| Many problems caused by energy crisis lead to the necessity of reforms of energy structure which relies on traditional energy source. New energy emerged as having the advantages of no pollution and recyclable is expected to replace conventional energy. Among different kinds of new energy sources, solar energy has many advantages such as large reserves, easy exploitation, without geographical restrictions, clean and pollution-free, thus it can go beyond the wind, hydro, geothermal potential to become highly efficient and can be widely applied in the clean energy field in the future. Solar cell is the main apparatus to achieve a photoelectric conversion, Silicon material accounted for the vast majority of the solar cell raw materials market, because of its large reserves, high photoelectric conversion efficiency, stable performance and so on. Wherein the polysilicon is the main raw material of silicon solar cells.At first, Siemens method was the main chemical purification method of polysilicon, it has the weak points such as high energy consumption, particular complicate equipment, and the purity of its product is not available for solar cells. As metallurgical method can solve all those problems of Siemens method, moreover it does mot involve the chemical reactions of silicon and its refining temperature is relatively low compared with chemical purification method which means its energy consumption is low and its equipments are simple, thus it is suitable for industrial application.In this paper, we study the purification of polysilicon by alloy slag refining, aiming at the refining under low temperature, and thus remove boron by low consumption and low-cost. In order to change the high temperature condition in traditional method of slag refining, the silicon was alloyed with tin to form Si-Sn alloy which was used in the slag refining experiments. In order to study the effects of different conditions on the effect of boron removal, we changed the addition amount of Sn, the slag agents, refining time, mass ratio of slag and alloy. The main conclusions are as follows:(1)When we use Na2SiO3-CaO-SiO2 slag, with the increase of the addition amount of tin addition, the mass ratio of slag and Si-Sn alloy and the refining time, boron content in silicon decreases and the removal fraction of boron increases. When the tin addition amount is 50at%, boron content in silicon decreases from 12.92ppmw to 0.786ppmw and the removal fraction is 93.93%. When the mass ratio increases to 1.75:1, the boron content in silicon reaches a steady state.(2) When we use Na2CO3-SiO2 slag, with the increase of the addition amount of tin addition, the mass ratio of slag and Si-Sn alloy and the refining time, boron content in silicon decreases and the removal fraction of boron increases. When the tin addition amount is 50at%, boron content in silicon decreases from 12.92ppmw to 0.9ppmw and the removal fraction is 93.03%. When the mass ratio increases to 1.25:1, the boron content in silicon reaches a steady state. With the increase of basicity, the boron content in silicon decreases at first and then increases, mainly because of the boron removal is related to both of the basicity and the oxygen partial pressure. When the basicity is small, it is the main factor, when it increases to some value; the oxygen partial pressure becomes the main factor.(3) When we use Na3AlF6-Al2O3 slag, because it’s volatile when we refining under 1200℃, a lot of bubbles in the system has a certain agitation, speed up the removal of boron. As the refining time increases, the weight loss of the slag agents increases, which mainly due to the volatilization of Na3AlF6. The slag has no significant influence to the micro morphology of the sample, and easy and sample separation; When we use Na3AlF6-20at% Al2O3 slag, Si-60at% Al, the mass ratio of slag to Si-Al alloy is set to 1:1, and refining under 1200 ℃ for 1h, and the removal fraction of boron increased from 62.24% to 77.94% compared with the Al-Si sample without slag addition. |
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