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Vol. 8. Issue 5.
Pages 4470-4476 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4470-4476 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.07.060
Open Access
Synthesis and separation of Si-Fe alloy to produce high-purity silicon
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Yang Liua, Shuai wanga,1, Shengnan Jianga,1, Xiaofeng Wanga, Jian Konga, Pengfei Xinga,
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xingpf@smm.neu.edu.cn

Corresponding author.
, Yanxin Zhuangb, Xuetao Luoc
a School of Metallurgy, Northeastern University, Shenyang 110819, PR China
b Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, PR China
c Department of Material Science and Engineering, Xiamen University, Xiamen 361005, PR China
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Table 1. Chemical composition and content of impurities in the raw materials and products.
Abstract

The metallurgical route is a prospective method to replace the Siemens process to produce silicon for solar cells. However, the imperfect removal efficiencies of boron (B) and phosphorus (P) from silicon are the major issue. Herein, we report a novel approach to produce high-purity silicon for solar cells by the reaction of SiC with SiO2 and pure FeO (with low B and P) in vacuum, which first forms Si-Fe alloy and is followed by milling and acid leaching. Pure FeO can not only reduce the silicon formation temperature but also provide an abundance of pure iron elements that serve as an impurities getter to capture B and P during the formation of silicon. Compared with the traditional metallurgical methods, this work could remove B and P from silicon directly during the formation of silicon. The refining ratios of B and P from the raw materials to the Fe-Si alloy were about 71.05% and 93.49%. Characterizations of the Si-Fe alloy indicate that many defects exist in the FeSi2 phase of the milled Si-Fe alloy, which is convenient for the elimination of Fe, B and P by acid leaching. Final silicon had a purity of 99.99 wt.% with 0.04 ppmw B and 0.07 ppmw P, which had met the requirement of B and P for solar cells (B ≤ 0.15 ppmw, P ≤ 0.35 ppmw). This strategy significantly decreased energy consumption and shortened production process of high-purity silicon and is thus an effective and scalable approach for the production of silicon for solar cells.

Keywords:
Synthesis
Silicon for solar cells
Formation
Si-Fe alloy
Acid leaching
Full Text
1Introduction

Solar-grade silicon (SoG-Si) is the dominant material for solar cells and is mainly produced by the Siemens process. However, the Siemens process need long procedure, high cost and energy consumption, which have hindered the large scale industrialization of silicon solar cells preparation [1,2]. Therefore, the metallurgical purification of metallurgical-grade silicon (MG-Si, 2 N) methods have been developed for SoG-Si preparation, which are low cost, high efficiency and environmental friendliness [3]. These methods include acid leaching [4], solvent refining [5], vacuum refining [6], slag treatment [7], and plasma treatment [8].

The metallic impurities (Fe, Al, Ca and so on) can be eliminated completely by directional solidification, whereas the nonmetallic impurities of boron (B) and phosphorus (P) are unresponsive to directional solidification. The reason for this is their large segregation coefficients between solid and liquid silicon (kB = 0.8 and kP = 0.35) [9]. B can be removed from silicon by slag treatment (via oxidation reactions) and P can be removed from silicon by vacuum refining based on vapor pressure difference [10]. To date, significant advances have been achieved in the removal of B and P from silicon; however, considering the high cost facilities, high energy consumption and the environmentally benign methods remain a research hotspot.

