Comparative study on superconducting properties and surface morphology analysis for Y0.85K0.15Ba2Cu3O7-δ and Y0.85Ca0.15Ba2Cu3O7-δ synthesized via thermal treatment method

A comprehensive investigation on the comparison of superconducting properties, normal state properties and surface morphology analysis for YBa2Cu3O7-δ (YBCO) substitution with alkali metals (K and Ca) at Y site was conducted via the simple thermal treatment method. The superconducting properties and surface morphology of the specimens were characterized by using electrical resistance measurement, Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-Ray Spectroscopy (EDX). From XRD result, the YBa2Cu3O7-δ (Y-123) substituted with K showed orthorhombic crystal structure with secondary phase BaCuO2 beside a small amount of (Y2BaCuO5) Y-211. However, for the case Ca substituted in Y-123, the phase of Y-211 was not seen. While, the superconducting properties improved after Ca substitution in the YBCO system with higher average grains size compared to that of the pure and K substitutions specimens Thus, it’s hole concentration, p posted the highest value among the three specimens with the lowest superconducting transition width, ΔTc = 8.02 K (Tc-onset = 82.12 K and Tc(R=0) = 74.10 K). This is due to the reduced oxygen content in the Cu-O chain in Ca substituted specimen, which led to the higher amount of hole concentration in the specimen. Hence, the Ca substituted specimen with better grain growth and compactness surface morphology exhibited higher p as designated better coupling among the grains with stronger Josephson junction. In the other points, that may also be due to the increment grain growth of the secondary phase of BaCuO2 that generates and introduced as effective flux pinning site and might attribute to the enhancement p or critical current density Jc. This propose ideas were confirmed by EDX result that element Ca seen in the area of the peak with 1.0 atomic % and 0.98 atomic % (cross section), while K peaks couldn’t be detected. Thus, we confirmed that Ca has fully introduced within the grain without altering the structural symmetry while K was decomposed in the sintering process in the high enough sintering temperature i.e. 920 °C. Moreover, this comparative study highlights the importance of alkali metals, K and Ca substitution in the YBCO system or bulk rare earth based (REBa2Cu3Oy (RE=bulk rare earth)) high temperature superconductors synthesized using thermal treatment method.