Applications of Superconducting Materials

Engineering applications

Based on their properties, they have the following applications.

Since there is no loss in power (zero resistivity) super conductors can be used for the transmission of power over very large distance.

Since, the super conducting property can be easily destroyed it can be used in switching devices.

It can be used in very sensitive electrical instruments. Example: galvanometer.

Since the current in a superconducting ring can flow without any change in its value (persistent current), it can be used as a memory (or) storage element in computers.

It can be used to manufacture electrical generators and transformers in small sizes with high efficiency.

They are used to design cryotron, Josephson devices, SQUID, magnetic levitated trains (MagLev), modulators, rectifiers, commutators, etc.,

Medical applications

Superconducting materials are used in NMR (Nuclear Magnetic Resonance) imaging equipments which is used for scanning purposes.

They are applied in the detection of brain wave activity such as brain tumour, defective cells etc.,

Using super conducting magnets one can separate the damaged cells from the healthy cells.

Josephson devices

Principle : Persistent current in DC voltage is the principle used in Josephson devices.

Working : A Josephson junction should be made up of superconductor-insulator-superconductor layer. Therefore, a thin layer of insulating material is placed in between two superconducting materials. Here, the insulator acts as a barrier to the flow of conduction electrons from one superconductor to the other.

If a voltage $V$ is applied across the superconductors, then current starts flowing between the superconductors by tunnelling effect. Due to the increase in voltage, more and more thermally excited electrons in the superconductor 1 will tunnel across the insulator into the superconductor 2 and the current increases.

The current has two components namely:

(a) DC component: This current component persists even after the external voltage is cut-off.

(b) AC component: This current component persists only up to which the external voltage is applied.

Superconducting Quantum Interference Device (SQUID)

Principle : Small change in magnetic field produces variation in the flux quantum.

Working: A SQUID is a double junction quantum interferometer formed from two Josephson junction mounted on a superconducting ring. Magnetic field is applied normal to the plane of the ring and results in the inducement of current, at the two junctions. The detection coils, the connecting wires and the SQUID input coils form a closed superconducting ring, so any change produced is detected, and is proportional to the change in magnetic flux.

The induced current flow around the ring so that the magnetic flux in the ring can have quantum values of flux, which corresponding to the value of magnetic field applied.

Therefore, SQUIDS are used to detect the variation in very minute magnetic signals in terms of quantum flux. They are used as storage devices for magnetic flux.

Applications:

They are also used in the study of earth quakes, removing impurities, detection of magnetic signals from the brain, heart etc.
The SQUIDs are used to study tiny magnetic signals from the brain and heart. They act as storage devices for magnetic field.
SQUID magnetometers can detect the paramagnetic response in the liver and give the amount of iron held in the liver of the body accurately.

Cryotron (or) Fasi electrical switching

Cryotron is used as a switching device.

Principle : The superconducting property disappears when magnetic fields is greater than critical field. (H_c).

Construction : It consists of two superconducting materials A and B as shown in fig, Let the critical magnetic field of material A is less than of B. (Hc_A < Hc_B).

Working: A current passing through material ‘B’, produces magnetic field which influences the material A also. If the current in B is such that the magnetic field produced is such that magnetic field produced is less than the critical field of A, hen the material A is in the superconducting state, allowing a current to flow through it.

But if the current in $B$ is such that the magnetic field produced is greater than the critical field of the material A, then the superconducting state of $A$ is destroyed and does not allow a current to pass through it.

Thus a current flowing in B controls the current in A. The whole device acts as a switch, because, the material B conducts in one case (equivalent to a switch in the ON state) and does not conduct in another case(equivalent to a switch in the OFF state). This device operates at liquid helium temperature. In computer, cryotron is used as a switching element similar to that of a telephone relay.

Explanation with example:

A superconductor possesses two states, the superconducting and normal.

The application of a magnetic field greater than Hc can initiate a change from superconducting to normal and removal of the field reverses the process. This principle is applied in development of switching element cryotron.

This consists of a tantalum core around which is would a niobium wire (Please note that both tantalum and niobium are superconductors). Tantalum $(Tc=4.5 K)$ is the gate and niobium $(Tc=9.5 K)$ is the control.

As shown in Fig current flows through Tantalum and this can be controlled by switching action.

Now allowing a control current to pass through the niobium winding, magnetic field sufficient to change tantalum from its superconducting to normal state is produced.

This closes gate for the flow of current through tantalum.

Removal of the control current reopens the gate.

Magnetic levitation

A superconducting material behaves as a diamagnetic material. Hence when a superconductor is placed over a magnetic field, the material floats. This effect is known as magnetic levitation.

Magnetic levitated train is the train which cannot move over the rail, rather it floats above the rails, under the condition, when it moves faster.

Principle

Electro-magnetic induction is used as the principle (i.e.,) when there is a relative motion of a conductor across the magnetic field, current is induced in the conductor and vice versa.

Explanation for Magnetic levitated train (MagLev)

This train consists of super conducting magnets placed on each side of the train.

The train can run in a guidance system which consists of a series of ‘8’ shaped coils as shown in Fig.

Initially when the train starts, they slide on the rails. Now, when the train moves faster, the super conducting magnets on each side of the train will induce a current in the ‘8’ shaped coils kept in the guidance system.

This induced current generates a magnetic force in the coils in such a way that the lower half of the 8-shaped coil has the same magnetic pole as that of the superconducting magnet in the train, while the upper half has the opposite magnetic pole.

Therefore, the total upward magnetic force acts on the train and hence the train is levitated (or) rised above the wheels (i.e.,) the train now floats above the air.

Now, by alternatively changing the poles of the super conducting magnet in the train alternating current can be induced in the ‘8’ shaped coils.

Thus, alternating series of north and south magnetic poles are produced in the coils, which pulls and pushes the super conducting magnets in the train and hence the train is further moved.

This magnetic levitated train can travel a speed of $500 km/$ hour, which is double the speed of existing fastest train.