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 conductor composed of a layer of bismuth-based high-temperature superconductor (BSCCO) bonded to a silver substrate. The analytical model assumes that the silver strip is continuous, and the superconductor is periodically discontinuous. The resistance at the discontinuities in the superconductor is assumed to be much greater than that of the silver substrate. The results show that if the BSCCO is superconducting, the solution depends on only one dimensionless parameter, lambda(s)L/2, which is equal to the square root of the ratio of the resistance of the silver 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 the current remains in the silver. Current is shared between the BSCCO and the silver when 0.1 < lambda(s)L/2 < 100. All of the current returns 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 is high but finite and there is some small dissipation in the system. In this case, the healing length (defined as the length at which most of the current has returned to the superconductor) is shown to be proportional to the characteristic length 1/lambda(s), which depends on the properties of the silver and that of the BSCCO/silver interface. Voltage 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 critical current of an experiment is that of the composite conductor (BSCCO plus Ag). It also suggests that in fabricating this type of composite 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 cracks on critical current density if the discontinuities are considered to be transverse cracks generated by axial strains.