Polymer silicon carbide fibers

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). 

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

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. 

The discovery by Yajima that polysilanes could be pyrolyzed to silicon carbide was mentioned in the introduction.7 In this process, either (Me2Si) or the cyclic oligomer (Me2Si)6 are synthesized from Me2SiCl2 and are then heated to near 450 °C (Scheme 5.10). This discovery has been commercialized by the Nippon Carbon Co. for the production of NICALON silicon carbide fibers. In this process, methylene groups become inserted into many of the Si-Si bonds to give a polycarbosilane polymer with the idealized 5.14. 

Fine-diameter, polymer-derived silicon carbide fibers generally have high... 

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

Sacks, M.D., A.A. Morrone, G.W. Scheiffele, and M. Saleem. 1995. Characterization of polymer-derived silicon carbide fibers with low oxygen content, near-stoichiometric composition and improved thermomechanical stability. Ceramic Engineering and Science Proceedings 16(4) 25-35. ... 

Silverman, L.A., W.D. Hewett, T.P. Blatehford, and A.J. Beeler. 1991. Silicon carbide fibers from slurry spinning. Journal of Applied Polymer Science Applied Polymer Symposium 47 99-109. ... 

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. 

Idesaki, A., Narisawa, M., Okamura, K., Sugimoto, M., Morita, Y., Seguchi, T., Itoh, M., Application of electron beam curing for silicon carbide fiber synthesis from blend polymer of polycarbosilane and polyvinylsilane. Radiation Physics and Chemistry 2001,60(4-5), 483-487. 

Hi-Nicalon/Celsian composites are stable up to use temperature of 1100°C in oxidizing environments and degrade at higher temperatures due to the instability of polymer-derived fibers. The stability of Celsian matrix composites may be extended to higher temperatures by more uniform and stable interface coating(s) and by reinforcement with more advanced silicon carbide fiber (Sylramic) for applications as hot components (combustion liner, air foil, nozzle, etc.) in turbine engines. 

Yajima [76] was the first to study the preparation of silicon carbide fibers from carbosilanes. These and other SiC-containing polymers were used to produce SiC powders with a crystallite size as small as several nanometers [77, 78]. The advantage ofthe production route from liquid to solid to produce SiC has also attracted attention for SiC film production in microelectronics or as protection layers. In this way, amorphous, polycrystalline films of high purity produced by the dip-coating of substrates in PCS solutions and subsequent pyrolysis in an inert gas atmosphere, have been prepared [115]. 

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. 

The generic polymer based process that yields oxygen containing silicon carbide fibers consists of five steps. (1) Polydimethylsilane, or PDMS, is synthesized. (2) PDMS is rearranged into polycarbosilane, or PCS. (3) PCS is melt spun and yields a solid, green, or... 

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. 

W. Toieki, C. D. Batich, M. D. Sacks, M. Saleem, G. J. Choi and A. A. Morrone, Polymer-derived silicon carbide fibers with low oxygen content and improved thermo-mechanical stability. Composites Sd. and Technology, 51,145-159 (1994). 

J. Lipowitz, J. A. Rabe, G. A. Zank, Y. Xu and A. Zangvil, Nanocrystalline silicon carbide fibers derived from organosilicon polymers, in Chemical Processing of Advanced Materials, L. L. Hench and J. K. West eds., John Wiley, New York. 767-776 (1992). 

Yajima et al. have reported the synthesis of continuous silicon carbide fibers from an organic polymer [30]. SiC fibers have been commercially produced by Nippon Carbon Co. under the trade name Nicalon , and similar SiC fibers containing Ti are produced by UBE Industries under the trade name Tyranno fiber [31]. 

The commercially produced continuous and multifilament Nicalon (Hercules) fiber is produced from polydimethylsilane however other organosilicon polymers have been used for the production of silicon carbide fiber. Polydimethylsilane is first distilled to remove the low molecular weight components, and polymer of average molecular weight 1500 is melt spun at 280°C and cured in air at 200°C. The fiber is then heat treated between 800 and 1500°C in nitrogen or vacuum. Optimum mechanical properties are achieved at ca 1250°C. Listed properties of the Nicalon fiber are modulus 200 GPa and tensile strength 2.8 GPa (1). 

Composites frequently consist of a material with embedded fibers, which in the example just given is epoxy plastic with embedded carbon fibers. The epoxy plastic is strong but subject to fracture. The embedded fibers impede a fracture that might start, thus enhancing the stability of the structure. Another example of a composite is silicon carbide ceramic with embedded silicon carbide fibers. These fibers are made by pyrolysis of a polymer of dimethylsilane, (CH3)2SiH2. By itself, silicon carbide ceramic is brittle, like most ceramics. The silicon carbide fibers, however, break up any fracture lines that may form, making the composite much less brittle. 

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