Engineering of pinning sites in melt-processed (Nd_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_y superconductors

Extremely fine normal-conducting particles could be uniformly dispersed in the superconductive matrix through a combination of Pt addition and a mixture of three different LRE (LRE: `light rare earths', i.e. Nd, Eu, Gd) elements in the LRE-Ba₂Cu₃O_y system. Although (Nd,Eu,Gd)₂BaCuO₅ powders about 3 μm in diameter were added to the precursor, we have found that Gd₂BaCuO₅ (Gd-211) inclusions about 0.1 μm in diameter were finely distributed in the (Nd,Eu,Gd)Ba₂Cu₃O_y matrix. The formation of such fine normal-conducting particles has not been achieved in any other bulk superconductors like YBa₂Cu₃O₇ or NdBa₂Cu₃O₇. The presence of small Gd-211 did not inhibit the function of flux pinning provided by composition fluctuations leading to a spatial scatter of the superconducting transition temperature, T_c, which is typically observed in LRE-Ba₂Cu₃O_y materials, which have a LRE_1-xBa_2-xCU₃O_y-type solid solution. Therefore, two different pinning centers are simultaneously active in one sample, which is also confirmed by the pinning analysis based on the scaling formalism, and leading to a record high current density for bulk high temperature superconductors at 77 K.