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Evaluation of the formation of the silicon-rich area of hypereutectic aluminum-silicon melts treated with alternating electromagnetic directional solidification
Materials Science in Semiconductor Processing 104 (2019) 104596
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Evaluation of the formation of the silicon-rich area of hypereutectic aluminum-silicon melts treated with alternating electromagnetic directional solidification
T
Yunfei Hea,b,c, Xi Yangb,d, Xiongdong Yanga,b,c, Ting Xiaoa,b, Yu Baoa,b, Wenhui Maa,b,c,*, Guoqiang Lva,b,** a
The National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming, 650093, China d Yunnan Provincial Energy Research Institute Co., Ltd., Kunming, 650000, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Si-rich formation Hypereutectic aluminum-silicon alloy Silicon separation mechanism Fick's second law
In order to replace the high energy consumption and high-polluting Siemens method, aluminum-silicon (Al-Si) solvent refining was used by many researcher to manufacture purified silicon. Aiming at evaluating the formation of the Si-rich areas of hypereutectic aluminum-silicon (Al-Si) melt materials, a series of experiments on the effects of silicon separation from hypereutectic aluminum-silicon under different conditions was carried out. Currently, some researchers used the mechanism to explain silicon formation and separation from hypereutectic Al–Si melts during directional solidification (DS), based on Fick's second law. However, the mechanism cannot explain some experimental phenomena. In the present study, the deficiencies of the mechanism were examined and are presented. To explain the formation of Si-rich areas in detail, combinations of hypereutectic aluminumsilicon melts were analyzed to obtain a better understanding of the properties of hypereutectic aluminum-silicon melts. The experiments were used to confirm that the Si separate from hypereutectic Al-Si melts is also a purification process. Moreover, when the sample dealt with a 0.9 mm/min pulling speed under 3 kHz, the silicon content of the Si-rich areas can reach over 80 wt%. Finally, a novel optimized device was developed to obtain better Si separation from the hypereutectic Al-Si melts.
1. Introduction Due to the shortage of sources of renewable and green energy, significant attention has been paid to the solar energy and photovoltaic industry, and it has shown booming growth in the past few decades [1]. The growing demand for clean energy materials on the part of various industries has prompted an increased need for solar-grade silicon (SOG–Si), and this material requirement involves high costs. Numerous conventional approaches may be adopted to manufacture SOG–Si. These techniques include the improved Siemens method, which provides high quality material but involves high costs, significant pollution and low silicon productivity [2]. Thus, a large number of studies have been conducted on ways to increase the separation and purification efficiency and decrease the production cost of the process by upgrading metallurgical-grade silicon (MG–Si) to solar-grade silicon (SOG–Si).
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Examples include plasma refining [3], slag refining [4], directional solidification [5], electron beam [6] and vacuum refining [7]. However, the high temperatures that the process requires suggest a consequently high energy cost, which can lead to energy waste. Employing a process of solvent refining to purify the MG-Si is a good choice. Solvent refining processes, including Al-Si [8], Sn-Si [9], Cu-Si [10], Fe-Si [11], Ni-Si [12,13] were studied, and these combinations were chosen to purify the MG-Si. These combinations were compared and discussed by Murray D [14]. Al–Si solvent refining presents many advantages. Using Al–Si alloys to refine silicon is a prospective method that can be used to produce solar cells [15]. Si solidified from a liquid melt can be purified, and the final yield be calculated [16]. In addition, when some elements are refined with Al–Si alloys, target impurities are removed, because their segregation coefficients can be reduced by the transition elements added [17], and they remain in the liquid phase to
Corresponding author. The National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, 650093, China. Corresponding author. The National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, 650093, China. E-mail addresses: [email protected] (W. Ma), [email protected] (G. Lv).
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https://doi.org/10.1016/j.mssp.2019.104596 Received 10 March 2019; Received in revised form 22 June 2019; Accepted 28 June 2019 Available online 01 August 2019 1369-8001/ © 2019 Published by Elsevier Ltd.
Materials Science in Semiconductor Processing 104 (2019) 104596
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Fig. 1. The schematic of the experimental devices: (a) Induction furnace. (b) Resistance furnace.
facilitate the purification of the first solid Si. Al–Si melts exhibit outstanding characteristics, such as a single eutectic structure, lower liquid temperature than that of silicon, and high solubility of the Si in the Al [18]. For these reasons, the Al-Si combination has attracted considerable research attention [19,20]. In recent decades, numerous studies have been conducted on purifying silicon via the solidification of Al–Si alloy melts under an alternating electromagnetic field [21–27]. Many works have focused on silicon purified from the Al-Si binary system. However, the Si-rich area formation of hypereutectic Al-Si melts that deals with alternating electromagnetic directional solidification has not been considered. In this work, the separation efficiency of Al-45 wt%Si melts under two different frequencies in furnaces with a pulling speed of 0.9 mm/ min was studied. By applying quenching operations on the samples, the Si-rich area formation of hypereutectic Al-Si melts under alternating electromagnetic directional solidification was investigated.
2. Experiment A resistance furnace and two induction furnaces were used in this study. The frequencies of the induction furnaces were 3 and 30 kHz. Fig. 1 presents images of the experimental apparatus. The furnace was equipped with a lifting machine, which could lower the samples at fixed speeds. The furnace body was evacuated using a vacuum pump (ultimate pressure < 10 Pa) so that the samples would not be oxidized, and then the furnace body was filled with argon gas (99.99 pct). This step was repeated three times to ensure that the oxygen was completely removed. When the experiments began, a dual-wavelength infrared pyrometer was used to monitor the surface temperature of the Al–Si melts through the prism in the alternating electromagnetic furnace body, and a PID system was used to control the induction furnace. The dropping rate of the pulling platform was precisely controlled by a PC. As much as 90 g of Si shot (99 pct) and Al shot were combined to form an Al–45 wt% Si alloy which was placed in a high-purity graphite crucible (O.D. = 36 mm, I.D. = 28 mm, length = 130 mm). The bottom 2
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Fig. 2. Samples treated with different conditions: (a) in a resistance furnace, without pulling. (b) Pulling at a speed of 0.9 mm/min, in resistance furnace. (c) In 30 kHz induction furnace, without pulling. (d) Pulling at a speed of 0.9 mm/min, in 30 kHz furnace. (e) In 3 kHz induction furnace, without pulling. (f) Pulling at a speed of 0.9 mm/min, in 3 kHz furnace.
