SILICON OXYCARBIDES VIA SOL-GEL ROUTE - CHARACTERIZATION OF THE PYROLYSIS PROCESS

Modified silicon alkoxides, RxSi(OR')4-x, appear as very promising precursors to introduce Si-C bonds inside a silica gel network and thus to obtain silicon oxycarbide glasses after pyrolysis under inert atmosphere. Copolymers between tetraethoxysilane and diethoxydimethylsilane have been prepared and the pyrolyzed products have been characterized mainly using infrared and magic angle spinning-nuclear magnetic resonance (Si-29, C-13 and H-1). The pyrolysis process can be divided in three stages. Up to 400-degrees-C, the gels are thermally stable. Between 400 and 600-degrees-C, redistribution reactions between Si-O and Si-C bonds occur, but with no destruction of the organic groups. The transition between a polymeric and an inorganic material occurs only between 600 and 800-degrees-C with decomposition of the methyl groups. Samples pyrolyzed at 1000-degrees-C show the presence of Si-C and can be described as a mixture of a silicon oxycarbide phase and free carbon.