Electrodeposition of Dense Tungsten Films from Molten KF-KCl-WO3 and Csf-Cscl-WO3

Abstract

Tungsten metal is known for its robustness, especially high melting and boiling points and hardness. However, its hardness and brittleness make it difficult to work. Thus, electrodeposition of dense tungsten films is worth investigating. There are a number of reports on the electrodepositon of tungsten from high temperature molten salts such as fluorides, chlorides, oxides, etc. It is known that dense and coherent tungsten deposits are more easily obtained from molten fluorides compared to molten chlorides. Although good tungsten deposits can be electrodeposited from molten oxides, the operation temperature tends to be high, typically over 1123 K. One drawback of typical fluoride melts like LiF-NaF-KF is difficulty in removing the adhered molten salts by water washing because LiF and NaF have limited solubility to water.From this background, we proposed new electrolyte baths, molten KF-KCl and CsF-CsCl. Since all the components salts of KF, KCl, CsF and CsCl have large solubility to water, adhered salts to deposited films can be easily removed by water washing.First, electrodeposition of tungsten was investigated in eutectic KF–KCl melt to which WO3 was added at 923 K. Cyclic voltammograms at copper and gold electrodes indicated the deposition of tungsten metal at more negative potential than 1.55 V vs. K+/K. The effects of electrolysis conditions on the morphology of deposits were studied by changing the cathodic current density from 12.5 to100 mA cm-2. As shown in Fig.1 (a), a dense tungsten film with a thickness of approximately 20 µm was obtained on a copper substrate electrode at 12.5 mA cm−2 for 120 min. From the Raman spectra of molten KF–KCl and molten KF–KCl–WO3 at 923 K, the coordination structure of W(VI) complex ion was indicated to be fac-[WO3F3]3−.Secondly, eutectic CsF-CsCl melt was used to lower the operation temperature. After adding 2.0 mol% of WO3, the electrodeposition was investigated using copper and gold electrodes at 823-923K. As shown in Fig.1 (b), a dense and smoother tungsten film was obtained by galvanostatic electrolysis at 12.5 mA cm−2 for 120 min at 873 K. Figure 1