(EDS), the EDS was used to confirm the elements. The Si content in the samples was determined by X-ray fluorescence spectrometry (XRFS). Elements in the special areas were characterized by EPMA, and their distribution was confirmed.
of the crucible was placed on the same level at the end of the induction coils, as shown in Fig. 1. The samples were heated to 1050 °C (1323 K) and then held for 30 min. Subsequently, samples were extracted at a fixed speed of 0.9 mm/min. After cooling, the samples were cut and polished. In order to characterize the microstructures of the solidified samples and other features, the ingots were cut vertically, and the surfaces were polished using metallographic sandpaper. The microstructures of the samples, especially the Si-rich areas, were observed by scanning electron microscopy (SEM), equipped with energy-dispersive spectroscopy
3. Results and discussion 3.1. Silicon separation effects under different conditions Fig. 2 shows the cross-sections of the samples treated with different 3
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Table 1 List of experiments. Series
Exp.No.
Frequency
Pulling speed
Series-I
I-1 I-2 II-1 II-2 III-1 III-2
0 0 3 3 30 30
0 0.9 mm/min 0 0.9 mm/min 0 0.9 mm/min
Series-II Series-III
conditions, which are summarized in Table 1. Fig. 2a, c and e present the samples without pulling, in the resistance furnace, and in the 3 and 30 kHz alternating electromagnetic furnaces, respectively. Fig. 2b, d and f show the samples pulling at a speed of 0.9 mm/min (with a corresponding cooling rate of 6.75 °C-11.40 °C/min), in the resistance furnace, and in the 3 kHz and 30 kHz induction furnaces, respectively. Samples d and f clearly exhibit an apparent separation at the bottom, whereas samples a, b, c, and e exhibit no separation. Moreover, the two typical separating samples (3 and 30 kHz samples) differed in separation efficiency. The 30 kHz sample had 3 areas. The silicon distribution showed a gradual change from the bottom to the top. The 3 kHz sample was compared with the 30 kHz sample, and it was found that the silicon-enriched area and Al-Si eutectic composition were clearly divided by a noticeable crack. Fig. 3 shows the samples treated with quenching. We can clearly see that when the sample pulling with 65 mm, the silicon separation can be achieved. To compare the separation efficiency, the ratios of Si content in the
Fig. 4. Silicon content in different positions of the two samples (d and f, shown in Fig. 2). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Si-rich areas were measured by the XRFS to detect the silicon content in the two separating samples (shown in Fig. 2 d and f). Fig. 4 shows the silicon content in different positions of the two samples. When a 30 kHz frequency was used, the silicon content exhibited a gradual change
Fig. 3. Samples subjected to quenching: (a) direct quenching, (b) pulling for 40 mm and then quenched, (c) pulling for 65 mm and then quenched. 4
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Fig. 5. The microstructures of different positions shown in Fig. 2d: (a) Position 1, in Fig. 2d; (b) Position 2, in Fig. 2d; (c) Position 3, in Fig. 2d; (d) Position 4, in Fig. 2d.
Moreover, Wang et al. [28], Lv et al. [29] and Nishi et al. [30] held that the silicon growth rate in Si-rich areas can be calculated by Fick's law and under the assumption that silicon growth is diffusion controlled. The corresponding equations of Si growth rate was used in their studies as follows:
from top to bottom. However, when the 3 kHz frequency was used, the distribution of silicon content was homogeneous in the two areas. The silicon content in the Si-rich areas reached over 80 wt%, and the silicon content in the Al-Si eutectic composition areas reached about 22 wt%, for few Si plates, particles and silicide exist. To compare the microstructures of the two separating samples (Sample d and f shown in Fig. 2). Fig. 5a–d shows the microstructures in the 30 kHz samples. Their corresponding positions are 1, 2, 3 and 4 in Fig. 2d. The densities of the silicon plates showed a gradual change. Fig. 6b–e shows the microstructures in the 3 kHz sample. Their corresponding positions are 1, 2, 3 and 4 in Fig. 2f. Fig. 6a is the microstructure in Fig. 2e. The Si distributions tendency of the two samples was in accord with the silicon content, as can be seen in Fig. 4. The Si-rich areas were obtained during the solidification process. During the refining stage, the temperature of the melts was 1050 °C, higher than the corresponding melting point, and the hypereutectic AlSi melts were stirred under alternating electromagnetic fields. The two main requirements must be reached to meet Si-rich conditions: 1. Proper temperature gradient; 2. Sufficient melt flow: Comparing the samples shown in Fig. 2, a, c and e, the three samples dealt without directional solidification, there is no separation results occurred. Because there is no temperature gradient exists when samples dealt without directional solidification. However, when samples dealt with directional solidification, with 0.9 mm/min (the corresponding cooling rate is 6.75 °C-11.40 °C/min), Fig. 2b, d and f, only the two samples dealt with alternating electromagnetic field, because hypereutectic AlSi melts are conductive, electromagnetic field has Lorenz force on melts and melts can be stirred, which will lead sufficient melts flow. Moreover, from previous studies [29,36] the simulations of hypereutectic AlSi melts under resistance furnace and induction furnace were carried out. There are two orders of magnitude exists in melts flow velocities of samples between resistance furnace and induction furnace.
