Superconductivity

Superconductivity simply states that there is no resistance or almost zero resistance in the material or any object. A material or an object that shows such properties is known as a superconductor. The conductivity referred to here is the electrical conductivity of a material.

When the electrical conductivity is to the full potential facing almost to completely zero resistance in a material any of the magnetic flux fields are expelled from the material. The zero resistance is achieved by lowering the temperature of the material which leads to a decrease in the resistance of the material and an increase in the conductivity.

In a superconductor as well, the same method is applied to achieve superior conductivity.

Who discovered Superconductivity and What is a Superconductor?

A brief introduction of superconductivity and superconductors is given. In order to explain superconductivity, it is necessary to note that materials possess certain physical properties that cause resistance to electrical conductivity through the material. This characteristic of the material varies with temperature changes. 

If the temperature of the material is increased the resistance increases whereas if the temperature of the material is decreased the resistance decreases. This phenomenon is exploited for achieving the highest conductivity of the superconductor.

In 1911, Heike Kamerlingh Onnes,  a Dutch physicist, discovered the superconductivity phenomenon. Currently, the research for the explanation of the phenomenon is done using quantum mechanics as it cannot be completely explained by the concept of perfect conductivity in classical physics.

One of the important physical properties exhibited by a conducting material exhibiting superconductivity is that there is no magnetic flux field present in the material as the presence of magnetic flux fields leads to a loss in energy and an indication of the presence of resistance in the material.

The use of superconductors in magnets is limited to one fact. When magnetic fields are super strong and above a certain critical value, it causes the superconductor to revert to its normal non-superconducting state, even when the material is kept well below the transition temperature. This varies from material to material used in superconductors.

Superconductor definition can be given as a material that incorporates the superconductivity meaning as a part of its physical properties. Normally when the temperature of a conductor is decreased there is an increase in conductivity as one moves to absolute zero temperatures. But superconductors are those special materials in which after a certain critical temperature the resistance drops to zero value and the conductivity thus reaches the maximum.

This is a critical point that has to be noted while defining what is a superconductor and explaining the superconductor definition. At this point, while decreasing the temperature below the critical temperature, the conductivity of the superconductor is maximum and there is the complete ejection of any magnetic field flux from the material as well.

In superconductivity, the conductivity of material becomes such that when an electric current is passed through a loop of such a superconductor the electric current will keep flowing through it indefinitely without any need of a power supply.

This can lead to the creation of self-sustaining energy sources solving innumerable problems such as power surges and costly electricity. And because there is no loss of energy due to the resistance of the material the electricity available will be much cheaper when such superconducting material sources are used as power sources.

There are two types of superconductors. They are called Type I and Type II superconductors. Type I superconductors transform abruptly from their normal state to superconducting state and vice versa at the transition temperature. These superconductors show complete Meissner’s effect below their transition temperatures.

However, type II does not show any abrupt change. Instead, they first show partial Meissner’s effect in between two critical values of applied magnetic field and later on show complete Meissner’s effect. There are wide applications of Meissner’s effect especially in designing levitation trains.

Properties of Superconductors

In the superconductor definition, the electrical properties arising due to unique and specialized physical properties play an important role, as what is a superconductor without any such interesting electrical properties. One of such properties is the zero electrical DC resistance present in the material.

This is a common property of all superconductors irrespective of physical properties of the material such as the heat capacity, critical temperatures (as they can be different for different materials), etc. Also as defined above in the superconductivity phenomenon key role is played by a decrease in the temperature.

Although different materials have different critical temperatures once the temperature drops down from the critical temperatures the resistance falls to absolute zero. Thus, it indicates that superconductivity in a superconductor is a thermal property and hence after having reached a superconducting state the phenomenon is independent of the physical properties of the material. All the superconducting materials behave in the same manner.

When material changes from a non-superconducting state to a superconducting state there are significant changes in the physical properties of the material which are the characteristics of phase transitions. When the temperature drops below a thermal superconductor there is an ejection of the magnetic field.

However, when there is an external magnetic field applied to the superconductor and which is more than the critical magnetic field, the superconductor leaves the superconducting state and starts to behave as a normal conductor. This change in the phase of the superconducting material occurs due to the changes in the Gibbs free energy.

In the superconducting phase, the Gibbs free energy of the conductor is lower than the normal non-superconducting phase of free energy. When a finite amount of free energy is applied externally to the superconductor through the external magnetic field, the free energy increases quadratically in the superconductor and reaches the normal free energy value. Thus a phase transition takes place in the conductor from the superconducting phase to the non-superconducting phase. 

Thermal Properties of Superconductors

The thermal properties of superconductors are vastly different from normal electric conductors. Some of the electrons in normal conductors are not bound to individual atoms but are free to move through the material,i.e, their motion constitutes an electric current. However, these so-called conduction electrons are scattered by impurities, dislocations, grain boundaries and lattice vibrations.

But in superconductors, there is an ordering among the conduction electrons that prevents this scattering. Because of this, electric current can flow with no resistance at all. The ordering of electrons is called Copper pairing. It includes the momenta of the electrons rather than their positions. The energy per electron that is associated with this ordering is quite small. 

One attribute that superconductivity remained unexplained for so long is the minute energy changes that happen during the transition between normal and superconducting states.

Hundreds of materials are said to become superconducting at low temperatures. 27 metals are superconductors in their usual crystallographic forms at low temperatures as well as pressure.

In addition to this, around 11 chemical elements including metals, semi-metals or semiconductors turn into superconductors at low temperatures and high pressures. Examples of these materials are uranium, cerium and selenium.

Also, most of the superconductors are alloys or compounds. A compound can be superconducting even if its constituent chemical elements are not. Examples include silver fluoride(Ag2F)) and a compound of carbon and potassium (C8K).

Certain semiconducting compounds become superconducting when they are properly doped with impurities. These compounds are called fullerenes. They have superconducting transition temperatures higher than that of classic temperatures.

Thus, these properties of the superconductor make it possible to be used for a variety of purposes. Superconductor examples and their applications are mentioned below.

Applications of Superconductors

Superconductors are noted for their zero DC electrical resistance. Hence, most of the applications of the superconductor examples are because of their properties which provide advantages such as low power loss because of less dissipation of energy, high-speed operations because of zero resistance and continuous flowing electrical current, and high sensitivity.

The usual and well-known superconductor examples are mercury superconductors, niobium-tin superconductors, lanthanum-barium-copper oxide superconductors, and yttrium-barium copper oxide superconductors. 

Examples of applications of superconductors include medical MRI/NMR devices, magnetic-energy storage systems, motors, generators, transformers, computer parts and sensitive devices for the measurement of magnetic fields, electrical currents, etc.

Future possible applications involve high-performance smart grids, electric power transmission, transformers, electric motors (in vehicles like maglev trains), magnetic levitation devices, superconducting magnetic refrigerators, etc.

Superconducting materials have come to be used experimentally to speed up connections in computer chips. Superconducting coils are used as electromagnets in MRI machines.

Disadvantages of Superconductors

Although useful in a diverse range of applications, Superconducting materials are active only when they are kept at low temperatures. Every superconducting material has a temperature below which it becomes active. This temperature is called transition temperature. Keeping them below the transition temperature involves a lot of expensive cryogenic technology. Hence, superconductors still do not show up in most everyday electronics. Scientists are researching ways that can make such superconductors operate at room temperatures.