Thermal and devitrification behavior of CaO–Ga2O3–SiO2 glasses

A study of the influence of the substitution of Ga2O3 for CaO, at constant O/Si ratio, on thermal properties and non-isothermal devitrification of 2.5CaO.2SiO2 is reported. Differential thermal analysis (DTA) and X-ray diffraction analysis were used. The X-ray diffraction pattern of the crystallized Ga2O3 base glass shows that the αCaO SiO2, 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, Tg, and softening, Ts, temperature decrease as Ga2O3 is substituted for CaO. This is the result of the substitution of the network modifying cation Ca2+, of higher coordination number, by a network forming cation Ga3+ in fourfold coordination, in a composition range of relative insensitivity to changes of covalent cross-linking density. The crystal growth activation energy, Ec, 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.