Diffusion Coefficient Of Atoms Research Articles

Path ahead: Tackling the Challenge of Computationally Estimating Lithium Diffusion in Cathode Materials.

In the roadmap toward designing new and improved materials for Lithium ion batteries, the ability to estimate the diffusion coefficient of Li atoms in electrodes, and eventually solid-state electrolytes, is key. Nevertheless, as of today, accurate prediction through computational tools remains challenging. Its experimental measurement does not appear to be much easier. In this work, we devise a computational protocol for the determination of the Li-migration energy barrier and diffusion coefficient, focusing on a common cathode material such as LiNiO2, which represents a prototype of the widely adopted NMC (LiNi1-x-y Mn x Co y O2) class of materials. Different methodologies are exploited, combining ab initio metadynamics, path sampling, and density functional theory. Furthermore, we propose a novel, fast, and simple 1D approximation for the estimation of the effective frequency. The outlined computational protocol aims to be generally applicable to Lithium diffusion in other materials and components for batteries, including anodes and solid electrolytes.

Multi-scale simulation of microstructural evolution for molten U–Zr–O ternary system

The morphological evolution of the molten corium is critical to the success of the “in-vessel retention” (IVR) strategy, which is of great significance for the safety of the nuclear industry under severe accidents. However, the direct observation of microstructural evolution by experiments is impossible in extreme conditions. Herein, a multi-scale simulation approach was employed to investigate the microstructural evolution and related mechanisms for the molten U–Zr–O ternary system. First, based on the reported calculated phase diagram, a simplified free energy formula describing a phase diagram of U–Zr–O was determined, where only liquid phases are focused. Then, the atomic diffusion coefficients were calculated by ab-initio molecular dynamics simulations. Based on such information, a homemade phase-field model was developed, which was coupled with Cahn–Hillard and Navier–Stokes equations. Accordingly, the microstructural evolution of molten U–Zr–O materials was systematically investigated. The results reveal that the evolution of the molten core was driven by both the decreasing free energy and gravity potential, accessing the final morphology of the system as a two-layer structure, including a metallic layer and an oxidized layer. Moreover, diverse intermediate morphologies could be observed from different initial component distributions, although their average compositions were identical. Finally, some intermediate simulated microstructural patterns agreed well with experimental observation, demonstrating the unstable structures captured by experiments and the validity of the simulations. The present study provides an effective approach to investigate the microstructural evolution of molten core and a guideline for optimizing the IVR strategy based on different morphologies in the corium.

High-efficiency TOPCon solar cell with superior P + and P++ layer via one-step processing

The boron diffusion process in the front field of N-type tunnel oxide passivated contact (TOPCon) solar cells is crucial for PN junction formation and the creation of a selective emitter. This study presents a theoretical model of boron diffusion in silicon using molecular dynamics. The research examines the mean square displacement and diffusion coefficient of boron atoms at varying temperatures, confirming their diffusion behavior. The simulations indicate predominant boron diffusion in the z-direction within the silicon matrix, with the diffusion depth being temperature dependent. The optimal temperature range for boron diffusion in silicon is identified as 950 °C to 1050 °C. Using boron-doped silicon paste and boron trichloride as dopants, thermal diffusion experiments were conducted to fabricate the front-field PN junction (p+ layer) and selective emitter (p++ layer) by one step. Subsequent processing and performance evaluation were performed on a production line. Experimental findings reveal a decrease in boron diffusion at higher temperatures, reduced sheet resistance, increased doping concentration, and deeper junction formation. The ideal boron concentration in the p+ layer is 8.68 × 1018 atom/cm3 with a depth of 0.53 μm, while the p++ layer is 2.35 × 1019 atom/cm3 and 0.82 μm. The efficiency of the optimized TOPCon + cell production line reaches up to 25.17 %, marking an improvement of 0.23 % over the standard cell production line. This research contributes to elucidating the mechanism of boron diffusion and offers insights for enhancing the efficiency of TOPCon solar cells.

