White Si-O-C ceramic: Structure and thermodynamic stability

While pyrolysis of a polysiloxane precursor in argon typically produces a black amorphous Si-O-C ceramic containing "free" carbon (sp² carbon), pyrolyzing the same precursor in hydrogen leads to a white amorphous ceramic with a negligible amount of sp² carbon and a considerable hydrogen content. ²⁹Si magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy confirms the existence of very similar bonding environments of Si atoms in the Si-O-C network for both samples. In addition, ¹H NMR spectroscopic measurements on both samples reveal that the hydrogen atoms are bonded mainly to carbon. For the thermodynamic analysis, the enthalpies of formation with respect to the most stable components (SiO₂, SiC, C) of the black-and-white Si-O-C samples obtained after the pyrolysis at 1100°C are determined using high-temperature oxidative drop-solution calorimetry in a molten oxide solvent. The white ceramic is 6 kJ/g-atom more stable in enthalpy than the black one. Although the role of hydrogen in the thermodynamic stability of the white sample remains ambiguous, the thermodynamic findings and structural analysis suggest that the existence of sp²-bonded carbon in the amorphous network of polymer derived Si-O-C ceramics does not provide additional thermodynamic stability to the ceramic.