Xiaolong Bai

Removal of Impurities from Metallurgical Grade Silicon During Ga-Si Solvent Refining

Purification of metallurgical grade silicon (MG-Si), using gallium as the impurity getter has been investigated. The technique involves growing Si dendrites from an alloy of MG-Si with Ga, followed by their separation by acid leaching. The morphologies of impurity phases in the MG-Si and the Ga-Si alloy were investigated during the solvent refining process. Effective segregation ratios of B and P in the Ga-Si system were calculated. Most metallic impurities formed silicides, such as Si-Fe-Ga-Mn or Si-Fe-Ga impurity phases, which segregated to the grain boundaries of Si or into the Ga phase during the Ga-Si solvent refining process. After purification, the refined Si is plate-like with < 111 > crystallographic orientation, and the removal fraction of B and P was 83.28 % and 14.84 % respectively when the Si proportion was 25 % in the Ga-Si alloy. The segregation ratios of B and P were determined to be 0.15 and 0.83 when the solid fraction of Si was 0.25. The effective removal of B and P by a solidification refining process with a Ga-Si melt is clarified.

Effect of Cooling Rate on Phosphorus Removal During Al-Si Solvent Refining

The effect of cooling rate on phosphorus removal during Al-Si solvent refining is studied during solar grade silicon purification. It is found that the phosphorus removal rate is controlled by kinetic factors. When the cooling rate decreases, the phosphorus removal rate increases. A concept of apparent segregation coefficient of phosphorus is introduced to characterize the phosphorous removal ability. It increases with the decrease in the average solidification temperature between 910.5xa0K and 1050.5xa0K (637.5xa0°C and 777.5xa0°C).

The Mechanism of P Removal by Solvent Refining in Al-Si-P System

To study P removal from Si by solvent refining in Al-Si-P system, series of quench experiments are carried out, which confirms a high P removal rate. An apparent segregation coefficient is introduced to characterize the segregation between primary Si and Al-Si melt, which are determined to be 0.00670, 0.0117, and 0.0201xa0when Si contents are 20, 30, and 40xa0wtxa0pct, respectively, at the cooling rate of 0.556xa0mKxa0s−1. A calculation of Gibbs free energy of AlP in the alloys is also carried out and proves AlP precipitates before the primary Si formation. The formation of the AlP particles causes the decrease of P content in the Al-Si melt and contributes to the high P removal. Models to explain the P contents in the refined Si of each sample in this work are presented. These include three different cases to describe the interaction between the AlP particles and the primary Si.

Distribution of Al in Al–Si Alloys during electromagnetic continuous casting with cylindrical open ended crucible

Electromagnetic continuous casting (EMCC) exhibits great potential for silicon purification by Al–Si method. Distribution of Al in Al–Si alloys with different aluminium contents during EMCC was investigated. The macrostructure of the Al–Si alloy ingots reveals that the content of primary silicon plates in hypereutectic alloys decreases from the bottom to the top of the ingots, and no obvious primary silicon plate exists in eutectic alloy. The fitting results of aluminium distribution in the alloys reveal two types of aluminium distribution patterns in hypereutectic Al–Si alloys: Scheil model for high temperature solidification and initial transient model for low temperature one. An apparent segregation coefficient of Al is proposed to characterise the solidification process, which is ∼0.83 in the hypereutectic alloys.

Distribution Behavior of B and P during Al-Si Melt Directional Solidification with Open-Ended Crucible

Abstract Distribution behavior of B and P during directional solidification of Al-20Si, Al-30Si and Al-40Si alloys has been investigated. Macrostructure of the Al-Si alloy ingots and concentration profile of elements B and P reveal that the elements segregate to eutectic Al-Si melt during growth of primary Si flakes, and P gradually segregates to the top of the ingots during directional solidification. An apparent segregation coefficient, ka, is introduced to describe the segregation behavior of B and P between the primary Si and the Al-Si melt and compared with thermodynamic theoretical equilibrium coefficients. The apparent segregation coefficients of B and P decrease with increase of solidification temperature.

Segregation Behavior of Metal Impurities During Al-Si Melt Directional Solidification with an Open Ended Crucible

The distribution characteristic and segregation behavior of metal impurities during directional solidification of Al-20Si, Al-30Si and Al-40Si alloys have been investigated. The morphologies of the alloys and impurity phases were observed by optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. The concentration profiles of representative metal impurities Al, Fe and Ti were measured by inductively coupled plasma optical emission spectrometry. The results indicate that the metal impurities segregate into the eutectic Al-Si melt during the growth of primary Si flakes and gradually segregate towards the top of each ingot during directional solidification. A concept of apparent segregation coefficient is proposed to characterize the segregation behavior of impurity elements. The apparent segregation coefficients of metal impurities decrease with increase in solidification temperature of the Al-Si alloys.

Mechanism of boron removal from Si–Al melt by Ar–H2 gas mixtures

Abstract A new method about purification of metallurgical grade silicon (MG-Si) by a combination of Si–Al solvent refining and gas blowing treatment was proposed. The morphologies and transformation of impurity phases, especially for boron and iron in Si–Al melt were investigated during Ar–H 2 gas blowing treatment. The mechanism of boron removal was discussed. The results indicate that gas blowing can refine grain size and increase nucleation of the primary Si. Boron can be effectively removed from MG-Si using the Ar–H 2 gas blowing technique during the Si–Al solvent refining. Compared with the sample without gas blowing, the removal efficiency of boron increases from 45.83% to 74.73% after 2.5 h gas blowing. The main impurity phases containing boron are in the form of TiB 2 , AlB 2 and VB compounds and iron-containing one is in the form of β -Al 5 FeSi intermetallic compound. Part of boron combines [H] to transform into gas B x H y (BH, BH 2 ) and diffuses towards the surface of the melt and is volatilized by Ar–H 2 gas blowing treatment under electromagnetic stirring.

Effect of Ga Addition on Morphology and Recovery of Primary Si During Al–Si Alloy Solidification Refining

Abstract The effect of Ga addition on alloy macrostructure, morphology and recovery rate of primary Si during the Al–Si–Ga alloy solvent refining process of silicon was studied in this work. The addition of Ga to Al–Si alloy could change the morphology of the primary Si. The average plate thickness of the primary Si increases with increase of Ga content. With the increase of Ga content, the average plate length of the primary Si crystals becomes larger when the Ga content is less than 5% in the Al–30%Si–xGa alloy, but becomes smaller when the Ga content exceeds 5%. Al–Si–Ga alloys consist of three types, primary Si, GaxAl1–x, (α-Al+Si+β-Ga) eutectic. (111) is the preferred growth surface of the plate-like primary Si. The recovery rate of the primary Si increases with the increase of Ga content. When the Ga content increased to 20% in Al–30%Si–xGa alloy, the relative recovery rate of the primary Si increased to 50.41% than that in Al–30%Si alloy.