Silicon carbide fibers, process

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). 

In the silicon carbide manufacturing process the major bioactive dusts identified are quartz particles and silicon carbide fibers generated in the process. In contrast to the silicon carbide fibers, silicon carbide particles were... 

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. 

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

In the final step of the multistep process, the carbon in the fiber is converted to graphite by heating at 1400-2500°C. Similarly, silicon carbide fiber can be made by heating fibers that contain long-chain molecules with alternating silicon and carbon atoms ... 

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. 

Silicon carbide fiber must be regarded as a major development in the field of ceramic fibers during the last quarter of the twentieth century. In particular, a process developed by the late Professor Yajima in Japan, involving controlled... 

F me 6.17 Chemical vapor deposition (CVD) process of making silicon carbide fiber. 

Another CVD type silicon carbide fiber available commercially, is called Sigma fiber. Sigma fiber filament is a continuous silicon carbide monofilament obtained by CVD on a trmgsten substrate. Figure 6.19 gives the flow diagram for the fabrication of this process. 

This process is carried out in Japan by Nippon Carbon Co. to make NICALON silicon carbide fibers, with high tensile strength and excellent temperature and oxidation resistance. It can also be used to generate coatings and solid objects. Modifications of the basic process include the addition of borosiloxanes as catalysts, and the incorporation of titanium, in the form of titanium alkoxides, to produce fibers containing titanium and oxygen as well as silicon and carbon. 

Silicon carbide fibers can be manufactured using the same process as boron fibers (see Section 5.2.7.2), if methyltrichlorosilane is used as the starting material. Deposition from the gas phase proceeds according to the following equation ... 

A continuous CVD fiber coating process is being explored for the preparation of a low cost, high strength, thermally stable silicon carbide fiber tow. By depositing a 5 pm thick layer of stoichiometric SiC onto each filament of a carbon fiber tow it is possible to prepare fibers that are 89 vol.% SiC which have twice the tensile strength of the commonly employed Nicalon fiber. In addition, the CVD fiber has superior resistance to creep. An economic analysis indicates that the fibers could be produced for 50/lb compared to 300/lb for Nicalon. 

Ichikawa, H., I. Yoshikazu, M. Takeda, and J. Sakamoto. 1995. Process for producing silicon carbide fibers. European Patent Publication 0653391A1 (Application No. 94117853.5). 

Laine, R.M., Z.F. Zhang, K.W. Chew, M. Kannisto, and C. Scotto. 1995. Synthesis and processing of silicon carbide fibers state of the art. Pp. 1979 1986 in Ceramic Processing Science and Technology, Vol. 51 in Ceramic Transactions, H. Hausner, G.L. Messing, and S. Hirano (eds ). Westerville, Ohio American Ceramic Society. 

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. 

As a result, the strength level for a stoichiometric or 100% converted silicon carbide yam was reported to be 1.11 GPa (uncoated) and 1.60 GPa (coated), and that for a 90% converted yarn was reported to be 1.70 GPa (uncoated). The individual silicon carbide fibers, multifilament yarns and fabric samples which were obtained were suitable for evaluation of mechanical properties from room temperature to 1500 C. A commercial process for the fabrication of continuous multi-filament silicon carbide yarns by this route has not yet been reported [29]. 

Chemical vapor infiltration (CVI) of carbon fibers with silicon monoxide provides a textbook example (Figure 14) about the relationship between percent conversion of carbon to silicon carbide (a process variable) and fiber strength at elevated temperatures (a product variable). 

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... 

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). 

Until recently, the great majority of ceramic fibers were made from oxides such as alumina or mullite. But in the last few years, much woric has been done to develop practical processes for the production of other fiber materials, especially the refr actory carbides and nitrides. This work is beginning to bear results especially with silicon carbide fibers vt4iich have now reached full-scale production. Other materials such as silicon nitride, boron nitride, aluminum nitride, titanium carbide, hafriium carbide, and hafiiium nitride are at die development stage or in pre-production.l d... 

Coatings were applied to bend bars made from various commercial types of silicon nitride or silicon carbide, fiber-reinforced, silicon carbide matrix composites. In some cases the silicon nitride bend bars had as-processed surfaces, but in most cases the surfaces were machined and ground. The surfaces of the SiCf/SiCn, bend bars were cleaned prior to coating, but no other treatment was applied to them. 

Aerospace applications of ceramic matrix composites to date have been limited. Perhaps the most significant are the aircraft engine flaps used on a French fighter. There are two types. Both use silicon carbide matrices. One is reinforced with carbon fibers, and the other with a multifllament silicon carbide fiber. Another application is a missile diverter thruster made of carbon fiber-reinforced silicon carbide. Again, the process used to make this part is CVI. The Space Shuttle Orbiter thermal protection system (TPS) makes extensive use of tiles composed of a three-dimensional network of discontinuous oxide fibers with silicate surface layers. While there is no continuous matrix for most of the tile, the surface region is a form of CMC. In a sense, this can be considered to be a type of functionally graded material. 

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