Diffusion Of Metal Atoms Research Articles

Single-Atom Catalysts through Pressure-Controlled Metal Diffusion.

Single-atom catalysts (SACs) open up new possibilities for advanced technologies. However, a major complication in preparing high-density single-atom sites is the aggregation of single atoms into clusters. This complication stems from the delicate balance between the diffusion and stabilization of metal atoms during pyrolysis. Here, we present pressure-controlled metal diffusion as a new concept for fabricating ultra-high-density SACs. Reducing the pressure inhibits aggregation substantially, resulting in almost three times higher single-atom loadings than those obtained at ambient pressure. Molecular dynamics and computational fluid dynamics simulations reveal the role of a metal hopping mechanism, maximizing the metal atom distribution through an increased probability of metal-ligand binding. The investigation of the active site density by electrocatalytic oxygen reduction validates the robustness of our approach. The first realization of Ullmann-type carbon-oxygen couplings catalyzed on single Cu sites demonstrates further options for efficient heterogeneous catalysis.

Novel modelling of metal atoms diffusion and ion transport in ECR plasma relevant to ion sources and in-plasma nuclear physics studies

Metals can be injected into electron cyclotron resonance ion sources (ECRIS) via different techniques, among which resistive ovens are used to vaporize neutral materials, later captured by the energetic plasma that will step-wise ionize them, hence giving multiply charged ion beams for accelerators. Recently, PANDORA, a novel ECR plasma trap, has been conceived to perform interdisciplinary research spanning from nuclear physics to astrophysics, where in-plasma high charge states of metallic species are demanded. However, a full knowledge on the vaporization method and on the coupling of neutral atoms with plasma and its overall dynamics is still not available. Simulations, hence, are of fundamental relevance to improve the overall efficiency, reduce consumption of rare expensive isotopes, and to improve the ion source performance. We present a numerical study about metallic species suitable for oven injection in ECRIS, focusing on metals diffusion, transport, and wall deposition under molecular flow regime. We studied the metal dynamics with and without plasma. Results underline the plasma role on a space-dependent conversion yield, reflecting the strongly inhomogeneous ECR plasma. The plasma and its parameters have been modelled using an established self-consistent particle-in-cell model. The numerical tool is conceived for the PANDORA plasma trap but can be extended to other ECR plasmas and traps. As test cases we studied the 134Cs and 48Ca radioisotopes, as metals of interest for the modern nuclear physics. A focus is given on the β-decaying 134Cs, as an application case for PANDORA, providing quantitative estimates of the γ-detection signal-poisoning effect by neutral metals deposition at the chamber wall.

Low-temperature fabrication of boron-doped amorphous silicon passivating contact as a local selective emitter for high-efficiency n-type TOPCon solar cells

Tunnel oxide passivating contact (TOPCon) solar cells (SCs) currently dominate the photovoltaic industry but grapple with efficiency challenges. A primary concern is the direct contact between front-sided metal electrodes and boron emitters, resulting in substantial carrier recombination losses and limiting further efficiency improvements. Here, we introduce a low-temperature approach to deposit a local boron-doped amorphous silicon [a-Si:H(p)] between front-sided metal electrodes and boron emitters. This method, avoiding issues associated with high-temperature processes, demonstrates excellent passivation and contact properties, featuring the lowest contact resistivity (< 1 mΩ·cm2) and a low saturation current density (< 400 fA/cm2). The outstanding passivation and contact properties remain robust even with variations in diborane flow rates during a-Si:H(p) fabrication, annealing temperatures, and sheet resistances of boron emitters. We elucidate the factors contributing to the enhanced passivation observed in boron emitters with a-Si:H(p) through a combination of simulations and experiments. The a-Si:H(p) layer between boron emitters and metal electrodes acts as a protective barrier, preventing the diffusion of metal atoms and suppressing carrier recombination. A heterojunction is formed between a-Si:H(p) and the boron emitter, facilitating electric-field passivation. Consequently, the TOPCon SCs incorporating a-Si:H(p) achieve an efficiency of 24.50%, surpassing their counterparts without a-Si:H(p) (23.11%). This work utilizes low-temperature technology to achieve fully passivated contact, providing insights for the development of high-efficiency TOPCon SCs.

