Showing papers by "G. Krabbes published in 2005"

Reluctance motors with bulk HTS material

Abstract: In recent years we have successfully designed, built and tested several reluctance motors with YBCO bulk material incorporated into the rotor, working at 77 K. Our last motor type SRE150 was tested up to 200 kW. The aim of our investigations is the construction of motors with extremely high power density and dynamics. In comparison to conventional motor types the advantage of HTS reluctance motors with respect to size and dynamics could be demonstrated. Some fields of possible future applications will be described. These motors show a significant improvement in performance using high quality HTS bulk elements in the rotor. Until now the motor parameters have been limited by the current density which could be obtained in the bulk material at 77 K and by the geometric dimensions of the segments available. Therefore we expect further improvements in the case of these materials. Since the total motor including stator and rotor is working at low temperature we have to optimize the windings and the magnetic circuit to these operation conditions. A new design of a 200 kW motor in order to achieve increased power density and the theoretical results of our calculations will be shown.

Influence of the lateral movement on the levitation and guidance force in the high-temperature superconductor maglev system

Abstract: After the levitation force relaxation was studied for different field-cooling height and working-levitation height, the high-temperature superconductor (HTS) bulk was horizontally moved in the lateral direction above the permanent magnet guideway. Both levitation and guidance force were collected by the measurement system at the same time. It was found that the decay of levitation force is dependent on both the maximum lateral displacement and the movement cycle times, while the guidance force hysteresis curve does not change after the first cycle. This work provided scientific analysis for the HTS maglev system design.

Novel methods to characterize bulk RE-BCO superconductors

Abstract: We report on new techniques for the investigation of the radial and the axial homogeneity of top seeded melt-textured RE–Ba–Cu–O (REBCO) monoliths. Whereas the traditional Hall probe scanning technique of trapped fields at the surface of the superconductor provides only global information on the patterns of supercurrent flow, a modified scanning technique employing sequentially increasing activation fields leads to more detailed information on the flux penetration process and, therefore, on possible radial inhomogeneities of the defect structure for flux pinning. Particularly valuable information on this subject can be deduced from a new technique, the magnetoscan, that allows the flow of the local supercurrent in large bulk monoliths on the scale of less than 1 mm to be investigated. Combination of the magnetoscan with optical or electron beam imaging of the sample surface results in full characterization of the bulk microstructure, developed during the growth process, and its effective flux pinning capability. We show that these techniques can also be used to investigate axial inhomogeneities, i.e. those developing along the c -axis (growth direction), by removing layers of the monolith step by step and by observing the microstructure and the local supercurrent flow in sequential layers. It turns out that the c -growth sector is always characterized by a lower current carrying capability and that characteristic features of the growth morphology appear at the a -growth sector boundaries. Finally, initial results on the current flow in multi-seeded bulks will be presented.

Flux pinning enhancement in melt-processed YBa2Cu3O7 with extended nanodefects

Abstract: Melt-processed YBa2Cu3O7 (YBCO), containing a random array of Y–Ba–Cu–Ir–O nanostructures (about 20–150nm in diameter and several μm in length) has been grown. The pinning efficiency of the resulting YBCO nanocomposite is superior to Li-doped or undoped melt-processed YBCO at higher magnetic fields. At 77K, the irreversibility field increases more than 8T in the presence of the extended nanodefects. The increase of the effective activation energy of the YBCO nanocomposite is nearly twice that of the Li-doped material.

Magnetic flux penetration into melt-textured superconductors

Abstract: Scanning Hall probe measurements were made on melt-textured Y-123 monoliths under sequentially increasing activation fields. The remnant field profiles are compared to measurements of the local response of the superconductor to a small magnetic field induced by a permanent magnet ('magnetoscan'). Optical microscopy is employed to relate the flux patterns with the microstructure of the sample.

Method for precisely analyzing the stoichiometry of Na x CoO 2 -type superconductor material

Abstract: An analytical procedure for precisely determining the stoichiometry of NaxCoO2-type superconductor material is presented. Sodium and cobalt contents, ranging between 3.5 and 11 mg L−1 and 18 and 32 mg L−1, respectively, were measured simultaneously using CID–ICP–OES. Sodium was found to significantly lower the emission intensity of cobalt, so the addition of 6.4 g L−1 of the ionization buffer LiCl was required to compensate for this effect. The recoveries and precisions of the measurements were significantly increased by internal standardization using yttrium: Co(II) emission intensities at 230.786 nm, 237.862 nm, and 238.346 nm can be corrected using Y ion emission intensities, as can the atomic emissions of Co at 345.351 nm and Na at 589.592 nm. The cobalt contents of three real superconductor samples were independently verified by complexometric titration using EDTA. The valence state of cobalt was determined with a relative uncertainty of ~0.5% by redox titration using sodium oxalate as reductive agent and Ce(SO4)2 solution. The final stoichiometries of the superconductor samples can be calculated using the Na and Co contents, and the Co valence state. Conclusions about the quality of the prepared samples in terms of phase purity and presence of side products are drawn.