Effect of helium and silver ions co-implanted into polycrystalline silicon carbide at 350 °C on structural evolution and migration behaviour of silver

Abstract

Safety of nuclear reactors strongly depends on the containment of fission products. In modern nuclear reactors, this is accomplished by coating the fuel with chemical vapour-deposited layers of carbon (C) and silicon carbide (SiC), in which SiC is the main barrier of fission products. During operation, at elevated temperatures, the SiC layer is subjected to various radiation in the presence of helium (He). Silver (Ag) is one of the fission products that is released by the coated fuel during operation, while He is known to form bubbles in SiC. These bubbles compromise the integrity of SiC as the main barrier of fission products. Hence, the effect of He bubbles in the migration of radiological important fission products needs to be understood. In this study, the effect of helium (He) and silver (Ag) ions co-implanted into polycrystalline silicon carbide at 350 °C on structural evolution and migration behaviour of silver was investigated. Ag ions of 360 keV were implanted into polycrystalline SiC to a fluence of 2×1016 cm-2 at 350 °C. Some of the implanted samples were then implanted with He ions of 17 keV to a fluence of 1×1017 cm-2 also at 350 °C. Both Ag and co-implanted samples were then isochronally annealed at temperatures varying from 1000 ℃ to 1300 ℃ in steps of 100 ℃ for 5 hours. The structural changes were characterized using Raman spectroscopy while morphological and topographical evolutions were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The elemental depth profiles and concentration of implants in as-implanted and annealed samples were monitored by heavy-ion elastic detection analysis (ERDA). Both individual and co-implantation retained some defects in SiC without amorphization. These retained defects were slightly more in the co-implanted samples. Annealing of samples resulted in the progressive healing of defects in SiC. Co-implantation of He resulted in the formation of blisters and some holes on the SiC surface indicating the formation of He bubbles accompanied by some exfoliation of bubbles. The exfoliation increased with annealing temperature. These holes increased in number with annealing temperature resulting in the decrease in number of blisters. The formation of He bubbles and holes in the co-implanted samples caused the migration of Ag towards the surface accompanied by the loss of both Ag He. Annealing at 1100 ℃ caused loss of He accompanied by neither further migration nor loss of implanted Ag. No migration of Ag was observed in the Ag implanted samples annealed at 1100 ℃. Therefore, the formation of He bubbles enhanced the migration of Ag while cavities trap Ag implanted into SiC.