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[Koki Asai](https://orcid.org/0009-0009-4196-6366), [Tsuyoshi Yagai](https://orcid.org/0000-0003-1842-7881), [Taku Moronaga](https://orcid.org/0000-0002-6915-0627), [Nobuya Banno](https://orcid.org/0000-0002-7141-541X)

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[Effect of Zn Addition on NbSn Layer Formation in the Nb/Cu-Sn-Ti Diffusion Reaction](https://mdr.nims.go.jp/datasets/cd5ec6b6-c7f7-4550-a2f8-9422acef0519)

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1 ASC2024-4MPo1B-08  Effect of Zn Addition on Nb3Sn Layer Formation in the Nb/Cu-Sn-Ti Diffusion Reaction   Koki Asai, Tsuyoshi Yagai, Taku Moronaga, Nobuya Banno.     Abstract—Enhancing the characteristics of Nb3Sn superconducting wire is essential for the development of magnets for fusion reactors like ITER and DEMO. It has been established that Ti-doping has a significant effect on enhancing the upper critical magnetic field (Bc2). Ti is generally doped into Nb or Sn-alloys in practical Nb3Sn superconducting wires, and it would be preferable to add Ti to the Sn side from a manufacturing perspective. However, Ti-doping on Sn sites could form some undesirable stable compounds layers at the interface with Nb as a diffusion barrier for Sn during the Nb3Sn layer formation. It is challenging to find a new reaction route that destabilizes these compound layers in the Nb3Sn formation process, which is expected to cause a dramatic improvement in Sn diffusion when Ti is doped to the Sn core. Nevertheless, there are few studies that fundamentally investigate this aspect. Studies have reported that Zn is effective for promoting Nb3Sn layer formations. Therefore, in this study, specific diffusion couples of Nb/Cu/Sn were fabricated with different combinations of Ti and Zn doping to Cu and Sn and their diffusion reaction behaviors in Nb3Sn layer formation were investigated. Besides, anticipating a potential grain refinement effect, an Mg and Zn co-doped sample was additionally fabricated. However, the grain refinement by Mg was not seen in the present wire configuration. The effect of Zn addition on promoting the Nb3Sn layer formation appeared at 650 ℃/150 h HT, while it was not visible well at 685 ℃/100 h HT.   Index Terms—Ti addition, Zn addition, Mg addition, Nb3Sn.  I. INTRODUCTION  b3Sn superconducting wires are candidates for next-generation high-field magnets for future circular collider (FCC) projects[1], [2] and demonstration power stations (DEMO)[3]. FCC requires a non-Cu critical density of 1500 A/mm2 at 4.2 K under a 16 T field for Nb3Sn wires[1]. Ti is well-known for its ability to enhance the upper critical magnetic field (Bc2) and the critical current density (Jc)  Submitted for review September 25, 2024 This work was supported in part by JSPS KAKENHI Grant Number JP23K04453. (Corresponding author: Nobuya Banno.) Koki Asai is with Sophia University, Tokyo 102-8854, Japan, and with the  Research Center for Energy and Environmental Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan (e-mail: kouki0204@eagle.sophia.ac.jp) Tsuyoshi Yagai is with the Sophia University, Tokyo 102-8554, Japan (email: tsuyoshi-yagai@sophia.ac.jp). Taku Moronaga is with the Research Network and Facility Services Division, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan    (email: moronaga.taku@nims.go.jp) Nobuya Banno is with the Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan (e-mail: banno.nobuya@nims.go.jp)  in Nb3Sn[4],[5],[6]. Nb or Sn alloys are typically doped with Ti in Nb3Sn superconducting wires. However, from a manufacturing perspective, Ti doping of Sn is preferable. Nevertheless, according to our previous study, in the internal tin process, the Ti doping of Sn caused undesirable NbSnTiCu quaternary phases at the diffusion interface between Nb3Sn and Sn[7]. These phases can act as diffusion barriers for the formation of Nb from Sn. However, only a few studies have reported improvements in this aspect of Ti doping. To reduce these phases and promote Sn diffusion into Nb, in this study, a Nb/Sn/Cu composite was fabricated with Zn addition to Cu and Ti additions to the Sn core. Zn addition to Cu is thought to increase the Sn chemical potential (driving force) in Cu, which results in promoting the formation of thick Nb3Sn layers[8], [9],[10],[11],[12].  