Significant enhancement of superconducting performances of Bi-2212 fibers through combined sodium substitution and LFZ process

This paper reports the microstructure, magnetoresistivity, electrical and superconducting properties of Bi-2212 fibers with Na⁺ ions incorporated into a superconducting matrix prepared by a polymer solution method and additionally textured through the laser floating zone process. XRD patterns showed that Bi-2212 phase is the major one with mostly (00ℓ) diffractions due to the grain alignment, independently of Na content. SEM micrographs showed that samples are composed of well-stacked and oriented grains. The irreversibility field (H_irr), upper critical magnetic field (H_c2), coherence length (ξ), and activation energies (U) have been calculated using magnetoresistivity measurements and explained based on the thermally activated flux flow (TAFF) model. Considering the resistivity-temperature graph for zero field, T_c values tend to increase from 84.8 K (for the pure sample) to 93.2 K (for 0.075Na sample), slightly decreasing for higher content. Besides, transition temperature width (ΔT_c = T_c^onset − T_c^offset) decreases with the increment in the Na content and reaches its minimum value (ΔT_c = 3.7 K) in 0.075Na sample. However, broadening of superconducting transition has been observed with applied field and T_c values decreased to 76.1 K for the pure sample and 86.8 K for 0.075Na sample. Likewise, the activation energies of the samples also decreases significantly with the increase of the magnetic field and the activation energies of the Na-containing samples are found to be higher than the pure sample at each magnetic field value. H_c2(0) values are calculated as 33.8, 43.8, 50.1, 33.1, and 21.4 T for 0.0, 0.075, 0.10, and 0.20 T Na samples, respectively. As a consequence, referring to all experimental results and theoretical findings, the superconducting characteristics improve regularly with Na-doping until x = 0.075 due to increment in the interaction of superconducting clusters, decrement in weak-links and stabilization of charge carriers in CuO₂ conducting planes.