Definition of superconductor, Meissner Effect, transition or critical temperature, types of superconductors, applications of superconductors
The phenomena of the sudden disappearance of electrical resistivity in materials, when cooled to sufficiently low temperature, is called superconductivity. The materials that exhibit superconductivity and are in the superconducting state are called superconductors. For example, Mercury and vanadium. The phenomena of superconductivity were observed by Dutch Physicist, Heike Kammerlingh One in 1911 while measuring the resistivity of mercury at low temperatures.
Meissner effect was observed by Walther Meissner and Robert Ochsenfeld in 1933. Meissner effect can be stated as
When a weak magnetic field is applied to a superconducting specimen at a temperature below transition temperature Tc, the magnetic flux lines are expelled. The specimen acts as a good diamagnet. This effect is the Meissner effect.
It is clear from figure.1 that in the case of a perfect conductor at greater than transition temperature and above the transition temperature in both cases conductor acts as a good magnet and magnetic field lines pass through it smoothly. But in the case of a superconductor when the temperature is greater then transition temperature superconductor acts as a good conductor and the magnetic field of lines passes through it as shown in the figure.1 but when a superconductor is cooled below critical temperature then the superconductor behaves like a diamagnet and a magnetic field of lines passes outwards the superconductor. This is due to the generation of the north pole towards the magnetic field lines. As magnetic field lines originate from the north pole and absorb in the south pole, so these lines are deflected by the north pole of the superconductor and pass through outward the superconductor.
Transition or Critical Temperature(Tc)
Transition or critical temperature is a temperature at which resistivity of material becomes nearly equal to zero is called Transition temperature or critical temperature (Tc). At this temperature or below this temperature material is said to be in superconducting state and at above critical temperature material is said to be in the normal state just like conductors etc.
The above graph is plotted between the resistivity and the transition temperature Tc. For normal metal, resistivity decreases gradually as temperature decreases but in the case of superconductor resistivity decrease gradually as temperature decreases but when the temperature reaches transition the temperature then resistivity of superconductor drops to zero as shown in above graph.
There are the following types of superconductor
1) Type-1 Superconductors
2) Type-2 Superconductors
1) Type-1 Superconductors
Type-1 superconductors are also called soft superconductors. For these superconductors, only one transition magnetic field exists, and they have very low critical field value. These superconductors obey the perfect and complete Meissner effect. Type -1 superconductor applications are limited in industries because they have very low field strength value. These superconductor losses their superconductivity easily when placed in a critical magnetic field after this these superconductor behaves like a metal.
above the graph is plotted for type-1 superconductors in between applied magnetic field and magnetization of superconductors. The graph shows that when applied magnetic the field is increased then magnetization of type-1 superconductor increases but in opposite direction, the magnetic field of lines passes from outward the super conductor but when the applied magnetic field reaches to transition field then suddenly magnetization of type-1 superconductors reaches zero. After transition field type-1 superconductors behave in a normal state as shown in above graph and magnetic field lines of applied magnetic field passes through specimen
Examples: Pb, Hg, Zn, etc.
Type-2 superconductors are also called hard superconductors. These superconductors have very high critical fields and have two critical fields Hc1 and Hc2 fields. These superconductor does not follow Meissner effect completely and perfectly and these superconductors are widely used in technology because of their high value of field strength.
The above graph is plotted for type-2 superconductors in between applied magnetic field and magnetization of type-2 superconductors. This graph shows that type-2 superconductors have two transition states Hc1 and Hc2. Initially when the applied magnetic field is increased then magnetization of superconductors increases in the opposite direction, when applied magnetic field reaches to first transition state Hc1 then magnetization reaches its maximum value as sown in the above graph at this stage material acts as in superconducting state after first transition field magnetization of material gradually decreases as the applied magnetic field are increased after the first transition state the material will be in the mixed state. A second transition stage magnetization of the material becomes zero and after this stage material will be in a normal state as shown in the above graph.
Examples: Nb3Ge, Nb3Sn, PbMos, etc.
Application of Superconductors
Following are some applications of superconductors
Superconductors are used in energy power systems just like a superconducting generator which are smaller in weight and size comparison with old generators.
Superconductor magnets are used to levitate trains above the rails. Which makes the train moving at a high speed and with minimum input energy.
Superconductor’s magnet is also used in the propulsion systems to launch satellites into orbits.
Superconductor’s magnets are used in high efficiency ore-separating machines.
Superconductors are used in transmission lines because in superconducting wires current flows without change in its magnitude.
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