S.P. Ashworth

Measurements of AC losses due to transport currents in bismuth superconductors

Abstract Measurements of the losses of BSCCO-2223 and BSCCO-2212 tapes carrying AC transport currents are reported. Measurements were made at 77 K at frequencies in the range 20–60 Hz. The voltage waveforms generated by the AC transport are presented as well as the AC losses. For peak transport currents below 90% of the DC critical current the losses of BSCCO-2223 tapes are shown to be well described using an equation due to Norris [5] which assumes the presence of a critical state in the superconductor, and the power losses are proportional to the AC frequency (hysteresis losses). When the AC peak current approaches the DC critical current flux-creep effects dominate the losses, and the power losses cease to depend on frequency. In the BSCCO-2212 sample flux-creep effects dominate at all currents. The examination of the voltage waveform generated in the BSCCO-2223 sample shows the presence of both hysteretic and flux-creep generated voltages.

Influence of external magnetic field and its orientation on transport AC losses in Bi-2223 and Tl-1223 silver sheathed tapes

In the paper we report measurements of energy losses caused by an AC transport current in silver sheathed multifilamentary Bi-2223 and mono core Tl-1223 tapes, as a function of the magnitude and orientation of the applied external magnetic field. The external magnetic field was rotated in the range of 0 to /spl plusmn/90 degrees with respect to the plane of the tapes, and was In all cases perpendicular to the transport current flow. The transport current losses in a range of DC applied fields are consistent with the Norris equation provided the DC critical current at the same applied field is used in his expression for the loss.

AC losses of Ag-(Bi,Pb)SrCaCuO-2223 tapes in combination of transverse external magnetic field and transport current

The authors present the results of measurements of the AC transport (self-field) and magnetic losses on silver sheathed (Bi,Pb)SrCaCuO-2223 tapes prepared by the powder in tube method. In the frequency range 30-540 Hz the transport current losses are hysteretic in nature. Losses arising from AC external magnetic field (frequency range 65-207 Hz, amplitude up to 60 mT) are compared with losses generated by AC transport currents. The measured transport loss voltage depends strongly on the position of the potential taps on the surface of the tapes. Losses due to combined AC applied magnetic fields and transport currents are also reported.<<ETX>>

Energy dissipation in high temperature ceramic superconductors

Abstract Losses in high- T c superconductors are of vital importance in practical applications. The mechanisms are similar to those in low- T c materials but there are a number of complications. In this paper the loss mechanisms in different regimes are summarized and simple approximate expressions obtained. Losses due to the transport current are analysed with the theory of Norris for elliptical conductors. This predicts different loss voltages according to where the voltage contacts are placed on the conductor which has been confirmed experimentally. This poses difficulties in turning experimental data into reliable loss measurements, but a series of experiments with different contact layouts and phase setting techniques is found to give consistent results, in good agreement with theory.

AC LOSSES AND CRITICAL CURRENTS IN AG/(TL,PB,BI)-1223 TAPE

Abstract The results of AC loss measurements of silver-sheathed (Tl,Pb,Bi)-1223 tapes prepared by the powder-in-tube method are presented. Losses arising from an external AC magnetic field are compared with those generated by AC transport currents (self-field losses). Experimental results, obtained by both methods, show that energy losses are purely hysteretic in nature in the range of applied frequencies (30–200 Hz). Critical current densities are derived from the magnetic-loss data and compared with those measured by the four-point transport method. Further, the dependence of the transport critical currents on the external DC magnetic field, for different geometrical configurations of the tapes with respect to the field, i.e. field parallel or perpendicular to the plane of the sample, are presented. The experimental results are compared with theoretical predictions based on the critical-state model.

Anisotropic critical current densities of silver sheathed BSCCO-2223 tape

Abstract A novel experimental arrangement is described in which current is constrained to flow perpendicular to the faces of (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x silver clad tape produced by the standard powder in tube technique. This configuration with “perpendicular ” current flow shows that significant voltages are developed at current densities less than 1% of the conventional, longitudinal, critical current density. The effect of applied magnetic fields on the perpendicular critical current density are also reported, with fields perpendicular to the plane of the tape being most effective at reducing the critical current density. From this data it is calculated that the average critical current of each grain boundary is 0.5 mA and

Transport AC losses in multifilamentary Ag/Bi-2223 tapes in low external DC magnetic fields

Abstract Experimental results of measurements of the AC transport current losses on silver sheathed (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x multifilamentary tapes are presented. In the frequency range 33–180 Hz the self-field losses are hysteretic. Transport loss changes due to low DC, applied transverse magnetic fields are also studied. External transverse DC magnetic field causes an increase in the transport losses due to lowering of the critical current I c of the superconductor. Critical current dependence on the external magnetic field extracted from loss data is compared with that measured directly by standard DC four-probe measurements.

Correlation between cold deformation and microcrack formation in BSCCO-2223 powder in tube superconductor

Abstract This paper describes the result of a cold rolling deformation path on the microstructure and the longitudinal and transverse critical currents of monofilamentary BSCCO-2223 tape produced via the oxide powder in tube (OPIT) process. The fully reacted BSCCO-2223 powder used in this experiment highlights the deformation mechanisms effect on the microstructure of the core, and hence the effect on the longitudinal and transverse critical currents. Tapes with different sheath properties are produced by controlling the hardness of the pure silver sheath either by annealing at 500 °C between rolling stages or by allowing the sheath to work harden throughout the deformation path. The results show that the ultimate core density depends on the hardness of the sheath and that the maximum core density is reached whilst the tape is still relatively thick. Microscopy shows that continued rolling after the core has reached its maximum density results in the production of transverse cracks in the core. The critical current measurements show that these cracks severely limit the potential longitudinal critical current of the superconductor. A softer sheath allowed a greater reduction in thickness before transverse microcracking became severe, whilst a harder sheath enabled a higher core density to be reached. This technique of controlling sheath hardness suggests possible improvements in the deformation processing of OPIT produced superconducting BSCCO-2223 tapes.

Transport and Magnetic AC Losses in Ag/Tl-1223 Tape and the Effect of Mechanical Damage

In this paper we present the results of AC loss measurements on silver clad monocore tapes of Tl-1223 prepared by the powder-in-tube method. Losses arising from an external AC magnetic field are compared with those generated by AC transport currents (self-field losses). Critical current densities are derived from magnetic loss data and compared with those measured by the four point transport method. It was found that mechanically induced stresses (introduced by bending) only have a significant effect on the transport losses, via the critical current degradation. Such changes are not seen in measured magnetic losses, which are independent of mechanical stress, and remain unchanged, at least in the stress range used in our experiment.

A comparative study of microstructure and transport critical current in cold rolled, cold uniaxially pressed and low load hot uniaxially pressed Ag-(Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tape

The microstructure and transport critical current of cold rolled, cold uniaxially pressed, and low load hot uniaxially pressed Ag-(Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/, tape has been investigated. The transverse and longitudinal transport current measurements show strong correlation with morphological non-uniformities induced by the cold deformation processes. The aim of low load hot uniaxial pressing is to increase the density of the ceramic core and reduce the anisotropic nature of the critical current induced during the cold deformation processes. The deformation mechanisms of the silver and ceramic core when under load have been studied, and a deformation mechanism map has been constructed. Experimental investigation of the low load region over a range of temperatures has been carried out.<<ETX>>