Partial melt processing of solid-solution Bi2Sr2CaCu2O8+δthick-film conductors with nanophase Al2O3additions

Abstract

Partial-melt processing of Bi2+xSr2-x-yCa1+yCu2O8+δ(Bi-2212) thick-film conductors with additions of nanophase Al2O3was studied for dual purposes of increasing flux pinning and inhibiting Sr—Ca—Cu—O phase defect formation. Nanophase Al2O3(<50% mole fraction) was added to Bi:Sr:Ca:Cu:O powders with four different compositions: three with Bi:Cu approximately 2:2 and one (Bi2Sr2.38Ca1.15Cu2.92O9.7+δ) closer to the ideal Bi-2223 composition. The effect of Al2O3addition on film microstructural and superconducting properties was studied for a range of partial-melt temperatures (865 to 900 °C). Results were compared to Al2O3-free films with compositions lying within the single-phase solid-solution 2212 region. Nanophase Al2O3reacted with 2212-type precursors to form a composite of micron size or smaller particles of solid-solution (Sr,Ca)3Al2O6in a solid-solution 2212 superconducting matrix. The Ca content of the (Sr,Ca)3Al2O6particles formed approximated that of the 2212 precursor (≤6% mole fraction difference). Addition of 6–25% volume fraction of (Sr,Ca)3Al2O6to Bi-2212 (by reaction between Al2O3and Bi-2212) only slightly reduced superconducting transition temperatures andc-axis texturing; however this addition improved film quality by reducing Sr—Ca—Cu—O defect volume fraction by factors of 2 to 6 and significantly increased the critical current density by over one order of magnitude for 0 to 2 T applied fields at 20 to 30 K.