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 tetrae thoxysilane has been converted through pyrolysis at 1000 degrees C int o a silicon oxycarbide glass. A previous study has shown that this sys tem can be described as a silicon oxycarbide matrix (SiC0.14O1.64) in which a free carbon phase (approximate to 8 wt %) is dispersed. Its ch emical evolution at high temperatures from 1000 to 1600 degrees C was investigated using various structural techniques: Si-29 solid State ma gic angle spinning nuclear magnetic resonance (Si-29 MAS NMR), Raman s pectroscopy and X-ray diffraction (XRD). In the 1000-1300 degrees C te mperature 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 carbo thermal reaction between the free carbon and the silicon oxycarbide ph ase. However, even at 1500 degrees C, the samples remain mainly amorph ous. The oxidation behavior of the glasses was also investigated using thermogravimetric analysis coupled with mass spectrometry: despite th e presence of a free carbon phase, the samples exhibit a high oxidatio n resistance.