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WITHDRAWN: The purification of metallurgical grade silicon by electron beam melting
Journal of Materials Processing Technology 169 (2005) 21–25
The purification of metallurgical grade silicon by electron beam melting Universidade Estadual de Campinas, Mechanical Engineering College, Department of Materials Engineering, P.O. Box 6122, Campinas, SP 13083-970, Brazil b Universidade S˜ ao Francisco, Engineering College, Itatiba, SP, Brazil c Faculdades Integradas de S˜ ao Paulo, S˜ao Paulo, SP, Brazil
AT
a
E
J.C.S. Pires a,∗ , J. Otubo a , A.F.B. Braga b , P.R. Mei a,b,c
Received 15 August 2001; received in revised form 17 April 2002; accepted 11 March 2004
Abstract
PL
IC
Metallurgical grade silicon (MG-Si) is obtained from the reduction of silica (SiO2 ) in a voltaic arc furnace. The impurities are inherent to the reduction process and they are also dependent on the quality of the initial materials. Among other applications, silicon is used as a substrate for photovoltaic conversion of energy and this conversion is as bigger as greater is the purity of the substrate. Researches are being carried out, in some countries, with the objective of searching for new processes of silicon purification or new materials that can be used as substrates for energy conversion. In this research, the technique of silicon purification in an electron beam furnace was used, where the melting occurs in a high vacuum and the impurities are extracted by evaporation. MG-Si in bulk form without leaching, with an initial purity of 99.88% in mass and ground and leached MG-Si, with an initial purity of 99.92%, were used as starting materials. The final purity obtained, in both materials, was above 99.999% in mass. These results demonstrate that this process is technically viable, while also eliminating the stages of chemical purification used in other techniques. © 2005 Elsevier B.V. All rights reserved. Keywords: Silicon purification; Electron beam melting; Metallurgical grade silicon
1. Introduction
D
U
The sun is the largest energy source available because it is clean and renewable. Silicon is the element most used for the conversion of solar energy into electric energy. The solar cell is the key technology for the solution of the energy problem in our planet. This technology is already quite usual for aerospace applications, however, for terrestrial applications it is still limited due to the high cost of the photovoltaic modules [1,2]. Researches are being carried out in some countries, with the objective of looking for new processes for the purification of silicon or new materials that can be used as substrate for the conversion of energy, always seeking for a decrease of the cost of production of the photovoltaic devices [3,4]. ∗
Corresponding author. Fax: +55 19 3298 3722. E-mail addresses: [email protected] (J.C.S. Pires), [email protected] (P.R. Mei). 0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.03.006
According to Choudhury and Hengsberger [5] silicon purification by electron beam melting furnace (EBM) has some advantages: • Melting in a vacuum of 10−4 to 10−2 Pa in refrigerated copper crucible without contamination. • High flexibility of melting rate and conditions for removal of volatile elements. • Almost unlimited melting temperatures. • High power density for local superheating. Previous research of our group [6–9] demonstrated the technical viability of the process, with excellent results in terms of final purity of the silicon. In this research, the results of the purification in EBM of the two materials are compared. One of them is ground and leached MG-Si and the other is the same MG-Si in the massive form without the acid leaching treatment.
J.C.S. Pires et al. / Journal of Materials Processing Technology 169 (2005) 21–25
22
Fig. 2. MG-Si block without previously being acid leached treated and with an initial purity of 99.88% in weight of Si.
