High surface area SiC/silicon oxycarbide glasses prepared from phenyltrimethoxysilane-tetramethoxysilane gels

Phenyltrimethoxysilane (PhTMS) was hydrolyzed or cohydrolyzed with tetramethoxysilane (TMOS) to make aerogels and xerogels. Porous SiC/silicon oxycarbide glasses were prepared by further pyrolyzing these gels in inert atmosphere up to 1500°C. The pore structure and chemical nature were studied by nitrogen and water sorption measurement, chemical analysis, X-ray diffraction and scanning electron microscopy. It has been found that the addition of PhTMS into TMOS gels decreased the surface area and porosity of TMOS gels, but enhanced their hydrophobicity and thermal stability. Pyrolyzing 25 mole% PhTMS-75 mole% TMOS aerogel in argon resulted in porous silicon oxycarbide glass which has a surface area of 581 m²/g at 1000°C. Pyrolyzing pure PhTMS gels at 1400°-1500°C produced porous SiC/C/silicon oxycarbide composites having surface areas in the range of 400-500 m²/g.