Long-term helium bubble evolution in sequential He+ and H+ irradiated Li4SiO4

Abstract

In present study, Li4SiO4 ceramics were sequentially irradiated at room temperature using 540 keV He+ and 250 keV H+ with ion fluences of 1 × 1016 ions/cm2 and 1 × 1017 ions/cm2, corresponding to 2.14 displacements per atom (dpa) and 21.4 dpa, respectively. The change of featured structures and the long-term evolution of helium bubbles in irradiated Li4SiO4 were investigated by grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and transmission electron microscope (TEM). For the as-irradiated Li4SiO4, the results revealed that the lattice parameter and the amorphous fraction increased and the crystallinity decreased with the increasing irradiation dose, and the anionic disordering was the mainly induced damage. When the irradiation dose reached to 21.4 dpa, high-density bubbles with a mean diameter of 1.6 nm were observed in Li4SiO4. The following 480-day aging at room temperature rendered bubble growth due to surface diffusion, which was demonstrated by the fact of an increase in bubble size (mean diameter of 18.4 nm) and a decrease in bubble density. The formation and growth of bubbles could benefit from the presence of amorphous structures and hydrogen atoms. Furthermore, the dynamic evolution of bubbles during annealing at 723 K was monitored by in-situ TEM, and the results indicated that the migration and coalescence mechanism dominated the growth of bubbles in Li4SiO4 under relatively low temperatures.