Amorphous Si(Al)OC ceramic from polysiloxanes: bulk ceramic processing, crystallization behavior and applications

Here we report on bulk Si–Al–O–C ceramics produced by pyrolysis of commercial poly(methylsilsesquioxane) precursors. Prior to the pyrolysis the precursors were cross-linked with a catalyst, or modified by the sol-gel-technique with an Al-containing alkoxide compound, namely alumatrane. This particular procedure yields amorphous ceramics with various compositions (Si1.00O1.60C0.80, Si1.00Al0.04O1.70C0.48, Si1.00Al0.07O1.80C0.49, and Si1.00Al0.11O1.90C0.49) after thermal decomposition at 1100 °C in Ar depending on the amount of Al-alkoxide used in the polymer reaction synthesis. The as-produced ceramics are amorphous and remain so up to 1300 °C. Phase separation accompanied by densification (1300–1500 °C) and formation of mullite at T > 1600 °C are the stages during heat-treatment. Bulk SiAlOC ceramics are characterized in terms of microstructure and crystallization in the temperature regime ranging from 1100 to 1700 °C. Aluminum-free SiOC forms SiC along with cracking of the bulk compacts. In contrast, the presence of Al in the SiOC matrix forms SiC and mullite and prevents micro cracking at elevated temperatures due to transient viscous sintering. The nano-crystals formed are embedded in an amorphous Si(Al)OC matrix in both cases. Potential application of polysiloxane derived SiOC ceramic in the field of ceramic micro electro mechanical systems (MEMS) is reported.