Optimization of sol–gel/pyrolysis routes to silicon oxycarbide glasses

The optimization of the synthesis of silicon oxycarbide ceramics via the sol–gel/pyrolysis route is described, starting from several alkylalkoxysilanes and vinylalkoxysilanes. The main aim was to achieve low weight losses during cure and pyrolysis and also a compact ceramics morphology. The theoretical composition of the pyrolysates was changed between SiO1.5C and SiO1.2C2.3, by varying monomer types and ratios. An assessment of the real composition was performed using energy-dispersive X-ray spectroscopy. The smallest weight losses were obtained for the resins based on vinyltriethoxysilane and dimethyldiethoxysilane which were additionally modified by the addition of a small amount of boric acid. Somewhat higher weight losses combined with the best toughness were found for resins based on non-vinylated silsesquioxane with a low content on linear dimethylsiloxane units. As an alternative, a precursor (SiO0.9C2.8) was prepared via hydrosilylation, based on cyclic dimethylsiloxane oligomers interconnected to a network, but its weight losses were higher than those of sol–gel silicon oxycarbide.

► Synthesis of glassy SiOC via sol–gel/pyrolysis of alkoxysilanes was optimized.
► Siloxane resins suitable for preparation of SiOC fibrous composites were obtained.
► Vinylated monomers yield the smallest weight losses, but reduce toughness.
► Addition of boric acid reduces the escape of linear monomer during pyrolysis.
► Precusors rich on linear D units yield interesting foamy pyrolysates.