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), [CH3Si(N=C=N)(1.5)](n), leads to amor phous silicon carbonitride (SiCN) ceramics. In the present study the polyme r-to-ceramic transformation is characterized using analytical and spectrosc opic methods. In particular, the structural transition from the molecular p recursor to the final amorphous SiCN ceramic was studied by solid-state NMR spectroscopy. The coordination of the silicon atoms changes from CH3-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 degrees C. T his result implies the formation of amorphous silicon nitride (Si3N4) The r emaining weight fraction is analyzed to be elemental carbon. According to b ulk chemical analysis, the poly(methylsilsesquicarbodiimide)-derived SiCN i s comprised of a mixture of I mol amorphous Si3N4 with 3 mol amorphous carb on. The molar ratio Si3N4/C = 1/3 is suitable for the formation of stoichio metric silicon carbide, which is expected thermodynamically at temperatures exceeding 1440 degrees C in 0.1 MPa N-2. The pyrolysis behavior of the pol y(silylcarbodiimide) and the formation of silicon carbonitride ceramics are compared to those of poly(silazanes).