Study on selective emitter fabrication through an innovative pre‐diffusion process for enhanced efficiency in TOPCon solar cells

AbstractTOPCon (tunnel oxide passivated contact) solar cell is the mainstream high‐efficiency crystalline silicon solar cell structure. However, the lack of efficient passivation contact mechanisms on the front surface restricts the electrical performance ability to improve further. Selective emitter (SE) technology, considered a potential solution, needs to be more mature. This work provides a unique thermal pre‐diffusion approach combined with laser treatment and post‐oxidation annealing to create SE structures in TOPCon solar cells. Times for the high‐temperature process are equivalent to those for a traditional homogenous emitter. The innovative thermal pre‐diffusion process created a unique boron doping profile, achieving a high surface concentration of nearly 1 × 1020 cm−3 with a shallow junction depth of approximately 0.25 μm. Laser treatment further activated boron and facilitated its diffusion, influenced by the boron silicate glass layer and surface boron atoms. Adjustments were made to improve the pre‐diffusion recipe, including an additional boron deposition step, increasing non‐activated boron atoms. Introducing larger pyramidal microstructures also improved the junction depth and surface concentration in the heavily doped region. Compared to homogeneous emitters, the SE structures exhibited lower surface concentration in the lightly doped region, reducing the recombination current density in the passivation region J0,pass values. The SE structures achieved higher junction depths, limiting metal atom diffusion and reducing the current recombination density in the metal contact region J0,metal values. The contact resistivity between metal and silicon was also decreased. Overall, introducing SE structures resulted in a batch‐average efficiency improvement of 0.26%, reaching an average efficiency of 25.22% for TOPCon solar cells, and has industrial mass‐producible.

1D selective confinement and diffusion of metal atoms on graphene

The role of moiré graphene superstructures in favoring confined adsorption of different metal atoms is an intriguing problem not yet completely solved. Graphene (G) grown on Ni(100) forms a striped moiré pattern of valleys, where G approaches the nickel substrate and interacts with it rather strongly, and ridges, where G stays far away from the substrate and acts almost free-standing. Combining density functional theory (DFT) calculations and scanning-tunneling microscopy (STM) measurements, we show that this peculiar moiré constitutes a regular nanostructured template on a 2D support, confining in 1D trails single metal atoms and few atoms clusters. DFT calculations show that the confinement is selective and highly dependent on the atomic species, with some species preferring to adsorb on ridges and the other showing preference for valleys. Co and Au adsorbates, for instance, have opposite behavior, as predicted by DFT and observed by STM. The origin of such disparate behavior is traced back to the electrostatic interaction between the charged adsorbate and the nickel surface. Moreover, the selectivity is not restricted to the adsorption process only, but persists as adsorbate starts its diffusion, resulting in unidirectional mass transport on a continuous 2D support. These findings hold great promise for exploiting tailored nanostructured templates in a wide range of potential applications involving mass transport along element-specific routes.

