Magnetic Field Sources - Magnetocaloric Energy Conversion: From Theory to Applications - page 65

refrigerators, this will be rather dif cult to perform at a particular research institute.

More likely, industry has the potential for such developments. Therefore, we would
like to encourage the industry involved in superconducting power applications to
invest in the development of large-scale superconducting magnetic chillers or heat
pumps.

3.3.2 Superconducting Magnets

The phenomenon of superconductivity has been known since the discovery of
Onnes in 1911 [ 18 ]. He observed (in a sample of mercury), that certain electrical
conductors display zero DC electrical resistance. Since then, a number of metals
and alloys have been found to possess superconductivity at different temperatures
from one degree of Kelvin up to much higher temperatures. In 1933 Meissner and
Ochsenfeld [ 19 ] discovered extraordinary magnetic properties in superconductors.
These, when cooled below their critical (superconducting transition) temperature
and under an applied magnetic

eld, acted as a magnetic shield by expelling the

magnetic nition of the Meissner effect

(Fig. 3.23 ), which states that in the superconductor, when this is in its supercon-
ducting state, the magnetic flux density is zero [ 20 ]. This effect is not related to the
zero electrical resistance, but represents an additional property of superconductors.
If we consider the equation for the magnetic

fl ux around the sample. This led to the de

fl ux density of a magnetic material

(Eq. 3.36 ) B = ʼ 0 (H + M), then it follows for B = 0 and H > 0 that the magnetization

M should be negative. Therefore, a superconductor under an applied magnetic eld

will possess a negative magnetization. This results from the electric current, owing

without resistance around the outer envelope (surface) of the superconductor.
There are type-I and type-II superconductors. In type-II superconductors, the
surface energy of the superconducting/normal

fl

interface is negative. Therefore,

under the applied magnetic eld, in a type-II superconductor, a negative magne-

tization will be produced by the surface current. The same happens with type-I
superconductors, but only until the lower critical

eld at which the magnetic fl ux

Fig. 3.23 Two different magnetic states of superconductors due to the Meissner effect