TYPES OF SUPERCONDUCTORS
Based on the property of Meissner effect, the super conductors are classified into two types. They are (i) Type - I or (soft) super conductor (II) Type – II or (Hard) super conductor.
(i) Type - I or (soft) super conductor:
The super conductors which can obey complete Meissner effect are known as Type – I super conductors.
Let us consider a cylindrical shaped super conductor and a weak magnetic field is applied to it. Then the magnetic lines are not passing through it and obeys perfect diamagnetism. If applied magnetic field is equal to critical magnetic field (Hc) then flux will penetrate then looses the super conducting properties and behaves as a normal conductor.
Again, if magnetic field is decreased to lower than Hc then it continue to show super conducting property. Such superconductors are known as Type I or soft super conductors.
They have only one critical magnetic field value ‘Hc’. i.e. above Hc the material is in normal state and below ‘Hc’ the material will be in superconducting state
Ex: Al, Zn, Ga, ……………….. etc
(ii)Type - II or (Hard) super conductor
The super conductors which can obey partial Meissner effect are known as Type II super conductors. They have two critical fields Hc1 and Hc2.
If the applied magnetic field is less than Hc1, then material expels the magnetic lines of force from it and hence it will be in super conducting state.
a) If the applied field is increased beyond Hc1 then magnetic lines slowly passes through the body thereby decreasing the diamagnetic nature of the body.
b) The body will be in a mixed state i.e., in super conductor and normal state up to the Hc2.
This state is called “Vortex State” or “Mixed state”.
c) When the applied field H is equal to Hc2, H= Hc2 the magnetic lines of force spreads through the entire material and the material will be in normal state. Hc1 is called lower critical field and Hc2 is called upper critical field. They are also called hard superconductors.
Ex: zirconium (Zr), Neobium (Nb) ,etc.