Preparation of High Tc Superconductors and Its Low Temperature Properties

1. Preparation of High Tc Superconductors and Its Low Temperature Properties Dr. Fanggao Chang Teaching Center of Experimental Physics, Henan Normal University

2. Outline Historical background. Fundamentals of superconductors. Applications of superconductors. The chemistry of superconductors. Preparation of superconducting pellets. Determination of critical temperature. Demonstration of Meissner Effect.

3. Historical Background Superconductivity was first discovered in 1911 by the Dutch physicist,Heike Kammerlingh Onnes who was awarded the Nobel Prize in 1913. 1933 Walther Meissner and R. Ochsenfeld discovered Meissner Effect. John Bardeen, Leon Cooper, and Robert Schrieffer, developed BCS throry in 1957 and was awarded the Nobel Prize in 1972.

4. Historical Background Brian David Josephson predicted the Josephson Effect In 1962 and shared the Nobel prize with Ivar Giaever in 1973. Georg Bednorz and Alex M�ller discovered high temperature superconductor in 1986 and shared Nobel Prize in 1987. In 2008, Fe-based high Tc superconductor was reported.

5. Fundamentals of Superconductors What is superconductivity?

6. The Meissner Effect A diamagnetic property exhibited by superconductors. End result is the exclusion of magnetic field from the interior of a superconductor. What is diamagnetism?

7. Applications of Superconductors Magnetic resonance imaging. Biotechnical engineering. SQUIDs Transistors Josephson Junction devices Particle accelerators

8. Magnets sensors and transducers magnetic shielding Power Generation Motors Generators Energy Storage

9. Transmission Fusion Transformers Inductors Transportation: Magnetically levitated vehicles Marine propulsion

10. Chemistry of Superconductors

12. Preparation of Superconducting Pellets-Materials. Dust mask and safety goggles Mortar and pestle 50 ml acetone 1.00g yttrium oxide, Y(2).O(3) 2.11g cupric oxide wire, CuO 3.50g barium carbonate BaCO(3)

13. Alumina boat, 90x17x11.5 mm Tube furnace, 1 in. diameter Pellet press Quartz tube (24 mm o.d.) equipped with air-tight connections made at one end Tank of oxygen, with regulator and tubing

14. Procedure for Pellet Preparation In the mortar and pestle, grind together the yttrium oxide, copper oxide, and barium carbonate with enough acetone. Place the powder in the alumina boat and heat in the tube furnace at 950�C for one hour. Regrind the dry powder in the mortar and pestle (do not use acetone). Return the powder to the boat, and heat it in the furnace for five hours at 950�C

15. Grind the now black powder for a third time in the mortar and pestle, this time using acetone. Use a pellet press to form pellets at about 50,000 lb/in2 pressure. Heat at 950 for one hour Allow the furnace to cool to 500-600�C, Pass pure oxygen over the pellets at a rate of about 10ml/min for 3 hours. Turn off the furnace and allow it to cool to room temperature while maintaining the flow of oxygen over the pellets.

16. Determination of Tc The effect of the contact resistance can be diminished by the use of a four point probe

17. Demonstration of Meissner Effect Magnet Superconductor pellets Liquid nitrogen

18. Diamagnetism? A superconductor is not only a perfect conductor (R=0), but a perfect diamagnet. It will tend to repel a magnet.

19. So, Superconductors are Perfect Diamagnets? If a superconductor was only a perfect conductor, would there be a Meissner Effect? Recall Faraday�s Law of Induction.

20. Faraday�s Law of Induction

21. Faraday�s Law of Induction A change in magnetic flux will induce an emf in a conductor. There will be no induced emf if the magnetic flux is constant with respect to time.

22. The Minus Sign What does the minus sign imply physically? The direction of the induced emf will be such that the magnetic field produced by the induced emf resists the change in magnetic flux. The presence of the minus sign is referred to as Lenz�s Law

23. Lenz�s Law If the magnetic flux is decreasing out of the page, which way will the induced emf be directed? (Note: the induced emf has the same direction as the induced current.)

24. The direction of the induced emf (or current), will be counterclockwise. This will generate an induced magnetic field out of the page, counteracting the decrease in flux. (Found from the right-hand rule for current carrying wires.)

25. Perfect Conductor Move this perfect conductor into a magnetic field. By Faraday�s Law of Induction, a current is induced. The magnetic field generated by this current would oppose the change of the applied field.

26. How long will the induced current flow? Recall P= I2R. The induced current would flow indefinitely. There is no I2R power loss. The induced magnetic field will continue to oppose the change in the applied field. Conversely, if the conductor is in a magnetic field which is then removed, an induced current and corresponding magnetic field would tend to oppose the removal of the applied field.

27. What Do We See? Would a magnet levitate over the surface of a perfect conductor? No, if a magnet is placed on top of a material which becomes a perfect conductor, there would be no effect on the magnet. There would only be an opposing force if the magnet was removed.

28. In Superconductors Faraday�s Law does not explain magnetic repulsion by superconductors. Below its critical temperature (Tc) a superconductor does not allow any magnetic field to enter it.

29. Circulating currents on the surface of the superconductor induce microscopic magnetic dipoles that oppose the applied field. The induced field repels the applied field, and the magnet associated with it. If a magnet is on top of a superconductor as it is cooled below its Tc, it would exclude the magnetic field of the magnet.

30. References Image 1: http://www.physics.ubc.ca/~outreach/p420_97/bruce/ybco.html Image 2: http://www.sci.kun.nl/hfml/froglev.html Image 3: http://physicsweb.org/article/world/11/12/6 Image 4: http://www.phys.warwick.ac.uk/supermag/Research/Superconductors/body_superconductors.html