Abstract :
La2O3 was substituted for CaO in a glass of composition 2.5CaO·2SiO2 and the effect of this substitution on thermal properties and non-isothermal behavior was studied. The glass samples were submitted to thermal analysis (DTA) and X-ray diffraction analysis. The trends with the composition of the glass transformation, Tg, and the activation energy of crystal growth, Ec, are the result of a type of competition between Ca2+ and La3+ ions each to achieve their preferred coordination. The lanthanum ion does not succeed in realizing its own average coordination. As a result Tg=810±5°C and the activation energy for crystal growth was Ec=650±25 kJ/mol, regardless of composition. Devitrification involved a mechanism of surface nucleation: however, because the samples softened and sintered before devitrifying, surface nuclei behaved as bulk nuclei. In all the devitrified samples CaO·SiO2 crystals form together with a secondary calcium silicate phase, while Ca3La6(SiO4)6 crystals form when ≅40% molar weight CaO is substituted. The pattern for the La2O3 free glass shows that the αCaO·SiO2 is also formed in the temperature range 900–1000°C, which should be stable only above 1125°C as reported in the phase diagram.