v = DSi in Si − Al solvent ×
∂χSi in Al − Si solvent ∂χ
(1)
v = DSi in Si − Al solvent ×
∂χSi in Al − Si solvent ∂T × ∂T ∂χ
(2)
DSi in Si − Al solvent is the diffusion coefficient of the silicon in the Si-Al melts, χSi in Al − Si solvent is the silicon content, χ is the distance of the growth direction and T is the temperature of the Si-Al melts. ∂T / ∂χ is the temperature gradient. However, the assumption that silicon growth is controlled by the diffusion of atoms shows some defects: 1. When the sample dealt with alternating electromagnetic directional solidification (AEM-DS) under 3 kHz, shown as Fig. 2f, the sample after directional solidification did not show a content of silicon change from bottom to top. If the Si-rich areas are formed by diffusion controlled, the silicon content of the Sirich areas will change little by little from bottom to top, because when silicon atoms separate from the Al-Si melts, from equation (2), the silicon concentration of the Al-Si melts ( χSi in Al − Si solvent ) will decrease. Instead, an obvious crack formed between the Si-rich area and the Al-Si eutectic composition. 2. By detecting the unseparated areas of the quenched sample (Fig. 3c), SEM was used to detect the microstructures of these samples silicon plates can be observed (Fig. 8a, b and c) and this phenomenon can be used to explore the middle process during AEM-DS, and quenching the samples can reveal the separation process. For a better understanding of silicon separation from hypereutectic Al-Si melts, the silicon content of different areas, shown as the cycle regions 1 to 6 in Fig. 3c, were measured by XRFS, as shown in Fig. 7. Moreover, the microstructures of positions 1 to 4 were detected by SEM. Approximately the same microstructures (silicon plates) were observed in the unseparated areas (shown as Fig. 8). The pulling sample (Fig. 3c) shows that most of the silicon plates were in the unseparated areas, while when the AEM-DS was completed, few silicon plates were
3.2. The evaluation of the formation of Si-rich areas Regarding the formation of Si-rich areas, Xue et al. [25] held that the Si-rich area was formed by the controlled silicon atoms diffusion. 5
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Fig. 6. The microstructures of different positions shown in Fig. 2e and f: (a) Position 1, in Fig. 2e; (b) Position 1, in Fig. 2f; (c) Position 2, in Fig. 2f; (d) Position 3, in Fig. 2f; (e) Position 4, in Fig. 2f.
observed in the Al-Si eutectic composition-the final solidification area (position 1 in Fig. 2f). From the above analysis, a conclusion can be made that the Si-rich areas are not be formed just because of controlled silicon atom diffusion. The main formation of Si-rich areas was because the transportation of Si plates and particles, they formed frame of the Si-rich areas. The melts flow benefits Si atoms diffusion and can enhance augment of Si content of Si-rich area. The formation of Si-rich areas should be discussed. An Al-Si binary phase diagram [31] (Fig. 9) was used to investigate the behavior of the Al-45 wt%Si component during the directional solidification. According to the theory about the directional solidification of the alloy [32]，the slope of the liquid line dividing the slope of the solid line is infinity, which means that it is much easier to segregate the silicon when the temperature decreases below the temperature of the liquid line. When the temperature decreases, silicon atoms will segregate into silicon plates, rather than just forming Si-rich areas by means of a controlled diffusion of silicon atoms, the controlled diffusion of silicon atoms
exists in the whole process. Jie et al. [18] conducted an investigation that explored as the liquid temperature decreased, the Si content of the liquid Al-Si melt also decreased. Their findings indicated that there was a segregation process during solidification: the Al-Si melts segregated into solid silicon plates when the temperature of the melts decreased, in accordance with Fig. 8a–c, and many silicon plates or particles were mixed in the Al-Si eutectic composition. Fig. 10 shows the EPMA mapping of the distribution of elements in the unseparated areas in the quenched sample (shown in Fig. 3c). The mapping of unseparated areas indicates that the Si plates were mixed in the hypereutectic Al-Si melt during the directional solidification when the temperature of the melts was below the liquid line. Impurities were distributed in the Al-Si eutectic compositions. The EPMA-mapping of Al-Si eutectic compositions (position 1 in Fig. 2f) after AEM-DS was shown in Fig. 11. Silicon plates cannot be detected, which means that as directional solidification finished, silicon plates would be transported to the lower temperature area and because 6
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Fig. 9. Al-Si binary phase diagram. Fig. 7. Silicon content of the six circle areas shown in Fig. 3c. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
impurities. However, while some impurities were enveloped in silicon plates, these enveloped impurities can not be removed thoroughly by acid leaching, which were discussed in the recent work [36].
of the high viscosity [33], and then form Si-rich area. With regard to the mapping of the Si-rich areas (the bottom of the sample), we found that they are occupied by Si, as can be seen in the previous study [34]. The formation of complex impurities in the compounds should also be discussed. It was interesting to find some impurities in the silicon plates, which can be seen in Fig. 8, as shown by the red arrow. The EDS results (Points 1 and 2, in Fig. 12b) show that impurities were enveloped in silicon plates. Moreover, the segregated silicon plates had high purity. In other words, the silicon segregation process is a purification process. Yu et al. [26] and Jie et al. [35] both demonstrated that the convection generated by the electromagnetic field caused impurities to accumulate in front of the liquid/solid interface and had a positive effect on removing
3.3. The principle of silicon plates separated from hypereutectic Al-Si melts Much research has been done on non-metallic inclusions separating from metal [37–44]. The process of how silicon separates from aluminum-silicon can be regarded as the model of how non-metallic inclusions separate from metal. Comparing the electrical conductivity of silicon (σSi ≈ 1 × 10−2S / m ) and hypereutectic Al-Si melts (σMelt ≈ 2.4 × 106S / m )σSi ≪ σMelt , we see that silicon plates can be regarded as non-conducting. In 1954, Leenov and Kolin [42] investigated the force on the nonconducting particles in conductive melts. The Lorenze force of melts can be expressed by equation (3):
F=J×B
Fig. 8. Typical microstructures of the quenched sample C (shown in Fig. 3c): (a) position 1 . (b) Position 2; (c) position 3; (d) position 4. 7
(3)
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Fig. 10. EPMA-mapping of distribution of elements in the unseparated area of quenched sample shown in Fig. 3c.
