Effect Of Cu On The Recovery Rate And Purity Of Primary Silicon Prepared By Al-Si Alloy Refining Method

In recent years,as the main raw material of photovoltaic industry,the purification method of solar grade polysilicon has gradually attracted the attention of researchers.At present,in the preparation of high-purity solar grade polycrystalline silicon,alloy purification has many advantages:low cost,environmental protection,less energy consumption,so it has become a very popular method in the laboratory.Among them,the research on Al-Si alloy system is the most extensive.In this alloy system,the content of Al impurity is too high,and it is easy to cause the waste of silicon raw materials.At the same time,there is no systematic study on the best composition of the third element in the alloy.In view of the above problems,this paper studies the yield change of primary silicon and the effect of impurity removal in Al-Si-Cu ternary alloy system.On this basis,the best alloy composition was selected and the electromagnetic directional solidification experiment was carried out.The main conclusions are as follows:First of all,Al-50Si and Cu-50Si alloys were taken as research objects,and the ingots were prepared by the same process,and the yield and purity of primary silicon were compared and analyzed.The results show that the primary silicon in the Al-50Si alloy is uniformly distributed in the lamellar structure in the Al-Si eutectic structure,while the primary silicon in the Cu-50Si alloy is distributed in the Cu3Si matrix.During the solidification process of Cu-50Si alloy,Cu3Si phase is formed,and the yield is increased because a lot of Si atoms are consumed(51.7%).Far lower than the actual yield of Al-50Si alloy(83.6%).The larger the grain size is,the better the purity is.The removal efficiency of Al-50Si alloy is better than that of Cu-50Si alloy,especially for non-metallic impurities B and P.Secondly,the study of Al-Si Cu ternary alloy shows that the impurity content is significantly reduced due to the strong affinity between Cu and other impurity elements.The addition of Cu can reduce the activity coefficient of A1 and increase the removal rate of Al to about 92%.The addition of Cu not only reduces the melting point of the alloy,but also reduces the eutectic composition and promotes the growth of the primary silicon grains.At the same time,the lower solidification rate increases the precipitation time of the primary silicon,which makes the grains fully grow,and finally increases the yield of the primary silicon by about 4%.When the content of Cu is 2%-3%,the removal effect of impurities is the best;the removal effect of Al is the most obvious,from 353.1 ppm to about 143 ppm,and the content of B,P,Ti,Zn,Mg is below 1 ppm.Finally,in the aspect of electromagnetic directional solidification,the best ternary alloy composition(Al-49 wt.%Si)-2 wt.%Cu was used,and the experiment was scaled up by electromagnetic melting directional solidification technology.The experimental results show that the primary silicon of the alloy ingot prepared under the electromagnetic action appears segregation.The width and content of primary silicon are different in different distribution areas.The width of primary silicon in the Al rich layer is the smallest,the growth in the<111>direction is the weakest,and the content of primary silicon is the least;the width of primary silicon in the directional solidified layer is the largest,the preferred growth in the<111>direction is the most obvious,and the content of primary silicon is the highest;the width of primary silicon in the Si rich layer is moderate,the preferred growth in the<111>direction is slightly weaker than that in the directional solidified layer,and the content of primary silicon is slightly less.In each distribution area,the impurity removal effect of primary silicon is also different.The impurity removal effect of primary silicon in Al rich layer is the best,while the impurity removal effect becomes worse when primary silicon is gradually distributed to the bottom.At the same time,the removal rate of Fe,Ti,Mg and Zn is more than 90%.To sum up,this paper aims to improve the yield of primary silicon and the effect of impurity removal by ternary alloy method.A new experimental scheme is proposed and the practical and theoretical research is carried out.First of all,the yield of primary silicon is increased by about 4%and the removal rate of Al,the main impurity element in the alloy,is up to 92%.Secondly,the best composition of the ternary alloy was produced in large scale by electromagnetic directional solidification technology.The results show that this method can effectively reduce the content of Al in the alloy.Combined with the research status at home and abroad,the research work of this paper has certain practicability and innovation,which lays a certain foundation for the large-scale production of Al Si alloy method.