Understanding Processing, Microstructure and Transport Relationships of Bi2Sr2CaCu2Ox/Ag Round Wires

Superconducting magnets generating magnetic fields above 25 T are needed for many scientific applications. Due to fundamental limitations to NbTi and Nb3Sn, such high-field superconducting magnets require alternative high-field conductors. One candidate conductor is round wire composites of Bi2Sr2CaCu2Ox sheathed in an Ag-alloy matrix (Bi2212/Ag). The performance of such wires is sensitive to the heat treatment, so improvements in the critical current density (Jc) require a thorough understanding of the processing-structureproperties relationships. Due to the complex microstructure-transport relationships, their performance is far from optimized, and the potential for further increase in Jc remains.;In this research a new heat treatment approach, saw-tooth processing (STP) is introduced based upon previous results showing that Bi2212 nucleation is site-saturation limited. STP increases Jc by 120% and 70% relative to partial-melt processing at 5 T and self-field respectively. The effects of STP heat treatment parameters on the microstructure and transport properties are discussed. It is shown that wires with the highest transport critical current densities primarily have filaments with two types of microstructures, one comprised primarily highly textured Bi2212 grains, and another with a noticeable amount of Bi2Sr2CuOX with the Bi2212.;After processing, multifilamentary Bi2212 round wires have complex microstructures. In melt processed Bi2212/Ag wires the primary impurity is Bi2Sr 2CuOx (Bi2201), which forms as mesoscopic grains and nanoscopic intergrowths. Microstructureproperties relationship studies are performed based on the hypothesis that Bi2201 plays critical roles in transport depending on the micron-size length scale. Mesoscopic microstructures are analyzed quantitatively using a statistical approach in which filaments are categorized based on the predominant phases observed by scanning electron microscope (SEM). A Matlab program is created to analyze the SEM micrographs and categorize over 100 filaments within each image. The majority of filaments (78% of all filaments classified) are either predominantly Bi2212 or containing-large-Bi2201 grains. Jc is directly proportional to the percentage of "predominatly-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.;To study the impact of nanoscale Bi2201 intergrowths on the superconducting properties, Bi2212 grains are extracted from filaments with and without significant Bi2201 grains present. By relating scanning transmission electron microscopy results to the Bi2212 coherence length, anisotropic magnetization behavior and magnetic-field dependent transport the effects of Bi2201 intergrowths on c-axis transport and magnetic flux pinning is studied. Wide Bi2201 (15-25 nm), intergrowths are barrier to c-axis transport within Bi2212 grains, whereas narrow half (1.3 nm wide),- and full-cell (2.5 nm wide) Bi2201 intergrowths are not detrimental to c-axis transport and are likely magnetic flux pinning centers. Microstructural, magnetization and transport studies designate effective contribution of c-axis transport in Bi2212/Ag multifilamentary wires; because the continuous favorable a--b plane current path is obstructed randomly.;In addition to conventionally-processed wires, 100 atm over pressured partial melt processed wires (OP-PMP) are studied on different length scales. OP causes grain growth in the preannealing, increases the peritectic melting temperature in the partial melt, improves oxygen uptake during solidification, and enhances the grain connectivity in sintering. The increase in transport of OP-PMP wire is related to increase in filament density. Yet OP-PMP wire shows higher area fraction of filaments with large Bi2201 grains. To even further improve the transport, optimization of OP-PMP is essential. This study confirms that after porosity, Bi2201 grains has the greatest negative impact on Bi2212 wires and further enhancements in Bi2212 wire performance requires either avoiding the formation of Bi2201 grains, or ensuring complete conversion of Bi2201 to Bi2212 grains.