A machine learned interatomic potential for AlN was developed using the ultra-fast force field (UF3) methodology. A strong agreement with density functional theory calculations in predicting key structural and mechanical properties, including lattice constants, elastic constants, cohesive energy, and surface energies has been demonstrated. The potential was also shown to accurately reproduce the experimentally observed atomic core structure of edge dislocations. Most significantly, it reproduced the experimentally observed wurtzite crystal structure in the overlayer during homoepitaxial growth of AlN on wurtzite AlN, something that prior potentials failed to achieve. Additionally, the potential reproduced the experimentally observed layer-by-layer growth mode in the epilayer. The combination of accuracy, transferability, and computational speed afforded by the UF3 framework thus makes large-scale, atomistic simulations of epitaxial growth of AlN feasible.

Nuclear waste repository designs require immobilizing contaminants, including pertechnetate (TcO4 -). Clay functionalized with organic cations (organoclay) has been shown to immobilize TcO4 -. The current work measures the physicochemical properties of organoclays, tests each organoclay's ability to retain TcO4 -, and provides computational data for the orientation of the alkylammonium cation within the interlayer as well as binding energies for the pertechnetate-alkylammonium-clay system. The results show consistency between experimental and computational interplanar spacings and orientations, with indications that alkylammonium cations are sorbed to both the clay edge and interlayer sites during functionalization. Pertechnetate-alkylammonium interactions are calculated, and implications for TcO4 - sequestration by organoclay are discussed.

Understanding the impact of helium bubbles on crack propagation is complex. A useful first study towards understanding bubble effects on fracture is to examine how voids impact fracture. In this work, we used phasefield fracture simulations to examine the influence of voids and their distribution on Mode I fracture in Fe. Assuming brittle fracture, two simulation configurations were considered: (1) nanoscale systems with one or two voids, and (2) nanoscale systems with an experimentally relevant distribution of voids, with up to 20 % void area. Results from simulations with one and two voids showed that voids within 10 nm of a crack tip reduce the stress required for crack growth, with the magnitude of reduction depending on void-to-crack orientation. Comparisons with linear elastic fracture mechanics and evaluation of one versus two void systems revealed deviations from linear superposition, implying complex interactions between void and crack tip stress fields. In multi-void simulations, as void sizes increase, the nearest void to the crack tip exerts a greater influence on fracture stress than the overall porosity. This study provides valuable insights into the relationship between void size and concentration, and the stress necessary for crack growth, marking a step forward towards understanding He bubble-induced fracture in ferrous materials.

Layered vanadium phosphate dihydrate, VOPO4·2H2O, has been chosen as a host for ferrocene intercalation to explore the influence of solvent-guest and solvent-host interactions on rates and product selection. After eliminating solvents that react directly with the VOPO4·2H2O host, ferrocene intercalation was evaluated in a series of seven solvents that do not themselves intercalate or otherwise alter the host structure. The intercalation kinetics are analyzed in acetone, butanone, pentanone, acetonitrile, propionitrile, butyronitrile, and DMF. No intercalation is observed in DMF, but all other solvents yield a mixture of stage 1 and stage 2 ferrocenium intercalation products with reaction rates and product ratios varying across the series of solvents. Using ex situ as well as in situ PXRD methods, faster reactions yield more stage 1 product, while a higher degree of stage 2 product is seen for slower kinetics. The results are interpreted in the context of competition between intercalation rates and the buildup of elastic strain between intercalated domains and unintercalated host. To understand solvent effects, both solvent-guest and solvent-host effects are considered. None of the typically considered solvent-guest effects, such as guest desolvation energy, dielectric constant, and guest oxidation potential, correlate with the observed reaction kinetics. On the other hand, different solvent environments are shown to change the internal pressure, Peff, within the layered hosts, a consequence of changing surface energy and surface tension (stress) in particles with nanometer scale dimensions. The VOPO4·2H2O interlayer spacing decreases when particles are suspended in DMF and organonitrile solvents, while the layers separate slightly in acetone, butanone, and pentanone. Using calculated elastic constants, the structural changes correspond to changes in Peff in the range 0.22 GPa > ΔPeff > -0.11 GPa across the series of solvents. A density functional theory analysis of the influence of pressure on ferrocene diffusion adds support for the idea that intercalation kinetics are altered by changing internal pressure. The results show solvent environments can be responsible for altering the effective pressure within intercalation hosts, influencing intercalation rates and product selection, even if the solvents do not react directly with the host solids.

