Weibull analysis of the electromechanical behavior of AgMg sheathed Bi2Sr2CaCu2O8+x round wires and YBa2Cu3O7-δ coated conductors

The development of superconducting magnets requires not only a conductor that is capable of carrying sufficient critical current density (J(c)) at high magnetic field, but also one that is mechanically robust and predictable. Here, the electromechanical behavior of AgMg sheathed Bi2Sr2CaCu2O8+x (Bi2212) round wires and YBa2Cu3O7-delta (YBCO) coated conductors is studied using a statistical approach based upon three-parameter Weibull statistics, where the three parameters alpha, beta, and gamma describe the scale, shape and location of the resulting distribution function. The results show that Bi2212 round wire has significantly different behavior than previously studied Bi2212 tape conductors, with evidence of an underlying mechanically strong but poorly connected electrical 'backbone' in the round wire that is not found in the tape conductor. Furthermore, the Bi2212 round wire results indicate a distribution in the dependence of critical current upon strain (I-c(epsilon)) at the microscopic level, consistent with reports that a complex network of interfilamentary bridges plays a key role in connectivity. Unlike the behavior of either Bi2212 round wire or tape, the YBCO coated conductor shows a universal behavior for strains below yield, consistent with the presence of a strong, stiff NiW substrate that dominates the mechanical behavior, and a high purity, high density, highly textured YBCO layer with reversible electromechanical properties. These results indicate that, in particular for Bi2212 conductors, the strain-dependence of the location parameter, gamma(epsilon), which defines the minimum critical current for any segment of conductor at a particular value of strain, is a more important function for magnet design than I-c(epsilon) or the critical strain, epsilon(c). Using the approach reported previously and applied here, this curve is readily obtained using a limited length of conductor, but provides an important level of conservatism to the design of magnets using long lengths of conductor.