Numerical and experimental investigation on intermetallic compounds formation in ultrasonic vibration enhanced friction stir welding of dissimilar Al/Mg alloys

In friction stir welding (FSW) of dissimilar Al/Mg alloys, inevitable formation of intermetallic compounds (IMCs) has deleterious effect on the performance of joints. Although ultrasonic vibration enhanced FSW (UVeFSW) was invented to suppress the IMCs formation, the mitigating mechanism is not elucidated. In this work, the measured maximum temperature in weld nugget zone (WNZ) confirmed that in butt FSW of Al/Mg alloys the IMCs are generated by diffusion of solid atoms. The acoustic softening and antifriction effects were taken into account for computation of temperature, strain rate and dislocation density which are main factors affecting the inter-diffusion coefficients of Mg and Al atoms. An atom-diffusion based model was developed, and the formation and growth of the IMCs were quantitatively analyzed in FSW/UVeFSW of Al/Mg alloys. It was found that as the checking point flows toward to the tool, Al12Mg17 layer is first formed at the bonding interface, and then Al3Mg2 is formed at the Al side and grows rapidly. Under the conditions used in this study, the predicted values of IMCs thickness at the WNZ center in FSW and UVeFSW are 3.9 µm and 1.8 µm, respectively. The calculated IMCs thicknesses at the selected locations on the transverse cross-section and horizontal cross-section agree well with the measured ones. Ultrasonic induced dislocation-annihilation and associated decreasing of dislocation density in UVeFSW cause declining of the atomic diffusion coefficients, so that the formation and growth of IMCs are inhibited with ultrasonic action.

Nanoporous gold with microporous structure prepared by sodium dodecyl sulfate-mediated electrochemical dealloying

Nanoporous gold (NPG) is a promising catalytic material for the oxidation of CO and methanol applications. However, NPGs are prone to extensive macroscopic cracking that often decrease mechanic properties of NPGs and depresses their catalytic action. To produce crack-free NPG with an ultra-finer porosity in room temperature, the anionic surfactant sodium dodecyl sulfate (SDS) was added in electrochemical dealloying process. SDS has the effect of reducing the surface diffusion of gold which hinder the initial coarsening of ligaments and prevents interior silver atoms from being exposed and dissolved. As a result, the pore and ligment size are finer, but higher residual silver of NPG samples. NPG with pore size down to 2 nm and the ligament 4.0 nm was successfully fabricated with 13.32 mM SDS in perchloric acid solution. The surface diffusion coefficient of Au atoms was 1.6 × 10−24 m2·s−1, nearly 3 orders of magnitude smaller than that of Au atoms in the absence of SDS (2.8 × 10−21 m2·s−1). Nanoindentation results demonstrated that high residual silver content made NPG samples harder and stiffer，the specific surface areas of NPG with 6.66 mM SDS was 190 m2 g−1 by BET. This work provided very important clues on how to control the crack free ultrafine nanoporous structure of other materials.

Atomic mobilities, diffusion coefficients, and kinetic coefficients in Ti-rich Ti–Mo–Nb system

Diffusion coefficient is an important physical property to control the microstructure, thus the establishment of atomic mobility databank for Ti-based alloy systems is vital to the design of novel Ti alloys. In the present work, ternary Ti–Mo–Nb diffusion couples within the single bcc phase were prepared and measured after annealing at 1273 K for 25 h, and interdiffusion coefficients of bcc Ti–Mo–Nb alloys at 1273 K were determined from the experimental composition profiles by using Matano–Kirkaldy method. Subsequently, the atomic mobility parameters of bcc Ti–Mo–Nb system under the CALculation of PHAse Diagrams (CALPHAD) framework were assessed by using recently proposed diffusion models, which were verified by the comparisons between the experimental interdiffusion coefficients/composition profiles and the model-predicted results. Moreover, diffusion models were carefully discussed for the ternary assessment. Finally, relationships among atomic mobility, kinetic coefficient, and diffusion coefficients were discussed and demonstrated via ternary contour maps. The present work contributes to the development of diffusion databank for computational design of biomedical Ti alloys.

