings demonstrate that there is a spatial correlation between the growth of these dislocation loops and growth strains, significantly influenced by the crystal size. To explain the expansion of the 〈a〉 axis and the contraction of the 〈c〉 axis in irradiated Zr, it is necessary to consider the diffusion anisotropy difference (DAD) of mobile interstitial species. We show that the PWR Kearns parameters, specifically fr = 0.63, ft = 0.32, fa = 0.05, confer enhanced irradiation resistance to Zr along the principal directions when compared to single crystals. Additionally, reducing the grain size to nanograins further enhances the resistance to irradiation-induced growth, particularly along the direction with the highest volume fraction of basal poles [0001].

A Highly Deficient Medium-Entropy Perovskite Ceramic for Electromagnetic Interference Shielding under Harsh Environment.

Materials that can provide reliable electromagnetic interference (EMI) shielding in highly oxidative atmosphere at elevated temperature are indispensable in the fast-developing aerospace field. However, most of conductor-type EMI shielding materials such as metals can hardly withstand the high-temperature oxidation, while the conventional dielectric-type materials cannot offer sufficient shielding efficiency in gigahertz (GHz) frequencies. Here, a highly deficient medium-entropy (ME) perovskite ceramic as an efficient EMI shielding material in harsh environment, is demonstrated. The synergistic effect of entropy stabilization and aliovalent substitution on A-site generate abnormally high concentration of Ti and O vacancies that are stable under high-temperature oxidation. Due to the clustering of vacancies, the highly deficient perovskite ceramic exhibits giant complex permittivity and polarization loss in GHz, leading to the specific EMI shielding effectiveness above 30dB/mm in X-band even after 100 h of annealing at 1000°C in air. Along with the low thermal conductivity, the aliovalent ME perovskite can serve as a bifunctional shielding material for applications in aircraft engines and reusable rockets.

Measurement of Charge State Dynamics in Nitrogen−Vacancy Centers Based on Microwave‐Pulses‐Assisted Longitudinal Relaxations Balancing of Spin Qutrit

AbstractThe nitrogen−vacancy (NV) center in diamond gains its versatility when negatively charged (NV−) but is mediocre when neutrally charged. Particularly, the charge states of NV centers are convertible under optical pumping and during the dark intervals, whose dynamics are mixed with the NV−s’ spin polarizations and relaxations, making them difficult to detect. Here, a microwave‐pulses‐assisted charge state dynamics (CSD) measurement method of NV centers in the dark time (DT) is proposed. The microwave pulses are designed to manipulate the populations of the NV−s’ ground state spin triplets (qutrit) to the equilibrium state before the DT. Thus, the longitudinal relaxations of the qutrit are balanced, and pure CSD can be detected. Interestingly, in an annealed bulk diamond, not only the traditional tunneling‐induced fast exponential CSD are observed, but also a slow and long‐term recharging process, which is probably attributed to the exchanging of the electrons between NV centers and the high‐energy‐level charge traps such as vacancy clusters. Furthermore, results demonstrate a 40% increase in NV−s’ contrast by properly extending the recharging DT. These results are significant for the in‐depth study of the NV centers’ CSD and can improve the sensing abilities of the NV− ensemble.

Enabling tailored nanoporous germanium by quantifying the evolution of structural properties during self-irradiation

Porous materials are essential for a wide range of applications in energy storage, catalysis, and environmental solutions. Material fabrication methods which can tailor the porous structures in a deterministic manner are highly valuable for the development of the field. In this paper, we use ion irradiation to synthesize large-scale germanium (Ge)-based porous structures. We furthermore demonstrate the structure to consist of pores interconnected by ultra-small channels that effectively allow transport of gaseous molecules throughout the whole network. Aside qualitative assessment of the evolution of the structure, the combination of transmission electron microscopy, microbeam Rutherford backscattering spectrometry and a Ge-on-Si sample design enables the quantification of properties such as mass density and sputtering yields during the evolution of porosity. We find progression of pore formation from the surface with a constant rate with increasing dose at otherwise constant density of the porous phase. Proximity to the substrate terminates pore formation well before reaching the interface with higher doses, eventually inducing a depth-independent densification of the porous structure. Experimental evidence challenges vacancy clustering as a commonly referred-to mechanism for pore-formation. Instead, relaxation in structurally weakened, i.e. near-surface volumes of the material by micro-explosions can conclusively explain all observations. The porous germanium synthesized in our approach is considered more resilient at high temperature and strong pH conditions than polymeric and hybrid materials, indicating great potential for applications in such environments.