Microwave Enabled Incorporation of Singlemetal Atoms in Holey Graphene

Single-atom catalysts (SACs) can simultaneously maximize the utilization of metal atoms and minimize side reactions, achieving high catalytic activity and selectivity, which are not attainable for the conventional nanoparticle-based catalysts. SACs have been intensively investigated for a wide range of reactions and great progress has been achieved. However, most of the carbon substrates supporting the SACs are highly defective carbon materials, which are not only low conductivity but also low stability against oxidation. In the recent review articles, carbon matrix oxidation has been identified as the most important degradation mechanism of PGM-free catalysts in PEMFC cathodes. The stability issue is also especially critical when the SACs are used in Fenton-like catalytic reactions. In this work, we present a novel approach to quickly fabricate SACs in which the single metal atoms are incorporated in highly conductive holey graphene. This novel approach takes advantages of the rapid heating capability of microwave when highly conductive holey graphene is used as the microwave absorber. The initial assumption of the method was based on the rapid heating/cooling capability of microwave heating to quickly carbonize the organic linkers in a metal-organic framework (MOF) into a conductive carbon network, leaving no time for the diffusion of the metal atoms, therefore aggregation of the metal atoms to form clusters can be largely avoided. Interestingly, instead of stopping the diffusion of metal atoms in the MOF, microwave heating appears causing the metal atoms flying out from the MOF particles and incorporated into the basal planes of the holey graphene. Using PCN222-Fe and PCN222-Cu MOF as the precursors, Fe and Cu SACs were fabrications, in which the Fe and Cu catalytic centers are incorporated into the basal planes of the holey graphene. It is worth to mention that the non-hole regions of the holey graphene are nearly defect free, which is in high contrast to the highly defect carbon materials, ensuring high stability of the catalysts against oxidation. The catalytic performance of thus fabricated Cu and Fe SACs are studied in oxygen reduction reaction. Most importantly, the hydrogen peroxide reduction/oxidation reactions were also studied. These results provide fundamental basis for these catalysts in Fenton-like catalytic reactions. Figure 1

Formation of Core–Shell AuCu@Ag Nanocrystals through the Nanoscale Kirkendall Effect

Polymetallic nanocrystals (NCs) consist of multiple metal elements. A powerful platform to achieve the flexible construction of polymetallic NCs is highly desired but challenging. Herein, we devise a model system that realizes metal atom diffusion between different NCs, resulting in the formation of polymetallic NCs. The differential bond strength between different metal atoms is proposed to initiate such metal atom diffusion, and the specific high surface-to-volume ratio of the NCs can expedite the diffusion process. Taking the Au-Cu-Ag trimetallic system as an example, core-shell AuCu@Ag NCs were successfully formed by combining AgCu NCs with Au NCs. The evolution process was explored, and the gradual fusion of simple NCs into AuCu@Ag NCs was unambiguously observed, which could be attributed to the larger bond strength of Au-Cu than that of Ag-Cu. This work offers an opportunity/platform in theory and experiment to expand the synthesis framework as well as the polymetallic NC list.

Effect of blocking layer on achieving low sheet resistances in low-E multilayer structures

Low-emissivity films consisting of a metal oxide/Ag/metal oxide structure improve the thermal insulation performance of windows. Deposition of an oxide layer on top of the Ag layer requires either the insertion of a blocking layer (BL) or the sputtering deposition of an oxide target under an argon atmosphere. In this study, we carried out a sequential sputtering deposition method for inserting a BL to achieve both high visible-light transmittance and low sheet resistance. The effects of metallic and oxide BLs on the sheet resistances were investigated, and metallic BLs were confirmed to cause an increase in sheet resistance due to the diffusion of metal atoms into the Ag layer. Oxide BLs deposited under low oxygen concentrations maintained the low sheet resistance. The most significant result was obtained with a TiO x BL deposited under 2.9% oxygen, which prevented an increase in sheet resistance even after the deposition of the top layer (TL). By optimizing the thickness of the TL, high visible-light transmittance and low sheet resistance were achieved in a glass/TiO2/ZnO/Ag/TiO x /ZnO multilayer. Thus, high-performance multilayers could be fabricated by selecting an appropriate BL, which is beneficial for the industrial fabrication of low-E films.

