Influence of thermal treatment conditions on the formation of phase-pure mullite derived from a nanoparticulate aluminosilicate precursor

The crystallization of mullite derived from homogeneous, proto-imogolite allophane-resembling nanoparticulate aluminosilicate sol–gel precursors, was investigated. The kinetics of the crystal growth was studied using differential thermal analysis and the evolution of the chemical structure of the precursor was studied by Fourier transform infrared spectroscopy. Dehydroxylation of the nanoparticulate hydrogel led to the formation of an intermediate structure containing isolated M–OH groups, which temporarily favoured the formation of a co-existing spinel phase. In gels containing polymers, the interparticulate surface condensation was delayed, and further rearrangements occurring in the temperature range of 900–1000 °C were found to be essential in order to induce the nucleation-growth of polymorphic mullite, without the formation of an intermediate spinel phase.