Metallurgical Reactions and Their Relationships to Enhanced Mechanical Strength in Zr-Bearing YBCO Composite Superconductors

The metallurgical reactions occurring during partial melting of the Zr-bearing YBCO composite superconductors have been investigated in detail by analyzing the DTA, XRD spectra and SEM micrographs of samples subjected to various heat treatments. It is shown that Zr reacts preferentially with Ba to form BaZrO₃. As a result, the 123-phase is partially disintegrated and yields CuO and the 211-phase as by-products. The CuO phase in contact with the 123-phase lowers the melting point of the 123-phase to 970°C. The Zr-bearing YBCO sample annealed above 970°C melts partially and contributes to the reduction of voids. In addition, fine BaZrO₃ particles precipitated along the grain boundaries form complicated three-dimensional networks. This skeletonlike structure prevents the sample from collapsing during partial melting and reinforces the mechanical strength of composites.