Investigation Of Solar Grade Silicon By Direct Electrolytic Reduction Of Silica Optical Fiber Wastes In Low Temperature Molten Salt

With economic development,oil and other traditional energy resources are depleted and environmental problems have received major concerns recently,all countries in the world strongly to research Development and utilization of New Energy Sources.Solar energy source,as a clean and persistent energy,is considered as one of the most potential renewable resource since it is a clean and safe,relatively sustained,steady and inexhaustible energy.The photovoltaic industry is growing at the rate of 30%annually and correspondingly solar grade silicon,the most important feedstock supply of photovoltaic industry,the demand will increase greatly.But to rely solely on traditional production process has been far from meeting the rapid development of photovoltaic industry,In 2004 it had been appeared the shortage of polycrystalline silicon raw materials Worldwide.So it will be significant to develop a new technique and new equipment with high efficiency and low cost for polycrystalline silicon not depending on the semiconductor industry.At present, the main new techniques which studied much for solar grade silicon are:improved-Siemens process,silane thermal decomposition process,metallurgical process,carbothermic reduction of silica,molten salt electrolysis and so on.This project is study a new way using optical fiber wastes as raw material for the preparation of electrolytic reduction of solid SiO₂ to Si in molten chlorides combining traditional metallurgical process was proposed to be a considerable technology to prepare solar grade silicon(SOG-Si).Using thermodynamics and electrochemical two methods had been calculated that the theory of decomposition of the silica voltage,as 1.10 V (vs.Ca²⁺/Ca).The feasibility of the method that using optical fiber wastes as raw material for the preparation of electrolytic reduction of solid SiO₂ to high purity Si in molten chlorides is proved by a series of experiments.The Raman spectrum of the sample indicates that Si had been prepared through the process of direct electrolysis reduction under the electrolysis voltage 2.8 V,3.0V.The main impurity in the sample is oxygen and the other impurities content are very low in particular boron,phosphorus.There are crystal silicon and amorphous silicon two forms in the samples.Based on the concept,a new process production of solar grade silicon(SOG-Si)using optical fibers wastes has been emphatically reviewed,which has some advantages such as low consumption,simple production flow,optical fibers wastes,environmental protection.Meanwhile to this kind "solid phase reduction reaction in situ" the reaction mechanism has carried on the analysis and the corresponding reaction model is established as the solid phase reaction in situ for solid SiO₂.And the electrolytic reduction process is: In the original SiO₂,oxygen is removed from the Si-O-Si bridges by electrochemical reduction,Si dangling bonds are formed.The Si in this state is regarded as,a reaction intermediate.The reaction intermediate combines with the other intermediate in the nearest neighbor,immediately forming amorphous silicon(a-Si).Crystal silicon(c-Si)is produced by a rearrangement ofa-Si at 850℃.Based on the reaction model,the rate-determining step in the SiO₂ reduction had been discussed.The rate-determining step of the total electrochemical reaction is considered to be O^2-diffusion in molten salt.Using the dioxide silica powders or fine particles replaces the quartz plate and the quartz block could to enhance reaction rate to some extent.On the other hand,the electrochemical reaction rate is also related to the electrolysis potential.SiO₂ is reduced to Si between 0.70 and 1.25 V(vs.Ca²⁺/Ca)by electrolytic reduction in molten salt,and the reduction rate of SiO₂ is higher at more negative potential.Molten salt electrolysis process of preparation of high purity silicon has lots of advantages compared with the traditional technology.It has attracted more and more attention and in the near future will develop into a new technique for prepare solar grade silicon,which has some advantages such as low consumption,low cost,environmental protection.It will deal with the shortage of SOG-Si in the world market.