Internally Oxidized Nb3Sn Strands with Fine Grain Size and High Critical Current Density

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Nb3Sn superconducting strands are the most practical conductors to generate high magnetic fields (12-16 T), and thus have significant applications in nuclear magnetic resonance (NMR), and great potential for fusion reactors and particle accelerator magnets. High critical current density (Jc) is a key parameter for such applications. Significant efforts towards optimization of various factors led to an 80% improvement in Jc from the early 1990s to 2003, when the 4.2 K, 12 T non-matrix Jc reached 3000 A/mm2 (corresponding to 5000 A/mm2 in Nb3Sn layer Jc).[1,2] However, further efforts over the past decade have failed to bring about further increase beyond this level,[3,4] leading some researchers to conclude that the Jc of conventional Nb3Sn strands had reached its maximum. Here, however, by applying an internal oxidation method, we reduce the grain size by a factor of three and nearly double the 12 T Jc. In this method, a Nb3Sn strand is fabricated with Nb-Zr alloy as starting material; with oxygen supplied properly via an oxide powder, the Zr atoms in the Nb-Zr alloy are internally oxidized, forming fine intra-granular and inter-granular ZrO2 particles in Nb3Sn layer, which effectively refine Nb3Sn grain size. At a reaction temperature of 625 °C, grain size down to 20-50 nm (36 nm on average) has been achieved. For this sample the 4.2 K, 12 T Nb3Sn layer Jc reached 9600 A/mm2.




Published version: Adv. Mater. 2015, 27, 1346–1350.