| Bi2Sr2Ca2Cu3Oio+s (Bi-2223) and YBa2Cu3O7-5 (YBCO) are the most promising high-temperature superconductors for electric power devices as well as high-field magnets operated at elevated temperature. In this thesis, we use advanced transmission electron microscopy to study the formation of these two important superconductors and their structural characterizations systematically .First, the alignment and the formation of Bi-2223 inside an Ag sheath were investigated for tapes from four kinds of different precursor powder. Microstructural characterization revealed that the kinetics depended strongly on the processing technique and the phase assemblage of the precursors. The alignment process was governed by the preferential grain growth of the Bi-cuprates along the a-b plane and the constraint applied by the Ag sheath. The formation mechanism of Bi-2223 was either the intercalation or the nucleation and growth, depending on whether excessive liquid is adjacent to the platelets. The structures of the intermediate phases observed during the conversion of (Bi,Pb)2Sr2CaCu2O8+s (Bi-2212) to (Bi,Pb)2Sr2Ca2Cu3Oi0+6 were also determined. The intermediate phase (Ca,Sr)5+x(Pb,Bi)3+yCuO8 has a c-centered monoclinic lattice with a=1.723nm, b=0.978nm, c=0.342nm, and ct=93.5? while both (Ca,Sr)CuO2 and (Ca,Sr)i4Cu24O4i+g are phase mixtures with nano-precipitates randomly imbedded in their matrix. The structure of the matrix for the former has face-centered monoclinic symmetry with a=1.105nm, b=0.643nm, c=0.347nm, and p=92.3? the precipitates for the latter are face-centered orthorhombic with a~1.147nm, b=1.339nm and c=0.277nm. Furthermore, we discussed the structural evolution of these intermediate phases and their relation to Bi-2212 and Bi-2223.Second, the nucleation and growth of YBCO on SrTiOs and CeO2 substrates from precursor films in so-called BaF2 process which is a postdeposition reaction process for the synthesis of epitaxial YBCO films were studied by examining quenched and fully processed specimens using transmission electron microscope techniques. The precursor films, a stoichiometric mixture of fine-grained Y, Cu and BaF2, were deposited using physical vapor deposition methods. An (Y,Ba)-oxy-fluoride and Cu2O formed from the precursor after a short time heating in an atmosphere of flowing process gas, N2, O2 and H2O. It is shown that the preferential nucleation of YBCO at the interface is due to the strong chemical affinity of the (Y,Ba)-oxy-fluoride to SrTiOa and the epitaxial alignment of its (111) planes with the (001) surface of the SrTiO3 which reduces the activation barrier for the formation of YBCO. In the thin films (<2-3urn) the YBCO nuclei, whose c axis are perpendicular to the SrTiOa surface, form directly from this aligned oxy-fluoride or ordered oxy-fluorides. In thick films (5^im), however, this oxy-fluoride decomposes into a disordered transitory cubic phase which then orders to form YBCO nuclei with three orientational variants, one with its c axis perpendicular and two with their c axes parallel to the (001) surface plane of SrTiOs. This fact explained the formation of a/b axes YBCO in the thick films. For the growth of c-axis YBCO, we found that once the c-axis YBCO covers substrate surface, a thin liquid layer forms between YBCO and unreacted precursor film. The mechanism for the growth of c-axis YBCO layer is shown to be the homo-epitaxial precipitation of YBCO onto the existing c-axis-orientated YBCO layer from the thin liquid layer containing Y, Ba, Cu and O. The (Y,Ba)-oxy-fluoride, whose approximate composition is (Yo.3,Bao.7)(Oo.i5,Fo.85)2, decomposes at its interface with the liquid releasing HF to the process gas. The decomposition of this phase and CuO provides necessary cations and oxygen to the liquid. The structure of thedisordered and ordered oxy-fluorides, transitory phase and a reacted phase between precursor and SrTiO3 substrate have been determined by electron diffraction, EDS, EELS and high resolution images.We developed a novel electron diffraction technique...
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