SAXS measurements of the condensation in mullite precursors

Mullite in the composition 3Al₂O₃.2SiO₂ is applied as refractory material. One way to make ultrafine and homogeneous mullite powders is the controlled hydrolysis and condensation of an alcoholic solution of chelated aluminium alkoxides and silicon alkoxides. The condensation has been followed with SAXS (small-angle X-ray scattering). The condensation rate and the gelation time depended significantly on the time between preparation of solution and addition of water. In order to explain this behavior, ²⁹Si NMR was pursued. The particles, formed immediately after the addition of water, had a slit-smeared radius of gyration between 34 and 21 Å depending on the age of the solution. Before the gelation point was reached, we found a slope of 2.19 in the Porod plot in the q region 0.02 Å^-1 < q < 0.1. Heat treatment of gels yielded ceramic powders which crystallized near 1260 K to mullite directly from the amorphous state.