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Formation mechanism of hollow silicon ingot induced by fountain effect
Renewable Energy 77 (2015) 463e466
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Technical note
Formation mechanism of hollow silicon ingot induced by fountain effect Dachuan Jiang a, b, Shuang Shi a, b, Shiqiang Ren a, b, H.M. Noor ul Huda Khan Asghar a, b, c, Yi Tan a, b, *, Pengting Li a, b, d, Jieshan Qiu e, Jiayan Li a, b a
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Key Laboratory for Solar Energy Photovoltaic System of Liaoning Province, Dalian 116023, China Department of Physics, Balochistan University of Information Technology, Engineering & Management Sciences, Quetta, Pakistan d Qingdao Longsun Silicon Technology Ltd., Qingdao 266000, China e Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, Dalian University of Technology, Dalian 116024, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2014 Accepted 19 December 2014 Available online 7 January 2015
A hollow silicon ingot was obtained by a solideliquid separation method inducted by the fountain effect and the formation mechanism of the ingot was also discussed. A layer of solidified shell was formed on the melt surface and the gas dissolved in the melt was separated out. Because of this, the thickness of the shell was gradually increased and expanded due to the sudden change of the chamber pressure leading to the silicon melt being squeezed out from the preset hole of the shell. During this process, the melt left behind contains a high concentration of impurities and can be separated or detached completely from the decontaminated solid. This novel approach has of great potential to inhibit the back diffusion of impurities and to produce a silicon ingot at a high yield rate. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Solar energy materials Hollow silicon ingot Solideliquid separation Fountain effect Back diffusion
1. Introduction In modern years, the demand for solar-grade silicon has been increased dramatically with the rapid development of the photovoltaic industry [1e4]. Cast multicrystalline silicon is the main raw material for the production of solar cells, which occupies more than 80% of the market share [5,6]. However, the yield rate of cast multicrystalline silicon is less than 70% due to the back diffusion behavior of metal impurities in silicon ingot, leading to near 10% of each ingot cannot be used for solar cells. The current global annual production of cast multicrystalline silicon is more than 100,000 t, so every year more than 10,000 t silicon is lost. During the casting process, contaminants are redistributed in the silicon ingot, but not eliminated. When silicon solidifies, the contaminated accumulation region is discarded and the purity of the rest is enough to meet the high performance standard necessary for solar cells. The allotment of contaminations shows an
* Corresponding author. School of Materials Science and Engineering, Dalian University of Technology, No 2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province 116023, China. Tel./fax: þ86 411 84707583. E-mail address: [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.renene.2014.12.048 0960-1481/© 2014 Elsevier Ltd. All rights reserved.
increasing trend from the bottom to the top due to the segregation from the liquid-to-solid phase in the central regions of the ingot. This brings into being high concentrations of what near the top of the ingot, which subsequently diffuse back into the ingot during the cooling process [7,8], leading to a significant reduction of the ingot yield. However, only few reports on the inhibition of back diffusion are available to date. If the concentration distribution by Scheil's equation is regarded as the initial condition, the diffusion flux at the bottom is considered to be 0 and the concentration at the top is considered to be constant, thus, the concentration of the impurities as a function of the height and time can be calculated, as shown in Fig. 1. According to Scheil's equation, the iron concentration under the ingot height of 79% is less than 0.1 ppm. After diffusion for 1800 s, the area where the iron concentration is less than 0.1 ppm only accounts for about 51% of the whole ingot. There is a strong back diffusion behavior occurring in the top of the ingot due to a large concentration gradient, leading to the decrease of the yield rate by 28%. If the melt with high impurity concentration is separated from the solid which has been purified, the yield rate of the ingot cannot be affected by the effect of impurity accumulation region.
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Fig. 1. Simulation of iron distribution in silicon ingot.
