Three- and four- μm -thick YBa ₂Cu ₃O ₇ layers with high critical-current densities on flexible metallic substrates by the BaF ₂ process

We report on the synthesis and performance of 3- and 4-μm -thick Y Ba2 Cu3 O7 films on buffered metallic tapes. The precursor films were deposited by vacuum coevaporation of Ba F2, Y, and Cu on the substrates and converted to Y Ba2 Cu3 O7 by the Ba F2 ex situ process at reduced processing gas pressures. The best value of critical-current density Jc for these films was ∼3.8× 103 A mm2 at 77 K and in 1 T external magnetic field perpendicular to the film plane. Also, estimated critical-current densities per width of tape Jcw at zero magnetic field were ∼60 and ∼80 Amm for 3- and 4-μm -thick films, respectively. In order to achieve these high- Jc values, the films were processed at high growth rates (∼0.7 nms) and the oxygen partial pressure p (O2) was varied to minimize the growth of the "granular" c axis and randomly oriented Y Ba2 Cu3 O7 grains. A simple thermodynamic argument is also given to describe the observed dependence of the nucleation of Y Ba2 Cu3 O7 with different orientations on p (O2). This result demonstrates the feasibility of fabricating coated conductors with a single Y Ba2 Cu3 O7 layer having the critical current of the order of 100 Amm at self-field and liquid- N2 temperature.