The BCS Theory: The molecular vibrations in the lattice slow down when the temperature goes down, bellow the critical temperature this lack of movement allows the flow of electrons without any obstacle which translates in superconductivity. An interesting factor of this theory is the appearance of Cooper pairs (the electrons move coupled in pairs).
In the early 1960s at the Rutherford-Appleton Laboratory in the UK, high-energy and particle accelerator electromagnets made of copper-clad niobium-titanium were then developed. They were also first employed in a superconducting accelerator at the Fermilab Tevatron in the US in 1987.
Brian D. Josephson, a graduate student from Cambridge University, predicted that electrical currrent would flow between 2 superconducting materials even when they are seperated by a non-superconductor or insulator. His prediction, once confirmed, then won him a Nobel Prize in Physics in 1973. This appearence is now known as the "Josephson effect" and is often applied to electronic devices eg. squid.
In 1986, Alex Muller and Georg Bednorz, researchers at the IBM research Laboratory in Ruschlikon, Switzerland, created a brittle ceramic compound that supercouducted at the highest temperature 30 degrees Kelvin. Ceramics are normally insulators and therefore, researchers did not consider ceramics as possible high temperature superconductors. However, Muller and Bednorz's discovery had entriged the researchers around the world to begin their research in high temperature conductors. Later, in January 1987, a research team at the University of Alabama-Huntsville achieved an impressive 92 degrees Kelvin. A material had been found that would superconduct at temperatures warmer than liquid nitrogen. The world record of 138 degrees Kelvin is now held by a thallium-doped, mercuric-cuprate comprised of the elements Mercury, Thallium, Barium, Calcium, Copper and Oxygen.
In these recent years, many discoveries about superconductivity have been made. In 2001, magnesium diboride was found to be an extraordinary new superconductor, 39 degrees Kelvin, it is by far the highest of any of the elemental superconductors.
Uses of Superconductors
Superconductors have many uses, they are indeed very efficient conductors. Super conducting magnets are also more efficient in generating electricity than conventional copper wire generators.
Maglev trains use superconductors to levitate the train above magnetic rails. This allows them to operate without friction, and therefore acheive unheard of speeds. The maglev train was first instralled in the US. Unfortunately, due to the short track it is on, it can only reach speeds of 40 miles per hour. Maglevs normally can be able to reach speeds over 300 mph. However, there are safety concerns about the strong magnetic fields used as these could be a risk to human health.
Superconductors are used in particle accelerator as they are used to make extremely powerful electromagnets to accelerate charged particles almost as fast as the speed of light. Superconductors used instead of normal wire in electric generators increase efficiency, and they can be used to stabilize pwer grids. Superconducting wire can also be used to save space in wiring for large cities. GBP250 of liquid-nitrogen cooled superconducting wire is enough to replace nine tones of conventional copper wire.
Life-saving is one of the uses of superconductors. Superconductors help doctors to understand things that are happening inside the human body. Magnetic Resonance Imaging (MRI) machines use superconductors to deliver a strong magnetic field so that hydrogen atoms in the body's fat and water molecules will pick up energy from the field which can then be detected by special instruments. The superconducting quantum interference device, also known as SQUID, can be used like an MRI, but work without the srong magnetic field. They can detect magnetic fields of infinitely small magnatudes. They can also be used for extremely precise motion detection.
Types of Superconductors
Superconductors come in two categories: Type I superconductors and type 2 superconductors. Type I superconductors exclude all applied magnetic fields which most elemental superconductors are in Type I. Type II superconductors exlude low applied magnetic fields, but only partially exclude high applied magnetic fields; their diagmagnetism is not perfect but mixed in the presence of high fields. Niobium is an example of an elemental Type II superconductor.
- Type I Superconductors
The Type I category of superconductors in mainly consist of metals and metalloids that show some conductivity at room temperature. Type I superconductors were the first to be discovered and generally pure metals and metal alloys. These are also known as low-temperature superconductors due to the fact that the highest critical temperature of a type I superconductor is only 23.2K. Type I superconductors are also often known as the conventional superconductors. They exhit a very sharp transition to a superconducting state and "perfect" diamagnetism which means they completely repel any magnetic field they are placed in.
A Type I sueprconductor in a magnetic field will completely repel all field lines - the Meissner effect. The perfect exclusion of a magnetic field can be explained by using Faraday's law - electromagnetic fields is proportional to the rate of change of flux linkage. Since no potential difference can exist in a superconductor, then the magnetic field inside a superconductor cannot change.
Moreover, the Type I superconductors have been of limited practical usefulness because the critical magnetic fields are so small and the superconducting state disappears suddenly at that temperature.
-Type II Superconductors
Type II superconductors show a gradual transition from a normal to a superconducting state across a region of "mixed state" behavior. Type II superconductors are also known as hard superconductor. They are all made up of metallic compounds and alloys. These compounds can usually attain higher temperature but the reason behind has not yet been discovered. Some Type II superconductors show higher critical temperatures making technological applications possible.