Diffusion of water in silica glass
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The diffusion of water into silica glass is modeled to result from the diffusion of molecular water into the glass and its reaction with the silicon-oxygen network to form SiOH groups. Equations for this diffusion-reaction mechanism are presented and compared with experimental diffusion profiles. At temperatures above about 500 °C the reaction goes to equilibrium, but at lower temperatures it does not, leading to a time dependence of the concentration of surface-reacted OH groups and of their apparent diffusion coefficient. At higher temperatures, the OH groups are nearly immobile, but diffuse far enough to sample neighboring OH groups, leading to a bimolecular reverse reaction. At lower temperatures only OH pairs react, giving a first-order reaction. When water tagged with O18 diffuses into silica, the O 18 exchanges with O 16 in the silicon-oxygen network of the glass. This process is also controlled by the rate of diffusion of molecular water into the glass, and the rate of O 1 8 -O 1 6 exchange. This diffusion-reaction mechanism gives a unified description of the diffusion of water in silica glass from 160 °C to 1200 °C at least.
I. INTRODUCTION Water in vitreous silica strongly influences many of its properties, such as fatigue resistance, viscosity, crystallization rate, and optical absorption. The diffusion of water at temperatures from about 600 °C to 1200 °C was studied by Moulson and Roberts1 and Burn and Roberts.2 Their results can be understood in terms of a mechanism of diffusion of molecular water into the glass and its reaction with the silicon-oxygen network of the glass to form SiOH groups.3 Si-O-Si + H 2 O = SiOH + HOSi.
(1)
In this mechanism the molecular water is the carrier of water into and out of the glass; the OH groups formed in the glass are relatively immobile. In the comparison of this mechanism with experimental data3 it was assumed that Eq. (1) had reached equilibrium; that is, the ratio of hydroxyl groups to molecular water at all points in the glass was given by the expression for the equilibrium constant K2 of reaction (1): Kl
~ [H 2 O] '
(2)
in which [OH] is the concentration of SiOH groups in the glass and [H2O] is the concentration of dissolved molecular water in the glass. Since the concentration of lattice groups S i - O - S i remains nearly constant, it is omitted from Eq. (2). The subscript 2 is used because the reverse reaction is bimolecular. This mechanism has been confirmed for the diffusion of water into vitreous silica in other studies.4"6 and in other silicate glasses and minerals.7 J. Mater. Res., Vol. 10, No. 9, Sep 1995 http://journals.cambridge.org
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Nevertheless, at temperatures below about 600 °C some different behavior of water in vitreous silica has been reported.6-8"10 The main different features were these: (i) the surface concentrations of OH groups increased with time, rather than being constant as at higher temperatures; (ii) the profiles of OH in the glass at different times did not superpose when plotted as a function of x/s
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