Vacuum Directional Solidification Study Calcium Aluminum Impurities In Silicon Purification Distribution Model

At present, there are two processes to make solar grade polysilicon, chemical process and metallurgical process. Chemical processe has more environmental load and high production costs, which has blocked the development of photovoltaic industry. While owe to lower cost, simple process, little equipment investment, metallurgy method has become hotspot research process in the photovoltaic industry. Directional solidification is the main part of metallurgy process, which is used to control the removal of impurities through the segregation of impurities in the solid-liquid interface and to control the growth of silicon crystal by the directional transfer of heat. The research on the mass transfer of impurities in the vacuum solidificaiton process, which directly affects the quality of the purified silicon, has not been reported.In this paper, the influences of the chamber pressure, temperature and solidificaiton rate on the removal of impurities are analyzed by the vacuum evaporation and solidification theory. The research results show that when the chamber pressure is very low (10-1～10-3Pa), the volatilication of impurity aluminum and calcium is molecular distillation, where the chamber pressure doesn’t affect the volatilization; raising temperature and decreasing solidification rate are benificial to the removal of impurities, and the effect of solidification rate is more significant than temperature.By analyzing the mass transfer of impurities during the vacuum directional solidification process, we propose a simple numerical model for distribution of impurities (Al, Ca) in silicon ingot during vacuum directional solidification, including segregation from silicon crystal to silicon melt as well as evaporation from silicon melt to vacuum atmosphere.The refining silicon experiments are carried out in vacuum directional solidification furnace. The concentration of impurities in the refined silicon ingot can be obtained by inductively coupled plasma emission spectrometer (icp-aes). In comparison with the theoretical model results, the experiment data agree well with this model, and the experiment result fluctuates around the theory calculated curve. According to the model, the effective segregation Keff and the total evaporation coefficient kT(Al) of Al under the experimental conditions are0.0137and2.6755×\10-6+m·s-1, respectively. The effective segregation Keff and the total evaporation coefficient kT(Al) of Ca under the experimental conditions are0.067and4.3302×10-6m/s, respectively.The influences of the chief factors such as temperature, solidification rate and the thickness of solid-liquid boundary layer on the concentration of aluminum in silicon are discussed. The aluminum content in silicon decreases with increasing temperature and with the decrease of solidification rate and the thickness of solid-liquid boundary layer. The optimal temperature and solidification in the experiment are estimated to be about1823K and10μm·s-1, respectively.