Boron carbide

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Boron carbide
Boron carbide
The structural formula for boron carbide, consisting of a carbon atom covalently bonded to all four boron atoms and each boron atom connected to two other boron atoms.
Names
IUPAC name
Boron carbide
Other names
Tetrabor
Identifiers
12069-32-8 YesY
ChemSpider 109889 YesY
Jmol 3D model Interactive image
PubChem 123279
Properties
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)
insoluble
Acidity (pKa) 6–7 (20 °C)
Structure
Rhombohedral
Vapor pressure {{{value}}}
Related compounds
Related compounds
Boron nitride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY verify (what is YesYN ?)
Infobox references

Boron carbide (chemical formula approximately B4C) is an extremely hard boroncarbon ceramic material used in tank armor, bulletproof vests, engine sabotage powders,[1] as well as numerous industrial applications. With a Vickers Hardness of >30 MPa, 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, however, its chemical formula was unknown. It was not until the 1930s that the chemical composition was estimated as B4C.[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 B12 icosahedra. These features argued against a very simple exact B4C empirical formula.[4] 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.

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]

Crystal structure

Unit cell of B4C. The green sphere and icosahedra consist of boron atoms, and black spheres are carbon atoms.[6]
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.[6]

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.[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 (B11C)CBC = B4C at the carbon-heavy end of the stoichiometry, but formulas such as B12(CBB) = B14C 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 B12(CBC) = B6.5C.[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 B4C composition[9] and the non-metallic electrical character of the B13C2 composition.[10]

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.[5] Its Vickers hardness (38 GPa), Elastic Modulus (460 GPa)[11] and fracture toughness (3.5 MPa·m1/2) approach the corresponding values for diamond (115 GPa and 5.3 MPa·m1/2).[12]

As of 2015, boron carbide is the third hardest substance known, after diamond and cubic boron nitride, earning it the nickname "black diamond".[13][14]

Semiconductor Properties

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

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 B4C and is accompanied by liberation of large amount of carbon monoxide:[15]

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.[16]

Uses

Boron carbide is used for inner plates of ballistic vests

See also

References

  1. Gray, Theodore. The Elements: A Visual Exploration of Every Known Atom in the Universe. Black Dog & Leventhal Publishers. Retrieved 6 May 2014.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  2. "Rutgers working on body armor". Asbury Park Press. August 11, 2012. Retrieved 2012-08-12. ... boron carbide is the third-hardest material on earth.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  3. Ridgway, Ramond R "Boron Carbide", European Patent CA339873 (A), publication date: 1934-03-06
  4. 4.0 4.1 Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
  5. 5.0 5.1 Weimer, p. 330
  6. 6.0 6.1 Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
  7. 7.0 7.1 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 149. ISBN 0-08-037941-9.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  8. 8.0 8.1 8.2 Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
  9. Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
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  13. "Boron Carbide". Precision Ceramics. Retrieved 2015-06-20.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  14. Lua error in Module:Citation/CS1/Identifiers at line 47: attempt to index field 'wikibase' (a nil value).
  15. Weimer, p. 131
  16. Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8

Bibliography

External links