CURRENT-LIMITING MECHANISMS IN INDIVIDUAL FILAMENTS EXTRACTED FROM SUPERCONDUCTING TAPES

Many large-scale applications of high-temperature superconductors depe nd crucially on the ability to achieve high critical-current densities J(c) (of the order of 10(5) A cm(-2)). Existing silver-sheathed (Bi,P b)(2)Sr2Ca2Cu3Ox (BSCCO) tapes have J(c) values that come within about 25% of this target(1-4), these values being limited by the fact that the supercurrent flows percolatively around barriers that occur over m any length scales(5). To elucidate the nature of these barriers, we sh ave measured the transport properties of individual filaments extracte d from very-high-J(c) multifilament tapes(6). We find that J(c) for in dividual filaments reaches at least 8 x 10(4)Acm-2-about 50% higher th an the average value over the whole cross-section of the wire. Althoug h we injected the current to flow along the crystalographic a-b planes of the material, we found that all filaments possessed local characte ristics of c-axis transport, indicating the presence of occasional nan ometre-to micrometre-scale barriers at basal-plane-faced grain boundar ies. An independent and much larger limiting influence on the critical current comes from unhealed cracks produced by deformation during the processing of the wires. These results provide direct evidence that b etter processing methods aimed at improving the c-axis alignment and a t inhibiting residual cracks should raise the accessible J(c) values t owards those needed for applications.