Through-process study of factors controlling the critical current density of Ag-sheathed (Bi,Pb)₂Sr₂Ca₂Cu₃O_xtapes

Abstract

The connectivity and flux pinning components of the critical current density J c in a high-J c Pb) 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) composite throughout important steps in its thermomechanical treatment were investigated by mass density measurement, microstructural observation and extensive superconducting property characterization. The mechanical integrity of the filaments was greatest after the first heat treatment, because damage produced by intermediate rolling was not completely erased by subsequent heat treatment. The final relative filament mass density was less than 75%, even though J c more than tripled to reach ∼50 kA cm −2 (77 K, 0 T) after final heat treatment. Filament resistance and J c data showed that the electrical connectivity improved significantly throughout the process, although ultrasonic fracturing, magneto-optical images and density measurements all showed that the mechanical and physical connectivity degraded. Our study suggests that the attainable J c of Bi-2223 is determined by the electrical connectivity of each filament which is itself an uncertain compromise between minimizing the significant porosity produced by the Bi-2223 formation reaction and the extensive crack network that porosity reduction by intermediate rolling produces.

Significantly enhanced critical current density in Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox composite conductors prepared by overpressure processing in final heat treatment

“…11,12 A significant amount of porosity, ϳ20% as indicated by mass density measurement, remains in multifilament 2223 tape after HT2 or FHT at 1 bar. 13,14 These residual cracks and porosity interrupt the grain connectivity and current paths, thus reducing J c . It is generally accepted that 2223 forms from 2212 and alkaline earth cuprate ͑AEC͒ phases in the presence of a transient liquid phase.…”

Local measurement of current density by magneto-optical current reconstruction in normally and overpressure processed Bi-2223 tapes

“…Poor inter-grain connectivity is one reason advanced for the less than optimum current carrying capacity [5], but prior magneto-optical reconstructions suggest that problems with connectivity have many sources. For example, even very high-multifilament Bi-2223 tapes can contain as much as 20% porosity [6]. Such pores, cracks and other detrimental microstructural features are at least partially caused by the multi-step thermomechanical treatment needed to react and to densify the starting Bi-2212 precursor as it converts toward a largely Bi-2223 phase assemblage.…”

Microstructure and J/sub c/ improvements in overpressure processed Ag-sheathed Bi-2223 tapes

“…Furthermore, the filaments can dedensify when 2223 forms in HT2. It has been observed that even the best multifilamentary tapes still contain 10-30% porosity [3]. The pores and unhealed deformation damage from IR, which are inherent defects in the currently practiced conventional 2223 processing, destroy the Manuscript received August 6, 2002; revised October 7, 2002 integrity of grain connectivity, limiting supercurrent flow in the filaments.…”