Superconductor

A superconductor is a material that exhibits no resistance to the flow of an electric current. Once a flow of electrons is started in such a material, that flow continues essentially forever.

Superconductivity is an unheard-of property in materials at or near room temperatures. All substances that conduct an electric current—copper, silver, and aluminum are among the best conductors—exhibit at least some resistance to the flow of electrons. This resistance is somewhat similar to the friction that one observes in sliding a smooth wooden block across a smooth wooden floor.

Resistance is, in most cases, an undesirable property for conductors. When an electric current is passed through a wire, for example, some of the energy represented by that current is wasted in overcoming the resistance of the wire. Only a fraction, even if it is a large fraction, of the energy can actually be put to useful work.

Superconducting materials have the potential for revolutionizing electrical devices. Since they do not resist the flow of an electric current, all the energy represented by that flow can be used for practical purposes.

History

The story of the development of superconducting materials is an especially interesting one. Superconductivity was first discovered by Dutch physicist Heike Kamerlingh Onnes (1853–1926) in 1911. While studying the properties of materials near absolute zero (0 K, −273°C, or −459°F), Kamerlingh Onnes found that some materials lose all resistance to the flow of electric current at these temperatures.

Words to Know

Absolute zero: The temperature at which all atomic and molecular motion ceases. Absolute zero is about 0 K (Kelvin), −273°C, or −459°F.

Cryogenics: The production of very low temperatures and the study of the properties of materials at those temperatures.

Electric current: A flow of electrons.

Electrical resistance: The property of a material that opposes the flow of an electric current.

Electromagnetism: A form of magnetic energy produced by the flow of an electric current through a metal core.

Particle accelerator: A device for accelerating subatomic particles to very high speeds for the purpose of studying the properties of matter at very high energies.

Kamerlingh Onnes's discovery was, for more than 70 years, a subject of purely theoretical interest. As useful as superconducting materials would be in the everyday world, no one was able to find a way to produce that effect at temperatures much above absolute zero.

Then, in 1986, a remarkable breakthrough was reported. Karl Alex Müller (1927– ) and Georg Bednorz (1950– ), two physicists working at the IBM Research Division in Zurich, Switzerland, found a material that becomes superconducting at a temperature of 35 K (−238°C). Within an amazingly short period of time, news of other high-temperature super-conducting materials had been announced. In 1987, for example, a team led by Chinese-American physicist Paul Ching-Wu Chu announced the discovery of a material that becomes superconducting at a temperature of 92 K (−181°C). Shortly thereafter, materials with superconducting temperatures as high as 150 K (−123°C) also were announced.

It may seem strange to call a temperature of 92 K a high temperature. The reason for that choice of terms is that the liquid most commonly used for cryogenic (low temperature) research is liquid nitrogen, with a boiling point of 77 K. The technology for making and storing liquid nitrogen is now well advanced. Many industrial operations can be conducted quite easily at temperatures this low.

Thus, finding a material that becomes superconducting at temperatures greater than 77 K means that such materials can be produced and used very easily. Chu's breakthrough converted the subject of

superconductivity from one of theoretical interest to one that could be applied to practical electrical problems.

Applications

The applications for superconducting materials fall into two general categories: electronics and magnets. All electronic devices will operate more efficiently if they are made from superconducting materials rather than from ordinary conducting materials. However, given the fact that those materials have to be kept at the temperature of liquid nitrogen, those applications have only a limited commercial application so far.

The situation is very different with magnets. The most powerful magnets are electromagnets—magnets that owe their magnetic properties to the flow of electric current through a metal core. The traditional way to make a more powerful magnet is to make the metal core larger and larger. The problem with this approach, however, is that the core needed to make very powerful magnets is larger than can be used on a practical basis. Using superconducting materials, however, the flow of electric current is more efficient, and a more powerful magnet can be made with a smaller metal core.

Perhaps the most famous application of superconducting magnets was the Superconducting Super Collider (SSC). The SSC was a machine designed to be used as a particle accelerator, or atom-smasher, an instrument to be used for the study of subatomic particles (particles smaller than an atom). The U.S. Congress approved the construction of the SSC in 1987 and funded the early stages of its construction. Seven years later, Congress canceled the project because of its escalating costs. The only reason the SSC was practical at all, however, was that the enormous magnets it needed for its operation could be made from superconducting materials.

[See also Cryogenics; Electrical conductivity; Electric current; Electromagnetism ]

Superconductivity

In the age of technology, with smaller and smaller electronic components being used in a growing number of applications, one pertinent application of mathematics and physics is the study of superconductivity. All elements and compounds possess intrinsic physical properties including a melting and boiling point, malleability , and conductivity. Conductivity is the measure of a substance's ability to allow an electrical current to pass from one end of a sample to the other. The measurement of resistivity (the inverse of conductivity) is called resistance and is measured in the unit ohms (Ω).

Ohm's Law

An important and useful formula in science is called Ohm's Law, E = IR. E is voltage in volts, I is current in amperes, and R is resistance in ohms. To visualize this, imagine a wire conducting electrons along its length, like a river flowing on the surface of the wire. The resistance acts like rocks in the river, slowing the flow. Current is equivalent to the amount of water flowing (or the number of electrons per unit time), and voltage is equivalent to the slope of the river. As the water hits the rocks, it splashes up and away. This is equivalent to resistance generating heat in a circuit. All normal materials have some sort of resistance, thus all circuits generate heat to a greater or lesser degree.