Graphite-reinforced AgCuTi/Cu foam composite filler to braze Al2O3 ceramic and 304 stainless steel

Al2O3 ceramic and 304 stainless steel were brazed with AgCuTi filler in this paper. Two kinds of composite filler were prepared by adding Cu foam and Cu foam covered with graphite (Gr-Cu foam) to relieve the residual stress of the joints. The effects of Cu foam pore size on the joints were investigated in detail, and the evolution mechanism of the joints was analyzed. Graphite-reinforced Cu foam was fabricated by spraying graphite for the first time. Results showed that the extremely low coefficient of thermal expansion of graphite and the good energy absorption performance of Cu foam were both conducive to reducing the thermal residual stress during the cooling process. Gr-Cu foam with pre-filled pores was used to solve the problems of structural dissolution of Cu foam and unfilled solder in Cu foam pores to a certain extent. And it played a key role in preventing the diffusion of metal atoms. The graphite on the surface of the Cu foam generated TiC in the brazing seam. The TiC and Cu foam promoted the formation of the TiCu strengthening phase. Furthermore, when the pore size of the Gr-Cu foam was 150 μm, the maximum shear strength of the joint was 246.84 MPa. This value was about 40% higher than that of Cu foam with the same pore size. The residual stresses of joint with and without graphite-reinforced were calculated, respectively, for both conditions. The reduction of residual stress was found after adding the graphite during brazing.

Graphene-confined ultrafast radiant heating for high-loading subnanometer metal cluster catalysts.

Thermally activated ultrafast diffusion, collision and combination of metal atoms comprise the fundamental processes of synthesizing burgeoning subnanometer metal clusters for diverse applications. However, so far, no method has allowed the kinetically controllable synthesis of subnanometer metal clusters without compromising metal loading. Herein, we have developed, for the first time, a graphene-confined ultrafast radiant heating (GCURH) method for the synthesis of high-loading metal cluster catalysts in microseconds, where the impermeable and flexible graphene acts as a diffusion-constrained nanoreactor for high-temperature reactions. Originating from graphene-mediated ultrafast and efficient laser-to-thermal conversion, the GCURH method is capable of providing a record-high heating and cooling rate of ∼109°C/s and a peak temperature above 2000°C, and the diffusion of thermally activated atoms is spatially limited within the confinement of the graphene nanoreactor. As a result, due to the kinetics-dominant and diffusion-constrained condition provided by GCURH, subnanometer Co cluster catalysts with high metal loading up to 27.1wt% have been synthesized by pyrolyzing a Co-based metal-organic framework (MOF) in microseconds, representing one of the highest size-loading combinations and the quickest rate for MOF pyrolysis in the reported literature. The obtained Co cluster catalyst not only exhibits an extraordinary activity similar to that of most modern multicomponent noble metal counterparts in the electrocatalytic oxygen evolution reaction, but is also highly convenient for catalyst recycling and refining due to its single metal component. Such a novel GCURH technique paves the way for the kinetically regulated, limited diffusion distance of thermally activated atoms, which in turn provides enormous opportunities for the development of sophisticated and environmentally sustainable metal cluster catalysts.

Effect of Ni-modified reduced graphene oxide on the mechanical properties of Sn–58Bi solder joints

Sn–58Bi solder has great research value as a low-temperature solder. However, it still requires strengthening due to a lot of brittle Bi-phase, which reduces mechanical reliability. In this study, Ni-modified reduced graphene oxide (rGO@Ni) was prepared by heat reduction, and mechanical stirring was used to manufacture rGO@Ni-reinforced Sn–58Bi composite solder. The distribution, phase composition, and structural changes of graphene and Ni nanoparticles during enhancer synthesis were characterized in detail by Scanning Electron Microscope, Energy Dispersive Spectroscopy Raman and Fourier Transform Infrared spectra, X-ray Photoelectron Spectroscopy, X-ray Diffraction, and Transmission Electron Microscope. Besides, the morphology of intermetallic compound (IMC) layers of the composite solder/Cu interface was investigated. The thickness of the IMC layer decreases with the increase of rGO@Ni content. The microstructure analysis demonstrated that the rGO@Ni distributed near the IMC layer, which limits the diffusion of metal atoms and decreases the IMC layer's growth rate during the reflowing process. Besides, compared with Sn–58Bi solder alloys, the addition of 0.01 wt% rGO@Ni can increase the shear strength by up to 15%. However, when the content of rGO@Ni is more than 0.05 wt%, it agglomerates inside the composite solder, resulting in a decrease in the shear strength of the composite solder joint.

