Effect of zirconium diffusion on the microstructural and superconducting properties of YBa2Cu3O7-delta superconductors
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This study reports not only the role of zirconium (Zr) diffusion on the superconducting, physical and microstructural properties of YBa2Cu3O7-delta (Y123) bulk superconductors prepared by the conventional solid-state reaction method with the aid of scanning electron microscopy (SEM), electron dispersive X-ray (EDX), X-ray diffraction (XRD), electrical resistivity (rho-T) and transport critical current density (J(c)) measurements but also the diffusion coefficient and the activation energy of zirconium in Y123 system for the first time. Zr diffusion is examined in the temperature range 500-945 degrees C via the variation of the lattice parameters evaluated from the XRD patterns. It is found that while the room-temperature resistivity reduces in the presence of the Zr ions in the system, the critical transition temperature values measured enhance. The onset (T-c(onset)) and offset (T-c(offset)) critical temperatures are found to be about 94.4 K and 91.5 K for the Zr-diffused sample as against 93.7 K and 89.4 K, respectively, for the pure sample. The increase in the critical temperatures is due to the improvement of crystallinity and the enhancement in the grain size. Likewise, the presence of the Zr atoms in the system, the J(c) value enhances from 128 A cm(-2) to 160 A cm(-2) at 77 K owing to the increase of the flux pinning mechanism stemming from the stacking faults, planar and micro-defects. Further, XRD measurements display that the samples produced in this work exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain the Y123 phase only, illustrating the incorporation of Zr atoms into the crystalline structure of the Zr-diffused sample. Based on the refinement of cell parameters done by considering the structural modulation, the Zr diffusion is confirmed by both a reduction of a and b lattice parameters and an enhancement of the cell parameter c (about 0.24%) of the sample in comparison with that of the pure sample. As for the SEM investigations, the crystallinity and grain connectivity improve with the Zr diffusion. According to the results obtained, the T-c(onset), T-c(offset) and J(c) values of the samples studied are found to depend strongly on the microstructure. Moreover, EDX measurements indicate that the elements used for the preparation of samples distribute homogeneously. Y3+ and Ba2+ cations may partly be substituted by Zr4+ ions and the substitution at the Cu site is less likely, presenting that the superconducting properties of the Zr-diffused sample are more superior to the pure sample. Further, the diffusion coefficient of the Zr in Y123 system in a range of 500-945 degrees C is determined to change from 4.12 x 10(-10) to 9.77 x 10(-8) cm(2) s(-1) with activation energy of about 1.010 eV, confirming that the Zr diffusion at lower temperatures is much less significant and the Zr ions primarily proceed through defects in the polycrystalline samples. (C) 2012 Elsevier B.V. All rights reserved.