Physical property change of heavily neutron-irradiated Si3N4 and SiC by thermal annealing

Changes in macroscopic length, lattice parameter and thermal diffusivity of neutron-irradiated Si3N4 and SiC ceramics up to a fluence of 4.2×1026 n/m2 were measured. Macroscopic length increase of Si3N4 was almost one half of that of SiC. Thermal diffusivity of both ceramics was reduced severely by the irradiation at 390–540°C. Slight increase in the a-axis and slight decrease in the c-axis lattice parameter were detected for Si3N4. The amount of lattice parameter change of Si3N4 was very small compared with the macroscopic length change. Changes in these properties due to post-irradiation thermal annealing up to 1500°C were measured. Large part of thermal diffusivity of Si3N4 was recovered by annealing, with small step at ∼1100°C, but macroscopic length did not significantly change by annealing. Change in lattice parameter showed a complicated trend. It is supposed that formation of interstitial loops on the planes parallel to the c-axis, formation of voids during annealing or difficulty of recovery of points defects/loops, or solid solution formation due to glassy grain boundary phase may influence the recovery behavior of Si3N4 ceramics. Changes in macroscopic length, lattice parameter or thermal diffusivity of SiC by annealing coincided with the results of previous works. The critical irradiation conditions for loop formation/XRD line broadening for SiC is discussed based on the present and previous results.