Pyrolysis study of methyl-substituted Si—H containing gels as precursors for oxycarbide glasses, by combined thermogravimetry, gas chromatographic and mass spectrometric analysis

Abstract

Monolithic and transparent gels were prepared by mixing various ethoxide silicon precursors containing Si—CH₃ and Si—H groups, the composition ensuring the same number of C—H and Si—H bonds. Pyrolysis of these samples was followed under helium flow by connecting thermogravimetry, gas chromatographic and mass spectrometric analysis, to study the conversion of the gels into oxycarbide materials. In addition to the usual direct thermal and mass spectra analysis (TG–MS), a TG–GC–MS arrangement, allowing gas chromatographic separation of the species simultaneously evolving during thermodecomposition followed by mass spectral analysis, was successfully achieved. Experimental results indicate that mass loss occurs in three steps, each characterized by specific reactions. At low temperatures, densification of the siloxane network derives from further condensation reactions. At intermediate temperatures, a remarkable rearrangement of the siloxane chains occurs, with the release of volatile silanes and several siloxane fragments due to Si—H and Si—O bond exchanges. At higher temperatures, the development of methane was detected and attributed to Si—C bond cleavage. Pyrolysis of gels containing only Si—CH₃ or Si—H groups was also studied for comparison.