Magnesium diboride

Magnesium diboride (MgB₂) is an inexpensive and simple superconductor. Its superconductivity was announced in the journal Nature in March 2001.^[1] Its critical temperature of 39 K is the highest amongst conventional superconductors. This material was first synthesized in 1953 but its superconducting properties were not discovered until half a century later.

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Though 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.

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.^[2] 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.

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.^[3]

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.

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