A New Concept of Molten Salt Systems for the Electrodeposition of Si, Ti, and W

ConspectusThis Account describes the results of the electrodeposition of film-like Si, Ti, and W by utilizing molten salts selected based on a new concept. The proposed molten salt systems, KF-KCl and CsF-CsCl, have high fluoride ion concentrations, relatively low operating temperatures, and high solubility in water.First, KF-KCl molten salt was used for the electrodeposition of crystalline Si films to establish a new fabrication method for Si solar cell substrates. The electrodeposition of Si films from the molten salt at 923 and 1023 K was successfully achieved using K₂SiF₆ or SiCl₄ as the Si ion source. The crystal grain size of Si was larger at higher temperatures, indicating that higher temperatures are advantageous for the application of Si solar cell substrates. The resulting Si films underwent photoelectrochemical reactions. Second, the electrodeposition of Ti films using the KF-KCl molten salt was investigated to easily impart the properties of Ti, such as high corrosion resistance and biocompatibility, to various substrates. Ti films with a smooth surface were obtained from the molten salt containing Ti(III) ions at 923 K. Electrochemical tests in artificial seawater revealed that the electrodeposited Ti films had no voids and cracks and that the obtained Ti-coated Ni plate had a high corrosion resistance against seawater. Finally, the molten salts were used for the electrodeposition of W films, which are expected to be used as diverter materials for nuclear fusion. Although the electrodeposition of W films was successful in the KF-KCl-WO₃ molten salt at 923 K, the surface of the films was rough. Therefore, we used the CsF-CsCl-WO₃ molten salt, which can be employed at lower temperatures than KF-KCl-WO₃. We then successfully electrodeposited W films with a mirror-like surface at 773 K. Such a mirror-like metal film deposition has not been reported before using high-temperature molten salts. Further, the temperature dependence of the crystal phase of W was revealed by the electrodeposition of W films at 773-923 K. β-W was obtained at 773 and 823 K, α-W was obtained at 923 K, and a mixed phase of α- and β-W was obtained at 873 K. In addition, single-phase β-W films with a thickness of approximately 30 μm were electrodeposited, which has not been reported before.The results show that our proposed molten salt systems are advantageous for electroplating Si, Ti, and W. Our approach is also expected to be applicable for the electrodeposition of other metals such as Zr, Nb, Mo, Hf, and Ta.