Transport critical current density of melt-processed (QMG) YBa₂Cu₃O_x bulk superconductors

Transport critical currents of melt-processed (QMG) YBa₂Cu₃O_x bulk superconductors were measured by a d.c. four probe method. The QMC bulk specimens have high J_c characteristics of ≈ 1 × 10⁴ A cm^-2 at 5 T for the magnetic field parallel to a-b plane. The angular dependence of J_c upon the direction of applied magnetic field showed some characteristic features which could not be described by simple flux pinning. This indicates that anisotropic pinning is relatively strong. The pinning caused by the layered structure, twin boundaries and oxygen deficiency, as well as pinning by the Y₂BaCuO₅ phase, play important roles in determining the J_c of the QMG YBa₂Cu₃O_x bulk superconductors.