Insight into the improvement of service performance of Sn/Cu solder joint by Pt doping in Cu6Sn5 interfacial intermetallic compound

The effects of Pt doping on the mechanical properties of (Cu,Pt)6Sn5 interfacial intermetallic compound (IMC), bonding properties of the (Cu,Pt)6Sn5/Cu interface, and growth behavior of (Cu,Pt)6Sn5 IMC during the aging stage were investigated using first-principles calculations. The results indicate that Pt doping improves both the deformation resistance and ductility of the (Cu,Pt)6Sn5 phase, and that Cu2Pt4Sn5 demonstrates the highest ductility, as indicated by the maximum B/G value (3.14) and Poisson’s ratio (0.356). This phenomenon can be attributed to the smallest standard deviation of the charge density of 0.097 e/Å3. Comparisons of the interfacial work of adhesion and tensile strength of the Cu2Pt4Sn5/Cu and Cu6Sn5/Cu interfaces show that Pt doping enhances interfacial atomic interaction. Based on semi-classical Boltzmann theory, the electrical conductivity of the Cu2Pt4Sn5/Cu interface is smaller than that of the Cu6Sn5/Cu interface, with an amplitude of 9.26%, owing to the enhanced electron localization after Pt doping and resulting in a slight increase in the service temperature. Nevertheless, Pt doping still significantly suppresses the diffusion of Cu atoms, as evidenced by the millions of times reduction in the diffusion coefficient of Cu atoms across the Cu2Pt4Sn5/Cu interface compared to that across the Cu6Sn5/Cu interface because of the latter interface’s larger diffusion activation energy (3.52 eV > 2.08 eV), which was investigated by climbing image nudged elastic band methods. Therefore, Pt doping not only strengthens the mechanical properties of the (Cu,Pt)6Sn5 phase and improves the bonding strength of the (Cu,Pt)6Sn5/Cu interface, but also suppresses the growth of IMC during the aging stage, playing a positive role in obtaining a highly reliable Sn/Cu solder joint.

The effect of nickel on the carbide precipitation behavior in Cr–Mo–V hot-working die steel

The addition of small amount of nickel to hot-working die steel has been found to be beneficial for hardenability and toughness. In this paper, the addition of nickel to Cr–Mo–V hot-working die steel was studied to improve hardenability, and the inherent mechanism of enhancing toughness was further explored. The research results indicate that nickel as a non-carbide-forming element does not alter the type and total amount of carbide precipitation at equilibrium, but it can dissolve into the matrix and form substitutional lattice distortions with Fe atoms, promoting the dispersion of secondary carbides during the tempering process. This mechanism can reduce intergranular precipitation and inhibit grain boundary embrittlement, thereby improving toughness. However, it can also accelerate the precipitation, aggregation, and growth of secondary carbides during prolonged tempering, which deteriorates the high-temperature anti-softening properties of die steels. The role of Ni has been revealed through thermodynamic and kinetic calculations. The results show that Ni significantly enhances the diffusion coefficient of Cr atoms in the face centered cubic (fcc) Fe matrix during tempering and also improves the phase transition driving force of various precipitates. This work provides new insights into the mechanism of Ni in hot-working die steels, and can provide guidance for designing novel hot-working die steel.

Evolution of Structure and Dynamic of Simulated CdTe Model According to the Melting Process

CdTe model containing 9955 atoms was built and simulated by the method of molecular dynamics (MD) with the Stillinger–Weber (SW) potential. The model was obtained by heating from 300K to 3000K at a rate of 1013 K/s. The structural properties of the model are investigated through the total energy per atom, the partial radial distribution functions (PRDFs), the coordination number distributions and the bond-angle distributions. Calculation results have shown that the model has a melting point of about 1370K and changes from tetrahedral to octahedral structure when the temperature of CdTe model is above the melting point. These results are consistent with the reported experimental and simulation results. In addition, the mobility of Cd and Te atoms is also researched through the mean squared displacement (MSD) and the diffusion coefficients of Cd and Te atoms.

