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

The connectivity problem in Bi-Ca-Sr-Cu-O superconductors is addressed. The bulk samples always exhibit a step in the resistivity plot, consequent to which zero resistance is obtained around 75 K, although the onset is above 110 K. Magnetic measurements indicate the presence of almost equal fractions of two superconducting phases with transition temperatures of 110 K and about 80 K. 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 Bi₂Sr ₂Ca_n-1Cu_nO_y, where n takes on values of 1,2,3, and, in some infrequent cases 4. The c-parameter and T_c increase with n. In almost all the superconducting grains, the lower T_c (n=1 or 2) polytypoid is observed at the grain boundaries. Thus, the 110 K polytypoid in each grain is not `connected´ to similar regions in other grains. The lower T_c 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 allows 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