| Among the known high T{dollar}rmsb{lcub}c{rcub}{dollar} superconductors (HTS), Bi-Sr-Ca-Cu-O tapes have demonstrated the greatest potential for large-scale high-current high-field applications because they retain high critical current density {dollar}rm(Jsb{lcub}c{rcub}>10sp5 A/cmsp2){dollar} values in very high fields (20 T). Nevertheless, recent studies suggest that only a small fraction of the tape cross section actually carries transport supercurrent, and that the active current path is defined by various defects, of which one of the most important is grain boundaries (GBs). A variety of bulk-scale bicrystals of {dollar}rm Bisb2Srsb2CaCusb2Osb{lcub}8+x{rcub}{dollar} (2212) have been studied to understand which types of boundaries dictate the current paths in tapes and to identify critical boundary parameters that control their coupling character. The crystallography and microstructure of the bicrystals were investigated using polarized light microscopy, backscattered electron Kikuchi diffraction patterns, scanning electron microscopy, and transmission electron microscopy. The electromagnetic properties of both the grain interiors and the grain boundary were measured and compared directly. This characterization included determination of the field dependence of J{dollar}rmsb{lcub}c{rcub},{dollar} the irreversibility field (H{dollar}sp*),{dollar} and resistance-temperature transitions. The principal findings are: (001) tilt GBs with misorientation angles ({dollar}theta){dollar} up to 15{dollar}spcirc{dollar} and (100) tilt GBs with {dollar}theta{dollar} up to 8{dollar}spcirc{dollar} having boundary planes across which the basal plane can continue exhibited single-crystal-like, strongly coupled behavior. Sintered high-angle (001) twist boundaries showed both strongly and weakly coupled character. A 16{dollar}spcirc{dollar} (902) mixed bicrystal having a basal-plane-faced boundary showed the signature of c-axis transport with low J{dollar}rmsb{lcub}c{rcub}{dollar} and weak coupling. The experimental bicrystal data is generally consistent with the Railway Switch model of current transport in polycrystalline BSCCO tapes. |
