Effect of Temperature on Grain Growth during Ti Electrodeposition in Lif–LiCl Eutectic Melt
Authors: Norikawa Yutaro , Yasuda Kouji , Nohira Toshiyuki 
Published: July 2022

1. Introduction
Ti has superior properties, such as high specific strength, corrosion resistance, biocompatibility, and is used for many fields such as airplane, chemical plant, consumer products. However, its high manufacturing cost and poor processability have hindered its further spread. Thus, a number of smelting processes and processing methods have been investigated to reduce the cost and spread the use of Ti. Among them, plating Ti metal on inexpensive substrates is one of the methods that can utilize the surface properties of Ti. Electrodeposition is a promising plating method from the viewpoints of the cost and the flexibility in shape of substrate. Therefore, electrodeposition of Ti metal in high-temperature molten salts has been investigated for a long time [1–3].
Our group has reported the electrodeposition of Ti films with compact and smooth surfaces in molten KF–KCl and LiF–LiCl containing Ti(III) ions at 923 K [4–7]. We also reported that the Ti films obtained in LiF–LiCl at 823 K had smoother surfaces [8]. In the present study, we investigated the effect of temperature on the grain growth and smoothness of Ti films electrodeposited in LiF–LiCl eutectic melt at 823–973 K.
2. Experimental
The experiments were carried out in eutectic LiF–LiCl melt (30:70 mol%, melting point 774 K) at 823–973 K. The melt was placed in a nickel crucible and the experiments were conducted in an airtight Kanthal container in an argon glove box. As Ti(III) ion sources, Li2TiF6 (2.00 mol%) and Ti sponge (1.33 mol%) were added to the melt. The added amounts of Ti sponge were twice the amounts necessary to generate Ti(III) ions by comproportionation reaction between Ti(IV) ions and Ti(0) according to Eq. 1.
Ti(0) + 3Ti(IV) → 4Ti(III) (1)
Ni plates were used as working electrodes, and Ti rods was used as counter and reference electrodes. The samples prepared by galvanostatic electrolysis were analyzed by XRD and SEM/EDX after washing with distilled water to remove adhered salts.
3. Results and Discussion
Fig. 1 shows the optical and surface SEM images of the samples obtained by galvanostatic electrolysis at 823, 873, 923, and 973 K. The cathodic current density was 50 mA cm−2 and electrolysis time was 20 min. All the samples have a metallic luster and are confirmed to be Ti metal by XRD analysis. However, the brightness differs between samples, with the Ti film at 823 K showing the highest brightness. From the surface SEM images, the crystal grain size of Ti increases as the temperature rises. This tendency is reasonably explained by previous work on the temperature dependence on grain growth of Ti [9]. Also, the value of surface roughness (Sa) measured by SEM increases as the temperature increases. The smoothest Ti film with Sa = 2.05 ± 0.22 μm was obtained at 823 K. These results show that Ti film with smoother surface can be electrodeposited by suppressing the grain growth of Ti at lower temperatures.
To obtain thicker films, electrolysis was conducted for longer time at 823 K, where grain growth is suppressed. Fig. 2 shows the optical and SEM images of the samples obtained by galvanostatic electrolysis at a cathodic current density of 50 mA cm−2 and charge densities of 60, 150, and 450 C cm−2. As shown in the SEM images, the thickness of the Ti film increases with the increase in charge density. Although some deposition onto the edge was observed at 450 C cm−2, the thickness of the Ti film reaches 100 μm.
Acknowledgement
A part of this work was supported by JSPS Fellows grant number 19J15015. A part of this study was conducted as a collaboration with Sumitomo Electric Industries, Ltd. The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto Univ.
References
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Figure 1