STRUCTURAL CHARACTERIZATION OF SOL-GEL DERIVED OXYCARBIDE GLASSES .2. STUDY OF THE THERMAL-STABILITY OF THE SILICON OXYCARBIDE PHASE

A gel resulting from cohydrolysis of dimethyldiethoxysilane and tetraethoxysilane has been converted through pyrolysis at 1000 degrees C into a silicon oxycarbide glass. A previous study has shown that this system can be described as a silicon oxycarbide matrix (SiC0.14O1.64) in which a free carbon phase (approximate to 8 wt %) is dispersed. Its chemical evolution at high temperatures from 1000 to 1600 degrees C was investigated using various structural techniques: Si-29 solid State magic angle spinning nuclear magnetic resonance (Si-29 MAS NMR), Raman spectroscopy and X-ray diffraction (XRD). In the 1000-1300 degrees C temperature range, redistribution reactions between Si-O and Si-C bonds occur, which lead to an enrichment in SiO4 and SiC4 units. Above 1400 degrees C, small silicon carbide crystallites are formed through carbothermal reaction between the free carbon and the silicon oxycarbide phase. However, even at 1500 degrees C, the samples remain mainly amorphous. The oxidation behavior of the glasses was also investigated using thermogravimetric analysis coupled with mass spectrometry: despite the presence of a free carbon phase, the samples exhibit a high oxidation resistance.