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

摘要

Mullite compounds with bismuth oxide in the SiO_2-Al_2O_3-Bi_2O_3 ternary system were synthesized from TEOS (C_2H_5O)_4Si, aluminum nitrate A1(NO_3)_3 centre dot 9H_2O and bismuth nitrate Bi(NO_3)_3. Thermal and structural transformations were studied at temperatures ranging from 1000 to 1400 °C. The coexistence of Al_4Bi_2O_9 and Bi_4Si_3O_(12) phases at temperatures up to 1000 °C was observed in compositions containing 5-31 mol% Bi_2O_3. Mullite is observed at temperature higher than 1000 °C in composition not exceeding 5 mol% of Bi_2O_3. Corundum coexist with a liquid above 1000 °C in all compositions containing more than 5 mol% Bi_2O_3. The liquid temperature is slightly above 1000 °C for all compositions. A tentative pseudo-binary diagram mullite-Bi_2O_3 is proposed. A similar system was studied with silico-aluminate compositions containing kaolinite and muscovite minerals. The occurrence of a liquid when Bi_2O_3 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.