AT
For the study of purification of silicon through melting in an EBM, furnace model EMO/LEW 80, made in Germany, 80 kW of power was used. This furnace is quite versatile, allowing the processing of materials of high melting points and products from some grams to ingots of 100 mm of diameter and 800 mm of length. Two types of silicon were used in this work. One of them was ground and leached MG-Si, with a granulation of 100–200 m and with an initial Si purity of 99.92% in weight. The other was MG-Si in the massive form without previously being acid leached treated and with an initial purity of 99.88% in weight. RIMA Industrial S.A. supplied both. The chemical composition of these materials was measured by Glow Discharge Mass Spectrometry in the Northern Analytical Laboratory in New Hampshire, USA. Three measurements were carried out on each sample and the standard deviation, for the three measurements, was less than 15%. The limit of detection of the equipment was 0.001 ppm for Mn, Ti and V and smaller than 0.01 ppm for the others impurities. In the samples after melting, the chemical analysis was performed in the border of the sample. Figs. 1 and 2 present the materials inside the crucible before melting. It can be seen in Fig. 2 that the MG-Si block was placed out of the center of the crucible to propitiate the drag of the liquid silicon during the melting. This procedure simulates the lateral feeding process, in which the material is melted drop by drop, which favors the extraction of the impurities by vacuum. Before being processed by EBM, the silicon was washed with acetone in an ultra-sound cleaner with the objective of removing possible solid residues from the surface. For each experience, a sample of approximately 280 g was placed inside the copper crucible and the evacuation of the whole furnace was initiated. The heating of the sample started with the gradual elevation of the electron beam power until the whole mass was melted, when the power was kept at a constant value
E
2. Experimental procedure
Table 1 Experimental parameters used in the melting Melting time (min) Beam power (kW) Internal pressure of the furnace (Pa)
20 15–17 10−4 to 10−2
D
U
PL
IC
for a certain time. After that interval of time, the power was slowly reduced until the extinction of the beam in the center of the sample had occurred, providing a temperature gradient from the border to the center of the sample that favored the segregation of impurities. This segregation was observed in a previous work [10]. The process conditions, used in this work, were established based on previous work [11] and are showed in Table 1.
Fig. 1. Ground and leached MG-Si, with a granulation of 100–200 m and with an initial purity of 99.92% in weight of Si.
Fig. 3. Top view of a ground and leached MG-Si sample after the melting in the EBM.
J.C.S. Pires et al. / Journal of Materials Processing Technology 169 (2005) 21–25
PL
IC
AT
After melting, the samples obtained in EBM presented the form of a disk with a diameter of 90 mm and a maximum thickness of 25 mm. The geometry of the crucible and the refrigeration favored the formation of temperature gradients from the bottom to the top and from the border to the center of the disk. This can be visualized through the rings (isotherms) formed on the surface. This demonstrates that there was a front of solidification from the border to the center of the sample. The center was the last area to be solidified. Another detail that can be observed in the silicon disks is the saliency formed in the center, due to expansion, during solidification.
This is a characteristic of silicon that, unlike metals, expands during the solidification process. Fig. 3 shows a top view and Fig. 4 shows a bottom view of one of these samples. It is noticed that due to the contact of the sample with the refrigerated copper crucible, the bottom of the disk did not melt properly. The results of chemical analysis are presented in Table 2. For both types of silicon used, great extraction of all impurities, except for the boron, can be observed. Due to impurities segregation [10], the sample purity is not homogeneous and the best purity was obtained in the border of the samples. The efficiency of extraction for all impurities can be observed in Fig. 5. Purification through evaporation is more effective for those elements that have a closer or higher vapor pressure than that of silicon. Silicon vapor pressure is 5 × 10−1 Pa at 1500 ◦ C. In Fig. 6, the removal efficiency for those impurities as a function of their vapor pressures can be observed. The boron was not extracted because its vapor pressure is much lower than the silicon vapor pressure. According to Ikeda and Maeda [12], the boron content should be smaller than 1 ppm, because this is one of the
E
3. Results and discussions
23
Fig. 4. Bottom view of a ground and leached MG-Si sample after the melting in the EBM.
Fig. 5. Efficiency of extraction of the impurities.
U
Table 2 Chemical analysis before and after silicon melting, final purity and efficiency of extraction Impurities
Final purity (%)
MG-Si in massive form without leaching
Before melting (ppmw)
After melting (ppmw)
Efficiency of extraction (%)
Before melting (ppmw)
After melting (ppmw)
Efficiency of extraction (%)
53.00 11.00 15.00 185.00 1.80 31.00 30.00 4.60 2.00 480.00 23.00 3.00 20.58 844.98
0.09 0.00 10.00 0.02 0.01 0.03 0.03 <0.01 0.003 0.04 0.41 0.002 1.12 1.766
99.83 100.00 33.33 99.99 99.20 99.90 99.92 >99.78 99.85 99.99 98.20 99.93 94.56 99.79
110.00 0.04 10.00 26.00 6.50 790.00 0.10 4.20 75.00 0.33 38.00 42.00 16.32 1108.49
0.44 <0.01 7.30 0.31 0.29 0.70 0.10 0.02 0.027 0.05 0.39 0.087 2.74 5.165
99.60 >75.61 27.00 98.81 95.54 99.91 0.00 99.50 99.96 83.94 98.97 99.80 83.21 99.53
D
Al Ba B Ca Cu Fe K Mg Mn Na P Ti Others Total
Powder of leached MG-Si
99.92
99.99982
99.88
99.9995
J.C.S. Pires et al. / Journal of Materials Processing Technology 169 (2005) 21–25
Fig. 6. Removal efficiency of the impurities as a function of their vapor pressures.
