Irradiation Defects in Neutron-irradiated 6H-SiC: Thermodynamic and High-temperature Recovery Kinetics

Silicon carbide (SiC) is a promising material for nuclear reactor structures due to its excellent radiation resistance and high-temperature performance. The behavior of irradiation damage and the mechanisms of high-temperature recovery in SiC directly affect its service performance and longevity in nuclear environments. This study investigated effects of neutron irradiation on properties of 6H-SiC, with a particular focus on high-temperature recovery mechanisms of irradiation-induced defects. Specifically, defect evolution and thermodynamic responses in nitrogen-doped (ND≈3.0×1019 cm-3) 6H-SiC subjected to neutron irradiation at about 150 ℃ and a fluence of 2.58×1020 n/cm2 followed by isochronal annealing were examined. Integrated techniques and first-principles calculations were employed to comprehensively analyze its structural and property evolution. The key findings were as follows. (1) Significant lattice swelling was observed during the irradiation, with a swelling rate of 0.416% along the a-axis, 0.430% along the c-axis, and 1.310% in the unit cell volume, while all maintaining integrity of the single-crystalline structure. (2) A 14.7% increase in specific heat capacity was recorded, with 375.4 J/g of stored irradiation energy being released during heating from 100 ℃ to 500 ℃. (3) A four-stage defect recovery kinetic model was proposed based on the recovery of lattice parameters and the evolution of Raman spectra: Stage I (room temperature (RT)-600 ℃), primarily dominated by close-range recombination of carbon Frenkel pairs driven by migration energy (Ea) of 0.14 eV; Stage II (600-850 ℃), recombination of silicon Frenkel pairs and migration of carbon interstitials (Ea=0.26 eV); Stage III (850-1200 ℃), lattice reconstruction (Ea=0.65 eV); Stage IV (1200-1500 ℃), long-range diffusion of carbon vacancies (VC) and dissociation of NCVSi complexes (Ea=1.50 eV). (4) The presence of nitrogen-stabilized NCVSi defect configurations was confirmed by a characteristic emission peak at 826 nm (634 cm-1 Raman shift) when excited with 785 nm light. This study quantitatively reveals the defect recovery pathways and migration energies in neutron-irradiated 6H-SiC, providing a critical foundation for evaluating radiation damage, predicting performance, and optimizing annealing processes in nuclear-grade SiC materials.