Influence of rare earth (RE) mixing in REBa2Cu3O7−x thin films, including RE2O3 nanoparticle formation, on in-field critical current density at 20K

The factors that influence high-field, low-temperature current carrying performance of coated conductors are complex. Using epitaxial thin films grown by pulsed laser deposition (PLD) made from ($Y_{0.8}$RE’${}_{0.2-x}$RE”${}_x$)Ba₂Cu₃ O${}_{7-y}$ (YREBCO), where RE = rare earth targets, with and without liquid (L) additions, this work decouples the different complex effects (both positive and negative) of RE mixing. In doing so, we understand the outcomes of the separate effects of this mixing, and the incidental formation of (Y,RE’,RE”)₂O₃ nanoparticles. The YREBCO composition is chosen because it permits growth conditions to be the same and near-optimum for all the targets, i.e., at the optimum temperature as for the growth of pure YBCO, the reference composition, to enable effective comparison of compositions. We find that a high RE ion size variance increases the RE₂O${}_3$ fraction but that $J_c$ (20 K, 8 T) is decreased, owing to the associated atomic disorder. On the other hand, we find that for moderate variance (and moderate RE₂O₃ nanoparticle formation), $J_c$ (20 K, 8 T) is increased. A liquid phase needs to be included to ensure minimal microstructural disorder of the YREBCO lattice caused by the RE₂O${}_3$ nanoparticle inclusions. Of the wide range of PLD target compositions explored, ($Y_{0.8}$Gd${}_{0.1}$Yb${}_{0.1}$)Ba₂Cu₃O${}_{7-y}$ is determined to have the optimum moderate ion size variance. An approximate doubling of $J_c$ (20 K, 8 T) compared to pure YBCO films results. Thus, the ($Y_{0.8}$Gd${}_{0.1}$Yb${}_{0.1}$)Ba₂Cu₃O${}_{7-y}$ composition with extra liquid phase addition represents an ideal base composition to which artificial pinning centres (APC) can be added for further $J_c$ enhancement.