Polycarbosilane silicon carbides

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... 

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... 

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

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. 

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. 

An important application of polydimethylsilane is as a source of silicon carbide (SiC) fibres, which are manufactured under the trade-name Nicalon by Nippon Carbon in Japan. Heating in an autoclave under pressure converts polydimethylsilane to spinnable polycarbosilane (-Me2Si-CH2-) with elimination of methane. The spun fibres are then subjected to temperatures of 1200-1400 °C to produce silicon carbide fibres with very high tensile strengths and elastic moduli." As a result of their conductivity, polysilanes have also been used as hole transport layers in electroluminescent devices. In addition, the photoconductivity of polymethylphenylsilane doped with Cgo has been found to be particularly impressive. ... 

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. 

Summary Polysilacarbosilanes and polysilasilazanes prepared according to a copolymer strategy offer an easy, coherent approach to polycarbosilanes and silazanes, precursors of SiC and SiCN-based materials with variable C/Si and C/Si/N ratios. In contrast with the polysilazane route which leads, upon pyrolysis, to carbon-containing silicon nitride, the synthesized polycarbosilazanes are finally converted into nitrogen-containing silicon carbide. 

Although it was one of the first, the Yajima polycarbosilane is not the only polymeric precursor to silicon carbide that has been developed. Another useful system that merits mention is the polysilane developed by Schilling and his co-workers at Union Carbide Corporation (iO). As already mentioned, a useful polymeric precursor for silicon nitride has been developed more recently by workers at Dow Corning Corporation (8). 

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). 

Synthesis by hydrosilylation of (R )(R2)SiH(HC=CH2) (R, R = H, Cl, alkyl, aryl, NR2) and the suitability of polycarbosilanes [(R )(R )Si-C2H4] obtained as precursors for silicon carbide/carbon composites were thoroughly investigated by Corriu et al. ... 

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. 

We here describe the synthesis of the polycarbosilane (H2SiCH2CH2) and its use as a preceramic compound for the preparation of silicon carbide in form of a powder or fibers. 

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

Polycarbosilanes can exhibit thermal stabilities greater than the analogous polysiloxane , contrary to their relative bond energies (Si—C, 78 kcalmol Si—O, 106 kcal mol" ). Thus polydimethylsilane is polymerized by heating the hexamer to 300-500°C, forming a product with thermal stability greater than 500°C and that can be spun into fiber, which in turn can be thermally oxidized, forming j5-silicon carbide, SifCHj) -,. 

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