(non-conductive). The applied electromagnetic force on the molten metal pushed the inductive melts in a certain direction. Moreover, the electromagnetic force only existed in the inductive aluminum-silicon melts, and the influence on the non-conductive solid silicon was ignored, because solid silicon can be considered non-conductive. When analyzing the force acting on the solid silicon, it can be
Where J is the induction current in the copper coils, B is the magnetic flux density; the latter is a vector and can be generated by the alternating current in the copper coils. F is the electromagnetic force. During our experiments, the segregated silicon particles were nonconductive as compared to the aluminum-silicon melts. Both were mixed. The molten metal (conductive) contained nonmetallic silicon
Fig. 11. EPMA-mapping of elements' distribution in Al-Si alloy, position 1, as shown in Fig. 2f. 8
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Fig. 12. (a) Microstructure of the unseparated area in Fig. 3c; (b) An enlargement of the rectangle region marked in (a); (c) EDS analysis of the formed impurities—intermetallic compounds enveloped in silicon plates; (d) EDS analysis of the silicon plates. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The viscous resistance can be calculated by equation (7):
regarded that the direction of the electromagnetic force on the solid silicon was opposite that of the electromagnetic force on the molten metal. The force can be expressed by equation (4):
Fp =
3(σl − σSi ) JBVSi 2(2σl + σSi )
Fn = ϕρdp2 ν 2
ϕ is the viscous damping coefficient, ν is the migrating rate of Si particles, and ρ is the density. The migrating equation of the Si particles in the Al-Si alloy melts is as follows:
(4)
Where Fp , σl , σSi , and VSi represent the total force of non-conductive Si particles, the electrical conductivity of the aluminum-silicon melts, the electrical conductivity of the solid silicon, and the volume of the solid silicon, respectively. Damoah and Zhang [44] investigated the use of an electromagnetic field for removing non-conductive particles from metals, and Jiang et al. [45] investigated the separation of silicon carbide inclusions by using a supersonic frequency magnetic field. Their conclusion was that if the particle is nonconducting (σSi =0) or the electric conductivity is far lower than that of the liquid metal (σSi «σl ), the particle will move in a direction opposite to the EM force; the authors concluded that the particles can move in a fixed direction, considering the magnetic force should be perpendicular to magnetic field, the particles will move to the edge of the sample. According to the research of Colin and others [46], the extrusion force on the impurity phases by an electromagnetic field can be calculated as follows:
Fp =
3 3 σl − σSi πdp F 2 2σl + σSi 6
Fp − Fn = m
3 3 πdp F 2 6
dv dt
(8)
Equation (8) reflects the relationship between the migrating velocity and viscous resistance of the Si particles, where m is the weight of the Si particles. According to equations (4)–(8), equation (9) can be obtained as follows: 3 πdp3 ρ dv 3 πdp Fn − ϕρdp2 ν 2 = 4 6 6 dt
(9)
Simplifying equation (9), it can be written as follows:
6ρϕv 2 dv 3 Fn − = = ρa πdp dt 4
(10)
where a is the migrating acceleration of the silicon particles, which equals the result of the electromagnetic force acceleration less the viscous resistance acceleration. The difference between the electromagnetic force and the viscous resistance (shown as equation (10), a = 0 ) can be calculated by equation (11):
(5)
where dp is the diameter of the silicon particles, and σSi ≈ 0 , so equation (5) can be written as follows:
Fp =
(7)
6ρϕv 2 3 Fn − =0 4 πdp
(11)
Finally, according to equation (11), the migrating velocity of the silicon particles can be calculated as v = 3Fn πdp/24ρϕ . From this, we can clearly see that the velocity is related to the electromagnetic force Fn , viscous damping coefficient ϕ , density ρ and diameter dp . Thus, a conclusion can be made that to improve the separation efficiency of Si,
(6)
As Si particles are very tiny, the buoyancy and gravity of these silicon particles can be neglected [47]. The Si particles obtained by viscous resistance should be considered when the Si rich areas are formed. 9
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Fig. 13. SEM photograph of the dividing line shown in Fig. 2f.
the velocity of the melts must be increased. Fig. 13 shows the SEM photographs of the dividing line in sample 2f, and the results reflect that the Si particles move quickly when under a 3 kHz alternating electromagnetic field because of the large melt flow. The flow field distribution of the melts can be seen in our previous work [34], in which the maximum flow rate reached 0.92 cm/s. Fig. 13 shows that some Si plates or particles were distributed around the crack, which represents the position 2 shown in Fig. 2f. In order to verify and discuss Si-rich area formed by particle migration or precipitation. The differences in density between solid Si (~2300 kg/ m3) and Si-Al melt (~2400 kg/m3) were compared, solid Si shows a lighter density and from the samples without pulling, the Si plates distribute uniformly (seen as Fig. 2a,b and c). The controlled diffusion of Si atoms exists in the whole AEM-DS process. From this analyses, we can concluded that the Si-rich areas were mainly formed by the migration of silicon plates or particles, not formed by the precipitation nor just controlled diffusion of Si atoms (known as Fick's second law). Considering the process involves complex Si plate growth, temperature gradient, electromagnetic field and movement with melt flow. The detailed and precise process requires more experimental and theoretical studies in the future.
Fig. 14. A sketch of an optimized device for purifying and separating silicon from hypereutectic Al-Si melts. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Shown as Fig. 14, the heating resource comes from resistive heaters. For the stirring effect, more subtle control may be provided by using coils. To avoid electromagnetic induction heating, a much lower frequency should be used.
3.4. Proposal of a novel optimized device for purifying and separating silicon from hypereutectic Al-Si melts efficiently
4. Conclusion
Fig. 14 shows an optimized device for purifying metals. It is different from the traditional alternating electromagnetic directional solidification devices. Traditional AEM-DS devices are equipped with an alternating electromagnetic field, and the electromagnetic field has two main effects: heating and stirring. As mentioned before, for some combinations (like the Al-Si combination), when the temperature is below the liquid line, segregation will occur, and it is a purifying process. These purified silicon plates should be transported to the lower temperature areas so as to collect the purified silicon and reduce the waste of silicon and aluminum. However, as the temperature of the melts decreases, the stirring effect will also decrease and the melt flow will be reduced. Considering these aspects, the stirring effect and heating effect should be separated so as to control the purifying process.
This study focused on the formation of Si-rich areas. By experiments and analyses, the formation of Si-rich areas was confirmed to be caused mainly by Si plates or particle migration, and not only by the controlled diffusion of atoms, known as Fick's law. Moreover, during the whole AEM-DS, the controlled diffusion of Si atoms exist. When using a 3 kHz alternating electromagnetic field with pulling at a speed of 0.9 mm/ min, the Si content of the Si-rich areas reached 80 wt%. The process of Si being segregated from the hypereutectic Al-Si was confirmed as a purification process, but some impurities were enveloped in Si plates. Moreover, a novel optimized device was proposed for better Si separation from the hypereutectic Al-Si melts. The process of Si separated 10
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from hypereutectic Al-Si melts with AEM-DS is such complex: melts flow, Si crystal growth, latent heat, Si plates move with melts, electrical properties of the melts and changing properties of melts and so on, they interweave each other. The detail precise quantitative analyses need more experimental and theoretical studies in the future.
