Page 411 - Electrical Properties of Materials

Some applications 393

14.8 Some applications
14.8.1 High-field magnets
For the moment the most important practical application of superconductivity
is in producing a high magnetic field. There is no doubt that for this purpose
a superconducting solenoid is superior to conventional magnets. A magnetic
flux density of 20 T can be produced by a solenoid not larger than about
12 cm × 20 cm. A conventional magnet capable of producing one-third of that
flux density would look like a monster in comparison and would need a few
megawatts of electric power and at least a few hundred gallons of cooling water
per minute.
What sort of materials do we need for obtaining high magnetic fields? Obv-
iously, type II superconductors—they remain superconducting up to quite high
magnetic fields. However, high magnetic fields are allowed only at certain
points in the superconductor that are surrounded by current vortices. When
a d.c. current flows (so as to produce the high magnetic field in the solenoid)
the vortices experience a J × B force that removes the vortices from the mat-
erial. To exclude the high magnetic field costs energy, and the superconductor
consequently becomes normal, which is highly undesirable. The problem is to
keep the high magnetic fields inside. This is really a problem similar to the one
we encountered in producing ‘hard’ magnetic materials, where the aim was to
prevent the motion of domain walls. The remedy is similar; we must have lots
of structural defects; that is we must make our superconductor as ‘dirty’ and
as ‘non-ideal’ as possible. The resulting materials are, by analogy, called hard
superconductors. Some of their properties are shown in Table 14.2.
There is, however, a further difficulty with vortices. Even if they do not
move out of the material, any motion represents ohmic loss, causing heating,
and making the material become normal at certain places. To avoid this, a good
thermal conductor and poor electrical conductor, copper—yes, copper—is
used for insulation, so that the heat generated can be quickly led away.
It must be noted that it is not particularly easy to produce any of these
compounds, and different techniques may easily lead to somewhat different

Table 14.2 The critical temperature and critical
magnetic field (at T =4.2 K) of the more important
hard superconductors

–1
Material T c (K) H c ×10 –7 (A m )
Nb–Ti 9 0.9
Pb 0.9 Mo 5.1 S 6 14.4 4.8
V 3 Ga 14.8 1.9
NbN 15.7 0.8
V 3 Si 16.9 1.8
Nb 3 Sn 18.0 2.1
Nb 3 Ga 20.2 2.6
Nb 3 (Al 0.7 Ge 0.3 ) 20.7 3.3
Nb 3 Ge 22.5 2.9