Fabrication of α-W and β-W Films By Using CsF–CsCl–WO3 Molten Salt: Effect of Oxygen Contents in W Films
Authors: Wang Haochen , Norikawa Yutaro , Nohira Toshiyuki 
Published: December 2024

1. Introduction
Tungsten(W) has a melting point of 3695 K, the highest among pure metals, and has excellent heat resistance. It also exhibits excellent properties such as high wear resistance, high corrosion resistance, low thermal expansion, and low sputtering rate. Because of these properties, W has been expected to be used as plasma-facing materials in fusion reactors [1]. However, its hardness and brittleness make it difficult to process into complex shapes, and conventional W processing methods are costly. Therefore, W electrodeposition in molten salts has been explored as an alternative processing method [2–4].
Recently, our group has reported that dense and mirror-like β-W films can be obtained in molten CsF–CsCl–WO3 at 773 K [5]. We have also found that the crystal structure of electrodeposited W has a temperature dependence between 773 and 923 K [6]. We predict that the oxygen content in the W films influences the crystal structure. Thus, in this study, the crystal structure and oxygen content of W films electrodeposited in molten CsF–CsCl–WO3 at 773–973 K were analyzed to investigate their relationship.
2. Experimental
Galvanostatic electrolysis was conducted in eutectic CsF–CsCl (50:50 mol%, melting point 713 K) containing WO3 (2.0 mol%) at 773–973 K in an Ar atmosphere glove box. For galvanostatic electrolysis, a Cu foil was used as the working electrode, a glassy carbon rod was used as the counter electrode and a Pt wire was used as the quasi-reference electrode. Surface morphology of the samples was observed by scanning electron microscopy (SEM). Phase identification of the samples was conducted by X-ray diffraction (XRD). Analysis of oxygen content in the samples was conducted by inert gas fusion-NDIR.
3. Results and Discussion
Fig. 1 shows the XRD patterns of the samples obtained by galvanostatic electrolysis at various temperatures. At temperatures of 773 K, 823 K, and 873 K, only β-W was detected. A mixture of α-W and β-W was detected at 923 K, and only α-W was detected at 973 K. Fig. 2 shows the oxygen contents in W films electrodeposited at various temperatures. The oxygen contents decreased from 6.65 at% to 0.24 at% as the bath temperature increased from 773 K to 973 K. These results suggest that the presence of oxygen in W films stabilizes β-W. Furthermore, our group has reported that the structure of the W(VI) ions in CsF–CsCl–WO3 is fac-[WO3F3]3− [5]. Thus, the electrochemical reduction of W(VI) ions can be described as the following formula:
[WO3F3]3− + 6e− → W(s) + 3O2− + 3F−

It is predicted that the covalent W–O bonds in [WO3F3]3
− partially remained during the electrochemical reduction and that oxygen atoms were incorporated into the W films. However, higher temperature is expected to accelerate the dissociation of covalent W–O bonds and lower the oxygen content in the W films, resulting in the formation of α-W.
To further confirm the relationship between crystal structure and oxygen content in the W films, post-annealing treatments were performed using β-W films electrodeposited at 823 K. The post-annealing treatments were conducted under vacuum conditions of 2 × 10−
3 Pa at 873 K or 973 K. The β-W films were heated to 873 K or 973 K for 10 minutes and then held for 3 hours. The XRD patterns of β-W as-electrodeposited at 823 K and after annealing at 873 K or 973 K are shown in Fig. 3. After the annealing treatments at 873 K and 973 K, the electrodeposited β-W films were both transformed to α-W. Moreover, the oxygen content in the samples annealed at 973 K were analyzed. As shown in Fig. 4, the oxygen content decreased from 1.82 at% to 0.38 at% accompanied by crystal phase change from β-W to α-W. The reason for the phase transformation from β-W to α-W is speculated to be the breaking down of the covalent W–O bonds in β-W, resulting in the release of oxygen.
References
[1] T. Hirai et al., Nuclear Materials and Energy, 9, 616-622 (2016).
[2] S. Senderoff and G. W. Mellors, J. Electrochem. Soc.,
114, 586 (1967).
[3] K. Koyama, M. Morishita, and T. Umezu., Electrochemistry, 67, 677 (1999).
[4] K. Nitta, T. Nohira, R. Hagiwara, M. Majima, and S. Inazawa, Electrochim. Acta, 55, 1278 (2010).
[5] X. Meng, Y. Norikawa, T. Nohira, Electrochem. Commun., 132, 107139 (2021).
[6] Y. Norikawa, X. Meng, K. Yasuda, T. Nohira, J. Electrochem. Soc., 169, 102506 (2022).

Figure 1