
maximum achievable levitation pressure is F
max
/A ¼
B
s
2
/2m
o
¼25 N cm
2
. The experimental values for
YBCO samples mentioned above achieve 50–72%
of this upper limit. Much larger levitation pressures
up to 5000 N cm
2
are expected if a superconducting
permanent magnet with a trapped field as high as
11 T were to be used instead of a conventional per-
manent magnet.
The most important problem which has to be
solved in order to realize YBCO magnets for super-
conducting permanent magnets or for other applica-
tions at temperatures lower than 77 K is to magnetize
the superconductor. Pulsed magnetic fields can be
used for magnetizing YBCO disks, however, large
viscous forces act on the penetrating magnetic flux,
especially in the case of short rise times (B1 ms) of
the pulsed field. Therefore, (i) higher pulsed fields are
needed in order to trap the same magnetic field in the
superconductor as by applying a static field and (ii)
heat generation due to the rapid motion of magnetic
flux strongly reduces the trapped field achievable at
temperatures lower than 77 K (Yanagi et al. 1998). At
77 K, promising results were achieved by using pulsed
fields with a relatively long rise time of 16 ms (Gruss
et al. 1999).
7. Conclusions
These new superconducting permanent magnets form
a new class of ultrahigh performance permanent
magnets. The trapped flux density can be higher by
an order of magnitude compared to the remanence of
the best ferromagnetic permanent magnets (see Rare
Earth Magnets: Materials). Since stored magnetic
energies and forces scale quadratically with the
trapped flux density, these new ‘‘cold magnets’’ of-
fer magnetic energies and forces two orders of mag-
nitude higher than conventional magnets, reaching
completely new fields of applications accessible for
permanent magnets. Two examples are the rotational
frictionless support, i.e., superconducting magnetic
bearings, and frictionless linear transportation. The
latter should allow transportation of extremely heavy
goods which can not easily be moved by wheeled
systems, like concrete ports or moving bridges. Ad-
ditionally, contactless transportation, which does not
create any wear debris, can be advantageously used in
clean rooms in the microelectronics industry (for fur-
ther reading, see Superconducting Permanent Mag-
nets: Potential Applications).
See also: Superconducting Machines: Energy Stor-
age; Magnetic Levitation: Superconducting Bearings
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