Microstructures and phase evolution in YBa2Cu3O7−x films grown on various substrates fabricated via a non-fluorine sol-gel route

YBa2Cu3O7−x films were grown on polycrystalline Ag and single crystalline YSZ, SrTiO3, and MgO substrates using non-fluorine sol-gel and spin coating methods. The effects of heat treatment conditions on the phase evolution and microstructures were investigated using optical microscope, X-ray diffraction, and atomic force microscope. For Ag substrates, Y123 phase started to form at 750 °C and higher sintering temperatures improved the degree of (0 0 1) texture. Mirror-like surfaces without any cracks were achieved for sintering at 750-900 °C. However, voids were observed for films grown on the Ag substrates at temperatures higher than 810 °C and their size and density increased as the temperature increased. For the films grown on single crystal substrates sintered at 800 °C, numerous microcracks with large crack widths were observed, while cracking is less of a problem for films grown on Ag substrates. Epitaxial films without any cracks were achieved for films grown on single crystalline substrates under optimized conditions. Possible mechanism for the formation of cracks is discussed.