Direct electrolytic reduction of SiO 2 was investigated in molten CaCl 2 at 1123 K as a fundamental study to develop a continuous process for solar-grade Si production. Several different types of SiO 2 granules, as well as SiO 2 pellets, were successfully reduced to Si on the bottom cathode of a Si plate. Three parameters were varied in the reduction of SiO 2 granules: electrode potential, layer thickness of the SiO 2 granules, and SiO 2 particle size. The reduction rate was evaluated by the magnitude of the reduction current. The main factor determining the reduction rate was the diffusion of O 2− ions inside the reduced porous Si layer filled with the electrolyte. Another factor which influenced the reduction rate was the contact resistance between Si granules.

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Sn-Cu films were electroplated on Cu seed layers of various thicknesses. The velocity of the diffusion of Cu atoms from a seed layer into the electroplated Sn-Cu film was investigated, focusing on the effect of microstructural properties of the film. It was found that the diffusion velocity of Cu from a Cu seed layer into the electroplated Sn-Cu film increased with a decrease in the thickness of the Cu seed layer. Moreover, the film microstructure was influenced by the thickness of the Cu seed layer. When the crystalline quality of the Cu seed layer was inferior, a large lattice mismatch developed at the interface between the electroplated Sn-Cu film and the Cu seed layer. As a result of this lattice mismatch, many lattice defects and stress may exist at the interface between the electroplated Sn-Cu films and the Cu seed layers. Therefore, the lattice mismatch was considered to be related to the high-diffusion velocity of Cu from the Cu seed layer. Furthermore, the high-diffusion velocity of Cu was found to cause whisker formation.

Direct electrolytic reduction of powdery SiO2 has been investigated in molten CaCl2 at 1123 K aiming at a low cost and mass production of solar-grade Si. SiO2 powder was set on a Si plate which was installed at the bottom of the crucible and connected to a graphite rod used as a current lead. SiO2 powder was reduced to Si by cathodic polarization of the Si plate. The reduction rate was evaluated by the magnitude of the reduction current, in which three parameters were varied: electrode potential, SiO2 layer thickness, and SiO2 particle size. As a result, the rate-determining step of the reduction is suggested to be the diffusion of O2- ions inside the reduced porous Si particles filled with the electrolyte. During the reduction process, however, other factors such as contact resistance between Si particles and porosity of SiO2 particles possibly influence the reduction rate.