EFFECTS OF NEUTRON-IRRADIATION ON THERMAL-CONDUCTIVITY OF SIC-BASED COMPOSITES AND MONOLITHIC CERAMICS

A variety of SiC-based composites and monolithic ceramics were charact erized by measuring their thermal diffusivity in the unirradiated, the rmal annealed, and irradiated conditions over the temperature range 40 0 to 1000 degrees C. The irradiation was conducted in the EBR-II to do ses of 33 and 43 dpa-SiC (185 EFPD) at a nominal temperature of 1000 d egrees C. The annealed specimens were held at 1010 degrees C for 165 d ays to approximately duplicate the thermal exposure of the irradiated specimens. Thermal diffusivity was measured using the laser flash meth od, and was converted to thermal conductivity using density data and c alculated specific heat values. Exposure to the 165 day anneal did not appreciably degrade the conductivity of the monolithic or particulate -reinforced composites, but the conductivity of the fiber-reinforced c omposites was slightly degraded. The crystalline SiC-based materials t ested in this study exhibited thermal conductivity degradation after i rradiation, presumably caused by the presence of irradiation-induced d efects. Irradiation-induced conductivity degradation was greater at lo wer temperatures, and was typically more pronounced for materials with higher unirradiated conductivity. Annealing the irradiated specimens for one hour at 150 degrees C above the irradiation temperature produc ed an increase in thermal conductivity, which is likely the result of interstitial-vacancy pair recombination. Multiple post-irradiation ann eals on chemical vapor deposited (CVD) beta-SiC indicated that a porti on of the irradiation-induced damage was permanent. A possible explana tion for this phenomenon was the formation of stable dislocation loops at the high irradiation temperature and/or high dose that prevented s ubsequent interstitial/vacancy recombination.