Microstructure, microhardness, and superconductivity of CeO₂-added Y–Ba–Cu–O superconductors

The CeO₂-added Y–Ba–Cu–O oxides were prepared by two different processes, the conventional solid-state reaction process and the partial melt process using powders, to examine the effect of the dopant on microstructure, microhardness, and superconductivity. In the solid-state reacted sample, most of the added CeO₂ was converted to a form of BaCeO₃, but some might enter into the 1-2-3 phase, resulting in the orthorhombic-to-tetragonal phase transition that accompanied the disappearance of twin structure in 1-2-3 grains. In the partially melted sample, however, the phase change was not observed up to 5 wt. % of CeO₂. All the added CeO₂ in these samples was consumed to form only BaCeO₃ which was finely dispersed in large 1-2-3 grains during the peritectic reaction stage. The zero resistance temperature (T_c) of the solid-state reacted sample gradually decreased with increasing CeO₂ content due to the phase change and the formation of BaCeO₃, whereas the T_c of the partially melted sample was nearly constant regardless of CeO₂ content up to 5 wt. %, owing to the separation of the second phase from the 1-2-3 superconducting phase. Microhardness of the partially melted sample increased with increasing CeO₂ content. The strengthening effect appears to come from the composite system where the fine BaCeO₃ particles are dispersed in a 1-2-3 matrix.