Comparison of growth texture in round Bi2212 and flat Bi2223 wires and its relation to high critical current density development

摘要

Why Bi₂Sr₂CaCu₂O_x (Bi2212) allows high critical current density J_c in round wires rather than only in the anisotropic tape form demanded by all other high temperature superconductors is important for future magnet applications. Here we compare the local texture of state-of-the-art Bi2212 and Bi2223 ((Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀), finding that round wire Bi2212 generates a dominant a -axis growth texture that also enforces a local biaxial texture (FWHM c -axes of its polycrystals to rotate azimuthally along and about the filament axis so as to generate macroscopically isotropic behavior. By contrast Bi2223 shows only a uniaxial (FWHM c -axis texture perpendicular to the tape plane without any in-plane texture. Consistent with these observations, a marked, field-increasing, field-decreasing J_c(H) hysteresis characteristic of weak-linked systems appears in Bi2223 but is absent in Bi2212 round wire. Growth-induced texture on cooling from the melt step of the Bi2212 J_c optimization process appears to be the key step in generating this highly desirable microstructure.