Axial tension test is often used in characterization of superconductor tapes. However, because of the thermal stresses and composite nature of the conductor, the stress distribution in the tape cannot be directly obtained from the measurement. Here, a computational procedure based on two-dimensional finite element method was developed in order to model the multi-axial stress distribution in Bi-2223/Ag tapes during cooling from the annealing temperature to measurement temperature and in axial tension test. In order to increase the compatibility with other codes the method was implemented in Matlab/Femlab environment. Elastic and elasto-plastic material models with temperature dependent Young’s modulus and yield strength were used for superconductor filaments and for silver matrix. The uncertainty of Young’s modulus of the filaments complicated the computations and made it necessary to perform calculations with several values. With a justifiable choice of the modulus, the results corresponded well with the measured ones. A comparison to the widely used one-dimensional analytical model showed that both models predict principally similar stress curve for the measurement.

An experimental study of the axial strain on Ic degradation of multifilamentary Bi(2223)/Ag tapes by different methods has been performed. The tapes were stressed by three kinds of tensioning set-up (expanded turn, short straight sample and U-shape spring) and the transport current versus strain Ic(ε) was measured. A comparison of Ic(ε) curves showed that the applied tensioning system may influence the measured sensitivity of Bi-2223 filaments, apparently against the tension strain. The lowest strain limit for critical current degradation (εirr) was observed for the expanded turn and the highest limit for the U-shape spring system. The observed differences in Ic(ε) curves are discussed and analysed.

An eccentric rolling (ER) technique has been applied during the intermediate deformation of commercial multicore Bi-2223/Ag tapes. The J(c)(B) performance of the tapes deformed by ER and standard 2-high rolling technique (FR) have been compared. It was shown, that at 77 K, the self-field J(c) can be increased by 12.6% and J(c)(1 T) by 21% if an optimum ER deformation is applied. The tapes subjected to ER show smaller J(c) drop in low magnetic fields (B-ex < 0.2 T) as a consequence of reduced number of transverse cracks and improved filament density. This is a vital aspect for further J(c) improvements by an optimal ER intermediate deformation applied for multicore Bi-2223/Ag tapes. ER tapes also have lower J(c)-anisotropy ratio at B-ex = 0.5 T than FR ones: which may be advantageous to HTS coil manufacturers for various applications. (C) 2001 Elsevier Science B.V. All rights reserved.

The nature of the BSCCO compound and application of the powder-in-tube technique usually lead to non-uniform quality across and/or along the ceramic fibres and finally to variations in the critical current and its irregular distribution in the Bi(2223)/Ag tape. Therefore, the gliding four-probe method and contactless field monitoring measurements have been used for homogeneity studies. The gliding potential contacts moved along the tape surface and a sensitive system based on an integrated Hall probe array containing 16 or 19 in-line probes supported by PC-compatible electronics with software allowed us to make a comparison of contact and contactless measurements at any elements of Bi(2223)/Ag sample. The results of both methods show very good correlation and the possibility of using a sensitive Hall probe array for monitoring the final quality of Bi(2223)/Ag tapes.

A set of small inner bore (8.5 mm) single pancake coils was wound from multifilament Bi(2223)/Ag tapes using the `wind and react' (W&R) technique. W&R means sintering heat treatment of the Bi(2223)/Ag tape wound to a pancake coil. During the winding procedure, the tape was insulated by a mixture of Al2O3 or ZrO2 powders with varnish. After the first sintering step (841 °C/50 h), the printed insulation was easily removed and the tape with a clean surface was subjected to intermediate deformation by rolling. After the final heat treatment (841 °C/150 h), the coils were impregnated by epoxy resin. Electrical measurements of individual pancakes certified the good quality of interturn insulation. Some variations of transport currents among the pancakes were measured. The selected coils were connected in series or parallel and current distribution and I-V curves of individual coils were measured. Comparison of both connections is discussed in detail. The results of a small magnet arranged by connection of five single pancake coils with partial compensation of the radial field component is also presented. The compensation of the radial field component allows us to increase the magnetic field in the bore axis by 215%. Further lowering of temperature to 65 K allows us to generate a magnetic field nearly five times higher than in the case of the non-compensated winding at 77 K.

An experimental study of multicore Bi(2223)/Ag tapes, roll-sintered by different methods and subjected to bending and tension stresses has been performed. The tapes, of various technological histories, were bent and tensioned and subsequently the transport current was measured at each stressed state. Comparison of degradation curves shows that applied rolling may influence the sensitivity of Bi-2223 filaments against the mechanical stress. The existence of transverse microcracks caused by intermediate rolling leads to a higher sensitivity of the tape to bending. A lowering of critical current degradation was observed for two-axially rolled tapes having a higher filament density and better homogeneity prior to sintering treatment.