Pyrolysis chemistry of polysilazane precursors to silicon carbonitride

Abstract

The chemistry of pyrolytic conversion has been studied for three polysilazanes, (ViSiHNH) _n , (ViSiHNMe) _n and [(ViSiHNH) _0.5 –(MeSiHNH) _0.5 ] _n , precursors to silicon carbonitride ceramics. ¹³ C and ²⁹ Si MAS and CPMAS NMR spectroscopies were used to clarify the processes leading to the formation of the silicon-based mineral network as well as the segregation of a free carbon phase. The assignment of ²⁹ Si NMR signals corresponding to SiC _n N _4-n sites was essential to follow the number of Si–C and Si–N bonds that are cleaved or formed. It was shown that at the organic–mineral transition temperature (ca. 900 °C) the final amount of free carbon as well as the final composition of the silicon-based network were already reached. Above this temperature, redistribution reactions around silicon atoms inside the amorphous silicon-based matrix take place in order to favour nitrogen-rich environments, i.e. crystallized Si ₃ N ₄ regions. Above 1400 °C, all ceramics contain a similar amorphous silicon carbonitride structure, whose composition is close to SiN _0.85 C _0.35 and which coexists with a crystallizing Si ₃ N ₄ phase. Finally, the relative amounts of the three possible final carbon states, at 1400 °C, i.e. gas products, carbon incorporated in the silicon-based network and free carbon, could be related to the nature of the carbon-containing substituents in the precursor backbones and to the occurrence of the cross-linking reactions below the mineral transition temperature.