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Magnesium Diboride

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Magnesium diboride (MgB2) is a metal with some very special properties. At a temperature of 39 K[1] (about –234º Centigrade), it becomes what is known as a superconductor.

You may have heard that as metals decrease in temperature, their resistance decreases. How ever, in some materials, the pattern of a gradual decrease in resistance is broken (right[2]). In substances, such as MgB2, there is a ‘critical temperature’ (here denoted as Tc) at which the resistivity of the material drops suddenly to zero.  This sudden drop to a complete lack of resistance is what defines a material as a superconductor.image05.pngimage06.jpgimage03.png

Superconductivity is a very useful property,

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, 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 you’re 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 system[3]).

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 picture[5].

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[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 country’s infrastructure.image04.jpg

[1] Page 635, The History of Science and Technology: A Browser's 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://superconductors.org/Index.htm 30-12-05

[3] http://www.sciencewatch.com/may-june2003/sw_may-june2003_page6.htm 30-12-06

[4] http://en.wikipedia.org/wiki/Diamagnetism 08-01-06

[5] Image from http://en.wikipedia.org/wiki/Image:Diamagnetic_graphite_levitation.jpg 08-01-06

[6] Page 309, The Quantum Quark, Andrew Watson, Cambridge University Press, October 2004

[7] http://groups.yahoo.com/group/superconductivity/message/2 9-01-06

[8] Image from http://www.fys.uio.no/super/levitation/ 08-01-06

[9] Image from http://physicsweb.org/objects/news/7/5/1/MgB2.jpg 30-12-05

[10] http://en.wikipedia.org/wiki/Magnetic_levitation_train 9-01-06

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