Progress in melt-processed (Nd-Sm-Gd)Ba₂Cu₃O_y superconductors

This comprehensive survey deals with the melt-growth process and characterization of the novel class of ternary materials, (Nd,Sm,Gd)Ba₂Cu₃O_y 'NSG-123'. Microstructure observations by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) revealed the presence of sub-micron secondary phase (RE₂BaCuO₅, 211) particles uniformly dispersed in the 123 matrix that enhance flux pinning at low fields and improve the performance of this material. The energy-resolved TEM-EDX analysis clarified that small particles consist mainly of Gd₂BaCuO₅ 'Gd-211', irrespective of the initially added secondary phase (Gd-211 or NSG-211). Dark-field TEM observations showed that the NSG-123 matrix had a high density of RE-rich RE_1+xBa_2-xCu₃O_y (RE-123ss) clusters 3-10 nm in size. These small clusters enhance pinning at intermediate fields. The scaling analyses of the volume pinning force density at 77 K made on NSG-123 samples with varying 211 contents of various secondary phases showed the secondary peak at normalized fields as high as 0.45, which might indicate the activation of ΔT_c pinning mechanism. Trapped field measurements proved that silver was the key element to improve the mechanical strength of NSG-123 materials. Record trapped-field values at high magnetic fields can be obtained at 77 K with appropriately large sized NSG-123 samples.