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Enhancing B removal from Si with small amounts of Ti in electromagnetic solidification refining with Al–Si alloy
Journal of Alloys and Compounds 666 (2016) 406e411
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhancing B removal from Si with small amounts of Ti in electromagnetic solidification refining with AleSi alloy Yun Lei a, b, *, Luen Sun b, Wenhui Ma a, b, **, Kuixian Wei b, Kazuki Morita c a
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, PR China National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, PR China c Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 November 2015 Received in revised form 7 January 2016 Accepted 17 January 2016 Available online 20 January 2016
Small amounts of Ti (<2037 ppma) were employed as an additive to enhance B removal from Si in electromagnetic solidification refining with an AleSi alloy. The B removal process was investigated by varying the initial concentration of Ti, the cooling rate, and the liquidus temperature of the AleSi alloy. Experimental results show that the small amounts of Ti were primarily responsible for enhancing B removal. This B removal process was more efficient at lower cooling rates combined with the use of AleSi alloy solvents having lower liquidus temperatures. With the addition of 2037 ppma Ti, the residual B in the refined Si could be controlled to 1.2 ppma, which meets the requirements for manufacturing solar cells. The added Ti could be simultaneously removed because of its extremely low segregation coefficient between solid Si and liquid AleSi alloy. Although trace amounts of Ti and Al were present in the refined Si, they are expected to be removed in the next processing step, which is directional solidification in vacuum. Finally, the mechanism of this B removal process is discussed. © 2016 Elsevier B.V. All rights reserved.
Keywords: B removal Ti addition Solidification refining AleSi alloy Electromagnetic force Solar-grade Si
1. Introduction Multi-crystalline Si is the main rough material for manufacturing solar cells. The concentration of impurities in multicrystalline Si has to be extremely low because their presence will decrease the conversion efficiency from solar to electric energy. Normally, the purity of multi-crystalline Si should be greater than 99.9999% (solar-grade Si or SoG-Si). Upgrading metallurgical-grade Si (MG-Si, purity 98%e99%) to SoG-Si with metallurgical technologies is a promising route for producing low-cost SoG-Si, which is important for decreasing the manufacturing cost of solar cells. Boron is a typical impurity in MG-Si, and its concentration should be reduced to 0.26e3.9 parts per million atoms (ppma) (0.1e1.5 parts per million by weight (ppmw)) [1] for solar cell applications. However, it is difficult to remove B from MG-Si efficiently with metallurgical methods because of its large segregation coefficient (0.8 at 1687 K) [2] and low vapor pressure (lower than that of Si) [3]. Si purification with H2O-added gas [4,5] and slag treatment [6e9] are effective for B removal. However, a further
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Lei), [email protected] (W. Ma). http://dx.doi.org/10.1016/j.jallcom.2016.01.127 0925-8388/© 2016 Elsevier B.V. All rights reserved.
reduction in cost and a more environmentally friendly Si refining process are still necessary. Recently, a process using an AleSi alloy as the refining solvent combined with electromagnetic solidification has shown outstanding results in refining Si more economically and efficiently [10,11]. The precipitated Si crystals can be purified during solidification of the AleSi alloy from a given temperature to its eutectic temperature because segregation ratios of impurities between solid Si and liquid AleSi alloy become smaller at lower liquidus temperature [10]. Induction heating was employed in this refining process because the precipitated Si crystals can be agglomerated at the bottom of the liquid AleSi alloy with electromagnetic force, which can reduce the consumption of Al and that of the acid solution in the following leaching process. Some researchers attempted to improve this B removal and Si-agglomeration process and make it more efficient with modifications, such as using AleSieZn (<40 mol%) [12] and AleSieSn (10e30 mol%) [13] as the solvents, combining SneSi and AleSi melts, and improving the electromagnetic field [14,15]. Our research group also tried to agglomerate Si crystals more efficiently with electromagnetic force [16] and proposed a mechanism of Si agglomeration [17]. However, further study is still required to control the concentration of residual B to an acceptable level for manufacturing SoG-Si. If the B
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removal process can be enhanced by adding small amounts of metal additives, it will make this Si refining process more practical, because it will be efficient with lower refining costs. Small amounts of Zr (<1057 ppma) and Ti (<933 ppma) have been employed as additives to enhance B removal in our previous study [18] and in Yoshikawa's work [19], respectively, and their presence yields outstanding results in B removal in the electromagnetic solidification refining process. Zr and Ti were employed as additives because of their strong affinity for B [20]. Moreover, both of them are expected to be simultaneously removed in the refining process because of their extremely low segregation coefficients (Zr: 1.6 108, Ti: 2.0 106, at 1687 K) [1] and extremely low solubilities in solid Si (both of them are less than 0.00001 ppma below 1173 K) [21]. When using Zr as the additive to enhance B removal, we found that the cooling rate and liquidus temperature of the AleSi alloy can influence this B removal process significantly [18]; however, the effect of these parameters on B removal with small amounts of added Ti have not been clarified yet. In this study, we attempt to investigate the effects of the cooling rate, the liquidus temperature of AleSi alloy, and the initial concentration of Ti on the B removal process in order to better understand this Ti-addition refining process. In this study, we investigate B removal with small amounts of Ti addition (<2037 ppma) in electromagnetic solidification refining of Si using AleSi alloy as a solvent. 2. Experimental An induction furnace combined with a stepping motor was used to carry out the electromagnetic solidification refining process. A schematic of the experimental set-up is shown in Fig. 1. Ten grams of Al shots (99.999%) and Si sheets (99.9999%) were put into a dense, high-purity graphite crucible (17-mm I.D., 25-mm O.D., 65mm length). Powdered Sie1 wt% B and Sie5 wt% Ti alloys were used to add the B and Ti in order to precisely control their small initial concentration (B: 153 ppma, Ti: 203e2037 ppma). The powdered Sie1 wt% B and the Sie5 wt% Ti alloys were prepared by pre-melting bulk Si with B powder (99.9%) and with Ti lumps (99.5%), respectively, followed by grinding the melted mixtures into
Fig. 1. Schematic of the induction furnace combined with a stepping motor: 1. Ar gas (99.99%) tank; 2. gas flow meter; 3. two-way valve; 4. gas inlet; 5. gas outlet; 6. induction coils; 7. graphite crucible; 8. AleSi melt; 9. prism; 10. ball screw; 11. stepping motor; 12. infrared pyrometer; 13. vacuum pump; 14. three-way valve; 15. porous alumina holder; 16. silicone plug; 17. bubble checking; 18. quartz chamber; 19. stainless steel plate; 20. vacuum meter.
