Statistical analysis of the relationship between electrical transport and filament microstructure in multifilamentary Bi₂Sr ₂CaCu₂O_x/Ag/Ag-Mg round wires

After processing, multifilamentary Bi₂Sr₂CaCu ₂O_x (Bi2212) round wires have complex microstructures. In this study, the microstructures are analyzed quantitatively using a new statistical method in which filaments are categorized based on the predominant phases observed by cross-sectional scanning electron microscopy (SEM). A Matlab program is created to analyze the SEM micrographs and categorize over 100 filaments within the image. In total, 26 wires, each heat treated differently so as to vary the critical current density (J_c), are studied. In some wires, two distinct cross-sectional areas are analyzed, so a total of 41 cross-sections and 5506 filaments are characterized. Five filament types are defined: filaments containing predominantly Bi2212, filaments containing relatively large Bi₂Sr₂CuO_x (Bi2201) grains, filaments containing relatively large alkaline earth cuprate (AEC) grains but no significant other non-Bi2212 phases, filaments containing relatively large copper-free (CF) grains but no other significant non-Bi2212 phases, and filaments containing relatively large AEC and CF grains. The majority of filaments (78% of all filaments classified) are either predominantly Bi2212 or containing-large-Bi2201 grains. Clear correlations between the number of these two types of filaments and the wire J_c are found; J_c is directly proportional to the percentage of 'predominantly-Bi2212' filaments. Although typically 70-90% of the containing-large-Bi2201 filament cross-sections is actually Bi2212 phase, Jc is inversely proportional to the percentage of this type of filament. Surprisingly, the correlations between Jc and the other filament types are weak or non-existent. Furthermore, using high-angle annular dark-field imaging in a scanning transmission electron microscope, Bi2201 intergrowths are found within Bi2212 grains, and results suggest possible differences in the Bi2201 intergrowth densities within Bi2212 grains extracted from predominantly Bi2212 filaments and those from Bi2201-containing filaments. These results indicate that significant enhancements in Bi2212 wire performance require either avoiding the formation of Bi2201, or ensuring complete conversion of Bi2201 to Bi2212.