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Superconductivity

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*6/26/2013 
from 6/26/2013

Maro Encyclopedia

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2. “Superconductors, whether polymeric or ceramic, are innovative materials capable of intrinsic properties such as levitation and conduction of electrical currents without resistance. These properties are due to the electrons being grouped in Cooper pairs, behaving as bosons.

Typically, mixed metal oxide materials exhibit high-temperature superconductivity as illustrated for example, in U.S. Pat. No. 6,794,339.  The rare-earth-metal-alkaline-earth-metal-copper oxide superconductor materials are generally of the formula RBa.sub.2Cu.sub.3O.sub.7-x. However, the ceramic nature of the superconductor materials pose a number of problems for the manufacture of high critical temperature (Tc) superconducting shaped products such as magnetic levitation components and magnetic shielding devices. Because the mixed oxide superconducting materials are susceptible to degradation by moisture and chemicals such as reducing agents, their use in various applications are limited. In addition, conventional ceramic superconductors are difficult to be molded, easily breakable and thus, difficult to be molded or folded in different forms such as tubes, cylinders, cubes, electric cables and wires, toroids (for use in transformers), train tracks, nucleus for industrial performers, etc.

In contrast, magnesium diboride (MgB2) is a simple, inexpensive ionic binary compound superconducting material. MgB2 has a Tc of 39 K (-234.degree. C.; -389.degree. F.), the highest amongst conventional superconductors, as shown for example, in U.S. Pat. No. 7,338,921.  MgB2 is a phonon-mediated superconductor, its electronic structure is such that there exists two types of electrons at the Fermi level with widely differing behaviors, one of them (sigma-bonding) being much more strongly superconducting than the other (pi-bonding). This is at odds with usual theories of phonon-mediated superconductivity which assume that all electrons behave in the same manner.

MgB2 typically only shows competitive properties at relatively low magnetic field values which are useful for biomedical applications such as conductors for MRI magnets. However, the effort and success in improving MgB2 in high magnetic fields remains desirable. Therefore, enhancing the critical current of MgB2  in high magnetic fields such as tracks for levitation trains, tubes and wires, remains one of the main goals to be pursued through the control and manipulation of the structure at a nanometer level to increase flux pinning.

In addition, methods which provide superconductor composite materials that facilitate the cost effective manufacture of high Tc superconducting products of various shapes also remain desirable.”

[Superconductors, US Patent 8,470,743 (6/25/2013)]

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1. “Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It is characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics.

The electrical resistivity of a metallic conductor decreases gradually as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of a normal conductor shows some resistance. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing through a loop of superconducting wire can persist indefinitely with no power source.

In 1986, it was discovered that some cuprate-perovskite ceramic materials have a critical temperature above 90 K (−183 °C).  Such a high transition temperature is theoretically impossible for a conventional superconductor, leading the materials to be termed high-temperature superconductors.  Liquid nitrogen boils at 77 K, and superconduction at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures. In conventional superconductors, electrons are held together in Cooper pairs by an attraction mediated by lattice phonons.  The best available model of high-temperature superconductivity is still somewhat crude. There are currently two main hypotheses – the resonating-valence-bond theory, and spin fluctuation which has the most support in the research community.  The second hypothesis proposed that electron pairing in high-temperature superconductors is mediated by short-range spin waves known as paramagnons.”

(Superconductivity, Wikipedia, 6/26/2013)

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Interested!!
Bookmark this page to follow future developments!.
(RDC 6/5/2012)

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Roger D. Corneliussen
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Copyright 2013 by Roger D. Corneliussen.
No part of this transmission is to be duplicated in any manner or forwarded by electronic mail without the express written permission of Roger D. Corneliussen
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* Date of latest addition; date of first entry is 6/26/2013.