Solvent refining (alloying) has been proposed as a potential metallurgical method for the removal of B and P from silicon, which involves melting silicon with a metal to form a phase that has higher affinity for B and P than does silicon. The added metal together with B and P absorbed from silicon must then be separated from silicon, either by chemical or physical methods [5]. Several metals have been investigated as a candidate for the role of getter metal in solvent refining of silicon, such as Fe, Al, Ca, Ti or Zr) [11–15]. Considering the price and purity, Fe is the best candidate for the proposed process due to its primarily characteristics of high affinity towards B and P, abundance, and low cost, as well as the recyclability of the remaining alloy (ferrosilicon) [16]. However, previous studies utilized normal Fe with higher contents of B and P to purify MG-Si, and the B and P from normal Fe can be incorporated into the silicon. To the best of our knowledge, pure Fe with low B and P has never been used as the impurity getter to purify MG-Si, not even to purify silicon during the formation of silicon. The formation mechanism of silicon shows that silicon is mainly generated by the carbothermic reaction of SiC with SiO2[17–19]. Additionally, iron oxide is a major impurity in the SiC and SiO2, and the Fe originating from iron oxide can dissolve in the silicon easily during the formation process of silicon, however, the concentrations of Fe in the SiC and SiO2 are less than 1 wt.%, which is not enough to capture the B and P from silicon. As is well known, the B and P in molten Fe can be removed to an ultralow level by oxygen blowing and vacuum refining. Namely, the iron oxide fabricated by the pure Fe can also exhibit ultralow contents of B and P. Therefore, adding an excess of pure iron oxide can directly provide sufficient pure Fe to favor B and P during the formation process of silicon.

In the present work, therefore, with excess pure Iron oxide (FeO, with low B and P) added and mixed with SiC and SiO2, a complete production of high-purity silicon with low B and P for solar cells was successfully realized by the reaction of SiC with SiO2 and FeO that firstly which forms a Si-Fe alloy, and is followed by milling and acid leaching. FeO was used as the impurity getter to capture B and P during the formation process of silicon. The characterizations of the milled Si-Fe alloy powder were analyzed. The mechanisms of gaining silicon with low B and P were analyzed, and the new strategy was compared with the Siemens process.

2Experimental2.1Materials

The raw materials of SiC (6H-SiC, 99.99 wt.%, average grain size of 12.38 μm), quartz sand (a-quartz, 99.99 wt.%, average grain size of 0.15 cm) and FeO (99.99 wt.%, average grain size of 1.46 μm) were commercially available and directly used without further purification. A SiC plate consist of 99.5 wt.% SiC and 0.5 wt.% Fe2O3 was also used in this work. Carboxy methyl cellulose (CMC) was used as the binder. Furthermore, reagents-grade chemical and deionized water were used.

2.2Process

Fig. 1a–c shows the experimental setup and process for the synthesis of silicon by the carbothermic reaction of SiC with SiO2 and FeO. In a typical experimental procedure (Fig. 1c), a mixture of SiC, quartz sand, FeO, CMC and high-purity water was firstly mixed with vigorous stirring for 4 h. The molar ratio of SiC to SiO2 was 2:1 and the proportion of FeO is 15 wt.%. The amounts of CMC and high-purity water were 0.1 wt.% and 5.0 wt.%, respectively. Then, the as-prepared mixture was pelletized at 45 MPa for 3 min. The dimensions of the uniform cylinder pellets were 20 mm × 25 mm. Afterwards, the pellets were dried at 60℃ for 25 min. Subsequently, the dried pellets were placed into a small graphite crucible (99.99 wt.% purity) with a SiC cover plate, and loaded into a large graphite crucible (99.99 wt.% purity). Finally, the large graphite crucible was introduced to the chamber of a graphite resistance furnace (a layer of 6H-SiC was used under the pellets). Prior to heating, the furnace chamber was evacuated to a pressure of 10−2 Pa. The temperature was monitored by an optical pyrometer (infrared radiation thermometer), which was calibrated by a tungsten-rhenium thermocouple. The specimens were heated from room temperature to 1950 ℃ at a rate of 25℃/min and maintained at the temperature for 90 min, followed by furnace-cooling to the ambient temperature. The product was milled by carnelian, and the powder was added into a 10 wt.% HF solution at 75 ℃ for four hours to remove the surface oxide layer on the surface of the silicon and to dissolve the metallic element. Acetic acid was also added (20% vol) to enhance the wetting of the fine particles [20]. The residue after leaching, being silicon, was then rinsed thoroughly with deionized water and dried in a vacuum at 80℃ for three hours to obtain the final product.

Fig. 1.

(a, b) Schematic diagrams of the experimental setup and interior of the large graphite crucible. (c) Experimental procedure.

