Argon solubility and diffusion in silica glass: Implications for the solution behavior of molecular gases

Abstract

Argon solubility and diffusivity in SiO_2 glass have been determined from experiments at pressures of 200 to 3725 bars and temperatures of 400 to 900°C. Samples contained in unsealed capsules were exposed to Ar gas used as the pressure-transmitting medium in cold-seal reaction vessels. Electron microprobe analysis of diffusion-controlled concentration gradients of Ar in the quenched samples allows determination of Ar diffusivity and solubility over the range of investigated conditions. The temperature dependence of Ar diffusivity (D) between 400 and 900°C at ∼ 1200 bars is well described by the Arrhenius relationship D = D_o · exp(−E/RT) with Log_(10)D_o = −5.06 ± 0.31 (cm^2/sec) and an activation energy for diffusion E, of 24.1 ± 1.3 kcal/mol. Argon diffusivity appears to be concentration independent up to at least ∼ 1.3 wt% Ar, and any pressure effect on Ar diffusivity is within the error of the measurements. Argon solubility at 700°C increases linearly with increasing pressure up to ∼ 1500 bars (0.35 wt%) and can be described by Henry's law (f_(Ar) = K_H· X_(Ar)) with K_H = 8.06 (±0.2)·10^5 bar,f_(Ar) = Ar fugacity in bars, and X_(Ar) = mole fraction dissolved Ar in Si_(0.5)O. Above 1500 bars the solubility is lower than would be predicted from a linear (Henrian) extrapolation of the low P data. At constant pressure Ar solubility decreases with increasing temperature. Thermodynamic analysis of the Ar solubility data using the assumption that Ar activity is equivalent to X_(Ar) yields an estimated partial molar volume for Ar in silica glass of 16.4 ± 1.4 cm^3/mol and an enthalpy of solution of −4.76 ± 0.26 kcal/mol at 1 bar. The data may be equally well described by a solution model in which the gas atoms are considered to occupy a certain population of sites or holes that are available to them in the glass structure. Fitting of the solubility data at 700°C to such a model with the assumption of ΔV = 0 indicates 5.74 · 10^(20) sites/cm^3 of SiO_2 glass, corresponding to ∼5% of the total number of holes in a randomized cristobalite-like structure. However, without an independent estimate of the number of available sites in the glass this solution is non-unique, and the data may be fit by a family of models with both positive and negative values for ΔV. Higher pressure solubility studies offer the best means to further investigate the applicability of such solution models for molecular or atomic gases dissolved in melts, glasses, and crystals.

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