407
powders in an agate mortar to achieve composition homogenization and passing the powders through a 186-mm mesh filter. Their compositions were confirmed by inductively coupled plasmaatomic emission spectroscopy (ICP-AES). The bottom of the graphite crucible was located level with the lower end of the coils (6 turns, 95-mm O.D., 80-mm I.D., 70-mm length). Before induction heating, the air in the quartz chamber was evacuated by a mechanical pump and then refilled with high-purity Ar gas (99.99%) to avoid oxidation of the samples. This process was repeated three times to ensure the air in the chamber was fully eliminated. After melting and holding at 1473 ± 20 K for 30 min, the samples were cooled by lowering the graphite crucible to carry out the electromagnetic solidification refining. The agglomerated Si crystals were separated from the solidified AleSi alloy by mechanical cutting. A portion of them was analyzed by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) to observe and determine the precipitated phases. The remainder was crushed into powder (by passing through a 186-mm mesh filter) followed by treating with aqua regia containing H2SO4 (HCl:HNO3:H2SO4 ¼ 3:1:1) at 343 K for 6 h to remove impurities. Thereafter, the concentrations of residual impurities in the refined Si were determined by ICP-AES. 3. Results and discussion Table 1 lists the initial composition of the AleSi alloy and the concentrations of residual impurities in the refined Si. The initial concentration of Ti in the AleSi alloy was chosen to be less than 2037 ppma to lower the refining cost. The concentration of B in typical MG-Si is normally between 14.5 and 130 ppma (5.6 and 50 ppmw) [22]. However, in consideration of possible contamination from Al in practical Si purification processes, its initial concentration in the AleSi alloy in this work was set to 153 ppma (60 ppmw). The initial composition of AleSi was set to Al-36 at% Si, Al-45 at% Si and Al-55 at% Si to achieve low-temperature refining, because their liquidus temperatures are 1173 K, 1273 K and 1373 K, respectively, which are much lower than the melting point of Si (1687 K). Fig. 2(a) shows a cross-section of a sample after the electromagnetic solidification refining. The precipitated Si crystals were successfully separated from the AleSi alloy and agglomerated at its bottom because of the electromagnetic force. The mechanism of this separation and agglomeration process has been elaborated in other studies [11,17]. An enlargement of the circled region in Fig. 2(a) is shown in Fig. 2(b) to exhibit its morphology clearly. Fig. 3 shows the initial concentration of Ti in the AleSi alloy and its effect on B removal with a lowering rate of 0.55 ± 0.02 mm/min. Al-45 at% Si alloy was used as the refining solvent. The B removal fraction increases significantly when the initial concentration of Ti is larger than 407 ppma. The B removal fraction without Ti addition was 59.5%; however, this value increased to 99.2% with the addition of 2037 ppma Ti, i.e., the concentration of B could be reduced from 153 ppma to 1.2 ppma, which meets the acceptable concentration (0.26e3.9 ppma) [1] for producing solar cells. In Yoshikawa's study, the removal fractions of both B and Ti were much larger than those obtained in this study, for example, the removal fraction of B was 99.4% (the B content decreased from 170 to 1.1 ppma) with the addition of 933 ppma Ti [19] (Al-45 at% Si solvent, cooling rate of 5.0e10.0 K/min). This significant difference can possibly be attributed to the different electromagnetic fields employed in the two studies (20 kHz in this study, 50 kHz in Yoshikawa's study). Electromagnetic field induced flow and the electromagnetic stirring can affect the precipitation behavior of Si crystals [14], which indicates these factors can also affect the distribution behavior of Ti and B between the solid Si crystals and the liquid AleSi melt. For example,
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Y. Lei et al. / Journal of Alloys and Compounds 666 (2016) 406e411
Table 1 Composition of the liquid AleSi alloy before refining and concentration of residual impurities in the refined Si with different lowering rates. No.
Lowering Rate, mm/min (±0.02)
1 2 3 4 5 6 7 8 9 10 11
0.55 0.55 0.55 0.55 0.55 0.55 0.55 1.36 0.22 0.55 0.55
Composition of the AleSi solvent before refining
Residual impurities in the refined Si, ppma
B, ppma
Ti, ppma
Al, at%
B
Ti
Al
153 153 153 153 153 153 153 153 153 153 153
e 203 407 814 1018 1527 2037 1018 1018 1022 1014
55.0 55.0 55.0 55.0 55.0 55.0 55.0 55.0 55.0 45.0 64.0
62.0 61.2 45.1 41.3 37.1 6.4 1.2 58.5 1.5 55.7 4.6
e
317 540 450 570 745 792 868 780 510 501 684
1.2 1.7 96.3 248 889 1530 294 118 243 119
Fig. 2. (a) Cross-section of an AleSi alloy after electromagnetic solidification refining and (b) an enlargement of the circled region marked in (a).
Removal fraction, %
100 80 60 40 20 0 0
B Ti
500 1000 1500 2000 2500 Initial Ti content, ppma
Fig. 3. Relationship between the initial concentration of Ti in the AleSi alloy and the removal fractions of B and Ti after electromagnetic solidification refinement of Si. (The lowering rate was 0.55 ± 0.02 mm/min and the refining solvent was the Al-45 at% Si alloy.)
Ban et al. [14] reported that electromagnetic stirring could significantly affect B removal at the cooling rate of 1.5 K/min. Therefore, it is difficult to compare the results obtained by Yoshikawa's and our work because of the different electromagnetic fields.