(0.63MB).
2.3Characterization

Inductively coupled plasma-mass spectrometry (ICP-MS) was used to measure the Fe, B and P. The milled specimens were characterized by the X-ray diffractometer (XRD), Raman, X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) with an energy-dispersive X-ray spectroscopy (EDS).

3Results and discussion3.1Characterization of the silicon product (ferrosilicon)

As shown in the inset of Fig. 2a, a silicon ingot forms in the bottom gap between cylindrical pellets after the carbothermal process, and an abundance of pores are observed on the surface of the cylindrical pellets. XRD analysis is commonly adopted to probe the crystal structure in depth [21,22]. Fig. 2a also displays the XRD patterns of the milled silicon ingot. As shown in the XRD patterns, the diffraction peaks of crystalline Si (crystal phase PDF#27-1402) and the FeSi2 crystal (PDF#35-0822) are observed in the silicon ingot (Si-Fe alloy). Crystalline Si is formed by the carbothermic reaction of SiC with SiO2. The reaction process firstly generates SiO(g) (Reaction 1, higher than 1879℃), and then, the SiO(g) reacts with SiC to nucleate silicon on the SiC surface (Reaction 2, higher than 1937℃ ). In fact, the real formation temperature of silicon could be significantly lower due to the presence of FeO and the vacuum environment. Based on the equilibrium diagram of the SiOC system [23], the FeO (or Fe2O3 from the SiC plate in Fig. 1b) plays the roles of reducing the concentration of CO(g) (Reaction 3) and lowing the formation temperature of silicon. In addition, the SiC plate serves to protect the graphite resistance against corrosion by SiO(g) (SiO(g) + 2C(s) = SiC(s) + CO(g)). Once the silicon forms, the molten iron can react with the molten silicon to produce FeSi2 (via the dissolution of silicon atoms into the iron lattice), and subsequently the following produced silicon is just the crystalline Si after the molten iron is exhausted. Moreover, the peak widths of the crystalline Si are clearly thinner and their intensity larger than those of the FeSi2 phase, which indicates that the crystallinity of the crystalline Si in the milled Si-Fe alloy is better than that of FeSi2 phase.

SiC(s) + 2SiO2(l) = 3SiO(g) + CO(g)
SiC(s) + 2SiO2(l) = 3SiO(g) + CO(g)
FeO(s) + CO(g) = Fe(l) + CO2(g)

Fig. 2.

(a) XRD patterns of the typical milled product and silicon ingot produced from the pellets (inset). (b) Raman spectrum of the milled powder. High-resolution spectra of the Fe 2p (c) and Si 2p (d).

(0.49MB).

The structure property of materials can be examined by Raman spectroscopy [24,25]. The Raman spectrum of the milled Si-Fe alloy is shown in Fig. 2b. The peak at 520 cm−1 is due to the first-order scattering of the optical phonon of silicon, and the sharp peak of silicon displays that the silicon has better crystal quality. The results are also consistent with those from the XRD analysis. The two weak peaks at 188 cm−1 and 237 cm−1 related to FeSi2 and are absent in the spectrum of the milled Si-Fe alloy (Fig. 2b), indicating that the FeSi2 exhibits a worse crystal quality. A slight shift of the peaks to lower wave numbers compared with the Raman peaks of bulk FeSi2[26,27] may be due to the small crystallite size in the samples, as the samples were milled into powders for measurement, and a decrease in crystallite size is expected to result in an increase of the peak width and cause a position shift of the peak to lower wave numbers. In addition, this observation, might also be attributed to lattice imperfections related to Fe defects in the polycrystallites of FeSi2.

