Structural relaxation of densified silica glass by thermal annealing

Thermal relaxation of glass structure has been studied on silica glasses densified by hot isostatic pressing. Density of the glasses relaxed toward the value of an undensified glass by thermal annealing. Relaxation rates of density of the glasses were measured after the annealing at several temperatures. Fast and slow relaxation processes were found from the analysis by using a stretched exponential relaxation function Φ(t)=exp{-(t/τ_eff)^β}. The slow process becomes dominant after the fast process. Raman scattering spectrum also has been measured through the thermal relaxation. The width of the main band at 450cm^-1 increased by the annealing and recovered the value for the undensified glass after the fast process. The bands at 1060 and 1200 cm^-1 shifted back to the positions for the undensified glass. The high density state (Δρ/ρapprox.0.5%), however, was maintained even after the fast process. From these results, it is deduced that the fast process is due to the recovery of the O₃Si-O-SiO₃ tilt angle and Si-O-Si bond angle to the mean values for the undensified glass.