High-temperature oxidation of a 21-2N steel alloy in a dry atmosphere produces a multilayered oxide film containing MnCr2O4, Cr2O3, and Mn3O4. To understand how this multilayered film evolves over time, it is crucial to identify the prevalent defect types and their respective high mobility diffusion pathways. In this work, the thermodynamics and kinetics of Mn and Cr point defects in the Cr2O3 system are evaluated with density functional theory calculations. High-temperature conditions consistent with the application domain are modeled by adjusting the ab initio chemical potential values to fit relevant thermochemical data. Mn is found to readily form substitutions on the Cr sublattice, thereby enabling rapid vacancy-mediated diffusion of MnCr defects. Furthermore, an interstitialcy mechanism is investigated as an alternative diffusion pathway for both Cr and Mn. Finally, formation energy and migration barrier heights from this work and others are synthesized to produce a mechanistic model of diffusion in the multilayered film. This model aims to predict changes in relative thickness at long exposure times and the specific interface at which oxide growth takes place.

Nuclear waste repository designs require backfill materials to contain long-lived radionuclides, including technetium-99, present as the pertechnetate anion (TcO4-) under oxic repository conditions and mobile in the environment. Bentonite has been proposed as a suitable backfill material, and it is composed of montmorillonite clay, which can be functionalized to optimize its performance for TcO4- sorption. In the current work, bentonite clay is functionalized with organic and inorganic moieties, and the impact of dual functionalization on TcO4- sorption is investigated. The results show that the ordering of the functionalization is essential and that adding metal to an organoclay improves TcO4- sorption compared to either the reverse ordering or the organic/inorganic clay alone. Furthermore, the TcO4- sorption to the organometallic clays was consistent with either chemisorption or cooperative sorption, with a multi-step mechanism determining the kinetics of sorption.

Solar thermochemical hydrogen production using redox-active metal oxides is a promising pathway for the production of green hydrogen and synthetic fuel precursors. Herein, the perovskite material (La0.6Sr0.4)(0.95)Mn0.8Ga0.2O3-delta (LSMG6482) is identified as a promising metal oxide for thermochemical water splitting. LSMG6482, along with more-established water splitters ceria and (La0.6Sr0.4)(0.95)MnxAl1-x O3-delta (LSMA) perovskites, is experimentally characterized via thermogravimetric (TGA) analysis and high-temperature water splitting in a reactor simulating solar concentrating conditions. TGA analysis demonstrated that LSMG6482 has high and stable oxygen exchange capacity under controlled pO(2 )redox cycling, demonstrated by large changes in oxygen nonstoichiometry (delta) relative to ceria. Water splitting experiments using laser heating (T red = 1400 degrees C, Tox = 1200 degrees C) resulted in H-2 yields of 165.1 mu mol g(-1) for the candidate LSMG6482 composition, exceeding that of all benchmark materials tested. Under high conversion oxidation conditions, where H2 is cointroduced with H2O (150 <= nH2O/nH2 <= 500), H2 yields were greatest for LSMG6482 and LSMA6482, up to four times that of ceria at the highest nH(2)O/nH(2) conditions. Crystallographic analysis showed that over the course of experimentation, there is some secondary phase growth for all perovskite compositions, except for LSMA6482, but there was no observable degradation in H2 yields.

We present a molecular dynamics study of the thermal transport properties of PbTe/PbSe (1 1 1) and PbTe/PbSe (1 0 0) interfaces at room temperature. The PbTe/PbSe heterostructures are obtained through simulations of the kinetic processes of direct bonding of PbTe and PbSe crystals. The atomic-scale dislocation core structures and the misfit dislocation networks in the heterostructures obtained in the simulations are found to closely match experimental data. Two types of heat transfer experiments are then simulated: a heat-sink heat-source experiment and an ultrashort heat pulse experiment. Thermal boundary resistance is calculated for three distinct interface types: coherent, semi-coherent, and semi-coherent with pinned dislocations. Both types of simulations consistently capture the significant role of the misfit dislocations on thermal resistance. The effect of the mobility of dislocations on thermal resistance is demonstrated for the first time through comparing the thermal boundary resistance of interfaces containing pinned dislocations and with those containing unpinned dislocations. In addition, the thermal boundary resistance is found to strongly depend on the length of the specimen and the area of the interface.

