Superconductors are materials that lose all resistance to electric current when cooled to a certain temperature. This temperature depends on the structure and composition of the material.
The advantage is that larger electric currents can be carried through thinner wire, with minimal energy losses. For example, the large step-down transformers that are part of the urban electrical supply system could be replaced with smaller, lighter and more efficient models.
In 1911, Dutch scientist Heike Kammerlingh Onnes discovered that, when the temperature of a sample of pure mercury was lowered, its electrical resistance became less. If taken down to -269°C by immersing it in liquid helium, the resistance disappeared completely.
In 1986, Swiss and German scientists discovered a new class of superconductors – ceramic materials made from copper oxides mixed with rare earth elements such as lanthanum and yttrium. The temperature at which these materials became superconductive was close to the boiling point of liquid nitrogen (-196°C).
In 1987, a major breakthrough in the superconductor field occurred. A team of scientists from the University of Houston, Texas, developed a calcium titanium oxide ceramic material that became a superconductor at -183°C. Such an improvement was especially significant because it allowed inexpensive liquid nitrogen to be used as the coolant rather than liquid helium.
In March 1988, New Zealand scientists Jeff Tallon and Bob Buckley, working at IRL in Wellington, correctly identified the structure and composition of an exceptionally high-performing ceramic compound. The material itself is a metal oxide consisting of bismuth, strontium, calcium, copper and oxygen – known as BSCCO-2223. It becomes superconductive at -163°C, which is regarded as ‘high temperature’ in the realm of superconductors.
This material is the only substance being used commercially in the world today for the production of high-temperature superconductor (HTS) wire. Large amounts of copper wire are used in the construction of electric motors, generators and transformers. By replacing the copper wire with HTS wire, huge gains in efficiency of operation of these devices are possible.
Superconductors have the ability to conduct electricity without the loss of energy. How does this happen?
With current in an ordinary conductor, such as copper wire, some of the electrical energy is converted into heat energy. This is due to the electrons in the metal structure colliding with atoms that are in their way.
At temperatures below or at the so-called ‘critical temperature’, the behaviour of electrons inside a superconductor changes. As the superconducting electrons travel through the conductor, they pass unobstructed through the complex lattice. There is no loss of energy.
A second property of superconductors
Superconductors expel magnetic fields when they become superconducting. This means that magnetic fields cannot penetrate the superconductors. As a result, they will repel magnets. This is known as the Meissner effect.
Potential applications of superconductors
A substantial fraction of electrical energy is lost as heat through resistance associated with traditional electrical transmission lines. If they were to be replaced with superconductors, these losses could be drastically reduced.
A large-scale shift to superconductivity technology depends on whether wires can be prepared from the ceramic materials as well as developing methods to cool them to -196°C.
The field of electronics holds great promise for practical applications of superconductors. The generation of heat and the charging time of capacitors due to the resistance of the interconnecting metal films limit the miniaturisation and increased speed of computer chips. The use of new superconductive films may result in more densely packed chips that could transmit information more rapidly.
The use of superconductors for transportation has already been established using liquid helium as a refrigerant. Prototype levitated trains have been constructed in Japan by using superconducting magnets.
Superconducting magnets are already crucial components of several technologies. Magnetic resonance imaging (MRI) is playing an ever-increasing role in diagnostic medicine. The intense magnetic fields that are needed for these instruments are a perfect application of superconductors.
Nature of science
One of the characteristics that binds scientists together is the desire to find out what has not been found out before. The scientists that make up the ceramic superconductor team at IRL in Wellington are a shining example of this.