Effect of the Diffusion of Copper Atoms in Polycrystalline CdTe Films Doped with Pb Atoms

The process of diffusion of labeled copper atoms in p-CdTe<Pb> coarse-block films with a columnar grain structure has been studied. The CdTe<Pb> film is a p-type semiconductor, where an increase in the Pb concentration in the composition of the CdTe films increases the resistivity ρ of the structure. When the Pb concentration in CdTe changes from 1018 to 5·1019 cm-3, the hole concentration decreases by more than 3 orders of magnitude at a constant operating level depth of EV + (0.4 ± 0.02) eV. This may indicate that the concentration of acceptor defects, which are formed in the films due to self-compensation upon doping with a PbCd donor, exceeds the number of the latter. Electrical measurements by the Hall method were carried out at a direct current and a temperature of 300 K. As a result, an increase in the temperature of films on a Mo-p-CdTe<Pb> substrate during annealing affects the electrical parameter of charge carrier mobility µ, it decreases significantly. X-ray diffraction analysis showed that on the diffraction patterns of samples of p-CdTe<Pb> films, all available reflections correspond to the CdTe phase and up to х = 0.08 do not contain reflections of impurity phases and have a cubic modification. Based on the results of the calculation, it was established that the low values of the diffusion coefficient of Cu atoms are due to the formation of associates of the A type , which are directly dependent on the concentration of atoms. Diffusion length Ln and lifetime τn of minority current carriers in large-block p-type cadmium telluride films, which can also be controlled by introducing lead atoms into cadmium telluride.

Dynamic heterogeneity and mechanical relaxation of Ti20Zr20Cu20Ni20Be20 high-entropy metallic glass probed by quasi-elastic neutron scattering and stress relaxation

Heterogeneity is an intrinsic property of metallic glasses (MGs) and is closely related to their physical properties. Relaxation dynamics provides a powerful approach to understanding the heterogeneity of MGs. State-of-the-art quasi-elastic neutron scattering (QENS) and stress relaxation are used to study the atomic relaxation dynamics, dynamic heterogeneity, diffusion coefficients and mechanical relaxation of Ti20Zr20Cu20Ni20Be20 high-entropy metallic glass (HE-MG). The dynamic susceptibility derived from the QENS self-intermediate scattering function was used to evaluate the presence of dynamic heterogeneity, which increases gradually with decreasing temperature. The average self-diffusion coefficient of Ni and Ti atoms in the Ti20Zr20Cu20Ni20Be20 HE-MG melt is much higher than that in the Ti13.8Zr41.2Cu12.5Ni10Be22.5 Vit1 MG melt. The Ti20Zr20Cu20Ni20Be20 HE-MG melt has higher atomic mobility with respect to the Vit1 MG melt. In addition, the activation energy for crystallization of the Ti20Zr20Cu20Ni20Be20 HE-MG is considerably larger than that of the Vit1 MG. In the stress relaxation test, both Ti20Zr20Cu20Ni20Be20 HE-MG and Vit1 MG exhibit a relaxation decoupling phenomenon, indicating a two-step relaxation process. The slow and fast relaxation processes of the two alloy ribbons have a stretched exponential decay. At the measured temperatures, the relaxation times for both of the relaxation processes for the Ti20Zr20Cu20Ni20Be20 HE-MG are larger than those for the Vit1 MG, meaning that the Ti20Zr20Cu20Ni20Be20 ribbon has a slower stress decay. This in-depth investigation will improve our understanding of the intrinsic heterogeneity and mechanical relaxation mechanism in HE-MG.

