Preceramic polymers synthesis

With the exception of reaction (3), the utility of all of the above reactions for the synthesis of oligo- and polysilazanes has been examined in varying detail. In the following paragraphs, we attempt to examine the pertinent studies in each area especially as it relates to the synthesis of preceramic polymers. 

Zank, G. A. Preceramic Polymer-Derived Silicon Oxycarbides. In Silicon-Containing Polymers. The Science and Technology of Their Synthesis and Applications Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 pp 697-726. 

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. 

The traditional synthesis route involves the direct reaction of silicon with nitrogen at temperatures above 1,300 °C, or by heating silica with carbon (coke) in a stream of nitrogen and hydrogen at 1,500 °C.41 However, as in the case of silicon carbide, the high processing and fabrication temperatures focused attention on the need for alternative access routes based on preceramic polymers. 

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. 

Keywords Hydroboration of Vinylchlorosilanes / Preceramic Polymers / Polyborosilazane / Thermal Stability / Synthesis... 

This chemical method has therefore opened new avenues for the synthesis of nonoxide ceramics with enhanced properties. The area of preceramic polymer chemistry is now about 20 years old. In this time a number of preceramic polymer systems have been developed. These systems include polymer precursors for Si3N4, SiC, AIN, BN, B4C, TiBa, and TiN. In this discussion, silicon-based nonoxide ceramics have been generally excluded they are dealt with more extensively in a related chapter by Professor Okamura. 

The following discussion covers the chemical synthesis of ceramics derived from organometallic polymers BN, AIN, TiN, TiC, and TiBa. It should be emphasized here that some of the syntheses involve starting materials, monomers, and intermediates, as well as polymers, that are oxidatively unstable and/or susceptible to hydrolysis. These syntheses therefore generally require inert atmospheres and the extensive use of vacuum (Schlenk type) line or dry-box techniques. This makes it obvious that collaborations between synthetic chemists and materials and ceramic scientists and engineers is important. Here we outline a selected number of synthetic routes to preceramic polymers. 

Characterization serves as an indispensable adjunct to synthesis and processing activities. To this end, the preceramic polymers must be intensively characterized and the cure kinetics of the pyrolytic processing studied extensively. The characterization portion is therefore generally composed of studies pertaining to (1) thermal processing and (2) materials properties. Table 1 is a summary of the pertinent instrumental techniques. 

Another area in which preceramic polymers can be utilized effectively is as binders for ceramic powders in near net shaping fabrication processes, such as compression or injection molding with subsequent sintering. Alternatively, an active filler and a polymer [67,68], as reported by Greil and Seibold, can be used in such fabrication. Other potential applications of preceramic polymers is in the general area of coatings, especially for carbon-carbon composites [69], and in the synthesis of nanostructured ceramic particles and composites [70-73]. 

DAVID WILSON is a research specialist with 3M and has more than 10 years of research and development experience in the synthesis of new ceramic fibers using sol-gel techniques and is the inventor of the Nextel 610 and Nextel 720 ceramic fibers. His experience includes the development of novel fiber precursor formulations, continuous fiber processing, and characterization of ceramic fibers at room and elevated temperatures. Current research activities include preceramic polymer-derived SiC fibers and MOCVD coating and composite fabrication for the Mullite Matrix Composite Consortium Program. Mr. Wilson holds four patents and is the author of several publications. 

Polymers included in this category are generally called polyborazylenes. Synthetic route to these polymers have been reviewed [8] thus, we provide an overview of typical synthesis procedures starting from borazine derivatives and leading to BN preceramic polymers. 

The Yajima et al. process [3] possesses general applicability to the preparation of ceramic materials from polymeric and oligomeric precursors via pyrolysis. In some cases even monomeric units can be used as precursors. Thus, the invention of the Yajima et al. process [3] has generated tremendous research activities in the synthesis of precursors and their pyrolytic conversion to ceramic powders and/or fibers, leading to the fields of what generally are known now as preceramic polymer chemistry and polymer pyrolysis technology [1,6]. 

Early work in the polymer pyrolysis technology area focused on the synthesis of preceramic polymers [7]... 

Silicon nitride has been obtained by the pyrolysis (in a stream of NHj) of the perhydropolysilazane prepared by the ammonialysis of the H SiClj-pyridine adduct [23, 24]. The gas stream employed during the pyrolysis of the preceramic polymer plays a crucial role [25]. Pyrolysis of [B, H,j diamine] polymers in an NHj stream gives BN [26]. TiN is similarly obtained by the pyrolysis of an amine precursor [27]. TiN has been prepared from titanazane [28]. Pyrolysis of Nicalon in NHj is reported to give SijN [24]. Besides single-phase ceramics, multiphase ceramics (e.g. composites of SiC and TiC, BiN and SijN ) have been prepared from precursors [29, 30]. Group 13 metal nitrides (GaN, AIN, hiN) have been prepared by the decomposition of urea complexes [31]. This method has been extended for the synthesis of BN, TiN and NbN [32]. 

The nonstoichiometric composition coupled with the microporosity and amorphous phase in the microstructure reduces the thermal stability of the fibers. Depending on the magnimde and duration of the applied stress, the use of the fibers may be limited to temperatures lower than 1000°C. To overcome this deficiency, recent efforts have been devoted to producing dense, polycrystalline, stoichiometric SiC through modification of the synthesis and processing conditions or through the synthesis of more useful preceramic polymers (63,67,68). 

Organometallic polymer precursors offer the potential to manufacture shaped forms of advanced ceramic materials using low temperature processing. Polysilazanes, compounds containing Si-N bonds in the polymer backbone, can be used as precursors to silicon nitride containing ceramic materials. This chapter provides an overview of the general synthetic approaches to polysilazanes with particular emphasis on the synthesis of preceramic polysilazanes. 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

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