Sn diffusion coefficient and activation energy determined by way of XRD measurement and evaluation of micromechanical properties of Sn diffused YBa₂Cu₃O_7−x superconducting ceramics

Abstract

This study manifests not only the effect of Sn diffusion on physical, electrical, mechanical, structural and superconducting properties of the bulk YBa₂Cu₃O_7−x (Y123) superconductors prepared by the conventional solid-state reaction route by use of electrical resistance, X-ray diffraction analysis (XRD), electron dispersive X-ray, scanning electron microscopy, transport critical current density (J _c ) and Vickers microhardness (H _v ) measurements but also the diffusion coefficient and the activation energy of tin (Sn) in the Y123 material for the first time. The diffusion coefficient and the activation energy of Sn are investigated in the temperature range 500–945 °C using the change of the lattice parameters extracted from the XRD patterns. The resistance (at room temperature), critical (onset and offset) temperature, variation of critical temperature, hole-carrier concentration, crystallinity, lattice parameter, texturing, surface morphology, lotgering index, element distribution, critical current density, oxygen content, load dependent microhardness, elastic modulus and yield strength values are obtained for the pure and Sn diffused samples and compared with each other. One can see that all superconducting parameters given above depend sensitively on the Sn diffusion on Y123 system. The obtained results exhibit that the room temperature resistance enhances with the Sn diffusion because of the hole filling when the onset $(T_{c}^{onset} )$ and offset $(T_{c}^{offset} )$ critical temperatures are obtained to be about 93.4 and 89.6 K for the pure sample as against 92.2 and 88.1 K for the Sn diffused sample, respectively. This may be attributed to the fact that the decrement in the critical temperatures is due to the deterioration of crystallinity and descend in the grain size. As for the critical current density measurements, J _c values are obtained to be about 125.4–65.3 A/cm², respectively, for the undiffused and Sn diffused materials. This may be led to the decrease of the flux pinning mechanism stemming from the stacking faults, planar and micro-defects. At the same time, XRD measurements display that the samples produced in this work exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines. Besides, the peak intensities belonging to major phase (Y123) decrease monotonously with Sn diffusion in the system; however, new peaks belonging to the minor (BaCuO₂) phases start to appear for Sn diffused sample confirming both the reduction of the grain size and degradation of the critical temperature. Moreover, the pure sample is confirmed by both enhancement of a and b lattice constants and the decrement of the cell parameter c of the sample in comparison with that of the Sn diffused sample. According to SEM examination, the crystallinity and grain connectivity suppress with the Sn diffusion. EDX measurements illustrate that not only do the elements used for the preparation of the Y123 superconductors with and without Sn diffusion distribute homogeneously but also the level of Cu element reduces with the Sn diffusion, presenting that the Cu²⁺ ions may partly be diffused by tetravalent tin (Sn⁴⁺) ions. Further, surprising results of the Vickers microhardness values demonstrate that the pure sample visualizes Indentation Size Effect (ISE) feature; however, the Sn diffused sample reports Reverse Indentation Size Effect (RISE) nature. Additionally, the diffusion coefficient is observed to increase from 1.11 × 10⁻⁹ to 2.82 × 10⁻⁸ cm² s⁻¹ as the diffusion-annealing temperature increases, verifying that the Sn diffusion at lower temperatures is much less significant as compared to the higher ones. Temperature dependence of the Sn diffusion coefficient and activation energy in the range of 500–945 °C is defined with the aid of the following equation: $D = 7.78 \times 10^{ - 6} { \exp }\left[ {\left( {( - 0.590 \pm 0.005){\raise0.7ex\hbox{${\text{eV}}$} \!\mathord{\left/ {\vphantom {{\text{eV}} {k_{B} T}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${k_{B} T}$}}} \right)} \right]$.