Elaboration of low shrinkage mullite by active filler controlled pyrolysis of siloxanes

The active filler controlled pyrolysis of polymers has been used to synthesize mullite from Al2O3-filled, Al-filled and Al/Al2O3-filled siloxanes. Due to the presence of transition alumina and to a finer scale of mixing of the mullite precursors provided from active Al-filler, the beginning of mullitization temperature was lowered in the two last systems (1200–1250°C) in comparison to the first one (1400°C). The mullite equiaxed-grains obtained from the polymer/Al2O3 (PAO) system contained a large amount of dislocations whereas in polymer/Al (PA) and polymer/Al/Al2O3 (PAAO) derived samples they were primarily dislocation free. The presence of small amounts of residual glassy phase was observed as glassy pockets at triple-junctions in the whole of the materials. The improved microstructure obtained with the Al/Al2O3-filled mixture indicates that this system seems to be a good choice for a low shrinkage and low defect mullite, in comparison to mullite obtained from Al-filled and Al2O3-filled siloxanes.