Effect of ZrO₂ doping on structure and superconducting properties of sintered DyBaCuO ceramics

The effect of ZrO₂ addition (1 wt% and 5 wt%) on the superconducting properties of sintered DyBa₂Cu₃O_y ceramics was studied. The doping was found to assist the formation of secondary phase inclusions such as Dy₂BaCuO₅ (211), BaZrO₃ and CuO. No traces of Zr in the 123 grains (matrix) were revealed. Doping by ZrO₂ favoured the formation of a fine-grain microstructure, the amount of clean boundaries strongly increasing compared with the undoped composition. Doping by 1 wt% ZrO₂ resulted in a substantial increase in the critical current density J_c. The samples with the addition of 5 wt% ZrO₂ showed the best flux pinning in a magnetic field. Such behaviour seems to be associated with a substantial increase in proportion of clean boundaries and the presence of fine facets and regularly distributed dislocations in these boundaries in the doped samples. The measured dependence of the critical current I_c on temperature close to T_c indicated that weak links at grain boundaries behave as a network of superconductor-insulator-superconductor (SIS) junctions in all compositions, except for the composition with 1 wt% ZrO₂ which showed superconductor-normal metal-superconductor (SNS) behaviour.