MODELS FOR LONG-RANGE CURRENT FLOW IN BULK OXIDE SUPERCONDUCTORS

Mitigating the weak-link behavior at grain boundaries, which limits cr itical current density (J(c)) in polycrystalline high-transition-tempe rature (high-T-c) superconducting materials, is required for all appli cations in magnetic fields. Although in general oxide superconductors in polycrystalline form have very poor in-field J(c), conductors conta ining the bismuth-based compounds Bi2Sr2CaCu2O8 (Bi-2212) and Bi2Sr2Ca 2Cu3O10 (Bi-2223) are very important exceptions. Long-range, strongly linked conduction has been demonstrated in just two additional polycry stalline materials: TlBa2Ca2Cu3O10 (Tl-1223) deposits prepared by spra y pyrolysis, and YBa2Cu4O8 (Y-124) conductor made by the oxidation of metallic precursor method. Determining the characteristics of these ma terials that permit strongly linked conduction is of critical importan ce to the development of better conductors. Models of strongly linked polycrystalline conductors are reviewed. Recent studies reveal that sm all-angle boundaries are present in much higher numbers than expected from calculations based on macroscopic texture, indicating a strong in fluence of grain boundary energy. It is proposed that strongly linked current flows through a connected network of low energy grain boundari es. Evidence has been obtained to support this model in Bi-2223, Y-124 and Tl-1223.