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Evaluation of the formation of the silicon-rich area of hypereutectic aluminum-silicon melts treated with alternating electromagnetic directional solidification
T
Yunfei Hea,b,c, Xi Yangb,d, Xiongdong Yanga,b,c, Ting Xiaoa,b, Yu Baoa,b, Wenhui Maa,b,c,*, Guoqiang Lva,b,** a
The National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China State Key Laboratory of Complex Nonferrous Metal Resources Cleaning Utilization in Yunnan Province, Kunming, 650093, China d Yunnan Provincial Energy Research Institute Co., Ltd., Kunming, 650000, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Si-rich formation Hypereutectic aluminum-silicon alloy Silicon separation mechanism Fick's second law
In order to replace the high energy consumption and high-polluting Siemens method, aluminum-silicon (Al-Si) solvent refining was used by many researcher to manufacture purified silicon. Aiming at evaluating the formation of the Si-rich areas of hypereutectic aluminum-silicon (Al-Si) melt materials, a series of experiments on the effects of silicon separation from hypereutectic aluminum-silicon under different conditions was carried out. Currently, some researchers used the mechanism to explain silicon formation and separation from hypereutectic Al–Si melts during directional solidification (DS), based on Fick's second law. However, the mechanism cannot explain some experimental phenomena. In the present study, the deficiencies of the mechanism were examined and are presented. To explain the formation of Si-rich areas in detail, combinations of hypereutectic aluminumsilicon melts were analyzed to obtain a better understanding of the properties of hypereutectic aluminum-silicon melts. The experiments were used to confirm that the Si separate from hypereutectic Al-Si melts is also a purification process. Moreover, when the sample dealt with a 0.9 mm/min pulling speed under 3 kHz, the silicon content of the Si-rich areas can reach over 80 wt%. Finally, a novel optimized device was developed to obtain better Si separation from the hypereutectic Al-Si melts.
1. Introduction Due to the shortage of sources of renewable and green energy, significant attention has been paid to the solar energy and photovoltaic industry, and it has shown booming growth in the past few decades [1]. The growing demand for clean energy materials on the part of various industries has prompted an increased need for solar-grade silicon (SOG–Si), and this material requirement involves high costs. Numerous conventional approaches may be adopted to manufacture SOG–Si. These techniques include the improved Siemens method, which provides high quality material but involves high costs, significant pollution and low silicon productivity [2]. Thus, a large number of studies have been conducted on ways to increase the separation and purification efficiency and decrease the production cost of the process by upgrading metallurgical-grade silicon (MG–Si) to solar-grade silicon (SOG–Si).
*
Examples include plasma refining [3], slag refining [4], directional solidification [5], electron beam [6] and vacuum refining [7]. However, the high temperatures that the process requires suggest a consequently high energy cost, which can lead to energy waste. Employing a process of solvent refining to purify the MG-Si is a good choice. Solvent refining processes, including Al-Si [8], Sn-Si [9], Cu-Si [10], Fe-Si [11], Ni-Si [12,13] were studied, and these combinations were chosen to purify the MG-Si. These combinations were compared and discussed by Murray D [14]. Al–Si solvent refining presents many advantages. Using Al–Si alloys to refine silicon is a prospective method that can be used to produce solar cells [15]. Si solidified from a liquid melt can be purified, and the final yield be calculated [16]. In addition, when some elements are refined with Al–Si alloys, target impurities are removed, because their segregation coefficients can be reduced by the transition elements added [17], and they remain in the liquid phase to
Corresponding author. The National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, 650093, China. Corresponding author. The National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, 650093, China. E-mail addresses: [email protected] (W. Ma), [email protected] (G. Lv).
**
https://doi.org/10.1016/j.mssp.2019.104596 Received 10 March 2019; Received in revised form 22 June 2019; Accepted 28 June 2019 Available online 01 August 2019 1369-8001/ © 2019 Published by Elsevier Ltd.
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Fig. 1. The schematic of the experimental devices: (a) Induction furnace. (b) Resistance furnace.
facilitate the purification of the first solid Si. Al–Si melts exhibit outstanding characteristics, such as a single eutectic structure, lower liquid temperature than that of silicon, and high solubility of the Si in the Al [18]. For these reasons, the Al-Si combination has attracted considerable research attention [19,20]. In recent decades, numerous studies have been conducted on purifying silicon via the solidification of Al–Si alloy melts under an alternating electromagnetic field [21–27]. Many works have focused on silicon purified from the Al-Si binary system. However, the Si-rich area formation of hypereutectic Al-Si melts that deals with alternating electromagnetic directional solidification has not been considered. In this work, the separation efficiency of Al-45 wt%Si melts under two different frequencies in furnaces with a pulling speed of 0.9 mm/ min was studied. By applying quenching operations on the samples, the Si-rich area formation of hypereutectic Al-Si melts under alternating electromagnetic directional solidification was investigated.
2. Experiment A resistance furnace and two induction furnaces were used in this study. The frequencies of the induction furnaces were 3 and 30 kHz. Fig. 1 presents images of the experimental apparatus. The furnace was equipped with a lifting machine, which could lower the samples at fixed speeds. The furnace body was evacuated using a vacuum pump (ultimate pressure < 10 Pa) so that the samples would not be oxidized, and then the furnace body was filled with argon gas (99.99 pct). This step was repeated three times to ensure that the oxygen was completely removed. When the experiments began, a dual-wavelength infrared pyrometer was used to monitor the surface temperature of the Al–Si melts through the prism in the alternating electromagnetic furnace body, and a PID system was used to control the induction furnace. The dropping rate of the pulling platform was precisely controlled by a PC. As much as 90 g of Si shot (99 pct) and Al shot were combined to form an Al–45 wt% Si alloy which was placed in a high-purity graphite crucible (O.D. = 36 mm, I.D. = 28 mm, length = 130 mm). The bottom 2
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Fig. 2. Samples treated with different conditions: (a) in a resistance furnace, without pulling. (b) Pulling at a speed of 0.9 mm/min, in resistance furnace. (c) In 30 kHz induction furnace, without pulling. (d) Pulling at a speed of 0.9 mm/min, in 30 kHz furnace. (e) In 3 kHz induction furnace, without pulling. (f) Pulling at a speed of 0.9 mm/min, in 3 kHz furnace.
