Gas Solubility and Diffusion in Oxide Glasses – Implications for Nuclear Wasteforms

Abstract

Gas solubility and diffusion in oxide glasses define the overall mechanisms of gas transport in these materials. There are numerous practical aspects of this transport with substantial commercial implications, e.g., gas separation, the outgassing and leak testing of vacuum systems, the thermal oxidation of silicon, and the focus of this conference: nuclear wasteforms. The fundamentals of gas transport are defined in regard to gas solubility and diffusion in rigid glasses below the glass transition temperature, including the case of nuclear wasteforms.Gas transport also provides an excellent tool for better understanding the atomic-scale structure of glasses, and, in turn, that understanding helps refine our understanding of the mechanisms of gas solubility and diffusion. This understanding has also had a significant influence on models of geological importance, such as volcanic eruptions and formation of the earth's atmosphere. Specifically, the statistical thermodynamics of gas solubility is analyzed in regard to the interstitial structure of the glass. The gas atoms or molecules serve as structural probes. The structural basis of gas solubility is best understood for vitreous silica, the most-studied noncrystalline solid that can be described reasonably well by the random network model. A detailed analysis of noble gas solubility has indicated that the distribution of interstitial size is log-normal in nature. This experimental result using gas solubility has been confirmed by the analysis of computer-generated models of vitreous silica. Also, the noble gas results have consequences for the transport of molecular species such as oxygen and water, e.g., during the thermal oxidation of silicon. The substantial amount of data for noble gas transport in vitreous silica also provides an especially useful comparison of the relative nature of solubility, diffusivity, and permeability of various sized noble gas atoms.There are also useful, empirical trends for the variation of gas solubility with modifier ion content in silicate glasses. The structural analysis of these data can provide general descriptions of interstitial site geometries. Structural modeling exercises have provided a more specific description of this interstitial geometry that is the basis for a more precise understanding of the atomic-scale mechanisms for solubility, diffusivity, and permeability of gases.Finally, the principles covered above will be discussed in terms of issues central to nuclear wasteforms, viz., the accommodation of helium produced by alpha decay and oxygen generated by radiolysis.