电磁定向结晶从过共晶铝硅合金中富集初晶硅及提纯硅的研究

【作者】 余文轴；

【导师】 戴永年； 马文会；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2014， 博士

【摘要】 由于全球经济危机的持续,各行各业都受到了一定的影响,光伏行业正面临严峻挑战。为了降低太阳能电池生产的成本,缓解光伏行业的压力,低成本太阳能级多晶硅的制备尤其关键。另一方面,目前粉煤灰等二次资源碳热还原后得到的一次粗铝硅合金处理困难,工业化应用面临诸多挑战,亟需新的工艺来解决现有存在的问题。在此背景下,本文提出电磁定向结晶从过共晶铝硅合金中富集初晶硅及提纯硅的想法,利用电磁搅拌结合定向结晶的技术对铝硅熔体中的初晶硅进行迁移控制,最终将富集后的初晶硅与共晶铝硅合金进行分离,既解决了一次粗铝硅合金难处理的问题,又为低成本太阳能级多晶硅的制备提供了思路。该技术为合金的分离提纯提供了一种新的途径,具有重要的学术意义和较大的实际应用价值。结合Al-Si二元相图、粘度梯度、定向凝固、固液界面推斥／吞没以及电磁搅拌等相关理论较全面地计算了电磁定向结晶过程中初晶硅在铝硅熔体中的迁移规律,预测了初晶硅在固液界面富集的条件。计算结果表明,电磁搅拌条件下,初晶硅颗粒在铝硅熔体中运动速率Vp变化的范围为7-26mm/s。本实验过程中,坩埚的拉升速率控制在7-200μm/s的范围内,且拉升速率即为凝固速率R,故无电磁场定向凝固过程条件下,由于vP-R≤0,所以随着初晶硅在铝硅熔体中逐渐析出,并不断被凝固界面吞没,而无法形成有效的富集。在有电磁场存在的定向凝固情况下,vP-R≥0,初晶硅颗粒虽然会被凝固界面推斥,但是在电磁搅拌和粘度梯度的共同作用下,最终可以使初晶硅形成富集。在理论计算的基础之上,实验研究结果表明,向下电磁定向结晶时,初晶硅在样品下部富集,向上电磁定向结晶时,初晶硅在样品上部富集。并且电磁定向结晶过程中,随着上拉或下拉距离的增加,初晶硅与铝硅合金的分离界面越来越清晰,初晶硅富集得越充分,但是当上拉或下拉距离超过10cm后,有部分初晶硅残留在铝硅合金中,不利于初晶硅的完全富集。由此确定,10cm为最佳上拉或下拉距离。电磁定向结晶实验过程中,降低拉速有利于初晶硅富集效率的提高,实验过程中采用7μm/s拉速条件下得到的初晶硅富集区中硅的含量为66.4wt%。电磁定向结晶过程中,增加感应电流可以使初晶硅的富集效率提高。但是随着感应电流增加,温度也随之增加,又反而不利于初晶硅在铝硅熔体中析出,从而使部分初晶硅最终残留在铝硅合金中。实验过程中采用先增加后降低感应电流两步法使初晶硅的富集效率显著提高,得到的样品初晶硅富集区中硅的含量为76.1wt%。在样品上方加水冷系统可以使样品轴向方向上的温度梯度增加,并且使初晶硅的富集效果变好,得到的样品中初晶硅富集区中硅的含量为82.0wt%。相同条件下,扩大实验比小型实验得到的初晶硅富集区中硅的含量更高。SEM-EDS结果表明,杂质元素主要分布在铝硅合金中以及铝硅合金和初晶硅的晶界上,所以初晶硅富集效率的提高使夹带的铝硅合金含量降低,有利于硅中杂质的减少,并且通过一次酸洗可以将硅中的杂质降低到50ppmw,二次酸洗后硅中杂质可以降低到约为10ppmw。初晶硅在铝硅熔体中晶体生长的形貌与硅的含量有关,硅的含量逐渐升高的过程中初晶硅的形貌从鱼骨状、板片状变为球状。球状初晶硅中由于铝的夹杂几率低而更有利于初晶硅纯度的提高。气泡吸杂提纯硅的研究结果表明,当向下电磁定向结晶,气泡向样品上方迁移,而当样品向上电磁定向结晶,气泡向着样品下方迁移。SEM-EDS分析结果表明,元素C.O.Si和Ca均聚集在气泡孔穴壁上,而在远离气泡的区域却没有发现这些元素的聚集。对于B.P.Ti等元素由于含量过低,所以在检测过程中并没有发现其分布的情况。对于元素Fe来说,由于其容易与基体形成Al-Si-Fe合金而没有聚集在气泡孔穴壁上。ICP-AES分析结果表明,远离气泡的区域比气泡壁上的杂质含量要低,由此可以证明气泡表面可以选择性吸附杂质。随着合金中铝含量的升高,电磁定向结晶后样品中的气泡孔穴逐渐增大,并且所得到的初晶硅中杂质的含量逐渐降低。随着上拉速率的降低,样品中出现的气泡孔穴逐渐增大,并且所得到的初晶硅中杂质的含量逐渐降低。电磁定向结晶改善铝硅合金性能的研究表明,电磁搅拌有利于抑制铝硅合金中初晶硅的长大,最终细化初晶硅。定向结晶的工艺有利于富铁相充分沉降并最终可以通过切除底部来达到富铁相去除的目的。电磁定向结晶处理对铝硅熔体中的气泡有较好的脱除效果,可以使气泡得到更充分地脱除。上述处理后,铝硅合金的力学性能得到明显改善。更多还原

【Abstract】 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.更多还原