Superconductors is materials thathave no resistance to the flow ofelectricity below a certaintemperature.
As the temperature drops below the critical point, Tc,resistivity rapidly drops to zero and current can flowfreely without any resistance. Thus, superconductorscan carry large amounts of current with little or noloss of energy.
1) Zero electrical resistivity.This means that an electrical current in asuperconducting ring continuesindefinitely until a force is applied tooppose the current.2) In front of external magnetic fieldit act like diamagnetic material.
In practically Resistivity: s ~ 4x10-23 cm forsuperconductor. Resistivity: m ~ 1x10-13 cm for nonsuperconductor metal (good conductor).
Does it obey the Ohm’s law??? R = V/IIf the voltage is zero, this means that theresistance is zero. Superconductorsare also able to maintain a currentwith no applied voltage whatsoever.
Mechanism inside thesuperconductor In a normal conductor, an electric current maybe visualized as a fluid of electrons movingacross a heavy ionic lattice. The electrons areconstantly colliding with the ions in the lattice,and during each collision some of the energycarried by the current is absorbed by the latticeand converted into heat. As a result, the energycarried by the current is constantly beingdissipated. This is the phenomenon of electricalresistance.The situation is different in a superconductor.
Mechanism inside thesuperconductor In a conventional superconductor, theelectronic fluid cannot be resolved intoindividual electrons. Instead, it consistsof bound pairs of electrons known asCooper pair. This pairing is caused by anattractive force(ΔE) between electronsfrom the exchange of phonons. There is a minimum amount of energyΔE that must be supplied in order toexcite the fluid.
Cont…Therefore,if ΔE > kT( Super fluid)ΔE< kT(not a super fluid)kT= thermal energy of the latticek = Boltzmanns constant (1.38066 x 10-23 J/K)T = temperature of the latticeThe fluid will not be scattered by the lattice.The Cooper pair fluid is thus a superfluid,meaning it can flow without energy dissipation.
Meissner effectWhen a superconductor is placed in aweak external magnetic field H, andcooled below its transition temperature, it"expels" nearly all magnetic flux and fromits interior; this is called the MeissnereffectThis constraint to zero magnetic field insidea superconductor
Magnetic Levitation Magnetic fields are actively excludedfrom superconductors (Meissner effect). If a small magnet is brought near asuperconductor, it will be repelledbecause induced super currents willproduce mirror images of each pole. If a small permanent magnet is placedabove a superconductor, it can belevitated by this repulsive force.
BCS Theory (1957)John Bardeen, Leon Cooper, and John SchreifferThe theory asserts that, as electrons pass through acrystal lattice, the lattice deforms inward towards theelectrons generating sound packets known as"phonons". These phonons produce a trough ofpositive charge in the area of deformation thatassists subsequent electrons in passing through aconductor will attract nearby positive charges in thelattice. This deformation of the lattice causes anotherelectron, with opposite "spin", to move into the regionof higher positive charge density. The two electronsthen become correlated.
Types I Superconductors There are pure metals which exhibit zeroresistivity at low temperature. They are called Type I superconductors(Soft Superconductors). The superconductivity exists only belowtheir critical temperature and below acritical magnetic field strength.
Types II Superconductors Type 2 category of superconductors becomposed of metallic compounds andalloys They were found to have much highercritical fields and therefore could carrymuch higher current densities whileremaining in the superconducting state.
High Temperature Superconductor (HTS) Discovered in 1986, HTS ceramics are working at 77K, saving a great deal of cost as compared topreviously known superconductor alloys. However, as has been noted in a Nobel Prizepublication of Bednortz and Muller, these HTSceramics have two technological disadvantages: they are brittle and they degrade under commonenvironmental influences.
HTS Ceramics HTS materials the most popular isorthorhombic YBa2Cu3O7-x (YBCO)ceramics
Nobel Prize for Superconductivity 1913 Heike Kamerlingh Onnes on Matter at lowtemperature 1972 John Bardeen, Leon N. Cooper, J. RobertSchrieffer on Theory of superconductivity(BCS) 1973 Leo Esaki, Ivar Giaever, Brian D. Josephsonon Tunneling in superconductors 1987 Georg Bednorz, Alex K. Muller on High-temperature superconductivity 2003 Alexei A. Abrikosov, Vitaly L. Ginzburg,Anthony J. Leggett on Pioneering contributions tothe theory of superconductors and superfluids.
ApplicationMaglev vehicles:Magnetically levitated vehicles are called “maglev”vehicles.The principle of repulsion of magnetic flux can beused in magnetic lavation application
The coaches of the train do not slide oversteel rails, but float on a four inch abovethe track, using superconductingmagnets.Eliminates losses due to friction.400km/hr-500km/hr
APPLICATIONS: Power The cable configuration features aconductor made from HTS wireswound around a flexible hollow core. Ba2Ca2Cu3O10 (BCCO) discovered inJapan. Sumitomo Electric is the worldsfirst company to produce long bismuth-based superconducting wire Liquid nitrogen (77K) flows through thecore, cooling the HTS wire to the zeroresistance state. The conductor is surrounded byconventional dielectric insulation. Theefficiency of this design reduceslosses.
APPLICATIONS: MedicalThe superconducting magnet coils produce a large anduniform magnetic field inside the patients body.MRI (Magnetic Resonance Imaging) scans produce detailedimages of soft tissues.
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