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Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification
Journal of Alloys and Compounds xxx (xxxx) xxx
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Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification Jingfei Hu a, Kuisong Zhu a, b, c, Kuixian Wei a, *, Wenhui Ma a, c, **, Tianlong Lv a a
The National Engineering Laboratory for Vacuum Metallurgy, State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China b Panzhihua School of Vanadium and Titanium, Panzhihua University, Panzhihua, 617000, China c Key Laboratory for Nonferrous Vacuum Metallurgy of Yunnan Province, Engineering Research Center for Silicon Metallurgy and Silicon Materials of Yunnan Provincial Universities, Kunming University of Science and Technology, Kunming, 650093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2019 Received in revised form 17 September 2019 Accepted 8 October 2019 Available online xxx
To analyze the effects of pulling rate on the removal of impurities from a Ti-90 wt% Si alloy during the Si refining process, five samples with different pulling rates were examined. At a low pulling rate, the contents of Fe, Al, and Ca could be reduced to 9 10 4 wt%, 21 10 4 wt%, and 10 10 4 wt%, representing impurity removal rates of 99.68%, 98.76%, and 95%, respectively. Under our experimental conditions, we determined that a pulling rate of 5 mm/s can reduce the time taken for directional solidification and provide excellent impurity removal efficiency. The average impurity removal rates for Fe, Al, and Ca in the silicon enrichment layer were 98.431%, 99.262%, and 85.833%, respectively. The migration and removal mechanism of Fe during the directional solidification refining process was also identified. Due to the low energy of grain boundary segregation of impurities, Fe segregate at the grain boundary of TiSi2 grains, such that Fe eventually forms FeTiSi2 with TiSi2 at the grain boundary. The analysis of our study provides theoretical and experimental support for the purification of polycrystalline silicon from TieSi alloys, which further broadens the application of TieSi alloys and lays the foundation for comprehensive utilization of high-titanium blast furnace slag. © 2019 Elsevier B.V. All rights reserved.
Keywords: TieSi alloys Directional solidification Metal impurity removal Pulling rate Silicon refining
1. Introduction With the rapid development of the photovoltaic industry, several potential methods for recycling solar-grade silicon are currently under development. In the photovoltaic industry, Sibased solar cells have attracted significant attention due to their efficient performance, abundant raw materials, and long service life. The main raw material in the Si-based solar cells is solar-grade Si [1,2]. The impurity content of solar-grade Si has a significant impact on the photoelectric conversion efficiency of solar cells [3,4]. Fe, Al, and Ca are the most common metal impurities in Si and their existence affects the minority carrier lifetime of solar cells [4e6]. Additionally, Al and Ca also directly affect the resistivity of solar
* Corresponding author. ** Corresponding author. The National Engineering Laboratory for Vacuum Metallurgy, State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China. E-mail addresses: [email protected] (K. Wei), [email protected] (W. Ma).
cells, which affects their power generation efficiency [5,6], indicating that they must be removed to meet the performance requirements of solar cells. Scholars from around the world have conducted research on the removal of impurities from solar-grade Si using methods such as vacuum refining [7,8], directional solidification [9], slag refining [10,11], and alloy refining [12,13]. Among these methods, the preparation of solar-grade Si using alloy refining is one of the most popular solutions [13e15]. Research on alloy refining largely concentrates on AleSi alloys; however, the separation interface for AleSi alloy refining [14,15] is not clear. Additionally, primary Si is distributed in the Al matrix after refining, which necessitates the use of acid leaching to obtain primary and eutectic Si. The acid consumption for this process is significant with the presence of crystal boundary segregation of impurities in the obtained Si. Additionally, it is well known that directional solidification is suboptimal for removing nonmetallic impurities, such as B and P. Bai [16] and Lei [17] conducted studies on the addition of titanium (Ti) to remove B from AleSi alloys and obtained promising removal results. Microscopic morphology analysis revealed that the crystal
