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a substance in which the electrical resistance disappears when the substance is cooled below a certain critical, or transition, temperature Tc. In other words, a superconductor is a substance that exhibits superconductivity. Superconductive properties are not shown by Cu, Ag, Au, Pt, the alkali metals, the alkaline-earth metals, and the ferromagnetic metals; most of the remaining metallic elements are superconductors (seeMETALS). The elements Si, Ge, and Bi become superconductors when cooled under pressure. Several hundred alloys and compounds and some heavily doped semiconductors can also go into the superconducting state. There exist superconducting alloys in which some or all of the components are not themselves superconductors. For virtually all known superconductors, Tc lies within the temperature range of the existence of liquid hydrogen and liquid helium (the boiling point of hydrogen is 20.4° K).
Another important parameter characterizing the properties of a superconductor is the critical magnetic field Hc, above which the superconductor goes into the normal, or nonsuper-conducting, state. As the temperature increases, the value of Hcd decreases monotonically; when T ≥ Tc, Hc = 0. Hc takes on its maximum value H0 at absolute zero. Table 1 gives the value of H0 for a number of superconductors, as determined by extrapolation from experimental data.
As of 1974, the highest known Tc was possessed by the compound Nb3 Ge, which is prepared by a special process.
Although the fundamental causes of the occurrence of superconductivity are well established, present-day theory is unable to calculate the values of Tc and Hc for known superconductors or to predict the values for a new superconducting alloy. How-
|Table 1. Transition temperature and critical magnetic field for a number of metals, superconductors, alloys, and compounds|
|Superconductor||Transition temperature Tc(°K)||Critical field H0 (oersteds*)|
|*1 oersted = 79.6 amperes/meter Above Tc these compounds are semiconductors|
|Alloy 65 BT (Nb-Ti-Zr)||9.7||≈100,000|
|(Nb3 AI)4 Nb3 Ge||20.0||—|
|Pb1.0 Mo5.1 S6||≈15||≈600,000|
ever, as a result of the accumulation of experimental material, a number of empirical regularities have been established that can be used as guides in the search for alloys with high values of Tc and Hc. The most important regularities were introduced by B. T. Matthias of the USA in 1955. Matthias’ rules can be summarized as follows: the highest Tc is observed for alloys with z valence electrons per atom, where z ~ 3, 5, and 7; there is a preferred type of crystal lattice for each z. In addition, Tc increases with increasing atomic volume and decreases with increasing atomic mass.
Superconductors can be divided according to their magnetic properties into two groups: type I and type II. In the case of a cylindrical type I superconductor in a magnetic field H oriented parallel to the length of the sample, the penetration of the field into the superconductor occurs in an abrupt manner simultaneously with the appearance of electrical resistance when H > Hc. In type II superconductors, a longitudinal magnetic field, under similar conditions, begins penetrating at substantially weaker fields and before the appearance of the resistance. A distinction is accordingly made in the case of type II superconductors between the lower critical field Hc1 and the upper critical field Hc2. At Hc1, the penetration of the magnetic field begins. At Hc2, the magnetic field completely penetrates the interior of the superconductor, and the electrical resistance acquires the value characteristic of the normal state. (The values of Hc2 are given for the type II superconductors in Table 1.) With the exception of V and Nb, all the pure superconducting metals are type I superconductors. Some alloys that have a low content of one component are also type I superconductors. Type II superconductors are more numerous and include most compounds with high Tc, such as V3 Ga and Nb3 Sn, and some alloys with a high content of doping impurities.
Type II superconductors include the group of hard superconductors. Such materials are characterized by a large number of structural defects, which are a result of special preparation processes. Examples of defects are vacancies, dislocations, and inhomogeneity of composition. The movement of magnetic flux in hard superconductors is greatly inhibited by the defects, and the magnetization curves exhibit a marked hysteresis. For the same reasons, strong direct currents can flow in such materials without losses—that is, without resistance—for field values up to Hc2 regardless of the orientation of the current and the magnetic field. A different situation is presented by an ideal superconductor, which is entirely free of defects (this situation can be approached by lengthy annealing of an alloy): when H > Hc1 an arbitrarily small current is accompanied by losses caused by the movement of the magnetic flux, for any nonlongitudinal orientation of the field and current.
The upper critical field Hc2 is usually many times greater than Hc1. Hard superconductors are consequently of interest with regard to technological applications—the electrical resistance of such superconductors is practically equal to zero even at very high field values. Hard superconductors are used, for example, in producing windings for superconducting magnets. Because, however, of the brittleness of hard superconductors, it is difficult to fabricate wires or ribbons of such materials for the windings of superconducting magnets. This drawback applies particularly to compounds with high values of Tc and Hc, such as V3Ga, Nb3Sn, and Pb1.0, MO5.1 S6. The manufacture of superconducting magnetic systems from such materials is a complicated technological problem.
REFERENCESSverkhprovodiashchie materialy. Moscow, 1965. (Collection of articles translated from English.)
Metallovedenie sverkhprovodiashchikh materialov. Moscow, 1969.
I. P. KRYLOV
superconductorA material that has little resistance to the flow of electricity. Traditional superconductors operate at absolute zero (-459.67 degrees Fahrenheit; -273.15 degrees Celsius). Experiments in the 1980s raised the temperature to -321 degrees Fahrenheit. By the late 1990s, superconductivity was demonstrated at -164 degrees Fahrenheit.
The major use for superconductors, made of alloys of niobium, is for high-powered magnets in medical imaging machines, particle accelerators and quantum computers. The magnets are cooled with liquid helium or liquid nitrogen. If superconductors can ever be made to work at reasonable temperatures, they will have a dramatic impact on the future of computing as well. In the meantime, the extra cost and bulk required to cool the circuits makes superconductive computers very expensive. See Josephson junction, semiconductor and quantum computing.