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

Abstract

Nb 3 Sn 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 (J c ) is a key parameter for such applications. Significant efforts towards optimization of various factors led to an 80% improvement in J c from the early 1990s to 2003, when the 4.2 K, 12 T non-matrix J c reached 3000 A/mm 2 (corresponding to 5000 A/mm 2 in Nb 3 Sn layer J c ). [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 J c of conventional Nb 3 Sn 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 J c . In this method, a Nb 3 Sn 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 ZrO 2 particles in Nb 3 Sn layer, which effectively refine Nb 3 Sn 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 Nb 3 Sn layer J c reached 9600 A/mm 2 .

Ta, Ti and Hf effects on Nb3Sn high-field performance: temperature-dependent dopant occupancy and failure of Kramer extrapolation

“…This implies that the Nb-Ta-Hf alloyed Nb3Sn does not have a suppressed actual HIrr as was observed in Nb-Zr alloyed Nb3Sn wire. 26,27 In fact we measured µ0HIrr (4.2 K) ∼23.4 T, about 1 T larger than the best Ta-doped RRP sample studied in this work, making Nb-Ta-Hf alloy a promising material for high-field Nb3Sn optimization.…”

Grain-boundary structure and segregation in Nb3Sn coatings on Nb for high-performance superconducting radiofrequency cavity applications

“…In superconducting magnets, defects are purposely engineered into the Nb 3 Sn wires at GBs. Impurities, such as normal conducting copper [16] or insulating oxides [17,18] may be added to degrade locally the superconducting properties (e.g., superconducting order-parameter and superconducting energy gap [11,19]) associated with the GBs, which may create an energetically favorable volume for vortices, where they become pinned. In contrast, the magnetic-field direction in SRF cavities changes on a nanosecond time-scale, leading to significant dissipation of heat and a corresponding decrease in the quality factor if the flux penetrates into the superconductor (including penetration into the GBs) [20].…”

GLAG theory for superconducting property variations with A15 composition in Nb3Sn wires

“…By this way, new physical insights into the importance of the composition inhomogeneity are provided. As for practical engineering significance, tin composition and possible additives are important for the very high J c now achieved in commercial strands 16, 17 . Therefore, describing the superconducting properties dependence on the A15 composition in theory will facilitate the understanding of the optimization for the critical current density J c .…”