Study On The Effect Of Solidification Method And Adding Element Zr On The Purification Of Polycrystalline Silicon By Aluminum Silicon Alloy Method

With the rapid development of the photovoltaic industry,the purification and preparation technology for high-purity silicon raw materials is also developing.Among them,alloy purification of polysilicon,with its high impurity removal rate,environmentally friendly,and other advantages,has become an up-and-coming technology for preparing solar-grade high-purity polysilicon.Among this technology,one of the bottlenecks for industrial application is how to improve the yield of primary silicon and reduce the impurity content.In this thesis,the purification process was first optimized,and then different solidification methods were selected to analyze the purification behavior of Al-Si alloys with different alloy compositions.Furthermore,for the removal of impurity B,experiments on the addition of Zr element to the alloy were conducted to elucidate the role of Zr in the grain growth of primary silicon and impurity removal,and a better ratio of Zr element addition was obtained.The main research content of this paper is as follows:First,the effects of two purification methods,resistance heating solidification and electromagnetic induction directional solidification,on the purification of Al-Si alloys with different alloy compositions were compared using alloys of different compositions as the study objects.From the experimental results,it can be seen that under the case of resistance heating solidification,with the increase of Si content,the morphology of the initial Si grains gradually changes from needle-like to disk-like,and the overall yield increases from 48.14%to 80.77%,then decreases to 71.32%.The impurities are mainly gathered in the eutectic region,Fe is gathered in an iron-rich phase,the remaining impurities are distributed more uniformly,and the removal rate of metallic impurities can reach more than 90.00%.The removal rate of non-metallic impurities is more than 70.00%.Under the electromagnetic induction directional solidification conditions,the morphology of the primary silicon also shows needle-like and disk-like shapes and is biased at the bottom of the crucible,but the grain thickness is significantly increased.With the increase of Si content in the alloy,the primary silicon grains keep getting thicker and shorter due to the generation of more fragmented crystals under the action of electromagnetic force.Meanwhile,the yield of primary silicon increases from 45.73%to 76.16%,then decreases to 64.81%,similar to the case of resistance heating.Additionally,the impurity content in the upper region(mainly the eutectic region)is significantly higher than in the agglomerated region.Fe-rich phase,mainly concentrated in the eutectic region,and the removal rate of metallic impurities is calculated to be above 95.00%.The removal rate of non-metallic impurities is up to 80.00%.Secondly,a study was conducted to add a third element,Zr,because the content of non-metallic impurity B did not meet the requirements for solar-grade polysilicon feedstock.The solidification process was optimized according to the Al-Si binary phase diagram,and 1200℃was used as the better sintering temperature.Next,the removal behavior of impurity B was analyzed by adding 500 ppmw,2000 ppmw,4000 ppmw,6000 ppmw,8000 ppmw,and 10000 ppmw of Zr to Al-50 wt.%Si alloy.The experiments showed that Zr formed the Zr-B phase with impurity B in silicon,which was irregularly massive at higher B content,and the morphology of the Zr-B phase changed significantly with the addition of B,gradually shifting to a dendritic shape and agglomerating.In addition,the excess Zr was detrimental to the growth of the primary silicon,which affected both the morphology and the yield of the primary silicon.The size of the primary silicon decreased with the increase of Zr addition,and the yield decreased from 80.77%to 74.76%.The optimum addition of Zr was found to be 6000-8000 ppmw when the alloy’s B content was 400 ppmw.When it was applied to the purification of Al-Si alloy,the B removal rate reached more than 96.00%.In summary,this thesis is based on the purification of polysilicon by the Al-Si alloy method.Firstly,the solidification process parameters were optimized.Then,essential index parameters such as growth morphology,purity,and yield of primary silicon were compared when purifying the alloy with different compositions using different solidification methods.Finally,the effect of Zr addition on the removal of impurity B and the effect on the growth morphology of primary silicon is investigated.The research work in this thesis is practical and can promote the development of the solar-grade polysilicon purification industry.