Helium migration in glass-forming oxides

Permeation, diffusion, and solubility of helium in vitreous B2O3, SiO2, and GeO2 have been measured. The permeability and diffusivity decrease in the order B2O3>SiO2>GeO2, whereas the solubility decreases in the order SiO2>B2O3>GeO2. Differences in the activation energy for diffusion between vitreous SiO2 and GeO2 are due to the greater strain energy necessary to distort the GeO2 structure sufficiently to allow passage of a helium atom from one interstice to another. Changes in the frequency factor for diffusion appear to be primarily related to the activation entropy. The solubility appears to be a function of the distribution of interstice sizes, but cannot be predicted by present techniques.