Properties and chemical stability of hot-rolled Ag(7 at. % Cu)-sheathed Bi₂Sr₂Ca_0.64Cu_1.64O_x powder-in-tube tapes

Bi₂Sr₂Ca_0.64Cu_1.64O_x (nominally Bi2212) powders were fabricated into powder-in-tube Ag- and Ag(7 at. % Cu)-sheathed tapes by cold and hot rolling to investigate the effects of sheath composition and rolling conditions on their microstructural development and superconducting properties. Bi2212 tapes with Ag(Cu) sheaths exhibited improved grain alignment and interfacial uniformity, as well as enhanced formation of the Bi-free phase (≊Sr_7.5Ca_6.5Cu ₁₄O_x), relative to the Ag-sheathed specimens. The hot-rolled Ag(Cu)-sheathed tapes displayed superior critical current densities (J_c), where magnetization J_cm=1.5×10⁶ (H∥c) and 4.6×10⁵ A/cm² (H⊥c) at T=5 K, H=1 T. Correspondingly, these specimens had transport critical current densities (J_ct) of 6.7×10⁴ A/cm² (H∥c) and 5.4×10⁴ A/cm² (H⊥c) at T=4.2 K, H=0 T and 2.2×10⁴ A/cm² (H∥c) and 3.0×10⁴ A/cm² (H⊥c) at T=4.2 K, H=14 T. The chemical stability of the Ag(Cu) sheath regions during the partial melting process was also studied. Rapid oxidation of copper produced Cu₂O precipitates in the sheath at 885 °C, and subsequently a Cu₂O-free zone developed near the core/sheath interface. A theoretical analysis of Cu₂O precipitate formation and decomposition during thermal processing is presented.