ANALYSIS AND INTERPRETATION OF CRITICAL-CURRENT EXPERIMENTS FOR BISMUTH-BASED HIGH-TEMPERATURE SUPERCONDUCTORS MADE BY POWDER-IN-TUBE PROCESSING

Current distribution and voltage drop were analyzed for a composite co nductor composed of a layer of bismuth-based high-temperature supercon ductor (BSCCO) bonded to a silver substrate. The analytical model assu mes that the silver strip is continuous, and the superconductor is per iodically discontinuous. The resistance at the discontinuities in the superconductor is assumed to be much greater than that of the silver s ubstrate. The results show that if the BSCCO is superconducting, the s olution depends on only one dimensionless parameter, lambda(s)L/2, whi ch is equal to the square root of the ratio of the resistance of the s ilver to the interfacial resistance between the silver and the BSCCO. When lambda(s)L/2 is very high (>100), most of the current returns to the superconductor after it has been diverted to the silver substrate by the discontinuity. When lambda(s)L/2 is very low (<0.1), most of th e current remains in the silver. Current is shared between the BSCCO a nd the silver when 0.1 < lambda(s)L/2 < 100. All of the current return s to the superconductor only if lambda(s)L/2 approaches infinity. The situation most likely to occur in an experiment is when lambda(s)L/2 i s high but finite and there is some small dissipation in the system. I n this case, the healing length (defined as the length at which most o f the current has returned to the superconductor) is shown to be propo rtional to the characteristic length 1/lambda(s), which depends on the properties of the silver and that of the BSCCO/silver interface. Volt age in the BSCCO is found to be equal to one-half the voltage drop in the silver over a length of L. The present results suggest that the cr itical current of an experiment is that of the composite conductor (BS CCO plus Ag). It also suggests that in fabricating this type of compos ite conductor, such as those made by powder-in-tube processing, it is desirable to reduce the interfacial parameter to below a certain value , thus ensuring that most of the current returns to the superconductor and reducing the amount of resistive heating (and healing length) in the composite conductor. The results of the analysis can be applied to interpret critical current density experiments if the discontinuities in the superconductor are considered to be grain boundaries, and the results can also be employed to predict the effect of transverse crack s on critical current density if the discontinuities are considered to be transverse cracks generated by axial strains.