CRYSTALLIZATION OF A COMBUSTION-DERIVED MULLITE PRECURSOR

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 3Al₂O₃·2SiO₂ — 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 AlO₄, AlO₅, and AlO₆ 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.

mullite 3Al₂O₃·2SiO₂, xerogel combustion synthesis, mullite precursor, crystallization, ²⁹Si NMR, ²⁷Al NMR, thermal analysis, effective activation energy