Ba(OH)2 Equilibria in the System Ba-O-H-F, With Application to the Formation of Ba2YCu3O6.5 + x From BaF2-Precursors

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: [Formula: see text] 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: [Formula: see text] 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 p total. 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 [Formula: see text] (Pa) values from ≈4 to 5, provided log p HF (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 [Formula: see text], 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.