Boron carbide

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Boron carbide
Boron carbide
Names
IUPAC name
Boron carbide
Other names
Tetrabor
Identifiers
3D model (
JSmol
)
ChemSpider
ECHA InfoCard
100.031.907 Edit this at Wikidata
UNII
  • InChI=1S/CB4/c2-1-3(2)5(1)4(1)2 checkY
    Key: INAHAJYZKVIDIZ-UHFFFAOYSA-N checkY
  • InChI=1/CB4/c2-1-3(2)5(1)4(1)2
    Key: INAHAJYZKVIDIZ-UHFFFAOYAS
  • B12B3B4B1C234
Properties
B4C
Molar mass 55.255 g/mol
Appearance dark gray or black powder, odorless
Density 2.50 g/cm3, solid.[1]
Melting point 2,350 °C (4,260 °F; 2,620 K)[1]
Boiling point >3500 °C[1]
insoluble
Structure
Rhombohedral
Hazards
Safety data sheet (SDS) External MSDS
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).
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Boron carbide (chemical formula approximately B4C) is an extremely hard

tank armor, bulletproof vests, engine sabotage powders,[2]
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.[3]

History

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.[4] 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.[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 B12CBC units.

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

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.[5][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 extremely high hardness (about 9.5 up to 9.75 on

elastic modulus (460 GPa)[12] and fracture toughness (3.5 MPa·m1/2) approach the corresponding values for diamond (1150 GPa and 5.3 MPa·m1/2).[13]

As of 2015[update], boron carbide is the third hardest substance known, after

cubic boron nitride, earning it the nickname "black diamond".[14][15]

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

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:[16]

2 B2O3 + 7 C → B4C + 6 CO

If magnesium is used, the reaction can be carried out in a graphite crucible, and the magnesium byproducts are removed by treatment with acid.[17]

Plastic embedded with boron carbide used as shielding in neutron experiments at the Atomic Energy Research Establishment, UK

Applications

ballistic vests

For its hardness :

For other properties :

  • nuclear reactors
    (see below)
  • ramjets

Nuclear applications

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[18] and from anti-personnel neutron bombs. Nuclear applications of boron carbide include shielding.[11]

See also

References

  1. ^ .
  2. ^ Gray, Theodore (2012-04-03). The Elements: A Visual Exploration of Every Known Atom in the Universe. Black Dog & Leventhal Publishers. . Retrieved 6 May 2014.
  3. ^ "Rutgers working on body armor". Asbury Park Press. Asbury Park, N.J. August 11, 2012. Retrieved 2012-08-12. ... boron carbide is the third-hardest material on earth.
  4. ^ Ridgway, Ramond R "Boron Carbide", European Patent CA339873 (A), publication date: 1934-03-06
  5. ^ a b Balakrishnarajan, Musiri M.; Pancharatna, Pattath D.; Hoffmann, Roald (2007). "Structure and bonding in boron carbide: The invincibility of imperfections". New J. Chem. 31 (4): 473. .
  6. ^ .
  7. ^ .
  8. ^ (PDF) on 27 December 2014. Retrieved 23 July 2015.
  9. .
  10. .
  11. ^ a b Weimer, p. 330
  12. .
  13. .
  14. ^ "Boron Carbide". Precision Ceramics. Archived from the original on 2015-06-20. Retrieved 2015-06-20.
  15. .
  16. ^ Weimer, p. 131
  17. American Society for Testing Materials
    , 1959

Bibliography

External links