Moreover, some studies have reported that Mg significantly affects Nb3Sn grain refinement. Togano et al. reported that a small amount of Mg-doping (0.5at%) in a Cu–Sn matrix significantly affected the grain refinement[13]. Smathers fabricated Modified Jelly Roll (MJR) process wire with Sn‒Mg alloy and reported that Mg-doping refined grain size and improved current carrying capacity [14]. In addition, McKinnell et al. also reported that Mg-doping increased Jc over 20% in the internal Sn process [15]. Yu et al. demonstrated that a small amount of Mg-doping (0.2wt%) to the Cu–Zn brass matrix resulted in the grain refinement of Nb3Sn, which led to an improvement in Jc in the internal Sn process[16]. Nevertheless, the mechanism of grain refinement by Mg-doping has not yet been fully elucidated so far. In this study, the co-addition of Mg with Zn to Cu was also tested, expecting to promote Sn diffusion and simultaneously enhance Nb3Sn grain refinement.  II. EXPERIMENTAL METHOD We fabricated a simple diffusion couple structure of Nb/Cu/Sn to simulate the internal Sn processed wires [17],[18],[19],[20]. Zn and Mg were doped into the Cu matrix and Ti was doped into Sn and Nb (for comparison). By analyzing the layer thickness, grain size, and superconducting properties (Ic and layer Jc), the effects of adding Zn and Mg were compared with those of Ti doping during the internal tin process.  A. Samples Four types of single diffusion coupled wires were fabricated. These four samples were named (A), (B), (C), and (D). The Cu matrix and Sn cores were inserted into the Nb-alloy tubes. The Nb/Cu/Sn alloy composite with the Cu N 2 ASC2024-4MPo1B-08  stabilizer was swaged down to 0.6 mm-diameter wires. For samples (B) and (C), Cu matrix was replaced by Cu–15wt%Zn and Cu–12wt%Zn–0.2wt%Mg. The Nb-alloy tubes were annealed at 650 ℃ for 3 h before drawing.  The heat treatment (HT) was performed under vacuum at 500 ℃ for 100 h for Sn/Cu mixing and 650 ℃ for 150 h or 685 ℃ for 100 h to form the Nb3Sn layer. Samples (A) and (D) were the same specimens used in our previous study[7].   (A)N–C–ST:     Nb/Cu/Sn–1.6wt%Ti  (B)N–CZ–ST:   Nb/Cu–15wt%Zn/Sn–1.6wt%Ti (C)N–CZM–ST:  Nb/Cu–12wt%Zn–0.2wt%Mg/Sn–1.6wt%Ti  (D)NT–C–S:        Nb–0.8wt%Ti/Cu/Sn B. Microstructural and microchemical analysis.  The mechanically polished cross section of each sample was observed using scanning electron microscopy (SEM) after HT. The grain size was determined by dividing a given area by the number of grains present in the fractured sample images captured using SEM in the fine-grained layer.   C. Ic measurements  The critical current Ic was measured using a standard four-probe resistive method with a magnetic field ranging from 6 to 18 T at 4.2 K. The Ic was determined by an electrical criterion of 1 µV/cm. The Jc layer was determined by dividing Ic by the area of Nb3Sn, which was measured using image analysis.  D. S/TEM-EDS analysis  To investigate the extent of Mg diffusion into the Nb3Sn phase, microstructural observations were performed using scanning TEM.  The sample (C) heat treated at 650 ℃ for 150 h were cut into 10-20 µm in width and 5-10 µm in thickness using a focused ion beam (Thermo Scientific Scios 2 DualBeam) from fine polished transverse cross-sections of the Nb3Sn area. The microstructures were observed using S/TEM (JEM-ARM300F) with an acc of 300 kV) that has been installed at the Research Network and Facility Services Division of NIMS. The composition was analyzed using energy-dispersive X-ray spectroscopy (EDS). III. RESULTS AND DISCUSSION A. Nb3Sn layer formation  Fig. 1 shows the SEM images of typical cross-section of samples (A)-(D) after heat treatment at 650 ℃ for 150 h and at 685 ℃ for 100 h. The boundaries of the Nb3Sn layer were determined from the results of EDS compositional analysis. The overall Nb3Sn layer thicknesses for the fine-grained area are summarized in Table. Ⅰ. After heat treatment at 650 ℃ for 150 h, a stable NbSnTiCu quaternary phase layer appeared in sample (A)N-C-ST at the interface between the Nb3Sn and Cu-Sn phase. This phase becomes a diffusion barrier for Sn to enter Nb, which leads to a thinner Nb3Sn layer formation. Similar quaternary phases were observed in sample (B). However, these compounds were isolated and did not function as diffusion barriers for Sn. This could account for the thicker Nb3Sn layer observed in sample (B). In sample (C), the Mg-co-doped sample showed a slightly thinner Nb3Sn layer than sample (B). Considering that the Mg co-addition sample exhibited a reduced effect of layer formation compared to the Zn-only addition sample, Mg seems to suppress the effect of Zn addition on the diffusion of Sn to Nb. In sample (D), NT-C-S exhibited a mixed area of coarse Nb3Sn grains and Nb6Sn5 between the Nb3Sn and Sn phases. The Sn diffusion has not progressed sufficiently at 650 ℃ heat treatment.  