Optimization of microstructure and properties of laser sintered Ni30Cr25Al15Co15Mo5Ti5Y5 high-entropy alloy coatings via controlling plasma

An active suppressing method of laser-induced plasma was proposed to optimize the crystal structure and tribological properties for the surface laser sintering (SLS) of Ni30Cr25Al15Co15Mo5Ti5Y5 high-entropy alloy (HEA) coatings using a pulsed laser with various shielding gases. The results indicate that electron temperature (Te) and the plasma jet volume with no shielding gas or in He atmosphere were higher than those in Ar−He atmosphere. Well- defined phase transitions and significant metal atom diffusion occurred in SLS, and metallurgical bonding occurred with the precipitation of α-AlFe3, α-AlNi, and γ-AlFe2Ni. The lower energy absorbed by the plasma via inverse bremsstrahlung (IB) strengthened interactions between laser and HEA, increasing the microhardness, and reducing the dynamic friction coefficient. This elucidates the crucial influence of plasma control on laser-based additive manufacturing.

In Situ Formation of Nanoparticles on Carbon Nanofiber Surface Using Ceramic Intercalating Agents

Nickel silicide nanoparticles were prepared in situ on carbon nanofibers through pyrolysis of electrospun fibers containing poly(acrylonitrile) (PAN, carbon fiber precursor), silazane (SiCN ceramic precursor), and nickel chloride (nickel source). SiCN ceramics produced in carbon nanofibers during the pyrolysis expanded the graphitic interlayer spacing and facilitated the diffusion of metal atoms to the fiber surfaces, leading to the formation of nickel silicide nanoparticles at a reduced temperature. In addition, nickel silicide nanoparticles catalyzed an in situ formation of carbon nanotubes, with carbon sourced from the decomposition of silazane. The method introduces a simple route to produce carbon supported metal nanoparticles for catalysis and energy storage applications.

An Ab-initio study of the Y decorated 2D holey graphyne for hydrogen storage application

Expanding pollution and rapid consumption of natural reservoirs (gas, oil, and coal) led humankind to explore alternative energy fuels like hydrogen fuel. Solid-state hydrogen storage is most desirable because of its usefulness in the onboard vehicle. In this work, we explored the yttrium decorated ultra porous, two-dimensional holey-graphyne for hydrogen storage. Using the first principles density functional theory simulations, we predict that yttrium doped holey graphyne can adsorb up to seven hydrogen molecules per yttrium atom resulting in a gravimetric hydrogen weight percentage of 9.34, higher than the target of 6.5 wt% set by the US Department of Energy. The average binding energy per H2 and desorption temperature come out to be −0.34 eV and ∼438 K, respectively. Yttrium atom is bonded strongly on HGY sheet due to charge transfer from Y 4d orbital to C 2p orbital whereas the adsorption of H2 molecule on Y is due to Kubas-type of interactions involving charge donation from H 1s orbital to Y 3d orbital and back donation with net charge gain by H 1s orbital. Furthermore, sufficient energy barriers for the metal atom diffusion have been found to prevent the clustering of transition metal (yttrium) on HGY sheet. The stability of the system at higher temperatures is analyzed using Ab-initio molecular dynamics (AIMD) method, and the system is found to be stable at room and the highest desorption temperature. Stability of the system at higher temperatures, presence of adequate diffusion energy barrier to prevent metal–metal clustering, high gravimetric wt% of H2 uptake with suitable binding energy, and desorption temperature signifies that Y doped HGY is a promising material to fabricate high capacity hydrogen storage devices.

