A method for improving nucleation of thick YBCO films in the ex-situ process

Some coated conductor applications require a YBCO layer at least 5 μm thick with high J_c. The growth of thick c-axis oriented epitaxial layers using the barium fluoride ex-situ technology is not always possible. Films over 3 μm thick have more undesirable a-oriented and random grains than thinner ones. We present an analysis of thick film nucleation and conclude that thick precursor layers impede out-diffusion of the ex-situ reaction product, HF. High impedance for HF diffusion results in large variations of the chemical potential of the growth reaction and disruption of the nucleation process. We conclude that a solution for c-axis growth is control of the precursor permeability. A method is described for controlling the permeability of precursor layers. Using this technique we were able to grow c-axis oriented 5-μm thick films on oxide-buffered Ni tape with J_c(0 T) = 4 × 10⁵ A/cm² and J_c(1 T) = 8 × 10⁴ A/cm².