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 resistivity 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 Bi/sub 2/Sr/sub 2/Ca/sub n-1/Cu/sub n/O/sub y/, where n takes on values of 1,2,3 and in some infrequent cases 4. The c-parameter and T/sub c/ increase with n. In almost all the superconducting grains, the lower T/sub c/(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 T/sub c/ polytypoid is observed at the grain boundaries due to a reduction Ca and/or Cu. It is suggeted 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. 19 refs., 6 figs.
- Research Organization:
- Lawrence Berkeley Lab., CA (USA)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 6810375
- Report Number(s):
- LBL-25842; CONF-880812-7; ON: DE89000558
- Country of Publication:
- United States
- Language:
- English
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75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ALKALINE EARTH METAL COMPOUNDS
BISMUTH COMPOUNDS
BISMUTH OXIDES
CALCIUM COMPOUNDS
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OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
PHYSICAL PROPERTIES
POINT DEFECTS
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STRONTIUM OXIDES
SUPERCONDUCTORS
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VACANCIES