AT
doping elements in silicon solar cells. However, the removal of this element is very difficult because its vapor pressure is very low in relation to the vapor pressure of the silicon (10−4 Pa for boron and 10−1 Pa for silicon). This makes its extraction for vacuum processes difficult. Another difficulty regarding to boron extraction is that its segregation coefficient in silicon is near 1 (k ∼ = 1), which also makes its extraction using a unidirectional solidification process difficult. A process usually used for the boron removal is plasma melting in an oxidizing atmosphere (O2 , CO2 or H2 O). In this process, the boron is transformed into the oxide form, increasing its vapor pressure [3,12]. The remaining impurities in SoG-Si degrade the solar cell performance. In Fig. 7 [13], the influence of each impurity levels can be observed. Certain impurities are more deleterious to solar cell behavior, however the tolerable level of impurities depends on the growth technique as well as the cell fabrication processes involved [13]. The direct melting using rocks of massive MG-Si, without any previous treatment (grinding and leaching), considering that this material had a large amount of impurities showed the best results, in terms of overall costs. The results obtained in this work demonstrate that it is possible to avoid this previous treatment (grinding and leaching) for silicon purification. Rocks of massive MG-Si can be directly purified in EBM with excellent results.
This process can be extended to big samples by the use of the lateral feeding and drop-by-drop melting, however, additional studies should be done and some parameters should be modified. So, these results could certainly be still improved by lateral feeding of the massive MG-Si blocks. With the drop-by-drop melting, a larger superficial area would be in direct contact with vacuum, favoring the extraction of the impurities by evaporation. Is not so easy to compare the results of this work, in terms of cost and production efficiency, with the actual industrial process because this work was carried out in laboratory scale, but in a previous study, the energy cost was estimated in US$ 7/kg of purified silicon. It is necessary to say that this process is clear and does not cause any environmental impact.
E
24
4. Conclusions
D
U
PL
IC
Disregarding the boron, the results obtained with the purification of MG-Si, through melting in an electron beam furnace, demonstrated that this process is technologically viable to obtain material with low concentration of impurities (smaller than 2 ppm). Starting with ground and leached MG-Si, with 99.92% initial purity (845 ppm of impurities), it was possible to obtain, in a single melting, a material with almost six nines purity (99.9998%) in the border of the sample. Starting with rocks of massive MG-Si (not ground and not leached), with 99.88% initial purity (1108 ppm of impurities), it was possible to obtain, in a single melting, a material with five nines purity (99.9995%) in the border of the sample. Comparing these results, it can be concluded that the melting purification starting with rocks of massive silicon, without any previous treatment, is a more promising process due to the elimination of the grinding and leaching, which just increase the final costs of the process and do not increase the final purity in a significant way.
Fig. 7. The influence of the impurity levels on the performance of a p-type solar cell [13].
Acknowledgements To CAPES by financial support to J.C.S. Pires, (DS44/97). To FAPESP by financial support for the chemical analyses among others (Process no. 97/10654-3). References [1] T. Ikeda, M. Maeda, Purification of metallurgical grade silicon for solar grade silicon by electron beam button melting, ISIJ Int. 32 (5) (1992) 635–642. [2] T. Ikeda, M. Maeda, Refining of silicon for solar cells, in: First International Conference on Processing Materials For Properties, Honolulu, HI, USA, 1993, pp. 441–445. [3] K. Suzuki, T. Kumagai, N. Sano, Removal of boron from metallurgical-grade silicon by applying the plasma treatment, ISIJ Int. 32 (5) (1992) 630–634.
