Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000°C of gel precursors obtained by cohydrolysis of triethoxysilane, HSi(OEt),, and methyldiethoxysilane, HMeSi(OEt), . The oxycarbide structures have been carefully characterized by means of different techniques such as 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500"C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano-sized p-Sic crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500°C for the samples in which the presence of Si-Si bonds was postulated at 1000°C.

Silicon oxycarbide glasses have been fabricated, in the shape of thin rods suitable for flexural test experiments, by pyrolysis in an inert atmosphere at 1000° and 1200°C of solgel precursors containing Si–CH3 and Si–H bonds. The amount of carbon in the silicon oxycarbide network has been controlled by varying the carbon load in the precursor gel. Density and surface area analysis revealed that all of the samples pyrolyzed at 1200°C were well‐densified silicon oxycarbide glasses whereas for the glasses treated at 1000°C, compositions with low carbon loads showed the presence of a residual fine porous phase. The elastic modulus (E), flexural strength (MOR), and Vickers hardness (Hv) increase markedly with the amount of carbon in the oxycarbide glasses reaching the maximum values (E∼ 115 GPa, MOR ∼ 550 MPa, and Hv∼ 9 GPa) for samples with the highest carbon content. The experimental elastic modulus values of the silicon oxycarbide glasses compare well with the theoretical estimations obtained using the Voigt and Reuss models assuming the disordered network formed by the corresponding thermodynamic compositions.

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Gels have been prepared via hydrolysis-condensation reactions of triethoxysilane (TREOS) and methyldiethoxysilane (MDES) in various ratios. The hydrolysis-condensation reactions of the precursors have been investigated by 29Si and 170 solution NMR. Both precursors present a very high reactivity towards hydrolysis and condensation reactions, and formation of co-condensed species will be discussed. These highly reactive precursors lead to transparent gels over a large composition range, 0.1 < MDES:TREOS < 10. Their structure was investigated by "Si MAS NMR, FTIR and DSC: the gels appear as highly condensed homogeneous systems in which the difunctional and trifunctional units are randomly distributed. The microstructure of the gels was also investigated through the measurement of specific surface area and skeletal density. Finally, some physical properties have also been measured such as thermal stability in O2 and inert atmosphere (Ar), coefficient of thermal expansion (a) and elastic modulus (€).

Monolithic and transparent gels were prepared by mixing various ethoxide silicon precursors containing Si-CH, 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-0 bond exchanges. At higher temperatures, the development of methane was detected and attributed to Si-C bond cleavage. Pyrolysis of gels containing only Si-CH, or Si-H groups was also studied for comparison.