Intergranular Jc determination in toroidal high temperature superconductors using a soft-core closed magnetic circuit

An examination of weak-link coupling between grains has been made using a noncontact magnetic technique. Intergranular J_c (T) measurements for unaligned and grain-aligned sintered YBa₂Cu₃O_7-x (YBC) toroids are reported. Supercurrent is deduced from measurement of the net current encircling the core, and J_c is found from supercurrent saturation in contrast to onset pickup voltage criteria. Agreement with recently reported direct transport measurements on the aligned sample suggests that the conduction mechanism is the same as for direct transport. A universal normalized J_c(T) behavior which follows the same junction critical current temperature dependence previously reported in bicrystal transport measurements is found. It is proposed that this technique may be extended for the measurement of critical currents in thin-film samples and that present bulk intergranular current densities appear to be high enough to produce a soft-material limited superconducting electromagnet at 77 K