Chengshan Li

Total AC loss of Ag-sheathed Bi2223 tapes with various filament arrangements carrying AC transport current in AC parallel transverse magnetic field

For the applications of Bi2223 tapes as the strands to power cables with multilayer structures, the tapes must carry the AC transport current in an AC parallel transverse field with various field amplitudes depending on the layer positions. In this study, we experimentally investigated the total AC loss characteristics under parallel transverse field for Bi2223 tapes with various filament arrangements in their cross sections. The tapes with different filament arrangements in the final tape sections were prepared by a conventional powder-in-tube (PIT) process with drawing using round dies and a rectangular deformation process using a two-axial rolling (TAR) technique. Total AC losses in the tapes carrying AC transport current in an AC parallel transverse field were measured by means of the electromagnetic method. The measured loss values in the tapes were compared with several analytical models. Based on the experimental results, the influence of filament arrangements on total AC loss behaviour is discussed, under the different operating conditions with various amplitudes of current and applied field.

Fabrication and characterization of Bi2223 tapes with interfilamentary SrZrO3+Bi2212 barriers for AC loss reduction

To suppress interfilamentary coupling in (Bi, Pb)2Sr2Ca2Cu3Ox (Bi2223) multifilamentary tapes under an AC external magnetic field it is necessary to increase the transverse resistivity by using a high-resistivity Ag alloy sheath as a matrix. Alternatively, oxide layers can be introduced as resistive barriers among the filaments, together with filament twisting. In this paper, we fabricated twisted Bi2223 tapes with interfilamentary oxide barriers and evaluated the effect of the introduction of such a barrier on the AC magnetization loss properties in a perpendicular transverse magnetic field. The conventional powder-in-tube method was used for tape fabrication. In order to avoid affecting the Bi2223 phase during the sintering process, SrZrO3 was selected as the barrier material. Moreover, 20 wt% Bi2Sr2CaCu2Oy (Bi2212) was mixed with SrZrO3 to improve its ductility for cold working. Monocore Ag-sheathed rods were coated with an oxide barrier using a slurry, before being stacked into honeycomb structures. By twisting the filament with a twist pitch length below 9 mm and introducing interfilamentary barriers, the coupling frequency ( fc) in a perpendicular field, which is inversely proportional to the decay time constant (τc) of coupling current, exceeded 100 Hz and perpendicular field losses around the power-grid frequency were reduced from the analytical prediction for fully coupled filaments. Avoiding physical connections among the irregular deformed filaments in twisted barrier tapes should lead to further loss reduction in a perpendicular field.

Fabrication and characterization of Bi2223/Ag tapes with interfilamentary oxide barriers for reducing AC losses

We prepared and characterized Bi2223/Ag tapes with interfilamentary oxide barriers for AC loss reduction in external AC magnetic field. Ca2CuO3, Al2O3, and SrZrO3 were selected as barrier materials. 20-30wt% Bi2212 were added to Ca2CuO3 and SrZrO3 powders to improve their deformation properties for cold working. The multifilamentary tapes with interfilamentary oxide barriers were fabricated with a conventional powder-in-tube method. The influence of introducing different oxide barriers on the final tape properties such as the phase formation in filaments, critical current density Jc and its uniformity along a tape length, was evaluated. Based on the results of the preliminary studies, we determined the oxide materials for fabrication of barrier tapes with twisted filaments. AC losses in barrier tapes with twisted filaments were measured at 77 K in transverse AC magnetic field. The AC loss characteristic of the tape with barrier was compared to the one of the tape without barrier.

Evaluation of Self-Field Distributions for Bi2223 Tapes With Oxide Barriers Carrying DC Transport Current

For AC loss reduction in Bi2223 tapes subjected to an AC external magnetic field, twisting the superconducting filaments and/or introducing oxide layers as highly resistive barriers around each filament are required. However, the structure of barrier tape becomes very complex and longitudinal uniformity of both transport property and tape structure could be easily deteriorated. To improve the current transport capability and its longitudinal uniformity for barrier tape for AC use, simple and non-destructive techniques to characterize them should be indispensable. In this paper, we examined the self-field distributions for Bi2223 tapes with oxide barriers carrying DC transport current by a scanning Hall-probe microscopy (SHM). Non-twisted and twisted 19-filamentary tapes with SrZrO3+Bi2212 barriers were prepared by powder-in-tube process. The distributions of self-field in perpendicular to the broader face of a tape carrying DC current below critical current Ic was measured at 77 K by SHM with an active area of 50 μm × 50 μm, at 0.5 mm away from the tape surface. Based on the measured self-field distributions, longitudinal non-uniformity of transport properties and the presence of local defects to obstruct the current transport in barrier tapes were investigated.

