The Study Of Polysilicon Purification And Temperature Field And Stress Field During Cold Crucible Directional Solidification

Polysilicon as the main raw material of photovoltaic solar energy industry has high practical value and broad prospects. Silicon solar battery materials occupy a large proportion in the cost of solar power generation. So it is very important to improve the purity and efficiency of silicon to improve the photoelectric conversion rate of solar energy and improve the competitiveness of the industry.In the process of preparation polycrystalline silicon, the presence of a variety of metal and non-metallic impurities makes the electromagnetic conversion rate of polysilicon decreased. Therefore, before obtaining the directional solidification of polycrystalline silicon, we need to purify silicon raw materials to achieve a higher purity and then get a better photoelectric conversion rate. In this paper, the vacuum arc melting of metallurgical grade silicon was carried out and the X- ray fluorescence spectrum of the purified sample was analyzed. Vacuum arc melting on the impurities in polysilicon Fe, Al, Mg have a more obvious removal effect. With the increase of time, the purification effect is improved, and the samples can be turned over to improve the purification effect. The increase of current can improve the purification effect, but the effect is relatively weak.Based on previous studies on this topic, a 3D physical model of electromagnetic cold crucible was created and numerical simulation calculation of electromagnetic field in cold crucible, temperature field and stress field by ANSYS finite element was made. The experiments of vacuum arc melting and purification of metallurgical grade silicon were carried out and the purification results were analyzed.The simulation results show that with the increasing of electric current and reducing of the frequency, the electromagnetic field intensity in the crucible increases. Under the condition of full load, the magnetic induction intensity of the materials is mainly concentrated in the crucible along the height of the induction coil and decreases when closing to the crucible wall. By the numerical simulation calculation of different heating power and cooling rate in the distribution of temperature field, the result showing that with the increasing of heating current, it can reach a higher temperature and obtain the more efficient melting results in the same period. At the same time, the internal melt temperature presents the declining trend from the corner to center. When cooling, the temperature field of the melt follows strict directional solidification and the trend can be described as top-bottom temperature is gradually reduced step by step. The temperature field measurements verify the reliability of the simulation.The distribution of the thermal stress on the melt was calculated and directional solidification samples at different cooling rates were analyzed. With the increasing of cooling rate, silicon ingot surface stress difference is greater, and the stress concentration is more obvious. The strain value is greater. By using the directional solidification experiments, comparing the crack on the surface of silicon ingot with different parameters, the simulation results are verified.