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High temperature oxide superconductors

The phase relationships and the thermochemistry of the yttrium barium copper oxygen system were not known at the time of the discovery of the oxide superconductors by Bednorz and Mueller. Phase diagrams were available for only very few of the binary and ternary subsystems. Research has been carried out in order to provide a self consistent set of the Gibbs energies as function of temperature and, if applicable, composition for all phases of the system yttrium barium copper oxygen.

Experimental phase equilibria and thermochemical properties of the phases in the yttrium barium copper oxygen system taken from the literature have been augmented by crucial experiments involving calorimetry electromotive force (EMF) measurements, differential thermal analysis (DTA) and thermogravimetry (TG). For the superconductor phase (123) these experimental values have been supplemented by ab initio calculations of phase equilibria. Self consistent datasets for the subsystems barium copper, barium yttrium, copper yttrium, barium oxygen, copper oxygen, yttrium oxygen, barium copper yttrium, barium copper oxygen, barium yttrium oxygen, copper yttrium oxygen and the system yttrium barium copper oxygen with special emphasis on the solution range of the superconducting phase YBa2Cu3O6+x have been derived. With these data the standard types of phase diagram for the respective systems have been produced by way of equilibrium calculations. The data also have been applied successfully in the analysis of the thermochemical behaviour of a metallic 123-precursor and in the optimization of a chemical vapour deposition (CVD) process.

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