At the end of the casting process, a lot of contaminants concentrate in the liquid phase which has not yet solidified. If this liquid can be separated from purified silicon crystal, then the driving force for the back diffusion of impurities can be greatly weakened. This mechanism is regarded as a promising way to inhibit back diffusion and enhance the yield of the ingot. In this paper, a solideliquid separation method is executed by the induction of the fountain effect to form a giant gas hole in a silicon ingot, which can condense the contact area of the contaminations accumulation region and the purified region so that the back diffusion behavior is inhibited. 2. Principle of solideliquid separation by fountain effect A solideliquid separation method initiated by the fountain effect is proposed to inhibit the back diffusion behavior of the impurities. The schematic illustration of this method is shown in Fig. 2. First of all, silicon is melt entirely in a crucible under low vacuum condition. Consequently the melting power is reduced so that silicon melt start to solidify slowly from the bottom to top. If the silicon solidifies completely in such a way, a normal directional solidified silicon ingot can be obtained and a small part of silicon
melt could be squeezed out of the top due to volume expansion during the cooling process. If when the crystal goes up to a preset height, the vacuum system is established to diminish the chamber pressure quickly, so that silicon evaporates rapidly into the gas phase. It takes away huge heat, so that a deposit of solidified shell forms on the melt surface due to a sudden temperature drop of the melt. At this flash, a quartz pushrod which is set above the melt is used to prod a hole in the center of the solidified shell. Soon after, the power is shut off so that the left behind melt solidifies hurriedly. During this process, the gas suspended in the melt will be separated out, accumulated and expanded. The gas cannot be vented due to high viscosity of silicon melt at the temperature close to the melting point. Hence, the melt is squeezed out from the center hole of the shell and separated from the purified region. After the melt solidifies completely, a big gas hole forms inside the ingot, which reduces the contact area of the contamination accumulation region and the purified region to inhibit the back diffusion behavior of the impurities. The solideliquid separation process looks like a fountain, why it is called the fountain effect. 3. Experimental Two casting experiments were carried out in a vacuum induction melting furnace. 900 g MG-Si feedstock was used in this experiment. Prior to processing by melting and solideliquid separation, it was washed adequately with supersonic wave cleaner in alcohol to remove potential solid residues and superfluous contaminations from the surface. Consequently, the silicon was melted in the quartz crucible with a diameter of 10 cm, which was carried out in an argon gas atmosphere. After melted completely, the molten silicon was set to temperature of 1773 K and a set chamber pressure of 4 104 Pa for 1 h. For one of the experiments, the crucible was pulled downward out of the induction coil so that the silicon solidified slowly from the bottom to the top. When the silicon solidified completely, the power was shut off. For another experiment, the crucible was pulled downward out of the induction coil so that the silicon solidified slowly from the bottom to the preset height. After that, the vacuum system was started to rapidly reduce the chamber pressure to 2 103 Pa. When a solidified shell formed, a quartz pushrod was used to poke a hole in the center of the shell, and then the power was shut off.
Fig. 2. Schematic illustration of solideliquid separation method induced by fountain effect.
D. Jiang et al. / Renewable Energy 77 (2015) 463e466
465
Fig. 3. Macro morphologies of the vertical section: (a) the normal directional solidification ingot; (b) the solideliquid separation ingot.
4. Results and discussion The silicon ingot obtained by normal directional solidification had a diameter of 10 cm and a height of about 5 cm. The sidewall adheres to some quartz due to the nucleation of silicon on the inner wall of the crucible. A protuberance with an irregular shape can be observed at the top center of the ingot. The ingot created solideliquid separation had a diameter of 10 cm and a height of about 6 cm. From the side of the ingot, an interface can be clearly observed at the height of 75% to divide the whole ingot into two areas. The area under the interface was considered to be the area where silicon solidified from the bottom to the top and part of the sidewall adheres to some quartz. While, the area above the interface was considered to be the area where silicon was squeezed out and then solidifies induced by the fountain effect, so no quartz is adhered to the sidewall of this area. The macro morphologies of the vertical section are shown in Fig. 3. As for the normal directional solidification ingot, columnar crystals can be