The valence state and surface chemical composition of materials can be checked by XPS [28]. Fig. 2c and d represent high-resolution XPS spectra of the milled Si-Fe alloy. In the Fe 2p spectrum (Fig. 2c), in addition to the two main peaks at 707.4 and 720.4 eV, which are related to the FeSi2, a weak shoulder peak appears at 711.2 eV as a result of the oxidation of iron (Fe2O3) on the FeSi2 surface after its exposure to air. In the Si 2p spectrum (Fig. 2d), a shoulder peak is observed at 102.7 eV attributable to silicon oxide (SiO2). It can be concluded that the Si-Fe alloy after milling (powder) possesses small amounts of SiO2 and Fe2O3 due to the oxidation of the dangling bonds of Si and Fe. The peaks centered at 99.4 eV and 99.9 eV correspond to pure silicon and the silicon in FeSi2, respectively (Fig. 2d). The chemical composition found by XPS suggests an approximate pure silicon: iron ratio of ˜1.4:1, indicating that the pure silicon is rich in the silicon ingot.

Fig. 3a reveals the backscattered SEM images of the obtained silicon ingot, which exhibits coarse platelets and other irregular morphologies. The EDS analysis (Fig. 3b) shows that the grey region and black area belong to the FeSi2 and pure silicon, respectively. Interestingly, no carbon content is detected, confirming that the presence of FeO could enhance the consumption of CO(g) (Reaction 3). Compared the pure silicon with FeSi2 on the same scale, the surface of the pure silicon is smooth (good crystal quality) and that of the FeSi2 is full of cracks and pores (bad crystal quality), which is in accordance with the outcome of the XRD and Raman analyses. The result of the EDS microanalysis is in accordance with that of the XPS analysis. To verify the distribution of the silicon and iron elements in the silicon ingot, the sample was investigated by EDS mapping (Fig. 3c).The distribution of the iron element is regularly distributed, while some pure silicon element clusters are aggregated, as shown in Fig. 3c.

Fig. 3.

(a) Backscattered SEM image of the typical product. (b) EDS analysises of the grey region and black area in Fig. 3a. (c) The elemental mapping images of iron and silicon in (a).

(0.6MB).
3.2Purity and mechanisms

It is well known that metal impurities in silicon (Al, Mg, Ca and so on) can be efficiently removed by directional-solidification purification because of the extremely small segregation coefficients of the impurities, and thus, most metals do not contaminate the silicon crystals. Therefore, only Fe, B and P are studied in the present work. Table 1 shows that the Fe-Si alloy has less B and P than the raw materials of SiC, SiO2 and FeO. The refining ratios of B and P from the raw materials to the Fe-Si alloy are approximately 71.05% and 93.49%. During the carbothermal reaction of SiC with SiO2 at 1950℃ under vacuum in a graphite resistance furnace, the P could be reduced easily due to its higher saturated vapor pressure. In addition, the flow of SiO(g) and CO(g) can also remove the B and P during the formation process of silicon. The oxidation of B by molten SiO2 and the volatilization of a boron oxide (BO, B2O, BO2, B2O2, or B2O3) may be another main reason for this phenomenon. The refining ratio of B is evidently higher than the traditional silicon refining by vacuum [5], indicating that the low concentration of B in the final silicon product is mostly attributed to the effects of alloying, blowing refining or oxidation refining, and it is not due to the purity of the raw materials.

Table 1.

Chemical composition and content of impurities in the raw materials and products.

Sample  SiC (wt.%)  SiO2 (wt.%)  Si (wt.%)  Fe (wt.%)  B (ppmw)  P (ppmw) 
SiC  99.99  –  –  –  0.14  0.83 
Silica sand  –  99.99  –  –  0.09  0.94 
FeO  –  –  –  77.77  0.01  0.01 
Fe-Si alloy  –  –  69.18  30.81  0.11  0.17 
Final silicon  –  –  99.99  0.00  0.04  0.07 

