Boron nitride
From Wikipedia, the free encyclopedia
| General | |
|---|---|
| Name | Boron nitride |
| Chemical formula | BN |
| Appearance | White solid |
| CAS Number | 10043-11-5 |
| Physical | |
| Formula weight | 24.818 g/mol |
| Melting Point | 2967 ° C |
| Boiling point | 3273 °C |
| Density | 2.18 ×103 kg/m3 |
| Crystal structure | hexagonal or cubic |
| Solubility | insoluble |
| Thermochemistry | |
| ΔfH0gas | 476.98 kJ/mol |
| ΔfH0solid | -250.91 kJ/mol |
| S0gas, 1 bar | 212.36 J/mol·K |
| S0solid | 14.77 J/mol·K |
| Heat of fusion | 3263.8 J/g |
| Safety | |
| Risk phrases | R36 R37 |
| Ingestion | ? |
| Inhalation | ? |
| Skin | ? |
| Eyes | ? |
| More info | ? |
| SI units were used where possible. Unless otherwise stated, standard conditions were used. | |
Boron nitride (BN) is a binary chemical compound, consisting of equal proportions of boron and nitrogen. The empirical formula is therefore BN. Boron nitride is isoelectronic to the elemental forms of carbon and isomorphism occurs between the two species. That is boron nitride possess three polymorphic forms; one analogous to diamond, one analogous to graphite and ones analogous to the fullerenes. The diamond-like allotrope of boron nitride, known as Borazon, is one of the hardest materials known but is softer than materials such as diamond, ultrahard fullerite, and aggregated diamond nanorods.
Contents |
[edit] Cubic boron nitride
The diamond-like allotrope of boron nitride, 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. Such usefulness is derived from the insolubility of boron nitride in iron, nickel and related alloys at high temperatures (unlike diamond). Like diamond, it has good thermal conductivity, caused by phonons; this is a difference against metals, where the mediators are electrons. In contact with oxygen at high temperatures, BN forms a passivation layer of boron oxide.
Commercial products are known eg. under names Borazon (by General Electric Corporation), and Elbor or Cubonite (by Russian vendors).
A crystal modification of boron nitride is w-BN, the superhard hexagonal phase of the wurzite structure. It occurs at high pressures.
Polycrystalline c-BN (PcBN) is used for wear applications. It is superior to diamond in applications requiring high temperatures in oxidizing atmosphere, and contact with iron and its alloys; c-BN abrasives are therefore used for machining steel, while diamond abrasives are preferred for aluminium alloys, ceramics, and stone.
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. Ceramic binders can be used as well.
For grinding applications, softer binders, eg. resin, porous ceramics, and soft metals, are used.
Sintered cubic boron nitride can be used in electronics as an electrically insulating heatsink material.
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 industrual 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, but somewhat challenging. 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.
The band gap of cubic BN is 6.2 eV, similar to that of diamond. Like diamond, cubic boron nitride can be synthetized with semiconductor material properties. Beryllium can be used as a p-type semiconductor dopant, sulfur or silicon yield p-type semiconductor. The resulting diodes can be used up to 600 °C, and boron nitride LEDs can operate in ultraviolet region.
[edit] Hexagonal boron nitride
The graphite-like allotrope of boron nitride, known as hexagonal boron nitride, h-BN, α-BN, or g-BN (graphitic BN), and sometimes called white graphite, is useful as both a very low temperature and high-temperature lubricant (up to 900 °C in oxidizing atmosphere) and/or in situations where the electrical conductivity or chemical reactivity of graphite would be problematic. As the lubricity mechanism does not involve water molecules trapped between the layers, boron nitride lubricants can be used even in vacuum, eg. for space applications.
Due to higher electronegativity of the nitrogen atoms, the electrons that in graphite form a delocalized system, are concentrated around nitrogen atoms, sequestered outside the conductivity band, therefore not playing role in conductivity nor absorbing visible light.
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 eg. bearings. Plastics filled with BN have decresaed thermal expansion, increased thermal conductivity, increased electrical insulation properties, and cause reduced wear to adjanced parts.
Hexagonal boron nitride is stable in temperatures up to 1000 °C in air, 1400 °C in vacuum, and 2800 °C in inert gas. 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 (eg. aluminium, copper, zinc, iron and steels, germanium, silicon, boron, cryolite, glass and halide salts). 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.
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 eg. 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 insulating properties, BN is used in electronics eg. as a substrate for semiconductors, microwave-transparent windows, structural material for seals, electrodes and catalyst carriers in fuel cells and batteries.
Fine-grained h-BN is used in some cosmetics, paints, dental cements, pencil leads, etc.
Hexagonal boron nitride is produced by the nitridation or ammonolysis of boron trioxide. 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 is used for production of ultrafine boron nitride for lubricants and toners.
[edit] Boron nitride fibers
Hexagonal BN can be prepared in the form of fibers, structurally similar to carbon fibers, sometimes called white carbon fiber. Two of the main methods of their synthesis are thermal decomposition of extruded borazine fibers with addition of boron oxide in nitrogen at 1800 °C, and thermal decomposition of cellulose fibers impregnated with boric acid or ammonium tetraborate in the mixture 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).
[edit] 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 cesium, 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.
[edit] Rhomboedral boron nitride
Rhomboedral boron nitride is similar to hexagonal boron nitride. It is formed transitionally during conversion of cubic BN to hexagonal form.
[edit] Other allotropes
The fullerene-like allotropes of boron nitride can be synthesized and resemble those of carbon. 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.
[edit] See also
[edit] External links
- National Pollutant Inventory - Boron and Compounds
- Fiz Chemie Berlin thermophysical database
- Materials Safety Data Sheet at University of Oxford
