Koji Amezawa

Mechanism of Direct Electrolytic Reduction of Solid SiO2 to Si in Molten CaCl2

The mechanism of direct electrolytic reduction of SiO 2 in molten CaCl 2 was studied. Morphological and crystallographic investigations were conducted on Si prepared by potentiostatic electrolysis of SiO 2 contacting electrode at 1.10 V (vs. Ca 2 + /Ca) for 1 h at 1123 K. X-ray diffraction confirmed that amorphous SiO 2 was reduced to crystalline Si. From scanning electron microscopy (SEM) and energy-dispersive X-ray results, it was proved that Si columns were formed perpendicular to the reaction interface between Si and SiO 2 and that vacant spaces were formed between the columns. It was found from field emission SEM observation that the Si column had basically a hexagonal prismatic and stacking structure. Transmission electron microscopy and electron diffraction results revealed that the Si column was a single crystal having {111} twin planes perpendicular to the axis of the column. It is explained that amorphous Si is first formed by electrochemical reduction and then thermally transformed to crystalline Si. The rate-determining step of the reduction was found to be O 2 - diffusion in the vacant space between the columns.

Electrical Conduction Properties of Sr-Doped LaPO4 and CePO4 under Oxidizing and Reducing Conditions

Electrical conduction properties of Sr-doped LaPO 4 and Sr-doped CePO 4 under H 2 O/O 2 and H 2 O/H 2 conditions were investigated with conductivity measurements. Conductivities of I mol % Sr-doped LaPO 4 were 10 - 5 . 2 -10 - 3 . 5 S cm - 1 at 500-925°C under wet reducing conditions and were close to those under wet oxidizing conditions. It was found from the H/D isotope effect on conductivity that the material showed dominant protonic conduction under wet reducing conditions. As for 1 mol % Sr-doped CePO 4 , conductivities were 10 - 2 . 8 -10 - 2 . 0 S cm - 1 at 500-925°C under wet oxidizing conditions and much higher than those of 1 mol % Sr-doped LaPO 4 under the same conditions. Such high conductivities of Sr-doped CePO 4 seemed attributable to electronic conduction due to partial oxidation of Ce 3 + to Ce 4 + caused by substituting Sr2 + for Ce 3 + . Under wet reducing conditions, however, conductivities of the material decreased to 10 - 5 . 2 -10 - 3 . 4 S cm - 1 at 500-925°C, which was almost comparable with those of 1 mol % Sr-doped LaPO 4 . It was concluded that Sr-doped CePO 4 showed mixed protonic and p-type electronic conductions under wet reducing conditions. Based on the results of conductivity measurements, defect structures in Sr-doped LaPO 4 and CePO 4 under wet oxidizing and reducing conditions are discussed.

Electrochemical Formation and Phase Control of Pr-Ni Alloys in a Molten LiCl-KCl-PrCl3 System

The electrochemical formation of Pr-Ni alloys was investigated in a molten LiCl-KCl-PrCl 3 (0.50 mol %) system at 723 K. A coherent and dense PrNi 2 film was rapidly formed on a Ni electrode by potentiostatic electrolysisat 0.50 V. When a thin Ni electrode (50 μm) was used, PrNi was formed at 0.50 V after the whole electrode became PrNi 2 . The formed PrNi 2 electrodes were transformed to other phases, i.e., PrNi 3 , Pr 2 Ni 7 , PrNi 5 , and Ni, by electrochemical displantation. The formation reactions of the alloys and the corresponding equilibrium potentials were clarified. From morphological and crystallographic investigations using transmission electron microscopy and electron diffraction, it has been suggested that the rapid PrNi 2 formation is due to the high Pr diffusivity in and near the grain boundaries.