Critical current and microstructure of uniaxially aligned, polycrystalline YBa2Cu3O7

Three processing routes that generate uniaxial alignment but otherwise yield very different microstructure and critical current are compared. Fine grain size and c-axis alignment are obtained in magnetically aligned ceramics, pyrolyzed thick films, and in situ deposited thin films. The dense, well-aligned microstructure of the in situ process produces the highest zero field critical current Jc > 104A/cm2 at 77 K. However, the critical current is suppressed in low magnetic field, suggesting that uniaxial alignment is not sufficient to avoid Josephson-type intergranular coupling. Above 1 T, the critical current of the aligned ceramic dominates in spite of its less ideal microstructure. The critical current in this high field region is one to two orders of magnitude greater than that of nonaligned material. This result implies the existence of a 3-d percolative network of strong links.