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Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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boundary segregation of impurities was also improved. Recently, Zhu et al. [18] proposed a novel TieSi alloy material for preparing Si using directional solidification. This material is prepared through the aluminothermic reduction (ATR) of titaniumbearing blast furnace slag. The advantage of this raw material is that TieSi alloys prepared from Ti slag through thermal reduction of metals do not contain B, P, or other nonmetallic impurities, thus, making this material conducive for the purification of primary Si [19e21]. Furthermore, the phase diagram of TieSi alloy [22] has a liquidus with a small slope on the high-Si side. During solidification, when a hypereutectic TieSi alloy is cooled from its liquid phase L1 at the eutectic temperature TM, the liquid phase initially decomposes into a pure solid Si phase and reaches the eutectic phase. As the melting point of Si is higher than that of the eutectic composition, TieSi alloy is preferentially nucleated at the tip of solidification, indicating that a large amount of Si can be precipitated when TieSi alloy is cooled within a certain temperature range. Zhu [18] separated Si from a Ti-85 wt% Si eutectic alloy through electromagnetically-induced directional solidification and studied the effects of electromagnetic stirring and pulling rate on the separation performance. In the absence of electromagnetic stirring, the mass transfer rate of Ti in the melt cannot keep up with the pulling rate, leading to incomplete separation of the silicon enrichment layer and the eutectic alloy. When electromagnetic stirring is performed, it enhances the flow of the TieSi melt, which improves the diffusion process in the melt. The resulting ingot separation interface is clear where the bottom is largely composed of Si phase that can be separated from the alloy using a diamond wire cutting machine, without the need for acid leaching, thus, reducing the environmental impact of the preparation of solargrade Si. As Ti-85 wt% Si alloy is not conducive to sampling analysis due to the thickness of the silicon enrichment layer following directional solidification, we performed experiments using electromagnetically-induced directional solidification of Ti-90 wt% Si alloys at different pulling rates. The microstructures of the ingots were observed using an electron probe microanalyzer (EPMA), energy-dispersive X-ray spectroscopy (EDS), and a wavelengthdispersive spectrometer (WDS). We also analyzed the impurity phase compositions and distributions of the ingots. The impurity contents (Ti, Fe, Al, Ca) at different heights in the silicon enrichment layer were measured using inductively coupled plasma optical emission spectrometry (ICP-OES). The composition of the impurity phase on the impurity enrichment region was analyzed using X-ray diffraction (XRD). 2. Experiments For industrial Si and Ti sponges with a mass ratio of 9:1 and weighing 40 g, the impurities introduced by the industrial silicon and titanium sponge were studied. The sample was placed in a graphite crucible with an inner diameter of 20 mm, an outer diameter of 28 mm, and a length of 120 mm, and subjected to a directional solidification rate of 1 mm/s, 3 mm/s, 5 mm/s, 7 mm/s, and 9 mm/s using an electromagnetically-induced furnace. Temperature was measured at the melt surface during solidification using a noncontact infrared thermometer, which was calibrated at the melting point of titanium with fluctuations of about ±15 C. Fig. 1a gives the schematic diagram of the experimental equipment. The parameters for directional solidification pull-down in this study were as follows: frequency of 15.3 kHz, vacuum of 10 Pa, current of 65 A, and power of 6.8 kW. The prepared Ti sponges and industrial Si were placed in a graphite crucible, pre-melted in an induction furnace, and kept at 1550 C for 10 min to ensure that the melts were sufficiently stirred
Fig. 1. Schematic diagram of the electromagnetic induction directional solidification furnace: 1. Gas outlet; 2. Gas inlet; 3. TieSi melt; 4. High-purity graphite crucible; 5. Mechanical pump; 6. Hydraulic elevator platform; 7. Supporting base; 8. Melt flowing; 9. Shell with cooling water; 10. Furnace chamber; 11. Cooling water; 12. Copper induction coil; 13. Viewport; 14. Furnace lid.
to attain uniformity. The Fe, Al, and Ca impurity contents in TieSi alloy ingots that were not subjected to directional solidification after pre-melting are listed in Table 1. After the pre-melting and warming stage, a temperature gradient of 0.5e2.5 C/mm was controlled using directional solidification at a pulling rate of 5 mm/s and a pulling down distance of 5.5 cm, which has been explained in our previous studies [18,21]. After each ingot was solidified and cooled, it was cut with a diamond wire cutter along its longitudinal cross-sectional center. Multi-point sampling was performed along the direction of solidification for each ingot and morphology analysis was carried out using EPMA. The distribution of Si, Ti, Fe, Al, and Ca in the ingots was analyzed via EPMA mapping. To analyze the presence of impurities, the impurity enrichment region of each ingot was analyzed using XRD for phase identification. The X-ray diffractometer used was equipped with a Cu Ka source that was operational at a wavelength of 0.1540 nm. The operating conditions were 40 kV and 30 mA. Scans were conducted at 5 /min with sampling interval of 0.03 . To study electromagnetic stirring and vacuum directional solidification coupling external field strengthening efficiency of impurity removal, three samples were collected along the growth direction of the ingot and the distribution of metal impurity content along the direction of solidification of the ingots were detected using ICP-OES.
