Origin of the enhanced Nb3Sn performance by combined Hf and Ta doping

Abstract

In recent years there has been an increasing effort in improving the performance of Nb3Sn for high-field applications, in particular for the fabrication of conductors suitable for the realization of the Future Circular Collider (FCC) at CERN. This challenging task has led to the investigation of new routes to advance the high-field pinning properties, the irreversibility and the upper critical fields (HIrr and Hc2, respectively). The effect of hafnium addition to the standard Nb-4Ta alloy has been recently demonstrated to be particularly promising and, in this paper, we investigate the origins of the observed improvements of the superconducting properties. Electron microscopy, Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) and Atom Probe Tomography (APT) characterization clearly show that, in presence of oxygen, both fine Nb3Sn grains and HfO2 nanoparticles form. Although EXAFS is unable to detect significant amounts of Hf in the A15 structure, APT does indeed reveal some residual intragrain metallic Hf. To investigate the layer properties in more detail, we created a microbridge from a thin lamella extracted by Focused Ion Beam (FIB) and measured the transport properties of Ta-Hf-doped Nb3Sn. Hc2(0) is enhanced to 30.8 T by the introduction of Hf, ~ 1 T higher than those of only Ta-doped Nb3Sn, and, even more importantly the position of the pinning force maximum exceeds 6 T, against the typical ~ 4.5–4.7 T of the only Ta-doped material. These results show that the improvements generated by Hf addition can significantly enhance the high-field performance, bringing Nb3Sn closer to the requirements necessary for FCC realization.

Effects of the oxygen source configuration on the superconducting properties of internally-oxidized internal-Sn Nb3Sn wires

“…At higher magnifications, brighter dots were observed on the exposed grains, as shown in figure 4. Based on previous observations and analyses [26,35,36], these dots are likely oxide nanoprecipitates (HfO 2 or ZrO 2 , depending on the third element added to the Nb alloy). Currently we are not able to determine the composition or exact size of these particles.…”

“…They appear nevertheless to confirm a lower size for the HfO 2 precipitates. Based on atomic probe tomography, Tarantini et al [36] reported HfO 2 precipitate sizes of approximately 3 nm and Nb 3 Sn grains below 70 nm (between 55 and 68 nm) in a wire manufactured by Balachandran et al [22] and reacted at 670 • C. Ortino et al [40] measured by transmission electron microscopy ZrO 2 precipitates of 4.6 nm and Nb 3 Sn grain sizes of 70 nm in APC-PIT reacted at 675 • C. This comparison based on literature data and our own results in this paper is unfortunately not sufficient to fully understand how the size and distribution of oxides precipitate affect the grain refinement. Moreover, the variation of the size distributions of the Nb 3 Sn grain across the reacted layer have rarely been systematically studied.…”

High temperature superconductors for commercial magnets

“…Although the calculated example above is an extreme case, the importance of a small grain-size was already recognized in the 1990's [22][23][24], while its impact on the pinningcurve was demonstrated later [19]. Attempts to evoke smaller grain-sizes and/or introduce artificial pinning centers in wires have only recently become successful [25], and this triggered active current research into this way to further optimize Nb 3 Sn [26][27][28][29][30][31]. planes as a result of [100] strain, and the resulting reduction in the electron density-of-states if the sub-lattice distortion is either allowed or suppressed.…”

Understanding the nanoscale chemistry of as-received and fast neutron irradiated Nb3Sn RRP® wires using atom probe tomography

“…The first report of APT analysis on Nb 3 Sn wires in 2013 [10] showed grain boundary segregation of Cu and Ti, quantified using the principle of Gibbsian excess at grain boundaries. More recently APT has been used to look at the size distribution and number density of nanoscale HfO 2 and ZrO 2 pinning centres in Nb 3 Sn [24,25] and to investigate the concentrations of species at the Nb/Nb 3 Sn interface in an internally oxidised Nb 3 Sn wire doped with Zr and O [26]. However APT analysis has not yet been applied to the characterisation of commercial RRP ® wires nor any form of irradiated Nb 3 Sn wires.…”