Anisotropy of the critical current in silver sheathed (Bi,Pb)2Sr2Ca2Cu3O10 tapes

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Abstract
The transport critical current of Bi:2223 tapes was measured in magnetic fields up to 15 T and at temperatures from 4.2 to 84 K. At high temperatures, the critical current density Jc is strongly anisotropic and the anisotropy increases rapidly with magnetic field, whereas at low temperatures the critical current is less anisotropic and the anisotropy is field independent above 1 T. In the former case we believe we are in a regime, where pinning limits Jc, at least within some parts of the tape, whereas in the latter case the limitation of Jc by Josephson weak links seems to be the dominant mechanism. In addition, a critical current hysteresis induced by flux trapping in a weak link network is observed, which is more pronounced at low temperatures. From transmission electron microscopy observations of the microstructure we find that the ‘‘brick’’ in the ‘‘brick wall’’ model turns out to be the colony instead of the grain inside the colony. Additionally it is found that colony boundaries parallel the ab plane and intersected boundaries occur much more frequently than boundaries parallel to the c axis, due to the misalignment of the colonies inside the tape. In a small region near the silver sheath colony misalignment is much smaller and boundaries parallel to the c axis may act as strong links at high temperatures as their interfaces are very clean and well matched.