Structural evolution of polycrystalline SiC and migration behavior of silver (Ag) following sequential implantation of silver and helium and subsequent annealing at 1200 °C and 1300 °C

This study reports on the effects of sequential silver (Ag) and helium (He) ion implantation, followed by heat treatment, on the structural evolution of silicon carbide (SiC) and the migration behavior of Ag. Polycrystalline SiC samples were implanted with 360 keV Ag ions at 350 °C to a fluence of 2×10¹⁶ cm⁻². A subset of these Ag-implanted samples was subsequently implanted with He ions at the same temperature to a fluence of 1×10¹⁷ cm⁻². Both Ag-only and Ag+He sequentially implanted samples underwent isochronal annealing at 1200 °C and 1300 °C for 5 hours. Ag implantation introduced and retained structural defects in SiC. In contrast, sequential He implantation resulted in the formation of helium bubbles in the implanted region, along with surface blistering and the development of holes. These surface features acted as diffusion pathways for Ag during He implantation, leading to approximately 8% Ag migration in the sequentially implanted samples. Graphite crystals were detected in sequentially implanted samples both before and after annealing; at high temperatures, these structures inhibited recrystallization, while Ag-only samples exhibited greater recovery. Limited recrystallization at high temperatures was also associated with the growth of He-induced cavities, which promoted the formation of larger Ag precipitates. At 1300 °C, enhanced atomic mobility, helium release, and cavity shrinkage produced a denser population of smaller and more uniform cavities that act as effective Ag trapping sites. This behavior is linked to the high stress fields present in the sequentially implanted samples following annealing. At 1300 °C, Ag in both sample types shifted further toward the SiC bulk, again due to elevated stress levels. While no Ag loss was observed in Ag-only samples, sequentially implanted samples exhibited significant Ag loss, reaching ~13% at 1200 °C and ~23% at 1300 °C. These findings highlight the critical influence of helium on defect evolution, cavity morphology, Ag redistribution, and the thermal stability of SiC under extreme high-temperature conditions, with direct relevance to fission-product retention and TRISO fuel integrity.