Boron carbide

Boron carbide (chemical formula approximately B₄C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders,<ref> Template:Cite book </ref> as well as numerous industrial applications. With a Vickers hardness of >30 GPa,<ref>Template:Cite journal</ref> it is one of the hardest known materials, behind cubic boron nitride and diamond.<ref> Template:Cite news </ref>

History

Boron carbide was first noticed by Henri Moissan (Nobel Prize of chemistry in 1906) in 1894<ref>Template:Cite journal</ref> 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.<ref>Ridgway, Ramond R "Boron Carbide" Template:Webarchive, European Patent CA339873 (A), publication date: 1934-03-06</ref> 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.<ref name=stoi> Template:Cite journal </ref> 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.<ref name=zhangyb28.5c4>Template:Cite journal</ref>

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: RTemplate:Overlinem (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.<ref name=zhangyb28.5c4/>

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.<ref name=stoi/><ref name="gr">Template:Greenwood&Earnshaw2nd</ref> 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) = B_6.5C.<ref name="Domnich2011">Template:Cite journal</ref> 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<ref>Template:Cite journal</ref> and the non-metallic electrical character of the B₁₃C₂ composition.<ref>Template:Cite journal</ref>

Properties

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.<ref name=w330>Weimer, p. 330</ref> Its Vickers hardness (38 GPa), elastic modulus (460 GPa)<ref>Template:Cite journal</ref> and fracture toughness (3.5 MPa·m^1/2) approach the corresponding values for diamond (1150 GPa and 5.3 MPa·m^1/2).<ref>Template:Cite journal</ref>

Template:As of, boron carbide is the third hardest substance known, after diamond and cubic boron nitride, earning it the nickname "black diamond".<ref>Template:Cite web</ref><ref>Template:Cite journal</ref>

Semiconductor properties

Boron carbide is a semiconductor, with electronic properties dominated by hopping-type transport.<ref name="Domnich2011" /> 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.<ref name="Domnich2011" /> The material is typically p-type.

Preparation

Boron carbide was first synthesized by Henri Moissan in 1894 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:<ref name=w131>Weimer, p. 131</ref>

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.<ref>Patnaik, Pradyot (2002). Handbook of Inorganic Chemicals. McGraw-Hill. Template:ISBN</ref>

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

Applications

Boron carbide is used for inner plates of ballistic vests

Boron's exceptional hardness can be used for the following applications:

• Padlocks
• Personal and vehicle ballistic armor plating
• Grit blasting nozzles
• High-pressure water jet cutter nozzles
• Scratch and wear resistant coatings
• Cutting tools and dies
• Abrasives
• Metal matrix composites
• In brake linings of vehicles

Boron carbide's other properties also make it suitable for:

• Neutron absorber in nuclear reactors (see below)
• High energy fuel for solid fuel ramjets

Nuclear applications

The property of boron carbide to absorb neutrons without forming long-lived radionuclides makes it an attractive neutron radiation shielding or absorbing material, such as in use for control rods in nuclear power reactors.<ref>Fabrication and Evaluation of Urania-Alumina Fuel Elements and Boron Carbide Burnable Poison Elements, Wisnyi, L. G. and Taylor, K.M., in "ASTM Special Technical Publication No. 276: Materials in Nuclear Applications", Committee E-10 Staff, American Society for Testing Materials, 1959</ref> Nuclear applications of boron carbide include shielding and reaction regulation (control rod).<ref name=w330>Weimer, p. 330</ref>

Boron carbide filaments

Boron carbide filaments exhibit auspicious prospects as reinforcement elements in resin and metal composites, attributed to their exceptional strength, elastic modulus, and low density characteristics.<ref>Template:Cite journal</ref> In addition, boron carbide filaments are not affected by radiation due to its ability to absorb neutrons.<ref>Template:Cite web</ref> It is less harmful than filaments made of other materials, such as cadmium.<ref>Template:Cite web</ref>

See also

• List of compounds with carbon number 1

Bibliography

• Template:Cite book

External links

• National Pollutant Inventory – Boron and compounds
• NIST Chemistry Database Entry for Boron Carbide