| In this dissertation, we used Bi-2212 synthetically and naturally grown bicrystals for grain-boundary studies of electromagnetic properties as well as structural characteristics of many [001] twist and [100] tilt grain boundaries, which are the two most abundant grain boundaries present in Bi-tapes. Electromagnetic properties were carefully measured in various temperature and magnetic field ranges. In the study of the [001] twist grain boundary, we found that the grain boundary in the Bi-2212 bicrystal can transport as much critical current as its constituent single crystal at magnetic fields up to 9 T regardless of misorientation angle. Microstructures of the [001] twist grain boundaries were characterized by 0.17 nm high-resolution TEM (HREM), energy dispersive spectrometry (EDS), and electron energy loss spectrometry (EELS) in several bicrystals. The results also strongly supported our observation. We found no lattice expansion along the c-axis, no detectable off-stoichiometric composition, or amorphous phases near the grain boundaries. Arrays of Josephson junctions continue uninterruptedly from one single crystal across the grain boundary to the next single crystal without changing the coupling strength due to the in-plane rotation of the Cu-O double layers. Studies also show that the pairing mechanism of [001] twist grain boundaries is not pure dx2-y 2 type since the values of critical current across the grain boundaries are not affected by the inplane rotation from 0° to 180°. To increase the flux pinning in our bicrystals to the level of those grains present in the Bi-tapes, we irradiated a dozen bicrystals with 0°, 45°, 90° [001] twist grain boundary by 2.2GeV Au-ion beams along the common c-axis at a fluence of 1.0 x 1011/cm2. Our results show that the Ic across the grain boundaries is not significantly enhanced in the low temperature region even though large improvement on the irreversible temperature has been found in both grain boundary and single crystals. Therefore, we suggest that heavy ion irradiation will not dramatically increase the Ic carrying ability of highly textured Bi-tape at low temperature if the c-axis twist grain boundaries are the only current conducting path in the tapes.; Many striking results were also found in the [100] non-basal-plane-faced tilt grain boundaries. Large Tc depression is usually observed in this type of grain boundary at zero. (Abstract shortened by UMI.)... |
