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Superconductivity 2012

Superconductivity 2012. Demonstration What did we see? High-T c materials (How to make superconductors) Some applications and important properties. Department of Physics, Umeå University, Sweden. How do we show superconductivity?. Superconductors

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Superconductivity 2012

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  1. Superconductivity 2012 Demonstration What did we see? High-Tc materials (How to make superconductors) Some applications and important properties Department of Physics, Umeå University, Sweden

  2. How do we show superconductivity? Superconductors 1. have an electrical resistivity that is exactly zero, 2. refuse magnetic fields to enter the superconducting volume. (Lab experiment) Let's try! Department of Physics, Umeå University, Sweden

  3. Meissner-Ochsenfeld effect “Perfect“ metal Superconductor Room temperature Room temperature, with magnetic field At low temperature (T<Tc), after cooling in a constant magnetic field Department of Physics, Umeå University, Sweden

  4. "Perfect conductor" effect “Perfect“ metal Superconductor Room temperature Low temperature (T<Tc)without magnetic field After applying a magnetic field at low temperature (T<Tc) Department of Physics, Umeå University, Sweden

  5. Why is the levitation stable? When you balance things on soft springs the situation is usually unstable. So why doesn't the magnet simply fall off? Because the field can penetrate! Take a ceramic: Department of Physics, Umeå University, Sweden

  6. Why is the levitation stable? Although the grains are superconducting, the boundaries are effectively thin "normal" films. Some field lines can find ways to penetrate the ceramic, but then get "locked" in place - they cannot move without crossing grains! Department of Physics, Umeå University, Sweden

  7. Two types of superconductors: Types I and II Different behaviours in magnetic fields (red): Weak B-fields are always repelled, by both types; strong fields destroy the superconductivity in type I, but penetrate type II in "vortex tubes" containing one flux quantum each! Type I Type II Department of Physics, Umeå University, Sweden

  8. Superconducting materials "Classical" superconductors: Metals and alloys! Hg 4.2 K Discovered by Heike Kammerling Onnes in 1911 (Nobel Prize 1913) Pb 7.2 K Nb 9.2 K (0.2 T - type II element!) NbTi 9.8 K 14 T (The "standard" superconductor) NbN 16.1 K 16 T (used in thin film applications) Nb3Sn 18 K 24 T (expensive and difficult to use) Department of Physics, Umeå University, Sweden

  9. High Transition Temperature Superconductors (HiTc:s) MgB2 Department of Physics, Umeå University, Sweden

  10. High Transition Temperature Superconductors (HiTc:s) Some representative ”heads of families” of HiTcs: La2-xSrxCuO4 38 K (Bednorz & Müller, 1986) YBa2Cu3O7-d 92 K (Wu & Chu, 1987) Bi2Ca2Sr2Cu3O10 110 K Tl2Ba2Ca2Cu3O10 125 K HgBa2Ca2Cu3O8 135 K Department of Physics, Umeå University, Sweden

  11. High Transition Temperature Superconductors (HiTc:s) Quite complicated structures! One of the simplest is YBa2Cu3Ox, "Y-1-2-3": The basic structure is tetragonal, with copper and oxygen forming a framework into which we insert Ba and Y. The formula is now YBa2Cu3O6, and this material is NOT superconducting! Department of Physics, Umeå University, Sweden

  12. High Transition Temperature Superconductors (HiTc:s) Quite complicated structures! One of the simplest is YBa2Cu3Ox, "Y-1-2-3": The basic structure is tetragonal, with copper and oxygen forming a framework into which we insert Ba and Y. To get a superconducting material we must add more oxygen, to obtain YBa2Cu3O7! Department of Physics, Umeå University, Sweden

  13. High Transition Temperature Superconductors (HiTc:s) Quite complicated structures! One of the simplest is YBa2Cu3Ox, "Y-1-2-3": CuO chain Ba spacer CuO plane Y spacer CuO plane Ba spacer CuO chain These are the metallic, superconducting parts! To some extent, more CuO planes mean higher Tc! Department of Physics, Umeå University, Sweden

  14. High Transition Temperature Superconductors (HiTc:s) How to make YBa2Cu3Ox, "Y-1-2-3": 1. Mix and grind Y2O3, BaCO3 and CuO for a long time. 2. Heat in an oven at 900-925 oC for at least 1 hour. 3. Crush, re-grind, and repeat 2. a few times. 4. Press into a cake, then heat in pure oxygen gas at 450 oC for at least 24 hours. 5. Time to test for superconductivity! Department of Physics, Umeå University, Sweden

