Synthesis of mullite precursors in molten salts. Influence of the molten alkali nitrate and additives

Silica gel and aluminium salt, in the stoichiometric ratio for mullite (Al/Si=3), have been used to prepare the mullite precursor by reaction in molten alkali nitrates at 450°C. The influence of the nitrate (NaNO 3, KNO 3 or mixture NaNO 3–KNO 3) and the additives (Ca 2+, Ba 2+, Bi 3+) on the phase transformations has been followed between 1000 and 1400°C. It appears that molten NaNO 3 favours the crystallization of cristobalite and α-Al 2O 3 whereas KNO 3 inhibits the formation of silica and mullite. The mixture NaNO 3–KNO 3 seems to be the best choice. Addition of foreign cations improves the mullite formation and favours the synthesis of θ-Al 2O 3, especially in the case of Bi 3+. But the crystallization temperature of mullite is not changed.