Super conducter

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Souvik Kumar Dolui Dept. of Electrical Engg.

SUPER CONDUCTOR

Superconductors is materials that have no

resistance to the flow of electricity below a certain

temperature.

What is super conductor?

As the temperature drops below the critical point, Tc, resistivity rapidly drops to zero and current can flow freely without any resistance. Thus, superconductors can carry large amounts of current with little or no loss of energy.

Superconductors have two outstanding features:1) Zero electrical resistivity. This means that an electrical

current in a superconducting ring continues indefinitely until a force is applied to oppose the current.

2) In front of external magnetic field it act like diamagnetic material.

In practically Resistivity: s ~ 4x10-23 cm for

superconductor. Resistivity: m ~ 1x10-13 cm for

non superconductor metal (good conductor).

Does it obey the Ohm’s law???

R = V/I

If the voltage is zero, this means that the resistance is zero. Superconductors are also able to maintain a current with no applied voltage whatsoever.

Mechanism inside the superconductor In a normal conductor, an electric current may

be visualized as a fluid of electrons moving across a heavy ionic lattice. The electrons are constantly colliding with the ions in the lattice, and during each collision some of the energy carried by the current is absorbed by the lattice and converted into heat. As a result, the energy carried by the current is constantly being dissipated. This is the phenomenon of electrical resistance.

The situation is different in a superconductor.

Mechanism inside the superconductor In a conventional superconductor, the

electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound pairs of electrons known as Cooper pair. This pairing is caused by an attractive force(ΔE) between electrons from the exchange of phonons.

There is a minimum amount of energy ΔE that must be supplied in order to excite the fluid.

Cont…

Therefore,

if ΔE > kT( Super fluid)

ΔE< kT(not a super fluid)

kT= thermal energy of the lattice k = Boltzmann's constant (1.38066 x 10-23 J/K)T = temperature of the lattice

The fluid will not be scattered by the lattice. The Cooper pair fluid is thus a superfluid, meaning it can flow without energy dissipation.

Conductor Superconductor

Meissner effect

When a superconductor is placed in a weak external magnetic field H, and cooled below its transition temperature, it "expels" nearly all magnetic flux and from its interior; this is called the Meissner effect

This constraint to zero magnetic field inside a superconductor

Meissner effect

Magnetic Levitation

Magnetic fields are actively excluded from superconductors (Meissner effect).

If a small magnet is brought near a superconductor, it will be repelled because induced super currents will produce mirror images of each pole.

If a small permanent magnet is placed above a superconductor, it can be levitated by this repulsive force.

Magnetic Levitation

BCS Theory (1957)

John Bardeen, Leon Cooper, and John Schreiffer

The theory asserts that, as electrons pass through a crystal lattice, the lattice deforms inward towards the electrons generating sound packets known as "phonons". These phonons produce a trough of positive charge in the area of deformation that assists subsequent electrons in passing through a conductor will attract nearby positive charges in the lattice. This deformation of the lattice causes another electron, with opposite "spin", to move into the region of higher positive charge density. The two electrons then become correlated.

BCS Theory

phononsAccording to BCS theory

Types I Superconductors

There are pure metals which exhibit zero resistivity at low temperature.

They are called Type I superconductors (Soft Superconductors).

The superconductivity exists only below their critical temperature and below a critical magnetic field strength.

Mat. Tc (K)

Be 0

Rh 0

W 0.015

Ir 0.1

Lu 0.1

Hf 0.1

Ru 0.5

Os 0.7

Mo 0.92

Zr 0.546

Cd 0.56

U 0.2

Ti 0.39

Zn 0.85

Ga 1.083

Mat. Tc (K)

Gd 1.1

Al 1.2

Pa 1.4

Th 1.4

Re 1.4

Tl 2.39

In 3.408

Sn 3.722

Hg 4.153

Ta 4.47

V 5.38

La 6.00

Pb 7.193

Tc 7.77

Nb 9.46

Type I Superconductors

Types II Superconductors

Type 2 category of superconductors be composed of metallic compounds and alloys

They were found to have much higher critical fields and therefore could carry much higher current densities while remaining in the superconducting state.

