Effect of Alkali Oxides on the Diffusion of Helium in a Simple Borosilicate Glass

Helium permeation rates, diffusivities, and solubilities in a simple borosilicate glass containing alkali oxides were measured. The major effect of increasing Na 2 O content from 2 to 13 mol% is a systematic decrease in permeation rate and diffusivity, indicating that the Na + ion is essentially filling space in the glass network. At 7 mol% alkali oxide, as the alkali‐ion size is increased, the changes in diffusivity and activation energy indicate that from Li + to Na + to K + , the larger ion more effectively plugs the network interstices. The size distribution of interstices in the base‐glass network is such that ions larger than K + are too large to simply fill the network and consequently tend to spread the network as well.