Temperature and hydroxyl concentration dependences of diffusion coefficients of hydroxyl groups in vitreous silica at temperatures of 850–1200 °C

Abstract

The hydroxyl concentration and temperature dependences of the diffusion coefficients of hydroxyl groups in vitreous silica were investigated by analyzing the change in the hydroxyl concentration distribution caused by heat treatment around the binding interface between silica glass plates with different hydroxyl concentrations. We confirmed experimentally that the diffusion coefficient is proportional to the hydroxyl concentration, which had been predicted theoretically, and obtained the empirical formula D(cOH) = [(4.9 ± 1.0) × 10−14 m2/s·wt ppm] × exp[−(8.1 ± 0.3) × 103 K/T] cOH with the hydroxyl concentration cOH and the absolute temperature T, which is valid at least in the temperature range of 850–1200 °C. Using the values calculated using this formula, we can reproduce the hydroxyl concentration profile after diffusion from a silica glass surface induced by heating in a water vapor atmosphere in the literature, which supports the validity of the empirical formula obtained in this study. The relationship between the effective diffusion coefficient in the literature and our result is discussed.

Silica glass devitrification enhancement by a drop of sodium hydroxide saturated solution

“…Therefore, the water molecules should be introduced from the surface along with the diffusion of sodium ions by the SSG+ process. We explained in earlier reports 34,35) that hydroxyl groups were diffused by the reaction.…”

“…31) Concentration of the surface hydroxyl groups in an equilibrium state is proportional to the square-root of the vapor pressure. 33,34) This fact implies that no silanol group is produced under vacuum at sufficiently high temperatures. Based on these facts and considering the melting point of sodium chloride, 801 °C, we proposed a model to describe these phenomena as follows.…”

“…Through this process, the amount of surface silanol groups determined by the vapor pressure. 34) Then, sodium hydroxide is produced by reaction between molten sodium chloride and surface silanol groups as…”

Promoting vitreous silica devitrification by placement on a NaCl grain at 800 °C–1150 °C

“…Anyway, the surface hydroxyl groups play an important role in devitrification because the hydroxyl concentration on the silica glass surface is determined by the water vapor pressure in the atmosphere during heat treatment and the amount of the crystallization is independent to the bulk hydroxyl concentration. [30][31][32][33] This relation is explainable through the hydroxyl group diffusion by the following process…”

“…This fact suggests that devitrification must proceed by reaction between molten NaCl and surface hydroxyl groups in the form of º --º Si OH HO Si . 31,32) A probable process for these phenomena is…”