Densification, microstructures, and mechanical properties of (Zr, Ti)(C, N) ceramics fabricated by spark plasma sintering

Dense (Zr, Ti) (C, N) ceramics were fabricated by spark plasma sintering (SPS) at 1900–2000 °C using ZrC, TiCN and ZrH2 powders as raw materials. A single Zr-rich (Zr, Ti)(C, N) solid solution was formed in Zr0.95Ti0.05C0.975N0.025 and Zr0.80Ti0.20C0.90N0.10 ceramics (nominal composition). A Ti-rich solid solution appears in Zr0.50Ti0.50C0.75N0.25 ceramics. The coaddition of TiCN and ZrH2 promoted the densification of (Zr, Ti) (C, N) ceramics by forming solid solutions and carbon vacancies, which could reduce critical resolved shear stress (CRSS) and promote carbon and metal atom diffusion. ZrC-45 mol% TiCN-10 mol% ZrH2 (raw powder composition) possesses good comprehensive mechanical properties (Vickers hardness of 24.5 ± 0.9 GPa, flexural strength of 503 ± 51 MPa, and fracture toughness of 4.3 ± 0.2 MPa·m1/2), which reach or exceed most ZrC-based (Zr, Ti) C and (Zr, Ti) (C, N) ceramics in previous reports.

Sequential Growth of Vertical Transition-Metal Dichalcogenide Heterostructures on Rollable Aluminum Foil.

Vertical van der Waals heterostructures (vdWhs), which are made by layer-by-layer stacking of two-dimensional (2D) materials, offer great opportunities for the development of extraordinary physics and devices such as topological superconductivity, robust quantum Hall phenomenon, electron-hole pair condensation, Coulomb drag, and tunneling devices. However, the size of vdWhs is still limited to the order of a few micrometers, which restricts the large-scale roll-to-roll processes for industrial applications. Herein, we report the sequential growth of a 14 in. vertical vdWhs on a rollable Al foil via chemical vapor deposition. By supplying chalcogen precursors to liquid transition-metal precursor-coated Al foils, we grew a wide range of individual 2D transition-metal dichalcogenide (TMD) films, including MoS2, VS2, ReS2, WS2, SnS2, WSe2, and vanadium-doped MoS2. Additionally, by repeating the growth process, we successfully achieved the layer-by-layer growth of ReS2/MoS2 and SnS2/ReS2/MoS2 vdWhs. The chemically inert Al native oxide layer inhibits the diffusion of chalcogen and metal atoms into Al foils, allowing for the growth of diverse TMDs and their vdWhs. The conductive Al substrate enables the effective use of vdWhs/Al as a hydrogen evolution reaction electrocatalyst with a transfer-free process. This work provides a robust route for the commercialization of 2D TMDs and their vdWhs at a low cost.

Solid state dewetting of Ni-Co bilayers on sapphire during slow heating and cooling

We studied the solid state dewetting of thin Ni films and Ni-Co bilayers deposited on sapphire substrate during slow heating to several target temperatures. We demonstrated that during initial stages of dewetting and bilayers intermixing, Ni atoms from the top layer diffuse along the interface between the bottom Co layer and sapphire, simultaneously with Co atoms diffusion on the surface of Ni layer. This alloying of the top surface layer with Co slows down the rate of thermal grain boundary grooving and leads to the increase of the thermal stability of the bilayers with increasing Co content. We also observed that during intermediate stages of dewetting the regions of the untransformed film with increased thickness are dotted with small surface pores of 20–30 nm in diameter reaching to the level of as-deposited film. We related the formation and extraordinary stability of these pores with the modalities of surface diffusion of metal atoms in the presence of segregated impurities. In particular, we emphasized the role of Ehrlich-Schwoebel barrier for adatoms attachment to the edge of (111) terrace in formation of the pores. Our results indicate that the fine features of surface diffusion and surface thermodynamics, together with the presence of impurities and defects in the film lead to a rich variety of film morphologies during solid state dewetting.

