Thermal and devitrification behavior of CaO–Ga 2O 3–SiO 2 glasses

Abstract

A study of the influence of the substitution of Ga 2O 3 for CaO, at constant O/Si ratio, on thermal properties and non-isothermal devitrification of 2.5CaO.2SiO 2 is reported. Differential thermal analysis (DTA) and X-ray diffraction analysis were used. The X-ray diffraction pattern of the crystallized Ga 2O 3 base glass shows that the αCaO SiO 2, that should be stable only above 1125°C, forms in the temperature range 900–1000°C. A new ternary crystalline phase, whose reflections are not reported in the JCPDS cards, was found to form during crystallization of the glass. The glass transformation temperature, T g, and softening, T s, temperature decrease as Ga 2O 3 is substituted for CaO. This is the result of the substitution of the network modifying cation Ca 2+, of higher coordination number, by a network forming cation Ga 3+ in fourfold coordination, in a composition range of relative insensitivity to changes of covalent cross-linking density. The crystal growth activation energy, E c, decreases with substitution; this is the consequence of the decrease of the structural rigidity and of the shift of crystallization to a higher temperature range. Devitrification involves a mechanism of surface nucleation; surface nuclei behaving as bulk nuclei in samples that soften and sinter before devitryfing.