Role of c-axis field components in making the transport E(J) characteristics of (Bi,Pb)2Sr2Ca2Cu3Ox tapes insensitive to the direction of field within the tape plane

Abstract

We report extended ͑over four decades͒ electric field versus current density characteristics of ͑Bi,Pb͒ 2 Sr 2 Ca 2 Cu 3 O 10 tapes in magnetic fields applied along each of the three principal tape axes at 77 K, and along the two principal axes in the tape plane at 4.2 K. The characteristics are almost independent of field direction in the tape plane at both temperatures. Similar insensitivity of the critical current to magnetic field direction observed in other polycrystalline high temperature superconductors has been ascribed to the percolative nature of current flow, while the insensitivity in ͑Bi,Pb͒ 2 Sr 2 Ca 2 Cu 3 O 10 ͑2223͒ films and Bi 2 Sr 2 CaCu 2 O 8 single crystals has been attributed to c-axis conduction across Josephson junctions. We show that the insensitivity of the critical current density to the field direction of these tapes follows naturally from the fact that the dissipation depends only on the field component lying along the c axis of individual 2223 grains. Lorenz-force driven motion of the pancake vortices that form within each 2223 grain provides a dissipation mechanism that involves only the c-axis components of the field. The angular dispersion of the grains in the tape ensures a c-axis field component in most grains, regardless of the applied field direction. This produces the remarkable insensitivity of E(J) over some four orders of magnitude of the electric field.

High-temperature superconductors in high-field magnets

“…Small-angle grain boundaries are the least limiting, so the preferred microstructure is one with aligned grains. Intra-grain current density is largest within the plane of the platelets and flux pinning is optimized if the flux lines are within the same plane [59]. Considering that the magnetic flux lines penetrating the conductor in an HTS insert magnet is predominantly parallel to the tape we arrive at a desired microstructure in which all grains are parallel to the flat surface of the tape.…”

“…In layer wound solenoids there is an axial pitch per turn of conductor introducing a small magnetic field component parallel to the transport current. This applies to the transition turns of double pancake coils as well but this effect is ignored as it is not expected to have a large effect on J e [59]. For conductor E, the Bi-2212 round wire, only the magnitude of the magnetic field is relevant.…”

“…Measurements on samples of older batches of Bi-2212 tapes from the same manufacturer using a bending spring [117] and a Lorentz-force device [76] show a slope of 2 to 7 in normalized-J e versus strain up to 0.4% and significant degradation above 0.4%. Considering the similarities in the conductor and data, it seems reasonable to assume a that a comparable slope exists in J e versus strain below ε crit for conductors B, C and D. 59 The heat treatment conditions of the Bi-2212 conductors considered here are optimized to maximize J e at 4.2 K, at the expense of T c . 60 As introduced in section 2.4.…”