The removal fraction of Ti decreases from 99.4% to 24.9% with the increase of its initial concentration from 203 ppma to 2037 ppma, as shown in Fig. 3. The residual concentrations of Ti in the refined Si crystals are between 1.2 ppma to 1530 ppma, which are much larger than its solubility in solid Si (less than 0.00001 ppma below 1173 K [21]). Therefore, this refining process is actually a non-equilibrium process, so it is natural to inquire how the kinetic process changes with different cooling rates. For example, the removal fractions of both B and Ti significantly increase by decreasing the lowering rate from 0.55 ± 0.02 mm/min to 0.22 ± 0.02 mm/min. Similarly, these values significantly decrease by increasing the lowering rate to 1.36 ± 0.02 mm/min, as shown in Fig. 4 (using Al-45 at%Si alloy with 1018 ppma Ti as the refining solvent). The corresponding cooling rates for lowering rates of 0.22 ± 0.02 mm/min, 0.55 ± 0.02 mm/min and 1.36 ± 0.02 mm/min are 1.6e2.7 K/min, 4.2e7.1 K/min and 12.5e20.6 K/min, respectively. Cooling rate at a constant lowering rate is not a constant because of the induction heating [18]. The concentrations of residual B and Ti in the refined Si with a lowering rate of 0.22 ± 0.02 mm/min were 1.5 ppma (below the maximum acceptable level for producing solar cells) and 118 ppma, respectively, while their values were 37.1 ppma and 248 ppma, respectively, with a lowering rate of 0.55 ± 0.02 mm/min. These results indicate that a lower cooling rate was more favorable for Ti and B removal, because a slower cooling rate provides more time for B
Y. Lei et al. / Journal of Alloys and Compounds 666 (2016) 406e411
Removal fraction, %
100 90
Ti B
80 70 60 50 0.0
0.3 0.6 0.9 1.2 Lowering rate, mm/min
1.5
Fig. 4. Relationship between the lowering rates and the removal fractions of B and Ti after electromagnetic solidification refinement of Si (using Al-45 at%Si alloy with 1018 ppma Ti as the refining solvent).
and Ti to segregate between the solid Si and liquid AleSi alloy, making the refining process much closer to its thermodynamic equilibrium. Fig. 5 shows the effects of the composition of the AleSi alloy on B and Ti removal. The lowering rate was 0.55 ± 0.02 mm/min, and the initial concentration of Ti was about 1018 ppma. The removal fractions of both B and Ti increase with the increase of the initial Al concentration in the AleSi alloy. These results indicate that the composition of AleSi alloy (specifically the liquidus temperature) has a significant influence on B and Ti removal because the segregation coefficients of both B and Ti are smaller at lower liquidus temperature (B: 0.8 at 1687 K and 0.22 at 1473 K; Ti: 2 106 at 1687 K and 3.8 109 at 1073 K [10]). SEM with EDS analysis was performed to reveal this efficient B removal with Ti addition. A TieSieAl phase was observed to precipitate along the boundary of the Si crystals and the liquid AleSi melt, as shown in Fig. 6. Qualitative analysis indicated that the mole ratio of Ti, Al, and Si was 0.34:0.09:0.57. This value is close to that of the ternary compound Ti0.33Al0.15Si0.52 in the TieAleSi ternary system [23]. According to Yoshikawa's study [19], TiB2 was found to form during the AleSi solvent refining process, and efficient B removal by Ti addition is attributed to the low solubility of TiB2 in
Removal fraction, %
90 80 70
50 40
AleSi. In that study, TiB2 was mostly found embedded in the acidleachable eutectic AleSi alloy. However, we did not find TiB2 in this study. The liquid AleSi alloy is thought to dilute the concentrations of Ti and B and thereby reduce the chance of reaction of Ti and B. Instead of TiB2, we found another B bearing particles precipitated along the boundary of Si crystals (black particles shown in Fig. 7(a)) and their X-ray mapping is shown in Fig. 7. Regardless of whether the formation of the B-containing particles occurs or not, the efficient removal of B with Ti addition can be attributed to the decrease of the segregation coefficient of B between solid Si and the liquid AleSi alloy. When B segregates between the solid Si and the liquid AleSi alloy (containing Ti), its chemical potential in both phases is identical at equilibrium. We have the following three relations:
BðsÞin solid Si ¼ BðlÞin AlSi melt
(1)
DG+B fus ¼ 50200 21:8 TJ=mol½20
(2)
RTlnaBðsÞ in solid Si ¼ DG+B fus þ RTlnaBðlÞ in AlSi melt
(3)
Therefore, we can express the segregation ratio of B as.
ln kB ¼ ln
XBðsÞ in solid Si XBðlÞ in AlSi melt
¼
gBðlÞ in AlSi melt DG+B fus þ ln RT gBðsÞ in solid Si
B Ti 45 50 55 60 65 Initial Al content, at %
70
Fig. 5. Relationship between the initial concentration of Al in the AleSi alloy and the removal fractions of B and Ti after electromagnetic solidification refinement of Si. (The lowering rate was 0.55 ± 0.02 mm/min, and the initial concentration of Ti was approximately 1018 ppma.)
(4)
where DG+B fus denotes the Gibbs energy change for fusion of B, and aB, XB, and gB are the activity, mole fraction, and activity coefficient of B, respectively. The character in the parentheses denotes the standard state. kB denotes the segregation coefficient of B between solid Si and liquid AleSi melt. As the initial concentrations of B and Ti in this study are small, the activity coefficients of B in solid Si and liquid AleSi melt at the liquid and solid standard states can be expressed to first order in the interaction parameters as follows:
lngBðsÞ in solid Si ¼ lng+BðsÞ in solid Si þ εBB in solid Si XB in solid S þ εAl B in solid Si XAl in solid Si þ εTi B in solid Si XTi in solid Si
(5)
lngBðlÞ in AlSi melt ¼ lng+BðlÞ in AlSi melt þ εBB in AlSi melt XB in AlSi melt þ εTi B in AlSi melt XTi in AlSi melt
100
60
409
(6)
where εiB is the interaction parameter of i with B per mole fraction. As the segregation ratio of Ti between solid Si and AleSi melt is extremely small (2 106 at 1687 K and 3.8 109 at 1073 K [24]) and the solubility of Ti in solid Si is also extremely small (less than 0.00001 ppma below 1173 K [21]), XTi in solid Si XTi in AlSi melt at equilibrium. The effect of Ti on ln gBðsÞ in solid Si can be ignored, i.e., the term εTi X in Eq. (5) can be ignored. Therefore, B in solid Si Ti in solid Si according to Eq. (4) through Eq. (6), the effect of Ti addition on kB is mainly represented by the term εTi X in Eq. B in AlSi melt Ti in AlSi melt (6). Under the assumption that the value of εTi is negative B in AlSi melt because Ti has a strong affinity for B, the value of kB will decrease with the increase of XTi in AlSi melt . This assumption agrees well with our experimental results, which indicate that the removal fraction of B increases as the concentration of Ti increases. Therefore, the efficient B removal with Ti addition is due to the negative value of εTi , although there is yet no data on the value of B in AlSi melt this parameter. A smaller value of kB implies that much more B can be released from solid Si and enter into the AleSi melt or accumulate at grain boundaries of Si crystals (for example, stabilize as
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Y. Lei et al. / Journal of Alloys and Compounds 666 (2016) 406e411
Fig. 6. SEM-EDS analysis of the TieSieAl phase in the sample with 2037 ppma Ti and 153 ppma B. (a) morphology image, X-ray mapping of (b) Ti, (c) Si, (d) Al, and (e) B, and (f) Xray analysis of points 1#, 2# in (a).