J.C.S. Pires et al. / Journal of Materials Processing Technology 169 (2005) 21–25
E
[9] J.C.S. Pires, et al., Obtaining solar grade silicon through electron beam melting, in: CONEM-2000—Mechanical Engineering National Congress, Natal, RN, Brazil, August 7–11, 2000. [10] J.C.S. Pires, et al., Profile of impurities in sample of polycrystalline silicon purified in electron beam melting furnace, in: Proceedings of 55th Annual Congress of ABM (Brazilian Association of Metallurgy and Materials), Rio de Janeiro, RJ, Brazil, July 24–28, 2000. [11] A.F.B. Braga, Study of the potential of electron beam melting technique to the metallurgical grade silicon purification, Ph.D. Thesis, Mechanical Engineering College, State University of Campinas, 1997, 155 pp. [12] T. Ikeda, M. Maeda, Elimination of boron in molten silicon by reactive rotating plasma arc melting, Mater. Trans., JIM 37 (5) (1996) 983–987. [13] B.R. Bathey, M.C. Cretella, Review: solar-grade silicon, J. Mater. Sci. l7 (1982) 3077–3096.
D
U
PL
IC
AT
[4] Y. Sakagushi, M. Ishizaki, T. Kawahara, et al., Production of high purity silicon by carbothermic reduction of silica using AC-arc furnace with heated shaft, ISIJ Int. 32 (5) (1992) 643–649. [5] A. Choudhury, E. Hengsberger, Review: electron beam melting and refining of metals and alloys, ISIJ Int. 32 (5) (1992) 673–681. [6] A.F.B. Braga, J. Otubo, P.R. Mei, The electron beam melting influence on the metallurgical grade silicon purification for solar-grade silicon, in: Proceedings of Ninth CIMTEC International Meeting, Florence, Italy, 1998. [7] A.F.B. Braga, J. Otubo, P.R. Mei, The purification of leached metallurgical grade silicon by electron beam melting, in: The Third Pacific Rim International Conference on Advanced Materials and Processing, vol. 1, Honolulu, HI, USA, 1998, pp. 1057–1062. [8] J.C.S. Pires, et al., Upgrade of the purification of silicon through electron beam melting furnace, in: XV COBEM—Brazilian Congress ´ of Mechanical Engineering, Aguas de Lind´oia, S˜ao Paulo, Brazil, November 22–26, 1999.
25
The purification of metallurgical grade silicon by electron beam melting Universidade Estadual de Campinas, Mechanical Engineering College, Department of Materials Engineering, P.O. Box 6122, Campinas, SP 13083-970, Brazil b Universidade S˜ ao Francisco, Engineering College, Itatiba, SP, Brazil c Faculdades Integradas de S˜ ao Paulo, S˜ao Paulo, SP, Brazil
AT
a
E
J.C.S. Pires a,∗ , J. Otubo a , A.F.B. Braga b , P.R. Mei a,b,c
Received 15 August 2001; received in revised form 17 April 2002; accepted 11 March 2004
Abstract
PL
IC
Metallurgical grade silicon (MG-Si) is obtained from the reduction of silica (SiO2 ) in a voltaic arc furnace. The impurities are inherent to the reduction process and they are also dependent on the quality of the initial materials. Among other applications, silicon is used as a substrate for photovoltaic conversion of energy and this conversion is as bigger as greater is the purity of the substrate. Researches are being carried out, in some countries, with the objective of searching for new processes of silicon purification or new materials that can be used as substrates for energy conversion. In this research, the technique of silicon purification in an electron beam furnace was used, where the melting occurs in a high vacuum and the impurities are extracted by evaporation. MG-Si in bulk form without leaching, with an initial purity of 99.88% in mass and ground and leached MG-Si, with an initial purity of 99.92%, were used as starting materials. The final purity obtained, in both materials, was above 99.999% in mass. These results demonstrate that this process is technically viable, while also eliminating the stages of chemical purification used in other techniques. © 2005 Elsevier B.V. All rights reserved. Keywords: Silicon purification; Electron beam melting; Metallurgical grade silicon
1. Introduction
D
U
The sun is the largest energy source available because it is clean and renewable. Silicon is the element most used for the conversion of solar energy into electric energy. The solar cell is the key technology for the solution of the energy problem in our planet. This technology is already quite usual for aerospace applications, however, for terrestrial applications it is still limited due to the high cost of the photovoltaic modules [1,2]. Researches are being carried out in some countries, with the objective of looking for new processes for the purification of silicon or new materials that can be used as substrate for the conversion of energy, always seeking for a decrease of the cost of production of the photovoltaic devices [3,4]. ∗
Corresponding author. Fax: +55 19 3298 3722. E-mail addresses: [email protected] (J.C.S. Pires), [email protected] (P.R. Mei). 0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.03.006
According to Choudhury and Hengsberger [5] silicon purification by electron beam melting furnace (EBM) has some advantages: • Melting in a vacuum of 10−4 to 10−2 Pa in refrigerated copper crucible without contamination. • High flexibility of melting rate and conditions for removal of volatile elements. • Almost unlimited melting temperatures. • High power density for local superheating. Previous research of our group [6–9] demonstrated the technical viability of the process, with excellent results in terms of final purity of the silicon. In this research, the results of the purification in EBM of the two materials are compared. One of them is ground and leached MG-Si and the other is the same MG-Si in the massive form without the acid leaching treatment.
