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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Lattice structure and misfit between substrate/buffer/YBCO conductor Conductive buffer layers

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Twisted conductor - the filaments must be transposed (or twisted) at least one time in the middle of the length provided that the magnetic field is exactly symmetric along both half lengths -twisted decoupled filaments - ac loss reduction coefficient - large circulating currents can exist among the filaments due to connections at the ends (in current leads) -ac losses may increase by one order of magnitude or more in dependence on the distance between the filaments and the resistance at the end.

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki 3) multifilamentary tapes in a transverse magnetic field - hysteresis losses in perpendicular magnetic field - about 2-3 orders of magnitude higher than in parallel field H o - amplitude of the applied magnetic field w - tape width t - tape thickness -for decoupled filaments - ac loss reduction directly proportional to the number of filaments (proved experimentaly) HoHo

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Initial design of multifilamentary CC for Supergenerator

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Multifilamentary YBa 2 Cu 3 O 7 Coated Conductor CC-20 filaments CC-1filament 1cm Single filament < 500 m IRC in Superconductivity testing

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki AC field AC current anisotropy at the IRC in Superconductivity Magnet requirement for YBCO coated conductors in supergenerators is about 2T/400 Hz Q I /Q || ~100 (high precision of angle required in parallel field - of the order of 0.1 degree) Current AC magnet capability In phase and out of phase measurements for transformers and generators

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Division to smaller filaments essential for H a perp. to ab plane

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Transport critical current vs angle + Ca doping

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Current percolation through the dislocations I Important factors: conductor aspect ratio magnetic field angle sample history shunt layers, contacts width length HREM of LAGB [N.D. Browning et al, Micron 30, 425 (1999)]

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Applied Superconductivity Research Influence of misoriented Ni grains on YBCO layer CuO precipitates NiO layer grown by surface oxidation epitaxy CSTO grown by PLD via an amorphous route YBCO grown by PLD

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Length (no. of grains) J c /J c0 w=3 w=5 w=9 w=17 w=33 w=129 w=513 1 filament 5 filaments 11 filaments Recommended division of the coated conductors to narrower tracks/filaments is a compromise between reduction of the current in the longer tracks and gain in reduction of AC losses. 2° 4° 6° 8° The EBS maps above show regions misoriented by 2°, 4°, 6°, and 8° in a small area of the NiFe substrate. Using a model with a 2D array of R rows and C columns of hexagonal grains (shown above left), a number of parameters may be assessed over a range of threshold angles. Current percolation in In-plane and out of plane misoriented GB

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki p=5 p=1 A New Grain Structure Model MCS = 2MCS = 25MCS = 100 A Monte-Carlo grain growth model has been used to simulate more realistic grain structures. The grains are initially made up of single square pixels. Each pixel has an energy based upon the number of neighbours which are in the same grain. High energy pixels are consumed by neighboring grains. As the simulation progresses, grain structures such as those below develop. After N Monte-Carlo Steps (MCS) each pixel will, on average have been considered N times. The average grain size in pixels (p) is related to MCS. The figures above shows grain structures for p=1 (simple square model) and p=5, both for samples 25 grains long and 10 grains wide. 2-D Grain growth modelling IBAD and RABIT

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki I c vs length and width of Coated Conductor I c (1/N L ) 1/N W … a useful working approximation is 10 km 1 cm NLNL NWNW IcIc [Rutter and Goyal MRS 2003] N w =300 grains N w =100 grains N w =30 grains N w =10 grains

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Orientation of the GB in respect to the applied magnetic field is important In plane critical current vs magnetic field measurements on YBCO thin films: (a) angular dependence of the critical current crossing low angle grain boundary at 8T. For the higher GB angle the minimum is wider and the absolute valuses are substantially lower; =90 o represents Lorentz force-free configuration. Hexagons represent grains whereas black outlines of hexagons represent grain boundaries; (b) schematic of the J c vs (B, ) in plain measurements. Elongated hexagonal grains have the better percoative properties than the simple hexagonal ones. There is a difference in the response of hexagonal grains if all of them are aligned.

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Solution for the high magnetic field and low temperature magnet applications NbTi + Nb 3 Sn + NbTi 3 (Sn,Ta) = 22.59T additional magnetic field generated by Bi-2212 coil = 1.46T ; total field > 24T High magnetic field superconducting electromagnet. (a) schematic cross section of the multi- section hybrid electromagnet. The materials used are NbTi + Nb 3 Sn + NbTi 3 (Sn,Ta) resulting in Tesla and if additional magnetic field is generated by internal coil in the centre it would generate an additional field. The total magnetic field in such a hybrid configuration, currently exceeds 24 Tesla. (b) schematic outline of the favourable grain structure of the internal HTS coil made from the YBa 2 Cu 2 O 7 coated conductor. (a)(b)

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Presence of the magnetic material can have a detrimental and also beneficial influence on the reduction of AC losses and increase of J c of superconductors. This problem is particularly important in case of the coated conductors and multifilamentary wires. The latest research on the multifilamentary conductors surrounded by magnetic material proved that losses can be reduced substantially according to eq.1 by coating individual filaments by magnetic material. By comparing losses in a standard multifilamentary superconductor, Q st, to losses in a multifilamentary superconductor with the magnetic covers around individual filaments, Q cov, at the same reduced current i, one can obtain magnetic decoupling loss reduction coefficient, K md, (eq.1); where i=I/I c I c1 =I c /N, N number of filaments. The parameters k(i) and are to be determined from experiment and represent individual filament. (eq.1) 3) multifilamentary tapes in a transverse magnetic field

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Applied Superconductivity Research - University of Cambridge B.A.Glowacki Minimisation of AC losses Magnetic decoupling Recommended division of the coated conductors to narrower tracks/filaments is a compromise between reduction of the current in the longer tracks and gain in reduction of AC losses.

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