The effects of long-term annealing on superconducting properties in the Bi-Sr-Ca-Cu-O system

The effect of long-term annealing on the relative proportions of the low- and high-Tc phases present in the Bi-Sr-Ca-Cu-O system was examined by resistivity and ac susceptibility measurements. The zero-resistivity offset was found to increase from 50 to 68 K and the susceptibility measurements showed the disappearance of the low-Tc , 63-K phase with the enrichment of the high-Tc , 110-K phase as a result of the annealing treatment. Also, the onset of diamagnetism of the high-Tc phase was enhanced by 3 K due to prolonged annealing. The high-Tc phase thus formed is poorly crystallized as determined by the x-ray diffraction patterns. Some impurities are observed in both x-ray diffraction patterns and scanning electron microscopy pictures and are assigned to the low-Tc , Sr, and Bi-doped Ca2CuO3 phases. This study shows that a complex ac susceptibility measurement technique can be a very powerful tool for studying the equilibrium superconducting phases present at a particular temperature in a multicomponent system such as Bi-Sr-Ca-Cu-O.