Processing, microstructure and irreversibility of large-grainNd-Ba-Cu-O

Abstract

The fabrication of Nd-Ba-Cu-O (NdBCO), which exhibitssignificant potential for high-field engineering applications, is extremelysensitive to firing atmosphere, undercooling and secondary phase contentand presents considerably greater processing challenges than that of other(RE)BCO superconductors. A detailed study of NdBCO grain growth rate hasbeen performed and a schematic process phase diagram constructed in theperitectic solidification region from isothermal growth experiments atvarious temperatures for several NdBCO compositions. This phase diagram hasbeen used to fabricate large, single grains up to 2 cm in diameter of NdBa2Cu3O7 (Nd-123) containing 15 mol% ofnon-superconducting Nd4Ba2Cu2O10 (Nd-422)phase inclusions using a top-seeded melt growth technique (TSMG) in reducedoxygen partial pressure. Magnetization measurements of samples cut from alarge grain exhibit a pronounced peak effect over a wide temperature andfield region and an anisotropic irreversibility line for fields appliedparallel (≈9 T at 77 K) and perpendicular (extrapolates togreater than 40 T at 77 K) to the crystallographic c-axis. Thetemperature dependence of both the measured critical current density,Jc, and that associated with extrinsic pinning in this materialhas been observed to follow a power law above ≈70 K, suggestingthat the density of pinning sites does not vary significantly withtemperature.