Boron nitride

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Boron nitride

| Section8 = }} Boron nitride (BN) is a binary chemical compound, consisting of equal numbers of boron and nitrogen atoms. Its empirical formula is therefore BN. Boron nitride is isoelectronic with carbon and, like carbon, boron nitrides exists as various polymorphic forms, one of which is analogous to diamond and one analogous to graphite. The diamond-like polymorph is one of the hardest materials known and the graphite-like polymorph is a useful lubricant.

Hexagonal BN

The graphite-like polymorph of boron nitride, known as hexagonal boron nitride, h-BN, α-BN, or g-BN (graphitic BN), and sometimes called "white graphite", is the most widely used polymorph. The hexagonal polymorph is composed of layers of hexagonal sheets, analogous to graphite. The interlayer "registry" of these sheets differs, however, from the pattern seen for graphite, because the atoms are eclipsed, with boron atoms laying over and above nitrogen atoms. This registry reflects the polarity of the B-N bonds. The diminished covalency in BN results in diminished electrical conductivity relative to graphite, which is a semimetal that conducts electricity through a network of pi-bonds in the plane of its hexagonal sheets. The diminished electron-delocalizaton in hexagonal-BN is indicated by its absence of color, which signals a large 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).

Fine-grained h-BN is used in some cosmetics, paints, dental cements, and pencil leads.

Preparation of hexagonal BN

Hexagonal boron nitride is produced by the nitridation or ammonolysis of boron trioxide. h-BN parts can be made by hot-pressing with subsequent machining; due to the mechanical hardness similar to graphite, the machining cost is low. The parts are made from boron nitride powders, using boron oxide as a sintering agent. Thin films of boron nitride can be obtained by chemical vapor deposition from boron trichloride and nitrogen precursors. Industrial production is based on two reactions: melted boric acid with ammonia, and boric acid or alkaline borates with urea, guanidine, melamin, or other suitable organic nitrogen compounds in nitrogen atmosphere. Combustion of boron powder in nitrogen 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 diamond and some related materials. Also like diamond, cubic boron nitride is an electrical insulator but an excellent conductor of heat. This diamond-like polymorph, known as cubic boron nitride, c-BN, β-BN, or z-BN (after zinc blende crystalline structure), is widely used as an abrasive for industrial tools. Its usefulness arises from its insolubility in iron, nickel, and related alloys at high temperatures, whereas diamond is soluble in these metals to give carbides. Polycrystalline c-BN abrasives are therefore used for machining steel, whereas diamond abrasives are preferred for aluminium alloys, ceramics, and stone. Like diamond, cubic BN has good thermal conductivity, caused by 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 synthetic diamond is produced from graphite. Direct conversion of hexagonal boron nitride to the cubic form occurs at pressures up to 18 GPa and temperatures between 1730-3230 °C; addition of small amount of boron oxide can lower the required pressure to 4-7 GPa and temperature to 1500 °C. Industrially, BN conversion using catalysts is used instead; the catalyst materials differ for different production methods, eg. lithium, potassium, or magnesium, their nitrides, their fluoronitrides, water with ammonium compounds, or hydrazine. Other industrial synthesis methods use crystal growth in temperature gradient, or explosive shock wave. The shock wave method is used to produce material called 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 chemical vapor deposition, boron trifluoride is used (cf. use of atomic hydrogen for selective etching of graphite during deposition of diamond films). Ion beam deposition, Plasma Enhanced CVD, pulsed laser deposition, reactive sputtering, and other 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 carbon fibers, sometimes called "white carbon fiber." They can be prepared by thermal decomposition of extruded borazine fibers with addition of boron oxide in nitrogen at 1800 °C. The material also arises by the thermal decomposition of cellulose fibers impregnated with 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..

See also Physorg article which explains why boron nitride nanotubes may be superior to carbon nanotubes for certain applications.

Boron nitride nanomesh

Boron nitride nanomesh is a new inorganic nanostructured two-dimensional material. It consists of a single layer of hexagonal boron nitride on rhodium or ruthenium, forming a highly regular mesh. The distance between two pore centers is 3.2 nanometers 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 clusters. These characteristics promise interesting applications of the nanomesh in nanotechnology.

Amorphous boron nitride

Layers of amorphous boron nitride (a-BN) are used in some semiconductor devices, eg. MISFETs. They can be prepared by chemical decomposition of trichloroborazine with caesium, or by thermal 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 fullerene-like forms of boron nitride can be synthesized and structurally resemble carbon carbon nanotubes. The recently discovered boron nitride nanotubes are an important development due to their homogeneous electronic behavior. That is, tubes of different chiralities are all semiconductor materials with the same (approximate) band gap.

Composites containing BN

Addition of boron nitride to silicon nitride ceramics improves the thermal shock resistance of the resulting material. For the same purpose, BN is added also to silicon nitride-alumina and titanium nitride-alumina ceramics. Other materials being reinforced with BN are e.g. alumina and zirconia, borosilicate glasses, glass ceramics, enamels, and composite ceramics with titanium boride-boron nitride and titanium boride-aluminium nitride-boron nitride and silicon carbide-boron nitride composition.

Due to its excellent dielectric and thermal 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.