TITLE: Understanding of Ion-Solid Interaction and Defect Evolution in Zinc-Blende Structured Materials

CHAIR: Prof. Fei Gao

ABSTRACT: Zinc-blende structured materials have received considerable attentions due to their excellent performance in many fields. The major benefit has attributed to high power space energy systems and nuclear reactors. Their applications can expose to high energy radiation, including neutrons, ions and cosmic rays. Under these conditions, defects are generated in materials in amounts significantly exceeding their equilibrium concentrations. The accumulation of defects can lead to undesired consequences, which may alter the performance of the materials. Therefore, the fundamental understanding of ion-solid interaction and defect evolution is a key factor to the success of both nuclear and electronic materials. This thesis focuses on the study of zinc-blende materials, including GaAs, GaN, InAs, and SiC for their possible applications in both nuclear and space fields.
SiC has its unique capability in the applications of nuclear fuel, cladding and fusion structure materials. In tri-structural isotropic (TRISO) fuel particles, SiC coating is considered as a major barrier for the release of fission products (FPs). However, the release of some metallic FPs (i.e. Ag, Pd, Ru, and I) from fully intact fuel particles raises serious concern on the safety of high temperature gas-cooled reactors (HTGRs). This thesis first addresses atomistic process of FP diffusion in SiC. Ab initio calculations are used to determine the defects configurations, migration energy barriers and pathways of FPs in SiC. Based on the ab initio results, the interatomic potentials of FPs in SiC are developed and evaluated to serve as a link between the density functional theory (DFT) and next coarser level. Classical molecular dynamics (MD) simulations have been employed to investigate FP accommodation in SiC, interactions with point defects and grain boundaries, and their diffusion kinetics. These findings lead to a conclusion that the grain boundary diffusion of FPs is faster than bulk diffusion with a strong segregation at the GBs. Analysis of the radiation enhanced diffusion obtained by experiments and diffusion by modeling work for Ru and I has suggested that the interstitial migration is likely to be a major mechanism under irradiation condition. Moreover, the diffusivities can vary by grain boundary types, whereas high energetic grain boundaries can provide the fastest paths for FPs to diffuse. An elevation of 1.5 eV in GB energy can result in 2-3 orders of magnitude difference in Ag diffusion coefficient.
We have further explored the defect production, clustering, and its evolution in GaAs, GaN, InAs, and SiC, and determined non-ionizing energy loss (NIEL) that indicates a rate of degradation in electronic devices in space applications. Nonlinear defect production is observed with an increasing of primary knock-on (PKA) energy in GaAs and InAs. This effect, which corresponds to the direct-impact amorphization, is observed for PKA energy over 2 keV. Gallium nitride is however different and presents a pseudometallic behavior (PMB) resulting in a majority of surviving defects to be single interstitials or vacancies. SiC also has a limited number and size of defect clusters due to the formation of multiple subcascades with low energy density. With the damage density evaluated from MD simulations, a model to determine NIEL has been developed, which can be used to qualify the radiation degradation in space application. The NIELs for proton, alpha, and Xe particles are then predicted, and provide a pathway to evaluate the capabilities for the space applications of these materials. The comparisons of defect creation, density, and effective NIEL suggest that GaN may be the best candidate as a radiation hard material for space applications at high-energy regime. For low incident particle energies at which the NIEL ratio of InAs-to-GaN is less than 1, the performance of InAs may be superior to that of GaN.