Boron carbide

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Boron carbide
Boron carbide
Identifiers
CAS number 12069-32-8 YesY
PubChem 123279
ChemSpider 109889 YesY
Jmol-3D images Image 1
Properties
Molecular formula B4C
Molar mass 55.255 g/mol
Appearance dark gray or black powder, odorless
Density 2.52 g/cm3, solid.
Melting point 2,763 °C (5,005 °F; 3,036 K)
Boiling point 3,500 °C (6,330 °F; 3,770 K)
Solubility in water insoluble
Acidity (pKa) 6–7 (20 °C)
Structure
Crystal structure Rhombohedral
Hazards
MSDS External MSDS
Related compounds
Related compounds Boron nitride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Boron carbide (chemical formula approximately B4C) is an extremely hard boroncarbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications. With a Mohs hardness of about 9.497, it is one of the hardest materials known, behind cubic boron nitride and diamond.1

Boron carbide was discovered in the 19th century as a by-product of reactions involving metal borides, however, its chemical formula was unknown. It was not until the 1930s that the chemical composition was estimated as B4C.2 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 B12 icosahedra. These features argued against a very simple exact B4C empirical formula.3 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 B12C2 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. 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.4

Crystal structure

Unit cell of B4C. The green sphere and icosahedra consist of boron atoms, and black spheres are carbon atoms.5
Fragment of the B4C crystal structure.

Boron carbide has a complex crystal structure typical of icosahedron-based borides. There, B12 icosahedra form a rhombohedral lattice unit (space group: R3m (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.5

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 B12C2 units.36 Some studies indicate the possibility of incorporation of one or more carbon atoms into the boron icosahedra, giving rise to formulas such as (B11C)CBC = B4C at the carbon-heavy end of the stoichiometry, but formulas such as B12(CBB) = B14C at the boron-rich end. A common intermediate, which approximates a commonly found ratio of elements, is B12(CBC) = B6.5C.7

Properties

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.4 Its Vickers hardness (38 GPa), Elastic Modulus (460 GPa)8 and fracture toughness (3.5 MPa·m1/2) approach the corresponding values for diamond (115 GPa and 5.3 MPa·m1/2).9

Preparation

Boron carbide was first synthesized by Henri Moissan in 1899,6 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 B4C and is accompanied by liberation of large amount of carbon monoxide:10

2 B2O3 + 7 C → B4C + 6 CO

If magnesium is used, the reaction can be carried out in a graphite furnace, and the magnesium byproducts are removed by treatment with acid.11

Uses

See also

References

  1. ^ "Rutgers working on body armor". Asbury Park Press. August 11, 2012. Retrieved 2012-08-12. "... boron carbide is the third-hardest material on earth." 
  2. ^ Ridgway, Ramond R "Boron Carbide", European Patent CA339873 (A), publication date: 1934-03-06
  3. ^ a b Musiri M. Balakrishnarajan, Pattath D. Pancharatna and Roald Hoffmann (2007). "Structure and bonding in boron carbide: The invincibility of imperfections". New J. Chem. 31 (4): 473. doi:10.1039/b618493f. 
  4. ^ a b Weimer, p. 330
  5. ^ a b Zhang F X, Xu F F, Mori T, Liu Q L, Sato A and Tanaka T (2001). "Crystal structure of new rare-earth boron-rich solids: REB28.5C4". J. Alloys Compd. 329: 168. doi:10.1016/S0925-8388(01)01581-X. 
  6. ^ a b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 149. ISBN 0080379419. 
  7. ^ Boron Carbide: Structure, Properties, and Stability under Stress. Vladislav Domnich, Sara Reynaud, Richard A. Haber, and Manish Chhowalla, J. Am. Ceram. Soc., 94 [11] 3605–3628 (2011) DOI: 10.1111/j.1551-2916.2011.04865. Full Text Online
  8. ^ Sairam, K., Sonber, J.K., Murthy, T.S.R.Ch., Subramanian, C., Hubli, R.C.,Suri, A.K., Development of B4C-HfB2 composites by reaction hot pressing. Int.J. Refr. Met. Hard Mater., 2012; 35: 32-40. http://www.sciencedirect.com/science/article/pii/S0263436812000546
  9. ^ 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. 
  10. ^ Weimer, p. 131
  11. ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8

Bibliography

External links