K. Kwasnitza

AC losses of superconducting high-Tc multifilament Bi-2223/Ag sheathd tapes in perpendicular magnetic fields

Abstract We have measured at 4.2 K and partly also at 77 K the hysteresis and coupling losses of three different superconducting c -axis oriented Bi-2223/Ag multifilament tapes. The inter- and intragranular contributions to the hysteresis losses were analyzed and their dependence on d B /d t was investigated. The penetration field of a very flat filament was calculated. We could separate the measured coupling losses from the also d B /d t dependent hysteresis losses. Further we calculated the geometry dependence of the coupling losses of a very flat multifilament tape. The measured coupling losses can be described by a general scaling law with a generalized frequency ωτ. A skin-effect-like behavior has been found for ωτ>1 and AC loss saturation occurs for large AC field amplitudes.

Alternating magnetic field losses in high-Tc superconducting multifilament tapes with a mixed matrix of Ag and BaZrO3

Alternating magnetic field losses for the new (Bi, Pb)2Sr2Ca2Cu3O10 multifilament tapes with a mixed Ag–BaZrO3 matrix for coupling loss reduction have been measured at 77 K and at frequencies up to 100 Hz. Due to the presence of oxide barriers around the filaments, the effective transverse electrical resistivity in the matrix is increased by a factor of eight, with a corresponding modification of the frequency dependence of the coupling losses. This result is a very promising step in the development of low loss high-Tc superconductors for 50 Hz applications. For larger ΔB values, it seems feasible to reduce the coupling losses at 50 Hz to a tolerable level by both, applying high-resistive barriers and changing the conductor dimensions, away from the extremely flat tape geometry. A saturation loss criterion is derived for twisted flat multifilament tapes.

Strong magnetic history dependence of magnetic relaxation in high-Tc superconductors

Abstract Very detailed flux creep and relaxation measurements of intragrain magnetization in sintered high-Tc superconductors at 4.2 K are presented. The M-relaxation rate is not a single valued function of the applied B-field but depends strongly on the prior magnetic field cycling procedure. The largest relaxation rate is measured on the envelope curves of the M-hysteresis area. On the transition curves (minor loops) the relaxation rate is reduced and even disappears locally. This is of great importance for applications of high-Tc superconductors in electronics. The results can also be applied to low-Tc superconductors like NbTi and are of interest for accelerator magnets with superconducting windings. Theoretical expressions for the relaxation process are derived for all physically relevant parts of the M-hysteresis curve on the basis of Beans model. As the grain M influences also the integrain critical transport current and microwave absorption granular high-Tc material, our results on M-relaxation behavior are a basis to explain the already observed time dependence of those phenomena.

Reduced filament coupling in Bi(2223)/BaZrO3/Ag composite tapes

Multifilamentary Bi(2223)/Ag tapes often exhibit AC loss levels comparable to those measured in monofilamentary samples, which is partly due to the large coupling currents induced in the low resistive sheath material. Surrounding the individual filaments by electrically insulating barrier layers suppresses these currents and strongly reduces the coupling. We demonstrate this effect with various types of magnetic and self-field AC loss measurements on a series of Bi(2223)/BaZrO3/Ag tapes. We also discuss the influence of barrier thickness, twist pitch length and filament arrangement on the measured losses in these composite conductors.

Reduction of alternating magnetic field losses in high-Tc multifilament Bi(2223)/Ag tapes by high resistive barriers

Abstract Flat High- T c multifilament tapes with an Ag matrix have very large coupling current losses in perpendicularly applied 50 Hz magnetic fields. By the application of ceramic isolating BaZrO 3 - and SrZrO 3 barriers around the filaments we could reduce at 77 K the coupling current decay time constant to 1.5 ms, leading to a significant loss reduction at 25 Hz and a noticeable loss reduction at 50 Hz. The coupling loss maximum for small Δ B values was shifted for a stack of six tapes to 110 Hz and for a single tape to 180 Hz. By a further combined application of several loss reduction measures the application of High- T c superconductors in 50 Hz magnets will be possible in the next future.

Hysteretic effects in the flux-flow state of granular high-Tc superconductors

Abstract When an intergrain transport current larger than I c is applied to sintered YBa 2 Cu 3 O 7 samples in an external magnetic field we find hysteretic flux-flow voltage. While the flux-flow resistivity is not hysteretic, we can explain the effects by the dependence of critical transport current on the magnetic history of the samples. We find also partly a decrease of flux-flow voltage with increasing magnetic field. Also this effect, which in literature has been called “negative magnetoresistance” is explained.

Methods for reduction of flux creep in high and low Tc type II superconductors

Abstract This paper reports the experimental investigations into two different methods of reducing flux creep and magnetic relaxation in high T c YBa 2 Cu 3 O 7 and low T c NbTi. For temperatures T ≤ 4.2 K temperature reduction leads to a considerable reduction of the flux creep rate. The second method for reducing relaxation is the choice of appropriate magnetic field cycling prior to measurements being taken. As already found for YBa 2 Cu 3 O 7 , NbTi also shows a strong dependence on magnetic history. These results are of interest for various practical applications.

Oxide barriers and their effect on AC losses of Bi,Pb(2223) multifilamentary tapes

The transverse electrical resistivity in multifilamentary Ag/Bi, Pb(2223) tapes is considerably enhanced after introducing inert oxide barriers, a new concept in which each single filament is surrounded by a highly resistive BaZrO/sub 3/ layer of <2 /spl mu/m thickness. With these oxide barriers, we have so far obtained a shift of the AC loss maximum from 5 Hz to >100 Hz. This corresponds to a marked lowering of AC coupling losses. The highest critical current density of these tapes is actually 15000 A/cm/sup 2/ at 77 K, 0 T, i.e. still below that of our tapes without barriers (35000 A/cm/sup 2/). The fabrication processes leading to Bi,Pb(2223) tapes with oxide barriers is described, with an emphasis on new deformation processes developed in our laboratory for the fabrication of long multifilamentary Bi,Pb(2223) tapes, comprising four roll (or two-axes) rolling and periodic pressing. The developed tapes with oxide barriers are promising in view of their use in transformers and cables.

Hysteretic time dependence of the critical intergrain transport current in sintered YBa2Cu3O7

Abstract In our experiments we have found that the intergrain critical transport current I c in sintered, non-oriented high- T c superconductors shows a hysteretic time relaxation behavior. As a function of prior magnetic field cycling the I c -relaxation rate ∂ c /∂ t be positive or negative with different magnitudes and can also be made to vanish. Not only I c but the whole E-I -characteristics is shifting with time. We explain these results qualitatively in the frame of a magnetic field superposition model at the grain boundaries, with our earlier measurements on the relaxation of the intragrain magnetization. This could also be of importance for superconducting electronic applications.

Currents, magnetization and AC-losses of YBa2Cu3O7 superconductors in rapidly changing magnetic fields

Abstract In YBa2Cu3O7 samples of different shape time dependent magnetization currents were induced at 4.2K by the application of rapid magnetic field changes. This contactless method allows the study of the intergrain and intragrain currents in the resistive flux flow state.