Effect of mechanical stirring on silicon purification during Al-Si solvent refining
Introduction
The sun is a virtually infinite reservoir of renewable energy. Photovoltaic (PV) modules, assembled by 60 solar cells in common, are the most ubiquitous solar power harvesting technology. The raw material for the solar cells is solar grade silicon. With the development of the PV market, the demand for the solar grade silicon is large.
The solar grade silicon is sensitive to impurity elements, which affect carrier lifetime in the solar cells. The solar grade silicon is purified from metallurgical grade silicon which contains many kinds of impurity elements, such as iron, aluminum, calcium, boron, and phosphorus. The metallurgical grade silicon can be refined in two kinds of processes, chemical process and metallurgical process [1]. The metallurgical grade silicon is transformed into silicon-based compounds, such as SiHCl3 and SiH4, and the compounds are purified and reduced into high purity silicon in the chemical process [1]. While no silicon compound is formed in the metallurgical process, and the impurity elements are removed for their special properties. Metal elements, such as iron, aluminum, and calcium can be removed by directional solidification because they have small segregation coefficients in silicon [2]. But boron and phosphorus cannot be removed by directional solidification because their segregation coefficients in silicon are high (value of 0.8 and 0.35) [2], and they are the key impurity elements to be eliminated. While boron can be removed by slag treatment for boron’s affinity with slag [3], and phosphorus can be removed by vacuum treatment because of its high vapor pressure [4]. But these metallurgical processes need high temperatures and large energy consumption. Also, boron and phosphorus cannot be removed at the same time.
Solvent refining can remove boron and phosphorus simultaneously at relatively low temperatures. It has three steps [5]. Firstly, metal acting as impurity getter and the metallurgical grade silicon are mixed and heated to form a hypereutectic melt. Secondly, the primary silicon phase grows in the melt, and impurity atoms stay in the remaining melt. Lastly, the primary silicon phase is separated and collected. Al-Si [6], Fe-Si [7], Cu-Si [8], Ca-Si [9], Ti-Si [9], Sn-Si [10], Zr-Si [11], and Ga-Si [12] are studied for silicon purification by now.
Al-Si solvent refining has many advantages and becomes the most popular solvent refining strategy. According to the Al-Si phase diagram [13], Al-Si a simple binary system with no intermetallic compounds. What is more, the maximum solubility of aluminum in silicon is 384 ppmw [14], and the aluminum segregation coefficient in silicon is 0.002 [2]. It indicates that aluminum contamination in the primary silicon phase can be removed by directional solidification. Besides, the price of aluminum is low, and it fits industrial production. Based on Al-Si solvent refining, Yoshikawa [15], Tang [16], Zhao [17], and Hu [18] studied boron and phosphorus segregation coefficients in the Al-Si melt by thermodynamic analysis and found that boron and phosphorus segregation coefficients in the Al-Si melt are smaller than that in silicon. It indicates that Al-Si solvent refining can remove boron and phosphorus efficiently.
When the primary silicon phase is growing during Al-Si solvent refining, impurity atoms enrich at the solid-liquid interface and diffuse into the remaining melt. Impurity content in the primary silicon phase is confirmed by the impurity segregation coefficient in the Al-Si melt and the impurity content at the solid-liquid interface, as shown in equation (1).where , and are impurity content in the primary silicon phase, the impurity segregation coefficient in the Al-Si melt, and impurity content at the solid-liquid interface respectively.
The impurity segregation coefficient in the Al-Si melt is temperature-dependent, and lowering impurity content at the solid-liquid interface is a good strategy to control impurity content in the primary silicon phase.
Electromagnetic stirring is an effective way to stir the Al-Si melt [19], [20], and the effect of electromagnetic stirring on impurity removal has been proved [21], [22]. It is shown that electromagnetic stirring in the Al-Si melt can enhance the migration of impurity atoms into the remaining melt, and it is beneficial to silicon refining. Mechanical stirring is another way to stir the Al-Si melt. Besides, the stirring force and frequency are more controllable and adjustable than electromagnetic stirring. But the effect of mechanical stirring in the Al-Si melt on silicon purification has never been reported, and it is studied in this work.
Besides, the separation of the primary silicon phases (purified silicon) from Al-Si alloy is another problem during Al-Si solvent refining. The primary silicon phases distribute in the Al-Si matrix after solidification, and acid leaching is used to get purified silicon in common. It is low efficiency and not suitable for industrial manufacture. The purified silicon is supposed to grow on a rotating rod in this work, which is beneficial for the separation of purified silicon from the remaining melt.
In this work, an apparatus for mechanical stirring in the Al-Si solvent refining was assembled. A rotating tube with an end closed was used to stir the Al-Si melt, and the purified silicon grew on the rotating tube. The macrostructure and microstructure of the sample were observed, and the boron and phosphorus contents in purified silicon were measured and discussed.
Section snippets
Materials and methods
The apparatus was assembled as shown in Fig. 1. There is a driving belt controlled by a turning motor to rotate an alumina tube with one end closed (inner diameter: 14 mm, outer diameter: 25 mm). There is a quartz pipe (inner diameter: 4 mm, outer diameter: 8 mm) settled in the middle of the alumina tube. Air controlled by an air compressor and a flowmeter can be introduced in the alumina tube through the quartz pipe. The raw materials can be put in an alumina crucible (height: 140 mm, inner
Macrostructure and microstructure of the sample
Fig. 2 shows the images of sample No.2, and the other samples show similar structures. Fig. 2(a) shows the macro photograph of the sample, and Fig. 2(b) shows the cross-section of the sample. Fig. 2(b) demonstrates that the color of the zone near the rotating alumina tube (A in Fig. 2(b)) is dark, while the color at the edge of the sample (B in Fig. 2(b)) is bright.
Fig. 3(a) and (b) show the SEM images of sample No.2. Fig. 3(a) is corresponding to A in Fig. 2(b), and (b) is corresponding to B
Conclusions
Mechanical stirring was introduced in the Al-Si solvent refining to dilute enriched impurity atoms at the solid-liquid interface. It is found that mechanical stirring is beneficial to impurity atoms removal. Boron and phosphorus contents in purified silicon with mechanical stirring are smaller than that without mechanical stirring. When stirring speed increases, boron and phosphorus contents in purified silicon decrease.
CRediT authorship contribution statement
Yanlei Li: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Formal analysis, Investigation, Validation. Lindong Liu: Resources, Formal analysis. Jian Chen: Conceptualization, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Doctoral Scientific Research Foundation of Xinyang Normal University, and the Nanhu Scholars Program for Yong Scholars of XYNU.
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