SUPERCONDUCTIVITY The definition of superconductivity. Superconductivity is a phenomenon displayed by certain conductors that show no resistance to the flow of electric current. Conductors are materials in which the electron current goes through. There are 4 different kinds of conductors.
Insulators, like glass or wood, have a very high resistance while semi-conductors, such as silicon, have a medium resistance. Conductors, like copper and other metals, have very low resistance, and superconductors, comprised of certain metals such as mercury and ceramics such as lanthanum-barium-copper-oxide, have no resistance. Resistance is an obstacle in the flow of electricity. Superconductors also have strong dimagnetism. In other words, they are repelled by magnetic fields.
Due to these special characteristics of superconductors, no electrical energy is lost while flowing and since magnetic levitation above a superconductor is possible, new technology in the future could include high-speed trains that travel at 483 km/h (300 mph) while levitating on a cushion of air, powerful medical systems that have many more capabilities than the CAT scan, or even magnetically driven ships that get their power from the ocean itself (Gibilisco 1993, p 28). Making materials become superconductors. When superconductivity was first discovered, it was established that the compounds needed to be cooled to within several degrees Kelvin to absolute zero (zero Kelvin). Zero degrees Kelvin is the same as -460 degrees Fahrenheit and -273 degrees Celsius.
The large amount of cooling was done by putting the compound in liquid helium. Helium, which is usually a gas, liquefies when its temperature drops to 4 K. Once the material had cooled to that temperature, it became a superconductor. However, using liquid helium to cool down material has been a problem.
Liquid helium is very expensive, and the cooling equipment is very large (Langone 1989, p 8). In the past, there was no economic incentive to replace ordinary conductors with superconductors because the cooling costs for superconductors were so high. Scientists have tried to find ways to overcome the cooling problems, and so far they have found 2. The first is to find a way to cool the material using something less expensive and less bulky than liquid helium.
The second way is to raise the temperatures that are necessary to cause superconductivity in the metals, or the critical temperatures. By combining materials into superconducting alloys, the temperature was raised slightly. By 1933, the critical temperature was at 10 K, and it wasn’t until 1969 when the critical temperature was raised to 23 K and scientists tried, unsuccessfully, to raise it again. Then, in 1986, 2 IBM researchers in Zurich found a complex ceramic material that was superconducting at 30 K.
After being increased to 39 K in late 1986, a critical temperature of 98 K was reported by Ching-WuChu and his research team at the University of Houston in 1987. A new coolant was then used. Liquid nitrogen liquefies at 77 K, is fairly inexpensive, and can even be carried around in a thermos (Mayo 1988, p 7). Liquid nitrogen costs about 50 cents a liter, while liquid helium costs several dollars a liter. Thanks to this new discovery, efficient and cost-effective superconductors could be created. HISTORY OF THE SUPERCONDUCTOR Discovery.
In 1911, the Dutch physicist Heike Kamerlingh Onnes discovered superconductivity while doing research on the effects of extremely cold temperatures on the properties of metals. While conducting his experiments, he discovered that mercury list all resistance to the flow of electricity when it was cooled to about 4 K. He then went on to discover superconductivity in other metals. In each case, the material had to be cooled to within several degrees Kelvin to absolute zero.
To further his experiments, Onnes once put a current in a superconductor that was formed in the shape of a ring, and cooled it in liquid helium. One year after removing the source of electricity, the current was still flowing at its original strength in the superconductor (Hazen 1988, p 31). The only downside to the new finding was that scientists were unable to explain how it worked. Many scientists had theories, but it was Albert Einstein who perhaps summed it up best when he said in 1922, “With our considerable ignorance of complicated quantum-mechanical systems, we are far from being able to formulate these ideas in a comprehensive theory. We can only attack the problem experimentally” (Simon and Smith 1988, p 70).
