Abstract
The microstructures of high- T c Bi(Pb)-2223 tapes in Ag sheaths doped with Hf have been examined by high resolution transmission electron microscopy. Different microstructural features are found in samples with different annealing times. The J c values increase with annealing time, reach a maximum after 150 h at 827°C and then decrease. The low J c after a short annealing time is attributed to a poor connectivity of the grains and the presence of frequent stacking faults due to an incomplete formation of Bi(Pb)-2223. For longer annealing times a deterioration of the superconductor is observed which starts at grain boundaries and leads to the formation of amorphous phase between adjoining grains. The samples with the highest J c reveal a well-developed texture with large plate-like grains strongly linked together. Many defects like dislocations, small voids, small and large angle grain boundaries and, in particular, tilt-type subgrain boundaries are possible pinning centers of magnetic flux.
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