Boron nitride
3370
223666760
2008-07-05T04:28:45Z
Shalomamigos
7418035
/* Cubic boron nitride */
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| Name = Boron nitride
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| CASNo = 10043-11-5
| EINECS = 233-136-6
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| Section2 = {{Chembox Properties
| Formula = BN
| MolarMass = 24.818 g mol<sup>−1</sup>
| Appearance = white solid
| Density = 2.18 g cm<sup>−3</sup>
| MeltingPt = 2700 °C
| Melting_notes = sublimes
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| Solubility = insoluble
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| Section3 = {{Chembox Structure
| CrystalStruct = [[hexagonal]] or [[tetrahedral]]-[[cubic crystal system|cubic]]
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| Section4 = {{Chembox Thermochemistry
| DeltaHf = 476.98 [[joule|kJ]] [[mole (unit)|mol<sup>−1</sup>]]
| DeltaHc = −250.91 kJ mol<sup>−1</sup>
| Entropy = 14.77 J mol<sup>−1</sup> K<sup>−1</sup>
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| Section8 = {{Chembox Related
| OtherAnions = [[boron phosphide|BP]], [[boron arsenide|BAs]]<br />[[boron carbide|B<sub>4</sub>C]], [[boron trioxide|B<sub>2</sub>O<sub>3</sub>]]
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'''Boron nitride''' ('''BN''') is a [[binary compound|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 [[Polymorphism (materials science)|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.<ref>Jochen Greim, Karl A. Schwetz “Boron Carbide, Boron Nitride, and Metal Borides” in Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH: Weinheim: 2005. DOI: 10.1002/14356007.a04_295.pub2</ref> 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 [[wetting|wetted]] by many melted materials (e.g. aluminium, copper, zinc, iron and steels, germanium, silicon, boron, cryolite, glass and halide salts).{{Fact|date=December 2007}}
Fine-grained h-BN is used in some [[cosmetics]], [[paint]]s, dental cements, and [[pencil]] leads.{{Fact|date=December 2007}}
====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 (physics)|plasma]] at 5500 °C yields [[Ultrafine particles|ultrafine]] boron nitride for lubricants and [[toner]]s.
<gallery>
Image:Boron-nitride-(hexagonal)-side-3D-balls.png|<center>α-BN, hexagonal </center>
Image:Boron-nitride-(hexagonal)-top-3D-balls.png|<center>α-BN, hexagonal</center>
Image:Boron-nitride-(sphalerite)-3D-balls.png|<center>β-BN, [[sphalerite]] structure</center>
Image:Boron-nitride-(wurtzite)-3D-balls.png|<center>BN, [[wurtzite]] structure</center>
</gallery>
==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 [[electrical insulation|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<ref>Tool coatings-new developments for forming, cutting and abrasive machining. Gaebler, J.; Bewilogua, K.; Biehl, S.; Brand, J.; Hoefer, M.; Keunecke, M.; Schaefer, L.; Thomsen, H.; Weber, M. Fraunhofer-Institut fuer Schicht- und Oberflaechentechnik IST, Braunschweig, Germany. Annual Technical Conference Proceedings - Society of Vacuum Coaters (2007), 50th 608-615. Publisher: Society of Vacuum Coaters</ref>. 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 [[phonon]]s. 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 bit]]s of [[cutting tool]]s. 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 fiber]]s, 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 material]]s, 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 (<sup>10</sup>B) into structural materials improves their radiation-shielding properties, due to <sup>10</sup>B's neutron absorption properties. Such <sup>10</sup>BN 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.<ref>http://wwwrsphysse.anu.edu.au/nanotube/bnnts.php</ref>
===Boron nitride nanomesh===
[[Image:nanomesh 3D.jpg|thumb|Perspective view of nanomesh (structure ends at the back of the figure)]]
[[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 [[nanometer]]s 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 <sup>o</sup>C. In addition, it shows the extraordinary ability to trap [[molecules]] and metallic [[cluster (physics)|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. [[MISFET]]s. 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 nanotube]]s. The recently discovered boron nitride [[nanotube]]s are an important development due to their homogeneous electronic behavior. That is, tubes of different [[chirality (chemistry)|chiralities]] are all [[semiconductor material]]s 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 glass]]es, [[glass ceramic]]s, [[vitreous enamel|enamel]]s, 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 cell]]s 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. [[bearing]]s. Plastics filled with BN have decreased thermal expansion, increased thermal conductivity, increased electrical insulation properties, and cause reduced wear to adjacent parts.<!--poor construction-->
==See also==
*[[Beta carbon nitride]]
*[[Borazon]]
*[[Boron phosphide]]
*[[Boron suboxide]]
*[[Aluminium nitride]]
*[[Wide bandgap semiconductors]]
==References==
{{Reflist}}
{{Refimprove|date=January 2008}}
==External links==
*[http://www.npi.gov.au/database/substance-info/profiles/15.html National Pollutant Inventory: Boron and Compounds]
*[http://www.fiz-chemie.de/infotherm/html/molpages/00%5C35%5C/mol3597.html Fiz Chemie Berlin] thermophysical database
*[http://ptcl.chem.ox.ac.uk/MSDS/BO/boron_nitride.html Materials Safety Data Sheet] at University of Oxford
[[Category:Boron compounds]]
[[Category:Ceramic materials]]
[[Category:III-V compounds]]
[[Category:Lubricants]]
[[Category:Nitrides]]
[[Category:Semiconductor materials]]
[[Category:Superhard materials]]
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