Densification of orthorhombic mullite derived by the sol-gel process

Abstract

The effect of thermal decomposition and crystallisation on the densification of mullite precursor gels was investigated by thermal analysis and x-ray diffraction techniques. The densification rate was sensitive to the crystalline phases present at a given temperature. Compacts containing amorphous silica, g-Al2O3 spinel and transitional mullite, after initial crystallisation at 980 °C, densified readily, but sintering virtually ceased once orthorhombic mullite crystallised. Complete densification of mullite using constant-rate heating required temperatures > 1650 °C. In contrast, the use of rate-controlled sintering, to promote densification prior to the formation of mullite, yielded fully-dense mullite at < 1600 °C. Densification was promoted by increasing soak time. The temperature profile and ultimate temperature significantly affected the density of the sol-gel derived orthorhombic mullite.