Magnesium diboride

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Magnesium diboride
Ball-and-stick model of the part of the crystal structure of magnesium diboride
Identifiers
CAS number [12007-25-9]
Properties
Molecular formula MgB2
Molar mass 45.93 g/mol
Density 2.6 g/cm3
Melting point

1300 °C (decomp.)
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references

Magnesium diboride (MgB2) is an inexpensive and simple superconductor. Its superconductivity was announced in the journal Nature in March 2001.[1] Its critical temperature (Tc) of 39 K is the highest amongst conventional superconductors. This material was first synthesized and its structure confirmed in 1953 [2] 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 MgB2 has not yet been achieved, particularly so in the presence of a magnetic field.

Contents

[edit] Synthesis

Magnesium diboride can be synthesized by several routes. The simplest is by high temperature reaction between boron and magnesium powders. 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 MgB2 inside. In the ex situ variant, the tube is filled with MgB2 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 (MgB2) thin films.[3] 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 MgB2 films with smooth surfaces, which are required to make reproducible uniform Josephson junctions, the fundamental element of superconducting circuits.

[edit] Electromagnetic properties

Properties depend greatly on composition and fabrication process. Many properties are ansiotropic due to the layered structure.

Tc 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 MgB2 with carbon (eg using 10% malic acid) can improve the upper critical field and the maximum current density.

[edit] Applications

Its superconducting properties and cheapness make magnesium diboride useful for a variety of applications. Recently, a 0.5 tesla open MRI system has been successfully designed and built using 18 km of MgB2 conductors. This MRI did not need any cryogenic liquids for cooling.[4]

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, MgB2 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 MgB2 in this sector as well.

[edit] References

  1. ^ Jun Nagamatsu, Norimasa Nakagawa, Takahiro Muranaka, Yuji Zenitani and Jun Akimitsu (1 Mar 2001). "Superconductivity at 39 K in magnesium diboride" (letter). Nature 410: 63-64. doi:10.1038/35065039. 
  2. ^ Morton E. Jones and Richard E. Marsh (5 Mar 1954). "The Preparation and Structure of Magnesium Boride, MgB2". Journal of the American Chemical Society 76: 1434-1436. doi:10.1021/ja01634a089. 
  3. ^ X.X. Xi et al, (14 February 2007). "MgB2 thin films by hybrid physical-chemical vapor deposition". Physica C 456: 22-37. doi:10.1016/j.physc.2007.01.029. 
  4. ^ Press release.pdf

[edit] External links