Influence of the distance of a collisional cascade to an edge dipole in [formula omitted]-Fe on dislocation mobility and defect production

In this work we studied the effects of 20 keV collision cascades in alpha iron with a 1/2〈111〉{110} edge dipole using molecular dynamics. We analysed three different cases: a) the primary knock-on atom (PKA) is centred between both dislocations, b) the PKA is closer to one of the dislocations, and c) the PKA is on top of one of the dislocations and directed towards it. Our calculations show enhanced formation of vacancy clusters in the bulk for intermediate distances due to the absorption of self-interstitials by the dislocation during the collision cascade reducing the recombination between vacancies and self-interstitials. This effect results in the formation of jogs at the dislocation and the consequent climb due to the absorption of the self-interstitials. As a result, there is an unbalance between vacancies and self-interstitials produced by the collision cascade, with a significantly larger number of vacancies than self-interstitials in the bulk and the formation of large vacancy clusters. When the collision cascade develops directly on top of the dislocation, jogs are formed due to both the absorption of vacancies and self-interstitials, inducing climb and with the total dislocation length increasing up to several nanometers.

Superior radiation resistance of ODS-RAFM steels revealed by Positron annihilation spectroscopy study

Oxide-dispersion-strengthened (ODS) reduced-activation ferritic/martensitic (RAFM) steels are considered as most promising structural materials for fusion reactors and other advanced nuclear systems due to its excellent irradiation resistance under high dose irradiations. However, the extreme complex synergistic effect between transmuted helium/hydrogen and displacement damage which most seriously affects the structure and mechanical properties of ODS-RAFM steels is very difficult to understand, especially the role of hydrogen still remains unclear. Taking advantage of very sensitive to small vacancy clusters of Positron annihilation spectroscopy (PAS) method, we conducted irradiation defect study on ODS-RAFM, RAFM and its corresponding model alloy Fe–9Cr and α-Fe to investigate their anti-swelling properties at the early stage of irradiation with hydrogen-helium synergism and the mechanism. Three distinct irradiation schemes including (1) single Fe ion beam, (2) simultaneous Fe and He (named Fe + He) ion beam, (3) subsequent H ion injection after Fe + He (named Fe + He/H) ion beam irradiation were performed. Hydrogen-helium synergistic effects on vacancy evolution within these steels were explored combining the first-principles calculation. The implantation of He was observed to significantly increase defect concentration among all four steels, while H exhibited interestingly complexity on affecting the evolution of helium bubbles. The H post-injection increased defect concentration obviously in the materials with abundant sinks, but improved poorly in ones with rare sinks. Through the systematic study of the interactions between H, He, and vacancies, it was revealed that even at the early irradiation stage, ODS-RAFM steels has already exhibited excellent irradiation resistance compared to other alloys across three irradiation schemes.

