Crystallisation of amorphous spray-dried precursors in the Al2O3–SiO2 system

The crystallisation of amorphous precursors has been studied in the whole range of composition in the Al2O3–SiO2 system. The amorphous precursors have been obtained by hydrolysing TEOS directly in a diluted aqueous solution of aluminium nitrate, spray drying the clear solution and heating the resulting powder. Up to 70 mol % Al2O3, only mullite crystallises around 980–1000 °C; between 70 and 80 mol % Al2O3 mullite and spinel crystallise together; and for more than 80 mol % Al2O3 only spinel is formed. In the 70–80 mol % Al2O3 range of composition, when both mullite and spinel crystallise, low heating favours the crystallisation of mullite and it is nearly possible to crystallise only mullite from a 75 mol % Al2O3 sample. By rapid heating it is also possible to crystallise only spinel from the same 75 mol % Al2O3 precursor. The enthalpy and the activation energy for crystallisation are maximum for 60–80 mol % Al2O3. Heating the samples up to 1700 °C for 1 h, the phase equilibrium is not reached, particularly when both mullite and spinel crystallise together, and θ-Al2O3 is still present.