THE CHEMICAL SYNTHESIS AND CRYSTALLIZATION SEQUENCE OF MULLITE

Mullite stoichiometry powders have been produced using a water free sol-gel approach. Materials with varying homogeneity have been produced using this fabrication procedure and structural studies of these materials have indicated that the level of homogeneity, on an atomic scale, plays a central role in the determination of the crystalline phases that are produced. The level of homogeneity is indicated by the amount of pentacoordinate aluminium. A crystallisation model based on the size of alumina-rich inhomogeneities has been developed. Pentacoordinate aluminium exists at the interface between the inhomogeneity and the surrounding amorphous silica-rich matrix. The 980-degrees-C exotherm is due to crystallisation of mullite and/or spinel, which is initiated by the transformation of the metastable pentacoordinate aluminium to octahedral and tetrahedral arrangements.