Time dependent diffusion coefficient of water into silica glass at low temperatures

The diffusion of water into silica glass at low temperatures (<1000°C) exhibits a peculiar time dependence: the diffusion coefficient decreases at first and then increases with time at a constant temperature under a constant water vapor pressure. Water solubility exhibits the opposite trend: it initially increases with time and subsequently decreases with time. The initial decrease in diffusion coefficient and increase in solubility with time is attributed to the decrease of the self-stress around the diffusing water molecules by stress relaxation of the surrounding glass. The subsequent increase in diffusion coefficient and decrease in solubility with time is attributed to the structural relaxation of the glass and the corresponding change of the glass–water reaction equilibrium constant.