Influence of mechanical reinforcement of MgB2 wires on the superconducting properties

Monofilamentary MgB2-wires with a mechanical tough Nb/Cu/steel or Fe/steel sheath were prepared and characterized. The steel content was varied to investigate the reinforcement effect and the consequences for the superconducting properties of the wires, which were heat treated to achieve dense and homogeneous filaments. The use of Nb as first sheath layer, having a smaller thermal expansion coefficient than MgB2, requires the application of higher amounts of steel to achieve compressive pre-stress on the filament in comparison to Fe as first wall material. With raised steel content in the sheath the critical transport currents show field dependent significant critical current and irreversibility field degradations. First Ic vs. axial stress and strain experiments confirmed this observation of pre-stress induced degradations. Consequences for improved wires and for future applications will be discussed.Comment: 14 pages, 7 figures, 1 table; Submitted to Physica C; presentation at ISS-2001, Kobe Japa

High transport currents in mechanically reinforced MgB2 wires

We prepared and characterized monofilamentary MgB2 wires with a mechanically reinforced composite sheath of Ta(Nb)/Cu/steel, which leads to dense filaments and correspondingly high transport currents up to Jc = 10^5 A/cm^2 at 4.2 K, self field. The reproducibility of the measured transport currents was excellent and not depending on the wire diameter. Using different precursors, commercial reacted powder or an unreacted Mg/B powder mixture, a strong influence on the pinning behaviour and the irreversibility field was observed. The critical transport current density showed a nearly linear temperature dependency for all wires being still 52 kA/cm^2 at 20 K and 23 kA/cm^2 at 30 K. Detailed data for Jc(B,T) and Tc(B) were measured.Comment: 21 pages, 13 figures, revised version, to be published in Supercond. Sci. Techno

A second superconducting energy gap of Nb_3Sn observed by breakjunction point-contact spectroscopy

We report on investigations of the superconducting energy gap of the A15 superconductor Nb_3Sn by point-contact spectroscopy of breakjunctions. The voltage-dependent differential conductance dI/dV reveals features of a second energy gap besides the energy gap known from previous tunnel measurements with maxima at \Delta_1=3.92 +/- 0.16 meV and \Delta_2=0.85 +/- 0.17 meV as derived from a histogram summarizing the data of more than 60 contacts. These findings are the first spectroscopic evidence that Nb_3Sn belongs to the class of two-band superconductors and they are in line with low-temperature specific-heat measurements on Nb_3Sn

Influence of the voltage taps position on the self-field DC and AC transport characterization of HTS superconducting tapes

The current-voltage (I-V) curve is the basic characteristic of a superconducting wire or tape. Measuring I-V curves is generally problematic when samples have poor stabilization. Soldering voltage taps to an active part of the conductor affects the effectiveness of the local cooling and/or can be difficult to do in certain devices such as fault current limiters and cables where the tapes are closely packed. In order to overcome these problems, voltage taps can be placed outside the active area of the superconductor. We proved both by simulations and experiments that this arrangement leads to the same results as the standard four point method and it provides more detailed information for sample protection. The same arrangement can also be used for AC transport loss measurement. However in this case particular care has to be taken because the eddy current loss in the current leads contributes to the total measured loss. We used numerical simulations to evaluate the contribution of the eddy current loss to the measured AC loss. With help of simulations one can determine whether the contribution of the eddy current loss is significant and possibly optimize the current leads to reduce that loss contribution

Influence parameters of impact grinding mills

Significant parameters for impact grinding mills were investigated. Final particle size was used to evaluate grinding results. Adjustment of the parameters toward increased charge load results in improved efficiency; however, it was not possible to define a single, unified set to optimum grinding conditions

Roebel cables from REBCO coated conductors: a one-century-old concept for the superconductivity of the future

Energy applications employing high-temperature superconductors (HTS), such as motors/generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (AC) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of AC loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA to up to 20-30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology (KIT) and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low AC losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Times are therefore mature for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cables

New Experimental Method for Investigating AC-losses in Concentric HTS Power Cables

The optimization of a HTS cable design with respect to AC-losses is of crucial importance for the economic viability of the respective concept. However the experimental determination of AC-losses is not straightforward since for short cable samples the distribution of current among the super-conducting tapes is mainly determined by the contact resistances of the individual tapes. The resulting inhomogeneous current distribution definitely falsifies the results. To solve this experimental problem we present a new experimental technique. The setup is a 2m-long three phase concentric cable model for which, within each phase, the superconducting tapes (up to 30) are connected in series. The Cu-braid backwards conductors were assembled in a rotational symmetric cage type arrangement, such that their self fields at the cable cancel. If experimental peculiarities of this setup, as the strong inductive coupling between the phases and the suitable positioning of the voltage contact leads, are correctly taken into account, the currents can be controlled independently and the electrical properties of the cable can be measured unambiguously. In this paper preliminary results are presented. The work is part of the German government funded cable project AMPACITY (1 km / 20 kV/ 2 kA