Doctoral Dissertations

Date of Award

12-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

William J. Weber

Committee Members

Yanwen Zhang, Gerd Duscher, Maik K. Lang

Abstract

The high temperature radiation resistance of nuclear materials has become a key issue in developing future nuclear reactors. Because of its mechanical stability under high-energy neutron irradiation and high temperature, silicon carbide (SiC) has great potential as a structural material in advanced nuclear energy systems.

A newly developed nano-engineered (NE) 3C SiC with a nano-layered stacking fault (SFs) structure has been recently considered as a prospective choice due to enhanced point defect annihilation between layer-type structures, leading to outstanding radiation durability.

The objective of this project was to advance the understanding of gas bubble formation mechanisms under irradiation conditions in SiC. In this work, microstructural evolution induced by helium implantation and ion irradiation was investigated in single crystal and NE SiC. Elastic recoil detection analysis confirmed that the as-implanted helium depth profile did not change under irradiation to 30 dpa at 700 °C. Helium bubbles were found in NE SiC after heavy ion irradiation at a lower temperature than in previous literature results. These results expand the current understanding of helium migration mechanism of NE SiC under high temperature irradiation environment.

No obvious bubble growth was observed after ion irradiation at 700 °C, suggesting a long helium bubble incubation process under continued irradiation at this temperature and dose. As determined by electron energy loss spectroscopy measurements, only 1 % of the implanted helium atoms are trapped in bubbles. Helium redistribution and release was observed in the TEM samples under in-situ irradiation at 800 °C. In-situ TEM analysis revealed that the nano-layered SF structure is radiation tolerant below a dose of about 15 dpa at 800 °C, but continued irradiation to 20 dpa under these in-situ conditions leads to loss of the stacking fault structure, which may be a manifestation of irradiating thin TEM foils. The irradiation stability of the SF structure under bulk irradiation remains unknown. This stacking fault structure is critical since it suppresses the formation of dislocation loops normally observed under these irradiation conditions. Systematic studies towards understanding the role of defect migration under irradiation on the evolution of helium bubbles in NE SiC were performed.

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