Intergranular J/sub c/ 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/sub c/(T) measurements for unaligned and grain-aligned sintered YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBC) toroids are reported. Supercurrent is deduced from measurement of the net current encircling the core, and J/sub 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/sub 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.