Formation of aluminum rich 9:1 mullite and its transformation to low alumina mullite upon heating

Abstract

The formation of aluminum rich mullites Al 4 + 2 x Si 2 − 2 x O 10 − x with x > 2 3 has been studied at annealing temperatures between 700 and 1650 °C. Calcination of the amorphous precursor at 700 °C yields a mullite with 88 mol% Al 2O 3 corresponding to an x-value of 0·809. Simultaneously, a γ-alumina phase is formed. Further increase of the annealing temperature yields an increase in the aluminum incorporation up to 92·1 mol% Al 2O 3 at 1000 °C derived from the refined lattice constants. This is the highest amount of Al observed so far in a mullite except the supposed end member ι-Al 2O 3 which, however, has not yet been established unambiguously. Above 1000 °C, the aluminum content in mullite is reduced. This is accompanied by a transformation of the spinel-type phase to a superstructure of a θ-alumina like phase. The final product at 1650 °C consists of 34 mol% of a ‘normal’ mullite with x = 0·32 and 66 mol% corundum.