Microstructure, microchemistry and the development of very high Nb3Sn layer critical current density

The non-Cu critical current density, J_c, in engineering quality Nb₃Sn strand has increased beyond 3000 A/mm² at 12 T 4.2 K. Strand of this type, fabricated by a rod-in-tube technique using Nb-Ta alloy has been used by the Superconducting Magnet Group at LBNL to successfully fabricate a 16 T (4.2 K) dipole accelerator magnet. The grain size of this strand has been measured across the A15 layer and was found to be small (130 nm diameter) and homogeneous in morphology and size. This is despite a broad A15 layer thickness of 10 to 20 μm. We have also measured the Sn concentration across A15 layers in this and two other high J_c strands. Although the Sn concentration in the A15 layers adjacent to the original Sn source were similarly high (∼24.5 at.%Sn) for all strands, the decline in Sn concentration across the A15 layer was markedly different. We found that the gradient in Sn concentration across the layer was reduced by higher temperature heat treatment. However the benefit to high field J_c of an improved overall irreversibility field, H^*, due to better A15 stoichiometry, is offset by the larger grain size produced at higher temperature and thus a lower density of flux pinning sites.