Silicon nitride and silicon carbonitride by the pyrolysis of poly(methylsiladiazane)
- McGill Univ., Montreal, Quebec (Canada). Dept. of Chemistry
Poly(methylsiladiazane) (PMSDZ) is synthesized by the copolymerization of MeHSiCl{sub 2} with H{sub 2}NNH{sub 2} in 1:1.4 molar ratio. The structure of the polymer is shown by NMR and IR spectroscopies to consist of tetraazadisiliacyclohexane rings bridged by MeHSi groups. The polymer gives a mixture of silicon nitride and silicon carbonitride when heated above 1,100 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 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 {sup 29}Si MAS NMR spectroscopies, elemental analysis, and X-ray powder diffraction. Thermal curing of PMSDZ at 150 C results mostly in increased cyclization of the reaction of bridging MeHSi moieties. At 300 C N-N bond cleavage occurs rapidly with the formation of a polysilazane structure, in which the silicon is mostly coordinated as MeHSi(N){sub 2} and MeSi(N){sub 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 C, to form MeSi(CH{sub 2})(N){sub 2} units. This step is critical for the incorporation of carbon into the ceramic product. At the same time Si(N){sub 4} fragments are also generated. Between 700 and 900 C a high concentration of persistent free radicals is formed. Amorphous silicon carbonitride (SiN{sub x}C{sub y}) and Si{sub 3}N{sub 4} are produced at 1,100 C. Even after heating to 1,500 C most of the product is still amorphous, only a small amount of crystalline {alpha}-Si{sub 3}N{sub 4} being detected by XRD.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 136498
- Journal Information:
- Journal of the American Ceramic Society, Journal Name: Journal of the American Ceramic Society Journal Issue: 11 Vol. 78; ISSN 0002-7820; ISSN JACTAW
- Country of Publication:
- United States
- Language:
- English
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