Criterion for estimating the number of phases in phase-separated glass of the Na2O-K2O-B2O3-SiO2 system by dilatometric analysis

The data of dilatometry and electron microscopy of four series of xNa2O-(8 − x)K2O-32B2O3-60SiO2, xNa2O-(8 − x)K2O-22B2O3-70SiO2, xNa2O-(6 − x)K2O-34B2O3-60SiO2, and xNa2O-xK2O-(40 − 2x)B2O3-60SiO2 phase-separated glass heat-treated at 550°C for 144 h (for glass containing 70 mol % SiO2) and 24 h (for glass containing 60 mol % SiO2) for separation on phases are summarized. The comparison of dilatometric data and electron microscopy allow one to conclude that glass with a difference between the onset deformation temperature and a glass transition temperature of more than 100°C is phase-separated; and glass with a difference of less than 65°C is single-phase. Curves for the glass transition temperature as a function of the K2O content reveal a mixed alkali effect, namely, minimums for glass containing 60% SiO2, and maximums for glass containing 70% SiO2.