Concentration-dependent diffusion of hydrogen in vitreous silica

We report diffusion experiments where hydrogen permeates through a 1mm wall of vitreous silica at 550°C with applied gas pressures of 70, 460, 840, and 1200mbars. For each pressure, and at steady state, the flux and the amount of hydrogen in the material are measured. Within the experimental accuracy we find that the flux is proportional to the pressure and that the hydrogen amount increases with a falling gradient with respect to pressure. The result is a relationship between flux and mean concentration. A careful evaluation of the flux versus concentration relationship by means of the steady-state diffusion equation shows that the hydrogen diffusivity in the silica wall is concentration dependent and increases linearly with local concentration.