Diffusion experiments were reported in which H permeated through a 1mm wall of vitreous silica at 550C, under applied gas pressures of 70, 460, 840 or 1200mbar. For each pressure, at the steady state, the flux and the amount of H in the material were measured. To within experimental accuracy, it was found that the flux was proportional to the pressure and that the amount of H increased with a falling gradient with respect to pressure. The result was a relationship between flux and mean concentration. A careful evaluation of the flux versus concentration relationship, using a steady-state diffusion equation, showed that the H diffusivity in the silica wall was concentration-dependent and increased linearly with local concentration.

Concentration-Dependent Diffusion of Hydrogen in Vitreous Silica. J.Rundgren, Q.Dong, G.Hultquist: Journal of Applied Physics, 2006, 100[10], 104902 (5pp)