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Structural Characterizations of Bi2Sr2CaCu2O8FATAL ERROR -- Missing abstract.plus;FATAL ERROR -- Missing abstract.dgr; Twist Boundaries Using Advanced Transmission Electron Microscopy

Recently, considerable efforts have been made in growing bi-axially aligned thick Yba2cu3O7 (YBCO) films on a flexible, textured metallic substrate for electrical power applications. The BaF2 post-deposition annealing process is one of the most promising methods. to understand the nucleation and growth mechanism of YBCO in this process, extended transmission electron microscopy analysis has been made. Here, we report on the evolution of Ba-Y oxy-fluoride which is pertinent to the epitaxial YBCO nucleation process on SrTiO3 (STO) substrate.The stoichiometric ratio of Y, Cu and BaF2 was evaporated on (00l)-cut STO substrate Subsequently, these films were annealed at 735°C in an O2-H2O-N2 gas atmosphere and quenched after 10, 20, or 40 minutes. The morphology and phase composition of the precursor changed drastically with the annealing times, e.g. after 10 minutes heat treatment, the precursor consisted of fine grained CU2O and Ba-Y(Cu) oxy-fluoride (cubic, a=0.618nm) as shown in fig.la.

The development of YBa2Cu3O7 (YBCO) thick films on a flexible, textured, metallic substrate has recently attracted considerable scientific and technical interest. It is well known that the interfacial microstructure plays an important role on the formation and alignment of the YBCO films. Thus, understanding the evolution of the phases near the substrate interface will help us to improve the quality of the films. In this study, BaF2, Cu, and Y were deposited on the (001) CeO2/LaAlO3 substrates without external heating in vacuum. Subsequently, a series of these films were annealed at 735°C for preselected time periods and then quenched. Similar to YBCO films on SrTiO3 substrates, (Ba, Y, Cu)(F,O)2 (precursor) and Cu2O phases were formed after annealing for 10 minutes. After 12 hours annealing, the formation of YBCO film was completed.

Recently, considerable efforts have been made in developing a fabrication procedure for YBa2 Cu3O7-δ superconducting films on a metallic substrate tape for electrical power applications. The central issue is to develop a method, which can produce the tape in sufficiently long lengths at a commercially viable cost. One possible approach is the so-called BaF2 post-deposition annealing process, which involves deposition of an unreacted mixture of a BaF2 -Y-Cu-O layer on a substrate by electron beam evaporation method, followed by heat-treatment of the layer in a mixed-gas atmosphere of a low partial pressure O2, saturated H2 O, and N2 . To fabricate thick and long films, it is important to understand the effects of the heat-treatment parameters on the. structure and the properties. Here, we report our structural analysis of the films annealed at different temperatures.The films annealed at 725°C, 750°C and 800°C have a similar phase assemblage which are composed ofYBa2 Cu3O7-δ and Y2O3, CuO and Ba-Ti-Sr-O precipitates, as shown in fig. la.

Over the last few years, considerable efforts have been made in developing fabrication processes for thick superconducting YBa2Cu3O7-δ film on a metallic substrate for electric power applications. Since these applications require the composite tape to be longer than 4-500m, any methods chosen need to be compatible with such length requirement. One possible approach is the so-called BaF2 process which involves deposition of a unreacted mixture of a BaF2-Y-Cu-O layer on a substrate by an electron beam evaporation method, followed by heat-treatment of the layer in a mixed-gas atmosphere of a low partial pressure O2, saturated H2O, and N2. Although it was shown previously that c-axis aligned films up to ∼ I m long can be produced under these conditions it is uncertain how thick the films can be made and this is of crucial importance as most applications require film thickness to be greater than ∼5μm to carry a large, dissipationless current.

We systematically investigated the structure and properties of [001] twist boundaries using Bi2Sr2CaCu2O8+δ (Bi/22112) bicrystals. Contrary to conventional wisdom, all these boundaries, regardless of their misorientation angle, carried the same critical current as their constituent single crystals in magnetic fields up to 9 tesla. Fig. 1 shows the ratio of the critical currents across a grain boundary to that within the grain interior as a function of misorientation of the boundaries. In striking contrast to the results of Dimos et al. with YBa2Cu3O7−δ, the twist boundaries in our bicrystals are not a limiting obstacle for supercurrent.The origin of the robust superconducting behavior at these twist boundaries was sought by detailed structural characterization using various TEM techniques. Several notable structural features were observed: 1) all the boundaries were clean, structurally intact without any visible amorphous materials; 2) nano-probe EDS and EELS measurements showed that there was no detectable off-stochiometric composition, including oxygen/hole concentration along and across the boundaries; 3) HREM image simulation revealed that the boundaries were located in the middle of the double BiO layers without exception (Fig.2); 4) there was no detectable boundary expansion, contrary to general expectation, and the inter-planar distance of the double BiO layer {dBio =0.309± 0.005nm, measured with line-scan (Fig.3)) at the boundary was the same as those far from the boundary within measurement error; and 5) very often, there was an intercalation of a Ca/CuO2 bi-layer near the boundary, either on one, or both sides, forming a local Bi/2223 structure (Fig.2).