observed in the most regions, which indicates the silicon solidifies directionally from the bottom to the top. The formation of the protuberance at the top center was derived from volume expansion, since the density of liquid being larger than that of solid silicon, shown in Fig 3(a). Impurities with low segregation coefficient could be concentrated in the top area after the silicon solidifies completely, however, these impurities could diffuse back during the subsequent cooling process. As for the solideliquid separation ingot, a big gas hole can be observed in the middle of the ingot with a diameter of about 7 cm and a height of 2 cm. Before the solideliquid separation process, the chamber pressure was kept at 4 104 Pa. When the vacuum system was started, the chamber pressure decreased rapidly. Due to the sudden drop in chamber pressure, the whole energy cannot be contained in the liquid as prudent heat and the heat surplus was converted into latent heat of vaporization [9]. The surface temperature of the melt was quickly decreased due to sudden vaporization of silicon, leading to a layer of solidified shell forming on the surface. After that, the gas dissolved in the melt will be separated out inside the melt, accumulated and expanded. When a hole was made in the center of the shell, the melt is squeezed out from the hole like a fountain at the action of the gas and then started to solidify. After complete solidification, a big hole full of gas forms
inside the ingot, shown in Fig. 3(b). Many small gas holes can be observed on the interface from the local enlarged picture. The melt was squeezed out, flowed towards the outside and solidified immediately, so these gas holes were retained in the ingot. The distributions of calcium in these two ingots were measured by inductively coupled plasma optical emission spectroscopy (ICPMS), as shown in Fig. 4. As for the normal directional solidification ingot, the distribution shows a typically increasing trend. However, the back diffusion behavior is not very obvious due to the rapid cooling rate with lab-scale equipment. When it is carried out in industrial-scale equipment, the impurity could have enough time to diffuse back with slow cooling rate. As for the solideliquid separation ingot, the impurity shows a low level at the position below the big gas hole and shows a high level above it. The big hole formed inside the silicon ingot was separated the impurity accumulation region and the purified region which decreased their contact area. This process inhibits the back diffusion behavior of the impurities. The area above the gas hole contains large quantities of impurities, which must be discarded. Then
Fig. 4. Measured impurity distribution in silicon ingot.
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D. Jiang et al. / Renewable Energy 77 (2015) 463e466
the rest part was purified silicon. This method is considered to be a potentially efficient way to inhibit the back diffusion behavior and enhance yield rate. 5. Conclusions In this research, a silicon ingot with a big gas hole inside is obtained using the solideliquid separation method induced by the fountain effect. A solidified shell forms on the outer layer shell of the silicon melt by rapidly reducing the chamber pressure. At the same time, a great number of gases separate out inside the melt, which will accumulate and expand; so that the un-solidified melt was squeezed out from the center hole of the shell and separated from the purified region, leading to the reduction of their contact area. It is considered a potentially efficient way to inhibit the back diffusion behavior of the impurities and obtain a silicon ingot with high yield rate. Acknowledgments The authors appreciatively acknowledge financial support by China Postdoctoral Science Foundation (Grant No. 2013M530124), National Natural Science Foundation of China (Grant No. 51304033
and U1137601) and National Key Technology R&D Program (Grant No. 2011BAE03B01).
References [1] Tanay F, Dubois S, Enjalbert N, Veirman J. Low temperature-coefficient for solar cells processed from solar-grade silicon purified by metallurgical route. Prog Photovoltaics 2011;19:966e72. [2] Schutz-Kuchly T, Sanzone V, Veschetti Y. N-type solar-grade silicon purified via the metallurgical route: characterisation and fabrication of solar cells. Prog Photovoltaics 2013;21:1214e21. [3] Jiang DC, Tan Y, Shi S, Dong W, Gu Z, Zou RX. Removal of phosphorus in molten silicon by electron beam candle melting. Mater Lett 2012;78:4e7. [4] Martorano MA, Ferreira Neto JB, Oliveira TS, Tsubaki TO. Refining of metallurgical silicon by directional solidification. Mater Sci Eng B 2011;176:217e26. [5] Mei PR, Moreira SP, Cardoso E, Cortes ADS, Marques FC. Purification of metallurgical silicon by horizontal zone melting. Sol Energy Mater Sol Cells 2012;98: 233e9. [6] Jeong KP, Kim YK. Electrical properties of the multi-crystalline silicon ingots grown with UMG (upgraded metallurgical grade) silicon materials. Sol Energy Mater Sol Cells 2012;107:201e4. [7] Macdonald D, Cuevas A, Kinomura A, Nakano Y, Geerligs LJ. Transition-metal profiles in a multicrystalline silicon ingot. J Appl Phys 2005;97:033523. [8] Kvande R, Geerligs LJ, Coletti G, Arnberg L, Di Sabatino M, Ovrelid EJ, et al. Distribution of iron in multicrystalline silicon ingots. J Appl Phys 2008;104: 064905. [9] Saury D, Harmand S, Siroux M. Flash evaporation from a water pool: influence of the liquid height and of the depressurization rate. Int J Therm Sci 2005;44: 953e65.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Technical note
