Silicon carbide fibers, polycarbosilanes

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... 

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 earliest work on silicon carbide fibers was done by Yajima and co-workers [3]. Yajima applied the Kumada [4] rearrangement to Burkhard s [5] dimethylpolysilane - an insoluble and infusible compound - (Eq. 1) and obtained by thermolysis at 400 - 450°C or by catalysis with polyborodiphenyl-siloxane at 350°C a melt spinnable and soluble polycarbosilane (Eq. 2). 

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. 

Polycarbosilanes have also attracted much interest as starting material for silicon carbide fiber production. Reactive metal such as Mg88, Cu86,89,90 and Al91 electrodes have been shown to be highly effective for electrochemical synthesis of polycarbosilanes, as shown in Table 21. 

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. 

Shimoo, T., Morisada, Y., Okamura, K., (2002), Active-to-passive oxidation transition for polycarbosilane-derived silicon carbide fibers heated in Ar-02 gas mixtures , J. Mater. Sci., 37, 1793-1800. 

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. 

Su, Z. M., Tang, M., Wang, Z. C., Zhang, L. T., Chen, L. F., Processing of silicon carbide fibers from polycarbosilane with polypropylene as the additive. Journal of the American Ceramic Society 2010,93(3), 679-685. 

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. 

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... 

M. Takeda, Y. Ymai, H. Ichikawa, T. Ishikawa, N. Kasai, T. Seguchi and K. Okamura, Thermal stability of the low oxygen silicon carbide fibers derived from polycarbosilane, Ceram. Eng. Sd. Proc., 13 [7-8], 209-217 (1992). 

M. Sugimoto, T. ShinxK), K. Okamura and T. Seguchi, Reaction mechanisms of silicon carbide fiber synthesis by heat treatment of polycarbosilane fibers cured by radiation I Evolved gas analysis, J. Amer. Ceram. Soc., 78 [4], 1013-17(1995). 

K. Yoshida, M. Imai and T. Yano, Improvement of the mechanical properties of hot-pressed silicon-carbide-fiber-reinforced silicon carbide composites by polycarbosilane impregnation, Compos. Sci. Technol., 61,1323-1329. 

As these experiments indicate, polysilanes can in some cases be converted to silicon carbide directly, without the necessity for formation of polycarbosilane, fractionation, or oxidation. For example, polysilastyrene copolymers can be formed into films or fibers and then crosslinked by irradiation with UV light. The crosslinked polysilane forms silicon carbide when heated to 1100°C in vacuum. (1U This method can be used in a "printing" mode, if a film of polysilane is cast onto a ceramic or metal substrate, then... 

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. 

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