Ceramic processing using polysilazane

Organometallic polymer precursors offer the potential to manufacture shaped forms of advanced ceramic materials using low temperature processing. Polysilazanes, compounds containing Si-N bonds in the polymer backbone, can be used as precursors to silicon nitride containing ceramic materials. This chapter provides an overview of the general synthetic approaches to polysilazanes with particular emphasis on the synthesis of preceramic polysilazanes. 

West (p. 6), Miller (p. 43), Zeigler (10), and Sawan (p. 112) outline the synthesis of a wide variety of soluble, processable polydiorganosilanes, a class of polymers which not long ago was thought to be intractable. Matyjaszewski (p. 78) has found significant improvements in the synthetic method for polydiorganosilane synthesis as well as new synthetic routes to unusual substituted polydiorganosilanes. Seyferth (p. 21, 143) reports synthetic routes to a number of new polycarbosilanes and polysilazanes which may be used as precursors to ceramic materials. 

Access to phase pure silicon nitride materials via processable precursors is limited to just three approaches. The first, shown in reaction 6, provides one of the first oligomers exploited as a preceramic polymer24,253. This simple polysilazane, containing only Si, N and H, is known to be relatively unstable and will crosslink on its own to give intractable gels. Furthermore, it does not offer the 3Si I4N stoichiometry required for Si3N4. Nonetheless, it is useful as a binder and for fiber-reinforced ceramic matrix composites (CMCs)31. 

The initial polysilane is rearranged to a polycarbosilane by thermolysis at 743 K. The resulting polymer has no well-defined structure. The use of polysilazanes, having a -[RjSi-NH]- structural unit, allowed the production of mixed SiC/Si3N4 ceramics.12 By a variation of the processing conditions, both compounds can be selectively... 

H3C)3Si-NH-Si(CH3)3 (HMDS). It is commercially available and synthesized in large quantities by ammonolysis of trimethylchlorosilane. Costs per kilogram are less than US 10. HMDS cannot be directly used as a precursor for ceramics because it is a nonprocessable volatile liquid (boiling point 125°C). Nevertheless, it is an important source for the synthesis of polysilazanes by transamination reactions (see below) and also a valuable single-source precursor for the preparation of Si-C-N coatings by CVD processes. ... 

Borazine-Modified Hydridopolysilazanes. Polysilazanes have been shown to be excellent polymeric precursors to silicon nitride or SiNC ceramic materials. Chemical modification of polysilazanes has also been proposed as a means of modifying and enhancing e properties of the polymer and/or the final ceramic materials. For example, the incorporation of boron in the polysilazane has been claimed to decrease the crystallinity of the silicon nitride deriv from these polymers and thereby extend the effective use-temperature of these ceramics. Given the potential importance of SiNCB ceramics, investigations of the generation of new classes of hybrid polymers that might serve as processable precursors to such composites were initiate. 

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