Effects of irradiation and post-irradiation annealing on the thermal conductivity/diffusivity of monolithic SiC and f-SiC/SiC composites

Laser flash thermal diffusivity measurements were made on high-purity monolithic CVD-SiC and 2D f-SiC(Hi-Nicalon™)/ICVI-SiC composite samples before and after irradiation (250–800 °C, 4–8 dpa-SiC) and after post-irradiation annealing composite samples to 1200 °C. For irradiated CVD-SiC, the defect concentrations at saturation were estimated to range from 25300 appm (250 °C) down to 940 appm (800 °C). The transverse thermal conductivity ratios after-to-before irradiation (Kir/Ko) determined at the irradiation temperatures ranged from: 0.044 (250 °C) up to 0.12 (800 °C) for irradiated CVD-SiC and 0.18 (330 °C) up to 0.29 (800 °C) for the irradiated Hi-Nicalon™ composite. Analysis of thermal diffusivity values for the Hi-Nicalon composite measured in air, argon, helium and vacuum indicated that thermal conductivity degradation occurred primarily due to point defect accumulation in the matrix component. After annealing to 1200 °C and cooling to ambient, fiber/matrix debonding occurred due to net shrinkage in the fiber and PyC interface components.