An interesting thing happens as the temperature of the wire changes. As the temperature elevates, the resistance increases; as the temperature lowers, the resistance decreases, but only to a point, then it goes back up again. In 1911 Kamerlingh Onnes discovered that mercury cooled to 4 kelvin (4K) (that is −269.15°C , about −453° F) suddenly loses all resistance. He called this phenomenon superconductivity. Superconductivity is the ability of a substance to conduct electricity without resistance. If applied to Ohm's Law, a voltage (E ) is applied, the current (I ) should continue on its own if the voltage is then removed and the resistance is zero. This makes sense in terms of Ohm's Law, as E (0) = I (X ) ×R (0). When tested, it was found that this does indeed take place, with the current value (X ) dropping over time as a function of the voltage applied, with this current being referred to as a Josephson current.

At the time Onnes discovered superconduction , it was believed that superconductivity was simply an intrinsic property of a given material.* However, Onnes soon learned that he could turn superconduction on and off with the application of a large current, or strong magnetic field. Other than a lack of resistance, it was believed for many years that superconducting materials possessed the same properties as their normal counterparts. Then in 1933, it was discovered that superconducting materials are highly diamagnetic (that is, highly repelled by and exerting a great influence on magnetic fields), even if their normal counterparts were not. This led one of the discoverers, W. Meissner, to make scientific predictions regarding the electromagnetic properties of superconductors and have his name assigned to the effect, the Meissner effect.

*The zero point of the kelvin scale is the temperature at which all molecular motion theoretically stops, sometimes called absolute zero.

Meissner's predictions were confirmed in 1939, paving the way for further discoveries. In 1950 it was demonstrated for the first time that the movement of electrons in a superconductor must take atomic vibrational effects into account. Finally in 1957 a fundamental theory presented by physicists J. Bardeen, Leon Cooper, and J. R. Schrieffer, called BCS theory, allowed predictions of possible superconducting materials, and the behavior of these materials.

BCS Theory

BCS theory explains superconductivity in a manner similar to the river of electrons example. When the material becomes superconducting, the electrons become grouped into pairs called Cooper pairs. These pairs dance around the rocks (resistance), like two people holding hands around a pole. This symmetry of movement allows the electrons to move without resistance.

The first superconductors were experimental rarities, for research only. During the first 75 years of research, the temperature at which materials could be made to superconduct did not rise very much. Before 1986 the highest temperature superconductor worked at a temperature of 23 K. Karl Muller and Johannes Bednorz found a material that had a transition temperature (the temperature where a material becomes superconducting) of nearly 30 K, in 1986. Their research and discovery allowed even higher temperature superconductors to be made of ceramics containing various ratios of, usually, barium or strontium, copper, and oxygen. These ceramics allowed superconductivity to be done at liquid nitrogen temperatures (77 K)— a much more obtainable temperature than 4.2 K for liquid helium. However, ceramics are difficult to produce, break easily, and do not readily lend themselves to mass production. The newest generation of superconductors are nearing −148°C (125 K), which is the high temperature record as of 1998.

Superconductivity already touches the world, with its use in MRI (magnetic resonance imaging) magnets, chemical analytical tools such as NMR (nuclear magnetic resonance) spectroscopy, and unlimited electrical and electronic uses. If a high temperature superconductor could be mass produced cheaply, it would revolutionize the electronics industry. For example, one battery could be made to last years. In the future, people may look back at this basic research and compare it to the first discovery of fire.

see also Absolute Zero; Temperature, Measurement of.

Brook E. Hall

Bibliography

Mendelssohn, K. The Quest for Absolute Zero: The Meaning of Low Temperature Physics, 2nd ed. Boca Raton, FL: CRC Press, 1977.

Shachtman, T. Absolute Zero and the Conquest of Cold. New York: Houghton Mifflin Co., 1999.

Superconductivity

In 1911, Dutch physicist Heike Kamerlingh-Onnes discovered that some materials, when cooled to very low temperatures—within a few degrees of absolute zero—become superconductive, losing all resistance to the flow of electric current. Potentially, that discovery had enormous practical significance because a large fraction of the electrical energy that flows through any appliance is wasted in overcoming resistance. Kamerlingh-Onnes's discovery remained a laboratory curiosity for over 70 years, however, because the low temperatures needed to produce superconductivity are difficult to achieve. Then, in 1985, scientists discovered a new class of compounds that become superconductive at much higher temperatures (about -74°F [-170°C]). The use of such materials in the manufacture of electrical equipment promises to greatly increase the efficiency of such equipment.

su·per·con·duc·tiv·i·ty / ˌsoōpərˌkänˌdəkˈtivitē/ • n. Physics the property of zero electrical resistance in some substances at very low absolute temperatures.DERIVATIVES: su·per·con·duct / -kənˈdəkt/ v.su·per·con·duct·ing / -kənˈdəkting/ adj.su·per·con·duc·tive / -kənˈdəktiv/ adj.

superconductivity Electrical behaviour in metals and alloys that are cooled to very low temperatures. In a superconducting circuit, an electric current flows indefinitely because there is no electrical resistance. This results from electrons becoming paired to move through the material. Otherwise, one passing electron can create a distortion that can effect the passage of the next electron. Research continues to develop superconductors that function at higher temperatures. This electrical behaviour was discovered in 1911 by the Dutch physicist Heike Kamerlingh-Onnes.