3. Results and discussion 3.1. Morphology analysis The structures of the final TieSi alloys were found to be similar for all the five different directional solidification pulling rate Table 1 Initial elemental impurity contents in alloy ingots. Element Content/10
4
wt%
Fe
Al
Ca
2800
1700
260
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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conditions. Fig. 2(a) shows the typical structure of a Ti-90 wt% Si alloy ingot with a pulling rate of 7 mm/s. It can be seen that the TieSi alloy is clearly divided into three layers after directional solidification. From the bottom to the top, the layers are silicon enrichment layer, TieSi eutectic alloy, and impurity enrichment region. Unlike in the case of directional solidification of AleSi alloys [16,17], in this case, as can be seen from Fig. 2(a), there is a clear silicon enrichment layer at the bottom, allowing Si with higher purity to be obtained without acid washing. The black and green dotted lines are the dividing lines, the area above the black dotted line is the impurity enrichment region, the area between the black and green dotted lines is the TieSi eutectic alloy layer, and the area below the green dotted line is the silicon enrichment layer. There are clear cracks between the silicon enrichment layer and TieSi eutectic alloy layer, which aids in the separation of the silicon enrichment layer and TieSi eutectic alloy layer. The ingot was cut using a diamond wire cutter to obtain block L1~L4, R1, and R2 samples, where the height of L1~L3 was 7 mm and the height of L4 and R2 was 5 mm, as shown in Fig. 2(a). After the process of sanding and polishing, the microstructures of samples R1 and R2 was analyzed by EPMA. In addition, the distribution of Si, Ti, Fe, Al, and Ca was analyzed by using wavelength-dispersive spectrometer mapping. The samples L1~L4 were ground into powder after homogeneous mixing; the L1~L3 powder samples were selected for ICP detection to analyze the longitudinal distribution of impurities in the silicon enrichment layer. XRD analysis was carried out on the powder samples of L4 to identify the phase formed by the impurities. In our previous research [21], we investigated the influence of pulling rate on the height of the silicon enrichment layer; and found that the height exhibited an increasing trend with a decrease in the pulling rate. Fig. 3 shows the height and purity of the silicon enrichment layer measured in this experiment. The blue value indicates the average purity of silicon in the silicon enrichment layer and the black value indicates the height of the silicon enrichment
Fig. 2. Structure of TieSi alloy: (a) macro view of TieSi alloy ingot center cross section; (b) SEM ingot diagrams of directional solidification of TieSi alloys: (A-1)~(E 1) under 200 times magnification; (A-2)~(E 2) under 500 times magnification.
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layer. It can be found that the height ratio of the silicon enrichment layer gradually decreases at first, and then, tends to flatten. Moreover, there is a minimum limit on the height ratio. At this point, the purity of silicon does not rise. This is because the pulling rate is too rapid and the rate of advancement upwards of the liquid-solid interface is less than the pulling rate, causing the formation of a mushy zone between the liquid-solid interface and the melt. The appearance of the mushy zone greatly affects the growth of silicon crystals and the mass transfer rate of impurities during impurity segregation at the liquid-solid interface. The faster the pulling rate, the greater is the thickness of the mushy zone followed by a greater effect on the growth of silicon crystals and the mass transfer rate of impurities. Furthermore, the higher the height of the silicon enrichment layer, the more impurities remain in the silicon enrichment layer followed by lower purity of silicon in this layer. The lowering of the pulling rate reduces the thickness of the mushy zone and facilitates the upward migration of impurities, thereby, facilitating the purification of silicon in the silicon enrichment layer. When the pulling rate is reduced to 5 mm/s, the rate of upward advances of the liquid-solid interface was basically equal to the pulling rate. When the pulling rate continues to decrease, the upward rate of the liquid-solid interface can only be equal to this value, but cannot exceed it. Eventually, the height of the mushy zone is reduced to the lower limit and the pulling rate has minimal effect on the growth of silicon crystals and the mass transfer rate of impurities. Finally, the pulling rate is no longer the main factor for impurity removal, the height ratio of the silicon enrichment layer is no longer reduced, and the