Crystalline phase development during vacuum conversion of thin barium fluoride precursor films on metallic substrates

In situ X-ray diffraction has been used to study the conversion of ∼ 3000 Å thick, evaporated barium fluoride precursor films to YBCO on buffered metal substrates for a range of conversion time, ramp rate, water vapor pressure, oxygen pressure, and temperature. All films were taken from the same continuously processed tape. Critical current densities as high as 1.34 MA/cm² have been obtained for a precursor reaction rate of 2.0 Å/s. Higher reaction rates (up to 17 Å/s) lead to lower critical current densities and incompletely developed YBCO. The reaction rate increases with increasing P_H2O, increasing T, and decreasing P_O2. The reaction rate increases linearly with increasing P_H2O for the range of pressures used here (0.0125-5.0 mTorr). The formation of non-epitaxial YBCO is generally associated with lower T and higher reaction rate, whereas the formation of barium cerate, which results from a reaction of the precursor and the CeO₂-capped substrate, is generally associated with higher T and lower reaction rate. General trends in the formation of crystalline phases during conversion may serve as a baseline for development of higher rate conversion processing for thicker, higher performance YBCO.