Nucleation and crystal growth in commercial LAS compositions

Thermal cycles for crystal nucleation of two commercial compositions of the Li2O . Al2O3. SiO2 system, Ceran(R) and Robax(R), were determined by DTA. Both glasses show two exothermic peaks, the first peak corresponding to cry stallisation from the glassy phase and the second one to a phase transforma tion. Crystalline phases formed on heating up to the first maximum exotherm ic peak temperature are a lithium alumino-silicate (LixAlxSi1-xO2) and an u nidentified phase, which then transform to beta -spodumene. tetragonal sili ca and an unidentified phase at the second maximum exothermic peak temperat ure. Kinetic parameters for crystallisation were determined. For both glass es, the Avrami parameter (n) calculated from the Ozawa equation is between 1 and 3, indicating that surface and volume crystallisation occur simultane ously. The activation energies calculated from the Kissinger model yield 19 5.8 kJ/mol for Ceran(R) and 113.2 kJ/mol for Robax(R). For each glass, the thermal cycle for nucleation was optimised by studying the influence of rim e and temperature on the position of the maximum crystallisation peak tempe rature. It was observed that sintering completion requires temperatures hig her than the crystallisation onsets, hindering powders' full densification by pressureless sintering. (C) 2001 Elsevier Science Ltd. All rights reserv ed.