Interstitial Atom Behavior in Neutron-Irradiated Beta-Silicon Nitride

抄録

Silicon nitride, β-Si₃N₄, is an attractive material for several nuclear applications. This paper describes the macroscopic and microscopic behavior of β-Si₃N₄ neutron-irradiated in the experimental fast reactor JOYO. In our previous papers, the thermal diffusivity changes with annealing were discussed, and the specimen irradiated at lower temperature (373°C) showed almost complete recovery while recovery was imcomplete for others irradiated at higher temperatures (395 and 580°C). In this study, high resolution TEM observation was applied to these specimens to clarify the difference from the viewpoint of microscopic behavior. As a result, few interstitial dislocation loops were found in the specimen irradiated at 373°C to 0.5×10²⁶ n/m² (E_n>0.1 MeV). On the other hand, dense and large loops were observed when irradiated at 395°C to 1.4×10²⁶ n/m², and a few were observed when irradiated at 580°C to 0.4×10²⁶ n/m². The degree of recovery of thermal diffusivity corresponded to amount of interstitial dislocation loops. Furthermore, this difference suggested that there was a threshold condition to create interstitial dislocation loops during the irradiation. At least, loops in the specimen irradiated at 395°C indicated that the threshold temperature where interstitial atoms in β-Si₃N₄ began to move was lower than 395°C.