That is exactly what the scientists did, because before they could explain the behavior of superconductors, they had much to learn. Theories. Since the discovery of superconductivity in 1911, scientists have attempted to explain why superconductors act the way they do. In 1957, 3 researchers, John Bardeen, Leon Cooper, and J. R.
Schrieffer, came up with a theory that explained how superconductors worked. The theory, known as the BCS theory, helped the 3 researchers receive a Nobel Prize for its development. The BCS theory states that as electrons flow through the superconductor, they join up in pairs (called Cooper Pairs). These electron pairs are put together by phonons, which create a kind of glue-like substance (Mayo 1988, p 29).
As a pair flows through the lattice structure of the superconductor, it leaves a wake behind it. The wake would then act as a pathway through the lattice structure in which other electrons could follow, so they would then avoid collisions with other particles that would disrupt the flow and create resistance. The BCS theory also explains how a superconductor loses its ability to conduct an electric current without resistance when its temperature is greater than its critical temperature. According to the theory, as the temperature of the superconducting material rises, the atomic vibrations within the material increase to the point where the lattice structure begins to vibrate too much. The increased vibration causes the electron pairs to break apart and the wake to be disrupted, causing a loss of superconductivity.
However, the temperatures needed to cause superconductivity in 1957 were a lot lower than the critical temperatures today, so the BCS theory seems to no longer explain why superconductivity occurs in these new materials. Even though the temperatures are higher, scientists still feel that the electrons must pair up. There are theories now that say the electron pairing is now due to an atomic mechanism that is much stronger than the phonons of the BCS theory. Scientists call that mechanism the exciton. The BCS theory suffices for the older superconductors, but a new theory must be found for the newer high-temperature superconductors. Because new superconducting materials with even higher critical temperatures are now being developed, a new theory of superconductivity will probably not be widely accepted for some time.
PROPERTIES The Meissner effect. If a superconductor is cooled below its critical temperature while in a magnetic field, the magnetic field surrounds but doesn’t affect the superconductor (Hazen 1988, p 17). This property is known as the Meissner effect and was first discovered in 1933. However, if the magnetic field is too strong, the superconductor returns to its normal state, even though it is cooled below its critical temperature. Figure 1 shows the current that the magnet induces in the superconductive material creating a counter-magnetic force that causes the 2 metals to repel. Using a superconductor’s ability to “expel” a magnetic field (or flux) as a criteria, superconductors can be divided into 2 groups.
Type I superconductors are pure, simple metals such as tin and lead. They release a magnetic field until the field reaches a certain strength. This strength is called the critical field, and the critical field varies for each superconductor. Once the magnetic field is higher that the critical field, the superconductor returns to its normal state and loses its superconducting properties. Type II superconductors behave in a slightly different way. Type II superconductors are more complicated materials, often transition-metal alloys.
Transition-metals are a group of related elements in the Periodic Table (Chu 1995, p 1). In a type II superconductor, there is a second critical field that is higher in value than the first critical field. Once the magnetic field is more than the value of the first critical field, the superconductor no longer repels the entire field; however, the superconductor does continue to conduct electricity without resistance until the magnetic field exceeds the value of the second critical field. Right now, scientists are mostly interested in the type II superconductors.
Current density. Applying a large magnetic field is not the only way to eliminate superconductivity once a superconductor has been cooled below its critical temperature. The passing of a large current through the superconductive material may also cause the superconductor to return to its normal state (Langone 1989, p 96). The amount of current that a material can conduct while remaining superconductive is called the current density. The current density is measured in amperes per area. For example, a typical value for the current density of a superconducting wire might be 100,000 amperes per square centimeter.
If a larger current would pass through the superconductor, it would lose all of its superconducting properties. Most normal conductors like copper are isotropic, meaning they conduct current equally well in both directions (Mayo 1988, p 28). With an isotropic conductor or a superconductor wire, it doesn’t matter which end of the wire is connected to the positive and negative terminals of an electrical source. However, many of the new high-temperature superconductors are anisotropic, meaning they conduct an elect …
