  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). However, further efforts over the past decade have
failed to bring about further increase beyond this level, 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 {\deg}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.

Materials Science