Phase transformations and thermodynamic behavior of the YBa2Cu3O(sub)x system via EMF measurements
Abstract
EMF measurements of oxygen fugacities as a function of stoichiometry in YBa2Cu3Ox were made in the temperature range 650-750 C by means of an oxygen titration technique with a ZrO2 electrolyte doped with Y2O3. From plots of equilibrium oxygen pressures versus x, no sign of a change in curvature or discontinuity was observed at an oxygen stoichiometry of x = 6.5 where copper is divalent. A change in slope was observed when x approximates 6.3. The EMF measurements were extended to an oxygen stoichiometry of x = 6.15; after slow cooling to room temperature, X-ray examination of the samples indicated that the original orthorhombic phase was completely transformed to tetragonal. A rapid increase in the partial molar entropy of oxygen, delta bar S sub O sub 2, was found in the composition range x = 6.3 to 6.15. The increase in delta bar S sub O sub 2 values appears to reflect the differences between distributions of oxygen atoms and vacancies in the relatively disordered tetragonal structure and the more ordered orthorhombic structure. The results to date indicate that the important orthorhombic structure can exist over an extended oxygen composition range, namely, approximates 6.3-7.0 at temperatures in the range of 650-750 C.
- Publication:
-
Presented at the 90th Annual Meeting of American Ceramic Society
- Pub Date:
- 1988
- Bibcode:
- 1988acs..meet.....T
- Keywords:
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- Barium Oxides;
- Copper Oxides;
- Partial Pressure;
- Phase Transformations;
- Temperature Effects;
- Yttrium Oxides;
- Electromotive Forces;
- Entropy;
- High Temperature Superconductors;
- Oxygen Atoms;
- Stoichiometry;
- Superconductivity;
- Thermodynamics;
- Solid-State Physics