PYROLYSIS STUDY OF METHYL-SUBSTITUTED SI-H CONTAINING GELS AS PRECURSORS FOR OXYCARBIDE GLASSES, BY COMBINED THERMOGRAVIMETRY, GAS-CHROMATOGRAPHIC AND MASS-SPECTROMETRIC ANALYSIS

Monolithic and transparent gels were prepared by mixing various ethoxi de silicon precursors containing Si-CH3 and Si-H groups, the compositi on ensuring the same number of C-H and Si-H bonds. Pyrolysis of these samples was followed under helium how by connecting thermogravimetry, gas chromatographic and mass spectrometric analysis, to study the conv ersion of the gels into oxycarbide materials. In addition to the usual direct thermal and mass spectra analysis (TG-MS), a TG-GC-MS arrangem ent, allowing gas chromatographic separation of the species simultaneo usly evolving during thermodecomposition followed by mass spectral ana lysis, was successfully achieved. Experimental results indicate that m ass loss occurs in three steps, each characterized by specific reactio ns. 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 relea se of volatile silanes and several siloxane fragments due to Si-H and Si-O bond exchanges. At higher temperatures, the development of methan e 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 .