Contrasting effects of metal oxide dopants on the superconductivity of YBa₂Cu₃O_7-δ ceramics

Bulk superconductor samples of YBa₂Cu₃O_7-δ (YBCO or Y-123), with additions of 0.15 wt% MoO₃, TiO₂, RuO₂ and two different types of Al₂O₃ compounds, are synthesized by a solid state reaction route. Structural, magnetic and transport properties of the samples are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), magnetic (M-H, M-T) and magneto-resistivity (MR-T) measurements. SEM investigations show that the addition of the Al₂O₃ obtained from the alumina crucible (Al₂O₃-cru) decreases the grain size of the sample which, in turn, degrades the transition temperatures (T_c) and magnetic critical current densities $\left(J_c^{mag}\right)$. On the contrary, the addition of the other metal oxides enhances $J_c^{mag}$ of the samples by a few times and also increases T_c by up to 3 K. EDX analyses of the samples exhibit homogeneous distribution of additions, except for Ru-heterogeneities located around superconducting Y-123 grains. The highest J_c and T_c values obtained on the RuO₂ added sample were attributed to the agglomeration of the Ru-particles, which led to the optimal porosity that supports oxygenation process and intergrain coupling of the structure. From the DC magnetization data, we estimated $J_c^{mag}$ of 3.1 × 10⁵ A/cm², 2.3 × 10⁵ A/cm², 2 × 10⁵ A/cm², 1.5 × 10⁵ A/cm², 10⁵ A/cm² and 5.1 × 10⁴ A/cm² for the RuO₂, TiO₂, MoO₃, Al₂O₃, undoped and Al₂O₃-cru added samples at 5 K under 0.25 T, respectively.