J.C.S. Pires et al. / Journal of Materials Processing Technology 169 (2005) 21–25
22
Fig. 2. MG-Si block without previously being acid leached treated and with an initial purity of 99.88% in weight of Si.
AT
For the study of purification of silicon through melting in an EBM, furnace model EMO/LEW 80, made in Germany, 80 kW of power was used. This furnace is quite versatile, allowing the processing of materials of high melting points and products from some grams to ingots of 100 mm of diameter and 800 mm of length. Two types of silicon were used in this work. One of them was ground and leached MG-Si, with a granulation of 100–200 m and with an initial Si purity of 99.92% in weight. The other was MG-Si in the massive form without previously being acid leached treated and with an initial purity of 99.88% in weight. RIMA Industrial S.A. supplied both. The chemical composition of these materials was measured by Glow Discharge Mass Spectrometry in the Northern Analytical Laboratory in New Hampshire, USA. Three measurements were carried out on each sample and the standard deviation, for the three measurements, was less than 15%. The limit of detection of the equipment was 0.001 ppm for Mn, Ti and V and smaller than 0.01 ppm for the others impurities. In the samples after melting, the chemical analysis was performed in the border of the sample. Figs. 1 and 2 present the materials inside the crucible before melting. It can be seen in Fig. 2 that the MG-Si block was placed out of the center of the crucible to propitiate the drag of the liquid silicon during the melting. This procedure simulates the lateral feeding process, in which the material is melted drop by drop, which favors the extraction of the impurities by vacuum. Before being processed by EBM, the silicon was washed with acetone in an ultra-sound cleaner with the objective of removing possible solid residues from the surface. For each experience, a sample of approximately 280 g was placed inside the copper crucible and the evacuation of the whole furnace was initiated. The heating of the sample started with the gradual elevation of the electron beam power until the whole mass was melted, when the power was kept at a constant value
E
2. Experimental procedure
Table 1 Experimental parameters used in the melting Melting time (min) Beam power (kW) Internal pressure of the furnace (Pa)
20 15–17 10−4 to 10−2
D
U
PL
IC
for a certain time. After that interval of time, the power was slowly reduced until the extinction of the beam in the center of the sample had occurred, providing a temperature gradient from the border to the center of the sample that favored the segregation of impurities. This segregation was observed in a previous work [10]. The process conditions, used in this work, were established based on previous work [11] and are showed in Table 1.
Fig. 1. Ground and leached MG-Si, with a granulation of 100–200 m and with an initial purity of 99.92% in weight of Si.
Fig. 3. Top view of a ground and leached MG-Si sample after the melting in the EBM.
J.C.S. Pires et al. / Journal of Materials Processing Technology 169 (2005) 21–25
PL
IC
AT
After melting, the samples obtained in EBM presented the form of a disk with a diameter of 90 mm and a maximum thickness of 25 mm. The geometry of the crucible and the refrigeration favored the formation of temperature gradients from the bottom to the top and from the border to the center of the disk. This can be visualized through the rings (isotherms) formed on the surface. This demonstrates that there was a front of solidification from the border to the center of the sample. The center was the last area to be solidified. Another detail that can be observed in the silicon disks is the saliency formed in the center, due to expansion, during solidification.
