The Influence of Nano-Sized SnO₂ Doping on Physical and Magnetic Properties of the Bi₂Sr_2-x(SnO₂)_xCa₁Cu_1.75Na_0.25O_y Superconductors

In this study, we have investigated the effect of nano-sized SnO₂ (50 nm) doping on the superconducting properties of Bi-2212 ceramics. In the first stage of the the experiment, the composition Bi₂Sr_2-xSn_xCa₁Cu_1.75Na_0.25O_y where x = 0, 0.05, 0.1 and 0.2 is selected due to the positive effect of sodium substitution on the grain sizes of the Bi-2212. X-ray diffraction results indicate that all the samples have considerable amount of Bi-2212 phases. Scanning electron microscopy (SEM) analysis of all samples clearly shows significant grain growth due to lower crystallization temperature formed by sodium. The lowest resistivity in the nano-sized SnO₂-doped samples at 150 K is obtained for the sample of x = 0.05. The M-H hysteresis loops for all the samples have been measured within ± 2 T externally applied magnetic field range at T = 15 and 25 K, respectively. The undoped and the sample with x = 0.05 SnO₂ have both large M-H loops, indicating the improvement of flux pinning properties of the Bi-2212 as well as enhanced intergrain connectivity. However, the width of the hysteresis loop in samples including high (x > 0.05) SnO₂ contents significantly decreased, indicating deterioration in superconducting properties of the Bi-2212 system. Additionally, the critical current densities (J_c) of all the samples at 15 K are calculated from their hysteresis loop measurements by using Bean's critical state model. When compared with other samples, a slight increase in J_c is obtained for x = 0.05 SnO₂. The results indicate that the optimal contents of sodium (x = 0.25) and nano-sized SnO₂ (x = 0.05) in the Bi-2212 system are effective for achieving enhanced superconductivity properties.