380 Chapter Ten
type I superconductors. Because Bc 2 can be quite high (Table 10.4), type II supercon-
ductors are used to make high-field (up to 20 T) magnets for particle accelerators, fusion
reactors, magnetic resonance imagery, and experimental maglev(magnetic levitation)
trains in which magnetic forces provide both propulsion and frictionless support.D
espite much effort, until 1986 no superconductor was known whose critical tempera-
ture was higher than 27 K. In that year Alex Muller and Georg Bednorz, working in
Switzerland, studied a class of ceramic materials that had never before been suspected of su-
perconducting behavior. They discovered an oxide of lanthanum, barium, and copper for which
Tcwas 30 K, and soon afterward others extended their approach to produce superconductors
with critical temperatures of as high as 134 K (139°C) for an oxide of mercury, barium,
calcium, and copper. (This material has an even higher critical temperature when under pres-
sure.) Although still extremely cold by everyday standards, such temperatures are above the
77-K boiling point of liquid nitrogen, which is cheap (cheaper than milk) and readily avail-
able, unlike the liquid helium needed for earlier superconductors.
The new superconductors are all type II and some have high Bc 2 values. The ceramic crys-
tals consist of layers of copper oxide sandwiched between layers of the other metal oxides. The
superconduction occurs in the copper oxide, normally an insulator. Despite much study, the ex-
act mechanism of current flow remains unknown, but it is definitely not the same as in ordinary
superconductors.
A number of problems have prevented the wide use of the new superconductors thus far.
For instance, like other ceramic crystals they are brittle and difficult to make into wires, cannot
carry high currents, and tend to be unstable over long periods. However, methods have been
devised to overcome or sidestep these difficulties; one is to encase granules of superconduct-
ing material in silver tubes that are then drawn into thin filaments and finally bundled into ca-
bles or ribbons. For electric power transmission, the superconducting cables are placed in an
insulated pipe through which liquid nitrogen is circulated. The result is not necessarily cheaper
than a copper cable that can carry the same current but it is much smaller and lighter. This
makes superconducting pipes attractive in such applications as adding electric distribution ca-
pacity by replacing copper cables in places where cable ducts are already full, a common situ-
ation in cities.
A material that is superconducting at room temperature would revolutionize technology. In
addition, by reducing the waste of electrical energy (about 10 percent of the electrical energy
generated in the United States is lost as heat in transmission lines), the rate at which the world’s
resources are being depleted would be reduced. Since 1986 such a material no long seems
inconceivable.High-Temperature Superconductors
Table 10.4 Critical Temperatures and
Upper Critical Magnetic Fields (at T
0) of Some Type II SuperconductorsSuperconductor Tc, K Bc 2 (0),TNb 3 Sn 18.0 24.5
Nb 3 Ge 23.2 38
Nb 3 Al 18.7 32.4
Nb 3 (AlGe) 20.7 44
V 3 Ge 14.8 2.08
V 3 Si 16.9 2.35
PbMoS 14.4 6.0bei48482_ch10.qxd 4/8/03 20:19 Page 380 RKAUL-7 Rkaul-07:Desktop Folder:bei: