Super conductors,properties and its application and BCS theory

superconductors:-Introduction, definition, type1,type2 and atypical. Preparation of high temperature super conductor-Y1 Ba2Cu3Ox±δ, BCS theory and general application of high temperature super conductors.

1. Super conductors Introduction, definition, type1,type2 and atypical. Preparation of high temperature super conductor-Y 1 Ba 2 Cu 3 O x± δ , BCS theory and general application of high temperature super conductors. Presented by G SMITHA Assistant professor MES COLLEGE OF ARTS , COMMERCE AND SCIENCE. Malleshwaram, Bengaluru.

2. Definition : A superconductor is a material that can conduct electricity or transport electrons from one atom to another with no resistance . OR Superconductors are the material having almost zero resistivity and behave as diamagnetic below the superconducting transiting temperature.

3. Discovery of the Superconductor Superconductivity was first discovered in 1911 when mercury was cooled to approximately 4 degrees Kelvin by Dutch physicist Heike Kamerlingh Onnes, which earned him the 1913 Nobel Prize in physics. In the years since, this field has greatly expanded and many other forms of superconductors have been discovered, including Type 2 superconductors in the 1930s.

4. General properties of superconductors 1. Electrical resistance : Virtually zero electrical resistance . 2. Effect of impurities : When impurities are added to superconducting elements , the superconductivity is not loss but the T c is lowered . 3. Effect of pressure and stress : certain material exhibits superconductivity on increasing the pressure in superconductor, the increase in stress result in increase of the T c value . 4. Isotope effect : The critical or transition temperature T c value of a superconductors is found to vary with its isotopic mass i.e.”the transition temperature is inversely proportional to the square root of isotopic mass of single superconductors .” 5. Magnetic field ( H c ) : If strong magnetic field is applied to superconductors below its T c the superconductors undergoes transition from superconducting state to normal state .

5. 6. Meissner effect : When a material makes the transition from the normal to superconducting state, it actively excludes magnetic fields from its interior; this is called the Meissner effect. (or) The complete explosion of all the magnetic field by a superconducting material is called the Meissner effect. When a superconduting material is placed in a magnetic field (H > H c ) at room temperature The magnetic field is found to penetrate normally throughout the material. If the temperature is lowered below T c and with H < H c the material is found to be all the Magnetic field penetrating through it .

6. Types of superconductors Depending upon the behaviour of superconductors in external magnetic field, they have been classified into two categories. 1. Type I superconductors.(Soft superconductors) 2. Type II superconductors.(Hard superconductors)

7. 1) Type I superconductors : a) They show magnetization curve as shown in figure. Type I superconductors are those superconductors which loose their superconductivity very easily or abruptly when placed in the external magnetic field . As you can see from the graph of intensity of magnetization (M) versus applied magnetic field (H), H c Applied magnetic field when the Type I superconductor is placed in the magnetic field, it suddenly or easily looses its superconductivity at critical magnetic field ( H c ) (point A) after H c , the Type I superconductor will become conductor. Intensity of Magnetic field

8. b). Type I superconductors are also known as soft superconductors because of this reason that is they loose their superconductivity easily. c) They are perfectly diamagnetic. d) Type I superconductors perfectly obey Meissner effect. e ) The value of H c for these is very low.(0.1 T) f) These superconductors find very limited application because of their lower H c value . e) Example of Type I superconductors: Aluminium ( H c = 0.0105 Tesla), Zinc ( H c = 0.0054) 2) Type II superconductors : a) Type II superconductors are those superconductors which loose their Intensity of magnetization H c1 H c2 applied magnetic field superconductivity gradually but not easily or abruptly when placed in the external magnetic field. As you can see from the graph of intensity of magnetization (M) versus applied magnetic field (H), when the Type II superconductor is placed in the magnetic field, it gradually looses its superconductivity. I II III I = superconducotr state II = mixed state (vortex state) III = normal state

9. Type II superconductors start to loose their superconductivity at lower critical magnetic field ( H c1 ) and completely loose their superconductivity at upper critical magnetic field ( H c2 ). b) Type II superconductors are also known as hard superconductors because of this reason that is they loose their superconductivity gradually but not easily. c ) The state between the lower critical magnetic field (Hc1) and upper critical magnetic field (Hc2) is known as vortex state or intermediate state. After H c2 , the Type II superconductor will become conductor. d ) They show perfect magnetism for field less than H c1 . e ) Since large magnetic field is required to destroy the superconducting properties so they called hard supeconductors . f ) Type II superconductors obey Meissner effect but not completely. g ) Example of Type II superconductors: NbN ( Hc = 8 x 10 6 Tesla), BaBi 3 ( Hc = 59 x 10 3 Tesla) h) Because of property (d) they have many applications.

10. Types of super conductors Type I Type II It also called SOFT SUPERCONDUCTORS . Soft superconductors are those which can tolerate impurities without affecting the superconducting properties. Only one critical field exists for these superconductors. Exhibits perfect and complete Meissner effect. The current flows through the surface only. These material have limited technical applications because of very low field strength value. e.g. Pd, Hg, Zn etc It also called HARD SUPERCONDUCTORS . Hard superconductors are those which cannot tolerate impurities i.e., the impurity affects the superconducting properties. Two critical fields Hc1 (lower) & Hc2 (upper) for these. Don’t exhibits perfect and complete Meissner effect. It is found that current flows throughout the material. These material have wider technology of very high field strength value . e.g. NbN , BaBi 3

11. Types of S uperconductors on the basis of temperature Low temperature superconductor or LTS : those whose critical temperature is below 30K(-243.2 C) . And must be cooled using liquid helium in order to achieve Superconductivity. High-temperature superconductors or HTS : those whose critical temperature is above 30 K and can be cooled conductivity using liquid nitrogen.

12. Preparation of high temperature super conductor-Y 1 Ba 2 Cu 3 O x± δ , For the preparation of YBCO sample, the following steps are adopted- 1. Stoichiometry weighing 2. Grinding 3. Calcinations 4. Pelletization 5. Sintering 6. Annealing 7. Cooling The chemical equation involved in the synthesis of YBCO is- 0.5Y 2 O 3 (s) + 2BaCO 3 (s) + 3CuO(s) → YBa 2 Cu 3 O 7-δ (s) + 2CO 2 (g)

13. The precursors were mixed properly by grinding them thoroughly with the help of a mortar and pestle for about one and a half hours till the mixture becomes homogenous and grey in colour . To increase homogeneity, the mixed precursors were calcined ( calcination as "heating to high temperatures in air or oxygen“) twice at 920°C for 24 hours followed by grinding. By following the above procedure, deep grey coloured YBCO powder was formed. The powder sample was again ground for about 2hours. After that, pellets were prepared by using hydraulic press pelletizer ( Pelletizing is the process of compressing or molding a material into the shape of a pellet at a pressure )of 75KN/(1.5cm) 2 . To increase the hardness of the sample, the prepared pellets were sintered ( Sintering is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction )at 920°C for 24 hours. The sintered pellets were black incolour .

14. To enhance the phase purity and to achieve long range ordering in the sample, annealing ( annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable )of the pellets were done in a tube furnace at 900°C for 24hours were black in colour. and further at 600°C for next 24 hours with a flow of oxygen at a rate of 1 bubble per second (approx.). After that, the sample was left for cooling with the oxygen flow. The cooled pellets were black in colour.

15. BCS Theory   This theory was developed by Bardeen, Cooper and Schrieffer . This theory states that the electrons experience special kind of attraction interaction, overcoming the coulomb forces off repulsion between them, as a result Cooper pairs .At low temperature, these pairs move without scattering i.e. without any resistance through the lattice points and the material become Superconductor. Here the electron-lattice-electron interaction should be stronger than electron-electron interaction. Electron-Lattice-Electron interaction : When an electron [1 st } moves through the lattice, it will be attracted by the core(+ ve charge) of the lattice. Due to his attraction, ion core is disturbed and it is called as lattice distortion. The lattice vibration are quantized in terms of Phonons . The deformation product a region of increased positive charge. Thus if another electron[2 nd ] move through this region as shown in fig. It will be attracted by the greater concentration of positive charge and hence the energy of the 2 nd electron is lowered.Hence the two electros interact through the lattice (or) the phonons field resulting in lowering of energy of the electron.  

16. This lowering of energy implies that the force between the two electrons are attractive. This type of interaction is called Electron-lattice-electron interaction . This interaction is strong only when the two electrons have equal and opposite momentum and spins.

17. Explanation Suppose an electron having energy (k) approaches a positive ion core. It suffers attractive coulomb interaction. It will loose its energy (q) to ion core. Thus the energy of electron will be k-q. Due to this attraction ion core is set in motion and thus distorts that latticed electron. Let a second electron having energy (k’) come in the way of distorted lattice and interaction between the two occurs and the energy gained by ion core (q) will be transferred to second electron. The second electron will have energy k’+q . Thus there is exchange of virtual energy called phonon in between two electrons through the positive ion crystal. The two electrons therefore interact indirectly, via lattice distortion or the phonon field, thus lowering the energy of electrons, This type of interaction is called as electron-electron interaction through phonons.  

18. Cooper Pairs . The fundamental postulate of BCS theory is that the superconductivity occurs when an attractive interaction mentioned above, between two electrons by mean of a phonon exchange, dominate the usual repulsive coulomb interaction. Two such electrons having equal and opposite spin and momentum which interact attractively in the phonon field are called cooper pair. Coherence length . The paired electrons are not scattered and can maintain their coupled motion up to certain distance called the coherence length. It is a measure of the distance within which the gap parameter does not change very much in varying magnetic field.

19. General application of high temperature super conductors . Magnetic Levitation : Magnetic levitation (maglev) or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields . 2.

20. 2. Power transmission

21. 3. Superconducting magnets in generators 4. Energy storage devices Superconducting magnetic energy storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil. There are two superconducting properties that can be used to store energy: zero electrical resistance (no energy loss!) and Quantum levitation (friction-less motion).  5. Particle accelerators A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for basic research in particle physics. 6. Rotating machinery 7. Magnetic separators. Magnetic separation can also be used in electromagnetic cranes that separate magnetic material from scraps and unwanted substances. Another application, not widely known but very important, is to use magnets in process industries to remove metal contaminants from product streams.  

22. Extra explanation of application