Effects of neutron irradiation on polymorphs of silicon nitride and SiAlON ceramics

Silicon nitride and related ceramics are excellent engineering ceramics and of interest for nuclear applications because of their excellent mechanical and other properties. Silicon nitride and related ceramics include a wide range of compounds, which are categorized into two basic crystal structures, α- and β-Si3N4. Reports discussing neutron irradiation effects on SiAlON ceramics are very limited, and almost no data is available on effects of their polymorphs. In this research, α and β phase of Si3N4 and SiAlON were prepared by hot-pressing to evaluate neutron irradiation effects. Si3N4 and SiAlON ceramics were neutron-irradiated in the Japan Materials Testing Reactor up to a fluence of 8.5 × 1024 n/m2 (E > 0.1 MeV) at 563 K. Changes in macroscopic length and lattice parameter due to the irradiation were measured. Changes in macroscopic length of α- and β-SiAlON and α-Si3N4 were similar in magnitude (0.11–0.14%), and almost double that of β-Si3N4 (0.06%). The lattice parameter changes of SiAlONs showed a complicated trend, expansion of the a-axis and contraction of the c-axis in both phases. No obvious microcracking, void formations, or loop formations were observed in SiAlONs microstructures.