Study On The Separation And Purification Of Primary Si During Electromagnetic Solidification Of Hypereutectic Al-Si Alloy

With the increasing serious issue of global energy crisis,the development and utilization of solar cells has become extremely essential and urgent.However,as the main raw material used for solar cells,high-purity polycrystalline Si which has high cost of purification and further limited the development and application of solar power generation technology.In order to promote the sustainable development of Photovoltaic(PV)industry,the development of a low-cost,high efficiency and process stability solar grade Si refining method is significantly important.Compared to conventional metallurgical processes,the solidification refining method of Al-Si alloy can play an important role in removal of metallic and non-metallic impurities B and P during one-time solidification refining process,which has various advantages,such as low refining temperature,high efficient in removing impurities and low energy consumption.Therefore it becomes one of the extremely competitive and most promising technology.However,because of the limitation of non-equilibrium solidification,the segregation behavior of impurity elements need to be further enhanced.In addition,another serious problem is that it is difficult to separate the primary Si from Al-Si melt because of their similar densities.Therefore,how to enhance the segregation behavior of impurity elements and develop the separation method of primary Si during the alloy solidification refining process become two main urgent issues to be solved.In this thesis,the hypereutectic Al-Si alloys are used as solvents to refine primary Si,during which the effect of forced convection on the segregation behavior of impurities as well as the corresponding separation mechanism is discussed.The separation of the primary Si during the solidification of hypereutectic Al-Si is controlled under the coupling effect of electromagnetic field/temperature field to reveal the underlying separation mechanism.Furthermore,a new continuous separation technology of the primary Si is put forwarded during electromagnetic semi-continuous casting to achieve the high efficiency collection of the high-purity crystalline Si.The results are showed below:(1)The effect of Al32 and AlP phases on the nucleation and growth of primary Si is investigated,furthermore the feasibility of removing non-metallic impurity B and P is also verified during the alloy solidification refining.In the hypereutectic Al-Si alloy solidification process,the formed AlB2 phase cannot serve as the effective nucleation particles of primary Si and tends to uniformly distribute in the eutectic Al-Si alloy matrix,which hence will not cause the pollution of primary Si.The introduction of impurity P in metallurgical grade silicon is not enough to result in the formation of AlP phase prior to primary Si,thus it cannot act as the nucleation particles of primary Si and then can effectively be removed during the growth of primary Si.(2)The enhancing effect of forced convection on the segregation behavior of impurity elements can be confirmed according to the step-like distribution feature of impurity elements.Compared to conventional solidification,application of rotating magnetic field can effectively reduce the effective segregation coefficient(keff)of impurity B and P.keff,B is determined from 0.41 to 0.23 and keff,p is determined from 0.47 to 0.30.The rotating magnetic field introduces a strong convection to carrie away the impurities accumulated in front of the solid/liquid interface,which has a positive effect on impurities removal.(3)The separation of primary Si during solidification process of hypereutectic Al-30wt.%Si alloy under electromagnetic stirring is studied,which has the potential to effectively collect high-purity Si.The effect of magnetic field type,cooling rate and alloy composition on the separation of primary Si is discussed.The results show that in comparison with traveling magnetic field and intermediate frequency magnetic field,application of rotating magnetic field(RMF)can effectively promote the separation of primary Si to form a Si-rich layer with 65 wt.%Si content on the periphery of the ingot.In order to achieve efficient separation of primary Si,cooling rate of alloy melt should be controlled in the range of 10?33 0C/min,temperature gradient with 1?1.8 ?/mm needs to be established in the axial direction of alloy,and H/D of alloy melt should be kept at above 1.(4)The separation mechanism of primary Si during solidification of Al-30wt.%Si alloy is investigated under coupling effect of electromagnetic field and temperature field.The electromagnetic separation of primary Si is a complex coupling process of cooling condition,flow field and crystal growth.The Al-Si melt flow under RMF and the temperature field of the liquid metal are the two dominant conditions for the formation of Si-rich layer.The intense melt flow,i.e.,secondary flow and Taylor-Gortler vortices,carries the bulk liquid with higher Si content to promote the growth of the primary Si phase formed close to the inner wall of the crucible where the temperature is low,finally resulting in the remarkable segregation of primary Si phase.(5)The continuous separation technology of primary Si is proposed.The continuous separation of primary Si is realized by electromagnetic semi-continuous casting of hypereutectic Al-30wt.%Si melt.Then the ingot with Si-rich layer structure(60 mm in diameter and 550 mm in height)is prepared successfully,which can achieve more efficient separation/collection of the primary Si.The contents of almost all impurities in the primary Si can be reduced to the solar grade level through three times solidification refining,indicating that the method may provide a potential way to solve the Si supply for the PV industry.