  15. High Transition Temperature Superconductors (HiTc:s) Higher values for Tc can be found for other materials, based on Bi, Hg or Tl. These are also layered, often with many parallel internal layers of CuO: Tl2Ba2CuO6 Tl-2201 (single CuO) 85 K Tl2Ba2CaCu2O8 Tl-2212 2 layers 105 K Tl2Ba2Ca2Cu3O10 Tl-2223 3 layers 125 K (Bi-2223  110 K, Hg-1223  135 K) Department of Physics, Umeå University, Sweden

  16. A new star: MgB2 Superconductivity in MgB2 was discovered in 2001 with Tc = 39 K, the highest for any "classical" superconductor. The material is cheap, easy to handle, non-poisonous, and easily formed into wires or films/tapes. Problem: The practical critical field seems to be limited to 3.5 T. Department of Physics, Umeå University, Sweden

  17. An even newer star: iron arsenides In 2008, another type of layered, exotic superconductors, based on iron and arsenic, was discovered. Takahashi et al., Nature 453, 376 (2008) Department of Physics, Umeå University, Sweden

  18. An even newer star: iron arsenides In 2008, another type of layered, exotic superconductors, based on iron and arsenic, was discovered. Another family is BaxKyFe2As2. Critical temperatures up to above 55 K have been reported when changing the La to heavier rare earths. Again, the material is cheap and fairly easy to handle, but As is clearly poisonous! Department of Physics, Umeå University, Sweden

  19. Applications for superconductors There are basically two types of applications: Power circuits and electronics/measurements. Most practical applications use type II superconductors. Existing and future commercial devices: Power transmission components, power storage devices, electric motors and generators, frictionless bearings, permanent magnets and electromagnets, voltage standards, fast computers and electronics, microwave filters, ......... Department of Physics, Umeå University, Sweden

  20. Applications for superconductors In electronics, one possible application is in fast computers. Clock pulses must be synchronized in a computer, but at 3 GHz light travels only 10 cm during one clock pulse! Shrinking a computer means more concentrated heating, killing the CPU. The obvious solution is a cool superconducting computer! Department of Physics, Umeå University, Sweden

  21. Electronics and measurements: tunnelling Tunneling between two superconductors (”SIS”) can be used as the basis for many devices. In principle, both electrons and pairs can tunnel through a Josephson junction, so the real behaviour can be either bistable (logic 1/0!) or continuous. Department of Physics, Umeå University, Sweden

  22. Electronics and measurements: the SQUID A particularly useful device is the SQUID: Superconducting QUantum Interference Device or With a SQUID it is possible to routinely measure magnetic fields down to well below 10-16 T! Department of Physics, Umeå University, Sweden

  23. Electronics and measurements: the SQUID The SQUID can be used for measurements (as a sensor). Superconducting loop Josephson junctions, called ”weak links” External connections Department of Physics, Umeå University, Sweden

  24. Electronics and measurements: the SQUID Each Josephson junction has a maximum supercurrent I = I0 sin g, so the maximum current that can run through the device is 2I0. 2I0 Department of Physics, Umeå University, Sweden

  25. Electronics and measurements: the SQUID If we apply a very weak external magnetic field, a circulating shielding currentwill appear and no field will exist inside the loop! The external current must decrease to avoid exceeding the maximum supercurrents in the junctions. Department of Physics, Umeå University, Sweden

  26. Electronics and measurements: the SQUID When the magnetic field corresponds to exactly ½ magnetic flux quantum inside the ring, the circulating current has its maximum and the external current its minimum value. Department of Physics, Umeå University, Sweden

  27. Electronics and measurements: the SQUID If the field increases further, one flux quantum is admitted through a weak link, and the circulating currentreverses! It can easily be shown that the external current is a periodic function Imax = 2I0cos(pF/F0) Department of Physics, Umeå University, Sweden

  28. But how do you make ceramic "wires"? There are two ways: 1. Thin films on a metal or ceramic substrate 2. "Powder-in-tube" technology Stainless Deposition of Oxygen treatment Storage steel band ceramic film in hot oven Department of Physics, Umeå University, Sweden

  29. But how do you make ceramic "wires"? There are two ways: 1. Thin films on a metal or ceramic substrate 2. "Powder-in-tube" technology Fill a silver tube with superconductor powder, then draw to desired shape, then heat treat ("anneal"). Department of Physics, Umeå University, Sweden

