Microstructure and pinning in high-T_c and large-J_c (Nd, Eu, Gd)-123 superconductors prepared by OCMG process

The effects of (Nd, Eu, Gd)₂BaCuO₅ (NEG-211), Pt and CeO₂ additions on OCMG-processed (Nd, Eu, Gd)Ba₂Cu₃O_y ('NEG') composites have been investigated in detail using a combination of scanning electron microscopy and measurements of critical current densities, J_c. Microstructural observations clearly indicated that the platinum addition was effective in refining the NEG-211 secondary-phase particles, which led to a dramatic increase in J_c in the low-field as well as in the high-field region. TEM-EDX analyses revealed that the small secondary-phase particles (diameter ≈0.1 μm) consist mainly of Gd in the rare earth site, while the large particles contain Nd, Eu and Gd in the rare earth site with an even ratio, which is identical to the nominal composition of the precursor powder. For an NEG sample with 40 mol% NEG-211 additions, a large critical current density of 60000 A cm^-2 at 77 K was observed in a magnetic field of 3 T applied parallel to the c-axis of the sample. The critical current density measurements showed that the zero field and peak J_c values are similar in samples with 30 and 40 mol% of NEG-211 added. In contrast to this, the combination of CeO₂ and Pt addition was not as effective in refining the NEG-211 particles, and also the J_c values were found to decrease in the entire field region.