One of the most important breakthroughs in experimental physics was the discovery by the Dutch physicist Heike Kamerlingh Onnes in 1911 that electrical resistance in mercury vanishes when it is cooled to temperatures close to absolute zero, which is 0 K (-273.15°C or -459.7°F). This phenomenon is known as superconductivity. Soon it was found that other metals and alloys also become superconducting at very low temperatures. For example, if one cools a ring of lead below 7.22 K (-265.93°C or -446.67°F) and induces a current in it, this current will keep flowing indefinitely.
It was not until 1957, more than 45 years after the discovery of superconductivity, that a theory explaining the phenomenon was articulated. According to the BCS theory, named after the American physicists John Bardeen, Leon N. Cooper, and J. Robert Schrieffer, the superconducting current is carried by electrons linked together through lattice vibrations (phonons), a combination that does not dissipate energy through electrical resistance.
The temperature at which a metal becomes superconducting is called the critical temperature. The higher the critical temperature of a metal or alloy, the easier the material is to use in technical applications. Superconductors have only found a marginal application in technology, however, because of the need for extreme cold. Metallic superconductors, such as the alloy of niobium and titanium often used in coils for magnets, require liquid helium at 4 K (-269°C or -452°F) for cooling. Liquid helium is expensive and superconductors remain restricted primarily to applications where the high cost of their operation is justified, as in deflection magnets used in very large particle accelerators and in magnets used for nuclear magnetic resonance imaging.
In 1986 two scientists working for IBM in Switzerland, Georg Bednorz and Alex Müller, found that certain ceramic materials that normally are electrical insulators become superconducting at low temperatures. The materials they first experimented with were made from copper oxide, lanthanum, and barium. The greatest surprise of all was the temperature at which these materials became superconducting, 30 K (-243°C or -405°F), which was higher than for any metallic superconductor. The importance of this discovery can be judged by the fact that Bednorz and Müller were awarded the Nobel Prize in physics just a year later.
An impressive series of breakthroughs followed, and for a time the discovery of a new material with a higher critical temperature was announced every few weeks. In February 1987 a team at Houston University created a superconductor with a critical temperature of 93 K (-180°C or -292°F), and in June 1988 scientists at the IBM Almaden Research Center achieved a critical temperature of 127 K (-146°C or -231°F). It looked as if the series of superconducting compounds with increasingly higher critical temperatures would go on indefinitely and that the creation of room-temperature superconductors would become an attainable dream. So far, however, this has not happened. In 2001 Jun Akimitsu of Aoyama Gakuin University in Tokyo and coworkers discovered that the common, simple compound magnesium diboride is a superconductor at temperatures as high as 39 K (-234°C or -389°F), which is considerably higher than the ordinary BCS superconductors, but not quite as high as the ceramic high-temperature superconductors. So far, the record critical temperature is from mercury-thallium-barium-calcium-copper-oxygen that becomes superconducting at 138 K (-135°C or -211°F).
From a technical point of view, the most striking advantage of ceramic superconductors is that they can operate when cooled by liquid nitrogen. The boiling point of nitrogen is 77 K (-196°C or -321°F). Liquid nitrogen is much cheaper than liquid helium (used to cool ordinary superconductors), and the required cooling machinery is much easier to operate. Shortly after the discovery of the high-temperature superconductors, it seemed possible that ceramic superconductors would soon replace the conventional superconductors cooled with liquid helium. However, a series of difficulties with the ceramic materials have largely delayed applications on a grand scale. During the early 1990s, several successes in the attempts to apply high-temperature superconductors have restored hope that they will play an important role in technology in the near future.
One of the biggest problems with ceramic superconductors is that it is very difficult to shape them into forms that are useful. One way of preparing ceramic superconductors employs the sintering techniques commonly used for manufacture of other ceramic materials. Reagents in powder form are mixed, ground, and baked. A solid-state reaction forms the final compound. This material is very brittle and therefore not very suitable for making wires or coils.
Another way of obtaining superconducting materials is the creation of films. Films are not only more flexible but can be used in electronic circuits. Superconducting films are produced by a technique called physical vapor deposition. In one method the superconducting material is vaporized by a laser beam and is allowed to condense on a surface of a substrate that is placed close to the superconducting material. Such films are more flexible, and scientists create wires and coils from them by bundling strips of film together. Some of these films can carry several ten-thousands of amperes per square centimeter when no magnetic field is present.
The most important and immediate applications of superconducting films are in electronic circuits. One such application is in microwave circuits. Microwave signals flow in a very thin layer on the outer surface of a conductor and therefore are affected considerably by the conductor's resistance.
The Josephson junction is the most likely active electronic device based on superconductors that will find applications in technology. A Josephson junction consists of two superconducting regions that are very close together, separated by an insulating or nonsuperconducting layer. Because of a quantum-mechanical effect, superconducting charge carriers can pass from one region to the other. Josephson junctions have several interesting properties. One of them is that the junction can occupy two distinct voltage states, and thus can be integrated in flip-flop or switching circuits. Because circuits based on Josephson junctions can operate much faster than those based on semiconductors, scientists expect that they will be used in the future in very fast computers.
Josephson junctions can also sense extremely small electric currents and voltages and are therefore used in SQUIDs (an acronym for Superconducting QUantum Interference Device). SQUIDs have been previously developed with conventional superconductors but now also exist made from high-temperature superconducting materials. A SQUID consists of a tiny ring of superconducting material that incorporates one or two Josephson junctions; it can detect very small magnetic fields, such as those created by the human heart or brain.
Besides electronics, high-temperature superconductors will probably cause a revolution in energy creation and transport, making the high-voltage lines that now disfigure the perimeters of large cities as much a thing of the past as the steam pipes of the industrial cities of a hundred years ago.
The most important application of superconducting wires and coils will be in the transformation and storage of energy. Magnets made with superconducting coils produce much stronger magnetic fields than those currently used in motors and generators; they are used in a number of large particle accelerators. Superconducting coils are used in prototype motors and transformers, allowing their size to be reduced considerably.
Because a current in a superconducting closed loop flows indefinitely, such a loop can be used for energy storage. The stored energy can be used when required by breaking the closed circuit. Plans exist for building huge superconducting coils mounted underground to store thousands of megawatts of energy.
Scientists are also investigating how they can apply an interesting property of superconducting materials, that of being able to remain suspended in a magnetic field, a phenomenon called magnetic levitation. Obvious candidates to make use of magnetic levitation are maglev trains, trains that move suspended above "magnetic rails." Another application under investigation are magnetic bearings in which the rotating parts do not make any mechanical contact with the outer casing of the bearing.