Progress in Reducing AC Losses of Bi2223 Tapes with Interfilamentary Resistive Barriers

This paper presents our recent progress for the development of low-AC loss Bi2223 tapes with interfilamentary oxide barriers. For the compatibility with Bi2223 phase formation during sintering, SrZrO3 was selected as barrier materials. Moreover, small amount of Bi2212 was mixed with SrZrO3 to improve its ductility for cold working. Although some breakages of barrier layers still existed, the effective transverse resistivity was approximately 10 times higher than a tape with pure Ag matrix. By controlling barrier thickness, reducing a tape width below 3 mm and twisting the filaments with its length below 5 mm , coupling frequency fc attained to 260 Hz in an AC perpendicular transverse field. Critical current densities Jc of our twisted barrier tapes were ranged in 12-15 kA/cm2 at 77 K and self-field, which was 25-30 % lower than non-twisted one (=18 kA/cm2). In our knowledge, this is the first report to achieve both Jc >; 12 kA/cm2 and fc 250 Hz simultaneously in a single Bi2223 tapes. Our barrier tapes showed 60-70% lower perpendicular field losses than a conventional 4 mm-width tape with fully coupled filaments at 50 mT and 50 Hz. These results are promising for remarkable improvement in AC performance for Bi2223 tapes in future.

Evaluation of remanent field distributions on Bi2223 tapes with oxide barriers by using scanning Hall-probe microscopy

Introducing oxide barriers around the superconductor filaments is an effective way to increase the matrix resistivity for AC loss reduction in Bi2223 tapes. However, because of the poor ductility of barrier oxides, the uniformity of the transport property along a tape length could be easily deteriorated. To qualify the uniformity along a length of barrier tape, therefore, simple and non-destructive evaluation techniques should be urgently required. In this study, we measured the remanent field distributions on the surface of Bi2223 tapes with Ca2CuO3 barriers by using scanning Hall-probe microscopy (SHM). By moving the permanent magnet on the tape surface along a length direction, an external field in perpendicular to the broader face of a tape was applied at 77 K. After removing the field, the distributions of remanent field Brz in perpendicular to a tape surface were measured by SHM. The results indicated that the Brz at a fixed distance of 0.5 mm away from a tape surface shows the maximum near the centre of transverse tape section, but the magnitude and uniformity of the maximum Brz was degraded significantly by introducing barriers. We also confirmed that there were strong correlation between the distributions of the maximum remanent field and the transport critical current Ic along a tape length. This indicates that the longitudinal uniformity of transport property for a barrier tape could be investigated simply and non-destructively by SHM.

Reduction of AC Losses in Bi-2223 Tapes by Introducing Interfilamentary Oxide Barriers

AC loss reduction in Bi-2223 tapes is one of the key issues for the realization of power cables and transformers. The large loss generation is mainly attributed to the strong electromagnetic coupling between the filaments in AC external magnetic field. In this paper, we fabricated twisted Bi-2223 tapes with interfilamentary oxide barrier and evaluate their AC loss properties. To suppress the side effect on Bi-2223 phase formation during sintering, Ca2CuO3 or SrZrO3 powders were used as barrier materials. Furthermore, small amounts of Bi-2212 powders were mixed with them to improve their ductility for cold working. AC loss characteristics were examined at 77 K in parallel and perpendicular transverse magnetic fields with various field amplitudes and frequencies. The effect of introducing different oxide barriers on AC loss reduction at different operating condition is discussed.

AC Loss Properties in Bi2223 Multifilamentary Tapes with Enhanced Transverse Resistivity by Introducing Oxide Barriers

Bi2223 multifilamentary tapes with enhanced transverse resistivity by introducing Ca2CuO3 and SrZrO3 as interfilamentary barriers were prepared and their AC loss properties were examined under AC external magnetic field. To improve the deformation properties of these oxide powders, 20-30wt% Bi2212 powders were mixed with them. The mixed oxide powders were introduced among the twisted Bi2223 filaments by using dip-coating method. AC loss properties under AC parallel or perpendicular field were measured and compared with the results for the tape without barriers. The loss measurements were carried out by changing both the amplitude and the frequency of external field. Based on the experimental results, the effect of barrier introduction on the loss reductions was discussed.

AC Loss Characteristics in Bi2223/Ag Tapes With Interfilamentary Oxide Barriers for Filament Decoupling

In this paper, we prepared Bi2223/Ag tapes with interfilamentary oxide barriers for filament decoupling and also for reducing AC losses under AC external magnetic field. To suppress the side effect on Bi2223 phase formation inside the filaments and superconducting properties in final tapes by introducing oxide barriers, we selected Ca2CuO3 as barrier materials. In addition, the 30 wt% Bi2212 powders were mixed into Ca2CuO3, to improve their deformation properties for cold working such as drawing and flat rolling. The mixtures of Ca2CuO3 and Bi2212 were introduced around the twisted Bi2223 filaments. AC losses in barrier tapes under parallel transverse magnetic field were measured at 77 K, and compared with the several analytical models. Based on the results, the effect of barrier introduction on the AC loss characteristics in twisted multifilamentary tape is discussed.