Defects annihilation behavior of neutron-irradiated SiC ceramics densified by liquid-phase-assisted method after post-irradiation annealing

Numerous studies on the recovery behavior of neutron-irradiated high-purity SiC have shown that most of the defects present in it are annihilated by post-irradiation annealing, if the neutron fluence is less than 1 × 1026 n/m2 (>0.1 MeV) and the irradiation is performed at temperatures lower than 973 K. However, the recovery behavior of SiC fabricated by the nanoinfiltrated and transient eutectic phase (NITE) process is not well understood. In this study, the effects of secondary phases on the irradiation-related swelling and recovery behavior of monolithic NITE-SiC after post-irradiation annealing were studied. The NITE-SiC specimens were irradiated in the BR2 reactor at fluences of up to 2.0-2.5 × 1024 n/m2 (E > 0.1 MeV) at 333-363 K. This resulted in the specimens swelling up ∼1.3%, which is 0.1% higher than the increase seen in concurrently irradiated high-purity SiC. The recovery behaviors of the specimens after post-irradiation thermal annealing were examined using a precision dilatometer; the specimens were heated at temperatures of up to 1673 K using a step-heating method. The recovery curves were analyzed using a first-order model, and the rate constants for each annealing step were obtained to determine the activation energy for volume recovery. The NITE-A specimen (containing 12 wt% sintering additives) recovered completely after annealing at ∼1573 K; however, it shrank because of the volatilization of the oxide phases at 1673 K. The NITE-B specimen (containing 18 wt% sintering additives) did not recover fully, since the secondary phase (YAG) was crystallized during the annealing process. The recovery mechanism of NITE-A SiC was based on the recombination of the C and Si Frenkel pairs, which were very closely sited or only slightly separated at temperatures lower than 1223 K, as well as the recombination of the slightly separated C Frenkel pairs and the migration of C and Si interstitials at temperatures of 1223-1573 K. That is to say, the recovery mechanism was basically the same as that of pure SiC.