Boron carbide – wikipedia gas 0095

Boron carbide (chemical formula approximately B 4C) is an extremely hard boron– carbon ceramic, and covalent material used in tank armor, bulletproof vests, engine sabotage powders, [1] 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. [2]

Boron carbide was discovered in 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 B 4C. [3] There remained, however, controversy 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 B 12 icosahedra. These features argued against a very simple exact B 4C empirical formula. [4] Because of the B 12 structural unit, the chemical formula of "ideal" boron carbide is often written not as B 4C, but as B 12C 3, and the carbon deficiency of boron carbide described in terms of a combination of the B 12C 3 and B 12CBC units.

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, control rod and shut down pellets. Within control rods, boron carbide is often powdered, to increase its surface area. [5]

Boron carbide has a complex crystal structure typical of icosahedron-based borides. There, B 12 icosahedra form a rhombohedral lattice unit (space group: R 3m (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 B 12 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 B 12 icosahedron and the B 6 octahedron. Because of the small size of the B 6 octahedra, they cannot interconnect. Instead, they bond to the B 12 icosahedra in the neighboring layer, and this decreases bonding strength in the c-plane. [6]

Because of the B 12 structural unit, the chemical formula of "ideal" boron carbide is often written not as B 4C, but as B 12C 3, and the carbon deficiency of boron carbide described in terms of a combination of the B 12C 3 and B 12C 2 units. [4] [7] Some studies indicate the possibility of incorporation of one or more carbon atoms into the boron icosahedra, giving rise to formulas such as (B 11C)CBC = B 4C at the carbon-heavy end of the stoichiometry, but formulas such as B 12(CBB) = B 14C at the boron-rich end. "Boron carbide" is thus not a single compound, but a family of compounds of different compositions. A common intermediate, which approximates a commonly found ratio of elements, is B 12(CBC) = B 6.5C. [8] 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 B 4C composition [9] and the non-metallic electrical character of the B 13C 2 composition. [10] Properties [ edit ]

Boron carbide is known as a robust material having high hardness, high cross section for absorption of neutrons (i.e. good shielding properties against neutrons), stability to ionizing radiation and most chemicals. [5] Its Vickers hardness (38 GPa), Elastic Modulus (460 GPa) [11] and fracture toughness (3.5 MPa·m 1/2) approach the corresponding values for diamond (115 GPa and 5.3 MPa·m 1/2). [12]

Boron carbide is a semiconductor, with electronic properties dominated by hopping-type transport. [8] The energy band gap depends on composition as well as the degree of order. The band gap is estimated at 2.09 eV, with multiple mid-bandgap states which complicate the photoluminescence spectrum. [8] The material is typically p-type. Preparation [ edit ]

Boron carbide was first synthesized by Henri Moissan in 1899, [7] by reduction of boron trioxide either with carbon or magnesium in presence of carbon in an electric arc furnace. In the case of carbon, the reaction occurs at temperatures above the melting point of B 4C and is accompanied by liberation of large amount of carbon monoxide: [15] 2 B 2O 3 + 7 C → B 4C + 6 CO

• ^ a b c Domnich, Vladislav; Reynaud, Sara; Haber, Richard A.; Chhowalla, Manish (2011). "Boron Carbide: Structure, Properties, and Stability under Stress" (PDF). J. Am. Ceram. Soc. 94 (11): 3605–3628. doi: 10.1111/j.1551-2916.2011.04865.x . Retrieved 23 July 2015.

• ^ Ektarawong, A.; Simak, S. I.; Hultman, L.; Birch, J.; Alling, B. (2014). "First-principles study of configurational disorder in B 4C using a superatom-special quasirandom structure method". Phys. Rev. B. 90 (2): 024204. arXiv: 1508.07786 . Bibcode: 2014PhRvB..90b4204E. doi: 10.1103/PhysRevB.90.024204.

• ^ Ektarawong, A.; Simak, S. I.; Hultman, L.; Birch, J.; Alling, B. (2015). "Configurational order-disorder induced metal-nonmetal transition in B 13C 2 studied with first-principles superatom-special quasirandom structure method". Phys. Rev. B. 92: 014202. arXiv: 1508.07848 . Bibcode: 2015PhRvB..92a4202E. doi: 10.1103/PhysRevB.92.014202.

• ^ Solozhenko, V. L.; Kurakevych, Oleksandr O.; Le Godec, Yann; Mezouar, Mohamed; Mezouar, Mohamed (2009). "Ultimate Metastable Solubility of Boron in Diamond: Synthesis of Superhard Diamondlike BC5". Phys. Rev. Lett. 102 (1): 015506. Bibcode: 2009PhRvL.102a5506S. doi: 10.1103/PhysRevLett.102.015506. PMID 19257210.

• ^ A. Sokhansanj; A.M. Hadian (2012). "Purification of Attrition Milled Nano-size Boron Carbide Powder". 2nd International Conference on Ultrafine Grained & Nanostructured Materials (UFGNSM). International Journal of Modern Physics: Conference Series. 5: 94–101. Bibcode: 2012IJMPS…5…94S. doi: 10.1142/S2010194512001894.