Oxide and Metallic Precursor Powders for Bi2Sr 2CaCu2Ox/Ag Round Wires

Many future magnet applications for high energy physics and nuclear magnetic resonance require a superconducting conductor capable of generating magnetic fields above 30 T. Low temperature superconductors, such as NbTi and Nb3Sn, can only generate magnetic fields up to 10.5 T and 20 T. High temperature superconductor Bi2Sr2CaCu2Ox (Bi2212), with upper critical field greater than 100 T, is a promising candidate. As the only high field superconductor available as an isotropic round wire, Bi2212 Ag/Ag-alloy sheathed wires are of particular interest. Wire transport properties depends not only on wire heat treatment conditions, but also largely on precursor powder properties. Thus, a thorough study on precursor powder synthesis and the understanding of the key process-microstructureproperties relationships between precursor powder and wire performance is essential for further improvements in conductor performance.;In this research, a new method combining NanoSpray Combustion processing and solidstate calcination is developed to synthesize Bi2212 oxide precursor. Precursor powders with precisely controlled stoichiometry and chemical homogeneity containing over 99 vol% of Bi2212 single crystals are synthesized. Alkaline-earth cuprate (AEC) are found to be the only impurity phase in the precursor powders. Phase transformation, carbon release and grain growth during calcination are studied extensively through a series of quench studies. Effects of particle size, surface area, stoichiometry, chemical homogeneity and microstructures of the starting materials on Bi2212 formation and wire transport properties are discussed. With improved precursor powder properties, critical current density Jc is enhanced to 2520 A/mm2 (4.2 K, 5 T). The oxide-powder-in-tube (OPIT) route, however, has important shortcomings that limit the final Bi2212 wire performance, such as low tap-density, bubbling and poor electromechanical properties. In this research, an alternative method using metallic precursor with the potential to address these issues is developed and studied. A homogeneous metallic precursor with controlled stoichiometry containing Bi, Sr, Ca, Cu, Ag, and its alloys is produced by mechanical alloying. Both bulk samples and Ag-sheathed multifilamentary metallic wires are fabricated to study the correlations between metallic precursor synthesis, heat treatment conditions, phase transformations, microstructure development and the final wire transport properties.;Three pathways of metallic precursor oxidation and the cause of Bi flow are identified in Agsheathed wires. A thorough study on oxidation mechanism, phase transformation and microstructural development during heat treatment is performed. Advantages of metallicpowder- in-tube (MPIT) method include the formation of highly dense filaments containing micrometer Bi2212 grains before partial-melt processing (PMP) and the uni-axially textured Bi2212 grains within one filament after PMP. It is found that the formation of highly dense filament containing Bi2212 fine grains and Ag particles before PMP helps formation of large, c-axis textured Bi2212 filament during PMP. Furthermore, current-limiting factors including filament discontinuity, cation depletion and oxygen deficiency during oxidation stage are discussed. These results indicate that MPIT route has the potential to address the low tapdensity and bubbling issues that are intrinsic with OPIT route. Further improvements in Bi2212/Ag multifilament wires requires improved wire architecture, enhanced oxidation kinetics and increased filament density before PMP.