Engineering of pinning sites in melt-processed (Nd 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y superconductors

Abstract

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–Ba 2Cu 3O y system. Although (Nd,Eu,Gd) 2BaCuO 5 powders about 3 μm in diameter were added to the precursor, we have found that Gd 2BaCuO 5 (Gd-211) inclusions about 0.1 μm in diameter were finely distributed in the (Nd,Eu,Gd)Ba 2Cu 3O y matrix. The formation of such fine normal-conducting particles has not been achieved in any other bulk superconductors like YBa 2Cu 3O 7 or NdBa 2Cu 3O 7. 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–Ba 2Cu 3O y materials, which have a LRE 1+ x Ba 2− x Cu 3O 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.