Neutron-irradiation-induced crystalline defects in β-silicon nitride and their thermal stability

A hot-pressed Si3N4 was neutron-irradiated in the JOYO experimental fast reactor. The ranges of irradiation fluence and temperature were (0.4–3.7) × 1026 n/m2 (E > 0.1 MeV) and (377–727) °C, respectively. Based on high-resolution electron microscope observation, several kinds of tiny interstitial dislocation loops on the { 1 0 1 ¯ 0 } or { 1 1 2 ¯ 0 } planes were identified. It was clarified that area density and average size of each planar defect was greater after irradiation to higher doses at higher temperatures. The density of planar defects on { 1 0 1 ¯ 0 } was higher than that of on { 1 1 2 ¯ 0 } , regardless of irradiation conditions. After annealing, size distribution of the defects on the { 1 0 1 ¯ 0 } planes was not changed up to 1350 °C in the case of the specimen irradiated at the lowest temperature (0.5 × 1026 n/m2 at 377 °C). On the other hand, the number of dislocations with diameter smaller than 4 nm decreased at over 900 °C and the loop size distribution shifted larger size after annealing at temperatures higher than 1350 °C in the case of higher irradiation temperature specimen (0.4 × 1026 n/m2 at 542 °C).