Investigation on superconducting properties of GdBa₂Cu₃O_7−δ added with nanosized ZnFe₂O₄

Highlights: • The effect of nanosized ZnFe₂O₄ on the Gd-123 phase was studied. • The phase formation and superconducting parameters were improved up to x = 0.06 wt.%. • The Vickers microhardness number increased as x increased. • The Vickers microhardness data agreed with the proportional specimen resistance model. - Abstract: The effect of nanosized ZnFe₂O₄, prepared by Co-precipitation method, on GdBa₂Cu₃O_7−δ superconductor was studied. Therefore, superconducting samples of type (ZnFe₂O₄)_xGdBa₂Cu₃O_7−δ, 0.0⩽x⩽0.1 wt.%, were prepared by the conventional solid-state reaction technique. Moreover, nanosized ZnFe₂O₄ was characterized using X-ray powder diffraction (XRD), transmission electron microscope (TEM) and magnetic hysteresis measurements. The results revealed that a nanosized ZnFe₂O₄ was achieved with average grain size 9 nm and a super-paramagnetic hysteresis. On the other hand, the superconducting samples were characterized and investigated through XRD, scanning electron microscope (SEM), electrical resistivity, ac magnetic susceptibility and Vickers microhardness. XRD results showed that the volume fraction of Gd-123 phase increases as x rises from 0.0 to 0.06 wt.% while the SEM results showed that the grain connectivity between the grains increases with increasing x. The superconducting transition temperature T_c, determined from electrical resistivity and ac magnetic susceptibility, increases up to x = 0.06 wt.%, and then it decreases. Moreover, electric field–current density (E–J) characteristic curves were measured at 77 K. An improvement of the critical current density was obtained with x increase up to 0.06 wt.%. Furthermore, the Vickers microhardness data were analyzed using Meyer's law, Hays–Kendall approach, elastic/plastic deformation model and proportional specimen resistance model. The results indicated that Vickers microhardness number H_V increases as x increases and it exhibits a normal indentation size effect. Moreover, the proportional specimen resistance model is found to be the best model to verify the experimental load independent Vickers microhardness values