Reactions between oxides and Ag-sheathed Bi₂Sr₂CaCu₂O_<i>x</i>conductors

Abstract

We studied reactions between Ag-sheathed Bi2Sr2CaCu2Ox (2212) conductors and a variety of oxides. We identified two intrinsic reactions: solid-state diffusion of a metallic species from the 2212 melt through the Ag sheath to react with an oxide, and direct reaction between Ag and an oxide. The dominant species that diffused through the Ag sheath was Cu because its solubility in Ag and diffusivity through Ag under 2212 melt processing conditions are the highest of the BSCCO metallic species. Al2O3, CeO2, pure SiO2, Y2O3, ZrO2, CaZrO3, and SrZrO3 were identified as non-reactive when heat treated in contact with Ag-sheathed 2212 tape. Fe2O3, MgO, and NiO reacted with Cu that diffused through Ag during the heat treatment. Cr2O3 and SiO2-based Pyrex glass reacted directly with Ag in the sheath during the heat treatment, which can create cracks and pinholes in the Ag sheath through which liquid can flow. Based on thermodynamic estimates, we calculated the Cu concentration in Ag at the 2212/Ag interface at 900 °C and 1 bar O2 to be between 0.15 and 0.22 at.%, which is consistent with 0.19 ± 0.02 at.% deduced from the experimentally measured diffusion flux.