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 s...