Crystal boron carbides

Boron carbide (B C) is a hard, black crystal that is used as an abrasive powder and as an additive to strengthen composite parts in aircraft. 

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). 

Small single crystals, such as those of potassium titanatc, are being used at an annual rate of over 10,000 tons for the reinforcement of nylon and other thermoplastics. These composites are replacing die-cast metals in many applications. Another microfiber, sodium hydroxycarbonate (Dawsonite), also improves the physical properties and flame resistance of many polymers. Many other single crystals, called whiskers, such as alumina, chromia, and boron carbide, have been used for making high-performance composites. 

The boron carbide-graphite bodies are heat-treated to 2000°C after extrusion or warm pressing while protected from oxidation. Heat treatment is limited to below 2200°C to prevent migration of boron into the graphite crystals, which would enhance radiation swelling of the matrix. 

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. 

The vaporization-cold-substrate condensation process of boron carbide melted by electron-gun heating and plasma torch deposition does not give a well-organized and crystalline B4C-type structure. In CH4 [1.33.10 Pa 1 torr)] an electron-gun evaporation of boron carbide contained in a graphite crucible, followed by a condensation on a water-cooled jacket, gives crystals. Sputtering by H is possible . 

Single crystals a few mm long are obtained by chemical vapor deposition (cf. 5.3.2.2.3), or by reduction of B2O3 by graphite in an electrical arc . A 6-mm diameter sintered boron carbide rod can be zone melted under Ar... 

Sodium borohydride is marketed in powdered or pellet form, and in solution, for use in fuel cells. Boron nitride can withstand temperatures of up to 650°C (1,202°E) when subjected to high pressures and temperatures, it forms cubic crystals whose hardness rivals that of diamond. Boron carbide, produced by reacting coke and boric acid at 2,600°C (4,712°E), is a highly refractory material and one of the hardest substances known. It has both abrasive and abrasion-resistant applications, and is used in nuclear shielding, see ALSO Davy, Humphry Gay-Lussac, Joseph-Louis Nuclear Chemistry. 

Acceptor dopants are introduced in the crucible either in elemental form or in the form of carbides. If a dopant is introduced in elemental form, it is placed in a special internal crucible with carbon or silicon carbide powder. This is required to prevent the dissolution of the crucible, in the case of aluminium doping, and to reduce the boron vapour pressure to the equilibrium value for the SiC-C system, in the case of boron doping. If elemental boron is placed in the vicinity of the substrate, this results in the formation of boron carbide on the crystal faces of SiC [46]. For moderate doping of crystals, grown at high temperatures, doped SiC sources also can be employed. 

In order to investigate the microstructure of boron carbide under field A and field B, TEM studies were performed. Fig.7 showed the microstructure of deposits under different temperature fields. Under field A, the space of (021) plane were 0.24nm, which showed the deposits were crystal B13C2. Under field B, no crystal phase can be found, which showed the deposits were amorphous. 

The above results showed that the morphology, composition and microstructure were different under two different temperature fields. Under field A, the deposits were crystal B13C2 with high boron concentration and crystalline-like morphologies. Under field B, the deposits were amorphous boron carbide with low boron concentration and cauliflower-like morphologies. The characteristics of deposits depended on deposition mechanism. It was apparent that there were different deposition mechanisms for boron carbide since the characteristics of deposits were very great different under... 

Fig. 45. (a) Load-displacement and (b) AGP vs contact depth plot for the indentation of boron carbide single crystal. 

Fig. 46. Raman spectra from (a) the polished surface and (b) a 5 N Vickers indentation in the (111) surface of boron carbide single crystal.

Crystal Structure of Boron Carbide and Isotopic Compounds... 

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