Removal of Tantalum Layer from Nb3Sn Superconducting Wire and Corrosion Mechanism
GAO Zhan1,2, ZHANG Zerong2(), CHENG Junsheng3, WANG Qiuliang2,3()
1 School of Rare Earths, University of Science and Technology of China, Hefei 230026, China 2 Ganjiang Innovation Academy, Chinese Academy of Sciences, Ganzhou 341119, China 3 Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China
Cite this article:
GAO Zhan, ZHANG Zerong, CHENG Junsheng, WANG Qiuliang. Removal of Tantalum Layer from Nb3Sn Superconducting Wire and Corrosion Mechanism. Acta Metall Sin, 2024, 60(7): 968-976.
High-quality superconducting joints play a key role in the construction and stable operation of superconducting magnets. The preparation of superconducting joints for Nb3Sn wires is often affected by the presence of an internal tantalum barrier layer. Thus, this study examined the effectiveness of different removal methods of the Ta barrier layer in Nb3Sn superconducting wire. For this, the superconducting wire prepared by an internal tin method was considered as the experimental object, and the corrosion behavior and characteristics of the wire in HF solution, HF atmosphere, mixed solution of HF and H2O2, and mixed solution of HF and HNO3 were investigated. The microstructure and corrosion morphology of the wire were analyzed using SEM and OM. Furthermore, high-purity Ta sheets were selected as the research object, and corrosion experiments were carried out in the above-mentioned media. The corrosion morphology, phase structure, and valence state of the elements of the specimens were analyzed using OM, XRD, and X-ray photoelectron spectroscopy (XPS) to reveal the corrosion mechanism of Ta. The results showed that the corrosion of the Ta layer was the fastest in the mixed solution of HF and HNO3, followed by the mixed solution of HF and H2O2, the HF atmosphere, and then the HF solution. Based on the effect after corrosion, the mixed solution of HF and H2O2 was found to be the best method to remove the Ta barrier layer in Nb3Sn superconducting wire. Moreover, the presence of an oxidation agent can accelerate the corrosion rate of Ta by HF by accelerating the formation of Ta2O5 film on the Ta surface.
Table 1 Corrosion modes of Nb3Sn superconducting wire in the acid solutions
Sample
Type of acid solution
Corrosion mode
B1
HF
Completely immersed
B2
HF (atmosphere)
Sample is above the liquid level
B3
HF + H2O2
Completely immersed
B4
HF + HNO3
Completely immersed
Table 2 Corrosion modes of high-purity tantalum sheet in the acid solutions
Fig.1 Cross-sectional SEM image and corresponding EDS element distributions of Nb3Sn superconducting wire prepared by internal tin method
Fig.2 Cross-sectional OM image of Nb3Sn superconducting wire layered structure
Fig.3 Cross-sectional OM imags of Nb3Sn superconducting wire before (a) and after (b) removing copper stabilization layer (Inset in Fig.3b shows the locally enlarged image of square area)
Fig.4 Corrosion modes of Nb3Sn superconducting wire in the acid solutions
Fig.5 Macroscopic morphology of A1 sample after 4 h corrosion by hydrofluoric acid solution
Fig.6 Cross-sectional OM images of Nb3Sn superconducting wire samples A1 (a), A2 (b), A3 (c), and A4 (d) after corrosion by different corrosion media (Inset in Fig.6c shows the locally enlarged image of square area)
Fig.7 Nb3Sn superconducting wire sample with exposed Nb filaments
Fig.8 Corrosion modes of high-purity tantalum sheet in the acid solutions
Sample
Type of acid solution
t / min
v/ (mg·min-1)
B1
HF
1800
0.0461
B2
HF (atmosphere)
1440
0.0576
B3
HF + H2O2
110
0.7545
B4
HF + HNO3
8
10.3750
Table 3 Corrosion rates of B1-B4 samples in different corrosion media
Fig.9 OM images of the tantalum sheet samples B1 (a), B2 (b), B3 (c), and B4 (d) after corrosion for 1 h (a, b) and 2 min (c, d) by different corrosion media
Fig.10 XRD spectra of uncorroded and corroded tantalum sheets
Fig.11 XPS of Ta4f (a) and F1s (b) in the corrosion solution of B1 sample
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