Fig. 7. SEM-EDS analysis of a group of precipitated particles in the sample with 2037 ppma Ti and 153 ppma B. (a) morphology image, X-ray mapping of (b) B, (c) Si, (d) Al, and (e) C.
an B containing phase in Fig. 7), making B easily removable by a leaching acid solution (HCl:HNO3:H2SO4 ¼ 3:1:1 in this study). Although there was some residual Ti and Al in the refined Si, they can be removed in the following process step, directional solidification in vacuum, because the segregation coefficient of Ti between solid and liquid Si is extremely small and the vapor pressure of Al is higher than that of Si [25]. Therefore, B removal from Si with small amounts of Ti in electromagnetic solidification
refining with AleSi alloy is a promising method for manufacturing SoG-Si, because this B removal process offers advantages of low energy consumption and low environmental impact (low acid solution consumption, and no waste slag or gas generation). 4. Conclusions Small amounts of Ti (<2073 ppma) were employed as an
Y. Lei et al. / Journal of Alloys and Compounds 666 (2016) 406e411
additive to enhance B removal from Si because of its strong affinity for B, extremely small segregation coefficient between solid Si and liquid AleSi alloy, and extremely low solubility in solid Si. The results show that small amounts of Ti were significantly efficient in enhancing B removal, and this B removal process will become more efficient at lower cooling rate combined with using a lower liquidus temperature of AleSi alloy as the refining solvent. The concentration of residual B in the refined Si could be reduced to less than 1.2 ppma (0.46 ppmw), which meets the requirements for manufacturing solar cells. The added Ti could be simultaneously removed because of its extremely small segregation coefficient between solid Si and liquid AleSi alloy. Although there is some residual Ti and Al present in the refined Si, these can be removed in the next processing step, directional solidification in vacuum. Consequently, B removal from Si with small amounts of Ti in the electromagnetic solidification refining with AleSi alloy is a promising method for manufacturing SoG-Si. Acknowledgments This study was sponsored by The National Natural Science Foundation of China (Grant Nos. 51504118 and 51334002) and The People's Cultivation Project of Yunnan Province (KKSY201452087). References [1] S.S. Zheng, J. Safarian, S. Seok, S. Kim, T. Merete, X.T. Luo, Elimination of phosphorus vaporizing from molten silicon at finite reduced pressure, Trans. Nonferrous Met. Soc. China (2011) 697e702. [2] F.A. Trumbore, Solid solubilities of impurity elements in germanium and silicon, Bell Syst. Tech. J. 39 (1960) 206e233. [3] J. Safarian, M. Tangstad, Vacuum refining of molten silicon, Metall. Mat. Trans. B 43B (2012) 1427e1445. [4] N. Nakamura, H. Baba, Y. Sakaguchi, Y. Kato, Boron removal in molten silicon by a steam-added plasma melting method, Mat. Trans. 45 (2004) 858e864. [5] J. Safarian, K. Tang, K. Hildal, G. Tranell, Boron removal from silicon by humidified gases, Metall. Mat. Trans. E 1 (2014) 41e47. [6] L. Zhang, Y. Tan, F.M. Xu, J.Y. Li, H.Y. Wang, Z. Gu, Removal of boron from molten silicon using Na2O-CaO-SiO2 slags, Separ. Sci. Technol. 48 (2013) 1140e1144. [7] L.A.V. Teixeira, Y. Tokuda, T. Yoko, K. Morita, Behavior and state of boron in CaO-SiO2 slags during refining of solar grade silicon, ISIJ Int. 49 (2009)
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777e782. [8] L.A.V. Teixeira, K. Morita, Removal of boron from molten silicon using CaOSiO2 based slags, ISIJ Int. 49 (2009) 783e787. [9] E.J. Jung, B.M. Moon, S.H. Seok, D.J. Min, The mechanism of boron removal in the CaO-SiO2-Al2O3 slag system for SoG-Si, Energy 66 (2014) 35e40. [10] T. Yoshikawa, K. Morita, An evolving method for solar-grade silicon production: solvent refining, JOM 64 (2012) 946e951. [11] T. Yoshikawa, K. Morita, Refining of Si by the solidification of Si-Al melt with electromagnetic force, ISIJ Int. 45 (2005) 967e971. [12] Y.Q. Li, Y. Tan, J.Y. Li, K. Morita, Si purity control and separation from Si-Al alloy melt with Zn addition, J. Alloy Compd. 611 (2014) 267e272. [13] Y.Q. Li, Y. Tan, J.Y. Li, Q. Liu, Y. Liu, Effect of Sn content on microstructure and boron distribution in Si-Al alloy, J. Alloy Compd. 