Growth rate limiting mechanisms of YBa₂Cu₃O₇ films manufactured by ex situ processing

YBa₂Cu₃O₇ films were fabricated on SrTiO₃ substrates using the BaF₂ ex situ process. Precursor films 1, 3 and 5 μm thick were processed in an atmospheric pressure reactor using a gas mixture of oxygen, nitrogen and water vapor. The films were processed at different water vapor pressures and it was observed that the film growth rate was independent of film thickness and proportional to the square root of the water vapor pressure. The dependence of the film growth rate on film area was also investigated for film areas varying from 10 to 160 mm². Surprisingly, it was observed that the growth rate was inversely proportional to square root of the area of the film. A theoretical model is developed and applied to the experimental results presented in this paper. The model correctly predicts the inverse square root dependence of the film growth rate on sample area. In addition, it predicts that it will be difficult to process long samples, such as tapes, in simple reactor geometries.