Insights into the effect of γ′- or γ′′-precipitated phase types on resistance against harmful atoms of heterogeneous interface in nickel-based superalloys

To improve the service performances of nickel-based superalloys by controlling the type and proportion of precipitate phases, the resistances against harmful atoms (weakening effect caused by harmful atoms and capacity for hindering intrusion of harmful atoms) of the representative γ-matrix/γ′- or γ′′-precipitated phase heterogeneous interfaces (γ-Ni/γ′-Ni3Al, γ-Ni/γ′-Ni3Ti, and γ-Ni/γ′′-Ni3Nb interfaces) was investigated. The results show that, the harmful atoms (H, O, and S atoms) can be stable adsorbed on heterogeneous interfaces and weaken the interfacial bonding strength, resulting in the fracture position change from γ′- or γ′′ slab to interface during tensile deformation. Because of the minimum changes on interfacial bonding characteristics after the intrusion of harmful atoms, γ-Ni/γ′′-Ni3Nb heterogeneous interface (HIγ″−Ni3Nb) demonstrates the strongest resistance against weakening effect, presented as the smallest deterioration of interfacial ideal work of adhesion and tensile strength. Meanwhile, the energy barrier of harmful atoms diffusing along HIγ″−Ni3Nb is much larger than those along HIγ″−Ni3Al and HIγ″−Ni3Ti, leading to a smallest diffusion coefficient for harmful atoms diffusing along HIγ″−Ni3Nb. The result indicates HIγ″−Ni3Nb also shows the strongest capacity for hindering the intrusion of harmful atoms. This phenomenon is attributed to the strongest interaction between harmful atoms on saddle point and the neighboring atoms on HIγ″−Ni3Nb during the diffusion process. To summary, the Ni-based superalloys with γ′′-Ni3Nb as the main precipitated phase demonstrate the most promising properties in resistance against harmful atoms. This work not only contributes to the further improvement of Ni-based superalloys, but also provides the creative ideas for the research of other superalloys.

Microscopic mechanism of big-white-spot defect formation in hot-based galvanized strip steel: Theoretical calculation and molecular dynamics simulation

By combining experimental, thermodynamic, dynamics, and molecular dynamics simulations, the microstructure characteristics and microscopic processes generated of big-white-spot defects on the surface of hot-based galvanizing were studied. OM, LCM, FESEM, EPMA, XRD are used to characterize the microstructure of the coating surface and cross-section from different angles, and electrochemical tests are used to test corrosion mechanism of the coating. Molecular dynamics is used to calculate the atomic diffusion coefficient. The distribution of the aluminum-rich layer in the big-white-spot coating is extremely uneven. And the aluminum-rich layer loses its hindrance to the diffusion of Fe and Zn. There are many holes inside the defect coating. The fracture of aluminum-rich layer leads to a large number of Fe2Al5 particles within the defect. The defects cause the transformation of Al5Sb3 phase from interdendritic segregation to particle distribution. Nucleation theory, diffusion model, coating growth model are used to analyze the influence of temperature on coating. The nucleation rate increases exponentially with temperature and is mainly controlled by diffusion. The simulation results indicate that diffusion coefficient of Al is greater than Fe, and unequal diffusion leads to an increase in holes. The intensification of element diffusion, increase of holes, and poor adhesion between aluminum-rich layer and substrate jointly contribute the fracture of aluminum-rich layer.

MD simulation of vacancy and interstitial diffusion in FeCr alloy

The diffusion mobility of iron and chromium atoms in Fe-9%Cr and Fe-20%Cr alloys for temperature up to 1000 K has been studied by molecular dynamics simulation. Both vacancy and interstitial migration of atoms has been considered. Corresponding diffusion coefficients of iron and chromium atoms, as well as the diffusion coefficients of a vacancy and self-interstitial configuration, have been evaluated. The values obtained for the vacancy diffusion satisfactory agree with experimental results taken from the literature.