583 (2014) 85e90. [14] B.Y. Ban, Y.L. Li, Q.X. Zou, T.T. Zhang, J. Chen, S.Y. Dai, Refining of metallurgical grade Si by solidification of Al-Si melt under electromagnetic stirring, J. Mat. Process Tech. 222 (2015) 142e147. [15] Q.C. Zou, J.C. Jie, S.C. Liu, T.M. Wang, G.M. Yin, T.J. Li, Effect of traveling magnetic field on separation and purification of Si from Al-Si melt during solidification, J. Cryst. Growth 429 (2015) 68e73. [16] W.Z. Yu, W.H. Ma, G.Q. Lv, H.Y. Xue, S.Y. Li, Y.N. Dai, Effect of electromagnetic stirring on the enrichment of primary silicon from Al-Si melt, J. Cryst. Growth 405 (2014) 23e28. [17] H.Y. Xue, G.Q. Lv, W.H. Ma, D.T. Chen, J. Yu, Separation mechanism of primary silicon from hypereutectic Al-Si melts under alternating electromagnetic fields, Metall. Mat. Trans. A 46 (2015) 2922e2932. [18] Y. Lei, W.H. Ma, L.E. Sun, Y.N. Dai, K. Morita, B removal by Zr addition in electromagnetic solidification refinement of Si with Si-Al melt, Metall. Mat. Trans. B 47 (2016) 27e31. [19] T. Yoshikawa, K. Arimura, K. Morita, Boron removal by titanium addition in solidification refining of silicon with Si-Al melt, Metall. Mat. Trans. B 36B (2005) 837e842. [20] E.T. Turkdogan, Physical Chemistry of High-temperature Technology, Academic Press, New York, 1980, p. 5, 22, 24. [21] K. Tang, E.J. Øvrelid, G. Tranell, M. Tangstad, Thermochemical and kinetic databases for the solar cell silicon materials, in: The Twelfth International Ferroalloys Congress, Sustainable Future, Helsinki, Finland, 2010, pp. 619e630. [22] C.P. Khattak, D.B. Joyce, F. Schmid, Production of solar grade (SoG) silicon by refining liquid metallurgical grade (MG) silicon, Cryst. Syst. Inc. Mass. (December 1999) 7. [23] Q. Luo, Q. Li, J.Y. Zhang, S.L. Chen, K.C. Chou, Experimental investigation and thermodynamic calculation of the Al-Si-Ti system in Al-rich corner, J. Alloy Compd. 602 (2014) 58e65. [24] K. Morita, T. Yoshikawa, Thermodynamic evaluation of new metallurgical refining processes for SOG-silicon production, Trans. Nonferrous Mater. Soc. China 21 (2011) 685e690. [25] K.X. Wei, D.M. Zheng, W.H. Ma, B. Yang, Y.N. Dai, Study on Al removal from MG-Si by vacuum refining, Silicon 7 (2015) 269e274.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhancing B removal from Si with small amounts of Ti in electromagnetic solidification refining with AleSi alloy Yun Lei a, b, *, Luen Sun b, Wenhui Ma a, b, **, Kuixian Wei b, Kazuki Morita c a
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, PR China National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, PR China c Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 November 2015 Received in revised form 7 January 2016 Accepted 17 January 2016 Available online 20 January 2016
Small amounts of Ti (<2037 ppma) were employed as an additive to enhance B removal from Si in electromagnetic solidification refining with an AleSi alloy. The B removal process was investigated by varying the initial concentration of Ti, the cooling rate, and the liquidus temperature of the AleSi alloy. Experimental results show that the small amounts of Ti were primarily responsible for enhancing B removal. This B removal process was more efficient at lower cooling rates combined with the use of AleSi alloy solvents having lower liquidus temperatures. With the addition of 2037 ppma Ti, the residual B in the refined Si could be controlled to 1.2 ppma, which meets the requirements for manufacturing solar cells. The added Ti could be simultaneously removed because of its extremely low segregation coefficient between solid Si and liquid AleSi alloy. Although trace amounts of Ti and Al were present in the refined Si, they are expected to be removed in the next processing step, which is directional solidification in vacuum. Finally, the mechanism of this B removal process is discussed. © 2016 Elsevier B.V. All rights reserved.
Keywords: B removal Ti addition Solidification refining AleSi alloy Electromagnetic force Solar-grade Si
1. Introduction Multi-crystalline Si is the main rough material for manufacturing solar cells. The concentration of impurities in multicrystalline Si has to be extremely low because their presence will decrease the conversion efficiency from solar to electric energy. Normally, the purity of multi-crystalline Si should be greater than 99.9999% (solar-grade Si or SoG-Si). Upgrading metallurgical-grade Si (MG-Si, purity 98%e99%) to SoG-Si with metallurgical technologies is a promising route for producing low-cost SoG-Si, which is important for decreasing the manufacturing cost of solar cells. Boron is a typical impurity in MG-Si, and its concentration should be reduced to 0.26e3.9 parts per million atoms (ppma) (0.1e1.5 parts per million by weight (ppmw)) [1] for solar cell applications. However, it is difficult to remove B from MG-Si efficiently with metallurgical methods because of its large segregation coefficient (0.8 at 1687 K) [2] and low vapor pressure (lower than that of Si) [3]. Si purification with H2O-added gas [4,5] and slag treatment [6e9] are effective for B removal. However, a further
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Lei), [email protected] (W. Ma). http://dx.doi.org/10.1016/j.jallcom.2016.01.127 0925-8388/© 2016 Elsevier B.V. All rights reserved.
