Quantitative description of a highJcNb‐Ti superconductor during its final optimization strain. I. Microstructure,Tc, Hc2, and resistivity

摘要

A most important step in the critical current density (Jc) optimization of Nb‐Ti is the large final drawing strain, in which agr;‐Ti precipitates, initially approximately equiaxed and 100–200 nm in diameter, are drawn into ribbons, whose thickness (1–2 nm) is less than the superconducting coherence length xgr; (4.2 K)∼5 nm. Using transmission electron microscopy, the precipitate thickness, spacing, cross‐sectional area, and circumference were measured over the whole final drawing strain range. Each of these parameters was found to have a simple power dependence on the wire diameter.Tc,Hc2, and the resistivity (rgr;n) were also change considerably during the refinement of the precipitates. Directly after precipitation,Tcincreased, and (dHc2/dT)Tcand rgr;nwere reduced from the single‐phase values. Drawing the wire returned these parameters to their single‐phase values, as the precipitate thickness was reduced to less than xgr;. This observation explains a long‐standing peculiarity in this system, namely that the optimumHc2of highJcconductors occurs for a composition close to Nb 46 wt. Ti, even when the precipitation of 18 vol  of agr;‐Ti shifts the matrix composition to a Nb‐rich composition of theoretically lowerHc2.