The Roles of Grain Boundary Refinement and Nano-Precipitates in Flux Pinning of APC Nb3Sn

Abstract

Internally oxidized Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires with artificial pinning centers (APC) have been developed by manufacturing a multifilament PIT-style wire, each filament consisting of a Nb-Ta-Zr alloy tube filled with a mixture of Sn, Cu, and oxide powders. During heat treatment, the oxide decomposes, and the oxygen goes into solid solution in the Nb alloy. Upon Sn diffusion into the Nb alloy, ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoprecipitates form which serve both to inhibit grain coarsening and directly pin magnetic flux. The finer grain structure and high concentration of non-superconducting precipitates serve as flux pinning sites and enhance high-field J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ; direct pinning by the precipitates also shifts the maximum pinning force to higher fields. To distinguish these two effects, an APC wire with a high heat treatment temperature (700 °C) was compared to a conventional PIT wire with very low heat treatment temperature (600 °C), resulting in similar grain size in both samples. The pinning force vs applied field (F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> -B) curve was deconvoluted into grain boundary and point pinning components. It was found that the grain boundary component of the APC wire was very close to the F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> -B curve for the PIT wire, demonstrating that the two pinning components in an APC wire are directly additive. It was then possible to show that, in the 15-20 T regime, direct pinning contributed 45-50% of the total pinning.