MICROSTRUCTURE EVOLUTION IN HIGH-T-C BI(PB)-2223 AG(HF) TAPES/

The microstructures of high-T-c Bi(Pb)-2223 tapes in Ag sheaths doped with Hf have been examined by high resolution transmission electron mi croscopy. Different microstructural features are found in samples with different annealing times. The J(c) values increase with annealing ti me, reach a maximum after 150 h at 827 degrees C and then decrease. Th e low J(c) after a short annealing time is attributed to a poor connec tivity 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 grai n boundaries and leads to the formation of amorphous phase between adj oining grains. The samples with the highest J(c) reveal a well-develop ed texture with large plate-like grains strongly linked together. Many defects like dislocations, small voids, small and large angle grain b oundaries and, in particular, tilt-type subgrain boundaries are possib le pinning centers of magnetic flux.