Relaxation processes of densified silica glass

Densified SiO2 glasses, obtained from different pressure and temperature routes have been annealed over a wide range of temperature far below the glass transition temperature (500$^\circ$C-900$^\circ$C). Hot and cold compressions were useful to separate the effects of pressure and the compression temperature. In-situ micro-Raman spectroscopy was used to follow the structural evolution during the thermal relaxation. A similar glass structure between the non-densified silica and the recovered densified silica after the temperature annealing demonstrates a perfect recovery of the non-densified silica glass structure. While the density decreases monotonically, the structural relaxation takes place through a more complex mechanism, which shows that density is not a sufficient parameter to fully characterize the structure of densified silica glass. The relaxation takes place through a transitory state, consisting in an increase of the network inhomogeneity, shown by an increase in intensity of the D2 band which is associated with 3 membered rings. The activation energy of these processes is 255$\pm$45 kJ/mol for the hot compressed samples. The kinetic is overall faster for the cold compressed samples. In that last case the relaxation is partially activated by internal stresses release.