Silicon carbide fiber precursor

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 first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by Yajima and his co-workers at Tohoku University... 

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. 

Yajima, S.. Ha.segawa, Y., Hayashi, J., limura, M. (1978). Synthesis of continuous silicon carbide fiber with high tensile strength and high Young s modulus, part I, synthesis of polycarbosilane as precursor. J. Mater. Sci. 13, 2569-2576. 

Twenty-five years later, Burhard reported the preparation of permethylated. polysilane (2). These materials were, however, highly crystalline, insoluble white solids which evoked little scientific interest until recently when it was discovered that silane polymers could be used as thermal precursors to / -silicon carbide fibers (3-5). In this regard, Yajima and co-workers reported that poly (dimethyl) silane could be converted by the two-step process shown below to / -silicon carbide, a structural material of considerable industrial importance. 

Another silicon carbide multifilament fiber, made via a polymeric precursor by Dow Corning Corp., US A, is called Sylramia According to the manufacturer, this textile grade silicon carbide fiber has a nanocrystalline, stoichiometric... 

Based on this approach, families of silicon carbide fiber and silicon ceramic composites are now being routinely produced based on polycarbosilane precursors [22] These new materials are finding a wide range of new applications, for example, as a hot zone component in the next generation turbojets where silicon carbide composite components now routinely service at operating temperatures well in excess of 1000°C under high static thrusts (up to 50,000 psi) and high sonic pressure (up to 800 DB). No metallic components survive under these conditions. 

Silicon Carbide Fibers from Organic Precursors... 

Among other topics, his research interests include the micro- and nanostructure of ceramic fibers derived from organosilicon precursors and polycrystalline silicon carbide fibers derived from organosilicon polymers. Dr. Lipowitz has nearly 20 U.S. patents and more than 50 publications to his credit. He is a member of several professional and honorary societies, including the American Ceramic Society, the Materials Research Society, Sigma Xi, and the New York Academy of Science. 

Narisawa, M., Idesaki, A., Kitano, S., Okamura, K., Sugimoto, M., Seguchi, T., Itoh, M., Use of blended precursors of poly(vinylsilane) in polycarbosilane for silicon carbide fiber synthesis with radiation curing. Journal of the American Ceramic Society 1999,82(4), 1045-1051. 

Polymer Pyrolysis-Derived Silicon Carbide Fibers (PP-Fibers) As shown in 1976 by Yajima [76], P-SiC fibers with a smaller diameter (8-30 pm) and without a central core can be manufactured by the solid-state pyrolysis of a PCS precursor fiber. 

Many continuous inorganic fibers can be formed directly from the liquid phase including melts (Chapter 4) and solutions (Chapter 5). Ceramic aluminate fibers are indirectly derived from a viscous liquid phase, which includes dispersions and sol-gels. These fibers as well as carbon fibers and silicon carbide fibers are initially obtained as nonfunctional precursor fibers. Since the final functional fibers are actually derived from solid precursor fibers, they will be covered in Chapters 8 to 10 dealing with advanced inorganic fibers derived from the solid phase. 

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). 

When the pyrolysis of PCS precursor fibers is carried out in the presence of hydrogen, or when boron or aluminum doped PCS precursor fibers are pyrolyzed, it is possible to obtain quasi-stoichiometric silicon carbide fibers. Alternatively, quasi-stoichiometric fibers are also obtained from precursor fibers consisting of SiC powder reinforced polymers. 

USGS. The delocalization of a electrons in polysilanes gives rise to unique electronic and optical properties. Also, several polysilanes have been foimd to function as useful thermal precursors to silicon carbide fibers and these materials have attracted attention with respect to microlithographic applications and as polymerization initiators (1,27,28). The use of these materials as hole transport layers in electroluminescent devices has also been explored (42). Indeed, the photoconductivity of poly(methylphenylsilane) doped with Geo has been studied and has been found to be comparable with the best materials available (43). 

In the field of thermostmctural composites, silicon carbide occupies a privileged place, whether it is for the production of ceramic matrices or fibers. Silicon carbide fibers prepared from organometallic precursors are the most stable at high temperature in oxidizing atmosphere. 

Yajima, S., Hasegawa, Y, Okamura, K., Matsuzawa, T. (1978b). Development of high-tensile strength silicon-carbide fiber using an organosiliconpol3mier precursor. Nature, 273(5663), 525-527. doi 10.1038/273525a0. 

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. 

Other organosilicon polymer precursors for ceramics have either been prepared or improved by means of transition metal complex-catalyzed chemistry. For instance, the Nicalon silicon carbide-based ceramic fibers are fabricated from a polycarbosilane that is produced by thermal rearrangement of poly(dimethylsilylene) [18]. The CH3(H)SiCH2 group is the major constituent of this polycarbosilane. 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

---