Hydrogen-Induced Microstructure Changes in Zr/Nb Nanoscale Multilayer Structures

Zr/Nb nanoscale multilayer coatings (NMCs) were studied after hydrogenation in a gaseous environment at 400 °C. The hydrogen distribution and content were determined by pressure and hydrogenation time. Increasing the pressure from 0.2 to 2 MPa resulted in different hydrogen distribution within the Zr/Nb NMCs, while the concentration remained constant at 0.0150 ± 0.0015 wt. %. The hydrogen concentration increased from 0.0165 ± 0.001 to 0.0370 ± 0.0015 wt. % when the hydrogenation time was extended from 1 to 7 h. The δ-ZrH hydride phase was formed in the Zr layers with Zr crystals reorienting towards the [100] direction. The Nb(110) diffraction reflex shifted towards smaller angles and the interplanar distance in the niobium layers increased, indicating significant lateral compressive stresses. Despite an increase in pressure, the nanohardness and Young’s modulus of the Zr/Nb NMCs remained stable. Increasing the hydrogen concentration to 0.0370 ± 0.0015 wt. % resulted in a 40% increase in nanohardness. At this concentration, the relative values of the Doppler broadening variable energy positron annihilation spectroscopy (S/S0) increased above the initial level, indicating an increase in excess free volume due to hydrogen-induced defects and changes. However, the predominant positron capture center remained intact. The Zr/Nb NMCs with hydrogen content ranging from 0.0150 ± 0.0015 to 0.0180 ± 0.001 wt. % exhibited a decrease in the free volume probed by positrons, as demonstrated by the Doppler broadening variable energy positron annihilation spectroscopy. This was evidenced by opposite changes in S and W (S↓W↑). The microstructural changes are attributed to defect annihilation during hydrogen accumulation near interfaces with the formation of hydrogen–vacancy clusters and hydrides.

Pseudo-nanostructure and trapped-hole release induce high thermoelectric performance in PbTe.

Thermoelectric materials can realize direct and mutual conversion between electricity and heat. However, developing a strategy to improve high thermoelectric performance is challenging because of strongly entangled electrical and thermal transport properties. We demonstrate a case in which both pseudo-nanostructures of vacancy clusters and dynamic charge-carrier regulation of trapped-hole release have been achieved in p-type lead telluride-based materials, enabling the simultaneous regulations of phonon and charge carrier transports. We realized a peak zT value up to 2.8 at 850 kelvin and an average zT value of 1.65 at 300 to 850 kelvin. We also achieved an energy conversion efficiency of ~15.5% at a temperature difference of 554 kelvin in a segmented module. Our demonstration shows promise for mid-temperature thermoelectrics across a range of different applications.

A hybrid rate theory model of radiation-induced growth

Differences in the diffusivity of radiation-produced defects along zirconium’s prismatic and basal planes result in radiation-induced growth (RIG) and creep. The difference in anisotropy of diffusion (DAD) model explains this behavior by assuming vacancies and self-interstitial atoms (SIAs) possess dissimilar ratios of in-basal-plane vs out-basal-plane diffusion coefficients. The self-interstitial cluster diffusion (SICD) model was proposed as an alternative to DAD, and relates growth to the formation of self-interstitial clusters which diffuse solely along the 〈a〉 direction. Here, we propose a hybrid model, in which the contribution of both effects are considered, which provides additional flexibility as compared to DAD or SICD taken individually. In this model, both anisotropic diffusing point defects and 1-dimensional (1-D) glissile SIA clusters are considered, which are removed in the internal and boundary sinks. Thirty-two DAD, SICD and hybrid models are fitted to a set of thirty-two RIG experiments. Good agreement between DAD and the experiments can be reached only by assuming unrealistically large SIA bias factors. In 9 of the 32 scenarios, good agreement between SICD and experiments could not be reached, indicating that the model lacks flexibility. Good agreement between the hybrid model and the experiments can be reached using realistic SIA bias factors; however, it requires assuming considerably lower vacancy bias factors than those found by ab initio calculations aimed at mono-vacancies, suggesting small vacancy clusters and loops play a more important role in RIG than previously thought.

Influence of vacancy type defects on the exchange integrals in ferromagnetic metals: a Monte-Carlo-based approach

Estimating exchange interactions in real systems, particularly with defects like vacancies and impurities, poses significant challenges. A classical Monte-Carlo spin simulation within an atomistic machine-learning framework is utilized to predict the pairwise exchange interactions in ideal and defective bcc Fe and fcc Ni systems. The results closely replicate the experimental Curie temperature, with a reduction in exchange interaction by ∼1 meV for the system with cluster vacancy concentration up to 6%. An exponential relationship is established between the classical rescaling parameter α and microscopic exchange interaction enhancing α up to 3.35 and 2.5 in Fe and Ni, respectively. Additionally, the spin wave stiffness constant calculated using the near neighbor Heisenberg model demonstrates a dampening in both Fe and Ni with the inclusion of cluster vacancies.