Mullite interaction with bismuth oxide from minerals and sol–gel processes

Mullite compounds with bismuth oxide in the SiO2–Al2O3–Bi2O3 ternary system were synthesized from TEOS (C2H5O)4Si, aluminum nitrate Al(NO3)3·9H2O and bismuth nitrate Bi(NO3)3. Thermal and structural transformations were studied at temperatures ranging from 1000 to 1400 °C. The coexistence of Al4Bi2O9 and Bi4Si3O12 phases at temperatures up to 1000 °C was observed in compositions containing 5–31 mol% Bi2O3. Mullite is observed at temperature higher than 1000 °C in composition not exceeding 5 mol% of Bi2O3. Corundum coexist with a liquid above 1000 °C in all compositions containing more than 5 mol% Bi2O3. The liquid temperature is slightly above 1000 °C for all compositions. A tentative pseudo-binary diagram mullite-Bi2O3 is proposed. A similar system was studied with silico-aluminate compositions containing kaolinite and muscovite minerals. The occurrence of a liquid when Bi2O3 is added highly favors the mullite growth at temperature below 1200 °C. It is favored by local concentrations at interfaces of a transient liquid phase, which enhance the mobility of species.