Silicon carbide precursors

R. West, X.-H. Zhang, I. P. Djurovich, andH. Stuger, "Crosslinking of Poly silanes as Silicon Carbide Precursors," in L. L. Henck andD. R. 

Other more complex polymers have been employed as silicon carbide precursors. For instance, the mixture of methylchlorodisilanes obtained as a by-product in the direct synthesis of Me2SiCl2 can be redistributed with catalysts to give a polycyclic, partially cross-Uuked polymer with the approximate composition shown in equation (44). Pyrolysis of this precursor produces silicon carbide in good yield. Partially cross-linked polymers made by condensing vinylmethyldichlorosilane and other methylchlorosilanes with sodium also are efficient precursors for silicon carbide. 

Several silicon carbide precursors were tested on the AFOSR/DARPA study, e.g., polycarbosilanes (PCSs) with backbones... 

The copolymer (PhMeSi) (Mc2Si) is also used as a silicon carbide precursor, especially for monolithic formed objects. 

The potential of silicon carbide pre-ceramic compounds was recognized especially after Yajima et al. had prepared silicon carbide-based ceramic fibers [1]. The development of tailor-made silicon carbide precursor molecules has to be seen in close relation to the tremendously fast growth of the field of... 

Scheme 1 presents important synthesis routes of silicon carbide precursor molecules. 

Applications silicon carbide precursor, wear coating and sealer coating for ceramic composites and precursor for porous composites for high-temperature filters and catalysts ... 

With the screening results obtained with the polycarbosilane it had become obvious that a polymer or a series of polymers that had bodi controllable carbon levels in their ceramic chars and respectable char yields (ca. greater than 40 %) was needed. By combining portions of two previous studies (vida infra) we began to investigate the use of silazanes as silicon carbide precursors and their use as binders for SiC powders. 

Since the development of the first useful organosilicon compound, a silicon carbide precursor, by Yajima [11], there has been extensive research on the design of polymers which transform to silicon—containing ceramics upon pyrolysis. In recent years, progress has been made in the synthesis of polymeric precursors of silicon nitride. Seyferth and coworkers [12] developed a methylsilazane compound, basically an ammonolysis product of methyldichlorosilane, and reported the... 

Vapor decomposition (14,15) iavolves dryiag, decomposiag, and vaporising a spray of salt precursor solution ia a plasma, and subsequentiy nucleating and growing ceramic particles ia the vapor. Silicon carbide [12504-67-5] SiC, powder is produced by this method. 

Turning now to other types of ceramic fibre, the most important material made by pyrolysis of organic polymer precursors is silicon carbide fibre. This is commonly made from a poly(diorgano)silane precursor, as described in detail by Riedel (1996) and more concisely by Chawla (1998). Silicon nitride fibres are also made by this sort of approach. Much of this work originates in Japan, where Yajima (1976) was a notable pioneer. 

The first type of polycarbosilane synthesized by using ADMET methodology was a poly[carbo(dimethyl)silane].14c Linear poly(carbosilanes) are an important class of silicon-containing polymers due to their thermal, electronic, and optical properties.41 They are also ceramic precursors to silicon carbide after pyrolysis. ADMET opens up a new route to synthesize poly(carbosilanes), one that avoids many of the limitations found in earlier synthetic methods.41... 

C3Hg) at 1350°C, ormethane (CH4) at 1800°C. Methyltrichlorosilane (CH351013) is the preferred precursor for silicon carbide. 

The process competes with the traditional method of fiber production in which the precursor material is melted, usually in an arc furnace, then drawn through spinnerets and spun or impinged by high pressure air. The melt-spin process is not well suited to materials with high melting points such as zirconia, silicon carbide, or pure alumina. 

The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of silicon atoms, which show unique properties resulting from easy delocalization of sigma electrons in the silicon-silicon bonds. Polysilanes may be useful as precursors to silicon carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors. 

Possible ways in which polysilanes may be useful include, 1. As precursors to silicon carbide ceramics 2. As photoinitiators in radical reactions 3. As photoconductive materials, and 4. As photoresists in microelectronics. The last of these uses will be treated in the chapter by Miller,(31) and so will not be covered here. 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

Another process for silicon carbide fibers, developed by Verbeek and Winter of Bayer AG [45], also is based on polymeric precursors which contain [SiCH2] units, although linear polysilmethylenes are not involved. The pyrolysis of tetramethylsilane at 700°C, with provision for recycling of unconverted (CHg Si and lower boiling products, gave a polycarbosilane resin, yellow to red-brown in color, which was soluble in aromatic and in chlorinated hydrocarbons. Such resins could be melt-spun but required a cure-step to render them infusible before they were pyrolyzed to ceramic... 

Soluble polydiorganosilane homo and copolymers have recently shown great potential in such areas as precursors for the preparation of silicon carbide fibers (1), as photoinitiators in alkene polymerization (2), as photoconductors (3), and as positive or negative self-developing photoresists for photolithographic applications (4). A number of copolydiorganosilane copolymers have been reported recently (5) in which the copolymer contained equal amounts of both monomers in the feed. 

The first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by S. Yajima and his coworkers at Tohoku University in Japan [5]. As might be expected on the basis of the 2 C/l Si ratio of the (CH3)2SiCl2 starting material used in this process, the ceramic fibers contain free carbon as well as silicon carbide. A typical analysis [5] showed a composition 1 SiC/0.78 C/0.22 Si02- (The latter is introduced in the oxidative cure step of the polycarbosilane fiber). 

The Yajima polycarbosilane, while it was one of the first, is not the only polymeric precursor to silicon carbide which has been developed. 

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