Superconductors are the materials that exhibit #superconductivity. The property of superconductivity is one of the most remarkable discoveries of the 20th century. Superconductors are those that possess negligible or zero resistance below a certain temperature.
Heike Kamerlingh Onnes observed the property of superconductivity for the first time in April 1911, while he was studying the resistance of solid Mercury under cryogenic conditions. Apart from offering zero resistance, the superconductors have a few special attributes that make them unique and beneficial. In this article, let's know about the properties of superconductors, their classification, current research on #superconductors, their applications, and the Meissner Effect.
The property of Superconductivity
Metals are the most common conductors known. Their resistance decreases with a decrease in temperature. But superconductors are the ones whose resistance decreases abruptly to zero when the temperature decreases beyond a certain value called the Critical Temperature. The below-depicted graph shows the temperature below which the resistance swiftly decreases to zero. This value of temperature is the Critical Temperature Tc.
How do materials exhibit superconductivity?
Usually, #metals have free electrons, that move along the conductor promoting the flow of current. But the electrons collide with the positively charged atoms in their course of motion, thereby dissipating heat. This is what we call heat dissipation due to resistance.
In the context of superconductors, materials can be #categorized as Fermions and Bosons. #Fermions are the substances that have a half-integer spin and thus are restrained by the Pauli Exclusion Principle. Thus, no two particles can have the same state. Whereas, the #bosons are particles with an integral spin and thus are not restrained by the Pauli Exclusion Principle. In a Boson, all the particles come together and possess the lowest level of energy.
When the temperature is decreased below the critical temperature (Tc), the fermions(electrons in this case) come together to form cooper pairs and hence act like bosons. This results in an entangled network of cooper pairs that travel as a group along the conductor. The network of electrons adjusts itself in such a way that there are no collisions with the positive atoms of the lattice and hence no heat dissipation occurs. The above approach is the BCS theory (Bardeen–Cooper–Schrieffer theory) that explains the zero resistance of superconductors.
Classification of Superconductors
They are classified as Type I and Type II superconductors, based on the response to a magnetic field.
Type I superconductors:
There exists a single critical value of the magnetic field, above which all the superconductivity is lost.
Examples include metals such as Aluminium(with a critical temperature of 1.2 kelvin), Mercury(with a critical temperature of 4.2 kelvin), and a few metal alloys.
They are malleable, ductile, possess physical strength, and are easy to make.
But, the temperature must be extremely low, which can only be achieved by liquid Helium.
Type II superconductors:
They have two critical fields, one above which the material allows partial penetration of magnetic flux into it and the other above which it loses the superconductivity.
They are usually oxides and ceramics.
An example is Y Ba2 Cu3 O7 with a critical temperature of 90 kelvin. Thus, liquid Nitrogen(Boiling point: 77 kelvin) is used to cool it below the critical temperature.
They have a higher critical temperature, which is an advantage.
They are not malleable and ductile and hence are difficult to work with.
There are few other classifications as well. The ones following the BCS theory are termed as conventional and the rest are Non-conventional. The materials with a critical temperature less than 30 kelvin are considered as Low-temperature superconductors and the ones with a critical temperature greater than 30 kelvin are named as High-temperature superconductors.
The Meissner Effect
Lenz law states that the direction of eddy currents induced in a conductor due to a magnetic field is in such a way that it opposes the magnetic field. Due to the negligible resistance of superconductors, the eddy currents induced in them are of high magnitude. Thus the repulsion to the magnetic field(as stated by Lenz law) is very high, and as a result, they do not allow the magnetic flux lines to move through them.
This property of the superconductors to not allow the magnetic flux lines through them is termed as the Messiner Effect. This phenomenon was discovered by the German physicists Walther Meissner and Robert Ochsenfeld in 1933. Thus a magnet placed near a superconductor levitates due to the Meissner Effect. This property is the basis for the modern high-speed bullet trains.
The extreme cool temperatures required for the materials to behave as superconductors can only be achieved by using liquid Helium, which is expensive. Hence, researchers have been striving to develop high-temperature superconductors and study their physical properties such as the electronic structure and elastic constants. Scientists have been able to create superconductors at 200 kelvin(-73 degree Celcius). More recently, superconductivity was observed at 250 kelvin(-23 degree Celcius) in a new class of hydrides under extremely high pressure of 150 to 170 gigapascals.
Also, active research is going on about the disordered superconductors, size effect in nanostructured superconductors, the spin polarization, and the Gap anisotropy in a class of superconductors called the quarternary borohydrides. The synthesis, impurity effects, and processing-property relationship of superconductors are also under study. There is still a lot of research going on in this field to discover many more properties of superconductors and to make them viable at room temperatures.
Applications of Superconductors
Powerful superconducting electromagnets are used in MRI(Magnetic Resonance Imaging), Magnetic levitation trains, and NMR(Nuclear Magnetic Resonance) machines.
Superconducting Quantum Interface Devices(SQUIDs) are capable of recognizing extremely small magnetic fields.
Lossless electric motors, generators, power cables, and fast fault current limiters.
High-speed computing supercomputers.
High sensitive particle detectors such as Bolometer, Transition edge sensor, and Kinetic inductance detector.
Superconductors are capable of transmitting power across very long lengths with negligible losses. They can also hold currents for thousands of years without any losses even after the removal of the source. They have the potential to revolutionize the whole power grid. They assure better transmission of power to isolated areas as well. Therefore, the development of superconductors that are viable at room temperatures is one of the major concerns of the contemporary world.