Characterizing the current carrying capacity of different high T c superconductors with as much sensitivity as versatility

To qualify quickly new or optimized versions of high-T _c ceramic superconductors that are not available in the shape of a wire, two simple and accurate methods have been set up. The first measures the transport critical-current density of small sintered specimens or films with no electrical contacts, using a standard magnetic-core transformer with a washer of superconducting material fitted as a shorted, one-turn secondary winding. The current is induced in the specimen by exciting the primary coil with AC, and the resulting magnetic flux is very accurately measured with a pick-up coil and a lock-in amplifier. A similar approach, in which a small superconducting disk is introduced to replace an air gap in the magnetic circuit, measures the AC susceptibility components. This method can be used to study the effect of both temperature and magnetic field (up to 80 Oe) by clearly discriminating between intra- and inter-grain conduction transitions. Results of measurements on YBa₂Cu₃O _7-δ and bismuth compounds are presented