Magnesium diboride

From Wikipedia, the free encyclopedia

Magnesium diboride
Image:Magnesium diboride.jpg
Systematic name	Magnesium diboride
Molecular formula	MgB₂
Molar mass	45.93 g/mol
Density	2.6 g/cm³
Solubility (water)	x.xx g/l
Melting point	1300 °C (decomp.)
Boiling point	xx.x °C
CAS number	[12007-25-9]
Disclaimer and references

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

Though a conventional 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 Celsius, however, since Magnesium metal melts at 652 Celsius, 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.

Engineering wire production is typically via the Powder In Tube process. Either (in-situ) a mixture of boron and magnesium is poured into a metal tube, or (ex-situ) magnesium diboride powder is poured into a tube. In both cases, the tube is reduced in diameter to form a wire by conventional wire drawing processes. Then the finished wire is heat-treated, either at the reaction temperature (in-situ) or at approximately 800 to 1000 degrees Celsius (ex situ). The ex-situ heat treatment is a conventional sintering process. In both cases, later hot isostatic pressing at approximately 950 degrees Celsius further improves the properties.

Its superconducting properties and cheapness make it useful for a variety of applications. Thin coatings can be used in superconducting radio frequency cavities to minimize energy loss and reduce the inefficiency of liqiuid helium cooled niobium cavities. Due to the low cost of its constituent materials, MgB₂, has promise for use in superconducting low/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.