Flux pinning mechanism of Bi-2223 bulk superconductors by tuning Bi-2212 and Bi-2234 intergrowths

Abstract

In this study, we have regulated the density of Bi-2212 and Bi-2234 intergrowth layers by tuning the Cu content. The phase composition and microstructural changes have been investigated by employing X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and aberration-corrected transmission Electron Microscope (AC-TEM) analysis. It has been noticed that with the increasing Cu content, a chemical reaction between the Bi-2212 phase and the Pb-rich phases can be promoted, thereby efficiently facilitating the formation of the Bi-2223 phase. Consequently, a remarkable 1.4-fold increase in self-field critical current density (self-field J_c) is observed, which can be attributed to the maximum volume fraction of the Bi-2223 phase (97.5%). With further increase of Cu content, a high density of Bi-2234 intergrowths was observed within Bi-2223 grains with a high Cu content (3%), resulting in a decline in flux pinning force (F_p). Conversely, Bi-2223 bulks with a 0% Cu addition, characterized by a high density of Bi-2212 intergrowth, demonstrate superior F_p (∼10⁹ N/m³) under high magnetic fields (4.2 K, 10 K, and 20 K). In summary, the phase composition and density of intergrowths within Bi-2223 grains can be systematically tuned with the change in Cu content. Compared to Bi-2212 intergrowth, the appearance of Bi-2234 intergrowth can decrease the superconducting uniformity (ΔT_c) and intra-granular pinning force (α_g(0)) of Bi-2223. Consequently, atomic point defects (cations) within the Bi-2223 grains may transform into Bi-2234 intergrowths. Since the flux pinning mechanism of Bi-2223 under high fields is dominated by point pinning, surface defects (high density of Bi-2234 intergrowths) cannot effectively serve as pinning centers, resulting in a rapid decline in current-carrying performance.