Ba(OH)2 Equilibria in the System Ba-O-H-F,With Application to the Formation of Ba2YCu3O6.5 + x From BaF2-Precursors |
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Authors: | L. P. Cook W. Wong-Ng R. Feenstra |
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Affiliation: | National Institute of Standards and Technology, Gaithersburg, MD 20899-8520;Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 |
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Abstract: | The ex situ process for fabricating Ba2YCu3O6.5 +x superconducting tapes from BaF2- based precursors involves a hydration/oxidation reaction at ≈730 °C to 750 °C generally written as:However, microscopic observations of partially processed films suggest the presence of a transient liquid phase during conversion. Alternatively, the conversion reaction can be rewritten as the sum of several intermediate steps, including the formation of a barium hydroxide liquid:To evaluate the possibility of a hydroxide liquid conversion step, thermodynamic calculations on the stability of Ba(OH)2(liq) have been completed from 500 °C to 900 °C at 0.1 MPa ptotal. Based on currently available data, the calculated phase diagrams suggest that a viable hydroxide reaction path exists in the higher part of this temperature range. The calculations indicate that Ba(OH)2(liq) may be stable at log pH2O (Pa) values from ≈4 to 5, provided log pHF (Pa) values can be maintained below 0 to −1. Limited experimental confirmation is provided by results of an experiment on BaF2(s) at 815 °C, 0.1 MPa pH2O, in which essentially all F at the surface was replaced by O. It is therefore possible that processing routes exist for producing Ba2YCu3O6.5 +x based on the presence of a Ba(OH)2 liquid, which might have an effect on conversion rates and texturing in the superconducting film. |
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Keywords: | Ba2YCu3O6.5 +x BaF2 Ba(OH)2 ex situ process liquid phase equilibria PH2O pHF processing superconductor thermodynamic calculations |
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