This sudden drop to a complete lack of resistance is what defines a material as a superconductor. Superconductivity is a very useful property, and magnesium diboride is particularly useful because of the temperature at which it becomes superconducting. -234i?? C might seem pretty cold to you and I, but in superconductor terms its especially warm. The majority of superconductors similar to MgB2 have a critical temperature of -269i?? C, or about 4 Kelvin.
Now, a difference of 35 degrees might not seem to be of much consequence, especially when dealing with such extreme temperatures, but because youre dealing with such low temperatures, those 35 degrees make the difference between using liquid helium (a very difficult and volatile substance to work with) and electrical closed-cycle refrigeration (a much less expensive system3). Diamagnetism is a form of magnetism that is only present when a material is put under an external magnetic field.
When a magnetic field is applied to a substance, it changes the orbital motion of the electrons; this change is signified by the production of a magnetic field that directly opposes the external one. 4 All materials show some diamagnetism, but in most the force is so tiny it has no real consequence. Some materials (for example pyrolytic graphite) have strong enough diamagnetism that small samples can be levitated above readily available rare earth magnets, as shown in the accompanying picture5.
However, even in these cases, the force exerted by the material is only a small fraction of the strength of the magnetic field applied to it. This is where superconductors differ from other diamagnetic materials. Superconductors arent only special because of their lack of resistance. They have some other very interesting properties. The property that is used in MagLev trains is known as perfect diamagnetism. Perfect diamagnetism is a very strange phenomenon.
When a magnetic flux is applied to a superconductor, the superconductor produces a so-called screening current. This current, which flows along the edge of the superconductor, creates a magnetic field that exactly counters the externally applied magnetic field. 6 Unlike normal diamagnetic materials, this force is not only opposite to that of the external magnet, but it also equal to it. Because the induced currents from the external magnet meet no resistance, they can persist in whatever magnitude necessary to
perfectly cancel the external field change. 7 The result of this is that a magnet can be seen to levitate above a superconducting material, without any forces applied other than the magnetic flux of the magnet. A small-scale example of this perfect diamagnetism in action is shown in the inset picture. 8. Magnesium diboride is what is known as a binary compound, which means it only consists of two types of atom: magnesium and boron. Magnesium diboride consists of two atoms of boron to each atom of magnesium.
The boron atoms are arranged in hexagonal planes, with magnesium separating the planes, positioned at the centre of the boron hexagons, as shown in the above diagram. 9 Because of the electron arrangement in the boron atoms, and the interaction between electron holes (an empty place where an electron could rest), magnesium diboride exhibits superconducting properties at a relatively high temperature. This superconductivity in turn causes the previously mentioned diamagnetism. One of the main applications of superconductors in use today is so-called MagLev trains.
These are trains that dont actually come into physical contact with the track; they levitate a few inches above. Superconductors in the train are used to repel the train from strong magnets in the track. Using this technique, there is no friction present between train and track. The only limiting factor is air resistance. This, along with the use of linear allows the trains to reach unprecedented speeds (a working, full-scale model in Japan has reached speeds of 581 km/h) 10, produce very little noise, and very little pollution when compared to traditional trains.
Although at the moment, cost can be prohibitive (as much as $60,000,000 per mile of track), in the future MagLev trains may well form the basis of a countrys infrastructure. 1 Page 635, The History of Science and Technology: A Browsers Guide to the Great Discoveries, Inventions, and the People Who Made Them from the Dawn of Time to Today, Bryan Bunch (Editor), Alexander Hellemans (Editor), Houghton Mifflin (April 16, 2004) 2 Image from http://www.sciencewatch.com