Silicon carbide/carbon fibers

Wawner, F.E., Jr. (1988). Boron and silicon carbide/carbon fibers. In Fiber Reinforcements for Composite Materials (A.R. Bunsell cd.), Elsevier, Amsterdam, pp. 371-425. 

The hot fiber (wire) CVD process has been commercially used for 30 years to produce continuous sheath/core bicomponent boron/tungsten and silicon carbide/carbon fibers. Since they are continuous fibers, they are discussed in Chapter 3.3. More recently, this process was used to produce discontinuous, i.e., short, experimental sheath/core diamond/carbon fibers by depositing a thick diamond sheath on short pieces of a potentially carbon fiber. 

The technology of manufacturing sheath/core bicomponent boron/ tungsten, boron/carbon, silicon carbide/tungsten, and silicon carbide/ carbon fibers is 40 years old and the relationships between process variables, structures, and properties have been authoritatively described in important review articles. One article deals mainly with their preparation [33] another correlates process variables with structures [34], and one explores potential correlations between structures and properties [30]. 

Continuous sapphire fibers (Chapter 4) and continuous sheath/core bicomponent silicon carbide/carbon fibers (Chapter 3) offer impressive performance as reinforcing fibers and in ceramic and metal matrix composites. Here are some noteworthy commonalties and differences. 

A considerable amount of subsequent research and process development has been carried out to produce silicon carbide with a reduced level of excess carbon via processes that allow more facile cross-linking.2 -32 Several hundred papers and patents on this topic exist in the literature, and only a few examples will be mentioned here. One process development involves the slurry spinning of fibers in place of melt spinning.33 In this process, silicon carbide powder, made by a conventional industrial process, is dispersed in a solution of carbosilanes in toluene. The syrupy paste is spun into fibers and then pyrolyzed to silicon carbide. These fibers are reported to be stable at 1,500 °C for 120 hours. 

The earliest work on silicon-carbide-related fibers was by Verbeek and Winter (4). Using the principles developed earlier by Fritz and co-workers (5), Verbeek and Winter (4) reported that the high-temperature pyrolysis of tetramethylsilane or methylchlorosilanes gives branched polycarbosilane (PCS) polymers containing a structure with alternating silicon and carbon atoms (equation 1). 

E. Tani, K. Shobu, and K. Kishi, Two-dimensional-woven-carbon-fiber-reinforced Silicon Carbide/Carbon Matrix Composites Produced by Reaction Bonding, J. Am. Ceram. Soc., 82 [5] 1355-57(1999). 

The major deficiency of carbon fibers is their sensitivity to oxidation even at relatively low temperatures. Although silicon carbide (SiC) fibers are also sensitive to oxidation, their oxidation starts at higher temperatures and yields a protective silica coating. In an oxidative environment, SiC and Si-C-0 fibers are generally more useful than carbon fibers [1-3]. Large diameter silicon carbide fibers are obtained by chemical vapor deposition (Chapter 4). Small diameter silicon carbide and oxycarbide fibers are derived from solid polydimethylsilazane precursor fibers (this chapter). 

The most mature CMCs consist of silicon carbide matrices reinforced with silicon carbide-based fibers (SiC/SiC) and silicon carbide reinforced with carbon fibers (C/SiC). 

The facile formation of ceramic materials from molecules has undoubtedly been one of the si ificant contributions made by chemistry to materials science (7). However, it is desirable not only to produce the ceramic per se but also to do so in a specific form, for example a fiber. Therefore, one of the key requirements for any ceramic precursor should be its processability. For this reason, there has been continued research effort aimed at the design of precursors with physical properties suitable for processing prior to pyrolysis. Two examples with sigr cant commercial application are polyacrylonitrile and polyorganosilanes, both of which may be spun into fibers, and upon pyrolysis allow for the manufacture of carbon-graphite (2) and silicon carbide (5) fibers, respectively. Despite much effort, the extension of this polymer-type precursor strategy to other ceramic systems has only met with limited success. 

Manufacture of P-Silicon Carbide. A commercially utilized appHcation of polysdanes is the conversion of some homopolymers and copolymers to siHcon carbide (130). For example, polydimethyl silane is converted to the ceramic in a series of thermal processing steps. SiHcon carbide fibers is commercialized by the Nippon Carbon Co. under the trade name Nicalon (see Refractory fibers). 

Non-metallic Materials Carbides, carbon, ceramic fiber, ceramic, cermet, composite, cork, elastomer, felt, fiber, glass, glycerin, non-metallic bearing material, rubber (natural), rubber (synthetic), silicone, wood, leather. 

Thistable(and Table 19.2below) shows that the major competitor to CVD SiC is carbon as both fibers have similar properties and are in the same cost bracket. Another competitor is boron but it is expensive and may eventually be replaced by silicon carbide. 

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). 

A great potential for new compounds is provided by structures with two carbon and two silicon atoms around the central silicon. These polysilanes with organic groups lead to silicon-carbide ceramics. A wide field of application would be opened up if one could make a polysilane as a plastic mass which could be extruded and modeled and if after pyrolysis silicon-carbide is formed without a strong contraction (this means a high ceramic yield). Polysilane fibers are only one product in a range of many... 

Carbon-doped silicon carbide, 22 535 Carbon electrodes, 12 305, 752-758 furnaces using, 12 753 grades of, 72 754 in open-arc furnaces, 72 301 production of, 72 755 properties of, 72 755-756 Carbon elimination, in steelmaking, 23 258 Carbon fiber, 77 214-215 Carbon fiber ceramic-matrix composites, 26 773... 

Pyrolysis analogous to polymer carbon formation has also been applied to methylchlorodisilane. This is converted to beta silicon carbide fibers of high tenacity. 

Fitzer. E., Fritz. W. and Gadow, R. (1984). Carbon fiber reinforced silicon carbide. In Proc. International Symp. on Ceramic Components for Engineering (S. Somiya, E. Kanai and K. Ando, eds.), Elsevier. London, pp. 505-518. 

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