Dynamic heterogeneity in Pd40Ni40P20 glass-forming metallic melt and crystallization behavior probed by quasi-elastic neutron scattering and ultrafast scanning calorimetry

The intrinsic heterogeneity of metallic glasses is hidden in long-range disordered structures. Due to the lack of a long-range ordered structure, it is difficult to construct the structure-property relationship in metallic glasses. However, relaxation dynamics provides a powerful approach to understanding metallic glasses. State-of-the-art quasi-elastic neutron scattering (QENS) and ultrafast scanning calorimetry are used to study the atomic relaxation dynamics, dynamic heterogeneity, diffusion coefficient and crystallization behavior of the Pd40Ni40P20 metallic glass. The presence of dynamic heterogeneity in the Pd40Ni40P20 glass-forming metallic melt is evaluated from the dynamic susceptibility, which is derived from the QENS self-intermediate scattering function. The dynamic heterogeneity increases gradually with decreasing temperature, and a dynamic crossover arises at approximately 1235 K, which is related to the cooperative motion of atoms. Ultrafast scanning calorimetry combined with conventional calorimetry provides heating and cooling rates over seven orders of magnitude, allowing us to study how the liquid thermal history affects glass formation, glass transition, and crystallization. The critical cooling rate required for glass formation is directly determined. A continuous-heating-transformation diagram for crystallization is constructed. This study will improve our understanding of the atomic relaxation dynamics mechanism and intrinsic heterogeneity in metallic glasses.

Interfacial bonding, corrosion, and wear behavior of Zr-based amorphous (Zr41.2Ti13.8Cu12.5Ni10Be22.5) alloy coatings prepared by plasma spraying technique

In this study, zirconium-based amorphous coating (Zr-AMC), a high-performance coating for industrial equipment that could adapt to the operating conditions of the reaction kettle under high temperature and pressure, was prepared by plasma spraying on AISI 316L. The bonding strength, wear behavior and corrosion mechanism of the corrosion-resistant and wear-resistant coatings were analyzed in detail using coating tensile, friction wear, and electrochemical and molecular dynamics (MD) simulations. The results reveal that the Zr-AMC coating has a uniform and dense structure, characterized by a high hardness of 689.92 HV and an elastic modulus of 181.98 GPa. Furthermore, the bond strength of the Zr-AMC coating on AISI 316L is 31.92 MPa. The corrosion rate of Zr-AMC coating is 0.056 mm·a−1 in 20 wt% H2SO4 solution, its corrosion resistance is six times that of AISI 316L, and the protection efficiency of Zr-AMC to AISI 316L is 82.15 %. Additionally, Zr-AMC has better corrosion resistance than Zr and Ti. The MD calculations reveal that the adsorption energy of H2SO4 molecules on Zr-AMC (−3.5 eV) is greater than that of AISI 316L (−17 eV). The binding energy of ZrO2 passivation film to Zr-AMC is higher than that of Cr2O3 passivation film and Fe, and the diffusion coefficient of oxygen atoms in ZrO2 (2.67 × 10−7 cm2s−1) is greater than that in Cr2O3 (1.5 × 10−9 cm2s−1). The Zr-AMC coating has a lower wear rate (1.24 × 10−7mm3N−1 mm−1) than that of Zr and Ti, and the wear mechanism is predominantly adhesive wear and fatigue wear. The development and preparation of Zr-AMC coating offer more material options for the use of reactors under extreme environmental conditions.

Study on diffusion kinetics of chromium and nickel electrochemical co-deposition in a NaCl–KCl–NaF–Cr2O3–NiO molten salt

Abstract The process of preparing surface composite by molten salt co-deposition is the result of the mass transfer of active particles in molten salt, electrochemical reduction, and solid diffusion. In this study, we prepared Cr–Ni alloy/low-carbon steel surface composites in NaCl, KCl, NaF, Cr2O3, and NiO melt salt system successfully, and analyzed the entire diffusion dynamics process, aiming to find out the limiting links and provide ideas for further improving the preparation efficiency. The results show that chromium and nickel ions are simultaneously reduced on the cathode surface through two and one steps, respectively. And an alloy layer with Fe content of 64.52 wt%, Ni content of 28.96 wt%, and Cr content of 6.52 wt% is formed on the surface of low-carbon steel substrate. The average diffusion coefficients of chromium and nickel atoms in the surface composites are 1.16 × 10−14 and 1.44 × 10−14 m2·s−1. The mass transfer process in molten salt is the limiting link in the whole preparation process.

