Polycarbosilazane precursors

D. Mocaer, R. Pailler, R. Naslain, C. Richard, J. P. Plllot, J. Dunogues, C. Gerardin and F. Taulelle, Si-C-N ceramics with a high microstructurai stability elaborated from the pyrolysis of new polycarbosilazane precursors. Part I. The organic-inorganic transition, J. Mater. Sci., 28,2615-31 (1993). 

Summary Polysilacarbosilanes and polysilasilazanes prepared according to a copolymer strategy offer an easy, coherent approach to polycarbosilanes and silazanes, precursors of SiC and SiCN-based materials with variable C/Si and C/Si/N ratios. In contrast with the polysilazane route which leads, upon pyrolysis, to carbon-containing silicon nitride, the synthesized polycarbosilazanes are finally converted into nitrogen-containing silicon carbide. 

There are several structurally different types or polymers that are suitable precursors for ternary Si-C-N ceramics. By far the most investigated precursors are polysilazanes of the general type [Si(R )(R°)N(R°)] (R, R°, R° = H, alkyl, aryl, alkenyl, etc.). In contrast to the limited number of starting compounds, H SiCl(4 ) (x = 0-3) as the silicon source and NH3 or H2N-NH2 as the nitrogen source for synthesis of polysilazanes as precursors for binary Si-N ceramics, the chemistry of polycarbosilazanes, that is, carbon-containing or modified polysilazanes, is very multifaceted. The attachment of various organic groups to the silicon atoms allows adjustment of their physicochemical properties, to control their thermolysis chemistry, and also to influence materials properties. The first... 

The model of Monthioux et al. describes the crystallization behavior of SiC also for more complex precursor systems Si-C-N, Si-C-0, and Si-C-N-0. In the case of nitrogen-containing systems it has been observed [158] that, beside the bulk mechanism based on BSUs discussed above another mechanism, a surface mechanism may also occur. Especially on surfaces of fibers derived from polycarbosilazanes, needle-like silicon nitride crystals are formed under a nitrogen atmosphere via gas phase processes at T > 1600 C. 

In recent years silicon-based polymers were investigated as precursors for SiC and Si3N4 ceramics, as well as for crystalline or amorphous Si/C/N and SiC/Si3N4 composite materials [1, 2]. This is due to the very interesting chemical and thermomechanical properties of silicon carbonitrides, such as high hardness, toughness and corrosion resistance. In most of these studies polycarbosilanes, polysilazanes and polycarbosilazanes were applied [3]. 

Si-C-N-0 and Si-C-N fibers are made by spinning, curing and pyrolyzing polysilazane (PSZ) or polycarbosilazane (PCSZ) precursors. PSZs have a Si-N backbone, carbon being present in pendent groups, whereas, in PCSZs, carbon is contained in a Si-C-N backbone. 

M. Birot, J. P. Pillot and J. Dunogues, Comprehensive chemistry of polycarbosiianes, poiysilazanes and polycarbosilazanes, as precursors of ceramics, Chem. Reviews, 95,1443-77 (1995). 

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