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Cation self-diffusion plays a key role in the microstructure evolution of a wide range of materials, including oxide nuclear fuels. Growing evidence indicates that small defect clusters can make a substantial contribution to cation transport in these oxides. Quantifying this contribution is nevertheless challenging because of complex defect chemistry, strong correlations between cation and anion migration, and the existence of multiple competing migration mechanisms. In this work, we compute bulk cation self-diffusion coefficients in stoichiometric UO2 and PuO2 by explicitly accounting for small vacancy clusters (cation-anion divacancies and Schottky defects) in addition to isolated cation vacancies. Our approach combines a systematic exploration of cluster migration pathways using ART-nouveau with the calculation of cluster transport coefficients through the KINECLUE code, which rigorously incorporates the influence of local atomic environments and kinetic correlations on defect migration. These transport coefficients are then coupled with equilibrium cluster concentrations to obtain self-diffusion coefficients directly comparable with experimental data. We find that small vacancy clusters are significantly more mobile than isolated cation vacancies and dominate the overall diffusion process. In UO2, the resulting self-diffusion coefficients are consistent with the most reliable experimental datasets, whereas in PuO2 the predictions are discussed in light of the limited data currently available. Although vacancy clusters exhibit higher mobility in PuO2, the overall self-diffusion coefficient is lower than in UO2 because of the larger cation vacancy formation energy predicted by our defect model. Beyond actinide oxides, this work demonstrates how defect clusters can be systematically integrated into diffusion calculations and provides transferable insights and input data for fuel behavior models.

Platinum-group elements (PGE) are most effectively concentrated via sulfide-silicate melt immiscibility; however, under sulfide-undersaturated conditions, PGE may exhibit highly siderophile behavior and occur as native metals and alloys. In this case, they can form micrometer-size inclusions within Cr-spinel, but also large nuggets (up to several kilograms), found in chromitites and related placer deposits. The exact formation mechanism of such large nuggets and accumulations of PGE unrelated to sulfide melts remains controversial due to mass balance issues. In this study of multiphase inclusions in Os-Ir-Ru nuggets from the Adamsfield placer district (Tasmania, Australia), we constrain their crystallization environment. Multiphase inclusions comprise variable proportions of hornblende, enstatite, quartz, anthophyllite, anorthite, chlorite and native iridium and coexist with single-phase olivine and Cr-spinel inclusions. The heterogeneity in phase and chemical composition of the inclusions indicates a complex origin from an inhomogeneous source media. This is corroborated by grain scale disequilibrium mineral assemblages, where forsterite and quartz are both included within a single osmium grain. Our proposed multi-stage origin of Adamsfield Os-Ir-Ru nuggets involved magmatic olivine-Cr-spinel-Os-Ir-Ru cumulates in peridotite bodies that were subsequently overprinted by various hydrothermal and metamorphic processes, including serpentinization or growth of Os-Ir-Ru from supercritical fluids. The final metamorphic stage resulted in the obliteration of Os-Ir-Ru zonation, culminating in the current assemblage of inclusions.

In this work, a hierarchical-linked batch-to-batch optimization based on transfer learning is proposed to realize the effective optimization of a new synthesis process. Optimization efficiency is especially crucial for batch processes to improve the product quality and maximize the economic benefits. The traditional hierarchical optimization method can achieve a better effect, but it may lead to low efficiency since it requires more iterations. To further improve the optimization efficiency of a new batch process with high operational cost, a hierarchical-linked batch-to-batch optimization based on transfer learning is proposed in this work. By introducing the linkage between hierarchies, the available information transmitting between hierarchies is addressed to assist and accelerate the modelling and optimization process. A performance assessment criterion based on the prior knowledge of similar processes is also proposed to further improve the optimization effect. Finally, the performance of the proposed method is verified through a simulation study of the cobalt oxalate synthesis process.

It is well known that stress and electromigration induced voiding is of major concern for integrated circuit interconnect reliability. However, there has been little systematic characterization of void morphology and crystallography in ever more technologically important narrow, “near-bamboo” conducting lines. Prior reports indicate that voids are typically wedge or slit shaped, with failure often associated with slit voids. Void face habit plane is most often reported to be {111}. Wedge and slit void morphology and crystallography have been studied in comb/serpentine and parallel line array test structures. In virtually all cases, void faces are {111} oriented. In contrast to earlier studies, intragranular wedge stress voids have been observed. All electromigration opens were due to slit voids; these were typically intragranular, in contradiction to current theories of void formation, and likely are mechanical fractures. Under accelerated test conditions, non-grain boundary diffusion paths appear to operate at distances of tens of micrometers. Relative displacement between wedge voids and attached grain boundaries occurs where a wedge face lies on a near common {111} plane for the two grains. It is suggested that slit voids are intragranular under both stress and electromigration conditions and likely associated with local interconnect depassivation. Based solely on appearance and crystallography, no void can uniquely be identified as due to stress alone or electromigration alone.