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Current distribution and microstructure studies of isolated filaments extracted from Ag-sheathed Bi(2223) tapes

All the filaments extracted from an Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O10+x tape, prepared by the oxide-powder-in-tube technique, have been isolated by chemical etching of the silver matrix, and individually characterized by keeping track of their position inside the conductor. Different types of measurements have been performed on each filament: magnetization hysteresis loops and zero-field cooled DC susceptibility by SQUID magnetometry; electrical resistivity by the four-probe method; phase purity and texture by x-ray diffraction analysis; and stoichiometry by scanning electron microscopy/energy dispersive x-ray analysis. The critical-current density jc of every filament has been calculated from the magnetization hysteresis loops. At 77 K it has been found that the innermost filament carries a jc of 53 kA cm-2, i.e. much larger than for all the filaments of the outer shell (with jc ranging between 25 and 40 kA cm-2). These results are consistent with the variation in room temperature electrical resistivity observed on the same samples. To some extent the observed current distribution can be imputed to the different amount of cold working experienced by the filaments. On the other hand, we have found that the average stoichiometry of the central filament matches well with that of the initial precursor powders, while in the filaments of the outer shell a consistent loss of Pb has been observed. The outer filaments also present a lower oxygen content.