reduction in cost and a more environmentally friendly Si refining process are still necessary. Recently, a process using an AleSi alloy as the refining solvent combined with electromagnetic solidification has shown outstanding results in refining Si more economically and efficiently [10,11]. The precipitated Si crystals can be purified during solidification of the AleSi alloy from a given temperature to its eutectic temperature because segregation ratios of impurities between solid Si and liquid AleSi alloy become smaller at lower liquidus temperature [10]. Induction heating was employed in this refining process because the precipitated Si crystals can be agglomerated at the bottom of the liquid AleSi alloy with electromagnetic force, which can reduce the consumption of Al and that of the acid solution in the following leaching process. Some researchers attempted to improve this B removal and Si-agglomeration process and make it more efficient with modifications, such as using AleSieZn (<40 mol%) [12] and AleSieSn (10e30 mol%) [13] as the solvents, combining SneSi and AleSi melts, and improving the electromagnetic field [14,15]. Our research group also tried to agglomerate Si crystals more efficiently with electromagnetic force [16] and proposed a mechanism of Si agglomeration [17]. However, further study is still required to control the concentration of residual B to an acceptable level for manufacturing SoG-Si. If the B
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removal process can be enhanced by adding small amounts of metal additives, it will make this Si refining process more practical, because it will be efficient with lower refining costs. Small amounts of Zr (<1057 ppma) and Ti (<933 ppma) have been employed as additives to enhance B removal in our previous study [18] and in Yoshikawa's work [19], respectively, and their presence yields outstanding results in B removal in the electromagnetic solidification refining process. Zr and Ti were employed as additives because of their strong affinity for B [20]. Moreover, both of them are expected to be simultaneously removed in the refining process because of their extremely low segregation coefficients (Zr: 1.6 108, Ti: 2.0 106, at 1687 K) [1] and extremely low solubilities in solid Si (both of them are less than 0.00001 ppma below 1173 K) [21]. When using Zr as the additive to enhance B removal, we found that the cooling rate and liquidus temperature of the AleSi alloy can influence this B removal process significantly [18]; however, the effect of these parameters on B removal with small amounts of added Ti have not been clarified yet. In this study, we attempt to investigate the effects of the cooling rate, the liquidus temperature of AleSi alloy, and the initial concentration of Ti on the B removal process in order to better understand this Ti-addition refining process. In this study, we investigate B removal with small amounts of Ti addition (<2037 ppma) in electromagnetic solidification refining of Si using AleSi alloy as a solvent. 2. Experimental An induction furnace combined with a stepping motor was used to carry out the electromagnetic solidification refining process. A schematic of the experimental set-up is shown in Fig. 1. Ten grams of Al shots (99.999%) and Si sheets (99.9999%) were put into a dense, high-purity graphite crucible (17-mm I.D., 25-mm O.D., 65mm length). Powdered Sie1 wt% B and Sie5 wt% Ti alloys were used to add the B and Ti in order to precisely control their small initial concentration (B: 153 ppma, Ti: 203e2037 ppma). The powdered Sie1 wt% B and the Sie5 wt% Ti alloys were prepared by pre-melting bulk Si with B powder (99.9%) and with Ti lumps (99.5%), respectively, followed by grinding the melted mixtures into
Fig. 1. Schematic of the induction furnace combined with a stepping motor: 1. Ar gas (99.99%) tank; 2. gas flow meter; 3. two-way valve; 4. gas inlet; 5. gas outlet; 6. induction coils; 7. graphite crucible; 8. AleSi melt; 9. prism; 10. ball screw; 11. stepping motor; 12. infrared pyrometer; 13. vacuum pump; 14. three-way valve; 15. porous alumina holder; 16. silicone plug; 17. bubble checking; 18. quartz chamber; 19. stainless steel plate; 20. vacuum meter.
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powders in an agate mortar to achieve composition homogenization and passing the powders through a 186-mm mesh filter. Their compositions were confirmed by inductively coupled plasmaatomic emission spectroscopy (ICP-AES). The bottom of the graphite crucible was located level with the lower end of the coils (6 turns, 95-mm O.D., 80-mm I.D., 70-mm length). Before induction heating, the air in the quartz chamber was evacuated by a mechanical pump and then refilled with high-purity Ar gas (99.99%) to avoid oxidation of the samples. This process was repeated three times to ensure the air in the chamber was fully eliminated. After melting and holding at 1473 ± 20 K for 30 min, the samples were cooled by lowering the graphite crucible to carry out the electromagnetic solidification refining. The agglomerated Si crystals were separated from the solidified AleSi alloy by mechanical cutting. A portion of them was analyzed by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) to observe and determine the precipitated phases. The remainder was crushed into powder (by passing through a 186-mm mesh filter) followed by treating with aqua regia containing H2SO4 (HCl:HNO3:H2SO4 ¼ 3:1:1) at 343 K for 6 h to remove impurities. Thereafter, the concentrations of residual impurities in the refined Si were determined by ICP-AES. 3. Results and discussion Table 1 lists the initial composition of the AleSi alloy and the concentrations of residual impurities in the refined Si. The initial concentration of Ti in the AleSi alloy was chosen to be less than 2037 ppma to lower the refining cost. The concentration of B in typical MG-Si is normally between 14.5 and 130 ppma (5.6 and 50 ppmw) [22]. However, in consideration of possible contamination from Al in practical Si purification processes, its initial concentration in the AleSi alloy in this work was set to 153 ppma (60 ppmw). The initial composition of AleSi was set to Al-36 at% Si, Al-45 at% Si and Al-55 at% Si to achieve low-temperature refining, because their liquidus temperatures are 1173 K, 1273 K and 1373 K, respectively, which are much lower than the melting point of Si (1687 K). Fig. 2(a) shows a cross-section of a sample after the electromagnetic solidification refining. The precipitated Si crystals were successfully separated from the AleSi alloy and agglomerated at its bottom because of the electromagnetic force. The mechanism of this separation and agglomeration process has been elaborated in other studies [11,17]. An enlargement of the circled region in Fig. 2(a) is shown in Fig. 2(b) to exhibit its morphology clearly. Fig. 3 shows the initial concentration of Ti in the AleSi alloy and its effect on B removal with a lowering rate of 0.55 ± 0.02 mm/min. Al-45 at% Si alloy was used as the refining solvent. The B removal fraction increases significantly when the initial concentration of Ti is larger than 407 ppma. The B removal fraction without Ti addition was 59.5%; however, this value increased to 99.2% with the addition of 2037 ppma Ti, i.e., the concentration of B could be reduced from 153 ppma to 1.2 ppma, which meets the acceptable concentration (0.26e3.9 ppma) [1] for producing solar cells. In Yoshikawa's study, the removal fractions of both B and Ti were much larger than those obtained in this study, for example, the removal fraction of B was 99.4% (the B content decreased from 170 to 1.1 ppma) with the addition of 933 ppma Ti [19] (Al-45 at% Si solvent, cooling rate of 5.0e10.0 K/min). This significant difference can possibly be attributed to the different electromagnetic fields employed in the two studies (20 kHz in this study, 50 kHz in Yoshikawa's study). Electromagnetic field induced flow and the electromagnetic stirring can affect the precipitation behavior of Si crystals [14], which indicates these factors can also affect the distribution behavior of Ti and B between the solid Si crystals and the liquid AleSi melt. For example,
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Table 1 Composition of the liquid AleSi alloy before refining and concentration of residual impurities in the refined Si with different lowering rates. No.
