P.X. Zhang

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.

Reduction in alternating-current transport losses of a (Bi, Pb)2Sr2Ca2Cu3Ox multifilamentary tape-form conductor by the introduction of resistive barriers

We succeeded in reducing alternating-current (ac) transport losses at 77 K and 50 Hz for the (Bi, Pb)2Sr2Ca2Cu3Ox tape-form conductor with 39 filaments, fabricated by a four-roller machine to make a rectangular deformation and also by the introduction of Bi2Sr2CuOx sheets as resistive barriers in an arrangement parallel to a tape surface. Although the main contribution to the losses comes from the hysteresis loss of the superconductor, the loss value for the conductor is reduced by approximately 70% compared with the value for the (Bi, Pb)2Sr2Ca2Cu3Ox multifilamentary tape prepared by an ordinary powder-in-tube process without the resistive barriers. Using numerical calculations we ascribe this loss reduction to the division of field-free-core under ac current transmission, caused by introduction of the barriers.

Alternating-current transport losses of Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox monocore round wires and tapes

Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox monocore round wires and tapes with different core geometry have been fabricated by a powder-in-tube method. The alternating-current transport losses of these wires and tapes were measured at 77 K as a parameter of frequency between 50 and 1000 Hz in a self-field. The round wires have lower transport losses than the tapes in the investigated frequency range, even in the cases of nearly the same critical currents. The hysteresis loss for the wires is derived from the experimental data and found to be well below the prediction for a thin-strip superconductor. This is attributed to the inhomogeneous Jc distribution in a radial direction in the core, which is expressed as a sharp rise of Jc in going from the core centre to the interface with the Ag sheath. An increase in frequency enhances the loss generation in the round wires because of significant eddy-current loss. In the tape samples, an increase in the aspect ratio accompanied by significant core deformation raises the loss value, so that it approaches the value predicted for an elliptic superconductor tape.

Transport a.c. losses of (Bi,Pb)-2223 multifilamentary tapes with Ca–Cu–O as resistive barrier

By means of doping Ca 2 CuO 3 powders into the Ag-sheathed (Bi,Pb)-2223 tape, we found that the Ca 2 CuO 3 powder has little side effect on the J c values of the investigated tapes. So the Ca 2 CuO 3 powder as resistive barriers between filaments was introduced into the (Bi,Pb)-2223 multifilamentary tape by powder-in-tube method. A rectangular deformation process by using a four-rollers machine has been applied to prepare the wires. The results indicate that the transport a.c. losses were decreased by introduction of the resistive barrier, while the J c of the tape with barriers was almost not deteriorated compared with the tape without barriers.

Critical current density and a.c. losses of Ag-sheathed Bi2223 tapes with Sr–V–O barriers

Abstract Ag-sheathed Bi2223 multifilamentary tapes with 19 filaments with Sr–V–O (SVO) barriers have been prepared using powder-in-tube method. The SVO barriers are arranged inside and/or outside each filament. The critical current density J c at 77 K strongly depends on the preparation conditions such as the heat-treatment temperature and intermediate rolling. For the barrier tapes the J c values are higher than those of the non-barrier tapes for all the barrier configurations. Furthermore, for all the barrier tapes the transport a.c. losses at 77 K in self-field are lower than those of the non-barrier tapes. In particular, the inside-barrier tapes will give us an excellent possibility to reduce the over all J c and the ease to fabricate to tape shape.

Fabrication and transport AC losses of (Bi,Pb)2223 multifilamentary tapes with resistive barriers

The Ag-sheathed (Bi,Pb)-2223 multifilamentary tapes with different filament distribution and resistive barriers have been fabricated by powder-in-tube (PIT) method. A rectangular deformation process by using a four-roller machine was applied to fabricate the multifilamentary wires. Bi-2201 and Ag-Cu alloy sheets were introduced into the tape as resistive layers to decrease the coupling between filaments. The transport AC losses were measured on these tapes at 77 K with power frequency between 50 Hz and 1000 Hz The results indicated that the behaviors of transport AC losses are influenced by both filament distribution and resistive barrier. The introduction of resistive barriers to divide the tape into several thin strips is effective to reduce the transport AC losses. For the prepared 39 filament tapes, when the resistive layers are configured parallel to the tape surface, the transport AC losses at 50 Hz measured at 77 K are greatly reduced compared to those of 37 filament tape without barriers fabricated by a conventional PIT method.

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.

Fabrication and Properties in Ag/Bi2223 Tapes With Ca-Cu-O as Interfilamentary Resistive Barriers

In this study, we fabricated Ag-sheathed Bi2223 tapes with interfilamentary resistive barriers by using standard powder-in-tube (PIT) method and evaluated their AC loss characteristics at 77 K. The mixture of Ca2CuO3 and 30 wt.% Bi2212 are used as new barrier materials for tape fabrication. Although some broken parts in barrier layers were observed in a final tape section, it was confirmed that the barriers were almost arranged around each Bi2223 filament. In addition, XRD results indicated that the barrier introduction among the filaments has little side effect for the formation of Bi2223 phase inside the filaments. By introducing the barrier layers, the Jc values at 77 K and self-field are degraded about 15%. From the results of the measurements for AC magnetization loss under parallel transverse field, it was suggested that introducing the mixture of Ca2CuO3 and Bi2212 as barriers is effective to increase the transverse resistivity, and also to suppress the electromagnetic coupling among the filaments.

Significant reduction in AC transport self-field losses of Ag-sheathed Bi2223 tapes achieved by changing filament arrangements

Significant reduction in AC transport self-field losses has been achieved at 77 K for Ag-sheathed Bi2223 multifilamentary tapes with m × n (m,n = 3 or 5) filament arrangements, fabricated via a rectangular deformation process using two-axis rollers. This achievement is not effected by introducing resistive barriers but by changing filament arrangements. Despite the main contribution being the hysteresis loss of the superconductor, the loss values are decreased by approximately 50–70%, compared with the values for multifilamentary tapes obtained by a standard powder-in-tube process. More complicated filament arrangements achieved via a rectangular deformation process lead to further reduction in the loss values. On the basis of numerical calculations, the loss reduction is mainly ascribed to a division of the field-free core in the filament groups near the tape edge under AC current transmission. The study should pave the way to removing an obstacle to loss reduction and to developing tape strands with low loss values satisfying practical demands.