Boron carbide (chemical formula approximately B₄C) is an extremely hard

boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

–

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

, a

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

material used in tank armor,

bulletproof vest A bulletproof vest, also known as a ballistic vest or bullet-resistant vest, is a type of body armor designed to absorb impact and prevent the penetration of firearm projectiles and explosion fragments to the torso. The vest can be either soft ...

s, engine

sabotage Sabotage is a deliberate action aimed at weakening a polity, government, effort, or organization through subversion, obstruction, demoralization (warfare), demoralization, destabilization, divide and rule, division, social disruption, disrupti ...

powders, 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 Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula B N. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexago ...

and

diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...

History

Boron carbide was discovered in the 19th century as a

by-product A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be cons ...

of reactions involving metal borides, but its

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

was unknown. It was not until the 1930s that the chemical composition was estimated as B₄C. Controversy remained as to whether or not the material had this exact 4:1

stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...

, as, in practice the material is always slightly carbon-deficient with regard to this formula, and

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

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. 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

Boron carbide has a complex crystal structure typical of icosahedron-based borides. There, B₁₂ icosahedra form a

rhombohedral In geometry, a rhombohedron (also called a rhombic hexahedron or, inaccurately, a rhomboid) is a special case of a parallelepiped in which all six faces are congruent rhombus, rhombi. It can be used to define the rhombohedral lattice system, a Ho ...

lattice unit (space group: ''Rm'' (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 In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector In mathematics, a unit vector i ...

, 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 In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...

. 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. 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. 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. 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 and the non-metallic electrical character of the B₁₃C₂ composition.

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 Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...

and most chemicals.Weimer, p. 330 Its Vickers hardness (38 GPa),

elastic modulus An elastic modulus (also known as modulus of elasticity (MOE)) is a quantity that describes an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. Definition The elastic modu ...

(460 GPa) and

fracture toughness In materials science, fracture toughness is the critical stress intensity factor of a sharp Fracture, crack where propagation of the crack suddenly becomes rapid and unlimited. It is a material property that quantifies its ability to resist crac ...

(3.5 MPa·m^1/2) approach the corresponding values for diamond (1150 GPa and 5.3 MPa·m^1/2). , boron carbide is the third hardest substance known, after

and cubic boron nitride, earning it the nickname "black diamond".

Semiconductor properties

Boron carbide is a

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

, with electronic properties dominated by hopping-type transport. The energy

band gap In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to t ...

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. The material is typically p-type.

Preparation

Boron carbide was first synthesized by Henri Moissan in 1899, by reduction of

boron trioxide Boron trioxide or diboron trioxide is the oxide of boron with the formula . It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called boric oxide or boria. It h ...

either with

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

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 Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

:Weimer, p. 131 :2 B₂O₃ + 7 C → B₄C + 6 CO If magnesium is used, the reaction can be carried out in a graphite

crucible A crucible is a container in which metals or other substances may be melted or subjected to very high temperatures. Although crucibles have historically tended to be made out of clay, they can be made from any material that withstands temperat ...

, and the magnesium byproducts are removed by treatment with acid. Plastic with boron carbide

Applications

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

Padlock Padlocks are portable Lock (security device), locks with a shackle that may be passed through an opening (such as a chain, chain link, or hasp staple) to prevent wikt:use, use, theft, vandalism or harm. Naming and etymology The term '':wikt: ...

s *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 composite In materials science, a metal matrix composite (MMC) is a composite material with fibers or particles dispersed in a metallic matrix, such as copper, aluminum, or steel. The secondary phase is typically a ceramic (such as alumina or silicon carb ...

s *In brake linings of vehicles Boron carbide's other properties also make it suitable for: *

Neutron absorber In applications such as nuclear reactors, a neutron poison (also called a neutron absorber or a nuclear poison) is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable ef ...

nuclear reactors A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction. They are used for commercial electricity, marine propulsion, weapons production and research. Fissile nuclei (primarily uranium-235 or plutonium-2 ...

(see below) *

High energy fuel High may refer to: Science and technology * Height * High (atmospheric), a high-pressure area * High (computability), a quality of a Turing degree, in computability theory * High (tectonics), in geology an area where relative tectonic uplift ...

for solid fuel ramjets

Nuclear applications

The property of boron carbide to absorb neutrons without forming long-lived

radionuclide A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

s makes it an attractive

neutron radiation Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new nuclides— ...

shielding or absorbing material, such as in use for control rods in nuclear power reactors. Nuclear applications of boron carbide include shielding and reaction regulation (control rod).Weimer, p. 330

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. In addition, boron carbide filaments are not affected by radiation due to its ability to absorb neutrons. It is less harmful than filaments made of other materials, such as cadmium.

References

Bibliography

External links

National Pollutant Inventory – Boron and compoundsNIST Chemistry Database Entry for Boron Carbide
{{Carbides Carbides Boron compounds Superhard materials Neutron poisons