Magnesium-Diboride Superconductors

Magnesium-diboride superconductors can tolerate twice the usual amount of magnetic field if you spike them with some carbon atoms. The main reason superconducting wires are used as the windings in magnets is not because they save energy, but because they can generate large magnetic fields by carrying large current densities without the resistive heating associated with ordinary copper wire, giving you a much more intense field for the same amount of volume employed in your MRI machine.

MgB₂ superconductors, which made their debut three years ago (see Update 530), become superconducting at around 40 K, in a colder regime than for the ceramic superconductors (which can be bathed in liquid nitrogen), but much warmer than traditional metal superconductors (such as niobium-tin) which must be cooled in liquid helium. Some consider that the MgB₂ materials (which can be chilled with refrigerators without the use of expensive liquid helium) might be advantageous in some applications where Nb₃Sn is presently used. For this to happen, the MgB₂ materials need to be able to stand up to high fields and high current densities.

At Iowa State, a new test of carbon-doped MgB₂ shows that the critical field can now be doubled, up to a value of 32.5 Tesla; this is the field at which superconductivity in unadulterated MgB₂ would be undone. This is now higher than the best value for Nb₃Sn. The researchers (contact Paul Canfield, canfield@ameslab.gov, 515-294-6270) would like MgB₂ to tolerate even higher fields, and to enhance the critical current too. (Wilke et al., Physical Review Letters, upcoming article.)