purity of silicon is no longer improved. Fig. 2(b) shows the magnified microstructure of the ingot after directional solidification of the TieSi alloy. The microstructure of region A in Fig. 2(a) corresponds to Figs. 2(A-1) and Fig. 2(A-2) in Fig. 2(b), and the corresponding rule of the B ~ E region is the same as region A. Figs. 2(A-1) - 2(E 1) show the microstructure with a magnification of 100 times, and Fig. 2(A-2) - 2(E 2) show the microstructure with a magnification of 500 times. It can be seen from Figs. 2(A-1) - 2(E 1) that the impurity has a tendency to decrease significantly in the longitudinal direction, and a highpurity Si phase region exists at the bottom of the silicon enrichment layer as shown in Figs. 2(B-1). However, large amounts of impurities are also found at the bottom of the ingot, that is, at the corners of the side wall and bottom of the crucible as shown in Figs. 2(A-1). This is because the polishing of graphite crucibles in these areas is not perfect, leading to a large amount of graphite debris. Moreover, as the energy of nucleation of impurities on these debris is low, it easily leads to heterogeneous nucleation that grows upward, thus, causing large amounts of impurity enrichment in these regions, eventually forming a small range of impurity phase enrichment regions. In Fig. 2(C-2), one can see that TiSi2 is largely dispersed on the Si matrix in the form of dendrites or rods with impurities coated onto the grain boundaries of some of this TiSi2. In the longitudinal direction in Fig. 2(B-2) - 2(E 2) from top to bottom, it can be seen that the TiSi2 content is significantly reduced. Additionally, the impurity phase tends to decrease gradually. In Fig. 2(D-2), one can see that TiSi2 is largely distributed in a strip shape or needle shape in the Si matrix with no obvious impurity phase. In Fig. 2(E 2), one can see that TiSi2 grains without impurities are uniformly distributed on the Si matrix in the form of needles or rods. TiSi2 grains with impurity enrichment at the grain boundaries are distributed irregularly in the Si matrix in the form of rods or spheres. One can see that the impurities are largely segregated and concentrated on the grain boundaries without any significant crystal segregation, indicating that the impurities concentrated in the grain boundaries can be effectively removed via pickling. In general, the ingots of TieSi are different from the ingots of AleSi alloy after directional solidification [16,17]. When TieSi
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Fig. 3. The height and purity of silicon enrichment layer in an ingot after directional solidification of TieSi alloy
alloy undergoes directional solidification, impurities are mainly enriched at the top of the ingot, and rarely precipitate in the silicon enrichment layer. There is a relatively pure silicon phase area in the silicon enrichment layer, therefore, this layer can be separated from the TieSi eutectic alloy layer with the use of a diamond wire cutting machine to obtain relatively highly pure silicon.
3.2. Impurity phase analysis As the impurity content in the silicon enrichment layers is low, the impurity phase cannot be analyzed accurately using XRD. However, the impurity enrichment regions at the top of the ingots contain a large amount of Fe, which facilitates characterization of the phase formed by Fe. Therefore, the impurity phase in the impurity enrichment region of an alloy and the impurity phase in the silicon enrichment layer were compared and analyzed. Fig. 4 presents the results of the EPMA surface analysis of the impurity phase of the silicon enrichment layer and impurity enrichment region. The results reveal that Fe, Al, and Ca are largely distributed on the grain boundaries of TiSi2 and not inside. Furthermore, the SieTieFe impurity phase is dominant. Compared to the directional solidification of AleSi alloys [16,17] and metallurgical grade Si [23], the impurities are clearly more concentrated on the grain boundaries of TiSi2 without obvious crystal segregation, which is beneficial for the purification of directionally solidified silicon enrichment layers via pickling to obtain highly purity silicon. Fig. 5 and Fig. 6 present the energy spectra of the impurity phase of the silicon enrichment layer and the impurity enrichment region, respectively. From Fig. 5, one can see that, after precipitation, TiSi2 gradually grows into a crystal phase (point 3; Ti:Si z 1:2). Thereafter, the grain boundaries form a FeeTieSi phase. From the inside to the outside, the Fe content gradually increases until it reaches a maximum of approximately 6.76 at.%. In Fig. 6, one can see that Fe content gradually increases in the alloy as well until it reaches a maximum value of approximately 7.72 at.%. This difference of
