Helium Migration in TiO ₂ ‐SiO ₂ Glasses

Helium permeability, diffusivity, and solubility were measured for a series of TiO 2 ‐SiO 2 glasses containing up to 10.3 mol% TiO 2 . The activation energies for helium permeation and diffusion decrease with increasing TiO 2 concentration. The enthalpy of solution is independent of composition, as are the infinite‐temperature values for the permeability, diffusivity, and solubility. It is concluded that the observed changes in activation energies result from changes in the strain energy necessary to distort the R‐O‐R’bond. The average diffusional jump distance appears to be essentially independent of composition, as would be expected from the small changes observed in the molar volume as TiO 2 replaces SiO 2 in these glasses.