Magnesium diboride
459872
223674095
2008-07-05T05:32:49Z
71.0.54.103
/* Electromagnetic properties */
{{Refimprove|date=September 2007}}
{{Chembox new
| Name = Magnesium diboride
| ImageFile = Magnesium-diboride-3D-balls.png
<!-- | ImageSize = 250px -->
| ImageName = Ball-and-stick model of the part of the crystal structure of magnesium diboride
| Section1 = {{Chembox Identifiers
| CASNo = 12007-25-9
}}
| Section2 = {{Chembox Properties
| Formula = MgB<sub>2</sub>
| MolarMass = 45.93 g/mol
| Density = 2.6 g/cm<sup>3</sup>
| Solvent = other solvents
| SolubleOther =
| MeltingPt = 1300 °C (decomp.)
| BoilingPt =}}
}}
'''Magnesium diboride''' (MgB<sub>2</sub>) is an inexpensive and simple [[superconductor]]. Its superconductivity was announced in the journal ''[[Nature (journal)|Nature]]'' in March 2001.<ref>{{cite journal | format = letter | title = Superconductivity at 39 K in magnesium diboride | author = Jun Nagamatsu, Norimasa Nakagawa, Takahiro Muranaka, Yuji Zenitani and Jun Akimitsu | journal = [[Nature (journal)|Nature]] | volume = 410 | pages = 63–64 | date = 1 Mar 2001 | doi = 10.1038/35065039}}</ref> Its [[critical temperature#Superconductivity|critical temperature]] (T<sub>c</sub>) of 39 K is the highest amongst [[conventional superconductor]]s. This material was first synthesized and its structure confirmed in 1953 <ref>{{cite journal | title = The Preparation and Structure of Magnesium Boride, MgB2 | author = Morton E. Jones and Richard E. Marsh
| journal = Journal of the American Chemical Society | volume = 76| pages = 1434–1436 | date = 5 Mar 1954 | doi = 10.1021/ja01634a089}}</ref> but its superconducting properties were not discovered until half a century later.
Though generally believed to be
a conventional ([[phonon]]-mediated) superconductor, it is a rather unusual one. Its electronic structure is such that there exist two types of electrons at the [[Fermi level]] with widely differing behaviours, one of them being much more strongly superconducting than the other. This is at odds with usual theories of phonon-mediated superconductivity which assume that all electrons behave in the same manner. For this reason, theoretical understanding of the properties of MgB<sub>2</sub> has not yet been achieved, particularly so in the presence of a [[magnetic field]]. In 2001 it was regarded as behaving more like a low-T<sub>c</sub> metallic than a high-T<sub>c</sub> [[cuprate]] superconductor <ref>http://arxiv.org/abs/cond-mat/0102216</ref> .
==Synthesis==
Magnesium diboride can be synthesized by several routes. The simplest is by high temperature reaction between [[boron]] and [[magnesium]] powders <ref>http://arxiv.org/abs/cond-mat/0102216</ref>. Formation begins at 650 °C; however, since magnesium metal melts at 652 °C, the reaction mechanism is considered to be moderated by magnesium vapor [[diffusion]] across boron grain boundaries. At conventional reaction temperatures, [[sintering]] is minimal, although enough grain recrystallization occurs to permit Josephson [[quantum tunnelling]] between grains.
Superconducting magnesium diboride wire can be produced through the [[powder in tube]] (PIT) process. In the ''in situ'' variant, a mixture of boron and magnesium is poured into a metal tube, which is reduced in diameter by conventional [[wire drawing]]. The wire is then heated to the reaction temperature to form MgB<sub>2</sub> inside. In the ''ex situ'' variant, the tube is filled with MgB<sub>2</sub> powder, reduced in diameter, and sintered at 800 to 1000 °C. In both cases, later hot isostatic pressing at approximately 950 °C further improves the properties.
[[Hybrid Physical-Chemical Vapor Deposition]] (HPCVD) has been the most effective technique for depositing magnesium diboride (MgB<sub>2</sub>) thin films.<ref>{{cite journal | title = MgB<sub>2</sub> thin films by hybrid physical-chemical vapor deposition | author = X.X. Xi et al, | journal = Physica C | volume = 456 | pages = 22–37 | date = 14 February 2007 | doi = 10.1016/j.physc.2007.01.029}}</ref> The surfaces of MgB2 films deposited by other technologies are usually rough and non-[[stoichiometric]]. Instead, the [[HPCVD]] system can grow high-quality ''in situ'' pure MgB<sub>2</sub> films with smooth surfaces, which are required to make reproducible uniform [[Josephson effect|Josephson junctions]], the fundamental element of superconducting circuits.
==Electromagnetic properties==
Properties depend greatly on composition and fabrication process. Many properties are anisotropic due to the layered structure.
T<sub>c</sub> up to 39 K.
As of 2008 :
[[Upper critical field]] (parallel to ab planes) approx 14.8 [[tesla]].
Upper critical field (perpendicular to ab planes) approx 3.3 tesla.
Upper critical field in thin films up to 74 tesla.
Upper critical field in fibres up to 55 tesla.
Various means of doping MgB<sub>2</sub> with carbon (eg using 10% malic acid) can improve the upper critical field and the maximum current density.
==Applications==
Its superconducting properties and cheapness make magnesium diboride useful for a variety of applications. Recently, a 0.5 tesla open [[magnetic resonance imaging|MRI]] system has been successfully designed and built using 18 km of MgB<sub>2</sub> conductors. This MRI used a closed-loop [[cryocooler]], without requiring externally supplied cryogenic liquids for cooling.<ref>[http://www.columbussuperconductors.com/Press%20release.pdf Press release.pdf]</ref>
Thin coatings can be used in superconducting radio frequency cavities to minimize energy loss and reduce the inefficiency of liquid helium cooled niobium cavities. Due to the low cost of its constituent elements, MgB<sub>2</sub> has promise for use in superconducting low to medium field magnets, electric motors and generators, fault current limiters and current leads. The relatively low working temperature (compared with high temperature superconductors) means that cooling costs make it an unlikely candidate for power lines, although the hope in the future hydrogen technology could enable the use of MgB<sub>2</sub> in this sector as well.
==References==
{{reflist}}
==External links==
* [http://www.esi-topics.com/mgb2/ Essential Science Indicators on MgB2]
* [http://www.eurekalert.org/features/doe/2001-12/dl-omm060502.php Old material makes a new debut], US Department of Energy Research News
[[Category:Borides]]
[[Category:Magnesium compounds]]
[[Category:Superconductors]]
[[de:Magnesiumdiborid]]
[[es:Boruro de magnesio]]
[[ja:二ホウ化マグネシウム]]
[[sr:Магнезијум диборит]]
[[zh:二硼化鎂]]