We have studied the microstructure and magnetic properties of oxygen-controlled-melt-growth (OCMG) processed (Nd, Eu, Gd)-Ba-Cu-O superconductors, which contain about 40 mol% of (Nd, Eu, Gd)2BaCuO5 (NEG211) second phase by varying the amount of Pt from 0.25 to 3 mol%. Scanning electron and transmission electron microscopic observations demonstrate that 0.5 mol% Pt is the optimum quantity to refine the 211 particles, leading to a dramatic increase in the critical current density. An addition of 2-3 mol% Pt results in the formation of relatively large NEG211 particles. Jc values increase with increasing Pt content up to 0.5 mol% and then decrease thereafter. A high critical current density of 70 000 A cm-2 at T = 77 K and an applied field of 2.2 T (H ∥ c axis) is achieved in the sample with 0.5 mol% Pt. Our results demonstrate that the Pt addition is critical to the refinement of the 211 particle size and thus for the enhancement of the critical current density.
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