After heat treatment at 685 ℃ for 100 h, all the samples exhibited thick Nb3Sn layer formation. Samples (A)–(C) developed a Nb3Sn layer with similar thickness. The Ti compounds phase that has been present largely at 650 ℃ decreased, which indicates that the NbSnTiCu quaternary phase can be decomposed in the temperature range from 650 ℃ to 685 ℃. Sample (D) formed a significantly thicker Nb3Sn layer. The NT sample did not form a quaternary phase during any of the heat treatment processes. Therefore, a thick Nb3Sn layer was formed.    Fig. 1 Cross sectional SEM images (BSE) of the reaction layer (acceleration voltage: 20 kV) for (A) N-C-ST, (B) N-CZ-ST, (C) N-CZM-ST, (D) NT-C-S after diffusion at 650 ℃ for 150 h and at 685 ℃ for 100 h.  Table. Ⅰ Overall Nb3Sn layer for fine grains layer thickness of sample (A)-(D).  Sample (A)N-C-ST (B)N-CZ-ST (C)N-CZM-ST (D)NT-C-S650℃/150h 12(µm) 25(µm) 18(µm) 11(µm)685℃/100h 23(µm) 25(µm) 27(µm) 41(µm)3 ASC2024-4MPo1B-08   B. Nb3Sn grain size Fig. 2 shows the SEM images of the fractured microstructure of the fine-grained Nb3Sn regions of each sample, and Table II summarizes the average grain size of Nb3Sn. Comparing samples (A) and (B), a slight grain refinement was observed in sample (B) owing to the addition of Zn in both heat treatment, that is, at 650 ℃ and at 685 ℃.  In contrast, in sample (C), where Zn and Mg were co-added, a slight grain refinement was observed at 685 °C compared to sample (A), whereas this effect was not evident at 650 ℃. Unlike expectations, the addition of Mg did not significantly affect the grain refinement [15]. This is assumed to reflect the smaller Mg diffusion in this sample, as shown later. In sample (D), the NT specimen showed the most refined Nb3Sn grains in both heat treatments, at 650 ℃ and at 685 ℃. It is notable that this trend differs from that of the bronze-processed Nb3Sn wires, where the grain size is reduced when Ti is doped into the bronze matrix[21][22][23].  C. Superconductivity properties Fig. 3 shows longitudinal cross-sectional optical microscope images for the single core sample and multifilamentary sample for (A) after the heat-treatment: the multifilamentary wires were fabricated by restacking 7 single-core wires with 0.6 mm in diameter into the Cu tube with outer/inner diameter of 3/2 mm and drawing down to 0.92 mm in diameter. The cross-sectional views of the precursor wires were sound for both wires along the wire axis. However, after the heat-treatment, the missing part of the Nb3Sn layer together with large void in the core was often found in places along the longitudinal direction in the single core wires as shown in Fig. 3(left). Reflecting this fact, the Jc properties of the single core wires were scattered from sample to sample even in the same type wires. Meanwhile, lack of Nb3Sn was minimal in the multifilamentary wires, which results in small scattering of Jc values. Therefore, Jc properties of the multifilamentary wires are discussed below.  Fig. 4 shows the characteristics of the Ic and Jc layer for multifilamentary samples (A)–(D) as a function of the magnetic field at 4.2 K. The Zn added sample (B) exhibited higher Ic and Jc properties at 650 ℃ compared to sample (A). This is owing to the thicker Nb3Sn layer and the smaller Nb3Sn grain size, as shown in Fig. 1 and Table. II. However, the Ic and Jc of Zn+Mg co-doping sample (C) were decreased from the properties of sample (B). This would be consistent with the fact of the smaller Nb3Sn layer thickness and large Table. Ⅱ Average Nb3Sn grain size of samples (A)-(D).   Sample (A)N-C-ST (B)N-CZ-ST (C)N-CZM-ST (D)NT-C-S650℃/150h 116(nm) 109(nm) 124(nm) 94(nm)685℃/100h 207(nm) 167(nm) 173(nm) 164(nm)  Fig .2 Fractured cross-sectional SEM images (acceleration voltage: 20 kV) in fine grain Nb3Sn regions with the average grain size for (A) N-C-ST, (B) N-CZ-ST, (C) N-CZM-ST, (D) NT-C-S after diffusion at 650 ℃ for 150 h and at 685 ℃ for 100 h.  