Crystallisation of amorphous spray-dried precursors in the Al 2O 3–SiO 2 system

Abstract

The crystallisation of amorphous precursors has been studied in the whole range of composition in the Al 2O 3–SiO 2 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 % Al 2O 3, only mullite crystallises around 980–1000 °C; between 70 and 80 mol % Al 2O 3 mullite and spinel crystallise together; and for more than 80 mol % Al 2O 3 only spinel is formed. In the 70–80 mol % Al 2O 3 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 % Al 2O 3 sample. By rapid heating it is also possible to crystallise only spinel from the same 75 mol % Al 2O 3 precursor. The enthalpy and the activation energy for crystallisation are maximum for 60–80 mol % Al 2O 3. 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-Al 2O 3 is still present.