Study On Removal Of Impurities In Silicon By Combining Electron Beam Melting With Directional Solidification

The fast development of photovoltaic industry (PV industry) in recent years drives the continued growth of the demand for solar grade silicon (SOG-Si). Metallurgical route for preparing SOG-Si with the characteristic of low cast, environment-friendly has get considerable development around the world, and lays the foundation for further development of the PV industry. Metallurgical route focuses on the removal of metal impurities, boron, and phosphorus, the removal principles and technologies of which depends on their properties and behaviors in silicon. As an important part of metallurgical route, electron beam melting (EBM) is used to remove impurities with high saturated vapor pressure. There are many guiding conclusions obtained by the research on EBM until now. However, the loss rate of silicon and energy consumption closing related to the purification process of EBM haven’t been reported so far. Meanwhile, EBM is still a single method and can’t be used to remove non-volatile impurities.The present work proposed an EBM model to analyze the relationship between the removal efficiency of phosphorus and loss rate of silicon under different beam power, In (1-ωi)=[(ki-ksi)/ksi]·In (1-η). The loss rate of silicon mainly depends on the removal efficiency of phosphorus and is unrelated with the beam power considering the removal of phosphorus. However, for aluminum, the loss rate of silicon increases with the increase of beam power and removal efficiency of aluminum. Energy consumption decreases with the increase of beam power under the same removal efficiency of phosphorus, but increases considering of aluminum. The removal of phosphorus and aluminum advances simultaneously during the melting process. Phosphorus removal process is slower comparing with aluminum in lower EBM power, needing longer melting time and limiting the whole removal process. The removal of aluminum turns into the limited step under high power conditions. The advantageous impact of the increase of the beam power on phosphorus is stronger than that on aluminum.The directional solidification experiment induced by exponential-type reduction of EBM is executed under18kW in the present work. Iron with segregation effect less than1can be removed effectively under the experiment with reducing tiem of beam power for65mins, and the proportion of area where the removal efficiency is more than90%measures up86%. Aluminum has both segregation effect and volatile effect. The propartion of the area where the removal efficiency is more than90%measures up92%in the experiment. The thickness of solid-liquid interface layer increases gradualy with decrease of beam power reducing rate, and reaches diffusion equilibrium. When the beam power reducing rate is fast, the enrichment effect of aluminum in molten pool is week by direactional solidification, which promotes the volatizilation of alminum weakly. While, the volatizilation of alminum enhances significantly through the enrichment of aluminum in molten pool when the beam power reducing rate becomes slow. Comparing with instantaneous solidification after electron beam melting, the removal efficiency of aluminum is higher under the following directional solidification experiment induced by exponential-type reduction of EBM, when the solidification (reducing electron beam) time is extended. The loss rate of silicon decreases and energy consumption lowers during the process achieving50%over simultaneously.