Transmission electron microscopy studies of YBCO coated conductors prepared by pulsed-laser deposition and multiple-stage chemical vapor deposition processes

Three types of YBCO films were deposited on metal tapes with biaxially textured CeO₂/Gd₂Zr₂O₇ multilayer. One YBCO film 4 μm in thickness was formed by pulsed-laser deposition (PLD) on the stationary substrate and another film 3 μm in thickness by PLD using a reel-to-reel substrate tape transferring system. I_c values of the films were 173 A and 293 A, respectively A further film 1 μm in thickness was fabricated by multiple-stage chemical vapor deposition (MSCVD). The I _c value of the MSCVD-YBCO film was 100 A. The microstructures of these YBCO films were characterized using transmission electron microscopy (TEM). In the PLD-YBCO films, the films less than 1 μm thick were predominantly composed of c-axis oriented grains which were considered to grow spirally, however, irregular instances of a-axis oriented grains were also found. The a-axis oriented grains size increased with increasing thickness of the YBCO films. In the 4 μm-thick PLD-YBCO film with an I_c value of 173 A, a-axis oriented grains were considered to nucleate on the CeO ₂. We found many Y₂O₃ and cupper oxides at the boundaries between a- and c-axes oriented grains and the orientation relationships between YBCO and Y₂O₃ were determined to be as follows: (0 0 1)YBCO//(0 0 1)Y₂O₃ and (1 0 0)YBCO//(1 1 0)Y₂O₃. We also found many gaps and pores between YBCO grains. Nucleation of a-axis oriented grains was completely suppressed on the CeO₂ in the 3 μm-thick PLD-YBCO film with an I_c value of 293 A. In the MSCVD-YBCO film, the YBCO film mostly consisted of c-axis oriented grains, however a-axis oriented grains and polycrystal also formed in the film. As the MSCVD-YBCO film became thicker, the a-axis oriented grains grew large, as same as in the PLD-YBCO films. The growth rate of a-axis oriented grain along the substrate normal was much higher than that of c-axis oriented grains in comparison to the difference in the growth rate between a- and c-axes oriented grains in the PLD-YBCO. In these specimens, the grain growth of the a-axis oriented grains was considered to be one of the most significant causes of degradation of J_c values in the YBCO films.