Transport Critical Current of Sintered YBa 2 Cu 3 O x in Time-Varying Magnetic Fields
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TRANSPORT CRITICAL CURRENT OF SINTERED YBa2Cu 3 Ox IN TIME-VARYING MAGNETIC FIELDS P. D. HAMBOURGER*, J. DiC1LLO*, M. GIBBERMAN*, J. MARINO*, L. P. VALEK*, AND MICHAEL A. CENTANNI** *Cleveland State University, Cleveland, OH 44115, USA **Ferro Corporation, Independence, OH 44131, USA ABSTRACT The transport critical current of bulk sintered YBa 2 Cu3 Ox has been measured at 77 K in the presence of static and sinusoidal ly-time-dependent (10-100 Hz) magnetic fields. In all cases, the field required for a given reduction of critical current density was independent of frequency to within -.10%. INTRODUCTION Several potential applications of high-temperature superconductors (e.g. generators and energy-storage coils) will require the transport of large currents in the presence of time-varying magnetic fields. However, several reports (1) on ac susceptibility measurements in bulk YBa 2 Cu3Ox (YBCO) have indicated that the transition-broadening effect of sinusoidally-time-dependent magnetic fields is much more severe than that of static fields of the same magnitude. It is therefore important to learn whether the influence of time-varying magnetic fields on the critical current density (Jc) is also more severe than that of static fields. In this paper we present data on transport Jc (using constant direct current and four-lead techniques) in the presence of sinusoidally-timevarying magnetic fields of frequency 4f) up to 100 Hz at 77 K. Our results indicate that the Jc-suppressing effects of these fields are essentially the same as those of static fields of the same magnitude. PROCEDURE Sintered bulk samples of YBCO were prepared by §tandard techniques and cut into samples measuring typically lxl.5xl5 mmr. Contact pads were formed from Ag paint and fired into the samples (2). Jc was measured using the circuit shown in Fig. 1. A constant diSAMPLE
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Fig. 1. Circuit used to measure Jc" Mat. Res. Soc. Symp. Proc. Vol. 169. t1990 Materials Research Society
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rect current, chosen to be smaller than the zero-field critical current, was applied to the sample, which was submerged in liquid N2 . The voltage drop between the potential contacts was plotted vs magnetic field on an X-Y oscilloscope. The field was supplied by an air-core coil outside the (glass) Dewar. Measurement of the oscillating magnetic field inside the Dewar with a pickup coil and phase-sensitive detector indicated that changes in amplitude and phase due to induced currents in the Dewar's metal-film radiation shields were negligible. For Jc measurements in static fields, the power amplifier and oscillator were replaced by a dc power supply and the wideband Y-axis preamplifier by a low-drift dc amplifier. RESULTS Jc data were taken on several samples with magnetic field either parallel or perpendicular to the sample axis. Since all measurements led to the same conclusions, we present data from just one sample, whose zerofield critical current density (J 0 ) was 350 A/cm2 . Ac susceptibility data for this sample are shown in Fig. 2 for th
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