Twin boundaries and critical current densities of YBa₂Cu₃O₇thick films fabricated by the BaF₂process

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2 µm thick epitaxial YBa 2 Cu 3 O 7 , YBCO, films on SrTiO 3 substrates were prepared at the reaction temperatures of 725, 750 and 800 °C by the BaF 2 process. The dc critical current densities of these films were found to vary by a factor of 2 at 77 K in perpendicular dc magnetic fields measured up to 1.5 T. The twin boundary spacing was essentially unchanged among these films while the size of Y 2 O 3 precipitates in the films varied by a factor of ∼5. Also, the measured ratios of the lattice parameters, a / b , of the films were surprisingly larger than those found in standard fully oxygenated YBCO. This unusual property of the films appeared to be caused by the presence of the Y 2 O 3 precipitates which provided a strain constraint in limiting the full tetragonal-to-orthorhombic phase transformation due to the oxygenation. Differences in the degree of the constraint possibly led to the observed difference in J c among these films.