Superconductivity
Superconductivity, the property that certain materials have of losing all resistance to an electric current. Such a material loses its electrical resistance when cooled below a temperature called the material's critical (or transition) temperature. Many pure metals and alloys are superconducting, but only at temperatures near absolute zero (0 K, or -273.15° C. [-459.67° F.]). Some niobium alloys have critical temperatures near 20 K (-253° C.). Several synthetic copper-oxide materials have higher critical temperatures; one such material, containing thallium, has a critical temperature near 125 K (-158° C.).
The most important use of superconducting materials, or superconductors, is for making powerful electromagnets. A common practical application of such magnets is in magnetic resonance imaging (MRI) devices, used for medical diagnosis. The operation of various types of research equipment also depends on superconducting magnets. The Tevatron, a powerful particle accelerator at the Fermi National Accelerator Laboratory in Illinois, uses hundreds of such magnets for its operation. Superconducting magnets are usually made of niobium alloys that can carry a very strong electric current without losing their superconductivity. Although the electromagnets must be cooled with liquid helium, requiring complex cooling apparatus, they consume much less electrical power than comparable conventional electromagnets.
Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes while he was studying the conductivity of mercury cooled to very low temperatures. In 1957, John Bardeen, Leon N. Cooper, and John R. Schrieffer developed a theory that successfully explains superconductivity in terms of an interaction between electrons that prevents them from scattering when they flow through a material at or below its critical temperature.
In 1986, J. Georg Bednorz and K. Alex Mller discovered superconductivity in a compound of lanthanum, barium, copper, and oxygen at a temperature near 35 K (-238° C.). This discovery, for which Bednorz and Mller were awarded the 1987 Nobel Prize in physics, sparked a flurry of research focused on copper-oxide materials, and, within little more than a year, scientists found similar materials having a critical temperature as high as 95 K (-178° C.).
These new materials showed great potential for a variety of uses, but were difficult to produce commercially. In the 1990's, researchers discovered ways to craft relatively high-temperature superconductors into useful magnetic components for research and for medical diagnostics.