Liquidus relations in Y–Ba–Cu oxides

The liquidus relations in the system YO_1.5–BaO–CuO_x in air in the compositional region near the superconducting oxide YBa₂Cu₃O_x were studied by differential thermal analysis, x-ray diffraction, electron microprobe analysis, and visual observation. The temperatures of 11 invariant points and the corresponding reactions were determined. YBa₂Cu₃O_x was found to melt incogruently at 1015 °C to Y₂BaCuO₅, which in turn melts incongruently to Y₂O₃ at 1270 °C. These reactions mean that preparing the superconducting phase by melting and rapid cooling will result in the presence of these two phases as well. The peritectic reaction YBa₂Cu₃O_x + CuO⇉Y₂BaCuO₅ + liquid at 940 °C accounts for the observation of partial melting, improved synthesis purity, and grain growth at temperatures of 950 °C. The determination of these invariant temperatures and reactions provide insight into optimal processing conditions.