Glass transition and fragility of Na 2O–TeO 2 glasses

Abstract

The heat capacity ( C p) change in the glass transition region for the xNa 2O · (100−x) TeO 2, mol%, glass forming melts with x=7.5, 11.1, 15.0, 20.0 and 25.0 was measured as a function of heating rate (2, 4, 6, 10 and 15 °C/min) using differential scanning calorimetry. The glass transition properties that have been measured and reported in this paper include the glass transition temperature ( T g), glass transition width (Δ T g), heat capacities in the glassy and liquid state ( C pg and C pl) and the activation enthalpy for glass transition ( ΔH ∗ ). T g for these sodium tellurite melts decreased and ΔH ∗ increased with increasing Na 2O. Values of the ratio C pl/ C pg ranged between 1.28 and 2.47, and the fragility parameter m (= ΔH ∗/RT g ) ranged between 100 and 130, suggesting that these glass forming melts may be classified as intermediate between typical strong and fragile liquids. The viscosity, η, calculated at a few selected temperatures near the glass transition region decreased with increasing Na 2O at any given temperature, which is also expected.