(EDS), the EDS was used to confirm the elements. The Si content in the samples was determined by X-ray fluorescence spectrometry (XRFS). Elements in the special areas were characterized by EPMA, and their distribution was confirmed.
of the crucible was placed on the same level at the end of the induction coils, as shown in Fig. 1. The samples were heated to 1050 °C (1323 K) and then held for 30 min. Subsequently, samples were extracted at a fixed speed of 0.9 mm/min. After cooling, the samples were cut and polished. In order to characterize the microstructures of the solidified samples and other features, the ingots were cut vertically, and the surfaces were polished using metallographic sandpaper. The microstructures of the samples, especially the Si-rich areas, were observed by scanning electron microscopy (SEM), equipped with energy-dispersive spectroscopy
3. Results and discussion 3.1. Silicon separation effects under different conditions Fig. 2 shows the cross-sections of the samples treated with different 3
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Table 1 List of experiments. Series
Exp.No.
Frequency
Pulling speed
Series-I
I-1 I-2 II-1 II-2 III-1 III-2
0 0 3 3 30 30
0 0.9 mm/min 0 0.9 mm/min 0 0.9 mm/min
Series-II Series-III
conditions, which are summarized in Table 1. Fig. 2a, c and e present the samples without pulling, in the resistance furnace, and in the 3 and 30 kHz alternating electromagnetic furnaces, respectively. Fig. 2b, d and f show the samples pulling at a speed of 0.9 mm/min (with a corresponding cooling rate of 6.75 °C-11.40 °C/min), in the resistance furnace, and in the 3 kHz and 30 kHz induction furnaces, respectively. Samples d and f clearly exhibit an apparent separation at the bottom, whereas samples a, b, c, and e exhibit no separation. Moreover, the two typical separating samples (3 and 30 kHz samples) differed in separation efficiency. The 30 kHz sample had 3 areas. The silicon distribution showed a gradual change from the bottom to the top. The 3 kHz sample was compared with the 30 kHz sample, and it was found that the silicon-enriched area and Al-Si eutectic composition were clearly divided by a noticeable crack. Fig. 3 shows the samples treated with quenching. We can clearly see that when the sample pulling with 65 mm, the silicon separation can be achieved. To compare the separation efficiency, the ratios of Si content in the
Fig. 4. Silicon content in different positions of the two samples (d and f, shown in Fig. 2). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Si-rich areas were measured by the XRFS to detect the silicon content in the two separating samples (shown in Fig. 2 d and f). Fig. 4 shows the silicon content in different positions of the two samples. When a 30 kHz frequency was used, the silicon content exhibited a gradual change
Fig. 3. Samples subjected to quenching: (a) direct quenching, (b) pulling for 40 mm and then quenched, (c) pulling for 65 mm and then quenched. 4
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Fig. 5. The microstructures of different positions shown in Fig. 2d: (a) Position 1, in Fig. 2d; (b) Position 2, in Fig. 2d; (c) Position 3, in Fig. 2d; (d) Position 4, in Fig. 2d.
Moreover, Wang et al. [28], Lv et al. [29] and Nishi et al. [30] held that the silicon growth rate in Si-rich areas can be calculated by Fick's law and under the assumption that silicon growth is diffusion controlled. The corresponding equations of Si growth rate was used in their studies as follows:
from top to bottom. However, when the 3 kHz frequency was used, the distribution of silicon content was homogeneous in the two areas. The silicon content in the Si-rich areas reached over 80 wt%, and the silicon content in the Al-Si eutectic composition areas reached about 22 wt%, for few Si plates, particles and silicide exist. To compare the microstructures of the two separating samples (Sample d and f shown in Fig. 2). Fig. 5a–d shows the microstructures in the 30 kHz samples. Their corresponding positions are 1, 2, 3 and 4 in Fig. 2d. The densities of the silicon plates showed a gradual change. Fig. 6b–e shows the microstructures in the 3 kHz sample. Their corresponding positions are 1, 2, 3 and 4 in Fig. 2f. Fig. 6a is the microstructure in Fig. 2e. The Si distributions tendency of the two samples was in accord with the silicon content, as can be seen in Fig. 4. The Si-rich areas were obtained during the solidification process. During the refining stage, the temperature of the melts was 1050 °C, higher than the corresponding melting point, and the hypereutectic AlSi melts were stirred under alternating electromagnetic fields. The two main requirements must be reached to meet Si-rich conditions: 1. Proper temperature gradient; 2. Sufficient melt flow: Comparing the samples shown in Fig. 2, a, c and e, the three samples dealt without directional solidification, there is no separation results occurred. Because there is no temperature gradient exists when samples dealt without directional solidification. However, when samples dealt with directional solidification, with 0.9 mm/min (the corresponding cooling rate is 6.75 °C-11.40 °C/min), Fig. 2b, d and f, only the two samples dealt with alternating electromagnetic field, because hypereutectic AlSi melts are conductive, electromagnetic field has Lorenz force on melts and melts can be stirred, which will lead sufficient melts flow. Moreover, from previous studies [29,36] the simulations of hypereutectic AlSi melts under resistance furnace and induction furnace were carried out. There are two orders of magnitude exists in melts flow velocities of samples between resistance furnace and induction furnace.
