Characterization of Macroscopic Properties and Crystalline Defects in Neutron-Irradiated Silicon Carbide

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Length change, mechanical properties, helium release behavior, ESR observation and microstructure of neutron-irradiated silicon carbide (SiC) were investigated. Changes in those properties due to annealing were also measured to clarify the relationship between crystalline defects induced by neutron irradiation and macroscopic properties. Firstly, the effects of sintering aids on irradiation and annealing behavior of three kinds of SiC ceramics were described. Swelling, starting temperature to decrease in length during annealing and recovery rate up to 1300℃ were not affected by sintering aids. On the other hand, the bending strength and length change above 1300℃ were influenced by the kind of sintering aids. Secondary, the effects of external stress on defect annihilation and bubble swelling during annealing of neutron-irradiated specimens were investigated. From those experiments, it is clarified that the decrease in length by annealing below 1300℃ was not affected by the external stress. However, annealing above 1300℃ led to an increase in length in B-containing SiC, and the compressive stress retarded the expansion along the loading direction. In the third section, the helium release behavior of neutron-irradiated SiC containing B was observed and it was clarified that the helium release rate of ceramic and its powder was different from each other. Defects characterization was carried out by ESR measurement and high-resolution electron microscopy. ESR signal related to the vacancy type defects having unpaired electrons markedly increased by higher fluence irradiation over 5x10^<24>n/m^2, and the intensity of signal decreased with increasing annealing temperature above irradiation temperature with slightly higher rate than that of length decrease. It indicated that the vacancies with unpaired electron detected by ESR selectively disappear at lower temperature than those detected by a macroscopic length measurement. High-resolution electron microscopy revealed that very small interstitial type Flank loops lying on {111}, having a Burgers vector b=1/3<111>, were formed in β-SiC which was heavily-neutron-irradiated in a fast reactor. Defect nuclei, a few nanometer in diameter, in hexagonal α-SiC were induced by lower doses in a thermal reactor. They are on the (0001) basal plane and have a Burgers vector b=1/6[0001].

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