Densification behavior and microstructure of mullite obtained from diphasic Al2O3-SiO2 gels

Linear shrinkage, density measurement and scanning electron microscopy (SEM) analyses were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 200 °C. It was found that the homogeneity level and amount of silica both have profound effects on the densification behavior. For the gels with high homogeneity level and thus low mullite formation temperature, the densification behavior was worse, owing to the overlapping with the temperature for densification by viscous flow and reaction sintering. In addition, the inhibitory effect for the densification of elongated mullite grains was stronger than that of equaxial ones. Herein, the derived ceramics containing elongated mullite grains and an excess amount of silica sintered to higher densities, since the presence of excess silica promoted the densification by viscous phase sintering.