The growth and superconducting properties of RE-Ba-Cu-O single grains with combined RE elements (RE = Gd and Y)

The superconducting properties, melting temperatures and crystal growth rates of single grain, RE-Ba-Cu-O [(RE)BCO] bulk superconductors (where RE = a rare earth element or yttrium) decrease with the RE-element sequence of Nd, Sm, Eu, Gd, Dy and Y. The mechanical properties of these technologically important materials, on the other hand, however, improve in the same sequence. Consequently, one promising approach for optimising the balance between mechanical and superconducting properties of bulk (RE)BCO superconductors, or for adjusting growth rate, is the use of combinations of different rare earth elements. In this study, we explore combinations of Gd and Y in the formation of (Gd-Y)-Ba-Cu-O single grains. We describe the optimisation of the growth process for this multi-RE element system and use optical and scanning electron microscopy to study the microstructure of both non-superconducting (Gd-Y)2BaCuO5 [(Y-Gd)-211] phase inclusions and the (Y-Gd)Ba2Cu3O7-δ [(Y-Gd)-123] phase matrix itself. We demonstrate that (Gd-Y)-Ba-Cu-O single grains can be fabricated reliably and that they exhibit reasonably good superconducting properties. We observe that there is an increase in RE-211 particle size in this mixed rare earth system, which, ultimately, limits sample performance, and conclude that this may be a general disadvantage of this approach to the synthesis of single grains for high field engineering applications