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.
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 electrical current better in one direction. Some high-temperature superconductors can carry current 30 times faster in one direction than in another direction (Simon and Smith 1988, p 102).
The Josephson effect. Another interesting property of superconductors is the Josephson effect. The Josephson effect is based on an occurrence called tunneling. Tunneling occurs when a thin oxide barrier is squeezed between 2 superconductors (Simon and Smith 1988, p 129). The 2 superconductors are coupled together and the current through them is measured. When the superconductors are exposed to different magnetic fields and radiation, the current flow sometimes changes because electrons jump through the oxide barrier. This is known as tunneling. This effect can be used to detect very faint magnetic fields in computer circuits. Recent studies have also shown that the Josephson effect might occur at temperatures higher than the critical temperature of the superconducting material.
Commercial superconductors. Right now, the largest commercial applications of superconductors use their capability to conduct electrical current without resistance. In order for a superconductor to be practical for commercial applications, it must be sturdy, reliable, and relatively easy to manufacture and form into shapes (Mayo 1988, p 31). There are 2 main types of commercially available superconductors: the ductile alloys and the intermetallic compounds.
The ductile alloys are a lot like normal metals in the fact that they can be drawn into wires and cables and are relatively malleable. The intermetallic compounds are much more brittle and, while they can be formed into shapes during the manufacturing process, they are not flexible (Gibilisco 1993, p 221). The ductile alloy superconductors are composed of the elements niobium and titanium. The more brittle intermetallic compounds are often made up of the elements vanadium and gallium.
Most superconductors are formed into wires that can be wound to make generators, motors, and electromagnets. These commercial superconductors have critical temperatures in the range of 10 K. They can generate very powerful magnetic fields, and they have a current density of around 2000 amperes per square millimeter. Most of the current superconductivity applications use the commercial niobium-titanium or vanadium-gallium superconductors (Mayo 1988, p 33).
Laboratory. The most recent high-temperature superconductors have been developed in research labs around the world, and some scientists decided to look for other materials and compounds that might become superconducting at higher temperatures. Several European researchers began to experiment with a type of crystal called perovskites. In 1986, Alex Muller and Georg Bednorz performed experiments with a perovskite and discovered that the compound became superconductive at a temperature higher than ever previously recorded. The 2 researchers eventually published their discovery, which was met with some skepticism until their experiments were repeated in other laboratories (Hazen 1988, p 182). In October of 1987, Muller and Bednorz were awarded a Nobel Prize for their discovery.
Manufacturing these new ceramic perovskite superconductors is relatively easy; they can be made in most moderately equipped laboratories (Mayo 1988, p 32). The first step in the process is mixing and heating the ingredients. Oxides of the metals Yttrium, Barium, and Copper are combined with citric acid and ethylene glycol. The mixture, after being heated to about 100 F, is placed in a furnace and heated to over 1500 F to vaporize the liquid components and cause the remaining material to crystallize into a black powder. The powder is compressed in a special furnace that generates about 2000 pounds of pressure psi. The resulting block of material is then gradually cooled over several hours. Once cooled, the material is placed in liquid nitrogen to test for superconductivity. A resistance meter is connected to the cooled material to measure the electrical resistance. If the meter registers no resistance, it indicates that superconductivity has probably been obtained. If the material also exhibits the Meissner effect, the material is a true superconductor.
Electricity. There are already several superconductive electric generators in existence. In addition to generators, a system known as magnetohydrodynamics might someday produce electricity from the by-products of burning coal. In 1983, General Electric scientists and engineers conducted the first full-load test of a superconducting electric generator (Mayo 1988, p 52). At full load, the experimental generator produced enough electricity for a community of about 20,000 people. This is about twice as much as electricity as could be produced by a conventional generator of the same size. By using superconductors, the generator can develop a much stronger magnetic field than a conventional generator, allowing the superconducting generator to be physically smaller for the same amount of power given off. Another advantage of the superconductors is that the electrical resistance normally associated with the flow of electricity in the rotor windings of a conventional generator is not there (Gibilisco 1993, p 332). The increase in efficiency could then reduce the operating costs of large generators by millions of dollars.