  30. But how do you make ceramic "wires"? The "powder-in-tube" method is simlar to what you do to "classical" superconductors: Basic procedure: - Make a Cu cylinder, - make a lot of holes along axis, - fill the holes with superconducting rods, - draw the whole cylinder to wire, as if it were massive Cu! This procedure works well with Nb-Ti, which is soft and ductile like copper! Department of Physics, Umeå University, Sweden

  31. But how do you make ceramic "wires"? All superconductor wires have similar internal "multi-strand" structures! NbTi wire High-Tc (BiSSC) wires Department of Physics, Umeå University, Sweden

  32. Using type II superconductors An obvious application for a superconductor is to transport electric current. What happens to electrons in a B-field ? Let us remember two laws: Fm = qv  B ("Maxwell")  F = 0 ("Newton") B-field There will be a force on the magnetic field lines! Current Department of Physics, Umeå University, Sweden

  33. Using type II superconductors Is this a problem ? A moving field ↔ changing flux; but - d/dt = E ! This gives two problems: 1. A voltage appears along the current flow; "resistance"! 2. This causes dissipation of heat, since P = UI B-field Current Department of Physics, Umeå University, Sweden

  34. Using type II superconductors Is this a problem ? A moving field ↔ changing flux; but - d/dt = E ! This gives two problems: 1. A voltage appears along the current flow; "resistance"! 2. This causes dissipation of heat, since P = UI Department of Physics, Umeå University, Sweden

  35. Using type II superconductors Or, if we measure voltage as a function of applied current at constant temperature: Department of Physics, Umeå University, Sweden

  36. Using type II superconductors Conclusion: We want to keep the flux lattice fixed in space! How do we do this? Flux lines prefer to go through non-superconducting regions, because it requires energy to create a vortex tube! So, we should insert impurity particles into the superconductor! This method is called flux pinning. Department of Physics, Umeå University, Sweden

  37. Using type II superconductors You have already seen a magnet fly ! You can also make a really good magnetic bearing, or ”freeze in” a field to make a permanent magnet – with a field which you can shape exactly as you want it! Department of Physics, Umeå University, Sweden

  38. Using type II superconductors BUT: Flux pinning also gives problems: There is a”friction force” that keeps them in place, and because J  X B, dBz/dx  Jc everywhere inside a type II superconductor! Increasing external field: Department of Physics, Umeå University, Sweden

  39. Using type II superconductors BUT: Flux pinning also gives problems: There is a”friction force” that keeps them in place, and because J  X B, dBz/dx  Jc everywhere inside a type II superconductor! Increasing external field: Department of Physics, Umeå University, Sweden

  40. Using type II superconductors BUT: Flux pinning also gives problems: There is a”friction force” that keeps them in place, and because J  X B, dBz/dx  Jc everywhere inside a type II superconductor! Decreasing external field: Department of Physics, Umeå University, Sweden

  41. Using type II superconductors BUT: Flux pinning also gives problems: There is a”friction force” that keeps them in place, and because J  X B, dBz/dx  Jc everywhere inside a type II superconductor! : This leads to a magnetic hysteresis, and to energy loss (= heating!). It can be shown that the loss is proportional to the thickness a of the superconductor! Department of Physics, Umeå University, Sweden

  42. A possible novel application The first practical application for high-Tc materials in power circuits is likely to be something that cannot be made without superconductivity. One such example is the superconducting current limiter: Consider a standard transformer (which you can find in any electronic device, at home or here): U1/U2 = N1/N2 = I2/I1, where 1 means "input" side, 2 "output" side, and N is the number of wire turns! http://www.yourdictionary.com Department of Physics, Umeå University, Sweden

  43. A possible novel application The first practical application for high-Tc materials in power circuits is likely to be something that cannot be made without superconductivity. One such example is the superconducting current limiter: Suppose we make a transformer with N2 = 1 (a single turn). If we short-circuit the output, U2=0, then U1 = NU2 = 0, for all currents! Usually this is just stupid, but what if we make the secondary one turn of superconducting wire? Department of Physics, Umeå University, Sweden

  44. A possible novel application Superconducting current limiter: Primary current I1 I2 = N I1; if the coil superconducts U1 = U2 = 0, and P = UI = 0 ! However, whenever I2 > Ic the secondary turns normal and R1 = U1/I1 = N2U2/I2 = N2R2 ! Because N can be made large and high-Tc materials have very large normal resistivities, this works as a "fuse"! Department of Physics, Umeå University, Sweden

  45. A possible novel application Superconducting current limiter: N1 = 500 N2 = 1 Ic ≈ 85 A at 77 K (measured!) Tc ≈ 110 K (Bi-2223) Department of Physics, Umeå University, Sweden

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