Ba(OH)₂ Equilibria in the System Ba-O-H-F, with Application to the Formation of Ba₂YCu₃O_6.5+x from BaF₂-Precursors

Journal Article · Sat Jan 01 04:00:00 EST 2005 · Journal of Research of the National Institute of Standards and Technology

DOI:https://doi.org/10.6028/jres.110.011· OSTI ID:976002

Cook, L. P. ^[1]; Wong-Ng, W. ^[1]; Feenstra, Roeland ^[2]

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1. National Institute of Standards and Technology (NIST)
2. ORNL

The ex situ process for fabricating Ba{sub 2}YCu{sub 3}O{sub 6.5+x} superconducting tapes from BaF{sub 2}-based precursors involves a hydration/oxidation reaction at {approx}730 C to 750 C generally written as: (2 BaF2 + Y + 3 Cu)(amorphous) + (2 H2O + 2.25 O2)(g) {yields} Ba{sub 2}YCu{sub 3}O{sub 6.5+x}(s) + 4 HF(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: (BaF{sub 2})(amorphous) + 2 H{sub 2}O(g) {yields} Ba(OH){sub 2}(liq) + 2 HF(g). To evaluate the possibility of a hydroxide liquid conversion step, thermodynamic calculations on the stability of Ba(OH){sub 2}(liq) have been completed from 500 C to 900 C at 0.1 MPa p{sub 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){sub 2}(liq) may be stable at log p{sub H{sub 2}O} (Pa) values from {approx}4 to 5, provided log p{sub HF} (Pa) values can be maintained below 0 to -1. Limited experimental confirmation is provided by results of an experiment on BaF{sub 2}(s) at 815 C, 0.1 MPa p{sub H{sub 2}O}, in which essentially all F at the surface was replaced by O. It is therefore possible that processing routes exist for producing Ba{sub 2}YCu{sub 3}O{sub 6.5+x} based on the presence of a Ba(OH){sub 2} liquid, which might have an effect on conversion rates and texturing in the superconducting film.

• Research Organization: Oak Ridge National Laboratory (ORNL)
• Sponsoring Organization: OE USDOE - Office of Electric Transmission and Distribution
• DOE Contract Number: AC05-00OR22725
• OSTI ID: 976002
• Journal Information: Journal of Research of the National Institute of Standards and Technology, Journal Name: Journal of Research of the National Institute of Standards and Technology Journal Issue: 2 Vol. 110; ISSN 1044-677X; ISSN JRITEF
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
BARIUM HYDROXIDES
HYDROXIDES
PHASE DIAGRAMS
PROCESSING
STABILITY
SUPERCONDUCTING FILMS
THERMODYNAMICS
TRANSIENTS