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
http://www.aip.org/enews/physnews/2001/split/530-2.html ), 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 NbSn 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 NbSn. 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; text at
www.aip.org/physnews/select )