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Silicon dioxide (SiO(2)) is conventionally reduced to silicon by carbothermal reduction, in which the oxygen is removed by a heterogeneous-homogeneous reaction sequence at approximately 1,700 degrees C. Here we report pinpoint and bulk electrochemical methods for removing oxygen from solid SiO(2) in a molten CaCl(2) electrolyte at 850 degrees C. This approach involves a 'contacting electrode', in which a metal wire supplies electrons to a selected region of the insulating SiO(2). Bulk reduction of SiO(2) is possible by increasing the number of contacting points. The same method was also demonstrated with molten LiCl-KCl-CaCl(2) at 500 degrees C. The novelty and relative simplicity of this method might lead to new processes in silicon semiconductor technology, as well as in high-purity silicon production. The methodology may be applicable to electrochemical processing of a wide variety of insulating materials, provided that the electrolyte dissolves the appropriate constituent ion(s) of the material.

In this paper we address several key aspects to the formation mechanism of self-organized oxide nanotube layers grown by anodization of valve metals and their alloys in fluoride ion containing electrolytes. We suggest that ͑i͒ the self-organized structure is produced as a result of an autocatalytic reaction, in which electrochemical oxidation and chemical dissolution of oxide accelerate each other; ͑ii͒ in the initial growth stage the competition for oxidizable area between neighboring initial growth spots is a key element in self-organization; and ͑iii͒ the diameter of the nanotubes on different materials and as a function of anodization voltage is strongly related with the anodic growth factor ͑nm/V͒ of the valve metal oxides. Additionally, for multilayer pore structure growth the present work provides insight into the sites of highest reactivity in repeated anodization experiments ͑bottom of the pores, in between pores͒.

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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+ /Ca͒ for 1 h at 1123 K. X-ray diffraction confirmed that amorphous SiO 2 was reduced to crystalline Si. From scanning electron microscopy ͑SEM͒ 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 ͕111͖ 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.Recently, electrolysis of metal oxides in molten chlorides such as CaCl 2 has come to attract attention as a substitute for the traditional metal production methods. Electrochemical reduction of metal oxide to metal in molten CaCl 2 was first reported by Okabe et al. 1 and recently also reported by Chen et al. 2,3 However, their works were restricted to metal oxides, which generally have nonstoichiometric oxide phases, and there was no report for oxides of nonmetallic elements.The present authors tried to electrochemically reduce SiO 2 to Si, where Si is classified as a nonmetal element, and found that direct electrolytic reduction of solid SiO 2 to Si was possible in molten CaCl 2 . 4 Despite the good insulating ability of SiO 2 , successful reduction was achieved by using a contacting electrode method in which a SiO 2 plate was wound by Mo wire. In the initial stage of the reduction, electrons are supplied directly to the SiO 2 through the Mo wire. The total reaction is written asThe reduction rate is extremely large in spite of it being a solid-state reaction, and the whole reduction of a SiO 2 plate ͑1 mm thick͒ has been achieved. 4 Recently, it was found that the reaction reaches a thickness of 200 m inside of the plate at 1.00 V ͑vs. Ca 2+ /Ca͒ for 1 h. 5 Applying this new electrochemical reaction, solar grade Si is expected to be produced by removing only oxygen from high-purity SiO 2 , called "solar grade SiO 2 " hereafter. This method can contribute to further widespread use of Si solar cells from the standpoints of both cost and productivity, because solar grade SiO 2 is not expensive and is easily purified. 6 With reference to our report, 4 Jin et al. recently reported that porous pellets of SiO 2 powder can be electrochemically reduced to Si in molten CaCl 2 . 7 Furthermore, by using this method, pinpoint reduction is possible at the neighborhood of the contacting point between the c...