Physical property change of heavily neutron-irradiated Si 3N 4 and SiC by thermal annealing

Abstract

Changes in macroscopic length, lattice parameter and thermal diffusivity of neutron-irradiated Si 3N 4 and SiC ceramics up to a fluence of 4.2×10 26 n/ m 2 were measured. Macroscopic length increase of Si 3N 4 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 Si 3N 4. The amount of lattice parameter change of Si 3N 4 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 Si 3N 4 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 Si 3N 4 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.