Ceramic materials, 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. 

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. 

To produce a ceramic material containing only Si3N4, the white solid polysilazane derived from DHCD of the oil obtained by ammonolysis of 6 1 CH3SiHCl2/CH3SiCl3 was pyrolyzed in a stream of ammonia (to 1000°C). A white ceramic residue containing only 0.36% by weight C resulted. 

This chapter gives an introduction to the preceramic polymer route to ceramic materials and focuses on the reasons why this new approach was needed and on the chemical considerations important in its implementation, with examples from research on organosilicon polymers. Novel polysilazanes have been prepared by the dehydro-cyclodimerization reaction, a new method for polymerizing suitably substituted cyclooligosilazanes. The living polymer intermediate in this reaction has been used to convert Si-H-containing organosilicon polymers that are not suitable for pyrolytic conversion to ceramics into useful preceramic polymers. 

The production of boron containing Si-C-N-ceramics provides an efficient system for materials with high thermal stability. Previously, the preparation of quaternary systems such as Si-B-C-N was realized by blending boron-containing compounds with polysilazanes This processing route leads to an inhomogenous elemental distribution in the finally received ceramic. In the last few years several investigations of ceramics derived from different polymers have been performed [1-3]. Therefore, the stoichiometry and basic structure units determine the properties of the final non-oxide ceramic materials [4-5]. 

Pyrolyses of these polysilazanes give ceramic materials . 

Polysilazanes represent another important class of ceramic materials that contain N-Si-N and C-Si-N units, sometimes wifli or without oxygen. The structural forms of polysilazanes may be chains, rings, crosslinked networks, or, more recently, dendrimers. Generally, the prepolymers contain sihcon bonded to nitrogen in the backbone and organic groups or hydrogen as substituents on Si and N (65). 

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. 

The spectrum of silicon based polymers is enriched by high tech ceramics like silicon nitride and carbide, respectively. These materials are produced by pyrolysis of appropriate polymeric precursors such as polysilanes, polycarbosilanes and polysilazanes (preceramics). These synthetic ceramics display a certain analogy to silicates, having SiC, SiN, or Si(C,N) as structural subunits instead ofSiO. 

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. 

On pyrolysis under argon to 1000 °C, the polysilazanes prepared from the systems just discussed gave black ceramic products whose elemental analysis could be explained in terms of the following formal compositions (in weight percent) 62-65% Si3N4, 15-20% SiC, and 14-16% free carbon. In general, the white materials obtained on pyrolysis in a stream of ammonia contained less than 1% carbon. 

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... 

Besides polysilazanes, PSCs are intensively investigated preceramic polymers for ternary Si-C-N materials. Their polymer backbone is composed of alternating Si-N=C=N nnits. Bis(silyl)carbodiimides, R3Si-N=C=N-SiR3 (R = alkyl) have been known since the early 1960s.Flowever, similar to low-molecular weight silazanes, they evaporate with heat treatment and are therefore not suitable as precursors for ceramics. PSCs were first obtained by Pump and Rochow ° in 1964 by metathesis reactions of dichlorosilanes and disilvercyanamide. 

Summary Precursor-derived quaternary Si-B-C-N ceramics frequently possess an enhanced thermal stability compared to SiC, SisN4 or Si-C-N ceramics. The stability of the materials towards crystallization and/or decomposition is directly coimected to the molecular structure and the elemental composition of the polymeric precursors. This paper highlights recent investigations on the synthesis of boron-modified polysilazanes and polysilylcarbodiimides. Hydroboration of polyvinylsilazanes and dehydrocoupling reactions of boron-modified silanes with ammonia or amines as well as cyanamide are described. It is shown that simple organosilicon chemistry provides a means to efficiently optimize ceramic yields and tune elemental composition as well as thermal properties of the polymer-derived ceramics. 

Another special type of silicone containing polymers is polysilazanes. These materials are used in for the preparation of high performance ceramics, silicon nitride, etc. Polysilazane can be prepared from substituted methylchlorodisilanes and gaseous ammonia in the following reaction [4] ... 

The possible compositions for the walls of the ordered mesoporous materials go beyond the field of inorganic chemistry, and materials with hybrid organosilica walls have been prepared [81-84], Some mesoporous benzene-silica hybrids are stable at a temperature higher than 500 °C [84], Mesoporous materials prepared from polysilazanes and nonionic surfactants can be activated to form silicon carbonilride ceramics, which retain an ordered mesoporosity up to 1500 °C [85],... 

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