Recovery behavior of neutron irradiated α- and β-SiAlON ceramics by thermal annealing up to 1473 K

Hot-pressed α- and β-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. Irradiated SiAlON ceramics were isothermally and isochronally annealed up to 1473 K in a precision dilatometer, and macroscopic length change of α- and β-SiAlON ceramics were examined. During isochronal annealing, the recovery of both SiAlONs started at a temperature about 50–100 K higher than the irradiation temperature. Although the macroscopic length change for isochronal annealing decreased gradually up to 1473 K, the recovery was not completed. The macroscopic length change of β-SiAlON decreased more continuously than that of α-SiAlON. During isothermal annealing, the length gradually recovered as increasing annealing time at each annealing temperature. The activation energy of macroscopic volume recovery at lower temperature range was calculated as the first-order reaction. For α- and β-SiAlON from 723 to 1073 K, the values were 0.09 and 0.06 eV, respectively. On the other hand, for higher temperature range, the activation energy was calculated as the second-order reaction, and to be estimated as 0.49 eV for α-SiAlON from 1273 to 1373 K, and 0.33 eV for β-SiAlON from 1123 to 1473 K, respectively. These values suggested that the recovery process was governed by the recombination of different type of Frenkel pairs and depended on its polymorphs.