Pyrolysis boron carbides

Other monomeric precursors similar to 6-hexynyl-decaborane such as 6-norbornenyl-decaborane (129) and 6-cyclooctenyl-decaborane (131) (Fig. 75) underwent ROMP in the presence of either first- or second-generation Grubbs catalysts to produce the corresponding poly(norbornenyl-decaborane) (130) (Fig. 75) and poly(cyclooctenyl-decaborane) (132) (Fig. 75) with Mn > 30 kDa and polydis-persities between 1.1 and 1.8.152 Electrostatic spinning and pyrolysis of poly (norbomenyl-decaborane) was discovered to produce nanoscale, free-standing porous boron-carbide/carbon, ceramic fiber matrices.153... 

On heating in air at 10°C per min, poly(m-carborane-siloxane) shows typically only 4% mass loss at 450°C and 7% mass loss at 600°C (see Fig. 4). In comparison, siloxanes without carborane units, show an approximate 50% mass loss at 450°C. As a consequence of the relatively high boron and carbon content of these materials, pyrolysis is expected to generate ceramic residues of boron carbide/silicon carbide. 

It is especially important to CTeate and develop terminology of nanochemistry as a part of a new area of science - nanology, or the science of the nanoworld (nanologists prefer this term to a more widely used word nanoscience). Figures 3.2-3.8, show the diversity of the morphology of nanostructures of carbon, sihcon and boron carbides, which were synthesised via hydrocarbon pyrolysis [2-5] or from elanental substances [6-10]. Morphologies of carbon nanotubes thus obtained are very unusual (Fig. 3.2). 

For the first time we have discovered transparent (painted in various colours) thread-like crystals of carbon among the products of hydrocarbon pyrolysis and during synthesis of silicon and boron carbides (Fig. 3.6) [12]. The X-ray spectral analysis has shown that the transparent threads consist of carbon (Fig. 3.7). 

The pentaborane cage structure -B5H9- has been used as a side group in the preparation of vinyl-type polymers, but only of relatively low molecular weight. Pyrolysis of this material gives primarily boron carbide, B4C. 

A preceramic, carrier polymer route to boron carbide has been reported via the pyrolysis of a polynorbomene that bears decaborane side groups.69 An important feature of this development is the ability to produce nanofibers of boron carbide in the following way. A solution of the poly(norbomenyldecaborane) in THF is subjected to the process of... 

A rhombohedral boron carbide Bi3C2 results from the pyrolysis of BBr3-CHLt-Hz mixtures on Ta or BN substrates at 900—1800°C. It has the crystal-chemical composition Bi2(CBC), i.e. Bi2 icosahedra and linear CBC chains.149 Excess carbon up to a resultant formula of B13C3 can be accommodated in the structure. 

Organic precursors can be used both polycarbosilane and a small amount of phenolic resin, giving CSi and carbon by in situ pyrolysis the resulting boron-carbide ceramies have high density (> 92 %) and contain no free carbon and a small amount of SiC( 5wt%) . 

The addition of free graphite yields fine-grained compounds near the theoretical density (94-100 %) . Carbon is better added by the in-situ pyrolysis of a phenolfor-maldehyde resin (i 9 wt Pressure-less sintering of boron-carbide is now... 

Controlled pyrolysis of a BBr3-CH4 H2 mixture over a BN surface at 1550—1650 C produces a rhombohedral boron carbide B13C2. This is believed to contain icosahedral B12 units and linear CBC chains. 

Very fine boron carbide powders of spherical shape and 20-30 nm in size have been prepared by chemical vapor deposition according to (iii). In an Ar-H2-CH2-BCI3 atmosphere a radio frequency plasma produces stoichiometries between Bi5 gC and B3 9C [33, 166]. Also laser-induced pyrolysis of similar gas mixtures with or without acetylene has been employed for the preparation of nano-sized particles [167]. With similar success, composites of B4C and SiC have been produced by the pyrolysis of boron-containing polysilanes [168]. 

FIGURE 20.17 SEM micrograph of boron carbide nanowires prepared via (a) a template-assisted pyrolysis method. (Adapted from Pender, M. J. and Sneddon, L. G. Chem. Mater., 2000, 12, 280-283.) (b) Vapor-liquid-solid (VLS) mechanism. (Adapted from (a) Ma, R. and Bando, Y. Chem. Mater., 2002, 14, 4403-4407 (b) Ma, R. and Bando, Y. Chem. Phys. Lett., 2002, 364, 314-317.)... 

A different approach has been adopted by Welna et al. to produce boron carbide nanofibers from a polymeric precursor. Poly(norbornenyldecaborane) was utilized via electrostatic spinning followed by pyrolysis to fabricate the nanofibers. The fiber diameter varied with the temperature of pyrolysis process the higher the temperature, the narrower the diameter. 

When the pyrolysis of PCS precursor fibers is carried out in the presence of hydrogen, or when boron or aluminum doped PCS precursor fibers are pyrolyzed, it is possible to obtain quasi-stoichiometric silicon carbide fibers. Alternatively, quasi-stoichiometric fibers are also obtained from precursor fibers consisting of SiC powder reinforced polymers. 

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. 

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