Fabrication of high performance Gd-Ba-Cu-O single grains in air using a practical melt processing technique

A practical processing route for the fabrication of LRE-Ba-Cu-O single grain superconductors has been developed at the University of Cambridge based on a generic, Mg-doped Nd-123 melt textured seed and suppression of the formation of the solid solution phase in air by enriching the precursors with higher Ba concentration. The processing of high performance Gd-Ba-Cu-O single grains using this processing route is described. The Mg-doped generic seed crystal has been used effectively to promote heterogeneous nucleation via a cold-seeding process. The Gd/Ba solid solution has been suppressed by enriching Gd-Ba-Cu-O precursor powders with two different Ba-rich compositions. This involved adding BaO2 and GdBa6Cu3Oy (Gd-163) (a novel Ba-rich second phase) to the precursor powders, respectively. The Gd-163 phase has been observed not only to suppress formation of the solid solution phase, but also to promote increased heterogeneous grain size. A detailed further study has been carried out with an initial aim of optimizing the BaO2 and Gd-163 phase content of the precursor composition to produce a single grain almost free of solid solution. Based on the optimized parameters, large single grain Gd-Ba-Cu-O superconductors have been fabricated in an air atmosphere and demonstrated to exhibit record trapped magnetic fields for this material melt processed in air in relatively small single grain samples. The trapped fields of samples produced in air atmosphere are at least comparable to those processed under reduced oxygen partial pressure.