Gerard Duperray

A.C. loss measurements in high-Tc superconductors

Abstract Low a.c. loss is a key issue for electrical engineering applications of high-Tc superconductors at industrial frequencies. Different material options are characterized by three different a.c. loss measurement techniques which approach as much as possible the application conditions of the conductors. Melt-textured-growth YBa2Cu3O7−x bulk samples, suitable for magnetic bearings, are characterized in an external a.c. field by magnetization measurements. Melt-casted Bi2Sr2Ca2Cu3O8−x samples and powder-in-tube Bi2Sr2Ca2Cu3O10−x tapes are suitable for current leads or power cables. In these applications the conductors are exposed to a.c. transport currents. The associated losses in self field are measured by a very sensitive electrical measurement technique. Finally, a calorimetric method is necessary when larger conductors (for instance Bi2Sr2Ca2Cu3O8−x tubes) are tested under transport currents It, generating a transverse magnetic field H α It, as encountered in magnet windings for SMES, transformers or generators. The results show that the a.c. losses are sufficiently low for self field applications at industrial frequencies and a comparison of the different high Tc superconductors is given. The results show further that the a.c. losses are essentially hysteretic and can be modeled using the Bean model.

Current transfer conditions from a metallic conductor to a high Tc superconductor

Abstract The quality of the electrical contact between a metal and a high T c superconductor is of primary importance for practical applications and may influence the result of transport J c measurements. In melt-textured growth (MTG) samples the current density in the contacts can be very high and dissipation may cause heating which can lead to faulty measurements or even to thermal runaway. The losses are determined by the resistance of the normal metal, the contact resistance and the transfer length in the YBaCuO. Methods to establish low resistivity contacts with a very low loss level at high current densities are currently being evaluated. Even when such low resistance contacts are realized, the sample often breaks under the Lorentz force or forces due to differential thermal contractions of the sample holder. To avoid this problem, a new sample holder has been developed which reinforces the sample and optimizes the geometry of the normal conducting part to achieve good heat exchange to the cryogenic liquid and to reduce the Joule heating. The influence of the sample holder on thermal stability is discussed and the J c values at different temperatures and fields are presented.

BSCCO Based Superconductors for Magnet Applications

Industries and Universities are developing various types of BSCCO conductors for magnet and for power applications at intermediate (20–30 K) and at liquid nitrogen temperatures. The optimum application domains of different conductors are briefly addressed. The Bi-2212 conductor represents today the optimum cost versus performance solution for magnet applications in the intermediate temperature range. A review will be given on fabrication of the precursor powders, on conductor fabrication process, on heat treatments and finally on performances of the conductors. Alcatel pursues the development of multifilamentary tape-conductors by the rectangular deformation route. The ultimate test of performances of conductors is of course their use in systems. First applications are today opening in the magnet technology domain. Some aspects on coil fabrication by the complementary R&W and W&R fabrication process are given.

Pre-industrial PIT conductor and coil development at Alcatel

In 1995 the PIT conductor development was strengthened at Alcatel and important progress has been achieved since. During 1997 the patented rectangular Alcatel route was upscaled and pre-industrial conductors have been fabricated in a wire drawing mill. After the successful transfer the first high performance conductors have been realized in kilometer lengths. The conductors are realized in cooperation with Aventis Research and Technologies using precursor powders optimized for the Alcatel conductor route. The simpler Bi-2212/AgPd matrix conductor is used as a working horse where engineering current densities of J/sub e/=65 kA/cm/sup 2/ have been reached at 4 K, 0 T. Significant progress has also been achieved on Bi-2223 conductors where critical current densities of J/sub c/=30 kA/cm/sup 2/ at 77 K, 0 T are reached. First solenoids generating a field up to 2.3 Tesla have been realized using Bi-2212 conductors. In a background field of 3 Tesla this magnet still generates a field of 2 Tesla. The results on test coils are consistent with J/sub c/(H) measurements on short samples showing that an important step towards high homogeneity conductors has been achieved.

Highly anisotropic filaments in BSCCO-based multifilamentary tapes

Published results indicate that high-current-carrying capacities of BSCCO-based conductors have been obtained under the following conditions: (1) monofilamentary conductors with a core thickness less than 30 µm and (2) the largest possible shape factor (30 to 70). When the first condition is feasible for multifilamentary tapes, it generally corresponds to large perturbations of the filament geometry. We succeeded in making several 10-m-long tapes with good filament geometry and a shape factor of more than 60. We present results of structural examination of the filaments, phase stability during thermal treatment, and critical current density. Typical results were 430 A/mm2 at 4.2 K and 15 A/mm2 at 77 K for 45-filament Bi-2223-phase tapes.