Effect of Temperature on Electrodeposition of Ti in LiF – LiCl Eutectic Melt (2020)

1. Introduction Titanium metal has superior properties such as high specific strength, high corrosion resistance, biocompatibility, etc. In addition to these properties, crustal abundance of titanium is more than 40 times larger than those of commonly used copper and nickel. However, titanium is not used widely and commonly due to two problems: the high cost of present smelting method and the poor workability of itself. Therefore, both a new smelting method and a processing method are required. Plating titanium metal on an inexpensive substrate is one of the methods to resolve these problems. Electrodeposition of titanium is a promising plating method from the viewpoints of the cost and the flexibility in shape of substrate. Thus, electrodeposition of titanium metal has been investigated for a long time using high-temperature molten salts [1–4]. We have already reported that compact, smooth, and adherent titanium films are electrodeposited in molten KF–KCl and LiF–LiCl containing Ti(III) ions at 923 K [5–8]. However, crystal grains of titanium become large with the increase of a film thickness, which results in a rough surface. Concerning the crystal grain size, Wei et al . electrodeposited titanium in molten LiCl–NaCl–KCl–K 2 TiF 6 (+ Ti metal) at 723–973 K and reported that the grain size became small as the temperature decreased [1]. This report suggests that titanium films are expected to have a smoother surface at a lower electrodeposition temperature. Therefore, in the present study, we investigated the effect of temperature on morphology and smoothness of electrodeposited titanium films in LiF–LiCl eutectic melt at 823–973 K. The effect of temperature on the electrochemical behavior of Ti(III) ions was also studied. 2. Experimental The experiments were conducted in LiF–LiCl eutectic melt (LiF:LiCl = 30:70 mol%, m.p.: 774 K [9]) in an Ar glovebox. The experimental temperature was varied from 823 K to 973 K. Li 2 TiF 6 (2.00 mol%) and Ti sponge (1.33 mol%) were added to the bath and Ti(III) ions were prepared by comproportionation reaction. Ni plate, Mo flag, and Au flag electrodes were used as the working electrodes. The counter and reference electrodes were Ti rods. The potential of the reference electrode was calibrated by Li + /Li potential measured at a Mo flag electrode. Samples prepared by galvanostatic electrolysis of Ni plate substrates were analyzed by SEM/EDX after washing with distilled water to remove adhered salts. 3. Results and Discussion Galvanostatic electrolysis was conducted at 823, 873, 923, and 973 K. Fig. 1 shows the optical and surface SEM images of the samples. The cathodic current density and electrolysis time were 100 mA cm −2 and 10 min, respectively. The surface roughness ( S a) was measured by SEM, which is also shown in Fig. 1. All the samples have a metallic luster. The sample at 823 K has the highest brightness with metallic luster. The surface SEM observations indicate that the crystal grain size increases as the temperature rises. This tendency is reasonably explained by a previous work on the temperature dependent grain growth of Ti [10]. The value of S a also increases as the temperature increases. The smoothest Ti film with Sa = 2.05 ± 0.22 μm was obtained at 823 K. These results conclude that Ti film with smoother surface can be electrodeposited at a lower temperature by suppressing the grain growth of Ti. In the presentation, more detailed analysis results and the effect of temperature on the electrochemical behavior of Ti(III) ions will be discussed. Acknowledgement A part of this work was supported by JSPS Fellows grant number 19J15015. The present address of Kouji Yasuda is Graduate School of Engineering, Kyoto Univ. References [1] D. Wei, M. Okido, and T. Oki, J. Appl. Electrochem ., 24 , 923 (1994). [2] H.Takamura, I. Ohno, and H. Numata, J. Jpn. Inst. Metals , 60 , 388 (1996). [in Japanese] [3] A. Robin and R. B. Ribeiro, J. Appl. Electrochem., 30 , 239 (2000). [4] V. V. Malyshev and D. B. Shakhnin, Mater. Sci., 50 , 80 (2014) [5] Y. Norikawa, K. Yasuda, and T. Nohira, Mater. Trans ., 58 , 390 (2017). [6] Y. Norikawa, K. Yasuda, and T. Nohira, Electrochemistry , 86 , 99 (2018). [7] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc ., 166 , D755 (2019). [8] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc ., 167 , 082502 (2020). [9] J. Sangster and A. D. Pelton, J. Phys. Chem. Ref. Data , 16 , 509 (1987). [10] K. Okazaki and H. Conrad, Metall . Trans ., 3 , 2411 (1972). Figure 1