This is a characteristic of silicon that, unlike metals, expands during the solidification process. Fig. 3 shows a top view and Fig. 4 shows a bottom view of one of these samples. It is noticed that due to the contact of the sample with the refrigerated copper crucible, the bottom of the disk did not melt properly. The results of chemical analysis are presented in Table 2. For both types of silicon used, great extraction of all impurities, except for the boron, can be observed. Due to impurities segregation [10], the sample purity is not homogeneous and the best purity was obtained in the border of the samples. The efficiency of extraction for all impurities can be observed in Fig. 5. Purification through evaporation is more effective for those elements that have a closer or higher vapor pressure than that of silicon. Silicon vapor pressure is 5 × 10−1 Pa at 1500 ◦ C. In Fig. 6, the removal efficiency for those impurities as a function of their vapor pressures can be observed. The boron was not extracted because its vapor pressure is much lower than the silicon vapor pressure. According to Ikeda and Maeda [12], the boron content should be smaller than 1 ppm, because this is one of the
E
3. Results and discussions
23
Fig. 4. Bottom view of a ground and leached MG-Si sample after the melting in the EBM.
Fig. 5. Efficiency of extraction of the impurities.
U
Table 2 Chemical analysis before and after silicon melting, final purity and efficiency of extraction Impurities
Final purity (%)
MG-Si in massive form without leaching
Before melting (ppmw)
After melting (ppmw)
Efficiency of extraction (%)
Before melting (ppmw)
After melting (ppmw)
Efficiency of extraction (%)
53.00 11.00 15.00 185.00 1.80 31.00 30.00 4.60 2.00 480.00 23.00 3.00 20.58 844.98
0.09 0.00 10.00 0.02 0.01 0.03 0.03 <0.01 0.003 0.04 0.41 0.002 1.12 1.766
99.83 100.00 33.33 99.99 99.20 99.90 99.92 >99.78 99.85 99.99 98.20 99.93 94.56 99.79
110.00 0.04 10.00 26.00 6.50 790.00 0.10 4.20 75.00 0.33 38.00 42.00 16.32 1108.49
0.44 <0.01 7.30 0.31 0.29 0.70 0.10 0.02 0.027 0.05 0.39 0.087 2.74 5.165
99.60 >75.61 27.00 98.81 95.54 99.91 0.00 99.50 99.96 83.94 98.97 99.80 83.21 99.53
D
Al Ba B Ca Cu Fe K Mg Mn Na P Ti Others Total
Powder of leached MG-Si
99.92
99.99982
99.88
99.9995
J.C.S. Pires et al. / Journal of Materials Processing Technology 169 (2005) 21–25
Fig. 6. Removal efficiency of the impurities as a function of their vapor pressures.
AT
doping elements in silicon solar cells. However, the removal of this element is very difficult because its vapor pressure is very low in relation to the vapor pressure of the silicon (10−4 Pa for boron and 10−1 Pa for silicon). This makes its extraction for vacuum processes difficult. Another difficulty regarding to boron extraction is that its segregation coefficient in silicon is near 1 (k ∼ = 1), which also makes its extraction using a unidirectional solidification process difficult. A process usually used for the boron removal is plasma melting in an oxidizing atmosphere (O2 , CO2 or H2 O). In this process, the boron is transformed into the oxide form, increasing its vapor pressure [3,12]. The remaining impurities in SoG-Si degrade the solar cell performance. In Fig. 7 [13], the influence of each impurity levels can be observed. Certain impurities are more deleterious to solar cell behavior, however the tolerable level of impurities depends on the growth technique as well as the cell fabrication processes involved [13]. The direct melting using rocks of massive MG-Si, without any previous treatment (grinding and leaching), considering that this material had a large amount of impurities showed the best results, in terms of overall costs. The results obtained in this work demonstrate that it is possible to avoid this previous treatment (grinding and leaching) for silicon purification. Rocks of massive MG-Si can be directly purified in EBM with excellent results.
This process can be extended to big samples by the use of the lateral feeding and drop-by-drop melting, however, additional studies should be done and some parameters should be modified. So, these results could certainly be still improved by lateral feeding of the massive MG-Si blocks. With the drop-by-drop melting, a larger superficial area would be in direct contact with vacuum, favoring the extraction of the impurities by evaporation. Is not so easy to compare the results of this work, in terms of cost and production efficiency, with the actual industrial process because this work was carried out in laboratory scale, but in a previous study, the energy cost was estimated in US$ 7/kg of purified silicon. It is necessary to say that this process is clear and does not cause any environmental impact.
