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

Conspectus This Account describes the results of the electrodepositionoffilm-like Si, Ti, and W by utilizing molten salts selected based ona new concept. The proposed molten salt systems, KF-KCl andCsF-CsCl, have high fluoride ion concentrations, relativelylow operating temperatures, and high solubility in water. First,KF-KCl molten salt was used for the electrodepositionof crystalline Si films to establish a new fabrication method forSi solar cell substrates. The electrodeposition of Si films from themolten 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, indicatingthat higher temperatures are advantageous for the application of Sisolar cell substrates. The resulting Si films underwent photoelectrochemicalreactions. Second, the electrodeposition of Ti films using the KF-KClmolten salt was investigated to easily impart the properties of Ti,such as high corrosion resistance and biocompatibility, to varioussubstrates. Ti films with a smooth surface were obtained from themolten salt containing Ti(III) ions at 923 K. Electrochemical testsin artificial seawater revealed that the electrodeposited Ti filmshad no voids and cracks and that the obtained Ti-coated Ni plate hada high corrosion resistance against seawater. Finally, the moltensalts were used for the electrodeposition of W films, which are expectedto be used as diverter materials for nuclear fusion. Although theelectrodeposition 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 moltensalt, which can be employed at lower temperatures than KF-KCl-WO3. We then successfully electrodeposited W films with a mirror-likesurface at 773 K. Such a mirror-like metal film deposition has notbeen reported before using high-temperature molten salts. Further,the temperature dependence of the crystal phase of W was revealedby the electrodeposition of W films at 773-923 K. beta-Wwas obtained at 773 and 823 K, alpha-W was obtained at 923 K, anda mixed phase of alpha- and beta-W was obtained at 873 K. Inaddition, single-phase beta-W films with a thickness of approximately30 mu m were electrodeposited, which has not been reported before. The results show that our proposed molten salt systems are advantageousfor electroplating Si, Ti, and W. Our approach is also expected tobe applicable for the electrodeposition of other metals such as Zr,Nb, Mo, Hf, and Ta.