Three silicon oxycarbide glasses (SiCO) with increasing C content were obtained through pyrolysis in inert atmosphere at 1000 °C of sol-gel derived siloxane networks containing Si-CH 3 and Si-H bonds. The glasses were further annealed at 1200, 1400, and 1500 °C to follow their evolution at high temperature. Quantitative information concerning the structure of glasses before and after annealing at high temperature was collected with a wide range of techniques (some of them used for the first time in this field) with the aim of probing the following: (i) the short-range order and chemical composition ( 29 Si and 1 H MAS NMR, RDF derived from X-ray and neutron scattering, inelastic neutron scattering, FT-IR, and elemental analysis), and (ii) the long-range order (X-ray and neutron diffraction) and microstructural features (HR-TEM combined with electron diffraction, Raman, porosity, and surface area measurements). This extensive collection of data, carried out on the same set of specimens, provided detailed and sound structural information on nearly-stoichiometric SiCO glasses and their high-temperature behavior.

SiCO glasses prepared from sol-gel precursors via pyrolysis in argon at temperatures ranging from 1000°to 1400°C were studied by transmission electron microscopy (TEM), in conjunction with electron energy-loss spectroscopy (EELS). EELS analysis showed that stoichiometric SiCO glass underwent phase separation, forming SiO 2 -and SiC-based environments. This process started at ϳ1200°C. However, at temperatures >1300°C, precipitation of nanometer-sized SiC particles embedded in vitreous SiO 2 was monitored by high-resolution TEM.

Crystallization behavior of Si-C-O glasses in the temperature range of 1000°-1400°C was investigated using transmission electron microscopy (TEM) in conjunction with electron energy-loss spectroscopy (EELS). Si-C-O glasses were prepared by pyrolysis of polysiloxane networks obtained from homogeneous mixtures of triethoxysilane, T H , and methyldiethoxysilane, D H . Si-C-O glass composition depended on the molar ratio of the precursors utilized. At a ratio of T H /D H ‫؍‬ 1, the formation of a carbon-rich glass was observed, whereas a ratio of T H /D H ‫؍‬ 9 yielded a Si-C-O glass with excess free silicon. Both materials were amorphous at 1000°C, but showed a distinct difference in crystallization behavior on annealing at high temperature. Although T H /D H ‫؍‬ 1 revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400°C, T H /D H ‫؍‬ 9 showed, in addition to SiC crystallites, numerous larger silicon precipitates (20 -50 nm), even at 1200°C. Both materials underwent a phase separation process, SiC x O 2(1-x) 3 xSiC ؉ (1 -x)SiO 2 , when annealed at temperatures exceeding 1200°C.

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