Development of the microstructure of uranium-doped Nd-Ba-Cu-O

Melt grown Nd-Ba-Cu-O (NdBCO) has been reported to exhibit higher values of critical current density, J/sub c/ and irreversibility field, H/sub irr/, than other (RE)BCO superconductors, such as YBCO. The microstructure of NdBCO typically contains 5-10 /spl mu/m sized inclusions of the Nd/sub 4/Ba/sub 2/Cu/sub 2/O/sub 10/ phase (Nd-422) in a superconducting NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// phase (Nd-123) matrix. The average size of these inclusions is characteristically larger than that of the Y/sub 2/BaCuO/sub 5/ (Y-211) inclusions in YBCO. As a result, there is scope to further refine the Nd-422 size to enhance J/sub c/ in NdBCO. Large grain samples of NdBCO superconductor doped with various amounts of depleted UO/sub 2/ and containing excess Nd-422 have been fabricated by top seeded melt growth under reduced oxygen partial pressure. The effect of the addition of depleted UO/sub 2/ on the NdBCO microstructure has been studied systematically in samples with and without added CeO/sub 2/. It is observed that the addition of UO/sub 2/ refines the NdBCO microstructure via the formation of uranium-containing phase particles in the superconducting matrix. These particles are of approximately spherical geometry with dimensions of around 1 /spl mu/m. The average size of the nonsuperconducting phase particles in the uranium-doped microstructure is an order of magnitude less than their size in un-doped Nd-123 prepared with excess Nd-422. The critical current density of uranium-doped NdBCO is observed to increase significantly compared to the undoped material.