Critical current densities and the structural quality of 3- and 4-μm -thick superconducting YBa₂Cu₃O₇ layers synthesized using the ex situ process

In this work, we used quantitative x-ray diffraction measurements and optical metallography to investigate the relationship between structural quality and critical current densities Jc for 3- and 4-μm -thick YBa2 Cu3 O7 (YBCO) films grown on CeO2 -buffered Ni-W substrates by the BaF2 process. The Jc of the films was shown to be approximately: (1) proportional to the intensity of the YBCO (006) diffraction line, and (2) inversely proportional to the average grain size of the c -axis oriented YBCO as determined from optical micrographs of polished surface of the films. We conclude that to achieve Jc s well above 106 A/ cm2 in self-field and at 77 K, it is critical to suppress the formation of randomly oriented YBCO grains while maintaining high crystallographic quality of the c -axis oriented part of the film. The quality of the c -axis oriented YBCO was found to be strongly dependent on the YBCO grains size-e.g., the grains, which are smaller than 10 μm, are required for high Jc films. The fine-grain high Jc films can be synthesized under processing conditions that promote a high rate of nucleation of c -axis oriented YBCO.