THE EFFECT OF PRECURSOR CHEMISTRY ON THE CRYSTALLIZATION AND DENSIFICATION OF SOL-GEL DERIVED MULLITE GELS AND POWDERS

Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder d iffraction and dilatometry techniques were used to investigate the the rmal decomposition, crystallisation and sintering of these mullite pre cursor gels. The method of preparation, by controlled hydrolysis of va rious mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single- phase or diphasic samples. The crystallisation sequence of the gels de pended on the composition and method of preparation. Single phase mull ite crystallised from homogeneous gels at 980 degrees C, while diphasi c gels initially formed of a mixture of gamma-Al2O3 spinel and mullite , or simple gamma-Al2O3 spinel, which subsequently transformed to mull ite at 1260 gamma C. Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepa red from gels precalcined at 500 degrees C. Varying the heating rates from 2 to 10 degrees C min(-1) had little effect on the densification to 1500 degrees C. However, the densification rate was sensitive to th e degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of gamma-Al2O3 spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica In the original gel, sintered to higher densities due to the presence of excess silica prom oting densification by viscous phase sintering.