TRANSPORT CRITICAL CURRENT OF ALIGNED POLYCRYSTALLINE Y1BA2CU3O7-DELTA AND EVIDENCE FOR A NONWEAK-LINKED COMPONENT OF INTERGRANULAR CURRENT CONDUCTION

A study of grain alignment and its effect on the dc transport critical current in fine-grained bulk Y1Ba2Cu3O 7-δ is reported in magnetic fields from 10-4 T to 26 T. Two features distinguish the critical current density Jc of aligned bulk Y1Ba2Cu3O7-δ from unaligned material. First, the effective critical field where the intergranular Jc approaches zero is about four times higher (30 T) for aligned samples with field parallel to the a, b planes, than it is for polycrystalline unaligned samples (7 T). Second, the nearly field independent plateau value of Jc between 10 mT and 1 T is one to two orders of magnitude higher than typical plateau values of Jc in unaligned bulk-sintered Y 1Ba2Cu3O7-δ, for field parallel to the a, b planes. A low-field (<10 mT) weak-link decrease in Jc with magnetic field is still observed, but it is much smaller than for unaligned material. These data clearly demonstrate that alignment alone significantly reduces the weak-link problem in fine-grained polycrystalline samples with low-aspect-ratio (4:1) grains (unlike melt-grown samples where there has been some ambiguity as to the relative importance of alignment versus large grain growth). Furthermore, the results provide strong evidence that there are two parallel components of intergranular current conduction, one consisting of weak-linked material, the other behaving like intrinsic intragranular material that is not weak-linked. A comparison with unaligned Y 1Ba2Cu3O7-δ indicates that the volume fraction of such nonweak-linked material is significantly enhanced by grain alignment, but still only 0.01%-0.1% of the grain boundary area. Field-cooled and force-free Jc data are also presented, along with detailed measurements of the shapes of the voltage-current characteristics.