Helium migration in lithium aluminosilicate glasses

Helium permeation, diffusion, and solubility have been measured for a number of lithium silicate and aluminosilicate glasses. Helium mobility in the lithium silicate glasses is controlled by the two-phase morphology of these glasses. Addition of as little as 2 mol% alumina strongly suppresses the tendency toward phase separation, i.e., yields homogeneous glasses. The compositional dependence of helium mobility in these glasses indicates that a major change in the glass structure occurs when the alumina-to-lithia ratio exceeds unity. Comparison of these results to those for analogous sodium aluminosilicates indicates that lithium has a greater effect than sodium on the free volume of the glass.