A comparative study of microstructural development in the sol–gel derived alumina–mullite nanocomposites using colloidal silica and tetraethyl orthosilicate

This paper compares the microstructure development of two alumina–15 vol% mullite composites during the sintering. The nanopowders of alumina–mullite composite were synthesized by the sol–gel method using aluminum chloride hexahydrate and two different silicon sources (colloidal silica in route 1 and tetraethyl orthosilicate in route 2). The alumina–mullite composites were prepared by pressing and sintering of the nanopowders. Although the intergranular mullites were observed in both routes, there were mullite particles in route 2 formed inside the alumina grains (intragranular mullite). Formation of the intragranular mullite is attributed to the drop in silica solubility, which occurs during the transition from metastable alumina to stable alumina. Compared to route 1, the relative density and the average grain size were increased and accelerated by route 2. The two-stage sintering is not useful for the mullite decomposition.