Critical current densities for the high temperature ceramic superconductors YBa2Cu3O7 and Bi2 Sr2Ca2Cu3O10+δ

A pulsed transport technique has been used to measure the zero-field critical current densities of YBa₂Cu₃O ₇ ceramic superconductors prepared under different sintering conditions. This low duty-cycle, pulsed technique, used on specially prepared samples with low-resistance current contacts, allows one to determine the entire superconducting/normal phase boundary without the problems associated with sample heating. The results can be interpreted in terms of two different critical-current densities. The intergrain critical current is low and is limited by the weak links between the grains; the intrinsic intragrain critical current is greater than 10⁴ A/cm² at 77 K. Critical-current values inferred from magnetic hysteresis measurements made on the same samples agree with the intrinsic intragrain critical currents obtained using the pulsed transport technique. In addition, the magnetization-derived critical-current density has been determined for the high T_c phase Bi₂Sr₂Ca₂Cu₃O_10+δ at 77 K