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 2.5 CaO.2SiO(2) is reported. Differential thermal analysis (DTA) and X-ray diff raction analysis were used. The Xray diffraction pattern of the crystallize d Ga2O3 base glass shows that the alpha CaO SiO2, that should be stable onl y above 1125 degrees C, forms in the temperature range 900-1000 degrees 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 gla ss transformation temperature, T-g, and softening, T-s, temperature decreas e as Ga2O3 is substituted for CaO. This is the result of the substitution o f the network modifying cation Ca2+, of higher coordination number, by a ne twork 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, E-c, decreases with substitution; this i s the consequence of the decrease of the structural rigidity and of the shi ft of crystallization to a higher temperature range. Devitrification involv es a mechanism of surface nucleation; surface nuclei behaving as bulk nucle i in samples that soften and sinter before devitryfing. (C) 2000 Elsevier S cience Ltd. All rights reserved.