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.