Superconductivity is an absence of measurable electrical resistance exhibited by some materials at extremely low temperatures. At room temperature, the same materials have normal ohmic resistance behavior—their resistance falls gradually as temperature decreases. This behavior ceases at a transition temperature, called the critical temperature (Tc). At that temperature, resistivity suddenly falls to zero, and the material passes into its superconducting state.
The value of Tc varies from material to material and can vary over a range of a few degrees for different samples of nominally identical material. This variation is due to small differences in impurity levels and the presence of varying densities of structural defects in the crystalline lattices that materials that can superconduct assume.
In the superconducting state, electrical currents can flow without generating the heat that would arise in a normal conductor as a consequence of its resistance. Since the heat generated in normal conductors represents a loss of electrical energy and, in extreme cases, could cause the conductor to melt, superconductors are prized for their ability to avoid heat generation.
The potential advantages of superconductors are offset by the need to cool them to extremely low temperatures for their superconducting property to come into effect. The earliest superconductors to be discovered had Tc values only a few degrees above absolute zero, which is –459.67°F (–273.15°C). For this reason, scientists in the field of superconductivity tend to use the Kelvin scale, in which absolute zero is 0 K and each degree interval is equivalent to 1°C (1.8°F).
Such low temperatures are achieved using liquid helium, which boils at 4.2 K (–452.1°F, –268.9°C). Great amounts of energy are required to obtain liquid helium by liquefaction and subsequent distillation of air, and thorough insulation is necessary to prevent ambient heat from boiling the helium at an unacceptable rate. These factors added expense, ensuring that early superconductors found few uses outside research laboratories.
Discovery
The basic phenomenon of superconductivity was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes. Using liquid helium, he cooled mercury to within a few degrees of absolute zero and discovered the sudden onset of zero resistivity that is typical of superconductors. To dispel the doubts of sceptics, he set a current flowing in a ring of superconducting mercury and then measured the current again after a year. The current had not diminished in the slightest, indicating that the resistance of mercury had truly fallen to zero.
At this point, it is convenient to explain how a current is set flowing in a ring and subsequently measured. Since a ring has no start or finish to which terminals can be attached, it is impossible to use the two outputs of a battery to kick start the current—current from the battery would simply short out through the superconductor.
Instead, a changing magnetic field is used to produce a current by induction. For example, a permanent magnet can be placed within the ring above Tc and then removed when the material has been taken into its superconducting state. When the permanent magnet is removed, the lines of magnetic flux that cut through the superconducting ring induce a current in it, rather as a changing magnetic field induces current in the windings of a generator. Once the current is established, it creates a magnetic field that passes through the center of the ring at right angles to the plane of the ring. The strength of this field can then be used to quantify the current flowing in the ring. Modern experiments using this technique have confirmed that superconductors have no resistivity above the lower limit of measurement, which is currently around 10–25 m.
Apart from discovering the existence of superconductivity and the critical temperature associated with its onset, Kamerlingh Onnes found that superconductivity depends on external magnetic fields. At any given temperature below Tc, there is a critical magnetic field strength, Hc, above which the material ceases to behave as a superconductor and normal conductive properties return. The value of Hc depends on how far the material is below its Tc—the colder the sample, the stronger the field needed to disrupt superconductivity.
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MAGLEV TRANSIT SYSTEMS |
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For years, scientists and engineers have been exploring the potential of magnetic levitation (maglev) to eliminate the friction between wheels and track that wastes much of the tractive power of conventional trains. The combination of maglev with linear-motor propulsion and braking also eliminates the susceptibility to poor adhesion that limits the performance of conventional trains. Maglev vehicles stay aloft because powerful electromagnets in their undersides temporarily induce magnetic fields in conducting loops in the track beneath them, and the opposition between the original and induced magnetic fields produces a mutual repulsion between vehicle and track. The strength of the repulsion increases as the gap between the track and vehicle becomes narrower, with the result that the vehicle floats at a height where the repulsive force matches its weight. The role of superconductors in maglev vehicles is in the coils in their electromagnets. If normal conductors were to be used, their coils would be around twice the size of superconducting magnets and they would waste much more energy. The only maglev vehicle to enter regular public service to date was a low-speed internal shuttle that operated at Birmingham International Airport, England, between 1986 and 1997. Nevertheless, speeds as high as 343 mph (552 km/h) have been attained in 1997 on the Yamanashi Maglev Test Line, Japan.
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