A mullite precursor was synthesized via combustion of a xerogel obtained from a mixture of aluminum nitrate, highly dispersed silica (AEROSIL), urea as the fuel (reducing agent), and hydrogen peroxide as an auxiliary additive. The crystallization process of the precursor — its transformation into mullite 3AlO·2SiO — was studied by thermal analysis, X- ray diffraction, IR spectroscopy, and NMR. The synthesized powder was amorphous and crystallized upon heat treatment at 1100 °C. Firing at 1200 °C yielded well-crystallized single-phase mullite. The combustion product contained AlO, AlO, and AlO groups. Using thermal analysis of the precursor at different heating rates, the effective activation energy of mullite crystallization was estimated from the positions of the exothermic peaks corresponding to this transition.
N.V. Filatova (Wed,) studied this question.