Thermal Decomposition of Poly(methylsilsesquicarbodiimide) to Amorphous Si−C−N Ceramics

This paper is based on the recent discovery that the thermal decomposition of poly(methylsilsesquicarbodiimide), [CH 3 Si(N C N) 1.5 ] n, leads to amorphous silicon carbonitride (SiCN) ceramics. In the present study the polymer-to-ceramic transformation is characterized using analytical and spectroscopic methods. In particular, the structural transition from the molecular precursor to the final amorphous SiCN ceramic was studied by solid-state NMR spectroscopy. The coordination of the silicon atoms changes from CH 3 −Si(−N C N−) 3 in the polysilylcarbodiimide precursor at room temperature to Si(−N<) 4 in the X-ray amorphous reaction product formed at 1200 °C. This result implies the formation of amorphous silicon nitride (Si 3 N 4 ). The remaining weight fraction is analyzed to be elemental carbon. According to bulk chemical analysis, the poly(methylsilsesquicarbodiimide)-derived SiCN is comprised of a mixture of 1 mol amorphous Si 3 N 4 with 3 mol amorphous carbon. The molar ratio Si 3 N 4 /C = 1 / 3 is suitable for the formation of stoichiometric silicon carbide, which is expected thermodynamically at temperatures exceeding 1440 °C in 0.1 MPa N 2 . The pyrolysis behavior of the poly(silylcarbodiimide) and the formation of silicon carbonitride ceramics are compared to those of poly(silazanes).