SOL-GEL SYNTHESIS OF A SILOXYPOLYCARBOSILANE GEL AND ITS PYROLYTIC CONVERSION TO SILICON OXYCARBIDE

A new ethoxy-substituted polycarbosilane (I) has been prepared from a highly branched chloropolycarbosilane [Si(OEt)x(Et)0.15ClyCH2]n. After hydrolysis, condensation, and drying, a gel having the general formula [CH2Si(OH)a(OEt)b(Et)0.15Oc]n (II) was obtained from I. The dried gel was pyrolyzed under argon to give a silicon oxycarbide glass. The gel and its pyrolysis chemistry were characterized by infrared and Si-29, C-13, and H-1 magic angle spinning nuclear magnetic resonance (MAS NMR). By 600-degrees-C the gel has developed a fully condensed structure and can be represented by the approximate formula [CH2SiO]n, but with a distribution of Si microenvironments ranging from X (SiC4) to Q (SiO4, minor component). The conversion of the polymeric network into a silicon oxycarbide glass occurs between 600 and 1000-degrees-C. A comparison between the chemical analysis data and the Si-29 NMR spectra simulations leads to an estimation of the carbon content and its distribution in the silicon oxycarbide. The results were compared with those obtained from a methylsilsesquioxane gel, CH3SiO1.5 (III). These results show that a higher proportion of the carbon in the initial precursor is retained in the oxycarbide product if the carbon atoms are initially bonded to two silicon atoms rather than one. Moreover, the amount of carbon that is incorporated into carbidic sites (CSi4) in the case of the ''CH2SiO'' precursor is found to be approximately twice that obtained by using the CH3SiO1.5 gel.