Temperature dependence of the strain of the AlGaN barrier layer under the gate in AlGaN/AlN/GaN HFETs

By a combination of electrical measurement, the Debye model and the basic theory of polarization, the strain of AlGaN barrier layer underneath the gate in two AlGaN/AlN/GaN HFETs at the same ratio of gate length (LG) to source-drain distance (LSD) but with different dimensions is investigated at temperatures from 300 ​K to 450 ​K. It is found that temperatures have important influence on this strain. Furthermore, the temperature dependence of the strain is found related to device dimensions, a phenomenon that can be explained by the Ohmic contact metal atoms diffusion nearby the Ohmic electrode and lattice mismatch underneath the gate electrode. This study is conducive to understanding the effect of temperatures on the polarization Coulomb field (PCF) scattering, which has been verified to be a significant scattering mechanism affecting the performance of AlGaN/AlN/GaN HFETs.

Interface mechanics of 2D materials on metal substrates

The chemical vapor deposition (CVD) of graphene on metal substrate serves as a robust method to synthesize large graphene flakes. Therefore, understanding the thermodynamics and kinetics of graphene–metal interface is an important step towards synthesizing defect-free graphene during CVD. The focus of this paper is on the phenomenon of surface reconstruction of a metal substrate during the deposition of a 2D material. Key features of surface reconstruction include the formation of straight and pyramidal facets of size ∼300 nm in width and ∼15–80 nm in height. In addition, discontinuities in facet directions under a single graphene flake distributed over two grains highlight the strong crystallographic influence on surface reconstruction.In this paper, we present an atomistically informed continuum model of a graphene–metal interface in three dimensions. Graphene is modeled as an elastic surface that is in contact with a rigid metal substrate. Due to the weak van der Waals interaction between graphene and metal, the kinematics of the model incorporates sliding of graphene. However, since we assume that graphene is always in contact with the substrate, its normal displacement is mediated by surface diffusion of metal atoms. Based on evidence from recent molecular dynamics simulations and experiments, we model the graphene-covered-metal surface energy to include in-plane elasticity and bending energy of graphene, and an interaction energy that depends on the orientation of graphene relative to the substrate. We compare the predictions of our model with surface reconstructions observed during the CVD of graphene on forged thin film palladium polycrystals. As we continue to unravel the atomic scale mechanisms responsible for surface reconstruction during CVD, we expect the current continuum framework and its generalizations will serve to bridge the atomic- and the meso-scales.

Compatibility of UN with refractory metals (V, Nb, Ta, Cr, Mo and W): An ab initio approach to interface reactions and diffusion behavior

Uranium mononitride (UN)-uranium dioxide (UO2) composites are being considered as an innovative advanced technology fuel option for light water reactors, where an optimal balance between the chemical advantages of UO2 and the thermal and neutronic properties of UN is struck. However, the effect and extent of chemical interactions between UN and UO2 during sintering and operation are still open issues of importance. A possibility to avoid these interactions is to protect the UN phase before sintering the UN-UO2 composites by encapsulating the UN. This protective material must have a high melting point, high thermal conductivity, and reasonably low neutron cross-section. Among many candidates, the use of refractory metals is a promising option. In this study, density functional theory calculations (DFT) were performed to study the interactions and kinetics at the UN-X interfaces respectively (X = V, Nb, Ta, Cr, Mo, and W). The diffusion behaviors in UN and in the metal were studied using the self-consistent mean field (SCMF) theory. Generally, the diffusion of metal atoms in UN is slow compared to the diffusion of N atoms in the metals. Furthermore, the DFT calculations predict that Ta and V may react with UN to form UTaN2 and V8N at the UN-X interfaces, respectively. In some cases, the formation of these phases also promotes the formation of point defects in the UN and metal phases. The interaction between W and Mo with the UN phase is largely prohibited. According to this work, Mo and W can be regarded as highly promising candidate materials for the fabrication of stable UN-UO2 composite fuel.