Si-incorporated alumina phases formed out of diphasic mullite gels

The diphasic mullite gel forms o-mullite on heating via intermediate spinel phase. Characterization of the latter phase with various physico-chemical techniques is concisely reviewed. It is noticeable that XRD intensity of both the amorphous scattering band and the diffraction peak of Al–Si spinel phase changes during each step of transformation processes of diphasic gel. Accordingly, the integrated area of the intensity peak of amorphous band and that of Al–Si spinel phase generated during heating diphasic gels were measured by XRD technique with the help of X’Pert Graphics and Profit softwares. The amount of free SiO2 (A) content present at various stages of heating diphasic gels was estimated by classical alkali leaching study standardized earlier. The results show that diphasic gel which forms an aluminosilicate (A) phase initially by dehydration and dehydroxylation, subsequently crystallizes to Al–Si spinel phase. In consequence, the ratio of XRD peak of spinal phase to that of amorphous band increases in the temperature range of 600–1000 C. This study confirms the earlier view of incorporation of silica into the alumina structure with formation of Al–Si spinal phase. Complementary alkali leaching study indicates the existence of non-crystalline silica-rich aluminous phase other than free non-crystalline silica during heating diphasic gel at *1000 C.