Microstructural characterization of thick YBa₂Cu₃O_7-δ films on improved rolling-assisted biaxially textured substrates

The microstructural changes associated with the reduced dependence of critical current density (J_c) versus thickness of thick, epitaxial YBa₂Cu₃O_7-δ (YBCO) films on rolling-assisted biaxially textured substrates (RABiTS) were investigated. Pulsed laser deposited YBCO films varying in thickness from 1.0 to 6.4 μm on RABiTS with an architecture of Ni-3 at.% W/Y₂O₃/yttrium-stabilized-zirconia/CeO₂/YBCO were prepared for cross-sectional transmission electron microscopy studies. Dramatic improvements in physical properties and microstructural quality were observed resulting from the use of Ni-3 at.% W substrates, which provided a sharper texture over earlier Ni substrates, and replacement of CeO₂ with Y₂O₃ as the seed layer within the buffers. The YBCO films showed exceptional orientation up to 6.4 μm thickness, with no misoriented grains or dead layers observed and only limited reaction between the YBCO and CeO₂ cap layer. The high quality of the films was also attributed in part to the formation of a tungsten oxide layer forming at the top of the Ni-3% W substrate, limiting the growth of deleterious NiO into the conductor.