Composition and connectivity variability of the A15 phase in PIT Nb₃Sn wires

Abstract

Powder-in-tube (PIT) Nb3Sn wires are competing with Restacked-Rod-Process (RRP®) for the realization of the high luminosity upgrade of the Large Hadron Collider (LHC) at CERN. These two conductors have different properties and microstructures that are in both cases averages of an inhomogeneous A15 microstructure. PIT has in general a smaller fraction of A15 in the non-Cu cross-section than RRP® and a lower non-Cu Jc (12 T, 4.2 K) (2500–2700 A mm−2 versus 2900–3000 A mm−2) but it can be made in smaller filament diameters, which is an important property for LHC magnets. Another characteristic of PIT A15 is that ∼25% is made up of ∼1–2 μm sized grains (typically ∼10 times the small grain (SG) diameter) and their contribution to transport is uncertain. Here we studied a 192 filament Ta-doped, 1 mm diameter PIT wire and combined multiple characterization techniques in order to distinguish the different wire components, to determine their individual properties and to identify which components are current-carriers. We found multiple evidence that the large A15 grains, which are also the highest-Tc grains, do not contribute to transport at high field and that the only current-carrying A15 is the SG with Tc <17.7 K. However, because of the high density of grain boundaries in the SG A15 layer, PIT has an exceptionally high SG-layer Jc and high specific grain boundary pinning force, QGB. These findings clearly show that it is essential to increase the ratio of small to large and disconnected grains in order to improve PIT performance.

Assessing composition gradients in multifilamentary superconductors by means of magnetometry methods

“…18 Tarantini et al report the presence of disconnected grains with critical temperature values extending up to 18.7 K in a PIT wire very similar to the one studied in this work. The AC magnetometry method is not sensitive to grains which do not form a percolative path around the sub-element core (a "shell", as we named it in section II A), hence the absence of such high T c values in Figure 3 is not surprising.…”

Pinning Improvement of A15 Applied Superconducting Materials

“…The formation reaction of the fine grain structured Nb 3 Sn layer in such wires takes place as a reactive diffusion process between NbSn 2 and Sn in presence of the Cu acting as a catalyst of fine grain formation as described previously in literature [13]. Such conductors are characterised by high density of grain boundaries in the small grain A15 layer, and such PIT conductor has an exceptionally high J c in the layer and also high specific grain boundary pinning force [2]. However, a substantial improvement of J c (B) was well documented, but there is no change in a pinning mechanism of the Nb 3 Sn layer which will shift the maximum pinning force position towards the higher magnetic flux density normalised values B/B c2 .…”

“…Research conducted by Tarantini et al [2] on the powder-in tube, PIT, Nb 3 Sn conductors, where mixture of NbSn 2 , Sn and Cu powders is inserted in Cu-clad Nb-7.5 wt% Ta tube and drawn in Cu matrix, address the fine grain structure layer formation of A15 phase influencing J c value [2]. The formation reaction of the fine grain structured Nb 3 Sn layer in such wires takes place as a reactive diffusion process between NbSn 2 and Sn in presence of the Cu acting as a catalyst of fine grain formation as described previously in literature [13].…”

“…Recent intensive research on powder-in-tube NbSn 2 -Cu-Sn composites results in exceptionally high J c of a very fine Nb 3 Sn layer formed [2]. However a substantial improvement of J c (B) is well documented but there is no change in a pinning mechanism of the Nb 3 Sn layers which will move the maximum pinning force towards the higher magnetic flux density normalised values B/B c2 .…”

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

“…Through the dissolution of the cooper sheath, the NbSn 2 phase reacts with the outer Nb tube forming Nb 6 Sn 5 . 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].…”