Preceramic polymers 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. 

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. 

The need for soluble or fusible precursors whose pyrolysis will give the desired ceramic material has led to a new area of macromolecular science, that of preceramic polymers [3]. Such polymers are needed for a number of different applications. Ceramic powders by themselves are... 

In the design of preceramic polymers, achievement of the desired elemental composition in the ceramic obtained from them (SiC and Si3N4 in the present cases) is a major problem. For instance, in the case of polymers aimed at the production of SiC on pyrolysis, it is more usual than not to obtain solid residues after pyrolysis which, in addition to SiC, contain an excess either of free carbon or free silicon. In order to get close to the desired elemental composition, two approaches have been found useful in our research (1) The use of two comonomers in the appropriate ratio in preparation of the polymer, and (2) the use of chemical or physical combinations of two different polymers in the appropriate ratio. 

Summary A brief review of the preparation of silicon containing preceramic polymers to prepare silicon carbide and silicon carbonitride fibers is given. Methylchlorodisilanes are converted to polysilanes and polysilazanes which yield ceramic fibers after meltspinning, curing, and pyrolysis. 

A variety of other ceramics are prepared by pyrolysis of preceramic polymers.32,38 Some examples are silicon carbide, SC, tungsten carbide, WC, aluminum nitride, AIN, and titanium nitride, TiN. In some cases, these materials are obtained by simple pyrolysis in an inert atmosphere or under vacuum. In other cases a reactive atmosphere such as ammonia is needed to introduce some of the atoms required in the final product. Additional details are given in Chapter 9. 

Figure 9.2 Illustrating the crucial need for cross-linking the preceramic polymer at an early stage in the pyrolysis to prevent loss of material in the form of volatile decomposition or depolymerization products. 

In the design of preceramic polymers, achievement of the desired elemental composition in the resulting ceramics (SiC and Si3N4 in the present cases) is a major problem. For instance, in the case of polymers aimed at the production of SiC on pyrolysis, solid residues usually are obtained after... 

In the pyrolysis of a preceramic polymer, the maximum temperature used is important. If the maximum temperature is too low, residual functionality (C-H, N-H, and Si-H bonds in the case of polysilazanes) will still be present. On the other hand, too high a pyrolysis temperature can be harmful because of solid-state reactions that can take place. For instance, if the polysilazane-derived silicon carbonitride contains a large amount of free carbon, a high-temperature reaction between carbon and silicon nitride (equation 1) (7) is a possibility. 

Characterization of Preceramic Polymers. The study of the pyrolysis products of preceramic polymers is not always straightforward. If crystalline species are produced (e.g., SiC and Si3N4 in the case of polysilazanes), then their identification by X-ray diffraction presents no problems. 

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