Refinement of secondary phase particles for high critical current densities in (Nd,Eu,Gd)-Ba-Cu-O superconductors

We have prepared (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy ('NEG') superconductors by means of melt-processing under a partial oxygen pressure of 0.1%. In these samples, we achieved a large critical current density, J(c), of 6.8 x 10(4) A/cm(2) at T = 77 K and in an applied magnetic field, H-a = 2.5 T. Furthermore, the irreversibility field, H-irr, exceeds 7 T at 77 K for fields applied parallel to the c-axis of the sample. As additional pinning sites, we introduced up to 40 mol% (Nd,Eu,Gd)(2)BaCuO5 (NEG-211) particles plus Pt in analogy to YBa2Cu3Oy (YBCO) melt-processed superconductors. Microstructural observations clarified that the Pt addition was effective in reducing the size of NEG-211 second-phase particles, which led to a dramatic increase in J(c) as compared to melt-processed YBa2Cu3Oy and NdBa2Cu3Oy (NdBCO). A pronounced secondary peak effect was observed in all magnetization curves obtained at 77 K, however, there is a considerable change in the shape of the magnetization loops as a function of NEG-211 content, which demonstrates the pinning provided by the NEG-211 particles in addition to the delta T-c-pinning caused by fluctuation in the transition temperature, T-c, like in ordinary NdBa2Cu3Oy superconductors. (C) 1999 Elsevier Science B.V. All rights reserved.