The effect of RO oxides on microstructure and chemical durability of borosilicate glasses opacified by P2O5

The microstructure, devitrification and chemical durability of borosilicate glass specimens opacified by P2O5, with the general composition SiO2 70, B2O3 12, Al2O3 2, P2O5 2, Na2O (13 − X), RO X (wt.%) (R = Ca, Mg, Ba, Zn) were investigated after being subjected to various heat treatment conditions, using DTA, XRD and SEM. It was shown that while heat treatments at 1073 K and >1123 K were generally detrimental for the hydrolytic resistance of glasses, due to the enhanced phase separation or formation of excessive amounts of cristobalite, heating at 1123 K for 1 h usually improved the resistance due to the partial crystallization or microstructural changes of specimens. It was also found that a progressive decrease in hydrolytic and alkaline resistance occurred during prolonged heat treatment at 1123 K due to the formation of exessive amounts of cristobalite. It was also revealed that ZnO and MgO had the worst effect on chemical durabilities of specimens containing 7 and 5.5 wt.% Na2O, respectively.