Polymer pyrolysis

Sihcon carbide is also a prime candidate material for high temperature fibers (qv). These fibers are produced by three main approaches polymer pyrolysis, chemical vapor deposition (CVD), and sintering. Whereas fiber from the former two approaches are already available as commercial products, the sintered SiC fiber is still under development. Because of its relatively simple process, the sintered a-SiC fiber approach offers the potential of high performance and extreme temperature stabiUty at a relatively low cost. A comparison of the manufacturing methods and properties of various SiC fibers is presented in Table 4 (121,122). 

In our research at MIT we have found transition metal chemistry a very useful aid in the preparation of silicon-containing ceramics by the polymer pyrolysis procedure. 

Experiments with antimony metal and DBDPO clearly demonstrated for the first time that antimony volatilization was possible in the complete absence of any source of hydrogen at polymer pyrolysis temperatures. 

O Flaherty, E.J. and W.C.Thomas. 1962. The cardiotoxicity of hydrogen cyanide as a component of polymer pyrolysis smokes. Toxicol. Appl. Pharmacol. 63 373- 381. 

Chlorine trifluoride is used in rocket propellant incendiaries and in processing of nuclear reactor fuel. It also is used as a fluorinating agent and as an inhibitor of fluorocarbon polymer pyrolysis. 

The conventional industrial method for the synthesis of a-silicon carbide is to heat silica (sand) with coke in an electric furnace at 2,000-2,500 °C. However, because of the high melting point of the product, it is difficult to fabricate by sintering or melt techniques. Thus, the discovery of a lower temperature fabrication and synthesis route to silicon carbide by Yajima and coworkers in 197526,27 proved to be an important technological breakthrough. This is a preceramic polymer pyrolysis route that has been developed commercially for the production of ceramic fibers. 

Figure 12. Cyclolinear methylsiloxane polymers pyrolysis products separated in OV-1 gas chromatographic phase [80],...

The need to develop fibers with better microstructural stability at elevated temperatures and ability to retain their properties between 1000-2000°C. The requirements of fiber properties for strong and tough ceramic composites have been discussed by DiCarlo.83 A small diameter, stoichiometric SiC fiber fabricated by either CVD or polymer pyrolysis, and a microstructur-ally stable, creep-resistant oxide fiber appear to be the most promising reinforcements. 

Much information about polymer ignition may be drawn from literature devoted to ignitability of gas mixtures and from the data on polymer pyrolysis products at temperatures developed on the surface during their combustion. 

Preceramic Polymer Pyrolysis. Chemical options also are available for the pyrolysis step. Certainly, the rate of pyrolysis, that is, the time-temperature profile of pyrolysis, is extremely important. However, the gas stream used in pyrolysis also is of great importance. Inert- or reactive-gas pyrolysis can be carried out. 

Char compositions can be controlled quite precisely through the proper choice of polymer composition, pyrolysis temperature, and atmosphere, as well as through an improved understanding of the mechanisms of polymer pyrolysis. 

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