v = DSi in Si − Al solvent ×
∂χSi in Al − Si solvent ∂χ
(1)
v = DSi in Si − Al solvent ×
∂χSi in Al − Si solvent ∂T × ∂T ∂χ
(2)
DSi in Si − Al solvent is the diffusion coefficient of the silicon in the Si-Al melts, χSi in Al − Si solvent is the silicon content, χ is the distance of the growth direction and T is the temperature of the Si-Al melts. ∂T / ∂χ is the temperature gradient. However, the assumption that silicon growth is controlled by the diffusion of atoms shows some defects: 1. When the sample dealt with alternating electromagnetic directional solidification (AEM-DS) under 3 kHz, shown as Fig. 2f, the sample after directional solidification did not show a content of silicon change from bottom to top. If the Si-rich areas are formed by diffusion controlled, the silicon content of the Sirich areas will change little by little from bottom to top, because when silicon atoms separate from the Al-Si melts, from equation (2), the silicon concentration of the Al-Si melts ( χSi in Al − Si solvent ) will decrease. Instead, an obvious crack formed between the Si-rich area and the Al-Si eutectic composition. 2. By detecting the unseparated areas of the quenched sample (Fig. 3c), SEM was used to detect the microstructures of these samples silicon plates can be observed (Fig. 8a, b and c) and this phenomenon can be used to explore the middle process during AEM-DS, and quenching the samples can reveal the separation process. For a better understanding of silicon separation from hypereutectic Al-Si melts, the silicon content of different areas, shown as the cycle regions 1 to 6 in Fig. 3c, were measured by XRFS, as shown in Fig. 7. Moreover, the microstructures of positions 1 to 4 were detected by SEM. Approximately the same microstructures (silicon plates) were observed in the unseparated areas (shown as Fig. 8). The pulling sample (Fig. 3c) shows that most of the silicon plates were in the unseparated areas, while when the AEM-DS was completed, few silicon plates were
3.2. The evaluation of the formation of Si-rich areas Regarding the formation of Si-rich areas, Xue et al. [25] held that the Si-rich area was formed by the controlled silicon atoms diffusion. 5
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Fig. 6. The microstructures of different positions shown in Fig. 2e and f: (a) Position 1, in Fig. 2e; (b) Position 1, in Fig. 2f; (c) Position 2, in Fig. 2f; (d) Position 3, in Fig. 2f; (e) Position 4, in Fig. 2f.
observed in the Al-Si eutectic composition-the final solidification area (position 1 in Fig. 2f). From the above analysis, a conclusion can be made that the Si-rich areas are not be formed just because of controlled silicon atom diffusion. The main formation of Si-rich areas was because the transportation of Si plates and particles, they formed frame of the Si-rich areas. The melts flow benefits Si atoms diffusion and can enhance augment of Si content of Si-rich area. The formation of Si-rich areas should be discussed. An Al-Si binary phase diagram [31] (Fig. 9) was used to investigate the behavior of the Al-45 wt%Si component during the directional solidification. According to the theory about the directional solidification of the alloy [32]，the slope of the liquid line dividing the slope of the solid line is infinity, which means that it is much easier to segregate the silicon when the temperature decreases below the temperature of the liquid line. When the temperature decreases, silicon atoms will segregate into silicon plates, rather than just forming Si-rich areas by means of a controlled diffusion of silicon atoms, the controlled diffusion of silicon atoms
exists in the whole process. Jie et al. [18] conducted an investigation that explored as the liquid temperature decreased, the Si content of the liquid Al-Si melt also decreased. Their findings indicated that there was a segregation process during solidification: the Al-Si melts segregated into solid silicon plates when the temperature of the melts decreased, in accordance with Fig. 8a–c, and many silicon plates or particles were mixed in the Al-Si eutectic composition. Fig. 10 shows the EPMA mapping of the distribution of elements in the unseparated areas in the quenched sample (shown in Fig. 3c). The mapping of unseparated areas indicates that the Si plates were mixed in the hypereutectic Al-Si melt during the directional solidification when the temperature of the melts was below the liquid line. Impurities were distributed in the Al-Si eutectic compositions. The EPMA-mapping of Al-Si eutectic compositions (position 1 in Fig. 2f) after AEM-DS was shown in Fig. 11. Silicon plates cannot be detected, which means that as directional solidification finished, silicon plates would be transported to the lower temperature area and because 6
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Fig. 9. Al-Si binary phase diagram. Fig. 7. Silicon content of the six circle areas shown in Fig. 3c. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
impurities. However, while some impurities were enveloped in silicon plates, these enveloped impurities can not be removed thoroughly by acid leaching, which were discussed in the recent work [36].
of the high viscosity [33], and then form Si-rich area. With regard to the mapping of the Si-rich areas (the bottom of the sample), we found that they are occupied by Si, as can be seen in the previous study [34]. The formation of complex impurities in the compounds should also be discussed. It was interesting to find some impurities in the silicon plates, which can be seen in Fig. 8, as shown by the red arrow. The EDS results (Points 1 and 2, in Fig. 12b) show that impurities were enveloped in silicon plates. Moreover, the segregated silicon plates had high purity. In other words, the silicon segregation process is a purification process. Yu et al. [26] and Jie et al. [35] both demonstrated that the convection generated by the electromagnetic field caused impurities to accumulate in front of the liquid/solid interface and had a positive effect on removing
3.3. The principle of silicon plates separated from hypereutectic Al-Si melts Much research has been done on non-metallic inclusions separating from metal [37–44]. The process of how silicon separates from aluminum-silicon can be regarded as the model of how non-metallic inclusions separate from metal. Comparing the electrical conductivity of silicon (σSi ≈ 1 × 10−2S / m ) and hypereutectic Al-Si melts (σMelt ≈ 2.4 × 106S / m )σSi ≪ σMelt , we see that silicon plates can be regarded as non-conducting. In 1954, Leenov and Kolin [42] investigated the force on the nonconducting particles in conductive melts. The Lorenze force of melts can be expressed by equation (3):
F=J×B
Fig. 8. Typical microstructures of the quenched sample C (shown in Fig. 3c): (a) position 1 . (b) Position 2; (c) position 3; (d) position 4. 7
(3)
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Fig. 10. EPMA-mapping of distribution of elements in the unseparated area of quenched sample shown in Fig. 3c.
