Boron carbide structure

W.R. Blumenthal and G.T. Gray III, Structure-Property Characterization of a Shock-Loaded Boron Carbide Aluminum Cermet, in 4th Oxford Conf. on Mech. Prop, of Mat. at High Rates of Strain, Int. Phys. Conf. Ser. 102, Oxford, 1989, 363 pp. 

Boron carbide is a non-metallic covalent material with the theoretical stoichiometric formula, B4C. Stoichiometry, however, is rarely achieved and the compound is usually boron rich. It has a rhombohedral structure with a low density and a high melting point. It is extremely hard and has excellent nuclear properties. Its characteristics are summarized in Table 9.2. 

Poly crystalline boron nitride films, with a structure similar to rhombohedral boron carbide and a ratio of boron to nitrogen of 3 1, were produced by hot-filament CVD. This work indicates the possible existence of other boron-nitride structures. 

Properties. Boron carbide has a rhombohedral structure consisting of an array of nearly regular icosahedra, each having twelve boron atoms at the vertices and three carbon atoms in a linear chain outside the icosahedra (3,4,6,7). Thus a descriptive chemical formula would be B12C3 [12075-36 4], Each boron atom is bonded to five others in the icosahedron as well as either to a carbon atom or to a boron atom in an adjacent icosahedron. The structure is similar to that of rhombohedral boron (see Boron, elemental). The theoretical density for B12C3 is 2.52 g/mL. The rigid framework of... 

Diamondlike Carbides. Silicon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be included. These materials have electrical resistivity in the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical structure, although its conductivity is low. 

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. 

As noted in Section 9, the structures of the R-B-C(N) compounds (Figure 21) are homologous to that of boron carbide which exhibits typical p-type characteristics. Boron carbide is the limit where the number of boron icosahedra and C-B-C chain layers separating the metal layers reaches infinity (i.e. no rare earth layers). It has been speculated that the 2 dimensional metal layers of these rare earth R-B-C(N) compounds are playing a role for the unusual n-type behavior, but the mechanism is not yet clear. 

Kravchik, A.E., Kukushikina, J.A., Sokolov, V.V., and Tereshchenko, G.F. Structure of nanoporous carbon produced from boron carbide. Carbon 44, 2006 3263-3268. 

Boron carbide (B4C) is also an extremely hard, infusible, and inert substance, made by reduction of B203 with carbon in an electric furnace at 2500°C, and has a very unusual structure. The C atoms occur in linear chains of 3, and the boron atoms in icosahedral groups of 12 (as in crystalline boron itself). These two units are then packed together in a sodium chloride-like array. There are, of course, covalent bonds between C and B atoms as well as between B atoms in different icosahedra. A graphite-like boron carbide (BQ) has been made by interaction of benzene and BC13 at 800°C. 

The utilization of the outstanding mechanical and wear properties of cubic BN and related superhard materials has been demonstrated, although further process improvements are necessary before a more general use can be expected. Superhard cubic BN exhibits superb inertness to hot steel surfaces under conditions in which diamond would be severely attacked. Other tetrahedral structures, for instance, BCN and B2O, are anticipated to display similar attractive properties. The high-pressure phase B2O is a semiconductor with a band gap of 0.25 eV and a hardness between that of boron carbide and diamond. 

People who make discoveries do not necessarily use the aesthetic side until afterward. Then they recognize it and say, yeah, that s beautiful. Of course, we were not the first to find the boron icosahedron either. That was found by Sevastyanov and Zhdanov in C3B12, boron carbide, in 1941. That was a missed opportunity because nobody thought then that you could make fragments of this polyhedron and arrive at a structure of decaborane. That was not an idea that was around. It s an idea we missed. 

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

Other binary carbides. Boron and carbon form a series of boron carbide phases of which B4C appears to be the most ordered. There are several competing structural models for boron carbide which have been investigated by B and C... 

Boron carbide pellets and structures can be produced by cold pressing and sintering (70-80% density) or by hot pressing. In the latter the B4C powder is first cold-pressed into pellet form and then hot-pressed in graphite dies at 2050-2300°C under 10.3 MPa (1500 psi). The density is controlled by varying the temperature and the pressure. 

At temperatures above 2000°C, the structural stability and strength of the boronated graphite materials degrade. Boron carbide melts at 2140-2450°C and reacts with the graphite 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. 

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