Acid leaching needs low-cost equipment and a small energy consumption and is known as a facile method to remove iron from the Si-Fe alloy. In addition, removal of the SiO2 films is necessary. Currently, the most widely practiced etchant of SiO2 is, without any doubt, HF solution. Although HF is a hazardous acid, it is irreplaceable for dissolving the SiO2 in present technology. Moreover, the final silicon (the Si-Fe alloy after acid leaching) presents a purity of 99.99 wt.% with 0.04 ppmw B and 0.07 ppmw P (Table 1), which meets the requirements of B and P for solar cells (B ≤ 0.15 ppmw, P ≤ 0.35 ppmw); whereas the other impurities (Al, Mg and Ca) can be readily removed by the directional solidification, and then, the 6 N silicon for solar cells can be obtained (not done in the present work). As shown in equations of (4) and (5), iron possesses a higher thermodynamic affinity for B and P than Si, thus, a liquid iron-rich phase, over the solid silicon, should normally favor these impurities. Namely, lower segregation coefficients are expected with the existence of iron. On the basis of the characterizations of the milled Si-Fe alloy, the FeSi2 phase in the Si-Fe alloy exhibits many defects, indicating that a large amount of impurities are exposed after milling, which is convenient for the elimination of iron by acid leaching. Different from the MG-Si, the content of iron is amplified by 30 times, ensuring sufficient pure iron to favor B and P.

Fe(l) + SiP(l) = FeP(l) + Si(l) ΔG1950 ℃ = –108.5 kJ/mol
SiB3(l) + 3Fe(l) = 3FeB(l) + Si(l) ΔG1950 ℃ = –155.7 kJ/mol

3.3Comparison of the new strategy with the Siemens process

The weights of the dried raw materials (pellets) and obtained Si-Fe alloy were 210 g and 48 g, respectively. After acid leaching, the final silicon presented a weight of 32 g, thus, the yields of the Si-Fe alloy and high-purity silicon for solar cells were approximately 23% and 15% respectively. The prices for the main raw materials are 0.89 $/kg SiC, 0.45 $/kg SiO2 and 0.74 $/kg FeO. The brief cost accounting of this process yields approximately about 5.10 $/kg, which is lower than 11.27 $/kg for the Siemens process. In addition, several unreacted SiC particles remain (Fig. 2), and the experiment is not a continuous process, suggesting that the yield of high-purity silicon has great potential to be raised if the process can be operated in an electric arc furnace (EAF). The MG-Si produced in an EAF could have a yield of about 80%, so the prospect of present strategy to produce silicon for solar cells is inestimable, but the cost accounting can be lowered to 2.55 $/kg. The absence of vacuum in EAF may be disadvantageous for the volatilization of boron oxide and P, however, the sufficiently pure iron elements could have the ability to capture B and P.

The environmental impact issues related to the manufacturing process can be controlled as HF solutions have been widely used in the wet chemical etching of silicon wafers. Furthermore, the wasted HF solutions can be recycled by CaO or other green methods after the experiment. Compared with the Siemens method with toxic intermediates and a high energy consumption, the application of a small amount of HF solution could relatively reduce the energy consumption and environmental pollution ensued to produce high-purity silicon for solar cells. Moreover, the concentrations of HF solutions for leaching can be reduced when the appropriate related parameters are gained.

4Conclusions

In summary, high-purity silicon with low B and P for solar cells has been prepared by the reaction of SiC with SiO2 and FeO that firstly which forms a Si-Fe alloy, which is followed by milling and acid leaching. Pure FeO can not only reduce the silicon formation temperature but also provide an abundance of pure iron to capture B and P during the formation process of silicon. Characterizations of the Si-Fe alloy powder show that many defects exist in the FeSi2 phase of the milled Si-Fe alloy powder, which is convenient for the elimination of Fe, B and P from silicon by acid leaching. The refining ratios of B and P from the raw materials to the Fe-Si alloy are about 71.05% and 93.49%. In comparison with the traditional metallurgical methods, this work could remove B and P from silicon directly during the formation process of silicon. This promising approach opens up a low-cost pathway toward the scalable production of SoG-Si to substitute the Siemens process, considering both economic and environmental points of view.

Conflicts of interest

There are no conflicts to declare.

Acknowledgments

This work was financially supported by the National Key Research and Development Program of China (Grant Nos. 2017YFB0310302, 2018YFC1901804 and 2018YFC1901805). In addition, Yang Liu wants to thank, in particular, the care and grammar support from Lu Liu.

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S. Wang and S. N. Jiang contributed equally to this work.

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