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

Abstract Stoichiometric and silica-rich diphasic Al 2 O 3 –SiO 2 gels were prepared through a sol–gel process. Structural evolution of the gels with heat-treatment up to 1300 °C was carried out by combination of differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. Results showed that the homogeneity level of the gels was the determining factor in the phase transformation sequence and the mullite formation temperature. For the gels with high homogeneity level, the crystallization to mullite occurred at low temperature. The presence of ammonium nitrate in the gels promoted the initial formation of alumina–silica spinal at about 950 °C, which further reacted with free silica to form elongated orthorhombic mullite grains at about 1250 °C. 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.