SILICON-NITRIDE AND SILICON CARBONITRIDE BY THE PYROLYSIS OF POLY(METHYLSILADIAZANE)

Poly(methylsiladiazane) (PMSDZ) is synthesized by the copolymerization of MeHSiCl(2) with H2NNH2 in 1:1.4 molar ratio. The structure of the polymer is shown by NMR and IR spectroscopies to consist of tetraazadisilacyclohexane rings bridged by MeHSi groups. The polymer gives a mixture of silicon nitride and silicon carbonitride when heated above 1100 degrees C under an inert atmosphere. Ceramic yields are increased relative to those obtained with the native polymer (67%) by cross-linking with a dimethyltitanocene catalyst, or by thermal curing at 150 degrees C (ceramic yields of 76% and 81%, respectively). The pyrolytic conversion of PMSDZ to ceramic products is studied by characterizing the solid residues by using FT-IR and solid-state Si-29 MAS NMR spectroscopies, elemental analysis, and X-ray powder diffraction. Thermal curing of PMSDZ at 150 degrees C results mostly in increased cyclization of the reaction of bridging MeHSi moieties. At 300 degrees C N-N bond cleavage occurs rapidly with the formation of a polysilazane structure, in which the silicon is mostly coordinated as MeHSi(N)(2) and MeSi(N)(3) fragments. A mechanism involving the formation and reaction of silylaminyl radicals is proposed to account for these structural changes. Some methylene insertion occurs at 500 degrees C, to form MeSi(CH2)(N)(2) units. This step is critical for the incorporation of carbon into the ceramic product. At the same time Si(N)(4) fragments are also generated. Between 700 degrees and 900 degrees C a high concentration of persistent free radicals is formed. Amorphous silicon carbonitride (SiNxCy) and Si3N4 are produced at 1100 degrees C. Even after heating to 1500 degrees C most of the product is still amorphous, only a small amount of crystalline alpha-Si3N4 being detected by XRD.