Engineering of pinning sites in melt-processed (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy 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–Ba2Cu3Oy 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)Ba2Cu3Oy 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, Tc, which is typically observed in LRE–Ba2Cu3Oy materials, which have a LRE1+xBa2−xCu3Oy-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.