approximately 1% can be attributed to different concentration fluctuations of Fe in the melts. As Fe has a small segregation coefficient in Si, directional solidification progresses to be largely distributed in the liquid phase. As the solid-liquid interface moves upward, the melt gets reduced and the concentration of Fe gradually increases. Due to lower energy impurity segregation, metal impurities segregate at the grain boundary of TiSi2, and Fe eventually combines with TiSi2 and forms FeTiSi2 at the grain boundary. Therefore, in the longitudinal direction of the ingot, TiSi2 is enriched in Fe, giving an increasing trend in terms of concentration. In addition, it can be noted from the energy spectrum results that the atomic ratios of Fe in the impurity phase of the silicon enrichment layer and impurity enrichment region are similar at approximately 4e8 at.%. Additionally, the phase compositions of the two samples are consistent. The powder in the impurity enrichment region at the top of the alloy was also subjected to XRD analysis. The results are shown in Fig. 7. The results demonstrate that the composition of the alloy is largely divided into four phases, namely a, b, g, and ε in the XRD pattern, which correspond to Si, TiSi2, FeTiSi2, and FeAl2.7Si2.3, respectively. Following full-spectrum fitting and finishing, the contents of the phases were found to be 47.4%, 22.8%, 34.2%, and 0.6%, respectively. The Si, Ti, Fe, and Al ratios were 66.92%, 20.73%, 12.12%, and 0.23%, respectively. In the Xray scanning region, Fe exhibited clear enrichment in the alloy with content as high as 12.12 wt%. The main composition phase is FeTiSi2 with a small amount of FeAl2.7Si2.3. Fig. 8 presents the migration mechanism of the main metal element in the Si melt during the pull-down process. Firstly, at high temperatures, impurities are uniformly distributed in the Si melt because of electromagnetic stirring [18,23]. After the initiation of pull-down, a small amount of TiSi2 crystals precipitate at the bottom of the crucible based on the corresponding temperature drop. This phenomenon occurs because the impurities undergo heterogeneous nucleation at the bottom of the silicon enriched layer, which is mainly due to poor polishing of the graphite at the corners between the bottom and side walls of the crucible. It is well known
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Fig. 4. Impurity distributions in Si measured via EPMA-mapping: (a) the upper part of silicon enrichment layer near the TieSi eutectic alloy and (b) the impurity enrichment region
Fig. 5. EDS results for residual impurities in silicon enrichment layer
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Fig. 6. Results of electron probe dotting in the impurity enrichment region.
Fig. 7. XRD results for the impurity enrichment region.
that the energy of heterogeneous nucleation is much lower than the required energy for homogeneous nucleation [24]. Therefore, the solidification of metals and alloys always occurs through heterogeneous nucleation. Based on the high temperature during the solidification stage, the precipitated TiSi2 crystals act as the impurity in the Si melt outside the Si element, which means that they will first heterogeneously nucleate in the melt, and then, grow excessively at the bottom, leading to a small amount of impurities at the bottom of the final ingot. As the segregation coefficients of metal impurities are generally low during the process of directional solidification, impurities will be redistributed at the solid-liquid interface, eventually leading to their upward migration. During this process, impurities will be evenly distributed by electromagnetic stirring. When directional solidification is almost complete, the impurity concentration reaches the partial condensation limit,
thus, leading to the impurities being precipitated together with the TieSi eutectic alloy at the top. Due to the low energy of impurity segregation, metal impurities segregate at the grain boundary of TiSi2, forming an ingot similar to that shown in Fig. 2(a). 3.3. Impurity removal in Si The electromagnetic stirring [18,21] mainly forms from the Lorentz force acting on the melt, combining the eddy current in the melt and the penetration depth of the magnetic induction. The Lorentz force is applied to the impurity atoms or clusters in the melt to promote their enrichment toward the solidification front. As the segregation coefficients [25] of metal impurities (Ti: 2 10 6, Fe: 6.4 10 6, Al: 8.0 10 2, Ca: 1.6 10 3) are smaller than 1, the impurities will segregate into the melt during directional
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Fig. 8. Mechanism of impurity removal in electromagnetically-induced directional solidification of TieSi alloys.
