Superconductivity Basics, Meissner effect, Classifications, Applications

After superconductivity was found many scientists started observing this concept and came up with theories like Meissner effect. With the time scientists found more superconducting materials. Those materials can be used in higher power applications, electronic applications and many other applications very effectively.

Written by: R.D. Jayathri Madhushika Ranasinghe (undergraduate), Department of Computer Engineering, University of Peradeniya

There are many materials that are categorized as good conductors. What is meant by good conductance is that those materials have less resiatance. But in higher temperatures this state changes and resistance increases. This happens due to more thermal vibration and electrons find it harder to flow through the material. So doing the opposite is leading to zero resistance in a material.

In 1911, a Dutch physicist Heike Kamerlingh Onnes discovered the Superconductivity with his observations. He observed this upon cooling mercury to 4.2 K and its drop of electrical resistance to zero. Also, Onnes found that when a strong magnetic field was applied to the mercury the superconductivity vanished quickly.

What is a superconductor?

A superconductor is a material that has no electrical resistance when the material is cooled below its critical temperature, Tc. When all electrical resistance is lost, the superconductor becomes perfect diamagnetic material.

Meissner effect

When a material is turning into the superconductivity state by cooling to its critical temperature, if an external magnetic field is applied, there is going to be an expulsion of the magnetic field from the superconductor during this process. This is called the Meissner effect. This unusual phenomenon was first observed experimentally by German physicists Walther Meissner and Robert Ochsenfeld in 1933 while measuring the magnetic field distribution of the superconductors; tin and lead.

behavior of the magnetic flux lines when a superconducting material and normal conducting material is placed in a magnetic field

Two classifications of superconductors

There are two superconductor classes as type I and type II according to the magnetic response.

Type I

While the superconducting material is completely diamagnetic, the magnetic field (H) is increased and the material stays as diamagnetic until magnetic field reaches to the critical magnetic field (Hc). At this point, the conductor becomes a normal conductor and magnetic flux penetrates through the material.

Ex: Aluminum, Lead, Tin and Mercury

critical temperature of elements and alloys in superconductors

Type II

With this type of transition from superconductive to normal is gradual in between lower critical and upper critical fields respectively Hc1 and Hc2. Magnetic flux begins to penetrate at Hc1 and keeps penetrating until the increase of the magnetic field reaches to Hc1. At Hc2, the field is completely penetrating. So, between the Hc1 and Hc2 the material is in a mixed state with both normal and superconducting properties.

Type II is mostly used in applications over Type I since they have higher critical temperatures and critical magnetic fields.

Ex: niobium-zirconium (Nb-Zr) alloys and niobium-titanium (Nb-Ti) alloys

High-temperature superconductors

With the time, superconductors with Tc > 77K were the interest of this field. One of the first discovery in high temperature superconductor is YBa2Cu3O7(yttrium barium copper oxide).

high temperature superconductor is YBa2Cu3O7 yttrium barium copper oxide

In this reaction, the oxygen content of the final material depends on the reaction conditions such as temperature and pressure.

Some high temperature superconductors and their critical temperatures are given below.

Table 1: High temperature superconductors and their critical temperatures

High temperature superconductors and their critical temperatures

Applications of superconductivity

  • Superconductive magnets are capable of providing a high magnetic field with low power supply. Therefore, those can be used in scientific tests and researches.
  • Used in Magnetic resonance imaging (MRI) in medical field.
  • For electrical power transmission through superconducting materials. Since power loss is very low, electrical equipment would be able to operate under low voltage levels.
  • Ability of higher speed switching and signal transmissions for computer.
  • Josephson junction which is made of two superconductors and thin oxide barrier at the middle is very sensitive to magnetic fields. This Junction is used in SQUID (superconducting quantum interference device) systems. This system is used for measuring very weak bio magnetic signals that generate through brain. Also, this system plays special role in detecting undersea mines.
  • High speed levitated trains work using the levitation results of the magnetic field repulsion that occurs in the superconductors. In this case, the train travels nearly 10 mm above its track so this motion is frictionless. First commercial train was in Shanghai in 2003 and it can reach to speed of 440kmh-1.

Problems in superconductors and solutions

First obstacle that development of superconductors faces is that material must be cooled to low temperatures to attain Tc. But with the development of higher temperature superconductors this has become less of a problem. But it still has not become easier in practical usage of superconductors.

Second problem that has to face is when superconductors is prepared as a bulk material, superconductors have unacceptably low critical current densities. So, material can only carry limited amount of current and after reaching to the limit, superconductivity is lost.

This problem occurs because of the presence of grain boundaries. This can be overcome by using thin films or using textured materials which can be gained through specialized crystallization techniques or mechanical working.

Future of superconductors

The first room-temperature superconductor

This is a daydream of future technologies. This found material has the ability to superconduct below temperatures of about 15ᵒC. However, these superconducting properties only appear at extremely high pressures. This limits the practical usage of this invention.

Physicist Ranga Dias of the University of Rochester in New York and his colleagues reported this in Nature on October 14, 2020. This superconductor was formed by squeezing C (Carbon), H (Hydrogen), and S (Sulphur) between two tips of diamonds. And the material was hit by laser light in order to induce the chemical reactions between materials. The pressure was about 2.6 million times that of earth’s atmosphere.

Now there is new aim for scientists to create a room-temperature superconductor that works without putting on a squeeze. If they are able to accomplish this one day it will be the revolution for energy loss.