Contribution to thermochemical studies of 0.8[xB2O3–(1 − x)SiO2]–0.2Na2O glasses

Calorimetric measurements in 0.8[xB2O3–(1 − x)SiO2]–0.2Na2O glasses and melts are performed in HF calorimetry at 298 K and lead borate calorimetry at 973 K, respectively. Mixing enthalpy is affected by both temperature and composition. At 298 K, the mixing enthalpies are slightly negative and components are miscible at that temperature in the whole composition range. At 973 K, the sign of the mixing enthalpies reflect the tendency to phase separation at silica-rich compositions, which is avoided for kinetic reasons as shown by SEM results. Therefore, one is tempted to consider the quenched 0.8[xB2O3–(1 − x)SiO2–0.20Na2O] glasses and melts as single-phase materials.