Date of Award
Doctor of Philosophy
Materials Science and Engineering
William J. Weber
Yanwen Zhang, Haixuan Xu, Maik K. Lang
Interfaces in solid materials are the so-called boundaries, separating crystals with the same structure and chemistry but different orientations, e.g. grain boundaries (GBs), different stacking sequences, e.g. stacking faults (SFs), or crystals with different structures and/or chemistries as well as orientations, e.g. the interface between substrate and thin film. In this study, first-principles calculations are used to investigate the defect behavior at different interfaces and in-plane strain fields, such as stacking fault (SF) in silicon carbide (SiC), in-plane strain field near interfaces in potassium tantalate (KTaO3), and grain boundary in ceria (CeO2).
Results show that the existence of SFs in SiC considerably affects the defect configurations, which modifies the local atomic and electronic structures. Both changes influence the local energy landscape, and thus affecting the formation and migration energy of defects in the SFs region. The lower barriers for Si interstitial diffusion near the faults may be responsible for the enhanced defect annihilation observed under irradiation in 3C-SiC with high densities of stacking faults.
In the KTaO3, the formation-dependent site preferences for oxygen vacancies are expected to occur under epitaxial strain, which can result in orders of magnitude differences in the vacancy concentration on different oxygen positions. The diffusion behavior of oxygen vacancy in strain fields is also considered. In contrast to the strain-enhanced intra-plane diffusion, it is found that the inter-plane diffusion, which perpendicular to the strained plane, is impeded under the strain field.
CeO2 is considered as the surrogate for mixed oxide fuel. In this case, fission gases, such as Xenon (Xe), are trapped near the GBs and form the gas bubbles. Our results show that the Xe segregation propensity is reduced as the size of trap sites increases. In the hyper-stochiometric conditions, the solubility of Xe trapped in the GB is significantly higher than that in the bulk, suggesting Xe concentration would be higher than that in the bulk. The activation energies for Xe diffusion in the GB are lower than those in the bulk, indicating that the mobility of Xe atom in the GB is higher than that in the bulk.
Xi, Jianqi, "First-Principles Study of Point Defect Behavior at Interfaces and In-Plane Strain Fields. " PhD diss., University of Tennessee, 2017.