solidification. At the same time, due to the obvious temperature gradient at the solidification front, the front end of the liquid-solid phase of the melt forms a thermoelectric potential. This leads to the formation of the thermal electromagnetic force in the axial and radial direction of the ingot. This promotes the agitation of the melt in the axial direction and the reflow of the melt in the radial direction at the solidification front, both of which improve the mass transfer characteristics of the atoms in the melt. And then this facilitates the migration and enrichment of metal impurity atoms or clusters toward the solidification front and eventually segregation into the liquid phase. In addition, controlling the rational directional solidification parameters is beneficial to reduce the impurity phase in the silicon enrichment layer. The decrease in the pulling rate increases the mass transfer time of impurities in the melt, such that metal impurity atoms or clusters have more time to migrate to the solidification front and accumulate, thereby, redistributing at the liquid-solid interface and finally, segregating into the liquid phase. Therefore, coupling to the electromagnetic field and directional solidification can improve the migration of impurities in the melt, thereby, achieving efficient removal of impurities in the silicon enrichment layer. The concentrations of impurities (Fe, Al, Ca, and Ti) at different heights in the silicon enrichment layer were measured using ICPOES. The results are shown in Fig. 9. The contents of each impurity element at different heights in the silicon enrichment layer exhibit trends similar to the Ti concentrations with different directional solidification pulling rates. When the pulling rate is 9 mm/s or 7 mm/s, due to the mushy zone, the microscopic migration rate of Ti is smaller than the advancing rate of the solid - liquid interface. This means that a large amount of Ti remains in the upper portion of the silicon enrichment layer. Additionally, Fe, Al, and Ca exhibit a similar distribution to Ti. Metal impurity contents above a pulling rate of 5 mm/s are typically of the same order of magnitude. Based on the analysis in Section 3.2, this is because Ti largely exists in the form of TiSi2 in Si and the impurities segregate on the grain boundaries of TiSi2. At a low pulling rate, the Fe content can be reduced from 2800 10 4 wt% to 9 10 4 wt%, the Al content can be reduced from 1700 10 4 wt% to 21 10 4 wt%; and the Ca content can be reduced from 200 10 4 wt% to 10 10 4 wt%.
These changes represent removal rates of 99.68%, 98.76%, and 95%, respectively. As shown in Fig. 9, in the range of 0e7 mm, the content of Ti decreases initially, then, begins to increase and a minimum value of 350 ppmw to exist, which is related to the heterogeneous nucleation illustrated in Figs. 2 and 8. Ti is the most concentrated element in the melt, apart from Si. At the bottom of the silicon enrichment layer, some Ti and Si combine to form TiSi2, which act as centers for heterogeneous nucleation. However, because of directional solidification, the segregation coefficient of Ti is much smaller than one, which means that most of the Ti will migrate upward. Following nucleation and excessive growth, TiSi2 is immediately coated with Si, which means that the bottom of the silicon enrichment layer has a Ti concentration lower limit (350 ppmw). So when the thickness of the silicon enrichment layer is in the range of 0e7 mm, heterogeneous nucleation is present making the content of Ti relatively high. The content of Ti intercepted in the upper area of the silicon enrichment layer is related to the pulling rate during directional solidification. The greater the pulling rate, the more Ti is intercepted. To reduce the time required for directional solidification and remove as much Ti as possible from the silicon enrichment layer, a pulling rate of 5 mm/s is appropriate. This is also consistent with the analysis of the height and purity of the silicon enrichment layer discussed in the previous section. At this pulling rate, the average Ti content remaining in the silicon enrichment layer is 583 ppmw. For a TieSi alloy with an initial Ti content of 10 wt%, the removal rate of Ti in the silicon enrichment layer is 99.4%. Fig. 10 shows the average removal rate of Fe, Al, and Ca in the silicon enrichment layer after directional solidification of a TieSi alloy. One can see that as the pulling rate decreases, the average removal rate of impurities increases and the gradually becomes flat. However, for Ca, excessively low pulling rates are not conducive to the removal of impurities; the specific reasons are still under further study. Under the experimental conditions in this study, the optimal pulling rate was 5 mm/s. At this rate, the removal rates of Fe and Al approach their limits and the removal rate of Ca shows significant improvement. Using an optimal rate can reduce directional solidification time and result in greater removal efficiency.
