Effect of water incorporation on the diffusion of sodium in Type I silica glass

The diffusion of sodium ions in Type I silica glass was studied experimenta lly by using Na-22 as a radioactive tracer at temperatures between 400 degr eesC and 700 degreesC. It was found that the sodium tracer diffusion coeffi cient in as-received glass was well described by an Arrhenius equation. All residual radioactivity profiles observed after pre-annealing glass samples in common and wet air at temperatures between 900 degreesC and 1100 degree sC and performing diffusion anneals at 650 degreesC cannot be described by a single diffusion coefficient. A strong residual radioactivity decrease oc curred near the surface, which was followed by a much smaller decrease in t he bulk below. This type of profile was attributed to the generation of ane ar-surface region with a reduced sodium mobility during the pre-annealing. A mathematical model for the analysis of the observed profiles was develope d and used to determine sodium tracer diffusion coefficients in the near-su rface region and in the bulk. It was found that the sodium tracer diffusion coefficient in the near-surface region was by about two orders of magnitud e smaller than that in the bulk. This immobilization of Na is believed to b e due to a water-induced structural relaxation in the near-surface region d uring pre-annealing. (C) 2001 Elsevier Science B.V. All rights reserved.