Densification of alkali silicate glasses at high pressure

The densification behavior of a number of alkali silicate glasses has been investigated using a Bridgman anvil high pressure device. The pressure range of the investigation was 10 to 60 kilobars; the temperature range was 100 to 400°C. In all cases, the densification was found to increase with increasing temperature and pressure. In the two systems where compositional variations were explored, Na₂OSiO₂ and K₂OSiO₂, a pronounced maximum in densification in the vicinity of 10 mole % alkali oxide was observed (for a given temperature and pressure). These maxima are taken to reflect a competition between two processes, one of which - the variation of molecular mobility with composition - should lead to increasing densification with increasing alkali concentration. Several possibilities are discussed for the decrease in densification with alkali oxide concentrations greater than about 10% (to values smaller than that of SiO₂ in some cases). The present results are related to those obtained in previous investigations of the densification of oxide and polymeric glasses.