TEM Microstructural Investigation of High Current Density YBCO Superconducting Thin Films Grown by Ultrafast Transient Liquid Assisted Growth (TLAG)

Among high temperature superconductors (HTS), YBa₂Cu₃O_7-δ (YBCO) based thin films and coated conductors (CCs) offer an unparalleled opportunity to be used in large-scale superconducting power applications and high field magnets due to their outstanding ability to carry high critical currents at high magnetic fields. In an essential need of high-performance and low-cost manufacturing, chemical-solution deposition (CSD) has become a very competitive cost-effective and scalable methodology to grow epitaxial YBCO films [1]. However, their growth rates are rather slow (0.5-1 nm/s). For this purpose, we have developed a novel growth approach, entitled, Transient Liquid Assisted Growth (TLAG) [2], which is able to combine CSD methodologies with ultra-fast growth rates (100-1000 nm/s) by facilitating a non-equilibrium liquid-mediated approach. Critical current densities up to 5 MA/cm² at 77K are already realized in TLAG-CSD grown thin films, but in order to further improve the current carrying properties, understanding of initial nano phases in pyrolysis process and fine tuning of growth parameters are essential to define a robust process, including thicker layers. Therefore, the microstructure of multi-deposited pyrolyzed and grown YBCO films, investigated via high-resolution transmission electron microscopy (HR-TEM), scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX), will be presented.<br/>Furthermore, the critical current density capabilities of high temperature superconductors (HTS) are determined by its microstructure and it can be enhanced by the presence of well-controlled nano defects inside the epitaxial superconducting matrix acting as vortex pinning centers. TLAG-CSD has demonstrated the growth of nanocomposites to increase flux-pinning at high magnetic fields by incorporating pre-formed NPs to the metal-organic initial inks [2]. Therefore, using aberration corrected STEM combined with high angle annular dark field (HAADF) and EELS, the detailed microstructure of grown TLAG-CSD YBCO films, nanocomposites, and coated conductors, with a focus on new defects landscape at the atomic level, secondary phases, and strain effects, will be presented. Finally, the recent progress in TLAG-CSD films and nanocomposites, in terms of microstructure correlation with growth mechanisms and growth rate will be discussed.<br/>[1] J. Gutierrez, A. et al., Nat. Mat., 6, 367 (2007)<br/>[2] L. Soler et al, Nat. Commun., 11, 344 (2020)