Superconductor Technology
A superconductor is a material that loses all resistance (called "zero resistance") to the flow of direct electrical current and nearly all resistance to the flow of alternating current when three conditions are met: (1) the material is cooled below a characteristic temperature, known as its critical temperature; (2) a current passing through a given cross-section of the material must be at, or below, its (characteristic) critical current density; and (3) the magnetic field to which the material is exposed must be below its (characteristic) critical magnetic field. Superconductivity of a material is activated only when those three critical conditions are met, which vary with the material used. Some examples of materials that are capable of exhibiting superconductivity are mercury, zinc, magnesium, lead, aluminum, iridium, tin, vanadium, and many alloys.
The research and development of superconductors (sometimes called cryogenic conductors) is called superconductor technology. Superconducting materials include both high-temperature superconductor (HTS) materials, which exhibit superconductivity at critical temperatures above 23 Kelvin (denoted 23K), and low-temperature (or classical) superconductor (LTS) materials, which exhibit superconductivity at critical temperatures equal to or below 23K materials. Both classes of materials need to be cooled to cryogenic (very low) temperatures in order to exhibit the property of superconductivity.
The discovery of superconductivity was made in 1911 by Dutch physicist Heike Kamerlingh Onnes, called "the gentleman of absolute zero," who found that the resistance of mercury suddenly dropped to zero at a temperature of 4.2K (-268.8°C/-451.8°F). The first successful theory pertaining to superconductivity was not realized until 1957, when American physicists John Bardeen, Leon Cooper, and John Schrieffer announced the BCS theory of classical superconductors. The three were awarded the 1972 Nobel Prize in physics for their theory that describes superconductivity as a quantum-mechanical phenomenon in which at very low temperatures electrons in an electric current move in pairs. This pairing allows them to move through a crystal lattice without their motion being interrupted by lattice collisions, resulting in zero electrical resistance. After the BCS theory there were a series of announcements about other LTS materials that became superconducting near absolute zero (0K, equaling -273°C/-459°F).
In 1962 British physicist Brian Josephson investigated the quantum nature of superconductivity, proposing the existence of oscillations in the electrical current flowing through two superconductors separated by a thin insulating layer in a magnetic or electric field. This Josephson effect was developed into a fast switching technology that uses superconductors where circuits are immersed in liquid helium to obtain near absolute zero temperatures. This "ultra-fast" switching takes place in a few picoseconds (one picosecond equals one-trillionth of a second). Although Josephson junctions have not materialized for computer circuits, they have been used for selected medical instruments.
Up until 1986, the critical temperatures for all known superconductors did not exceed 23K (-250°C/-418°F). These LTS materials only worked with refrigeration to a few degrees above absolute zero. These temperatures were achieved with liquid helium, an expensive, inefficient coolant that is costly to maintain. As a result, very low temperature operations place a severe constraint on the efficiency of a superconducting device. Thus, before the discovery and development of HTS materials, the use of superconductivity had not been practical for widespread commercial applications.
A breakthrough occurred in 1986 when scientists K. A. Müller and J. G. Bednorz, at International Business Machines Corporation's (IBM's) Zurich research laboratory, announced the discovery of a ceramic oxide compound that was shown to be superconductive at 30K (-243°C/-406°F). This created a new class of superconductors called high temperature superconducting (HTS) materials that contain copper and oxygen atoms, which form planes or chains of atoms in the crystal. Their superconducting properties are anisotropic; that is, dependent on the direction of current flow and of magnetic field with respect to the planes and chains of atoms. The work of Müller and Bednorz, which earned them the 1987 Nobel Prize in Physics, started the discoveries of related materials such as copper oxides, barium, lanthanum, and yttrium that have higher critical temperatures up to 125K (-148°C/-234°F). In 1987 American physicist Paul Chu, at the University of Houston, raised the critical temperature for a superconductor to 94K (-179°C/-290°F). This achievement, for the first time, showed that new superconductors could lead to lucrative new superconductor technologies, including a dramatic impact on the future of computing. Further discoveries at other universities and research laboratories began to radically alter the impracticability of using superconducting materials for commercial uses. Ceramic metal-oxide compounds containing rare earth elements were found to be superconductive at temperatures high enough to permit using liquid nitrogen as a coolant. Because liquid nitrogen, at 77 K (-196°C/-321°F), cools 20 times more effectively than liquid helium and is 10 times less expensive, a host of potential applications began to hold the promise of economic profitability.
The properties of these new HTS materials are sensitive, however, to the amount of oxygen in them. Problems of brittleness, instabilities in some chemical environments, and a tendency for impurities to segregate at surfaces of the crystals have yet to be overcome. Theoretically, superconducting wires provide significant advantages over conventional copper wires because they conduct electricity with little or no resistance and associated energy loss and can transmit much larger amounts of electricity than conventional wires of the same size. Thin films of normal metals and superconductors that are brought into contact can form superconductive electronic devices, which can replace transistors in some applications.
The discovery of better superconducting compounds is a significant step toward a far wider range of applications, including faster computers with larger storage capacities, magnetic energy-storage systems, computer parts, and very sensitive devices for measuring magnetic fields, voltages, or currents. The main advantages of devices made from superconductors are low power dissipation, high-speed operation, and high sensitivity.
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