Lowering Rate, mm/min (±0.02)
1 2 3 4 5 6 7 8 9 10 11
0.55 0.55 0.55 0.55 0.55 0.55 0.55 1.36 0.22 0.55 0.55
Composition of the AleSi solvent before refining
Residual impurities in the refined Si, ppma
B, ppma
Ti, ppma
Al, at%
B
Ti
Al
153 153 153 153 153 153 153 153 153 153 153
e 203 407 814 1018 1527 2037 1018 1018 1022 1014
55.0 55.0 55.0 55.0 55.0 55.0 55.0 55.0 55.0 45.0 64.0
62.0 61.2 45.1 41.3 37.1 6.4 1.2 58.5 1.5 55.7 4.6
e
317 540 450 570 745 792 868 780 510 501 684
1.2 1.7 96.3 248 889 1530 294 118 243 119
Fig. 2. (a) Cross-section of an AleSi alloy after electromagnetic solidification refining and (b) an enlargement of the circled region marked in (a).
Removal fraction, %
100 80 60 40 20 0 0
B Ti
500 1000 1500 2000 2500 Initial Ti content, ppma
Fig. 3. Relationship between the initial concentration of Ti in the AleSi alloy and the removal fractions of B and Ti after electromagnetic solidification refinement of Si. (The lowering rate was 0.55 ± 0.02 mm/min and the refining solvent was the Al-45 at% Si alloy.)
Ban et al. [14] reported that electromagnetic stirring could significantly affect B removal at the cooling rate of 1.5 K/min. Therefore, it is difficult to compare the results obtained by Yoshikawa's and our work because of the different electromagnetic fields.
The removal fraction of Ti decreases from 99.4% to 24.9% with the increase of its initial concentration from 203 ppma to 2037 ppma, as shown in Fig. 3. The residual concentrations of Ti in the refined Si crystals are between 1.2 ppma to 1530 ppma, which are much larger than its solubility in solid Si (less than 0.00001 ppma below 1173 K [21]). Therefore, this refining process is actually a non-equilibrium process, so it is natural to inquire how the kinetic process changes with different cooling rates. For example, the removal fractions of both B and Ti significantly increase by decreasing the lowering rate from 0.55 ± 0.02 mm/min to 0.22 ± 0.02 mm/min. Similarly, these values significantly decrease by increasing the lowering rate to 1.36 ± 0.02 mm/min, as shown in Fig. 4 (using Al-45 at%Si alloy with 1018 ppma Ti as the refining solvent). The corresponding cooling rates for lowering rates of 0.22 ± 0.02 mm/min, 0.55 ± 0.02 mm/min and 1.36 ± 0.02 mm/min are 1.6e2.7 K/min, 4.2e7.1 K/min and 12.5e20.6 K/min, respectively. Cooling rate at a constant lowering rate is not a constant because of the induction heating [18]. The concentrations of residual B and Ti in the refined Si with a lowering rate of 0.22 ± 0.02 mm/min were 1.5 ppma (below the maximum acceptable level for producing solar cells) and 118 ppma, respectively, while their values were 37.1 ppma and 248 ppma, respectively, with a lowering rate of 0.55 ± 0.02 mm/min. These results indicate that a lower cooling rate was more favorable for Ti and B removal, because a slower cooling rate provides more time for B
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Removal fraction, %
100 90
Ti B
80 70 60 50 0.0
0.3 0.6 0.9 1.2 Lowering rate, mm/min
1.5
Fig. 4. Relationship between the lowering rates and the removal fractions of B and Ti after electromagnetic solidification refinement of Si (using Al-45 at%Si alloy with 1018 ppma Ti as the refining solvent).
and Ti to segregate between the solid Si and liquid AleSi alloy, making the refining process much closer to its thermodynamic equilibrium. Fig. 5 shows the effects of the composition of the AleSi alloy on B and Ti removal. The lowering rate was 0.55 ± 0.02 mm/min, and the initial concentration of Ti was about 1018 ppma. The removal fractions of both B and Ti increase with the increase of the initial Al concentration in the AleSi alloy. These results indicate that the composition of AleSi alloy (specifically the liquidus temperature) has a significant influence on B and Ti removal because the segregation coefficients of both B and Ti are smaller at lower liquidus temperature (B: 0.8 at 1687 K and 0.22 at 1473 K; Ti: 2 106 at 1687 K and 3.8 109 at 1073 K [10]). SEM with EDS analysis was performed to reveal this efficient B removal with Ti addition. A TieSieAl phase was observed to precipitate along the boundary of the Si crystals and the liquid AleSi melt, as shown in Fig. 6. Qualitative analysis indicated that the mole ratio of Ti, Al, and Si was 0.34:0.09:0.57. This value is close to that of the ternary compound Ti0.33Al0.15Si0.52 in the TieAleSi ternary system [23]. According to Yoshikawa's study [19], TiB2 was found to form during the AleSi solvent refining process, and efficient B removal by Ti addition is attributed to the low solubility of TiB2 in
Removal fraction, %
90 80 70
50 40
AleSi. In that study, TiB2 was mostly found embedded in the acidleachable eutectic AleSi alloy. However, we did not find TiB2 in this study. The liquid AleSi alloy is thought to dilute the concentrations of Ti and B and thereby reduce the chance of reaction of Ti and B. Instead of TiB2, we found another B bearing particles precipitated along the boundary of Si crystals (black particles shown in Fig. 7(a)) and their X-ray mapping is shown in Fig. 7. Regardless of whether the formation of the B-containing particles occurs or not, the efficient removal of B with Ti addition can be attributed to the decrease of the segregation coefficient of B between solid Si and the liquid AleSi alloy. When B segregates between the solid Si and the liquid AleSi alloy (containing Ti), its chemical potential in both phases is identical at equilibrium. We have the following three relations:
BðsÞin solid Si ¼ BðlÞin AlSi melt
(1)
DG+B fus ¼ 50200 21:8 TJ=mol½20
(2)
RTlnaBðsÞ in solid Si ¼ DG+B fus þ RTlnaBðlÞ in AlSi melt
(3)
Therefore, we can express the segregation ratio of B as.
ln kB ¼ ln
XBðsÞ in solid Si XBðlÞ in AlSi melt
¼
gBðlÞ in AlSi melt DG+B fus þ ln RT gBðsÞ in solid Si
B Ti 45 50 55 60 65 Initial Al content, at %
70
Fig. 5. Relationship between the initial concentration of Al in the AleSi alloy and the removal fractions of B and Ti after electromagnetic solidification refinement of Si. (The lowering rate was 0.55 ± 0.02 mm/min, and the initial concentration of Ti was approximately 1018 ppma.)
