Boron nitride

Boron nitride (BN) is a binary chemical compound, consisting of equal numbers of graphite. The diamond-like polymorph is one of the hardest materials known and the graphite-like polymorph is a useful lubricant.

Hexagonal BN

The band gap.

Hexagonal BN is a lubricant at both low and high temperatures (up to 900 °C), even in oxidizing atmosphere). It is particularly useful lubricant in situations where the electrical conductivity or chemical reactivity of graphite would be problematic. Since the lubricity mechanism does not involve water molecules trapped between the layers, boron nitride lubricants can be used even in vacuum, e.g. for space applications.

Hexagonal boron nitride is stable in temperatures up to 1000 °C in air, 1400 °C in vacuum, and 2800 °C in an inert atmosphere. It has one of the best thermal conductivities of all electric insulators. It is fairly chemically inert and is not wetted by many melted materials (e.g. aluminium, copper, zinc, iron and steels, germanium, silicon, boron, cryolite, glass and halide salts).^{[citation needed]}

Fine-grained h-BN is used in some cosmetics, paints, dental cements, and pencil leads.^{[citation needed]}

Preparation of hexagonal BN

Hexagonal boron nitride is produced by the nitridation or plasma at 5500 °C yields ultrafine boron nitride for lubricants and toners.

Cubic boron nitride

Cubic boron nitride is extremely hard, although less so than phonons. In contact with oxygen at high temperatures, BN forms a passivation layer of boron oxide. Boron nitride binds well with metals, due to formation of interlayers of metal borides or nitrides. Materials with cubic boron nitride crystals are often used in the tool bits of cutting tools. For grinding applications, softer binders, e.g. resin, porous ceramics, and soft metals, are used. Ceramic binders can be used as well. Commercial products are known under names "Borazon" (by Diamond Innovations), and "Elbor" or "Cubonite" (by Russian vendors).

Sintered cubic boron nitride is an electrically insulating heatsink material of potential value in microelectronics.

Preparation of cubic BN

Cubic boron nitride is produced by treating hexagonal boron nitride at high pressure and temperature, much as heterodiamond, a superhard compound of boron, carbon, and nitrogen.

Low-pressure deposition of thin films of cubic boron nitride is possible. For selective etching of the deposited hexagonal phase during physical vapor deposition methods are used as well.

Other polymorphs of BN

w-BN

Known as w-BN, hexagonal boron nitride is a superhard phase that occurs at high pressures. This hexagonal phase differs from the layered graphitic material: it adopts the wurtzite structure.

Rhombohedral boron nitride

Rhombohedral boron nitride is similar to hexagonal boron nitride. It is formed transitionally during conversion of cubic BN to hexagonal form.

Boron nitride fibers

Hexagonal BN can be prepared in the form of fibers, structurally similar to boric acid or ammonium tetraborate in an atmosphere of ammonia and nitrogen above 1000 °C. Boron nitride fibers are used as reinforcement in composite materials, with the matrix materials ranging from organic resins to ceramics to metals (see Metal matrix composites).

Nanostructured BN

Boron nitride nanotubes

Like BN fibers, boron nitride nanotubes(BNNTs) show promise for aerospace applications where integration of boron and in particular the light isotope of boron (¹⁰B) into structural materials improves their radiation-shielding properties, due to ¹⁰B's neutron absorption properties. Such ¹⁰BN materials are of particular theoretical value as composite structural material in future manned interplanetary spacecraft, where absorption-shielding from cosmic ray spallation neutrons is expected to be a particular asset in light construction materials.[1]

Boron nitride nanomesh

  inorganic nanostructured two-dimensional material. It consists of a single layer of hexagonal boron nitride on ruthenium, forming a highly regular mesh. The distance between two pore centers is 3.2 nanometers (=0.0000032 mm!) and the pores are 0.05 nanometer deep. The boron nitride nanomesh is stable under vacuum, air and some liquids, but also up to temperatures of 796 ^oC. In addition, it shows the extraordinary ability to trap molecules and metallic nanotechnology.

Amorphous boron nitride

Layers of amorphous boron nitride (a-BN) are used in some chemical vapor deposition methods. Thermal CVD can be also used for deposition of h-BN layers, or at high temperatures, c-BN.

BN-based fullerenes

The semiconductor materials with the same (approximate) band gap.

Composites containing BN

Addition of boron nitride to silicon carbide-boron nitride composition.

Due to its excellent dielectric and insulating properties, BN is used in electronics e.g. as a substrate for semiconductors, microwave-transparent windows, structural material for seals, electrodes and catalyst carriers in fuel cells and batteries.

h-BN can be included in ceramics, alloys, resins, plastics, rubbers and other materials, giving them self-lubricating properties. Such materials are suitable for construction of e.g. bearings. Plastics filled with BN have decreased thermal expansion, increased thermal conductivity, increased electrical insulation properties, and cause reduced wear to adjacent parts.

See also

• Beta carbon nitride
• Borazon
• Boron phosphide
• Boron suboxide
• Aluminium nitride
• Wide bandgap semiconductors

References

Known to its aguculture, there are many types ofceramics that can harm animals in mouth contacts aprouch.