Formation mechanism of hollow silicon ingot induced by fountain effect Dachuan Jiang a, b, Shuang Shi a, b, Shiqiang Ren a, b, H.M. Noor ul Huda Khan Asghar a, b, c, Yi Tan a, b, *, Pengting Li a, b, d, Jieshan Qiu e, Jiayan Li a, b a
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Key Laboratory for Solar Energy Photovoltaic System of Liaoning Province, Dalian 116023, China Department of Physics, Balochistan University of Information Technology, Engineering & Management Sciences, Quetta, Pakistan d Qingdao Longsun Silicon Technology Ltd., Qingdao 266000, China e Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, Dalian University of Technology, Dalian 116024, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 March 2014 Accepted 19 December 2014 Available online 7 January 2015
A hollow silicon ingot was obtained by a solideliquid separation method inducted by the fountain effect and the formation mechanism of the ingot was also discussed. A layer of solidified shell was formed on the melt surface and the gas dissolved in the melt was separated out. Because of this, the thickness of the shell was gradually increased and expanded due to the sudden change of the chamber pressure leading to the silicon melt being squeezed out from the preset hole of the shell. During this process, the melt left behind contains a high concentration of impurities and can be separated or detached completely from the decontaminated solid. This novel approach has of great potential to inhibit the back diffusion of impurities and to produce a silicon ingot at a high yield rate. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Solar energy materials Hollow silicon ingot Solideliquid separation Fountain effect Back diffusion
1. Introduction In modern years, the demand for solar-grade silicon has been increased dramatically with the rapid development of the photovoltaic industry [1e4]. Cast multicrystalline silicon is the main raw material for the production of solar cells, which occupies more than 80% of the market share [5,6]. However, the yield rate of cast multicrystalline silicon is less than 70% due to the back diffusion behavior of metal impurities in silicon ingot, leading to near 10% of each ingot cannot be used for solar cells. The current global annual production of cast multicrystalline silicon is more than 100,000 t, so every year more than 10,000 t silicon is lost. During the casting process, contaminants are redistributed in the silicon ingot, but not eliminated. When silicon solidifies, the contaminated accumulation region is discarded and the purity of the rest is enough to meet the high performance standard necessary for solar cells. The allotment of contaminations shows an
* Corresponding author. School of Materials Science and Engineering, Dalian University of Technology, No 2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province 116023, China. Tel./fax: þ86 411 84707583. E-mail address: [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.renene.2014.12.048 0960-1481/© 2014 Elsevier Ltd. All rights reserved.
increasing trend from the bottom to the top due to the segregation from the liquid-to-solid phase in the central regions of the ingot. This brings into being high concentrations of what near the top of the ingot, which subsequently diffuse back into the ingot during the cooling process [7,8], leading to a significant reduction of the ingot yield. However, only few reports on the inhibition of back diffusion are available to date. If the concentration distribution by Scheil's equation is regarded as the initial condition, the diffusion flux at the bottom is considered to be 0 and the concentration at the top is considered to be constant, thus, the concentration of the impurities as a function of the height and time can be calculated, as shown in Fig. 1. According to Scheil's equation, the iron concentration under the ingot height of 79% is less than 0.1 ppm. After diffusion for 1800 s, the area where the iron concentration is less than 0.1 ppm only accounts for about 51% of the whole ingot. There is a strong back diffusion behavior occurring in the top of the ingot due to a large concentration gradient, leading to the decrease of the yield rate by 28%. If the melt with high impurity concentration is separated from the solid which has been purified, the yield rate of the ingot cannot be affected by the effect of impurity accumulation region.
464
D. Jiang et al. / Renewable Energy 77 (2015) 463e466
Fig. 1. Simulation of iron distribution in silicon ingot.
