202
Appendix
Since the superconducting region is limited, the condensation energy is relatively
low in spite of its very high critical temperature. This restricts the strength of flux
pinning, resulting in low critical current densities. In addition, these superconductors
are mostly used at high temperatures, the effect of thermal activation is big, and flux
creep occurs easily, which allows pinned flux lines to easily escape from the pinning
centers. Hence, the critical current density decreases even more, and the superconducting current, which has been believed to be a persistent current, decreases with
time. Thus, the irreversibility field at which the critical current density goes to zero
is much lower than the upper critical field at high temperatures. This trend is much
stronger in superconductors with larger anisotropy, and Bi-2212 superconductor is
a typical example. RE-123 (RE: rare earth element) superconductor has the lowest
anisotropy among the high-temperature superconductors, and the effect of flux creep
is not so strong. Hence, application of this superconductor is expected.
The flux pinning properties of high-temperature superconductors are complicated
on the temperature versus magnetic field plane in comparison with metallic superconductors. This is caused by the fact that the situation of the pinned flux line lattice
varies greatly. The variation depends strongly on the anisotropy, and the changes also
depend on the kind or strength of the pinning centers. The energies that determine
the behavior of the flux line system are the elastic energy U E of the flux line lattice,
the pinning energy U P , and the thermal energy U T . Hence, there are three kinds of
transition that are determined mostly by two energies. These are the melting transition between U E and U T , the glass-liquid transition between U P and U T , and the
order-disorder transition between U E and U P [9]. Thus, the mixed state in the phase
diagram on the magnetic field vs. temperature plane is divided up in a complicated
way, which is quite different from the simple mixed state discussed for metallic
superconductors.
Another characteristic point is that the superconductivity depends quite sensitively
on the carrier density. In polycrystalline superconductors, the superconductivity is
easily broken at grain boundaries due to a large distortion or a local deviation from
the stoichiometric composition that reduces the carrier density. For this reason, the
density of the superconducting current that can tunnel through grain boundaries is
quite low for polycrystalline superconductors. It is necessary to fabricate superconductors with highly aligned crystal axes to obtain a high critical current density as in
metallic superconductors. Hence, tapes of RE-123 superconductor are now fabricated
by using a highly developed thin film technique that can realize a polycrystalline long
tape about 1 km in length with a highly aligned tri-axial structure. On the other hand,
this property provides very strong pinning forces for point defects or dislocations,
which are known as weak pinning centers in metallic superconductors, because the
superconductivity will be lost in such defects due to variation in the carrier density.
Such a high crystalline alignment is not achieved in Bi-2223 tapes, and their critical
current density is lower by about two orders of magnitude in comparison with RE123. But the thickness of the superconductor is much larger than that of RE-123 thin
films, and the engineering critical current density divided by the cross-sectional area
of the tape is not significantly different between the two superconductors.
Appendix
Since the superconducting region is limited, the condensation energy is relatively
low in spite of its very high critical temperature. This restricts the strength of flux
pinning, resulting in low critical current densities. In addition, these superconductors
are mostly used at high temperatures, the effect of thermal activation is big, and flux
creep occurs easily, which allows pinned flux lines to easily escape from the pinning
centers. Hence, the critical current density decreases even more, and the superconducting current, which has been believed to be a persistent current, decreases with
time. Thus, the irreversibility field at which the critical current density goes to zero
is much lower than the upper critical field at high temperatures. This trend is much
stronger in superconductors with larger anisotropy, and Bi-2212 superconductor is
a typical example. RE-123 (RE: rare earth element) superconductor has the lowest
anisotropy among the high-temperature superconductors, and the effect of flux creep
is not so strong. Hence, application of this superconductor is expected.
The flux pinning properties of high-temperature superconductors are complicated
on the temperature versus magnetic field plane in comparison with metallic superconductors. This is caused by the fact that the situation of the pinned flux line lattice
varies greatly. The variation depends strongly on the anisotropy, and the changes also
depend on the kind or strength of the pinning centers. The energies that determine
the behavior of the flux line system are the elastic energy U E of the flux line lattice,
the pinning energy U P , and the thermal energy U T . Hence, there are three kinds of
transition that are determined mostly by two energies. These are the melting transition between U E and U T , the glass-liquid transition between U P and U T , and the
order-disorder transition between U E and U P [9]. Thus, the mixed state in the phase
diagram on the magnetic field vs. temperature plane is divided up in a complicated
way, which is quite different from the simple mixed state discussed for metallic
superconductors.
Another characteristic point is that the superconductivity depends quite sensitively
on the carrier density. In polycrystalline superconductors, the superconductivity is
easily broken at grain boundaries due to a large distortion or a local deviation from
the stoichiometric composition that reduces the carrier density. For this reason, the
density of the superconducting current that can tunnel through grain boundaries is
quite low for polycrystalline superconductors. It is necessary to fabricate superconductors with highly aligned crystal axes to obtain a high critical current density as in
metallic superconductors. Hence, tapes of RE-123 superconductor are now fabricated
by using a highly developed thin film technique that can realize a polycrystalline long
tape about 1 km in length with a highly aligned tri-axial structure. On the other hand,
this property provides very strong pinning forces for point defects or dislocations,
which are known as weak pinning centers in metallic superconductors, because the
superconductivity will be lost in such defects due to variation in the carrier density.
Such a high crystalline alignment is not achieved in Bi-2223 tapes, and their critical
current density is lower by about two orders of magnitude in comparison with RE123. But the thickness of the superconductor is much larger than that of RE-123 thin
films, and the engineering critical current density divided by the cross-sectional area
of the tape is not significantly different between the two superconductors.