| With the rapid deterioration of the living environment and sustainable growth of the world population, demand for energy increases. Solar clean pollution-free, contain lots of power, has the very good development prospect. Over the past decade, the rapid development of photovoltaic industry, lead to the shortage of cheap solar-grade polysilicon. In order to meet the needs of the development the PV industry, a method of low-cost, low energy consumption, low pollution which is used to produce solar-grade polysilicon-metallurgy method, came into being.Metallurgy method can remove the impurity in the metallurgical silicon in turn, according to the different nature of the impurities. Metal impurity and P impurity, respectively, can be removed by directional solidification and electron beam melting. But we still have not found an effective low-energy method to remove impurities B. Alloy refining process offers the possibility to remove B impurity with a low-cost method due to its high purification efficiency and low processing temperature.In order to overcome the drawbacks of Si-Al system, in this thesis, the Si-Al-Zn ternary alloy refining was used to remove impurities B by adding metal Zn to Si-Al binary alloy. B impurity removal efficiency was studied from morphology of primary silicon, which was changed by varying the atomic percentage of Zn and cooling rate; and was studied by changing the quenching temperature and cooling rate in the terminal stages of solidification. Conclusions are as follows:(1) The macro and micro morphology of primary silicon in Si-Al alloy was changed by adding metal Zn, which has an impact on B impurity removal. In macro level, primary silicon in Si-Al alloy mainly exists in the form of tabular. After addition of different atomic ratio of Zn, primary silicon becomes irregular shape, which has a long length and broad width. In micro level, the amount of binary and ternary eutectic wrapped in primary silicon, was increase gradually with the increasing amount of added Zn metal. That leading to the refining ratio of B impurities increase rapidly and removal efficiency of B impurity significantly reduced. The refining ratio reaches0.41and removal efficiency of B impurity drops to69.5%when adding30at%Zn.(2) With increasing Zn content, both theoretical and practical recovery of primary Si are increased, but the actual recovery rate6-10%lower than the theoretical. (3) The content of B impurity in refined Si gradually die down and removal rate increases gradually with the decrease of cooling rate within the range of900-600℃. The content of B impurity reduced to the1.3ppmw and removal rate added up to91.2%when the cooling rate was3℃/min. While, the larger the cooling rate, the less content of B impurity and the higher the removal rate within the range of600-20℃. The B content down to1.4ppmw and removal rate reach90.5%when quench the sample, which reach the best effect.(4) The B impurity content decreased and removal rate increased gradually with the decrease of quenching temperature when quenching higher than the binary eutectic temperature at the same initial temperature and cooling rate conditions. Nonetheless, the B impurity content increased slightly and the removal rate decreased slightly when quenching below the binary eutectic temperature. So in order to achieve the best purification effect, it is expected that quenching the alloy just a slightly higher than the binary eutectic temperature. |
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