Macroscopic properties and microstructure changes of heavily neutron-irradiated β-Si3N4 by annealing

Swelling and thermal diffusivity measurements have been carried out on sintered β-Si3N4 specimens that were heavily neutron-irradiated in an experimental fast reactor under various conditions. Changes in these parameters after post-irradiation isochronal annealing up to 1500 °C were measured. TEM observation showed that interstitial dislocation loops were formed in many specimens, and were stable after annealing at 1500 °C. Bubbles were formed in the specimens annealed at 1500 °C. Neutron-irradiated β-Si3N4 showed small swelling that was recovered very slightly due to annealing up to 1300 °C. The specimens showed very small expansion at 1200–1300 °C, which would be caused by He or H2 bubble formation. Recovery of the thermal diffusivity is higher in the specimen that did not contain many dislocation loops. The annealing behavior of the thermal diffusivity and macroscopic length in β-Si3N4 indicated that interstitial atoms are mobile at least at 380 °C, and vacancies begin to move around 1100 °C.