The yellow line indicates the area used for grain size analysis.  Fig. 3 longitudinal cross-sectional optional images for single core sample (left) and multifilamentary sample (right) for  (A) N-C-ST after diffusion at 650 ℃ for 150 h.  Fig .4 Characteristics of the Ic (above) and layer Jc (below) for multifilamentary samples (A)-(D) as a function of magnetic field at 4.2 K. 4 ASC2024-4MPo1B-08  grain size. In sample (D), the NT specimen showed the highest Ic and Jc values in both heat treatments presumably reflecting the small grain size, and also thick layer thickness for 685 ℃. In all samples, it can be seen that the lower temperature heat treatment leads to higher Jc properties, which is due to the reduced grain size as shown in Table. Ⅱ. The Kramer Bc2 estimated as the extrapolation of the Kramer plot (Jc0.5B0.25) to the horizontal axis (B: magnetic field) for samples (A)‒(D) heat-treated at 650 ℃ are 20.92T, 20.42T, 21.38T and 23.61T, respectively. The Kramer Bc2 of sample (B) was obviously small compared with (D). SEM-EDS revealed that Ti composition of the sample (B) was around 0.2at%, while that of the other samples was around 0.4at% (Nb and Sn compositions were 70-72at% and 19-21at% in all samples). The smaller Bc2 of (B) could be attributed to the smaller Ti content. The addition of Zn promotes Sn diffusion, while on the other hand may inhibit Ti diffusion. D. S/TEM-EDS analysis for Zn+Mg co-doped sample  Fig. 5 shows the STEM-BF images and EDS mapping (at%) of sample (C) N-CZM-ST heated at 650 ℃ for 150 h: (a) the interface between NbSnTiCu quaternary phase and Sn–Cu phase, (b) the interface between NbSnTiCu quaternary phase and Nb3Sn, and (c) the interface between Nb3Sn and Nb. Ti mappings of sample (C) N-CZM-ST heated at 650 ℃ for 150 h for (a) the interface between NbSnTiCu quaternary phase and Sn-Cu phase, (b) the NbSnTiCu phase, and (c) the interface between Nb3Sn and Nb are exhibited in Fig. 6. Ti can be detected strongly in NbSnTiCu phase and weakly in Nb3Sn.    The contrast of Mg (orange) has been slightly increased for better visibility. Mg were observed in the NbSnTiCu phase, but little in the Nb3Sn phase. In the EDS spectrum for the area shown in Fig. 5(c), Mg peaks were not detected. We could say that Mg did not almost diffuse into Nb3Sn at the reaction front. In the previous internal-tin wires with Mg addition, Ti was doped into Nb[14],[15],[24]. In contrast, in this work, Ti was doped into Sn, which resulted in the formation of NbSnTiCu phase. Mg seems to have been almost trapped in this phase and stabilize this phase. This would be the reason why no grain refinement effect was seen.  Ⅳ.CONCLUSION This study compared the effects of the addition of Zn and Zn–Mg to Cu on Ti-added samples. First, we found that Zn isolates NbSnTiCu phase at the interface of Nb3Sn and Cu‒Sn, which results in promoting the Nb3Sn phase formation. However, contrary to expectations, the NbSnTiCu phase could not be reduced by Zn addition. Grain refinement was found in Zn-addition. Mg was trapped in the NbSnTiCu phase in the case of Ti doping to Sn core and did not almost diffuse into Nb3Sn: no grain refinement effect by Mg doping was seen. If Mg can diffuse into Nb3Sn phase, the Zener pinning effect might be also expected for the grain refinement. However, whether Mg diffuses into Nb3Sn or not has not been clarified. This seems to be still an open question. The addition of Ti to the Nb sample (NT) yielded favorable results in terms of layer thickness and grain size. Combining the NT sample with Zn, higher Mg contents, and optimized heat treatments may be more effective in improving these properties.   (a) Interface between the NbSnTiCu quaternary phase and Sn-Cu phase.  (b) NbSnTiCu quaternary phase  (c) Interface between Nb and Nb3Sn Fig. 5 STEM-BF images and EDS mapping (at%) of sample (C) N-CZM-ST heated at 650 ℃ for 150 h for (a) the interface between NbSnTiCu quaternary phase and Sn-Cu phase, (b) the NbSnTiCu quaternary phase, and (c) the interface between Nb3Sn and Nb.  Fig. 6 EDS mapping (at%) of sample (C) N-CZM-ST heated at 650 ℃ for 150 h for  (a) the interface between NbSnTiCu quaternary phase and Sn-Cu phase, (b) the NbSnTiCu phase, and (c) the interface between Nb3Sn and Nb.  5 ASC2024-4MPo1B-08  REFERENCES  [1] A. 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