TY - JOUR KW - High temperature superconductors KW - Superconducting materials KW - Oxide superconductors KW - Microscopic Examination–Transmission Electron Microscopy KW - X-ray Analysis KW - Magnetic Measurements KW - Bismuth Calcium Strontium Copper Oxides KW - Polytypoid KW - Superconductor Connectivity Problem AU - Ramamoorthy Ramesh AU - G Thomas AU - Y Mej AU - H.L Luo AB - 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 BT - IEEE Transactions on Magnetics DO - 10.1109/20.92782 LA - eng M1 - 2 N1 - cited By 1 N2 - 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 PY - 1989 SP - 2360 EP - 2363 T2 - IEEE Transactions on Magnetics TI - Bi-Ca-Sr-Cu-O superconductors: The connectivity issue VL - 25 SN - 00189464 ER -