Overview: What Are Superconductors?
Superconductors are substances that do not resist electrical flow at all when they are cooled below a certain temperature. Electrical currents can flow indefinitely without using extra voltage; and no energy is lost to heat. Those properties make superconductors very efficient when used in devices such as MRI magnets, mass spectrometers, and the magnets used in particle accelerators. However, it has been difficult to measure how and why superconductors behave as they do.
How Do Conventional Conductors Work?
Conventional conductors, such as metals, conduct electricity along a current so that individual electrons can be measured while they are flowing in a continuous stream, until the current has stopped. This is the process for most electrical energy, currents, and wires. Copper, silver, and gold are the best conductors because they each have one electron in their outermost valence, which can easily be given up to join the flow of electrons. The disadvantage is that a certain amount of energy is lost along the way, so that power is expended in the process. In addition, a constant stream of electrons are needed to keep the stream flowing because of electrical resistance.
The Phase Transition
Some substances show a shocking transition when they are cooled below a certain temperature. Above that temperature, they show resistance to electrical current in a typical pattern of heat loss. At that temperature and below, they show none. That means that the electrons can flow freely, conducting electricity through the substance. Once the current is started, it can flow forever, with no added power, and no heat loss. This phase transition occurs for some substances near to absolute zero, and for others above the boiling point of liquid nitrogen. Chemically, the electrons in those substances, form pairs, called Cooper-pairs, that move freely from one part of the lattice to another. Copper, silver, and gold do not have a phase transition because they do not have free Cooper-pairs of electrons.
High-Temperature Superconductivity
The most useful substances for superconductivity are those that the phase transition occurs at higher temperatures, as they are easier to work with and keep chilled to the correct temperature. Liquid nitrogen boils at 770 K (-1960 C), and it is much easier to produce, store and use to cool compounds such as complex ceramics and other types of superconductors. Scientists have developed a process of studying the effects of magnetic fields in high-temperature superconductors to see how well the electrons align in pairs.
Magnetism and Superconductors
In many superconductors, layers of superconducting material are balanced with other types of material, such as in ceramics and alloys. A loop of continuous current appears to levitate above magnetic fields, which allow superconductors to be used in very powerful electromagnets, such as those used in MRI (Magnetic Resonance Imaging) machines, powerful magnetic trains, and nuclear accelerators. Future applications may include more efficient power grids, as superconducting materials are developed with phase transitions at higher temperatures.
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