Poly-Lactic Acid/Graphene Anode for Lithium-Ion Batteries Manufactured with a Facile Hot-Pressed Solvent-Free Process

For environmental and cost purposes, solvent-free electrode manufacturing techniques are needed for lithium-ion cell technology. In this work, we present a stand-alone lithium-ion anode, containing graphene and Poly-lactic acid (PLA) as active and binding material, respectively, manufactured in a free-solvent process. To this purpose, PLA and graphene were thoroughly mixed and a hot-press was used to form the resulting electrode. At a half-cell configuration, the electrodes exhibited a stable reversible specific capacity of more than 300 mAh g−1 at C/15 for over 450 cycles and a promising C-rate performance of around 90 mAh g−1 at 6 C of constant current mode. After cyclic voltammetry analysis of the electrochemical behavior and the kinetics of the prepared electrodes, the Li atom diffusion coefficient was calculated around 1.2 × 10−8 cm2/s during lithiation and 0.6 × 10−8 cm2/s during delithiation. Finally, we show that this electrode manufacturing technique can be upscaled for higher mass loading and corresponding areal capacity at least up to 1 mAh/cm2 and thus it can be considered for practical applications.

Single-size and cluster dynamics modeling of intra-granular fission gas bubbles in UO[formula omitted

We perform simulations of intra-granular fission gas bubble evolution in UO2 using both a relatively simple, computationally inexpensive single-size model and a detailed cluster dynamics model. Simulations encompass 36 experimental cases from 4 different databases, covering various temperature and burnup levels. We systematically compare results from the two models to each other and to post-irradiation experimental data of bubble average size and number density. Overall, the model-to-model comparisons reveal an excellent agreement across the set of simulations. This outcome indicates that, in spite of the underlying assumptions, the single-size model provides a good approximation of the complex physical behavior that is more rigorously described by the cluster dynamics model. Qualitatively, both models reproduce the trends of the experimental data with temperature and burnup correctly. Quantitatively, calculated results are either in good agreement with the data or within errors that appear consistent with the inherent uncertainties. Moreover, for the single-size model, we demonstrate and assess a multiscale approach whereby values for the fission gas atom diffusion coefficient from separate atomistic calculations are used. Systematic comparisons to experimental data point out a credible accuracy of the multiscale model.

Infrared Spectroscopic Studies of Oxygen Atom Quantum Diffusion in Solid Parahydrogen.

The thermally induced diffusion of atomic species in noble gas matrices was studied extensively in the 1990s to investigate low-temperature solid-state reactions and to synthesize reactive intermediates. In contrast, much less is known about the diffusion of atomic species in quantum solids such as solid parahydrogen (p-H2). While hydrogen atoms were shown to diffuse in normal-hydrogen solids at 4.2 K as early as 1989, the diffusion of other atomic species in solid p-H2 has not been reported in the literature. The in situ photogeneration of atomic oxygen, by ArF laser irradiation of an O2-doped p-H2 solid at 193 nm, is studied here to investigate the diffusion of O(3P) atoms in a quantum solid. The O(3P) atom mobility is detected by measuring the kinetics of the O(3P) + O2 → O3 reaction after photolysis via infrared spectroscopy of the O3 reaction product. This reaction is barrierless and is thus assumed to be diffusion-controlled under these conditions such that the reaction rate constant can be used to estimate the oxygen atom diffusion coefficient. The O3 growth curves are well fit by single exponential expressions allowing the pseudo-first-order rate constant for the O(3P) + O2 → O3 reaction to be extracted. The reaction rates are affected strongly by the p-H2 crystal morphology and display a non-Arrhenius-type temperature dependence consistent with quantum diffusion of the O(3P) atom. The experimental results are compared to H(2S) atom reaction studies in p-H2, analogous studies in noble gas matrices, and laboratory studies of atomic diffusion in astronomical ices and surfaces.