HIGH-TEMPERATURE CHEMISTRY OF THE CONVERSION OF SILOXANES TO SILICON-CARBIDE

The preparation of siloxane polymers as precursors to silicon carbide has been investigated. To make these polymers useful as binder materials for ceramic powders in the preparation of dense (sintered) ceramic monoliths, methods were developed to control the silicon carbide to carbon ratios in the polymer derived ceramic. Insights into the chemistry of the conversion of these high oxygen polymers to silicon carbide ceramics were obtained by following the pyrolysis products by elemental analysis, quantitative X-ray diffraction, Si-29 MAS NMR spectroscopy, Raman spectroscopy, and transmission electron microscopy. The pyrolysis proceeds by the formation of an amorphous SiCO material at 1200-degrees-C that continues to undergo an Si-O for Si-C bond redistribution so that trace amounts of beta-SiC are seen at 1400-degrees-C. By 1600-degrees-C the carbothermic reduction is well underway with only a small percentage of oxygen remaining in the material. At 1800-degrees-C the pyrolysis is complete. The final ceramic is composed of substantial amounts of crystalline beta-SiC and excess carbon that is present as turbostratic graphite.