Ceramic precursors polysilazanes

Pyrolysis of Preceramic Polysilazanes Prepared by the Dehy-drocyclodimerization Reaction. Whatever the structure of the polysilazanes obtained by KH treatment of the cyclo-(CH3SiHNH) oligomers, these polymers are excellent ceramic precursors. Examination of the polymers from various preparations by TGA (thermogravimetric analysis) showed the weight loss on pyrolysis to be between only 15 and 20%. Pyrolysis appears... 

Understanding pyrolysis mechanisms as a function of substituting groups and configurations and the steric arrangements of preceramic polysilazanes is extremely important for the better design of future ceramic precursors. The assessment of the chemical pathway... 

A number of processible nitrogen-containing ceramic precursors are also discussed. These include polyalazanes (Jensen, Chapter 32), cross-linkable vinyl substituted polysilazanes (Schwark, Chapter 5), mixed sol-gels fi om aminolysis reactions (Gonsalves, Chapter 16), and polyborazines (Sneddon and Paine, Chapter 27 and Kimura, Chapter 28) which serve as precursors to AIN, Si3N4, BN/AJN composites, and BN, respectively. Arylene and alkylene bridged polysilsesquioxanes (Loy, Chapter 11) and carborane-polysiloxanes (Keller, Chapter 31) have bIso been employed to make modified silicas. 

A further extension of the concept of thermosetting polysilazanes with peroxide initiators has been the preparation of new polysilazanes with peroxide groups bound directly to the polymer 20). Potential advantages of a peroxide-substituted polysilazane over systems in which the peroxide is simply admixed with the polymer include 1) segregation of the peroxide upon storage cannot occur, 2) dissolution or dispersion of a peroxide in the polysilazane is not necessary, and 3) homogeneous distribution of e peroxide in solid, as well as liquid, polysilazanes is possible. We have prepared a new class of peroxide-substituted polysilazanes by the reaction of a hydroperoxide with a poly(methylvinyl)silazane. The liquid polymers may be thermoset, even with extremely low levels of peroxide substitution. This chemistry provides access to a class of polysilazanes previously unknown as ceramic precursors. 

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. 

The introduction of small amounts of boron into precursors that produce silicon nitride have been known to improve the ceramic yields of silicon nitride and Si—B—C—N ceramics as first reported in 1986.110 Several reports have appeared in the past couple of years alone that utilize borazine precursors such as 2,4-diethylb-orazine and other cyclic boron precursors, such as pinacolborane, 1,3-dimethyl-1, 3-diaza-2-boracyclopentane, for their reactions with silanes, polysilazanes, and polysilylcarbodiimides for the high-yield production of Si—B—N—C ceramics.111... 

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. 

Highly branched and even cross-linked polysilazane polymers are also desirable, particularly for the formation of ceramic powders. The transamination of Tris and vinyl-Tris with ammonia is a convenient process for the preparation of these precursors. The reactions were effectively catalyzed by strong acids such as trifluoroacetic acid ... 

Preparative organosilicon chemistry offers manifold possibilities for the synthesis of precursors for nonoxide ceramics (Scheme 18.1). The focus has been on the synthesis of polymers such as polysilanes A, polysilazanes B, polycarbosilanes... 

In the following, the synthesis of different types of organosilicon polymers as potential precursors for ceramics is highlighted topologically, starting from polysilanes with Si-Si linkages, followed by polycarbosilanes. Subsequently polysilazanes with Si-N building blocks and polymers with multi-element sequences, such as PSCs (Si-N=C=N), will be discussed. 

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... 

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. ... 

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. 

SCHEME 18.27 Synthesis of C-B-C bridged polysilazane T(2)l as precursor to high-temperature Si-B-C-N ceramics by ammonolysis of tris(dichloromethylsilyl-ethyl-enefborane. ... 

Besides the continuous fibers, application of metallorganic polymers to heat-resistant coatings, dense ceramic moldings, porous bodies, and SiC matrix sources in advanced ceramics via polymer infiltration pyrolysis (PIP) have been developed. Novel precursor polymers have been synthesized and investigated for ceramics in addition to PCS (Table 19.1). For SiC ceramics, various Si-C backbone polymers have been synthesized. Their polymer nature (e.g., viscosity, stability, cross-linking mechanism, and ceramic yield) are, however, fairly different from PCS. On the other hand, polysilazane, perhydropolysilazane, polyb-orazine, aluminum nitride polymers, and their copolymers have been investigated... 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

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. 

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