Silicon-containing boron polymers

The most studied class of boron polymers containing boron-silicon bonds are the carboranylenesiloxane polymers. The history of these polymers was discussed at the beginning of this chapter. The more recent advances in this group of polymers and in silicon-containing borazine and boroxine polymers are discussed in later sections of this chapter. In this section, some remaining groups of silicon-containing boron polymers are discussed. 

Boron-containing nonoxide amorphous or crystalline advanced ceramics, including boron nitride (BN), boron carbide (B4C), boron carbonitride (B/C/N), and boron silicon carbonitride Si/B/C/N, can be prepared via the preceramic polymers route called the polymer-derived ceramics (PDCs) route, using convenient thermal and chemical processes. Because the preparation of BN has been the most in demand and widespread boron-based material during the past two decades, this chapter provides an overview of the conversion of boron- and nitrogen-containing polymers into advanced BN materials. 

In general, modification or after treatment of a given silicon containing polymer with an appropriate boron compound is a widely used method for the preparation of polymers containing silicon and boron. Already in 1984, Takamizawa et al. reported on the reaction of polydimethylsilane with B-trimethyl-N -triphenylborazine. Thermal treatment of a mixture of these educts results in decomposition and polycondensation reactions yielding an organoborosilicon polymer [65, 66]. 

In the last two decades, a new process has been developed, very similar to one used for carbon fibers [YAJ 76]. It consists of a thermal decomposition or pyrolysis of organometallic polymer fibers containing silicon or boron. This method is used to manufacture long weavable fibers with low diameter (8-15 pm), controlled composition and state of crystallization. This process generally includes five stages summarized below for silicon carbide fiber, which is currently the most developed. 

Finally, oxidation of boron-, silicon-, and aluminum-containing materials results in oxide particles. Along with polymers in the oil, these particles can plug lubrication ducts, and cause pump failure (755). An oil filter is often used to remove such particles, thereby extending pump life and increasing the time... 

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

Boron-containing organosilicon polymers have also been synthesized via a sodium coupling reaction of silicon and boron halides or alkyl boron halides in hydrocarbon solvents [37]. On pyrolysis, mixed ceramics comprising SiC, SiBjt, and Si-(B, C) can be obtained. 

A somewhat unconventional access to preceramic polymers containing silicon and boron has been reported by Schmidt et al. A borazine polymer and a silazane polymer are separately synthesized and dissolved in toluene, and then mixed together. The subsequent crosslinking of the borazine with the silazane components proceeds through hydrogen elimination and transamination reactions [54]. 

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