Preceramic polymers product

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. 

Potential Non-Cvcllc Precursors of Preceramic Polymers. Boranes such as bis(trimethylsilyl(aminotrimethylsilylaminochloroboranes can be viewed as monomers for preceramic polymer and, ultimately, boron nitride production. Intermolecular dehydrohalogenation of this borane would be thus expected to yield either the dimer or the polymeric system. 

Hence, for most applications, high ceramic yield precursors are essential. Consequently, it is important to formulate a preceramic polymer that contains minimal amounts of extraneous ligands that allow it to meet the processability criterion and yet provide high weight percent conversions to ceramic product. Thus, in many of the precursors discussed... 

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. 

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. 

To have a useful preceramic polymer, considerations of structure and reactivity are of paramount importance. Not every inorganic or organometallic polymer will be a useful preceramic polymer. Although preceramic polymers are potentially high-value products if the desired properties result from their use, the more generally useful and practical systems will be those based on commercially available and relatively cheap starting monomers. 

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... 

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. 

Preparation of Preceramic Polysilazanes. At MIT (Massachusetts Institute of Technology), our initial research on silicon-based preceramic polymers was aimed at developing a precursor for silicon nitride. To this end, we studied the ammonolysis of dichlorosilane, H2SiCl2 (ii). This reaction had already been carried out on a millimolar scale in the gas phase and in benzene solution by Stock and Somieski in 1921 (12). We found that this reaction gave a much better yield of soluble ammonolysis product when it was carried out in more polar solvents such as dichloromethane or diethyl ether (ii). 

Thus the chemistry leading to the desired ceramic product is quite satisfactory most of the requirements mentioned earlier are met. Initial evaluation of the polysilazane shows that it has promise in three of the main potential applications of preceramic polymers in the preparation of ceramic fibers and ceramic coatings and as a binder for ceramic powders. 

Due to their three-dimensional architecture, additional cross-linking of poly(silsesquiazane)s prior to thermolysis is not required. Nevertheless, there are two drawbacks that limit their applicability as preceramic polymers. First, the difficult workup, that is, removal of the couple product NH4CI from the polymer is complicated and time intensive. Second, polysilsesquiazanes are difficult to... 

The above examples show that preparative organometallic chemistry allows for the production of a wide variety of silicon-based molecular precursors for high-temperature ceramics. The desired physical chemical properties and appropriate thermolysis chemistry can be realized by an intelligent precnrsor design. Nevertheless, there is still a need for further development for example, investigations into the synthesis of precursors that release phase-pure ceramics or composites with tunable composition and properties. The focns will also be on designing preceramic polymers, which release functional materials. In this field, very little investigation has been performed so far. 

In principle, the simplest way to produce preceramic polymers for ternary silicon boron nitrides is to coammonolize mixtures of silicon and boron chloride. Dietz has applied for a patent for such a process, with Si/B ratios ranging from 9 1 to 1 9. There are two major disadvantages of this approach (1) the polymer is only accessible as a mixture with the by-product, ammonium chloride, and (2) the ceramics obtained are composites constituted of the binaries BN and Si3N4 [49]. 

Riccitiello et al. have synthesized preceramic polymers with Si—B bonds in their backbones by a Wurtz analogous reaction of dialkyldichlorosilanes and boron halides, either with or without adding methyl iodide for the control of molar masses of the condensation products. Most of the polymers obtained are solid, and soluble in hydrocarbons. Even though the Wurtz reaction is not specific, the IR spectra of the polymer clearly indicate that Si—B bonds have formed preferentially, but do not provide any evidence for the presence of Si—Si or B—B bonds. Based on these results the authors suggest that the backbones generated mainly consist of an alternating sequence of Si and B [55-59]. 

P. Colombo, G. A. Danko, and R. Silberglitt, Production of SiC and SiOC from preceramic polymers comparison of microwave and conventional heating, in Ceramics - Getting into the 2000s, Part C (Section F), Techna Sri, Faenza, 1998, pp. 353-360. 

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