Examination of effective nucleation centers for flux pinning of vortices and optimum diffusion annealing temperature for Au-diffusion-doped Bi-2212 polycrystalline compound

Abstract

This comprehensive study declares experimentally the effect of diffusion annealing temperature in a range of 600 °C–850 °C on the electrical, superconducting and microstructural properties of the Au-diffusion-doped Bi-2212 polycrystalline compounds with the aid of the available experimental methods such as bulk density, dc resistivity (ρ-T), transport critical current density (Jc), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and electron dispersive X-ray (EDX) measurements. The experimental findings of the dc electrical resistivity and XRD investigations present that the Au impurities are effectively entered into the crystalline structure of the Bi-2212 system. In fact, the EDX examinations confirm that the Au impurities may mostly be substituted for the Sr-sites in the Bi-2212 crystal matrix due to the increase of electronegativity. Besides, the Jc measurements present that highly dispersed Au impurities with higher electronegativity bind tightly to form the effective nucleation centers through the intra-grain and inter-grain boundaries for the flux pinning of the vortices in the Bi-2212 crystal structure. Furthermore, it is observed that the vital characteristics, being responsible for newly/novel and feasible market areas for the universe economy, improve significantly with the increment in the diffusion annealing temperature up to the value of 800 °C as a consequence of the retrogression in the disorders, dislocations, defects, lattice strains, misorientations and local structural distortions in the Cu-O2 consecutively stacked layers, leading to the development of the metastability (enhancement of the hole trap energy) belonging to the superconducting materials studied. However, after the critical temperature point, the properties recrudesce dramatically due to the penetration of excess Au foreign impurities in the Bi-2212 crystal structure. In more details, the XRD measurement findings display that the diffusion annealing temperature promotes significantly the high phase as a consequence of the increment in both the average crystallite size and c-axis length or decrement in the constant parameter a. In this respect, the solid compound produced at the annealing temperature of 800 °C obtains the maximum (minimum) c-axis length of 35.43 Å (5.32 Å) and average grain size of 68.7 nm. Similarly, the same sample exhibits the smoothest and densest surface morphology with the best interaction between the superconducting grains and largest crystalline distribution. Finally, the highest Tconset and Tcoffset values are noted to be about 89.78 K and 89.01 K, respectively. This is attributed to the fact that the Au nanoparticles penetrated transit from inherently overdoped nature of the Bi-2212 compound to optimally doped state. To sum up, the annealing temperature of 800 °C favors the Bi-2212 superconducting ceramics for usage in the large scale applications.