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 degrees C. The influence of the nitrate (NaNO3, KNO3 or mixture NaNO3-KNO3) and the additives (Ca2+, Ba2+, Bi3+) on the phase transformations has been followed between 1000 and 1400 degrees C. rt appears that molten NaNO3 favours the crystallization of cristobalite and alpha-Al2O3 whereas KNO3 inhibits the formation of silica and mullite. The mixture NaNO3-KNO3 seems to be the best choice. Addition of foreign cations improves the mullite formation and favours the synthesis of theta-Al2O3, especially in the case of Bi3+. But the crystallization temperature of mullite is not changed. (C) 1999 Elsevier Science Limited. All rights reserved.