Role of Gd content in Cu(1) and Cu(2) sites on electrical, microstructural, physical, mechanical and superconducting properties of YBa₂Cu_3−xGd_xO_7−δ ceramics

Abstract

This study deals with the effects of partial Gd³⁺ substitution for the Cu sites on the electrical, microstructural, physical, mechanical and superconducting properties of YBa₂Cu_3−xGd_xO_7−δ ceramic superconductors with x = 0, 0.025, 0.050, 0.100 and 0.150 with the aid of dc resistivity, transport critical current density (J _c ), X-Ray analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-Ray (EDX), Vickers microhardness (H _v ) and density measurements. The samples studied in this work are prepared by the standard solid-state reaction method. The resistivity (at room temperature), critical (onset and offset) temperature, variation of transition temperature, critical current density, hole-carrier concentration, grain size, phase purity, lattice parameter, texturing, surface morphology, element distribution, density, porosity, crystallinity, Vickers microhardness and elastic modulus (E) values of the samples are obtained and compared with each other. The obtained results show that the room temperature resistivity systematically increases with the increment of the Gd content as a result of the hole filling when the onset (T ^onset_c ) and offset (T ^offset_c ) critical temperatures determined from the resistivity curves are found to decrease from 95.2 to 93.6 K and 92.0 to 83.3 K, respectively, showing the presence of impurities and weak links between the superconducting grains. As for the critical current density measurements, the J _c values decrease from 132 to 34 A/cm² as the Gd doping increases. The XRD results give that although the Gd³⁺ ions substituted tend to occupy both the Cu(1) and Cu(2) sites, the ions are more favorable for the Cu(2) site as a consequence no change of the crystal structure. Besides, the peak intensities belonging to major phase (Y123) decrease monotonously with the increment of the Gd content in the system; however, new peaks belonging to the minor phases start to appear after the doping level of x = 0.0250 beyond which these peaks enhance monotonously, resulting in the decrement of the grain size. Further, the Lotgering indices calculated from the XRD patterns indicate that the texturing of the Y123 grains reduces systematically with the Gd content. According to the SEM investigations, the microstructures of the samples prepared degrade slightly with the content up to the doping level of x = 0.025 after which the morphology suddenly deteriorates due to the appearance of the different phases in the system. EDX measurements show that not only do the elements used for the preparation of the Y123 superconductors with and without Gd content distribute homogeneously but also the level of Cu element rapidly decreases with the increment of the Gd content compared to the other elements, illustrating that the Cu²⁺ ions may partly be substituted by Gd³⁺ ions. Moreover, the porosity analyses for the samples depict that the porosity increases with the Gd content, leading to the degradation of the grain connectivity. We also discuss on the mechanical properties of the samples to examine both the elastic modulus and the strength of connection between superconducting grains.