Nucleation and crystal growth in commercial LAS compositions

Thermal cycles for crystal nucleation of two commercial compositions of the Li2O·Al2O3·SiO2 system, Ceran® and Robax®, were determined by DTA. Both glasses show two exothermic peaks, the first peak corresponding to crystallisation from the glassy phase and the second one to a phase transformation. Crystalline phases formed on heating up to the first maximum exothermic peak temperature are a lithium alumino-silicate (LixAlxSi1–xO2) and an unidentified phase, which then transform to β-spodumene, tetragonal silica and an unidentified phase at the second maximum exothermic peak temperature. Kinetic parameters for crystallisation were determined. For both glasses, the Avrami parameter (n) calculated from the Ozawa equation is between 1 and 3, indicating that surface and volume crystallisation occur simultaneously. The activation energies calculated from the Kissinger model yield 195.8 kJ/mol for Ceran® and 113.2 kJ/mol for Robax®. For each glass, the thermal cycle for nucleation was optimised by studying the influence of time and temperature on the position of the maximum crystallisation peak temperature. It was observed that sintering completion requires temperatures higher than the crystallisation onsets, hindering powdersʹ full densification by pressureless sintering.