Cs-137 is a radionuclide fission product that poses a significant risk to life, making it crucial to develop effective methods for its separation and sequestration from nuclear waste streams. Zeolitic structures have emerged as promising materials. This work examines the influence of structure, exchange site energetics, and formation enthalpies of nascent and cation-exchanged Faujasite-X, -Y, and -HY zeolites in terms of their Cs-exchange selectivity. Their interplay was quantified with the application of high-temperature calorimetry, adsorption isotherms, X-ray diffraction and density functional theory (DFT) calculations. Greater efficacy of Cs+ exchange was demonstrated for the Na+-substituted Fau-Y (NaY) zeolite than that of the Fau-X (NaX) and Fau-HY (Na-HY) zeolites. This is explained by a higher amount of Na+ in un-exchangeable sites in the case of NaX and a lower stability in NaY that favors the ionic exchange with Cs+. Moreover, Cs+ incorporation in the structure increases the stability of each kind of zeolite. Correspondingly, structure and DFT analyses demonstrated site-exchange thermodynamic favorability as well as the contribution from cage cell, which resulted in an energy landscape far more conducive to Cs+ incorporation for NaY than either NaX or Na-HY.

Copper hexacyanoferrate (Cu-HCF) has been shown to be a good candidate for selectively sequestrating radioactive Cs. Cu-HCF is usually hydrated with water molecules. The water molecules that are typically incorporated into the framework can be categorized as coordinated water molecules, which bond to a specific atom, and zeolitic water molecules which, while penetrating the framework structure, are not chemically bonded to it. Here, we use density functional theory calculations to probe the distribution of water in hydrated Cu-HCF, the thermodynamics of the incorporation of alkali ions, and the exchange of Na+ and K+ ions by Cs+ ions. We find that the largest number of water molecules that can be incorporated into a unit cell of Cu-HCF is 16 with the balance between coordinated and zeolitic water molecules depending on the number of water molecules in the Cu-HCF framework. We also find that hydration enhances incorporation of alkali ions compared to nonhydrated Cu-HCF.

DFT reveals optimal configuration of ferrocene intercalation into VOPO 4 , and it also shows ferrocene prefers intercalating into VOPO 4 at V sites, driven by charge transfer and enhanced van der Waals forces.

Zeolites are versatile materials renowned for their extra-framework cation exchange capabilities, with applications spanning diverse fields, including nuclear waste treatment. While detailed experimental characterization offers valuable insight, density functional theory (DFT) proves particularly adept at investigating ion exchange in zeolites, owing to its atomic and electronic resolution. However, the prevalent occurrence of zeolitic ion exchange in aqueous environments poses a challenge to conventional DFT modeling, traditionally conducted in a vacuum. This study seeks to enhance zeolite modeling by systematically evaluating predictive differences across varying degrees of aqueous solvent inclusion. Specifically focusing on monovalent cation exchange in Na-X zeolites, we explore diverse modeling approaches. These range from simple dehydrated systems (representing bare reference states in vacuum) to more sophisticated models that incorporate aqueous solvent effects through explicit water molecules and/or a dielectric medium. Through comparative analysis of DFT and semi-empirical DFT approaches, along with their validation against experimental results, our findings underscore the necessity to concurrently consider explicit and implicit solvent effects for accurate prediction of zeolitic ionic exchange.

Phyllosilicate clay minerals have been proposed as a possible buffer material to be used in deep geological repositories containing high-level waste and used nuclear fuel. This work precisely characterizes ion interactions with two types of adsorption sites present in these clays: MgAl’ substitutions and undercoordinated edge surface atoms. A number of unique structural models were considered to represent the diverse local environments that ions in these systems are likely to encounter. Using molecular dynamics simulation with the CLAYFF potential, the spatial distribution, interlayer composition, and residence times of Na+ and Cl− ions as radionuclide analogs in pyrophyllite and montmorillonite clay models were investigated to identify the most favorable conditions for sequestration. The most significant factor impacting ion adsorption was found to be the localization of charge density at substitution sites. In a montmorillonite system in which substitution sites were distributed evenly to produce a low charge density, sequestration performance was found to be comparable to pyrophyllite.

We use density functional theory to investigate the interactions of cerium, americium, and curium cations with crown ethers. Our calculations reveal that the modeled structure of cerium integrated within the crown ether is in good agreement with experimental data, with the negative binding energy indicating that capturing the cerium nitrates is thermodynamically favorable. Our results demonstrate that crown ethers can also bind americium and curium, providing insights into the potential applications of crown ether in radionuclide sequestration. Finally, we explore the impact of the skeleton modification of different crown ethers through by substitution of nitrogen atoms in the core of the crown ether for oxygen atoms and find that this structural modification significantly increases the radionuclide binding energies. These findings provide insights on the potential for the use of organic linkers such as crown ethers to address the urgent needs in radionuclide sequestration, separation and sensing.

