Separation of primary Si and impurity boron removal from Al-30%Si-10%Sn melt under a traveling magnetic field

Abstract

C rystalline silicon is one of the materials used for solar cells, accounting for over 90% of the photovoltaic (PV) materials [1] . However, the PV industry has to face a serious low-cost feedstock supply shortage due to a rapid expansion of the solar cell industry. To lower the production cost of Si solar cells, a promising way is to upgrade metallurgical-grade silicon (MGSi) to solar-grade silicon (SoG-Si) with metallurgical treatments [2][3][4][5] . However, boron (B) element, an impurity in MG-Si, cannot be removed efficiently because of its low vapor pressure (lower than that of Si) [6] and large segregation coefficient (0.8 at 1,414 ºC) [7] . In recent years, considerable progress has been made in B removal from the MG-Si using hypereutectic Al-Si alloy solidification refining process at low temperatures [8][9][10][11] . In the alloy solidification process, the primary Si initially precipitates from supersaturated melt with decreasing Abstract: Separation of primary Si phase and removal of boron in the primary Si phase during the solidification process of the Al-30%Si-10%Sn melt under a traveling magnetic field (TMF) were investigated. The results showed that the agglomeration layer of the primary Si can be formed in the periphery of the ingot while the inner microstructures mainly consist of the eutectic α-Al+Si and β-Sn phases. The intense melt flow carries the bulk liquid with higher Si content to promote the growth of the primary Si phase which is first precipitated close to the inner wall of crucible with a relatively lower temperature, resulting in the remarkable segregation of the primary Si phase. The content of impurity B in the primary Si phase can be removed effectively with an increase in magnetic flux intensity. The results of electron probe microanalysis (EPMA) clearly indicated that the average intensity of the B Ka line in the α-Al phase region of Al-Si-Sn alloy is higher in the case of solidification under TMF than that of normal solidification condition, suggesting that the electromagnetic stirring can promote the B removal from the primary Si phase. temperature, during which more impurities, especially B, are enriched in the alloy by redistributing the impurities to the solid/liquid (S/L) interface. Adding pure Sn to the hypereutectic Al-Si alloy can effectively expand the crystallization temperature range to promote the growth of the primary Si [12,13] . However, considering the diffusion kinetics of the impurities at the nearby growing Si surface, a more effective enhanced S/L segregation tendency of impurities has not been achieved.In the present study, the traveling magnetic field (TMF) was applied during solidification of the Al-30%Si-10%Sn melt to separate the primary Si. Furthermore, the effect of the electromagnetic stirring on B removal from the primary Si during the solidification of the Al-30Si-10Sn melt was investigated. Experimental procedureCommercial purity Al (99.7 wt.%), Sn (99.99 wt.%) and MG-Si (99.7 wt.%) (all the compositions quoted in this work are in wt.% unless ot...

Distribution and Morphology of α-Al, Si and Fe-Rich Phases in Al–Si–Fe Alloys under an Electromagnetic Field

“…The currently observed Si separation in specimens, without stirring, supports findings regarding the important role of the occurring temperature gradient [88][89][90], whereas Si reduction under RMF seems to result from secondary azimuthal flow and the Si crystal movement. The occurrence of a purely eutectic center in the current study agrees with the majority of the before mentioned studies; however, it contradicts [73], where Si crystals were also observed in the center.…”

“…The occurrence of a purely eutectic center in the current study agrees with the majority of the before mentioned studies; however, it contradicts [73], where Si crystals were also observed in the center. In the current study, the observed reduction in Si crystals may come from flows reducing the temperature gradient in the melt throughout the sample, and as presented in [88][89][90], the gradient appears to be important. The effects of the temperature gradients, cooling rate and strength of magnetic field require more investigations.…”

“…Li, Ren and Fautrelle [87], in Al-18 wt.% Si, found that the high gradient magnetic field separated the Si solute and primary Si at top of the sample. Sun et al [88] in an Al-30 wt.% Si-10 wt.% Sn melt observed accumulation of primary Si at the bottom and side walls. Zou et al [89,90] explained, by numerical simulation, that RMF leads to the formation of a swirling flow in the azimuthal direction and then a secondary flow in the meridional plane, playing an important role in the transportation of solute atoms in the axial direction; they also proposed a method for the purification of metallurgical grade Si [91].…”

Effect of Strontium on Microstructure and Mechanical Properties of Semi-Solid A356 Al Alloy

“…The microstructure of hypoeutectic Al-Si casting alloys consists of the primary α-Al and the Al-Si eutectic which influence on the mechanical properties of the alloy [2]. Xugang et al [3] reported that the plate-like eutectic Si phase of the ascast hypoeutectic Al-Si alloy resulted in deleterious effect on mechanical properties. Alternatively, fine fibrous eutectic Si was found to improve mechanical properties of the Al-Si alloy, especially the ductility [4].…”