Study On Transportation And Removal Mechanisms Of Impurities During Silicon Purification By Electron Beam Melting

Solar grade silicon is the main raw material for solar cells.The solar grade silicon preparation method with high efficiency,low cost and stable process is the key to promote photovoltaic technology.Metallurgical route is an exclusive purification process for the preparation of solar grade silicon.Based on the differences of physicochemical properties between silicon and impurities,this method uses metallurgical silicon as raw material and integrates many metallurgical technologies,such as acid pickling,slag refining,vacuum refining,electron beam melting,directional solidification,etc.,to reduce the impurity content in silicon and obtain solar grade silicon.The differences of physicochemical properties between silicon and impurities are mainly in such characteristics as evaporation,segregation and oxidation.Each purification process in metallurgical route is based on only one of these characteristics,but not the combination of two or three of these characteristics.As a consequence,some impurities can not be removed deeply,or the driving force for their further removal is insufficient when they are removed to a certain level.Meanwhile,these technologies are independent of-each other.During each technology,silicon repeatedly undergoes melting,refining and solidifying process,leading to many problems such as long process,low efficiency and unstable quality,which increases the energy consumption and cost.During electron beam melting,material surface is bombarded by high speed electron beam.The kinetic energy of electrons are converted into hear energy to melt and refine the material.Electron beam melting has characteristics of high temperature and high vacuum.The temperature of the molten pool can be precisely controlled by adjusting the beam power,scanning pattern or frequency.In this study,it is proposed that the essence of impurity removal from silicon by metallurgical route is that impurity redistributes in different phases of silicon(solid,liquid and gas phase)or some other phases(slag or Si-X alloy)during metallurgical refining process.The characteristics of impurity in evaporation,segregation and oxidation are investigated.It is found that impurity has multiple characteristics,which can be used to removal them deeply.According to different types of impurities,electron beam is used to build special melting conditions and the transportation and removal mechanisms of these impurities are investigated.Based on the high temperature and high vacuum environment during electron beam melting,a full domain control model is established for impurities transportation in liquid phase,gas liquid interface and gas phase of silicon to analyze the dynamic mechanics of impurity removal.The results show that the overall mass transfer coefficient mainly depends on the temperature and the chamber pressure.Its value irncreases with the increase of temperature or the decrease of chamber pressure.Under the same melting condition,the order of the overall mass transfer coefficients for P,Al and Ca is kP>kAl>kCa,indicating P is easier to be removed by evaporation.During electron beam melting,the removal of P is controlled by both evaporation on gas-liquid interface and mass transfer in liquid boundary layer,and the removal of Al and Ca is controlled by evaporation on gas-liquid interface.Silicon also evaporates during impurity removal.When the removal efficiency of P is close to 100%,that of Al is still in a low level.A higher melting temperature or a longer melting time is needed to improve the removal efficiency,but leading to the increase of silicon loss.As for Al and Ca,these impurities have both the characteristics of segregation and evaporation.A special melting condition with unidirectional temperature gradient in molten silicon and high vacuum in gas phase is built to achieve directional solidification of silicon induced by electron beam.The results show that the distribution of Al and Ca in silicon show a segregation trend.The removal efficiency depends on the temperature of molten silicon and the solidification rate.A higher removal efficiency can be obtained by controlling the temperature and solidification rate.During melting process,the removal of impurity is controlled only by evaporation on gas-liquid interface.At 15 kW,the overall mass transfer coefficients of Al and Ca are 3.43×10-5 m/s and 2.04×10-5 m/s,respectively,and the removal efficiencies are 98%and 91%after 1800 s.During directional solidification process,the removal of impurity is controlled by the coupling of segregation on solid-liquid interface and evaporation on gas-liquid interface.The impurity content is further decreased to less than 0.7×10-4 wt.%.Compared with the traditional electron beam melting,the impurity removal rate is same or even higher and the energy consumption is lower due to the beam power decrease during solidification.In this experiment,compared with the traditional electron beam melting with the power of 15 kW and melting time of 3000 s,the energy consumption reduces by 20%with the melting time of 1800s and solidification time of 1200 s.P removal by electron beam melting and metal impurity removal by electron beam induced directional solidification are applied in an industrial-scale electron beam melting equipment.P content is decreased to less than 0.3×10-4 wt.%.The contents of Fe,Al and Ca are decreased to the magnitude of 10-6?10-5 wt.%,and the segregation effect be superior to that by the traditional solidification method.On this basis,from the perspective of energy utilization,the melting process is optimized by combining with the numerical simulation and experimental method.The energy utilization is improved through using a graphite substrate between silicon and copper crucible.The results show that the addition of graphite substrate is equivalent to increase the thermal resistance to reduce the heat loss.Compared with the traditional method,the temperature of the molten pool is higher at the same power to accelerate P removal.In the practical production,melting time can be reduced to achieve the same removal efficiency.The energy consumption decreases from 29.3 kW·h/kg to 19.5 kW·h/kg.