Superconductors.ppt

Super Conductors

Super ConductorsWhat is super conductivity? Why we need it?Where it is applied?

Super Conductivity: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.Electrical resistivity of a metallic conductor decreases gradually as temperature is lowered.Conductor whose resistance drops to abruptly zero ohms at its critical temperature is called as Super conductors.It is quantum mechanical phenomenon.Characterized by Meissener effect, the complete ejection of magnetic field lines from the interior of the conductors as it transitions into the superconducting state.Discovered by Dutch physisct Heike Kamerlingh Onnes in 1911.

Classification:

Response to magnetic fieldBy theory of operationBy critical temperatureBy material

Magnetic field:Type 1 it has single critical field above which all superconductivity is lost.Type 2-it has two critical field between which it allows partial penetration of magnetic field.

By theory of operation:Conventional explained by BCS theoryNon conventional-cant be expalined by BCS theoryBCS theory:BCS theory is the first microscopic theory of superconductivity since its discovery in 1911. The theory describes superconductivity as a microscopic effect caused by a condensation of Cooper pairs into a boson-like state. The theory is also used in nuclear physics to describe the pairing interaction between nucleons in an atomic nucleus. It was proposed by John Bardeen, Leon Cooper, and John Robert Schrieffer ("BCS") in 1957; they received the Nobel prize in physics for this theory in 1972.

conventional superconductors:Conventional superconductors are materials that display superconductivity as described by BCS theory or its extensions. This is in contrast to unconventional superconductors, which do not. Conventional superconductors can be either type-I or type-II.

Most elemental superconductors are conventional. Niobium and vanadium are type-II, while most other elemental superconductors are type-I. Critical temperatures of some elemental superconductors:eltTcAl1.20Hg4.15Mo0.92Nb9.26Pb7.19Sn3.72Zn0.88

By critical temperature:A superconductor is generally considered high temperature if it reaches a superconducting state when cooled using liquid nitrogen that is, at only Tc > 77 K) or low temperature if more aggressive cooling techniques are required to reach its critical temperature.

By material:Superconductor material classes include chemical elements (e.g. mercury or lead), alloys (such as niobium-titanium, germanium-niobium, and niobium nitride), ceramics (YBCO and magnesium diboride), or organic superconductors (fullerenes and carbon nanotubes; though perhaps these examples should be included among the chemical elements, as they are composed entirely of carbon).

Covalent superconductors:Covalent superconductors are superconducting materials where the atoms are linked by covalent bonds. The first such material was synthetic diamond grown by the high-pressure high-temperature (HPHT) method.[1] The discovery had no practical importance, but surprised most scientists as superconductivity had not been observed in DiamondExamples:DiamondSiliconSilicon carbideCarbon nano tubesIntercalated graphite

Carbon nano tubes:

Allotrophe of carbon with cylindrical nano structureProperties:High electrical conductivityHigh tensile strengthVery elastic and low thermal coeff.High thermal conductivity &large aspect ratio(length=1000*dia)

Gen apps:

Nobel Prizes for superconductivity

Heike Kamerlingh Onnes (1913), "for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium" John Bardeen, Leon N. Cooper, and J. Robert Schrieffer (1972), "for their jointly developed theory of superconductivity, usually called the BCS-theory" Leo Esaki, Ivar Giaever, and Brian D. Josephson (1973), "for their experimental discoveries regarding tunneling phenomena in semiconductors and superconductors, respectively," and "for his theoretical predictions of the properties of a supercurrent through a tunnel barrier, in particular those phenomena which are generally known as the Josephson effects" Georg Bednorz and K. Alex Mller (1987), "for their important break-through in the discovery of superconductivity in ceramic materials" Alexei A. Abrikosov, Vitaly L. Ginzburg, and Anthony J. Leggett (2003), "for pioneering contributions to the theory of superconductors and superfluids"