The characterisation of mullite-related phases by hydrothermal synthesis

Abstract

The aluminosilicate called mullite has been prepared by hydrothermal processing from aluminium- and silicon- acetates. The phase evolution of the products and the mechanism of formation of mullite from its precursors has been examined by x-ray powder diffraction, magic-angle spinning NMR spectroscopy, FTIR spectroscopy and electron microscopy. The product obtained from co-processing aluminium- and silicon- acetates was found to be a biphasic material with strong interfacial interactions between pseudoboehmite and amorphous silica-gel type phases. This interfacial interaction resulted in mullite being formed after calcination at 1250°C, which is a lower temperature than would normally be expected for the formation of mullite. Lattice parameter measurements of mullite calcined up to 1400°C suggested that a metastable state may be formed at the lower temperature. The effect of adding acid to the reagent mixture was examined and found to have no effect on the chemistry of the phase evolution of the products. The synthesis of iron-doped mullite has also been examined by x-ray powder diffraction, Mossbauer spectroscopy, EXAFS and electron microscopy. The iron-containing phases in dried intermediate materials exhibited superparamagnetism. Upon calcination iron-doped mullite formed at 1150°C with an iron content up to 14 mol%. EXAFS showed the iron to occupy octahedral sites in the mullite structure. The synthesis of magnesium-aluminium spinel has been examined using a similar methodology. The interfacial interaction between the initial precursors was found to be stronger than that observed in the aluminosilicate system. This increase in interfacial interaction is associated with the lower temperature required to form spinel (400°C).