Electroplating silicon and titanium in molten fluoride media

Abstract

The electrolytic reduction mechanisms of K₂SiF₆ and K₂TiF₆ solutions in LiF-KF and LiF-NaF-KF eutectic mixtures have been studied at temperatures between 550 and 850°C.

The reduction of K₂SiF₆ proceeds by two successive electron transfers,

$$Si(IV) + 2e \to Si(II) + 2e \to Si$$

coupled with an antidisproportionation reaction

$$Si(IV) + Si \underset{{k_b }}{\overset{{k_b }}{\longleftrightarrow}} 2Si(II)$$

Very pure thin silicon layers, up to 300 μm thick, were obtained on a silver substrate.

The cathodic reduction of TiF ²⁻₆ ions occurs in two well separated reversible steps,

$$TiF_6^{2--} + e \to TiF_6^{3--} + 3e \to Ti + 6F^--$$

Adherent coatings of pure titanium were found to be linked to the copper substrate by an interdiffusion sublayer comprising Ti₂Cu, TiCu, Ti₂Cu₃ and TiCu₄ which were formed in a narrow potential domain preceding titanium deposition.