Bi-Ca-Sr-Cu-O superconductors: The connectivity issue

The connectivity problem in Bi-Ca-Sr-Cu-O superconductors is addressed in this paper. Our bulk samples always exhibit a step in the resitivity plot, consequent to which zero resistance is obtained around 75K, although the onset is above 110K. Magnetic measurements indicate the presence of almost equal fractions of two superconducting phases with transition temperatures of 110K and about 80K. The microstructure of these samples has been examined using several transmission electron microscopy techniques. The superconducting phases exhibit a polytypoid type structure with the general formula of Bi2Sr2Can-1 CUnOy, where n takes on values of 1,2,3 and in some infrequent cases 4. The c-parameter and Tc increase with n. In almost all the superconducting grains, the lower Tc (n=1 or 2) polytypoid is observed at the grain boundaries. Thus, the 110K polytypoid in each grain is not “connected” to similar regions in other grains. The lower Tc polytypoid is observed at the grain boundaries due to a reduction of Ca and/or Cu. It is suggested that the controlled addition of PbO enables the formation of a Pb-rich liquid phase which makes the composition near the grain boundaries uniform. Consequently, the step in the resistivity plot disappears. © 1989 IEEE