At the end of the casting process, a lot of contaminants concentrate in the liquid phase which has not yet solidified. If this liquid can be separated from purified silicon crystal, then the driving force for the back diffusion of impurities can be greatly weakened. This mechanism is regarded as a promising way to inhibit back diffusion and enhance the yield of the ingot. In this paper, a solideliquid separation method is executed by the induction of the fountain effect to form a giant gas hole in a silicon ingot, which can condense the contact area of the contaminations accumulation region and the purified region so that the back diffusion behavior is inhibited. 2. Principle of solideliquid separation by fountain effect A solideliquid separation method initiated by the fountain effect is proposed to inhibit the back diffusion behavior of the impurities. The schematic illustration of this method is shown in Fig. 2. First of all, silicon is melt entirely in a crucible under low vacuum condition. Consequently the melting power is reduced so that silicon melt start to solidify slowly from the bottom to top. If the silicon solidifies completely in such a way, a normal directional solidified silicon ingot can be obtained and a small part of silicon
melt could be squeezed out of the top due to volume expansion during the cooling process. If when the crystal goes up to a preset height, the vacuum system is established to diminish the chamber pressure quickly, so that silicon evaporates rapidly into the gas phase. It takes away huge heat, so that a deposit of solidified shell forms on the melt surface due to a sudden temperature drop of the melt. At this flash, a quartz pushrod which is set above the melt is used to prod a hole in the center of the solidified shell. Soon after, the power is shut off so that the left behind melt solidifies hurriedly. During this process, the gas suspended in the melt will be separated out, accumulated and expanded. The gas cannot be vented due to high viscosity of silicon melt at the temperature close to the melting point. Hence, the melt is squeezed out from the center hole of the shell and separated from the purified region. After the melt solidifies completely, a big gas hole forms inside the ingot, which reduces the contact area of the contamination accumulation region and the purified region to inhibit the back diffusion behavior of the impurities. The solideliquid separation process looks like a fountain, why it is called the fountain effect. 3. Experimental Two casting experiments were carried out in a vacuum induction melting furnace. 900 g MG-Si feedstock was used in this experiment. Prior to processing by melting and solideliquid separation, it was washed adequately with supersonic wave cleaner in alcohol to remove potential solid residues and superfluous contaminations from the surface. Consequently, the silicon was melted in the quartz crucible with a diameter of 10 cm, which was carried out in an argon gas atmosphere. After melted completely, the molten silicon was set to temperature of 1773 K and a set chamber pressure of 4 104 Pa for 1 h. For one of the experiments, the crucible was pulled downward out of the induction coil so that the silicon solidified slowly from the bottom to the top. When the silicon solidified completely, the power was shut off. For another experiment, the crucible was pulled downward out of the induction coil so that the silicon solidified slowly from the bottom to the preset height. After that, the vacuum system was started to rapidly reduce the chamber pressure to 2 103 Pa. When a solidified shell formed, a quartz pushrod was used to poke a hole in the center of the shell, and then the power was shut off.
Fig. 2. Schematic illustration of solideliquid separation method induced by fountain effect.
D. Jiang et al. / Renewable Energy 77 (2015) 463e466
465
Fig. 3. Macro morphologies of the vertical section: (a) the normal directional solidification ingot; (b) the solideliquid separation ingot.
4. Results and discussion The silicon ingot obtained by normal directional solidification had a diameter of 10 cm and a height of about 5 cm. The sidewall adheres to some quartz due to the nucleation of silicon on the inner wall of the crucible. A protuberance with an irregular shape can be observed at the top center of the ingot. The ingot created solideliquid separation had a diameter of 10 cm and a height of about 6 cm. From the side of the ingot, an interface can be clearly observed at the height of 75% to divide the whole ingot into two areas. The area under the interface was considered to be the area where silicon solidified from the bottom to the top and part of the sidewall adheres to some quartz. While, the area above the interface was considered to be the area where silicon was squeezed out and then solidifies induced by the fountain effect, so no quartz is adhered to the sidewall of this area. The macro morphologies of the vertical section are shown in Fig. 3. As for the normal directional solidification ingot, columnar crystals can be observed in the most regions, which indicates the silicon solidifies directionally from the bottom to the top. The formation of the protuberance at the top center was derived from volume expansion, since the density of liquid being