Electromagnetic Directional Crystallization Study Of Crystalline Silicon And Aluminum-silicon Alloy Purified Silicon Eutectic Enrichment Beginning From Before

As the global economic crisis continues, worldwide businesses have been affected more or less, among which the photovoltaic (PV) industry is facing the serious challenge. In order to reduce the cost of the solar cell, the preparation of low-cost solar-grade silicon is significantly important. Additionally, the coarse Al-Si alloys obtained by the carbon thermal reduction of the coal fly ash and other resources can not be dealt with conveniently, which results in the difficulty of the industrial application. In this paper, the electromagnetic directional solidification process has been proposed for the Si purification by enrichment of primary crystal silicon from the hypereutectic Al-Si alloy. The migration of the primary silicon in Al-Si melt has been controlled by the electromagnetic stirring&directional solidification technology which separated the primary silicon and eutectic Al-Si alloy. This process can not only solve problems of the coarse Al-Si alloy, but also the preparation of low cost solar-grade silicon. Furthermore, this technology provides a new way for the separation and purification of the alloy, and it has important academic significance and practical value of application.According to the Al-Si binary phase diagram, viscosity gradient, electromagnetic stirring, directional solidification theory et al, this paper calculated migration features of the primary silicon in Al-Si melt during the electromagnetic directional solidification process, which provided a good theoretical basis for the experimental study. The results show that the velocity range of silicon particles in Al-Si melt is7-26mm/s under the electromagnetic stirring condition. The solidification rate of the melt is equal to the pulling rate which is controlled in the range of7～200μm/s. Because the Vp-R≤0, the primary silicon can be swallowed by the solidification interface when the electromagnetic field is absent. Under the electromagnetic field, although the primary silicon is rejected by the solidification interface, the combined effect of viscosity gradient and the electromagnetic stirring is beneficial to the enrichment of the primary silicon.Based on the theoretical calculation, the experimental results show that the primary silicon precipitates at the lower part of the sample by pulling down. While pulling up, it precipitates at the upper part. And with the increasing of the pulling-up or pulling-down distance, the primary silicon and Al-Si alloy can be separated more and more adequate, but when the pulling-up or pulling-down distance exceeds10cm, part of the primary silicon residue in Al-Si alloy. Therefore,10cm is the best pulling distance. With the decreasing of the pulling-up rates, silicon enrichment efficiency increased, and when the pulling-up rate is7μm/s, the content of primary silicon in silicon enrichment zone is66.4wt%. Meanwhile, with the increasing of the induction current, the enrichment efficiency of primary silicon increased. However, with the increasing of the induction current, the temperature increased which resulted in the residues of the primary silicon in Al-Si alloy. The two step process contains high and low induction current was proposed to save the problems mentioned above which can make the content of primary silicon in silicon enrichment zone up to76. lwt%. Furthermore, the content of primary silicon in silicon enrichment zone can up to82.0wt%by equipped a water cooling system above the sample due to the increase of the temperature gradient. The result phenomenon of the expand experiments may be similar to the small experiments, but the enrichment effect of primary silicon is better than that of the small experiments. The improvement of the primary crystal silicon enrichment efficiency can reduce the impurities in silicon, and the content of the impurities can be reduced to50ppmw after first acid leaching, while after the second times, the impurities can be reduced to about lOppmw. The morphology of primary silicon in the Al-Si melt is mainly decided by the silicon contents, and with the silicon content increasing, the morphology of primary silicon changes from fish-bone shape to plate-like, and then to spheroid which is considered to be beneficial for the purification of silicon due to the low Al entrapment in this morphology.Bubble adsorption results show that the bubble agglomerated at the lower part of the sample by pulling up. While pulling down, it agglomerated at the upper part. SEM-EDS analysis results show that the elements C, O, Si, and Ca were clustered in the bubble hole wall, and did not find these elements away from the bubble region. For B, P, Ti and other elements, it is not detected because the content is too low. For the element Fe, it can not aggregate in the bubble cavity wall due to its easy to form Al-Si-Fe alloy. ICP-AES analysis showed that the impurity content far from the bubble region is lower than that of the bubble wall. It is proved that the bubble surface can selectively adsorb impurities. With the increasing of the aluminum content in the alloy, the volume of the bubble increased which results in impurities content in primary silicon decreased. And with the decreasing of the pulling-up rates, the volume of the bubble increased which results in impurities content in primary silicon still decreased.The results show that the electromagnetic stirring is beneficial to the refinement of primary silicon. The process of directional solidification is attribute for removal of the iron rich phase. The electromagnetic directional solidification processing has good removing effect of the bubble in Al-Si melt. After the above treatment, the mechanical properties of Al-Si alloy improved.