MICROSTRUCTURES IN HIGH-TC BI(PB)-FAMILY-2212 SUPERCONDUCTORS AS REVEALED BY SCANNING AND TRANSMISSION ELECTRON-MICROSCOPY

Microstructures of Bi(Pb)-family 2212 superconductors, which were prep ared by a sol-gel method with three different compositions, were exami ned mainly by scanning and transmission electron microscopy. The magne tization of the specimens strongly depends on the ratio between Bi and Pb content, while Tc is almost constant. In specimen 1, prepared with the nominal composition of Bi/Pb = 9/1, small grains of 2212 phase ar e formed with a minor fraction of some impurity phases. In specimen 2, with Bi/Pb = 17/3, which is optimum from the viewpoint of magnetizati on, large grains of the 2212 phase are formed during heating at 800 de grees C, also with the impurity phases. In specimen 3, with Bi/Pb = 8/ 2, the 2212 grains are divided by layers of(Bi,,,, Pb-0.14) (Ca-0.7, S r-0.3)O-x. Moreover, plate-like 2212 crystals are severely bent so tha t small cracks appear often with an inclusion of amorphous substance b eing rich in Ca and Pb. These layers and cracks must degrade the magne tization. A modulated structure of Bi-type is formed in the 2212 grain s of specimens 1 and 2, while not only Bi-type but also Pb-type are fo rmed in specimen 3. The wavelength of Pi-type is different for each sp ecimen. (C) 1995 Wiley-Liss, Inc.