Critical current density and flux pinning in silver/superconductor composites and tapes

Abstract

Measurements of the transport critical current density, J c, and AC susceptibility, χ= χ′+ i χ″, are reported for silver/superconductor composites and silver-sheathed c-axis textures tapes fabricated from the (Bi 1.6Pb 0.4)Sr 2Ca 2Cu 3O 10−y compound. The field dependence of the χ″ intergrain loss peaks are used to obtain qualitative information on the pinning force density, and the Bean critical state model is used to determine the intergrain and intragrain critical current densities. It is found that silver enhances flux pinning in the intergranular regions with optimum critical current density occuring at 20–30 vol.% Ag. The temperature dependence of the transport J c of the tapes is controlled by flux creep, and analysis of the experimental data using flux creep theory gives a pinning energy U(0)≅90 meV. Measurements of the DC field dependence of J c show it to be anisotropic, e.g., J c =6.5×10 3 A cm −2 (77 K) in zero field, decreasing with field to ∼ 10 3 A cm −2 for H ⊥ c-axis and to ∼30 A cm −2 for H‖ c-axis at 0.4 T. Demagnetisation effects are considered and are though to account for most of this anisotropy.