Refinement of Nb ₃ Sn grain size by the generation of ZrO ₂ precipitates in Nb ₃ Sn wires

In this Letter, we demonstrate that if oxygen can be properly supplied to (Nb-Zr)-Sn wires, ZrO2 precipitates will form during the heat treatment, refining the Nb3Sn grain size markedly. Here, a Nb3Sn subelement was fabricated in which Nb-1Zr alloy was used, and oxygen was supplied via SnO2 powder. The results showed that such a design could supply sufficient oxygen to internally oxidize the Zr in the Nb-1Zr alloy, and that the sample reacted at 650 °C had grain sizes of ∼45 nm, less than half the size of the grains in present Nb3Sn conductors. Magnetic measurements showed that the peak of the pinning force vs. field (Fp-B) curve was shifted to ∼0.3Birr (the irreversibility field).