Superconductors

Presented By: Hirra Sultan 120101091Aastha Mahajan 120101006

Definition Of Superconductors

An element, inter-metallic alloy, or compound that will conduct electricity without resistance below a certain temperature, magnetic field, and applied current.

Definitions Involved

• Tc: This is the critical temperature at which the resistivity of a superconductor goes to zero. Above this temperature the material is non-superconducting, while below it, the material becomes superconducting.

• Bc: The scientific notation representing the "critical field" or maximum magnetic field that a superconductor can endure before it is "quenched" and returns to a non-superconducting state. Usually a higher Tc also brings a higher Bc.

Definitions

• Jc: The scientific notation representing the "critical current density" or maximum current that a superconductor can carry without becoming non-superconductive.

Superconductor Types

• Type I Exhibits perfect diamagnetism below transition temperature Tc and has only one critical magnetic field Bc.

• Type IITotally expels and excludes magnetic flux below lower critical field Bc1 and partially does so between Bc1 and upper critical field Bc2; all superconductors except elements are Type II. This type has a larger Tc than that of a Type I superconductor.

Types of Superconductors

Type I• Sudden loss of magnetization• Exhibit Meissner Effect

• One HC = 0.1 tesla

• No mixed state• Soft superconductor• Eg.s – Pb, Sn, Hg

Type II• Gradual loss of magnetization• Does not exhibit complete

Meissner Effect

• Two HCs – HC1 & HC2 (≈30 tesla)

• Mixed state present• Hard superconductor• Eg.s – Nb-Sn, Nb-Ti

-M

HHC

Superconducting

Normal

Superconducting

-M

Normal

Mixed

HC1 HCHC2

H

Electricity: Properties of Superconductors

• Below a critical temperature (Tc), the resistance of a superconducting material becomes almost zero causing current to flow indefinitely and with no power loss

• No voltage difference is needed to maintain a current.

• Above a current density, superconductivity is lost in the material.

• A supercurrent can flow across an insulating junction in what is called the Josephson Effect. Cooper pairs can do this due to quantum tunneling

Magnetism: Properties of Superconductors

Superconductors can be classified into two types according to their interaction with an external magnetic field:

Type I

• Type I superconductors expel all magnetic flux • Superconductivity ends when a critical flux is applied. Examples

include mercury, lead, and tin.

Magnetism: Properties of Superconductors

Type II

• Type II superconductors, unlike type I, have two critical fields.

• After the first critical field is reached, magnetic flux partially penetrates the material and it enters a state of mixed normal and superconductivity.

• After the second critical flux is passed, superconductivity abruptly ends. Type II superconductors usually have higher critical temperatures.

• Examples include YBCO, vanadium, and BSCCO

Fig: Comparison of superconductor and standard conductor in a magnetic field. The superconductor excludes itself from the field while the field passes through the conductor.

Superconductor Conductor• The phenomena of expelling magnetic flux

experienced by superconductors is called the Meissner Effect.

• The Meissner Effect can be understood as perfect diamagnetism, where the magnetic moment of the material cancels the external field or M = - H.

• The critical field and temperature are interdependent through:

Bc= B0[1-(T/Tc)2 ]

This is observed in Type I superconductors, but it can also be used to approximate the behavior of Type II

Magnetism: Properties of Superconductors

• The strange magnetic properties created by superconductors can cause the material to levitate in place over a magnet

• The superconductor will remain a certain distance from the magnet but will not flip over or reorient

• This video demonstrates this phenomena and potential for levitation applications

Magnetism:Properties of Superconductors

Applications of Superconductors

• Particle Accelerators

• Generators

• Transportation

• Power Transmission

• Electric Motors

• Military

• Computing

• Medical

• B Field Detection (SQUIDS)

The Yamanashi MLX01 MagLev train

Applications

• Large distance power transmission (ρ = 0)• Switching device (easy destruction of superconductivity)• Sensitive electrical equipment (small V variation large constant current)• Memory / Storage element (persistent current)• Highly efficient small sized electrical generator and transformer

Medical Applications

• NMR – Nuclear Magnetic Resonance – Scanning

• Brain wave activity – brain tumour, defective cells

• Separate damaged cells and healthy cells

• Superconducting solenoids – magneto hydrodynamic power generation – plasma maintenance