Three- and four-μm-thick YBa2Cu3O7 layers with high critical-current densities on flexible metallic substrates by the BaF2 process

Abstract

We report on the synthesis and performance of 3- and 4-μm-thick YBa2Cu3O7 films on buffered metallic tapes. The precursor films were deposited by vacuum coevaporation of BaF2, Y, and Cu on the substrates and converted to YBa2Cu3O7 by the BaF2 ex situ process at reduced processing gas pressures. The best value of critical-current density Jc for these films was ∼3.8×103A∕mm2 at 77K and in 1T 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 ∼80A∕mm for 3- and 4-μm-thick films, respectively. In order to achieve these high-Jc values, the films were processed at high growth rates (∼0.7nm∕s) and the oxygen partial pressure p(O2) was varied to minimize the growth of the “granular” c axis and randomly oriented YBa2Cu3O7 grains. A simple thermodynamic argument is also given to describe the observed dependence of the nucleation of YBa2Cu3O7 with different orientations on p(O2). This result demonstrates the feasibility of fabricating coated conductors with a single YBa2Cu3O7 layer having the critical current of the order of 100A∕mm at self-field and liquid-N2 temperature.