Critical-current measurements of high-<i>T</i> <i>c</i> superconductors using a transformer technique

Abstract

We report transport critical-current measurements of the YBa2Cu3Oy high-Tc superconductors using a contactless transformer technique. A critical-current density of 15.9±1.0 A/cm was observed at 77 K for samples fabricated by a standard preparation method for the high-Tc superconductors. The addition of a small amount of silver to samples of YBa2Cu3Oy has a dramatic degrading effect on the critical current. This is in sharp contrast to the Meissner-effect results, where silver addition has a minimal effect. No significant systematic frequency dependence of the critical-current determination was found between 10 and 5000 Hz. Visual observations of ‘‘blips’’ in oscilloscope traces and an extrapolation of signals to zero blip height gave consistent results and established an uncertainty of ∼3% for critical-current determination. The range of dimensions of the ring-shaped samples yielded an uncertainty of ∼3% for the cross-sectional area. The overall uncertainty for an absolute measurement of the critical-current density is consequently ∼6%. The uncertainty in relative measurements of the critical-current density is very much less.