Electronics. Josephson Junctions were developed in 1962 by a British researcher Brian Josephson. A Josephson Junction consists of 2 superconductors separated by a thin insulating barrier. Electrons are able to tunnel through the insulating barrier, creating a supercurrent. Josephson Junctions can also be used as electronic switches by varying the current levels (Gibilisco 1993, p 335). They operate at a much faster rate than transistors, which are used to control the flow of electric current, at less than 2 picoseconds (a picosecond is one trillionth of a second). These capabilities can create very fast electronic instruments, computers, and communication systems.
Medicine. There are many uses of superconductivity in medicine, and most of them revolve around 1 system called MRI. MRI, or Magnetic Resonance Imaging, is the medical term for a scientific system known as Nuclear Magnetic Resonance (NMR) Spectroscopy. In other words, MRI is a method for viewing the inside of the human body by noninvasive means (Mayo 1988, p 90). The MRI is similar to the CAT (Computerized Axial Tomography) scan in the fact that they both x-ray the body from many different angles, but the MRI is safer and better because the MRI is more sensitive to soft tissue and doesn’t expose the patient to x-ray radiation. The MRI works by exposing the body to a strong magnetic field generated by a superconducting electromagnetic coil. When the human body is exposed to a magnetic field, the protons in the water and other molecules align themselves relative to the magnetic field. A burst of radio frequency energy having the correct resonant frequency is applied, causing the protons to get excited. When the burst decays, the protons return to their former state with a release of energy. This energy field is detected and used to create an image. MRIs can also provide measurements of the blood flow through the veins and arteries in the human head and neck to diagnose strokes and other forms of cerebrovascular disease. This MRI technique is called projection angiography.
Another way superconductivity is used in the medical world is through Magnetoencephalogrophy, a technology used to help diagnose neurological disorders by measuring the extremely faint magnetic fields produced as a by-product when nerve cells generate electric signals. The instrument used to measure the fields is called a Superconducting Quantum Interference Device, or SQUID. SQUID can also be used by scientists or prospectors to get information of the material under the ground, and it can get information on things that are up to 6 miles underground. SQUIDs can also be used to take magnetocardiograms based on the magnetic fields generated by the electric currents in the heart (Hazen 1988, p 197).
THE FUTURE OF SUPERCONDUCTIVITY
A new state of matter. In August of 1995, some Colorado physicists cooled atoms of rubidium gas to such a low temperature that the particles entered a merged state, called the “Bose-Einstein condensate”. This phenomenon was first predicted about 70 years ago by theories of Satyendre Nath Bose and Albert Einstein. The condensate behaves like 1 atom, even though it is made up of thousands. The team used 2 techniques: first laser cooling and then evaporative cooling. The laser technique used a laser light whose frequency was tuned to slow the atoms down greatly. They then switched to evaporative cooling where the gas was “trapped” by a magnetic field that was at zero in its center (Chu 1995, p 1). Moving atoms wandered out of the field, while the coldest atoms stayed in the center. Very few atoms could escape the coldness at the center, and the center is what became the new state of matter.
Future developments. In the future, many scientists expect to have many new things due to superconductivity. Room temperature superconductivity would totally revolutionize the electrical power industry by making copper wires obsolete. Superconductivity would also improve transportation by changing the way trains, cars, and ships run. Magnetically levitated trains have the advantages of speed and quiet operation and the same magnetic levitation could be used with cars. Drivers would travel as fast as 150 mph on a highway and they would never have to worry about collisions. Ships propelled by superconducting motors would weigh less and would be more maneuverable (Simon and Smith 1988, p 308). In conclusion, superconductivity will have a tremendous impact on our future, totally revolutionizing our way of life.