Position-sensitive measurements of the local critical current density in Ag sheathed high-temperature superconductor (Bi, Pb)2Sr2Ca2Cu3Oy tapes

Suppression of magnetic granularity by transport current in (Bi,Pb)2Sr2Ca2Cu3Ox tapes

“…10,11 It was shown that J"r… could be deduced from the maps of B"r… and that the J c was markedly higher near the Ag-BSCCO interfaces, where colonies of well-aligned longer grains are mostly located. This observation verifies the inferences of previous microstructural 12,13 and electromagnetic 7,14 studies. Central parts of the tapes having smaller grains and more second phase exhibited a pronounced magnetic granularity characterized by percolative current patterns consisting of an array of current loops much larger than the crystallites themselves.…”

“…4 Extrinsic factors, such as irregularities of the Ag-BSCCO interface, 5 the grain-to-grain alignment, distributions of low-T c intergrowths 6 and cracks 7 play a significant role too. This results in strong inhomogeneities in J c (r), giving rise to percolative current flow [7][8][9][10] and an effective current-carrying cross section smaller than the real cross section of the tape. Thus the identification of current paths in BSCCO/Ag tapes is of great importance to understanding the real potential of high-temperature superconductors.…”

Evolution of the transport critical current density and irreversibility field as a function of heat treatment and pressing pressure during processing of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes

“…To understand what is happening at a fundamental level, it is also desirable to understand how the active cross-section is changing. This is a very challenging task, so far only accessible by experiments such as microslice 6,20 or MO imaging, [7][8][9] in which the local J c is probed on scales much smaller than the transport measurement. Unfortunately, it was not feasible to perform these experiments on these samples, although an extensive MO imaging experiment has been performed on a sample of the three heat treatment, two GPa tape.…”

“…Variations in J c defined in this way cannot be easily interpreted if the properties vary within a cross-section A, and/or if different current-limiting mechanisms dominate at different stages in the processing. Transport measurements on different regions of a tape core provide direct evidence for just such transverse inhomogeneity in optimized samples, 6 and magneto-optical ͑MO͒ images [7][8][9] reveal the resulting percolative current flow. Thus, there is an inherent uncertainty involved in optimizing J c ϭI c /A through a process which influences both the intrinsic flux pinning capability of the material and the connectivity of the polycrystalline crosssection.…”

Fabrication and testing of long length high-{Tc} composite conductors. Final report

“…To meet this requirement both high Jc and good control over the homogeneity of the transport current path must be achieved over long tapes. Larbalestier et a1 [40] pointed out that the problems caused by macroscopic defects must be first overcome before full benefits from the optimization of the structure of Bi-2223 ceramic can be expected. The conventional resistive four-probe method is not very convenient method t o obtain information about sample inhomogeneities.…”