Карбид бора: свойства и структура

Extremely hard ceramic compound
B4C
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| verifiedrevid = 430790006
| Name = Boron carbide
| ImageFile = File:Boron carbide. JPG
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| ImageName = Boron carbide
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| IUPACName = Boron carbide
| OtherNames = Tetrabor
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|Section2=Chembox Properties
| Formula = B4C
| MolarMass = 55.255 g/mol
| Appearance = dark gray or black powder, odorless
| Density = 2.50 g/cm3, solid. | Solubility = insoluble
| MeltingPtC = 2350
| MeltingPt ref =
as well as numerous industrial applications. With a Vickers hardness of >30 GPa, it is one of the hardest known materials, behind cubic boron nitride and diamond.

Boron carbide was discovered in the 19th century as a by product of reactions involving metal borides, but its chemical formula was unknown. It was not until the 1930s that the chemical composition was estimated as B4C. Controversy remained as to whether or not the material had this exact 4:1 stoichiometry, as, in practice the material is always slightly carbon deficient with regard to this formula, and X ray crystallography shows that its structure is highly complex, with a mixture of C B C chains and B12 icosahedra. These features argued against a very simple exact B4C empirical formula. Because of the B12 structural unit, the chemical formula of "ideal" boron carbide is often written not as B4C, but as B12C3, and the carbon deficiency of boron carbide described in terms of a combination of the B12C3 and B12CBC units.

Boron carbide has a complex crystal structure typical of icosahedron based borides. There, B12 icosahedra form a rhombohedral lattice unit (space group: Rm (No. 166), lattice constants: a = 0.56 nm and c = 1.212 nm) surrounding a C B C chain that resides at the center of the unit cell, and both carbon atoms bridge the neighboring three icosahedra. This structure is layered: the B12 icosahedra and bridging carbons form a network plane that spreads parallel to the c plane and stacks along the c axis. The lattice has two basic structure units – the B12 icosahedron and the B6 octahedron. Because of the small size of the B6 octahedra, they cannot interconnect. Instead, they bond to the B12 icosahedra in the neighboring layer, and this decreases bonding strength in the c plane. Quantum mechanical calculations have demonstrated that configurational disorder between boron and carbon atoms on the different positions in the crystal determines several of the materials properties – in particular, the crystal symmetry of the B4C composition and the non metallic electrical character of the B13C2 composition.

Boron carbide is known as a robust material having extremely high hardness (about 9.5 up to 9.75 on Mohs hardness scale), high cross section for absorption of neutrons (i. e. good shielding properties against neutrons), stability to ionizing radiation and most chemicals. Its Vickers hardness (38 GPa), elastic modulus (460 GPa) and fracture toughness (3.5 MPa·m1/2) approach the corresponding values for diamond (1150 GPa and 5.3 MPa·m1/2). as of 2015, boron carbide is the third hardest substance known, after diamond and cubic boron nitride, earning it the nickname "black diamond".

The ability of boron carbide to absorb neutrons without forming long lived radionuclides makes it attractive as an absorbent for neutron radiation arising in nuclear power plants and from anti personnel neutron bombs. Nuclear applications of boron carbide include shielding.