Study On Poly-silicon Preparation Directly By Molten Salt Electrolysis

With economics development, oil and other traditional energy resources are depletedand environmental problems have received major concerns recently. All countries in theworld research the development and utilization of new energy sources. Solar energysource, as a clean and persistent energy, is considered as one of the most potentialrenewable resource since it is a clean and safe, relatively sustained, steady andinexhaustible energy.This project is studied by the way, using silica powder as raw material for thepreparation of electrolytic reduction of solid SiO2to Si in molten chlorides combiningtraditional metallurgical process, which is proposed to be a considerable technology toprepare poly-silicon. The theory decomposition voltage of silica is 1.10 V (vs.Ca2+/Ca),calculated by thermodynamics and electrochemical methods. Pure Si was preparedthrough the method of direct electrolytic reduction of SiO2in the molten CaCl2system.X-ray diffraction, SEM and EDS were used to analyze the structure and propertres theproduction. The results show that crystal and pure Si can be achieved by electrolyzingSiO2at 2.6 V potential for 12 hours, and the proper temperature is 850℃. And cationicreaction occurs at 2.6 V potential, while calcium-thermal reduction reaction occurs at 2.8V.The influence of samples weight on current is discussed. When the sample weight is added,the current increases. The current efficiency of electrolysis can be affected by sinteringtemperature and electrolytic temperature. When the sintering temperature is 600℃andthe electrolytic temperature is 850℃, the current efficiency is the highest. The surface ofproduction is analysed after molten salt electrolysise. The main reasons of current increaseare graphite shedding and secondary reaction.At different temperatures, Si was Electrolysised successfully in CaCl2(50 mol%)–NaCl mixed molten salt, which proved the feasibility of molten salt electrolysis at lowtemperatures. Meanwhile, the reaction mechanism is analysed. And the electrolyticreduction process is: in the original SiO2, oxygen is removed from the Si-O-Si bridges byelectrochemical reduction, Si dangling bonds are formed. The Si in this state is regarded asa reaction intermediate. The reaction intermediate combines with the other intermediate in the nearest neighbor, immediately forming amorphous silicon. Crystal silicon is producedby a rearrangement of Si at 850℃.