Superconductor. Superconductor Materials Materials Eng. Dep. Kufa Univ. Dr. Sabah M. Thahab

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1 Superconductor Materials

2 What's a superconductor? Superconductors have two outstanding features: 1). Zero electrical resistivity. This means that an electrical current in a superconducting ring continues indefinitely until a force is applied lid to oppose the current. 2). The magnetic field inside a bulk sample is zero (the Meissner effect). When a magnetic field is applied current flows in the outer skin of the material leading to an induced magnetic field that exactly opposes the applied field. The material is strongly diamagnetic as a result. In the Meissner effect experiment, a magnet floats above the surface of the superconductor

3 What's a superconductor? Most materials will only superconduct, at very low temperatures, near absolute zero. Above the critical temperature, the material may have conventional metallic conductivity or may even be an insulator. As the temperature drops below the critical point,t c, resistivity itiit rapidly drops to zero and current can flow freely without any resistance.

4 What's a superconductor? Linear reduction in resistivity itiit as temperature t is decreased: ρ = ρ o (1 + α(t-tt o )) where ρ: resistivity and α: the linear temperature coefficient of resistivity. Resistivity: ρ s ~ 4x10-23 Ω cm for superconductor. Resistivity: ρ m ~ 1x10-13 Ω cm for nonsuperconductor metal.

5 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. This constraint to zero magnetic field inside a superconductor is distinct from the perfect diamagnetism which would arise from its zero electrical resistance. Zero resistance would imply that if we tried to magnetize a superconductor, current loops would be generated to exactly cancel the imposed field (Lenz s Law).

6 WHAT IS SUPERCONDUCTIVITY?? For some materials, the resistivity vanishes at some low temperature: they become superconducting. Superconductivity is the ability of certain materials to conduct electrical current with no resistance. Thus, superconductors can carry large amounts of current with little or no loss of energy.

7 Superconductivity Found in 26 metals and hundreds of alloys & compounds Mercury Copper (normal) 4.2 K Fig , Callister & Rethwisch 3e. T C = critical temperature = temperature below which material is superconductive

8 MEISSNER EFFECT When you place a superconductor in a magnetic field, the field is expelled below T C. B B T >T c T < T c Magnet Superconductor Currents i appear, to cancel B. i x B on the superconductor produces repulsion.

9 Non-superconductor B int = B ext

10 Superconductor Bext B int = 0

11 Magnetic Levitation Magnetic fields are actively excluded from superconductors (Meissner effect). If a small magnet is brought near a superconductor, it will be repelled becaused induced supercurrents will produce mirror images of each pole. If a small permanent magnet is placed above a superconductor, it can be levitated by this repulsive force.

13 Magnetic Levitation

14 Meissner-Oschenfeld Effect Superconductor B = 0 perfect diamagnetism: χ M = -1 B = µ 0( H + M ) = 0 M = χh = H Field expulsion unexpected; not discovered for 20 years. B/µ 0 -M Apply field Cool H c H H c H

15 Meissner Effect Superconductors expel magnetic fields normal superconductor Fig , Callister & Rethwisch 3e. This is why a superconductor will float above a magnet

16 Types I Superconductors There are 30 pure metals which exhibit zero resistivity at low temperature. They are called Type I superconductors (Soft Superconductors). The superconductivity exists only below their critical temperature and below a critical magnetic field strength. Type I superconductors: pure metals, have low critical field

17 Type I Superconductors Mat. T c (K) Mat. T c (K) Be 0 Gd* 1.1 Rh 0 Al 1.2 W Pa 1.4 Ir 0.1 Th 1.4 Lu 0.1 Re 1.4 Hf 0.1 Tl 2.39 Ru 0.5 In Os 0.7 Sn Mo 0.92 Hg Zr Ta 4.47 Cd 0.56 V 5.38 U 0.2 La 6.00 Ti 0.39 Pb Zn 0.85 Tc Ga Nb 9.46

18 Types II Superconductors Starting in 1930 with lead-bismuth alloys, were found which exhibited superconductivity; they are called Type II superconductors (Hard Superconductors). They were found to have much higher critical fields and therefore could carry much higher current densities while remaining in the superconducting state. Type II superconductors: primarily of alloys or intermetallic compounds.

19 Type II Superconductors

20 The Critical Field An important characteristic of all superconductors is that the superconductivity is "quenched" when the material is exposed to a sufficiently high magnetic field. This magnetic field, H c, is called the critical field. Type II superconductors have two critical fields. The first is a low-intensity field, H c1, which partially suppresses the superconductivity. The second is a much higher critical field, H c2, which totally quenches the superconductivity.

21 The Critical Field Researcher stated that the upper critical field of yttrium-barium-copper-oxide is 14 Tesla at liquid nitrogen temperature t (77 degrees Kelvin) and at least 60 Tesla at liquid helium temperature. The similar rare earth ceramic oxide, thuliumbarium-copper-oxide, was reported to have a critical field of 36 Tesla at liquid nitrogen temperature and 100 Tesla or greater at liquid helium temperature.

22 The Critical Field The critical field, H c, that destroys s the superconducting effect obeys a parabolic law of the form: H c = H o 1 where H o = constant, T = temperature, T c = critical temperature. In general, the higher T c, the higher H c. T T c 2

not superconducting H C = critical magnetic field - if H > H C not

23 Critical Properties of Superconductive Materials T C = critical temperature - if T > T C not superconducting J C = critical current density - if J > J C not superconducting H C = critical magnetic field - if H > H C not superconducting H C (T = H C (0) 1 T 2 C( ) C( T 2 C Fig , Callister & Rethwisch 3e.

