Study of oxygen diffusion and oxidation of silver alloys for Bi-2223 and their effect on tape fabrication

Abstract

Silver and silver alloys are essential sheathing materials for Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi-2223) superconducting tapes, due to their oxygen permeability, workability, and chemical compatibility with the highly reactive Bi-2223 powder. Alloying elements can enhance material strength, but oxidation triggered by oxygen diffusion during fabrication process affects their chemical and mechanical properties. This study investigates the oxygen diffusion and internal oxidation behavior of AgMg, AgMgNi, and AgMn alloys, which are commonly used as sheathing materials for Bi-2223 tapes. The oxygen diffusion kinetics of these alloys were analyzed under various temperatures and oxygen partial pressures to determine their diffusion rates and their effects on microstructure. The results indicate that AgMg-based alloys exhibit higher oxidation rates than AgMn, primarily due to the reactivity of Mg, which leads to significant mechanical strengthening but reduced ductility after complete oxidation. Oxidant precipitates observed at grain boundaries was found to influence the tensile behavior of the alloys. Moreover, Bi-2223 tapes fabricated with AgMg-based sheaths exhibited notable surface defects and reduced plasticity, particularly after heat treatment and intermediate rolling processes, attributed to the formation of MgO and CuO precipitates, which also altered the Cu content within the filament during heat treatment. These findings suggest that while AgMg-based alloys offer mechanical advantages, their oxidation characteristics may pose challenges in maintaining tape integrity during fabrication. It is essential to further optimize alloy compositions and processing conditions to achieve the best possible performance in superconducting tapes.