Evaporation Behavior Of Impurities During Electron Beam Melting Of Metallurgical Silicon

Accompanying the development of low-carbon economy in recent years, solar photovoltaic industry brings about a huge space for development. Therefore, as one of the raw materials for solar cells, the requirement of solar-grade silicon (SOG-Si) will be greatly increased. The traditional technology for preparing SOG-Si is Siemens method. However, the output of production can't meet requirement of the rapid development of photovoltaic industry, due to its high production cost and long production cycle for preparing SOG-Si, and severally hampered the development of solar cell. Under this background, the metallurgical method used for preparing solar grade silicon has drawn attention of many counties for its environmental protection and low cost. It is well known that the electron beam melting (EBM) characterized by high vacuum and high temperature has been widely used for refining materials that contain impurities with high vapor pressure. So the EBM is one of the key technologies applied to remove volatile impurities for solar grade silicon. The volatile impurities in silicon, such as P,Al and Ca, are known as "life-time killer" impurities, which shorten the lifetime of exited carriers in silicon solar cell and disturb electric generation. However, little is known about the dynamics behavior of those impurities in molten silicon, which is indispensable for considering the possibility and efficiency of impurities removal during EBM process.In order to establish reasonable parameters to decrease impurity content more effectively, thermodynamics and dynamics discussion is carried out in this research.The effects of melting time and melting power on impurity removal were studied in this research, meanwhile, the evaporation mechanism of impurities removed from molten silicon, rate-controlling step of impurities evaporation and evaporation loss of silicon were discussed in view of thermodynamics and kinetics of vacuum metallurgy. Based on the above obtained results, the conclusions can be drawn in the following:P,Al and Ca can be effectively removed during EBM.But as for Fe and B,no obvious effect is observed. The impurity contents decreased with the increase of the melting power. With a certain melting power kept, the contents of P,Al and Ca decreased rapidly when the completed melting began, and then, the removal rate became low with the further increase of the melting time. The effect of melting time on Fe and B removal was not obviously observed. The removal reaction of P from the molten silicon during EBM is controlled by the diffusion in molten silicon and the evaporation from molten silicon, while the diffusion in molten silicon is for Al and Ca. Accompanying the removal of these impurities, the loss of silicon is occurred. The balance between the decrease of the impurity contents and the reduction of the loss of silicon should be considered during EBM of metallurgical grade silicon..