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Fig. 9. Impurity content in the silicon enrichment layer
Ca in the silicon enrichment layer were 99.5%, 98.765%, and 85.83%, respectively. In addition, during the early stages of directional solidification, based on heterogeneous nucleation, impurities also exhibited a small amount of enrichment at the bottom of the silicon enrichment layer. Acknowledgments This research was supported by the National Natural Science Foundation of China project (No.U1702251), Special Funds of State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization (Grant No. CNMRCUTS1604), the Reserve Talents of Young and Middle-aged Academic and Technical Leaders in Yunnan Province (2018HB009), and the Program for Innovative Research Team in University of Ministry of Education of China (No. IRT17R48). Fig. 10. Average removal rate of impurities in the silicon enrichment layer at different pulling rates
The average removal rates of Fe, Al, and Ca in this study were 98.431%, 99.262%, and 85.833%, respectively. 4. Conclusions The mechanism of impurity removal in the silicon enrichment layer during directional solidification of Ti-90 wt%Si alloy was analyzed. Due to heterogeneous nucleation, TiSi2 first precipitated in the melt during directional solidification. Next, TiSi2 formed the FeTiSi2 phase with Fe, which was enriched on the grain boundaries of TiSi2. Following the directional solidification of TieSi alloys, impurities were found to be concentrated on the grain boundaries of TiSi2 without any significant crystal segregation. Furthermore, under the experimental conditions in this study, the optimal pulling rate was identified to be 5 mm/s. At this pulling rate, directional solidification time can be reduced and excellent removal efficiency can be achieved. The average impurity removal rates of Fe, Al, and
References [1] Y. Lei, L.E. Sun, W.H. Ma, K.X. Wei, K. Morita, Enhancing B removal from Si with small amounts of Ti in electromagnetic solidification refining with AleSi alloy, J. Alloy. Comp. 666 (2016) 406e411. [2] H. Chen, Y. Wang, W.J. Zheng, Q.C. Li, X.Z. Yuan, K. Morita, Model implementation of boron removal using CaCl2-CaO-SiO2 slag system for solar-grade silicon, Metall. Mater. Trans. B 48 (2017) 3219e3227. [3] A.A. Istratov, H. Hieslmair, E.R. Weber, Iron and its complexes in silicon, Appl. Phys. A-Mater. 69 (1) (1999) 13e44. [4] B. Gao, S. Nakano, K. Kakimoto, Influence of back-diffusion of iron impurity on lifetime distribution near the seed-crystal interface in seed cast-grown monocrystalline silicon by numerical modeling, Cryst. Growth Des. 12 (2011) 522e525. [5] X. Peng, W. Dong, Y. Tan, D.C. Jiang, Q. Wang, G.B. Li, Evaporation behavior of calcium in metallurgical silicon by an electron beam melting method, J. Funct. Mater. 41 (2010) 117e120. [6] S.P. Huang, W.H. Ma, K.X. Wei, S.Y. Li, K. Morita, A model for distribution of aluminum in Si refined by vacuum directional solidification, Vacuum 96 (2013) 12e17. [7] J. Safarian, M. Tangstad, Vacuum refining of molten silicon, Metall. Mater. Trans. B 43 (2012) 1427e1445. [8] Y. Tan, S.Q. Ren, S. Shi, S.T. Wen, D.C. Jiang, W. Dong, M. Ji, S.H. Sun, Removal of aluminum and calcium in multicrystalline silicon by vacuum induction melting and directional solidification, Vacuum 99 (2014) 272e276.
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J. Hu et al. / Journal of Alloys and Compounds xxx (xxxx) xxx [9] T.F. Ciszek, G.H. Schwuttke, K.H. Yang, Solar-grade silicon by directional solidification in carbon crucibles, IBM J. Res. Dev. 23 (2010) 270e277. [10] Y. Wang, K. Morita, Measurement of the phase diagram of the SiO2-CaCl2 system and liquid area study of the SiO2-CaO-CaCl2 system, Metall. Mater. Trans. B 47 (2016) 1542e1547. [11] H. Chen, X.Z. Yuan, K. Morita, Y.J. Zhong, X.D. Ma, Z.Y. Chen, Y. Wang, Reaction mechanism and kinetics of boron removal from molten silicon via CaO-SiO2CaCl2 slag treatment and ammonia injection, Metall. Mater. Trans. B 50 (2019) 2088e2094. [12] X. D Ma, T. Yoshikawa, K. Morita, Si growth by directional solidification of SieSn alloys to produce solar-grade Si, J. Cryst. Growth 377 (2013) 