The formation of superconducting phases in Bi(Pb)–Sr–Ca–Cu oxide/Ag microcomposites produced by oxidation of metallic precursor alloys

Abstract

Metallic alloy ribbons of Bi–Sr–Ca–Cu with nominal compositions of 2-2-1-2, 2-2-2-3, 2-2-2-4, and 2-2-3-4, with or without Pb, were made by vacuum induction melting followed by melt-spinning. Ag, at the level of 20–80 wt. %, was added to aid in the alloy melting and metallic ribbon formation processes. These ribbons were then subjected to a controlled atmosphere oxidation and annealing to produce superconducting oxide/silver microcomposites. Among the four alloy groups investigated in the present work, 2-2-3-4 alloys with 0.6Bi replaced by Pb possess the best superconducting properties after suitable treatment. These specimens exhibited zero resistance at T = 104–110 K and a critical current density of 600 A/cm² at 77 K in zero field with excellent reproducibility. The annealing conditions, namely the annealing temperature, time, and atmosphere, are discussed in terms of the formation kinetics of the superconducting phases. While the 85 K T_c “2212” phase formed over a wide temperature range (740 to 845 °C) after an ~2 h anneal, the 110 K high-T_c transition appeared only after an ~10 h anneal at 810–840 °C, suggesting that the “2223” phase grew from the 2212 phase controlled by diffusion. The microstructures of these superconducting microcomposites were observed and analyzed by a combination of optical and scanning electron microscopy, x-ray powder diffraction, and electron probe microanalysis. The high T_c “2223” phase exhibited plate-like grains which were distributed randomly in orientation. The silver addition improved the mechanical properties of the metallic precursor ribbons and the superconducting microcomposites, promoted the formation of the 2223 phase, and influenced the oxidation and the annealing conditions.