Investigation of Lu effect on YBa₂Cu₃O_7−δ superconducting compounds

This study reports the effect of Lu addition on the microstructural and superconducting properties of YBa₂Lu_xCu₃O_7−δ (Y123) superconducting samples with x = 0, 0.1, 0.3, 0.5 and 0.7 by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistivity and transport critical current density (J_c) measurements. The samples prepared by the liquid ammonium nitrate and derivatives are exposed to various annealing time (20, 40 and 60 h) and temperature (950, 960 and 970 °C), and the best ambient for the sample fabrication is determined to be 970 °C for 20 h. Zero resistivity transition temperatures (T_c), critical current densities (J_c), variation of transition temperatures, hole-carrier concentration, grain size, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the bulk superconducting samples prepared at 970 °C for 20 h are compared with each other. T_c and J_c values of the samples are inferred from the dc resistivity and the critical current measurements, respectively. The results show that the T_c value of the pure sample is about 90.6 K while the sample doped with 0.1 wt% Lu has the maximum T_c value (92.5 K). However, beyond x = 0.1, the T_c value is observed to decrease toward to 83.5 K with increment in the Lu addition. Similarly, the J_c values measured are found to reduce from 142 to 76 A/cm² with the addition. Moreover, XRD measurements show that both pure and Lu-doped samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain Y123 and Y211 phase, confirming the incorporation of Lu atoms into the crystalline structure of the samples studied. At the same time, comparing of the XRD patterns of samples, the intensity ratio of the characteristic (110) and (013) peaks on the sample doped with 0.1 wt% Lu is more than that on the other samples prepared. Additionally, SEM images display that the sample doped with 0.1 wt% Lu obtains the best crystallinity, grain connectivity and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.7). Further, EDX results demonstrate that the Lu atoms doped are successfully introduced into the microstructure of the Y123 samples studied and the maximum Cu element level is observed for the sample doped with 0.1 wt%, explaining that why this sample obtains the best superconducting properties compared to others. According to all the results obtained, it is concluded that the 0.1 wt% Lu addition into the Y123 system improves the microstructural and superconducting properties of the samples studied.