Superconductivity

Superconductivity is a phenomenon in which certain metals, alloys, organic compounds, and ceramics conduct electricity without resistance. Superconductivity in most of these materials occurs at temperatures near absolute zero, which is -273.15 ºC (-459.67 ºF). Lead and mercury become good superconductors at such temperatures. But some ceramics become superconductors at temperatures as high as -138 ºC (-216 ºF).

The modern theory of superconductivity was developed by three American physicists--John Bardeen, Leon N. Cooper, and John Robert Schrieffer. It is known as the BCS theory, after the men who developed it. They received the 1972 Nobel Prize for physics for their work. According to their theory, a superconductor has no electrical resistance because of an attractive interaction between its electrons that results in the formation of pairs of electrons. These electron pairs are bound to one another and flow without resistance around impurities and other imperfections. In an ordinary conductor, resistance occurs because its unbound electrons collide with imperfections and then scatter.

Superconductivity is used in the field of electromagnetics. Researchers have developed powerful superconducting magnets, which use much less electricity than ordinary electromagnets do. Superconducting magnets enable physicists to build more efficient particle accelerators, which are devices that increase the speed of atomic particles.

In 1986, West German physicist J. Georg Bednorz and Swiss physicist K. Alex Muller announced their discovery of superconductivity in a ceramic material. This material becomes superconducting at a higher temperature than metals or alloys. Bednorz and Muller received the 1987 Nobel Prize for physics for this discovery. Scientists have since found other ceramics that become superconducting at high enough temperatures to allow liquid nitrogen to be used for cooling them. Metals and alloys must be cooled to superconducting temperatures with liquid helium, which is far costlier and more difficult to handle than liquid nitrogen.

Today, scientists are investigating possible uses for the higher-temperature superconducting materials. For example, they are testing superconducting switching devices that control electronic circuits in computers. These devices operate extremely quickly and produce almost no heat. Superconducting materials may be used in devices that measure extremely small magnetic fields for medical diagnosis and other purposes. Superconductivity may also be useful for making more compact and efficient electric motors and generators. In addition, power lines made of superconducting materials could carry current over long distances without any loss of energy from electrical resistance. Some problems must be solved before ceramic superconductors can be used commercially. Many superconducting ceramics are hard to manufacture. Ceramics also are brittle and not easily formed into useful electrical wires. However, researchers have developed thin, flexible tapes that can carry large currents.

A version of the BCS theory of superconductivity may explain how superconductivity occurs in ceramic materials. But no complete theory of this phenomenon has yet been proposed. Scientists hope that a fuller understanding of this process will help them develop materials whose superconducting temperatures are much higher than those of today's ceramics.

The Dutch physicist Heike Kamerlingh Onnes discovered superconductivity in 1911. He made the discovery while measuring the electrical resistance of frozen mercury.

Contributor: Roger B. Poeppel, Ph.D., Director, Energy Technology Division, Argonne National Laboratory.

Superconductivity

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