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 Ba₂YCu₃O_{6.5 + x} superconducting tapes from BaF₂- based precursors involves a hydration/oxidation reaction at ≈730 °C to 750 °C generally written as: (2BaF2+Y+3Cu)(amorphous)+(2H2O+2.25O2)(g)→Ba2YCu3O6.5+x(s)+4HF(g).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: (BaF2)(amorphous)+2H2O(g)→Ba(OH)2(liq)+2HF(g).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 p_H2O (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 p_H2O, in which essentially all F at the surface was replaced by O. It is therefore possible that processing routes exist for producing Ba₂YCu₃O_{6.5 + 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.