Type II Superconductors

High Temperature Superconductor (HTS)

Discovered in 1986, HTS ceramics are working at 77 K, saving a great deal of cost as compared to previously known superconductor alloys.

However, as has been noted in a Nobel Prize publication of Bednortz and Muller, these HTS ceramics have two technological disadvantages: they are brittle and they degrade under common

environmental influences.

HTS Ceramics

HTS materials the most popular is orthorhombic YBa2Cu3O7-x (YBCO) ceramics

Some high-Tc superconductors

Formula Tc (K)

YBa2Cu3O7 92Bi2Sr2CuO6 20Bi2Sr2CaCu2O8 85Bi2Sr2Ca2Cu3O6 110Tl2Ba2CuO6 80Tl2Ba2CaCu2O8 108Ba2Ca2Cu3O10 125TlBa2Ca3Cu4O11 122HgBa2CuO4 94HgBa2CaCu2O6 128HgBa2Ca2Cu3O8 134

Nobel Prize for Superconductivity

1913 Heike Kamerlingh Onnes on Matter at low temperature

1972 John Bardeen, Leon N. Cooper, J. Robert Schrieffer on Theory of superconductivity(BCS)

1973 Leo Esaki, Ivar Giaever, Brian D. Josephson on 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 to the theory of superconductors and superfluids.

ApplicationMaglev vehicles:

Magnetically levitated vehicles are called “maglev” vehicles.

The principle of repulsion of magnetic flux can be used in magnetic lavation application

The coaches of the train do not slide over steel rails, but float on a four inch above the track, using superconducting magnets.

Eliminates losses due to friction.

400km/hr-500km/hr

APPLICATIONS: Power

The cable configuration features a conductor made from HTS wires wound around a flexible hollow core.

Ba2Ca2Cu3O10 (BCCO) discovered in Japan. Sumitomo Electric is the world's first company to produce long bismuth-based superconducting wire

Liquid nitrogen (77K) flows through the core, cooling the HTS wire to the zero resistance state.

The conductor is surrounded by conventional dielectric insulation. The efficiency of this design reduces losses.

APPLICATIONS: Medical

The superconducting magnet coils produce a large and uniform magnetic field inside the patient's body.

MRI (Magnetic Resonance Imaging) scans produce detailed images of soft tissues.

1)http://www.superconductors.org2)http://www.ornl.gov/info/reports/m/ornlm3063r1/pt4.html3)http://www.chem.ox.ac.uk/vrchemistry/super/default.html4)http://en.wikipedia.org/wiki/Superconductivity5)M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao,6)Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Letters, 1987. 58, 9087)J. File and R.G. Mills, Phys. Rev. Letters, 1963, 10, 938)J.G. Bednorz and K.A. Muller, Z. Phys., 1986, B64, 1899)J.M. Tarascon, L.H. Greene, W.R. McKinnon, G.W. Hall and10)T.H. Geballe, Science, 1987, 235, 137311)Chemistry in Britain, September 1994 - an issue devoted to the chemistry of 12)superconducting materials.13)P.A. Cox, Transition Metal Oxides, Oxford 199214)A.I. Nazzal, V.Y. Lee, E.M. Engler, R.D. Jacowitz, Y. Tokura and15)J.B. Torrance, Physica C, 1988, 153 & 136716)Ivar Giaever - Nobel Lecture. Nobelprize.org. Retrieved 16 Dec 2010. 17)http://nobelprize.org/nobel_prizes/physics/laureates/1973/giaever-lecture.html18)The BCS Papers:a)L. N. Cooper, "Bound Electron Pairs in a Degenerate Fermi Gas"b)J. Bardeen, L. N. Cooper, and J. R. Schrieffer, "Microscopic Theory of Superconductivity" (1957).c)J. Bardeen, L. N. Cooper, and J. R. Schrieffer, "Theory of Superconductivity"(1957).

References

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