MODELS FOR LONG-RANGE CURRENT FLOW IN BULK OXIDE SUPERCONDUCTORS

Mitigating the weak-link behavior at grain boundaries, which limits critical current density (J(c)) in polycrystalline high-transition-temperature (high-T-c) superconducting materials, is required for all applications in magnetic fields. Although in general oxide superconductors in polycrystalline form have very poor in-field J(c), conductors containing the bismuth-based compounds Bi2Sr2CaCu2O8 (Bi-2212) and Bi2Sr2Ca2Cu3O10 (Bi-2223) are very important exceptions. Long-range, strongly linked conduction has been demonstrated in just two additional polycrystalline materials: TlBa2Ca2Cu3O10 (Tl-1223) deposits prepared by spray pyrolysis, and YBa2Cu4O8 (Y-124) conductor made by the oxidation of metallic precursor method. Determining the characteristics of these materials that permit strongly linked conduction is of critical importance to the development of better conductors. Models of strongly linked polycrystalline conductors are reviewed. Recent studies reveal that small-angle boundaries are present in much higher numbers than expected from calculations based on macroscopic texture, indicating a strong influence of grain boundary energy. It is proposed that strongly linked current flows through a connected network of low energy grain boundaries. Evidence has been obtained to support this model in Bi-2223, Y-124 and Tl-1223.