STUDY OF EFFECT OF CATION CONCENTRATION AND SIZE ON HELIUM PERMEABILITY OF ALKALI‐SILICA GLASSES*

Abstract

AbstractA study was made of the effect of systematic variations in the alkali‐cation concentration and size on the permeability of alkali‐silica glasses to helium at 300°C. Permeability determinations were made on fused silica, Pyrex‐brand chemical glass, Na2O‐SiO2 glasses containing 20, 25, and 30 mol % Na2O, a K2O‐SiO2 glass containing 30 mol % K2O, and a Li2O‐Na2O‐SiO2 glass containing 5 mol % Li2O and 25 mol % Na2O.Additions of Na2O to SiO2 glass produced a rapid decrease in the permeability of the glass. The permeability was found to decrease exponentially over the range 0 to 30 mol % Na2O. The substitution, atom for atom, of potassium for sodium increased the permeability of the glass, whereas lithium in combination with sodium decreased the permeability as compared with the soda glass. This was attributed primarily to the difference in the polarizing power of these cations that affected the ionic radius of the surrounding oxygens and resulted in a change in the packing density of the ions in the glass.A comparative packing index was calculated for the glasses based on the percentage of the absolute volume of glass contributed by the constituent elements. For the Na2O‐SiO2 glasses, increase in packing index (indicating a more closely packed structure) was accompanied by a decrease in permeability. It is suggested that (1) the comparative packing index is a potentially valuable index of permeability only for glasses containing the same atoms, (2) new atoms introduce a new polarizing factor in the glass, and (3) the packing index ceases to be comparative for the two glasses.