Page 417 - Electrical Properties of Materials

High-T c superconductors 399

in a stoichiometric mixture of its constituents) and they are polycrystalline.
But if superconductive properties are anisotropic, how will they survive in
a polycrystalline material? The answer is that any departure from the single
crystal form is deleterious but not necessarily disastrous. Josephson tunnel-
ling comes to our aid in the sense that superconducting electrons may tunnel
across disoriented grain boundaries, provided the angle of disorientation is
small.
What about applications? What has become of the dazzling prospects of lev-
itated trains, electromagnetically propelled ships, and magnetic energy storage
devices? Not in the near future, is the answer. Some applications, however, are
bound to come quite soon, since there are obvious economic benefits to work-
ing at 77 K (using liquid nitrogen) in contrast with 4.2 K (using liquid helium).
Liquid nitrogen costs only as much as a cheap beer, whereas liquid helium is in
the class of a reasonably good brandy, so maintaining the samples at the right
temperature will be much cheaper. The application that is closest is probably
in electronic devices, and the property used is the lack of electrical resistance.
So total heat dissipated is reduced, which is good and particularly good in
preventing thermally activated damage like corrosion and electromigration of
atoms. In heavy current engineering, the most likely candidates for applica-
tions are underground cables. The present cables are made of copper and are
cooled by oil. In the future cables replacing them will most likely be made
of high-T c materials cooled by liquid nitrogen. Highly rated transformers and
coils for rotors in motors and generators are also close contenders.
At microwave frequencies, superconductors can no longer offer zero res-
istivities. However, their lower resistance is still a major advantage in mi-
crowave resonators. There were already some applications using conventional
superconductors, but chances have very much improved with the advent of
high T c superconductors. We would just like to mention one successful device,
the disk resonator shown in Fig. 14.23(a). The resonance occurs in the same
manner as in the Fabry–Perot resonator discussed in Section 12.5. The main
difference is that, in the present case, it is possible to excite a mode which

Fig. 14.23
(a) A microwave disc resonator (b) current distribution on the surface of the disc.