(Invited) Effect of Temperature on the Crystal Structure and Surface Morphology of W Films Electrodeposited from Csf–Cscl Eutectic Melt
Authors: Norikawa Yutaro , Meng Xianduo , Yasuda Kouji , Nohira Toshiyuki 
Published: July 2022

1. Introduction
Tungsten is a metal with excellent properties such as high heat resistance, high strength, and low thermal expansion, and is used for carbide tools, heat sinks, and fusion reactor diverters. However, due to its hardness and brittleness, tungsten is difficult to process into complex shapes and thin films. If tungsten plating on materials with good processability becomes possible, the range of applications will be greatly expanded. Therefore, as one of the plating methods, electrodeposition of tungsten in high-temperature molten salts has been investigated [1–3].
Our group has previously reported the electrodeposition of α-W films in molten KF–KCl–WO3 at 923 K and mixed phase films of α-W and β-W in molten CsF–CsCl–WO3 at 873 K [4]. We also reported that β-W films with mirror-like surface were electrodeposited in molten CsF–CsCl–WO3 at 773 K [5]. In the present study, we investigated the effect of bath temperature on the smoothness and crystal structure of W films electrodeposited from molten CsF–CsCl–WO3. We varied the temperature from 773 to 923 K and the cathodic current densities from 6 to 25 mA cm−2, and analyzed the electrodeposited W films on Cu plate substrates by XRD and SEM/EDX.
2. Experimental
The experiments were carried out in eutectic CsF–CsCl melt (50:50 mol%, melting point 713 K) with WO3 (1.0 mol%) additive at various temperatures in the range of 773–923 K. The melt was placed in a graphite crucible and the experiments were conducted in an argon atmosphere in an airtight SS container. Cu plates were used as working electrodes and a glass-like carbon rod was used as a counter electrode. A Pt wire was used as a quasi-reference electrode and the potential was calibrated by Cs+/Cs potential measured at a Ag electrode. Samples prepared at cathodic current density of 6–25 mA cm−2 were washed with distilled water to remove adhered salts, and then analyzed by XRD and SEM/EDX.
3. Results and Discussion
Fig. 1 shows the appearance of the samples obtained at 6–25 mA cm−2 and 773–923 K. No electrodeposits were obtained at 25 mA cm−2 and 773 K, which is due to the co-deposition of Cs metal fog because the potential during the electrolysis was close to 0 V with respect to Cs+/Cs potential. In other conditions, gray or silvery colored deposits were obtained. In particular, a mirror-like surface was obtained at 6 mA cm−2 and 773 K, indicating that the electrodeposited W film had a highly smooth surface. Fig. 2 presents the XRD patterns of the electrodeposits; all the XRD patterns were attributed to metallic W. However, only β-W was detected below 823 K, both α-W and β-W were detected at 873 K, and only α-W was detected at 923 K. This result indicates that the crystal structure of W obtained in molten CsF–CsCl–WO3 varies with bath temperature.
To investigate the smoothness of each obtained W film, we performed cross-sectional SEM observation and surface roughness (Sa) measurement. Fig. 3 shows the cross-sectional SEM images. At all bath temperatures, the lower the current density, the smoother the surface becomes, and the higher the current density, the more uneven the surface becomes. This is because the concentration gradient of W(VI) ions in the diffusion layer becomes larger, and the electrodeposition of W proceeds preferentially on the convex surface. It was found at the same low current density that the lower the bath temperature, the higher the smoothness. In particular, the mirror-like W film obtained at 6 mA cm−2 and 773 K showed almost no irregularity in the cross-sectional SEM image. The surface roughness Sa was as small as 0.32 μm, indicating that the W film was extremely dense and smooth. The reason for the smoother surface at lower bath temperatures is speculated to be due to the crystal structure of β-W and the suppression of crystal growth.
Acknowledgement
The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto Univ.
References
[1] S. Senderoff and G. W. Mellors, Science, 153, 3743 (1966).
[2] A. Katagiri, M. Suzuki, and Z. Takehara, J. Electrochem. Soc., 138, 767 (1991).
[3] Y. Liu, Y. Zhang, F. Jiang, B. Fu, and N. Sun, J. Nucl. Mater., 442, S585 (2013).
[4] T. Nohira, T. Ide, X. Meng, Y. Norikawa, and K. Yasuda, J. Electrochem. Soc., 168, 046505 (2021).
[5] X. Meng, Y. Norikawa, and T. Nohira, Electrochem. Commun., 132, 107139 (2021).

Figure 1