(non-conductive). The applied electromagnetic force on the molten metal pushed the inductive melts in a certain direction. Moreover, the electromagnetic force only existed in the inductive aluminum-silicon melts, and the influence on the non-conductive solid silicon was ignored, because solid silicon can be considered non-conductive. When analyzing the force acting on the solid silicon, it can be
Where J is the induction current in the copper coils, B is the magnetic flux density; the latter is a vector and can be generated by the alternating current in the copper coils. F is the electromagnetic force. During our experiments, the segregated silicon particles were nonconductive as compared to the aluminum-silicon melts. Both were mixed. The molten metal (conductive) contained nonmetallic silicon
Fig. 11. EPMA-mapping of elements' distribution in Al-Si alloy, position 1, as shown in Fig. 2f. 8
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Fig. 12. (a) Microstructure of the unseparated area in Fig. 3c; (b) An enlargement of the rectangle region marked in (a); (c) EDS analysis of the formed impurities—intermetallic compounds enveloped in silicon plates; (d) EDS analysis of the silicon plates. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
The viscous resistance can be calculated by equation (7):
regarded that the direction of the electromagnetic force on the solid silicon was opposite that of the electromagnetic force on the molten metal. The force can be expressed by equation (4):
Fp =
3(σl − σSi ) JBVSi 2(2σl + σSi )
Fn = ϕρdp2 ν 2
ϕ is the viscous damping coefficient, ν is the migrating rate of Si particles, and ρ is the density. The migrating equation of the Si particles in the Al-Si alloy melts is as follows:
(4)
Where Fp , σl , σSi , and VSi represent the total force of non-conductive Si particles, the electrical conductivity of the aluminum-silicon melts, the electrical conductivity of the solid silicon, and the volume of the solid silicon, respectively. Damoah and Zhang [44] investigated the use of an electromagnetic field for removing non-conductive particles from metals, and Jiang et al. [45] investigated the separation of silicon carbide inclusions by using a supersonic frequency magnetic field. Their conclusion was that if the particle is nonconducting (σSi =0) or the electric conductivity is far lower than that of the liquid metal (σSi «σl ), the particle will move in a direction opposite to the EM force; the authors concluded that the particles can move in a fixed direction, considering the magnetic force should be perpendicular to magnetic field, the particles will move to the edge of the sample. According to the research of Colin and others [46], the extrusion force on the impurity phases by an electromagnetic field can be calculated as follows:
Fp =
3 3 σl − σSi πdp F 2 2σl + σSi 6
Fp − Fn = m
3 3 πdp F 2 6
dv dt
(8)
Equation (8) reflects the relationship between the migrating velocity and viscous resistance of the Si particles, where m is the weight of the Si particles. According to equations (4)–(8), equation (9) can be obtained as follows: 3 πdp3 ρ dv 3 πdp Fn − ϕρdp2 ν 2 = 4 6 6 dt
(9)
Simplifying equation (9), it can be written as follows:
6ρϕv 2 dv 3 Fn − = = ρa πdp dt 4
(10)
where a is the migrating acceleration of the silicon particles, which equals the result of the electromagnetic force acceleration less the viscous resistance acceleration. The difference between the electromagnetic force and the viscous resistance (shown as equation (10), a = 0 ) can be calculated by equation (11):
(5)
where dp is the diameter of the silicon particles, and σSi ≈ 0 , so equation (5) can be written as follows:
Fp =
(7)
6ρϕv 2 3 Fn − =0 4 πdp
(11)
Finally, according to equation (11), the migrating velocity of the silicon particles can be calculated as v = 3Fn πdp/24ρϕ . From this, we can clearly see that the velocity is related to the electromagnetic force Fn , viscous damping coefficient ϕ , density ρ and diameter dp . Thus, a conclusion can be made that to improve the separation efficiency of Si,
(6)
As Si particles are very tiny, the buoyancy and gravity of these silicon particles can be neglected [47]. The Si particles obtained by viscous resistance should be considered when the Si rich areas are formed. 9
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Fig. 13. SEM photograph of the dividing line shown in Fig. 2f.
the velocity of the melts must be increased. Fig. 13 shows the SEM photographs of the dividing line in sample 2f, and the results reflect that the Si particles move quickly when under a 3 kHz alternating electromagnetic field because of the large melt flow. The flow field distribution of the melts can be seen in our previous work [34], in which the maximum flow rate reached 0.92 cm/s. Fig. 13 shows that some Si plates or particles were distributed around the crack, which represents the position 2 shown in Fig. 2f. In order to verify and discuss Si-rich area formed by particle migration or precipitation. The differences in density between solid Si (~2300 kg/ m3) and Si-Al melt (~2400 kg/m3) were compared, solid Si shows a lighter density and from the samples without pulling, the Si plates distribute uniformly (seen as Fig. 2a,b and c). The controlled diffusion of Si atoms exists in the whole AEM-DS process. From this analyses, we can concluded that the Si-rich areas were mainly formed by the migration of silicon plates or particles, not formed by the precipitation nor just controlled diffusion of Si atoms (known as Fick's second law). Considering the process involves complex Si plate growth, temperature gradient, electromagnetic field and movement with melt flow. The detailed and precise process requires more experimental and theoretical studies in the future.
Fig. 14. A sketch of an optimized device for purifying and separating silicon from hypereutectic Al-Si melts. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Shown as Fig. 14, the heating resource comes from resistive heaters. For the stirring effect, more subtle control may be provided by using coils. To avoid electromagnetic induction heating, a much lower frequency should be used.
3.4. Proposal of a novel optimized device for purifying and separating silicon from hypereutectic Al-Si melts efficiently
4. Conclusion
Fig. 14 shows an optimized device for purifying metals. It is different from the traditional alternating electromagnetic directional solidification devices. Traditional AEM-DS devices are equipped with an alternating electromagnetic field, and the electromagnetic field has two main effects: heating and stirring. As mentioned before, for some combinations (like the Al-Si combination), when the temperature is below the liquid line, segregation will occur, and it is a purifying process. These purified silicon plates should be transported to the lower temperature areas so as to collect the purified silicon and reduce the waste of silicon and aluminum. However, as the temperature of the melts decreases, the stirring effect will also decrease and the melt flow will be reduced. Considering these aspects, the stirring effect and heating effect should be separated so as to control the purifying process.
This study focused on the formation of Si-rich areas. By experiments and analyses, the formation of Si-rich areas was confirmed to be caused mainly by Si plates or particle migration, and not only by the controlled diffusion of atoms, known as Fick's law. Moreover, during the whole AEM-DS, the controlled diffusion of Si atoms exist. When using a 3 kHz alternating electromagnetic field with pulling at a speed of 0.9 mm/ min, the Si content of the Si-rich areas reached 80 wt%. The process of Si being segregated from the hypereutectic Al-Si was confirmed as a purification process, but some impurities were enveloped in Si plates. Moreover, a novel optimized device was proposed for better Si separation from the hypereutectic Al-Si melts. The process of Si separated 10
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from hypereutectic Al-Si melts with AEM-DS is such complex: melts flow, Si crystal growth, latent heat, Si plates move with melts, electrical properties of the melts and changing properties of melts and so on, they interweave each other. The detail precise quantitative analyses need more experimental and theoretical studies in the future.
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