Lecture 8 Applications and Devices

•Levitation

•Wire Applications and Superconducting Magnets

•Flux Pinning and Issues in High Tc , High Jc Wires

•Electronic Devices Using Josephson Junctions and SQUIDS

A superconductor displaying the MEISSNER EFFECT

Superconductors have electronic and magnetic properties. That is, they have a negative susceptibility, and acquire a polarization OPPOSITE to an applied magnetic field. This is the reason that superconducting materials and magnets repel one another.

If the temperature increases the sample will lose its superconductivity and the magnet cannot float on the superconductor.

Emsland, Germany

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

Superconducting Wire for Magnets

MgB2 PIT at 20K

Superconducting Transmission Cables

APPLICATIONS: Power

Superconducting Transmission Cable From American Superconductor

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.

Other Superconducting Wire Applications

Fault Current Limiters

APPLICATIONS: Medical

The superconducting magnet coils produce a large and uniform magnetic field inside the patient's body.

MRI (Magnetic Resonance Imaging) scans produce detailed images of soft tissues.

SQUIDS

Future: Marriage of

Superconductivity and Nanotechnology

Nanoporous

Membrane

Superconductor

Magnet

Ferroelectric

Semiconductor

LowTemperature SuperconductorNanowires

• Mesoscopic superconducting lead nanowires with high aspect ratio and diameter ranging from 40 to 270 nm have been grown by electrodeposition inside nanoporous polycarbonate membranes. Nanowires with a diameter less than 50 nm were insulators due to a poor crystal structure. The others are shown to be type II superconductors because of their small electronic mean free path, instead of being type I which is usual for the bulk form of lead. An increase in the thermodynamic critical field Hc is observed and is attributed to the small transversal dimension leading to an incomplete Meissner effect. Finally, it is demonstrated that this enhancement agrees with numerical simulations based on the Ginzburg–Landau theory.

Michotte et al

• High Temperature Superconductor Nanowires

• Single crystalline YBa2Cu3O7−δ nanowires from a template-directed sol–gel route

• Xu Jian, Liu Xiaohe and Li Yadong, , Department of Chemistry, Tsinghua University, Beijing 100084, PR China Department of Chemistry, Central South University, Changsha 410083

• Preparation of anodic aluminum oxide (AAO) template • Porous AAO membrane was prepared using a two-step anodizing process [19].

After degreasing in ethanol, high-purity Al foil (99.999%) was electropolished in a H3PO4–H2SO4–H2O (40:40:20 wt.%) solution for 5 min to reduce the surface roughness. The resulting Al sheet was washed with ethanol and distilled water, then mounted on a copper plate to serve as anode for anodization. In the first step, the anodizing of the Al sheet was carried out under constant voltages of 40 V in 0.3 M oxalic acid electrolyte for 6 h at 17 °C. Prior to the second anodization, first disordered porous alumina layer was removed in a mixture of H3PO4 (6 wt.%) and CrO3 (1.8 wt.%) at 60 °C for 3 h. The second step was following the same conditions as that of the first, except that the anodizing time was 3 h. A stepwise voltage reduction technique [28 and 29] was then utilized to thin the barrier layer. The as-obtained AAO membrane was immersed in 5 wt.% aqueous H3PO4 solution for pore widening and barrier layer dissolving, followed by washing and drying. Finally, the through-hole AAO membrane was detached from the Al substrate in a saturated HgCl2 solution for next use.

• Sol–gel process and fabrication of YBCO nanowires • YBa2Cu3O7−δ sols used for template synthesis can be obtained as follows. Firstly,

appropriate quantities of ammonia and ethylenediamine tetraacetic acid (EDTA) was added to copper acetate solutions. Meanwhile the solution of Ba2+ or Y3+ was prepared by dissolving Ba(OH)2·8H2O or Y2O3 into acetic acid solution, and then both were poured into the previous solution of Cu2+ to give a mixture with initial molar ratio of Y3+:Ba2+:Cu3+ = 1:2:3. The pH of this mixture solution was adjusted to 6.5 using ammonia and glacial acetic acid, thus preventing crystallization of copper acetate during gelation. The obtained solution was hydrolyzed to form a dispersed blue colloidal particles solution (sol) at 70 °C under continuous stirring.

• Then a piece of AAO membrane (φ 2 cm) was dipped into this transparent hot sol followed by ultrasonic treatment 10 min, and kept at ambient temperature overnight (12 h). The sol containing AAO membrane was gelled to shards by slow evaporation at 90 °C. To form the superconducting phase, the obtained gel/AAO composite was further heated at 50 °C h−1 to 950 °C and held for 2 h in air at ambient oxygen pressure. Subsequently, the specimen was cooled at 50 °C h−1 to ambient temperature. Finally, the AAO membrane incorporated with YBCO nanowires was picked out, and rubbed away YBCO powders covering both faces of the membrane by polishing alumina.

• Scanning electron microscopy (SEM) confirmed that the monodisperse cylindrical pores of alumina membrane were filled with nanowire arrays with a narrow distribution of diameters. The X-ray diffraction (XRD) pattern of the obtained nanowires was assessed as orthorhombic YBCO 123 lattice after heating gels at 950 °C for crystallization. As determined by transmission electron microscopy (TEM) and electron diffraction (ED) analysis, the nanowires were essentially single crystalline in nature. High-resolution transmission electron microscope (HRTEM) images further revealed the single crystalline nature of the as-prepared nanowires.

• Representative SEM images of (a) top-view of an empty AAO membrane; (b) cross-sectional view of AAO membrane; (c) top-view of an AAO membrane

filled with YBCO nanowires; (d) cross-sectional view of YBCO nanowires/AAO composite, which revealed the filling of YBCO wires within most of the AAO pores. The surface of membrane was covered with YBCO powders and could be removed by polishing alumina.

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