Nanostructure and Energetics of Carbon-Rich SiCN Ceramics Derived from Polysilylcarbodiimides: Role of the Nanodomain Interfaces

SiCN polymer-derived ceramics (PDCs) with different carbon contents have been synthesized by pyrolysis of poly(phenylvinylsilylcarbodiimide) and of poly(phenylsilsesquicarbodiimide), and their structure and energetics have been studied using Si-29, C-13, N-15, and H-1 solid state nuclear magnetic resonance (NMR) spectroscopy and oxide melt solution calorimetry. The structure of these PDCs at lower carbon content (35-40 wt %) and pyrolysis temperatures (800 degrees C) consists primarily of amorphous nanodomains of sp(2) carbon and silicon nitride with an interfacial region characterized by mixed bonding between N, C, and Si atoms that is likely stabilized by the presence of hydrogen. The average size of the carbon domains increases with increasing carbon content, and a continuously connected amorphous carbon matrix is formed in PDCs with 55-60 wt % C. The interfacial silicon-carbon and nitrogen carbon bonds are destroyed with concomitant hydrogen loss upon increasing the pyrolysis temperature to 1100 degrees C. Calorimetry results demonstrate that the mixed bonding between C, N, and Si atoms in the interfacial regions play a key role in the thermodynamic stabilization of these PDC. They become energetically less stable with increasing annealing temperature and concomitant decrease of mixed bonds and hydrogen loss.