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

The high temperature treatment of kaolinite-muscovite alternate layers doped by bismuth oxide was studied by TGA and DTA, X ray diffraction, and electron microscopy (SEM). Thermal analyses shows that the two main transformation stages are the dehydroxylation of phyllosilicates and the structural reorganization of the whole assembly. During dehydroxylation, a progressive decrease of the structural order of kaolinite and muscovite occurs. It is more significant at temperature above 1000°C and mullite, glass and Alrich oxides are formed. Mullite exhibits an accentuated acicular morphology along 3 preferential orientations in relation to the remaining structure in the basal (001)musc planes of the former muscovite. With addition of bismuth oxide, SEM observations point to the strong accentuation of mullite growth along the c axis, at the very low temperature of 1050°C. This behavior was studied with various mullite-bismuth oxide compounds at temperatures ranging from 1000°C to 1400°C. Solgel processing were performed with TEOS (C2H5O)4Si, aluminum nitrate Al(NO3)3.9H2O and bismuth nitrate Bi(NO3)3. From Rietveld refinements of X ray diffraction patterns it was found that bismuth doesn't form a specific phase with mullite during the mullite nucleation and growth. At low Bi2O3 content, mullite coexists with amorphous silico-aluminate phases, even at high temperature. When the Bi2O3 content increases above 12mol%, mullite is no longer crystallized. Above 750°C, aluminum bismuth oxide coexists with silicon bismuth oxide. The liquidus temperature is below 1100°C and decreases with Bi2O3 additions. Above the liquidus, only corundum and a liquid coexist. A tentative mullite-Bi2O3 binary phase diagram is then proposed. These results evidence the Bi2O3 role on the early and large mullite growth in kaolinite-muscovite materials. Particularly, a very low quantity of a low viscous phase favor the local mobility of diffusing species and promotes the nucleation and growth of very large mullite crystals. In this way, highly textured and dense ceramics are obtained at low sintering temperature. Thin substrates (<500μm) present specific mechanical behaviors with the deviation of cracks along mullite layers.