Diffusivity of helium in fused silica

Abstract

A statistical mechanical model of diffusion is applied to helium diffusion in fused silica. The number of gas atoms moving solubility sites is related to the number of atoms in those sites. The number of atoms in the excited state, between two adjacent solubility sites, relative to the number occupying solubility sites can be described by their partition functions. The process of diffusion is treated as a random walk process which results in a diffusion equation in terms of the temperature, fundamental constants, and material parameters. The model is compared to data reported in the literature on He diffusion in fused silica in order to determine the unknown parameters. The model also shows that absolute rate theory is not able to account for the nonlinearity observed in logD vs 1/T for this system.