We have studied the microstructure and critical current density (J c) of Gd-Ba-Cu-O and Y-Ba-Cu-O bulk superconductors. The size of RE2BaCuO5 (RE211) particles trapped in REBa 2Cu3Oy (RE123) matrix could be refined by employing ultra-fine RE211 powder as a precursor, which led to high J c values exceeding 1 × 105 A/cm2 at 77 K in self-field for Gd-Ba-Cu-O. In the case of Y-Ba-Cu-O, however, the size reduction of Y211 led to inhomogeneous distribution of Y211 particles, in that the volume fraction of Y211 phase largely decreased near a seed crystal. We also studied superconducting properties of (Gd,Y)-Ba-Cu-O bulk material fabricated from precursor of Gd123-Y211. The employment of finely-milled Y211 powders resulted in a dramatic enhancement of Jc values in this system. The maximum Jc value for (Gd,Y)-Ba-Cu-O reached 2.5×105 A/cm2 at 77 K.
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