Refinement of the Bi-2212 characteristics through sodium-lithium co-doping

The present study focuses on the micro-structural, electrical, superconducting, hysteresis properties of Na-Li codoped Bi-2212 bulk ceramic superconducting materials prepared by solid-state reaction method. The XRD patterns of the samples are largely consistent, primarily displaying Bi-2212 peaks along with some weaker peaks from Bi-2201 and Bi-2223 phases. The average crystallite size (D) increased with Li concentration, ranging from 50.11 nm to 56.93 nm. SEM photographs captured randomly oriented platelet-type grains revealing the typical morphological feature of the Bi-2212 system prepared by the solid-state method. The images also show that substituting lithium with copper increases the grain sizes. Lithium doping is known to lower the melting point and crystallization temperature of Bi-2212, resulting in the formation of larger grains and the expansion of molten regions, thereby improving the connectivity between superconducting grains. These regions of partial melting are more pronounced in backscattered electron (BSE) mode images for samples with higher lithium content, attributed to the eutectic formation of Li2O and Bi2O3 at 700 degrees C. EDS analysis identifies the primary phase as Bi-2212, while a secondary Bi-poor phase exhibiting varying concentrations of Sr, Ca, Cu, and O is also present. The resistivity-temperature graph for zero fields shows that the residual resistivity values (rho 0) gradually drop 44.35 m Omega cm to 0.81 m Omega cm, while the Tcoffset values tend to grow routinely from 71.1 K to 77.1 K with the Na-Li-content. All samples containing Na and Li possess higher flux pinning energy (U) than the pristine one, suggesting that Na and Li dopants enhance the energy barriers of the pinning centers. In the same vein, magnetic hysteresis loops confirmed that Na and Li-doping enhances flux pinning, as indicated by higher remanent magnetization (MR) values, which increased from 9.88 emu/cm3 in the pure sample to 21.41 emu/cm3 in the Na-Li3 sample at 10 K. Similarly, all co-doped samples exhibited higher Jc values, further confirming the role of Na and Li in improving grain morphology and intergranular connectivity. Overall, these findings suggest that Na and Li co-doping optimizes the superconducting properties of Bi-2212, enhancing its potential for practical applications.