We develop a phase field model of char oxidation during atmospheric entry in the ablation zone of the Phenolic Impregnated Carbon Ablator (PICA). The phase field model is coupled to a model of heat transport, to capture the interaction between the oxidation and temperature, and it is based on our previous model of carbon fiber oxidation. We explore the char oxidation behavior using three 2D examples. First, we simulate oxidation with an imposed temperature gradient for char structures with increasing amounts of initial porosity. The oxidation time decreases with increasing porosity and the oxidation rate fluctuates as pores are opened to oxygen transport. Next, we predict the decrease in temperature due to the endothermic oxidation reactions in thermally-insulated systems with and without transport of oxygen through the top boundary. In both cases, the temperature decreases until oxidation stops; it stops due to the decrease in temperature in the case open to oxygen transport and due to lack of oxygen in the case closed to oxygen. Finally, we explore the oxidation behavior under different heat fluxes. The oxidation reactions compensate for the heat flux until the char is fully consumed and then the temperature increases. Our results demonstrate the importance of considering the interactions between the oxidation reactions and the temperature when modeling char oxidation.

The fundamental interest in actinide chemistry, particularly for the development of thorium-based materials, is experiencing a renaissance owing to the recent and rapidly growing attention to fuel cycle reactors, radiological daughters for nuclear medicine, and efficient nuclear stockpile development. Herein, we uncover fundamental principles of thorium chemistry on the example of Th-based extended structures such as metal-organic frameworks in comparison with the discrete systems and zirconium extended analogs, demonstrating remarkable over two-and-half-year chemical stability of Th-based frameworks as a function of metal node connectivity, amount of defects, and conformational linker rigidity through comprehensive spectroscopic and crystallographic analysis as well as theoretical modeling. Despite exceptional chemical stability, we report the first example of studies focusing on the reactivity of the most chemically stable Th-based frameworks in comparison with the discrete Th-based systems such as metal-organic complexes and a cage, contrasting multicycle recyclability and selectivity (>97%) of the extended structures in comparison with the molecular compounds. Overall, the presented work not only establishes the conceptual foundation for evaluating the capabilities of Th-based materials but also represents a milestone for their multifaceted future and foreshadows their potential to shape the next era of actinide chemistry.

Zr-based metal-organic frameworks (Zr-MOFs) have been widely used as ion adsorbents for the removal or extraction of toxic and/or radionuclide species from aqueous solutions. However, the mechanisms by which uranyl cations (UO22+) interact with Zr-MOFs have not been established. In this work, the nature of the bonding of uranyl cations with a Zr-MOF was determined using density functional theory for nineteen structurally distinct candidate complexes. The results showed that in all cases the binding energy was of the order of 1.5 eV, but depended on the specific bonding site. The most stable structure involved coordination of the uranyl cation and two structurally distinct oxygens in the Zr-MOF metal node. It was also found that higher degree of deprotonation in Zr-MOF correlated with higher binding energy between the Zr-MOF and uranyl cations. These insights can aid in the design of Zr-MOFs with optimized features for efficient capture of uranyl cations.

A phase-field model is parameterized to study the effect of elastic stresses on the migration of He gas bubbles in Fe under a temperature gradient. Stresses caused by the gas bubble pressure and residual stress in the Fe matrix are considered. The dependence of He bubble migration velocity on the magnitude of the residual stress, average temperature, temperature gradient, and bubble size is measured. In agreement with a theoretical model based on surface diffusion, simulation results demonstrate that He bubbles move towards the high temperature region with velocities in Fe that are orders of magnitude faster than previously reported in UO2. It is found that local stresses in the matrix caused by the He bubble have negligible effect on the bubble migration process; however, residual stresses in the Fe matrix, potentially caused by processing or irradiation, can modestly modify bubble kinetics through pressure dependence of the He diffusion coefficients. Compressive residual stress decreases diffusion coefficients for bulk and surface diffusion mechanisms, thus reducing the migration velocity of the gas bubble. In contrast, tensile residual stress increases the diffusion coefficients, resulting in an increase in the gas bubble migration velocity. This pressure dependence is also consistent with a theoretical model. This phase field model lays the foundation for analysis of bubble coalescence-induced fracture in He bubble-containing steels.

While ceria is the standard material for two-step water splitting, perovskites are emerging as viable alternatives. In this work, based on the orthorhombic LaMnO3 supercell, we substitute Li Na K Rb Mg Ca Sr Ba on the A-sites (La sites) and Al Ga In Mg Zn on the B-sites (Mn sites) at a concentration of 37.5%. The range of temperature and oxygen partial pressure at which each composition is stable is predicted. For compositions that are stable in relevant temperature and pressure ranges, the oxygen vacancy formation energies are determined for all of the oxygen vacancy site positions available in the computational supercell. Mg, Ca, Sr, and Ba A-site-substituted LaMnO3 and Al and In B-site-substituted LaMnO3 meet these two criteria for candidates in solar-thermal water splitting applications. Oxygen vacancy formation energy can also be controlled by adjusting the doping strategy.