E
24
4. Conclusions
D
U
PL
IC
Disregarding the boron, the results obtained with the purification of MG-Si, through melting in an electron beam furnace, demonstrated that this process is technologically viable to obtain material with low concentration of impurities (smaller than 2 ppm). Starting with ground and leached MG-Si, with 99.92% initial purity (845 ppm of impurities), it was possible to obtain, in a single melting, a material with almost six nines purity (99.9998%) in the border of the sample. Starting with rocks of massive MG-Si (not ground and not leached), with 99.88% initial purity (1108 ppm of impurities), it was possible to obtain, in a single melting, a material with five nines purity (99.9995%) in the border of the sample. Comparing these results, it can be concluded that the melting purification starting with rocks of massive silicon, without any previous treatment, is a more promising process due to the elimination of the grinding and leaching, which just increase the final costs of the process and do not increase the final purity in a significant way.
Fig. 7. The influence of the impurity levels on the performance of a p-type solar cell [13].
Acknowledgements To CAPES by financial support to J.C.S. Pires, (DS44/97). To FAPESP by financial support for the chemical analyses among others (Process no. 97/10654-3). References [1] T. Ikeda, M. Maeda, Purification of metallurgical grade silicon for solar grade silicon by electron beam button melting, ISIJ Int. 32 (5) (1992) 635–642. [2] T. Ikeda, M. Maeda, Refining of silicon for solar cells, in: First International Conference on Processing Materials For Properties, Honolulu, HI, USA, 1993, pp. 441–445. [3] K. Suzuki, T. Kumagai, N. Sano, Removal of boron from metallurgical-grade silicon by applying the plasma treatment, ISIJ Int. 32 (5) (1992) 630–634.
J.C.S. Pires et al. / Journal of Materials Processing Technology 169 (2005) 21–25
E
[9] J.C.S. Pires, et al., Obtaining solar grade silicon through electron beam melting, in: CONEM-2000—Mechanical Engineering National Congress, Natal, RN, Brazil, August 7–11, 2000. [10] J.C.S. Pires, et al., Profile of impurities in sample of polycrystalline silicon purified in electron beam melting furnace, in: Proceedings of 55th Annual Congress of ABM (Brazilian Association of Metallurgy and Materials), Rio de Janeiro, RJ, Brazil, July 24–28, 2000. [11] A.F.B. Braga, Study of the potential of electron beam melting technique to the metallurgical grade silicon purification, Ph.D. Thesis, Mechanical Engineering College, State University of Campinas, 1997, 155 pp. [12] T. Ikeda, M. Maeda, Elimination of boron in molten silicon by reactive rotating plasma arc melting, Mater. Trans., JIM 37 (5) (1996) 983–987. [13] B.R. Bathey, M.C. Cretella, Review: solar-grade silicon, J. Mater. Sci. l7 (1982) 3077–3096.
D
U
PL
IC
AT
[4] Y. Sakagushi, M. Ishizaki, T. Kawahara, et al., Production of high purity silicon by carbothermic reduction of silica using AC-arc furnace with heated shaft, ISIJ Int. 32 (5) (1992) 643–649. [5] A. Choudhury, E. Hengsberger, Review: electron beam melting and refining of metals and alloys, ISIJ Int. 32 (5) (1992) 673–681. [6] A.F.B. Braga, J. Otubo, P.R. Mei, The electron beam melting influence on the metallurgical grade silicon purification for solar-grade silicon, in: Proceedings of Ninth CIMTEC International Meeting, Florence, Italy, 1998. [7] A.F.B. Braga, J. Otubo, P.R. Mei, The purification of leached metallurgical grade silicon by electron beam melting, in: The Third Pacific Rim International Conference on Advanced Materials and Processing, vol. 1, Honolulu, HI, USA, 1998, pp. 1057–1062. [8] J.C.S. Pires, et al., Upgrade of the purification of silicon through electron beam melting furnace, in: XV COBEM—Brazilian Congress ´ of Mechanical Engineering, Aguas de Lind´oia, S˜ao Paulo, Brazil, November 22–26, 1999.
25