The structural change of diphasic mullite gel studied by XRD and IR spectrum analysis

A systematic study on the structural change of diphasic mullite gel with heat treatment was carried out by combination of XRD and IR analysis. Aluminium ions are prevalently hexacoordinated in the as-received gel. After heated at high temperatures, part of them transforms to tetrahedral coordination. Phase segregation occurs at ≈1000 °C, resulting in the formation of γ-Al2O3 (containing 9–11 wt.% SiO2) and almost pure amorphous silica. Mullite forms at ≈1250 °C by reaction between (δ,θ)-Al2O3 and amorphous silica. The incorporation of SiO2 into γ-Al2O3 raises the transformation temperature of γ-Al2O3→(δ,θ)-Al2O3, the formation of pure amorphous silica and mullite render Si-O bands in IR spectrum shift stepwisely towards higher wave numbers.