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 x 10(26) n/m(2) were measured. Macroscopic length increase of Si3N4 was almost one h alf of that of SiC. Thermal diffusivity of both ceramics was reduced severe ly by the irradiation at 390-540 degreesC. Slight increase in the a-axis an d slight decrease in the c-axis lattice parameter were detected for Si3N4 T he amount of lattice parameter change of Si3N4 was very small compared with the macroscopic length change. Changes in these properties due to post-irr adiation thermal annealing up to 1500 degreesC were measured. Large part of thermal diffusivity of Si3N4 was recovered by annealing, with small step a t similar to 1100 degreesC, but macroscopic length did not significantly ch ange 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 recover y of points defects/loops, or solid solution formation due to glassy grain boundary phase may influence the recovery behavior of Si3N4 ceramics. Chang es in macroscopic length, lattice parameter or thermal diffusivity of SiC b y annealing coincided with the results of previous works. The critical irra diation conditions for loop formation/XRD line broadening for SiC is discus sed based on the present and previous results. (C) 2001 Elsevier Science B. V. All rights reserved.