24 Breakdown in a Magnetic Field A magnetic field breaks up the superconductivity. MM H CRITICAL magnetic field c = H H c H 2 T H c H/H T c 1 c0 1 c 1 T/T c

25 Example

26 Breakdown in a Magnetic Field M H c SOFT superconductor Low T c HARD superconductor High T c H c1 H c H c2 H

27 Type I Magnetization curves for ideal type I and type II superconductors. Type II superconductors are penetrated by the magnetic field between H c1 and H c2.

29 Current Flow and Magnetic Fields in Superconductors Type I superconductors are poor carriers of electrical current since current can only flow in the outer surface layer of a conducting specimen. The reason for this behavior is that the magnetic field can only penetrate the surface layer, and current can only flow in this layer. In type II superconductors below H cu magnetic fields behave in the same way. However, if the magnetic field is between H cl and H c2 (mixed state), the current can be carried inside the superconductor by filaments,

30 Electric current flow in outer surface of wire only Electric current flow within wire (a) Cross section of a superconducting wire carrying an electrical current, (a) Type I superconductor or type II under low field (H < «c1 ). (b) Type II superconductor under higher fields where current is carried by a filament network (H c1 < H < H c2 ). Magnetic (dipole) field Circulating supercurrent I Superconductor

31 Schematic illustration showing magnetic fluxoias in a type II superconductor with the magnetic field between H c1 and H c2. In type II superconductors when a magnetic field between H cx and H c2 is applied, the field penetrates the bulk of the superconductor in the form of individual quantized flux bundles called fluxoids. A cylindrical super-current vortex surrounds each fluxoid. With increasing magnetic-field strength, more and more fluxoids enter the superconductor and form a periodic array. At H c2 the supercurrent vortex structure collapses and the material returns to the normal conducting state

32 Fluxoid : A microscopic region surrounded by circulating supercurrents in a type II superconductor at fields between HC 2 and HC 1 High-Current, High-Field Superconductors Although ideal type II superconductors can be penetrated by an applied magnetic field in the H c) to H cl range, they have a small current-carrying capacity below T c since the fluxoids are weakly tied to the crystal lattice and are relatively mobile. The mobility of the fluxoids can be greatly impeded by dislocations, grain boundaries, and fine precipitates, and thus J c can be raised by cold working and heat treatments. Heat treatment of the Nb 45 wt % Ti alloy is used to precipitate Heat treatment of the Nb-45 wt % Ti alloy is used to precipitate a hexagonal a phase in the BCC matrix of the alloy to help pin down the fluxoids.

33 The alloy Nb-45 wt % Ti and the compound Nb 3 Sn have become the basic materials for modern high-current, high-field superconductor technology. In today's superconductor technology these superconductors are used at liquid helium temperature (4.2 K). The Nb-45 wt % Ti alloy is more ductile and easier to fabricate than the Nb 3 Sn compound and so is preferred for many applications even though it has lower T c's and H c2 's. = Applications for NbTi and Nb 3Sn superconductors include nuclear magnetic imaging systems for medical diagnosis and magnetic levitation of vehicles such as high-speed trains. High-field superconducting magnets are used in particle accelerators in the high-energy physics field.

34 Superconductor Ceramics The ceramic materials used to make superconductors are a class of materials called perovskites. One of these superconductor is an yttrium (Y), barium (Ba) and copper (Cu) composition. Chemical formula is YBa 2 Cu 3 O 7. This superconductor has a critical transition temperature t around 90K, well above liquid id nitrogen's 77K temperature.

35 High Temperature Superconductor (HTS) Ceramics Discovered in 1986, HTS ceramics are working at 77 K, saving a great deal of cost as compared to previously known superconductor alloys. However, as has been noted in a Nobel Prize publication of Bednortz and Muller, these HTS ceramics have two technological disadvantages: they are brittle and they degrade under common environmental influences.

36 HTS Ceramics HTS materials the most popular is orthorhombic YBa 2 Cu 3 O 7-x (YBCO) ceramics. Nonoxide/intermetallic solid powders including MgB 2 or CaCuO 2 or other ceramics while these ceramics still have significant disadvantages as compared to YBCO raw material.

38 Advances in Superconductivity Research in superconductive materials was stagnant for many years. Everyone assumed T C,max was about 23 K Many theories said it was impossible to increase T C beyond this value new materials were discovered with T C > 30 K ceramics of form Ba 1-x K x BiO 3-y Started enormous race Y Ba 2 Cu 3 O 7-x T C = 90 K Tl 2Ba 2Ca 2Cu 3O x T C = 122 K difficult to make since oxidation state is very important The major problem is that these ceramic materials are inherently brittle.

39 APPLICATIONS: Superconducting Magnetic Levitation The track are walls with a continuous series of vertical coils of wire mounted inside. The wire in these coils is not a superconductor. As the train passes each coil, the motion of the superconducting magnet on the train induces a current in these coils, making them electromagnets. The electromagnets on the train and outside produce forces that levitate the train and keep it centered above the track. In addition, a wave of electric current sweeps down these outside coils and propels the train forward. The Yamanashi MLX01MagLev Train

40 APPLICATIONS: Medical MRI (Magnetic Resonance Imaging) scans produce detailed images of soft tissues. The superconducting magnet coils produce a large and uniform magnetic field inside the patient's body.

41 APPLICATIONS: Power The cable configuration features a conductor made from HTS wires wound around a flexible hollow core. Liquid nitrogen flows through the core, cooling the HTS wire to the zero resistance state. The conductor is surrounded by conventional dielectric insulation. The efficiency of this design reduces losses. Superconducting Transmission Cable From American Superconductor

WHAT IS SUPERCONDUCTIVITY??

WHAT IS SUPERCONDUCTIVITY?? For some materials, the resistivity vanishes at some low temperature: they become superconducting. Superconductivity is the ability of certain materials to conduct electrical