Formation of nucleation centers for vortices in Bi-2223 superconducting core by dispersed Sn nanoparticles

Abstract

Abstract In this study, we investigate the effect of the Sn diffusion at different annealing temperature in the range of 650–850 °C on the microstructural, electrical, mechanical and superconducting properties of the Bi 1.8 Pb 0.4 Sr 2.0 Ca 2.1 Cu 3.0 O y materials prepared by the standard solid state reaction method with the aid of bulk density, dc resistivity (ρ-T), transport critical current density ( J c ), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Vickers microhardness ( H v ) measurements. It is found that all the properties given above are significantly dependent upon both the Sn nanoparticles inserted in the Bi-2223 superconducting matrix and diffusion annealing temperature. Namely, the dc resistivity measurements indicate that all the samples exhibit the metallic behavior as a result of the electron–phonon interaction in the Bi-2223 crystal structure or logarithmic divergence in density of states at the Fermi level. Further, the normal state resistivity decreases with the enhancement of the annealing temperature owing to the increment of the metallic connection between the superconducting grains due to the optimization of the hole density and possible changes in the lattice vibration. Likewise, onset and offset critical transition temperature values tend to enhance with the diffusion annealing temperature, confirming not only the improvement of crystallinity and especially connectivity between the superconducting grains but the increment in the average grain size and mobile hole concentration in the Cu–O 2 slabs of the Bi-2223 system, as well. The decrement in the degree of the broadening stems from the degradation of the porosity, grain boundary resistivity and grain boundary weak-links in the system. Moreover, the results of J c show that the Sn inclusions (highly dispersed nanoparticles) form the effective flux pinning centers for the vortices in the crystal structure, and tightly bound to these nucleation centers. Thus, the J c value steeply increases from 852 A cm −2 to 4307 A cm −2 with the diffusion annealing temperature. Similar findings are valid for the microstructural characteristics obtained from the SEM images belonging to the samples. The annealing temperature of 850 °C develops considerably the surface morphology and interaction between the superconducting grains. Hence, the sample annealed at 850 °C for 24 h exhibits the smoothest and densest surface morphology. Similarly, the bulk density analysis illustrates that the densities regularly rise from 5.582 to 6.102 g/cm 3 with increasing the annealing temperature, confirming that more and more Sn inclusions penetrate into the superconducting grains or over grain boundaries in the crystal structure. This is associated with the enhancement of the connectivity (interaction) between the superconducting grains. At the same time, the results of the XRD experiments display that all the samples composed of two superconducting phases (Bi-2223 and Bi-2212) present the polycrystalline superconducting phases but in different intensity of diffraction lines. With increasing the annealing temperature up to 850 °C, the intensity of diffraction lines belonging to the high- T c phase (Bi-2223) becomes stronger and so the best superconducting properties are observed for the sample prepared at the annealing ambient of 850 °C for 24 h, being even favored by the largest (lowest) cell parameter c ( a ). Furthermore, Vickers microhardness measurements conducted at different applied indentation test load (0.245 N ⩽ F ⩽ 2.940 N) demonstrate that all the Bi-2223 ceramics obey the typical Indentation Size Effect (ISE) behavior, and the mechanical properties increase with the diffusion annealing temperature. The long and short of it is that the Sn inclusions and especially diffusion annealing temperature up to 850 °C are favorable for the formation velocity of Bi-2223 phase.