Vacancy Clustering Phenomena in Silcon Carbide Reactor Components: A Multiple Time-Scale Atomistic Problem

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The goal of this dissertation was to elucidate the coalescence of vacancies in the cubic polytype of silicon carbide in a nuclear reactor relevant system. Large-scale empirical potential atomistic simulations were employed and the exploration of long time-scale behavior was performed through a new application of the parallel replica dynamics method. To differentiate this dissertation from earlier work, emphasis has been placed on the determination of a predictive description of initial damage states, the study of the impact of pre-existing voids, and the exploration of long-time behavior of the damage configuration. Further, the methodology employed here is sufficiently general to be applied to any material. From the simulations performed, several major conclusions were drawn. The simulations of 10 keV silicon primary knock-on atom cascades in a perfect silicon carbide crystal showed good qualitative agreement with previous simulations. No large clusters formed and most vacancies were isolated in the final state, in contrast to some similar studies in metals. Simulations at 1200~K yielded an approximate spatial distribution of vacancies, a completely new result for silicon carbide. The temperature effect study indicated the existence of a high temperature defect relaxation regime at 2000 K that reduced the damage caused by the cascade. After the addition of the initial void structures to the 10 keV cascade studies, the results did not exhibit significant void growth or structural change. It is suspected that boundary conditions contributed to this effect. In both the perfect crystal and initial void studies, it was concluded that a dramatically larger timescale or multiple irradiation events would be needed to observe formation of large vacancy clusters. Parallel replica dynamics calculations were performed that demonstrated the major pathways for vacancy migration in cubic silicon carbide, in agreement with previous ab initio studies. This was the first application of parallel replica dynamics to a cubic silicon carbide system with a large number of migrating vacancies. It was concluded that the computational requirements for a long time calculation of the full cascade geometry would be prohibitive even on a large modern supercomputer.

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  • 09/14/2018
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