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 mixtu re of three different LRE (LRE: 'light rare earths', i.e. Nd, Eu, Gd) eleme nts in the LRE-Ba2Cu3Oy system. Although (Nd,Eu,Gd)(2)BaCuO5 powders about 3 mu m in diameter were added to the precursor, we have found that Gd2BaCuO 5 (Gd-211) inclusions about 0.1 mu m in diameter were finely distributed in the (Nd,Eu,Gd)Ba2Cu3Oy matrix. The formation of such fine normal-conductin g particles has not been achieved in any other bulk superconductors like YB a2Cu3Oy or NdBa2Cu3O7. The presence of small Gd-211 did not inhibit the fun ction of flux pinning provided by composition fluctuations leading to a spa tial scatter of the superconducting transition temperature, T-c, which is t ypically observed in LRE-Ba2Cu3Oy materials, which have a LRE1+xBa2-xCu3Oy- type solid solution. Therefore, two different pinning centers are simultane ously active in one sample, which is also confirmed by the pinning analysis based on the scaling formalism, and leading to a record high current densi ty for bulk high temperature superconductors at 77 K. (C) 1999 Elsevier Sci ence S.A. All rights reserved.