Selenium doping in Bi-based superconductors

Six different compositions in Bi2Sr2Ca2Cu3Oy:Se-x with x=0.0, 0.05, 0.1, 0. 2, 0.3, 0.4 have been prepared by the solid state reaction method to explor e doping of selenium in the system. Sintering at 847+/-4 degrees C and subs equent annealing at 827 degrees C has been carried out for different time p eriods. The samples have been characterized by X-ray powder diffraction (XR D), scanning electron microscopy (SEM), and measurement of resistance below room temperature has been carried out by four-probe method. Superconductin g transition temperature (T-c) value shows dependence on Se concentration: the highest T-c(0)=94 K is observed for x=0.3. A detailed Auger electron sp ectroscopic (AES) analysis has been carried out to investigate the presence of Se in the grains of the superconductors. It has also been found that Se increases the T-c(0) value and promotes the formation of high temperature superconducting phase (2223), which coexists with low temperature phase (22 12) in the sintered samples.