Evaluation of critical current density and residual resistance ratio limits in powder in tube Nb₃Sn conductors

Abstract

High critical current density (J c ) Nb 3 Sn A15 multifilamentary wires require a large volume fraction of small grain, superconducting A15 phase, as well as Cu stabilizer with high Residual Resistance Ratio (RRR) to provide electromagnetic stabilization and protection. In Powder-in-Tube (PIT) wires the unreacted Nb7.5wt.%Ta outer layer of the tubular filaments acts as a diffusion barrier and protects the interfilamentary Cu stabilizer from Sn contamination. A high RRR requirement generally imposes a restricted A15 reaction heat treatment (HT) to prevent localized full reaction of the filament that could allow Sn to reach the Cu. In this study we investigate recent high quality PIT wires that achieve a J c (12 T, 4.2 K) up to ~2500 A/mm 2 and find that the minimum diffusion barrier thickness decreases as the filament aspect ratio increases from ~1 in the inner rings of filaments to 1.3 in the outer filament rings. We found that just 2-3 diffusion barrier breaches can degrade RRR from 300 to 150 or less. Using progressive etching of the Cu we also found that the RRR degradation is localized near the external filaments where deformation is highest. Consequently minimizing filament distortion during strand fabrication is important for reducing RRR degradation. The additional challenge of developing the highest possible J c must be addressed by forming the maximum fraction of high J c small-grain (SG) A15 and minimizing low J c large-grain (LG) A15 morphologies. In one wire we found that 15% of the filaments had a significantly enhanced SG/LG A15 ratio and no residual A15 in the core, a feature that opens a path to substantial J c improvement.

Beneficial influence of Hf and Zr additions to Nb4at%Ta on the vortex pinning of Nb3Sn with and without an O source

“…An interesting observation is the presence of very small equiaxed Nb6Sn5 grains of less than 100 nm diameter. This is one of the first observations of sub-micron Nb6Sn5, whose grain size is normally several microns in commercial PIT conductors [13].…”

Very-high thermal and electrical conductivity in overpressure-processed Bi2Sr2CaCu2O8+x wires

“…The lowest RRR of Ag (≈ 90) has been measured on the conductor reacted at 1bar. This RRR reduction of a factor of ≈ 2 is not so dramatic if we consider that, in the case of Nb 3 Sn wires, a complex conductor layout that includes reaction barriers around the filaments is necessary to keep the RRR above ≈ 100 [16]. Figure 4 shows the correlation between the RRR and the maximum of the κ(T) curve at B=0 for Ag.…”

The crystal structure of (Nb0.75Cu0.25)Sn2 in the Cu-Nb-Sn system

“…for (Nb 0.75 Cu 0.25 ) Sn 2 , provoking a structural transformation from CuMg 2 type to NiMg 2 type. (3) The phase evolution during Nb 3 Sn wire processing influences the microstructure in the fully processed wire, and strong performance improvements of Nb 3 Sn PIT wires are expected when a more homogeneous microstructure can be achieved [13]. Knowledge of the crystal structure of (Nb 0.75 Cu 0.25 )Sn 2 , which was unknown until now, is an important step towards a better understanding of the Cu-Nb- Sn system.…”

“…The desired fine-grained Nb 3 Sn is formed by solid-state diffusion of Sn into the Nb(Ta) tube, whereas large grained and poorly connected Nb 3 Sn is formed directly from Nb 6 Sn 5 [12]. The strongly different Nb 3 Sn microstructures across the Nb 3 Sn layer limits the wire overall critical current [13].…”

“…The ternary phase, which is sometimes called Nausite [8,13] because it was first observed by Naus [5], is typically encountered as sub-micrometre layer at the interface between the former Cu sheath [9] of the PIT wire and at the middle as well as on the edge of the core of the RRP samples [14]. During heating experiments on wires monitored by synchrotron X-ray diffraction (XRD), the ternary phase was observed between 350 C and 580 C [15].…”