larger than that of solid silicon, shown in Fig 3(a). Impurities with low segregation coefficient could be concentrated in the top area after the silicon solidifies completely, however, these impurities could diffuse back during the subsequent cooling process. As for the solideliquid separation ingot, a big gas hole can be observed in the middle of the ingot with a diameter of about 7 cm and a height of 2 cm. Before the solideliquid separation process, the chamber pressure was kept at 4 104 Pa. When the vacuum system was started, the chamber pressure decreased rapidly. Due to the sudden drop in chamber pressure, the whole energy cannot be contained in the liquid as prudent heat and the heat surplus was converted into latent heat of vaporization [9]. The surface temperature of the melt was quickly decreased due to sudden vaporization of silicon, leading to a layer of solidified shell forming on the surface. After that, the gas dissolved in the melt will be separated out inside the melt, accumulated and expanded. When a hole was made in the center of the shell, the melt is squeezed out from the hole like a fountain at the action of the gas and then started to solidify. After complete solidification, a big hole full of gas forms
inside the ingot, shown in Fig. 3(b). Many small gas holes can be observed on the interface from the local enlarged picture. The melt was squeezed out, flowed towards the outside and solidified immediately, so these gas holes were retained in the ingot. The distributions of calcium in these two ingots were measured by inductively coupled plasma optical emission spectroscopy (ICPMS), as shown in Fig. 4. As for the normal directional solidification ingot, the distribution shows a typically increasing trend. However, the back diffusion behavior is not very obvious due to the rapid cooling rate with lab-scale equipment. When it is carried out in industrial-scale equipment, the impurity could have enough time to diffuse back with slow cooling rate. As for the solideliquid separation ingot, the impurity shows a low level at the position below the big gas hole and shows a high level above it. The big hole formed inside the silicon ingot was separated the impurity accumulation region and the purified region which decreased their contact area. This process inhibits the back diffusion behavior of the impurities. The area above the gas hole contains large quantities of impurities, which must be discarded. Then
Fig. 4. Measured impurity distribution in silicon ingot.
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D. Jiang et al. / Renewable Energy 77 (2015) 463e466
the rest part was purified silicon. This method is considered to be a potentially efficient way to inhibit the back diffusion behavior and enhance yield rate. 5. Conclusions In this research, a silicon ingot with a big gas hole inside is obtained using the solideliquid separation method induced by the fountain effect. A solidified shell forms on the outer layer shell of the silicon melt by rapidly reducing the chamber pressure. At the same time, a great number of gases separate out inside the melt, which will accumulate and expand; so that the un-solidified melt was squeezed out from the center hole of the shell and separated from the purified region, leading to the reduction of their contact area. It is considered a potentially efficient way to inhibit the back diffusion behavior of the impurities and obtain a silicon ingot with high yield rate. Acknowledgments The authors appreciatively acknowledge financial support by China Postdoctoral Science Foundation (Grant No. 2013M530124), National Natural Science Foundation of China (Grant No. 51304033
and U1137601) and National Key Technology R&D Program (Grant No. 2011BAE03B01).
References [1] Tanay F, Dubois S, Enjalbert N, Veirman J. Low temperature-coefficient for solar cells processed from solar-grade silicon purified by metallurgical route. Prog Photovoltaics 2011;19:966e72. [2] Schutz-Kuchly T, Sanzone V, Veschetti Y. N-type solar-grade silicon purified via the metallurgical route: characterisation and fabrication of solar cells. Prog Photovoltaics 2013;21:1214e21. [3] Jiang DC, Tan Y, Shi S, Dong W, Gu Z, Zou RX. Removal of phosphorus in molten silicon by electron beam candle melting. Mater Lett 2012;78:4e7. [4] Martorano MA, Ferreira Neto JB, Oliveira TS, Tsubaki TO. Refining of metallurgical silicon by directional solidification. Mater Sci Eng B 2011;176:217e26. [5] Mei PR, Moreira SP, Cardoso E, Cortes ADS, Marques FC. Purification of metallurgical silicon by horizontal zone melting. Sol Energy Mater Sol Cells 2012;98: 233e9. [6] Jeong KP, Kim YK. Electrical properties of the multi-crystalline silicon ingots grown with UMG (upgraded metallurgical grade) silicon materials. Sol Energy Mater Sol Cells 2012;107:201e4. [7] Macdonald D, Cuevas A, Kinomura A, Nakano Y, Geerligs LJ. Transition-metal profiles in a multicrystalline silicon ingot. J Appl Phys 2005;97:033523. [8] Kvande R, Geerligs LJ, Coletti G, Arnberg L, Di Sabatino M, Ovrelid EJ, et al. Distribution of iron in multicrystalline silicon ingots. J Appl Phys 2008;104: 064905. [9] Saury D, Harmand S, Siroux M. Flash evaporation from a water pool: influence of the liquid height and of the depressurization rate. Int J Therm Sci 2005;44: 953e65.