Page 418 - Electrical Properties of Materials

400 Superconductivity

leads to very low losses, since the current disappears at the edges. The calcu-
lated current distribution is shown in Fig. 14.23(b). At a frequency of 4.7 GHz
and a temperature of 60 K, the measured Q (quality) factor was close to 20 000
in contrast with the 600 achievable by copper. The superconductor was one of
the TBCCO family deposited in thin film form by DC sputtering.
Will high T c superconductors make a big difference in the performance of
high-field magnets? They probably will in due course, but there are lots of
problems at present. It is difficult to reach high critical current densities be-
cause of the granular nature of these materials, as already mentioned. If we
have to rely on Josephson tunnelling across grain boundaries, this means that
the current cannot exceed the critical current that makes the tunnelling normal
(cf. Section 14.7).
The greatest success so far has been achieved with BSCCO, which may
have the composition of Bi 2 Sr 2 CaCu 2 O 8 (known as Bi-2212, T c ≈ 85 K) or
Bi 2 Sr 2 Ca 2 Cu 3 O 10 (known as Bi-2223, T c ≈ 110 K). It does not have particu-
larly good properties at 77 K, as may be seen in Fig. 14.24. The critical current
declines very rapidly with magnetic field (dotted lines). However, at 4.2 K
(i.e. well below its critical temperature) BSCCO has properties superior to tra-
5
ditional superconductors. It still has a critical current density of 10 Acm –2
over 20 T. These results were obtained with tapes with a high degree of crys-
tallographic alignment. Wires with this performance are not available as yet,
but a practical device capable of producing 20 to 25 tesla is clearly feasible.
A further useful property of BSCCO is that their critical currents are fairly
independent of temperature in the 4–20 K range; hence instead of being dipped
into liquid helium, they could be kept in the right temperature range using
refrigerators.
So what can we say about the applications of high-T c superconductors? The
initial euphoria has evaporated, but it still seems very likely that many use-
ful devices will appear in the fullness of time. We have to repeat what we
said in Section 9.1. Revolutions are few and far between, or, perhaps more
appropriately: all revolutions, sooner or later, reach their thermidors.

10 6

Critical current density (A/cm -2 ) 10 5 4 77K Nb–Ti 4.2K BSCCO

Fig. 14.24 10 Nb Sn
3
Critical current densities as a function BSCCO
of magnetic field at 77 K (- - -) and at 3
10
4.2 K(—) for BSCCO, Nb–Ti, and 0 5 10 15 20 25
Nb 3 Sn. Magnetic field (T)