(4)
where DG+B fus denotes the Gibbs energy change for fusion of B, and aB, XB, and gB are the activity, mole fraction, and activity coefficient of B, respectively. The character in the parentheses denotes the standard state. kB denotes the segregation coefficient of B between solid Si and liquid AleSi melt. As the initial concentrations of B and Ti in this study are small, the activity coefficients of B in solid Si and liquid AleSi melt at the liquid and solid standard states can be expressed to first order in the interaction parameters as follows:
lngBðsÞ in solid Si ¼ lng+BðsÞ in solid Si þ εBB in solid Si XB in solid S þ εAl B in solid Si XAl in solid Si þ εTi B in solid Si XTi in solid Si
(5)
lngBðlÞ in AlSi melt ¼ lng+BðlÞ in AlSi melt þ εBB in AlSi melt XB in AlSi melt þ εTi B in AlSi melt XTi in AlSi melt
100
60
409
(6)
where εiB is the interaction parameter of i with B per mole fraction. As the segregation ratio of Ti between solid Si and AleSi melt is extremely small (2 106 at 1687 K and 3.8 109 at 1073 K [24]) and the solubility of Ti in solid Si is also extremely small (less than 0.00001 ppma below 1173 K [21]), XTi in solid Si XTi in AlSi melt at equilibrium. The effect of Ti on ln gBðsÞ in solid Si can be ignored, i.e., the term εTi X in Eq. (5) can be ignored. Therefore, B in solid Si Ti in solid Si according to Eq. (4) through Eq. (6), the effect of Ti addition on kB is mainly represented by the term εTi X in Eq. B in AlSi melt Ti in AlSi melt (6). Under the assumption that the value of εTi is negative B in AlSi melt because Ti has a strong affinity for B, the value of kB will decrease with the increase of XTi in AlSi melt . This assumption agrees well with our experimental results, which indicate that the removal fraction of B increases as the concentration of Ti increases. Therefore, the efficient B removal with Ti addition is due to the negative value of εTi , although there is yet no data on the value of B in AlSi melt this parameter. A smaller value of kB implies that much more B can be released from solid Si and enter into the AleSi melt or accumulate at grain boundaries of Si crystals (for example, stabilize as
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Fig. 6. SEM-EDS analysis of the TieSieAl phase in the sample with 2037 ppma Ti and 153 ppma B. (a) morphology image, X-ray mapping of (b) Ti, (c) Si, (d) Al, and (e) B, and (f) Xray analysis of points 1#, 2# in (a).
Fig. 7. SEM-EDS analysis of a group of precipitated particles in the sample with 2037 ppma Ti and 153 ppma B. (a) morphology image, X-ray mapping of (b) B, (c) Si, (d) Al, and (e) C.
an B containing phase in Fig. 7), making B easily removable by a leaching acid solution (HCl:HNO3:H2SO4 ¼ 3:1:1 in this study). Although there was some residual Ti and Al in the refined Si, they can be removed in the following process step, directional solidification in vacuum, because the segregation coefficient of Ti between solid and liquid Si is extremely small and the vapor pressure of Al is higher than that of Si [25]. Therefore, B removal from Si with small amounts of Ti in electromagnetic solidification
refining with AleSi alloy is a promising method for manufacturing SoG-Si, because this B removal process offers advantages of low energy consumption and low environmental impact (low acid solution consumption, and no waste slag or gas generation). 4. Conclusions Small amounts of Ti (<2073 ppma) were employed as an
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additive to enhance B removal from Si because of its strong affinity for B, extremely small segregation coefficient between solid Si and liquid AleSi alloy, and extremely low solubility in solid Si. The results show that small amounts of Ti were significantly efficient in enhancing B removal, and this B removal process will become more efficient at lower cooling rate combined with using a lower liquidus temperature of AleSi alloy as the refining solvent. The concentration of residual B in the refined Si could be reduced to less than 1.2 ppma (0.46 ppmw), which meets the requirements for manufacturing solar cells. The added Ti could be simultaneously removed because of its extremely small segregation coefficient between solid Si and liquid AleSi alloy. Although there is some residual Ti and Al present in the refined Si, these can be removed in the next processing step, directional solidification in vacuum. Consequently, B removal from Si with small amounts of Ti in the electromagnetic solidification refining with AleSi alloy is a promising method for manufacturing SoG-Si. Acknowledgments This study was sponsored by The National Natural Science Foundation of China (Grant Nos. 51504118 and 51334002) and The People's Cultivation Project of Yunnan Province (KKSY201452087). References [1] S.S. Zheng, J. Safarian, S. Seok, S. Kim, T. Merete, X.T. Luo, Elimination of phosphorus vaporizing from molten silicon at finite reduced pressure, Trans. Nonferrous Met. Soc. China (2011) 697e702. [2] F.A. Trumbore, Solid solubilities of impurity elements in germanium and silicon, Bell Syst. Tech. J. 39 (1960) 206e233. [3] J. Safarian, M. Tangstad, Vacuum refining of molten silicon, Metall. Mat. Trans. B 43B (2012) 1427e1445. [4] N. Nakamura, H. Baba, Y. Sakaguchi, Y. Kato, Boron removal in molten silicon by a steam-added plasma melting method, Mat. Trans. 45 (2004) 858e864. [5] J. Safarian, K. Tang, K. Hildal, G. Tranell, Boron removal from silicon by humidified gases, Metall. Mat. Trans. E 1 (2014) 41e47. [6] L. Zhang, Y. Tan, F.M. Xu, J.Y. Li, H.Y. Wang, Z. Gu, Removal of boron from molten silicon using Na2O-CaO-SiO2 slags, Separ. Sci. Technol. 48 (2013) 1140e1144. [7] L.A.V. Teixeira, Y. Tokuda, T. Yoko, K. Morita, Behavior and state of boron in CaO-SiO2 slags during refining of solar grade silicon, ISIJ Int. 49 (2009)
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