Fabrication and Superconducting Properties of Aluminum Doped (Nd,Sm,Gd)–Ba–Cu–O Filaments

(Nd_0.33Sm_0.33Gd_0.33)_1.18Ba_2.12Cu_3.09O_y (NSG123) precursor filaments doped with Al were prepared by a solution spinning method and partially melted at 1050degC for 30 min in flowing 0.1% O₂ + Ar gas. Samples were cooled at various rate of 10-60degC/h, and then oxygenated in pure O₂ gas. It was found that the microstructural and superconducting properties were strongly influenced on the cooling rate from partial melting temperature to 900degC during OCMG processing. Although the sample without Al cooled at 20-50degC/h showed J_c value higher than 2.0 times 10⁴ A / cm² at 77 K and 0 T, the sample treated at fast cooling rate of 60degC/h exhibited the J_c value of about 3.0 times 10³ A / cm² at most. In contrast, the NSG123 doped with 0.05 at%Al even at a rate of 60degC/h cooling showed high J_c value of 1.5 times 10⁴ A / cm² at 77 K and 0 T, and exhibited a well aligned texture and a dense microstructure. From J_c measurements in applied magnetic fields, a J_c value higher than 10³ A / cm² was maintained up to 9 T for 0.05 at % Al doped filament. It was found that a small amount of Al doping into NSG123 was effective on refinement of the size of the NSG211 particle and reduction pores and cracks in microstructure for sample treated even at fast cooling, and thereby increased J_c values.