192e196. [13] W.Z. Yu, W.H. Ma, Z. Zheng, Y. Lei, W.Y. Jiang, J. Li, Si purification by removal of entrapped Al during electromagnetic solidification refining of Si-Al alloy, Metall. Mater. Trans. B 48 (2017) 2804e2811. [14] S.D. Hu, Y.C. Dai, A. Gagnoud, Y. Fautrelle, R. Moreau, Z.M. Ren, K. Deng, C.J. Li, X. Li, Effect of a magnetic field on macro segregation of the primary silicon phase in hypereutectic Al-Si alloy during directional solidification, J. Alloy. Comp. 722 (2017) 108e115. [15] W.Z. Yu, Y. Xue, J. Mei, X.Z. Zhou, M.L. Xiong, S.F. Zhang, Segregation and removal of transition metal impurities during the directional solidification refining of silicon with Al-Si solvent, J. Alloy. Comp. 805 (2019) 198e204. [16] X.L. Bai, B.Y. Ban, J.W. Li, Z.Q. Fu, Z.J. Peng, C.B. Wang, J. Chen, Effect of Ti addition on B removal during Si refining in Al-30% Si alloy directional solidification, Separ. Purif. Technol. 174 (2017) 345e351. [17] Y. Lei, L.E. Sun, W.H. Ma, K.X. Wei, K. Morita, Enhancing B removal from Si
[18]
[19]
[20]
[21]
[22]
[23] [24] [25]
9
with small amounts of Ti in electromagnetic solidification refining with AleSi alloy, J. Alloy. Comp. 666 (2016) 406e411. K.S. Zhu, J.F. Hu, W.H. Ma, K.X. Wei, T.L. Lv, Y.N. Dai, Effects of solidification parameters and magnetic field on separation of primary Si from hypereutectic Tie85 wt.% Si melt, J. Cryst. Growth 522 (2019) 78e95. Y. Lei, L.E. Sun, W.H. Ma, X.D. Ma, J.J. Wu, S.Y. Li, K. Morita, An approach to employ titanium-bearing blast-furnace slag to prepare Ti and AleSi alloys, J. Alloy. Comp. 769 (2018) 983e990. Q.Y. Huang, X.W. Lv, R. Huang, J.J. Song, Preparation of TieSieAl alloy by aluminothermic reduction of TiO2 bearing blast furnace slag, Can. Metall. Q. 52 (2013) 413e421. K.S. Zhu, W.H. Ma, K.X. Wei, Y. Lei, J.F. Hu, T.L. Lv, Y.N. Dai, Separation mechanism of TiSi2 crystals from a Ti-Si eutectic alloy via directional solidification, J. Alloy. Comp. 750 (2018) 102e110. Scientific Group Thermodata Europe (SGTE), Binary system Si-Ti, in: Ternary Steel Systems: Phase Diagrams and Phase Transition Data, Springer, Berlin, Heidelberg, 2012, 59-59. M.A. Martorano, J.B.F. Neto, T.S. Oliveira, T.O. Tsubaki, Refining of metallurgical silicon by directional solidification, Mater. Sci. Eng. B 176 (2011) 217e226. D. Turnbull, Kinetics of heterogeneous nucleation, J. Chem. Phys. 18 (1950) 198e203. Y. Lei, C. Wang, W.H. Ma, J.J. Wu, K.X. Wei, S.Y. Li, G.Q. Lv, K. Morita, A novel approach to prepare high-purity Si and Si/TiSi2 materials simultaneously using Ti-bearing blast furnace slag, J. Alloy. Comp. 798 (2019) 333e341.
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification Jingfei Hu a, Kuisong Zhu a, b, c, Kuixian Wei a, *, Wenhui Ma a, c, **, Tianlong Lv a a
The National Engineering Laboratory for Vacuum Metallurgy, State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China b Panzhihua School of Vanadium and Titanium, Panzhihua University, Panzhihua, 617000, China c Key Laboratory for Nonferrous Vacuum Metallurgy of Yunnan Province, Engineering Research Center for Silicon Metallurgy and Silicon Materials of Yunnan Provincial Universities, Kunming University of Science and Technology, Kunming, 650093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2019 Received in revised form 17 September 2019 Accepted 8 October 2019 Available online xxx
To analyze the effects of pulling rate on the removal of impurities from a Ti-90 wt% Si alloy during the Si refining process, five samples with different pulling rates were examined. At a low pulling rate, the contents of Fe, Al, and Ca could be reduced to 9 10 4 wt%, 21 10 4 wt%, and 10 10 4 wt%, representing impurity removal rates of 99.68%, 98.76%, and 95%, respectively. Under our experimental conditions, we determined that a pulling rate of 5 mm/s can reduce the time taken for directional solidification and provide excellent impurity removal efficiency. The average impurity removal rates for Fe, Al, and Ca in the silicon enrichment layer were 98.431%, 99.262%, and 85.833%, respectively. The migration and removal mechanism of Fe during the directional solidification refining process was also identified. Due to the low energy of grain boundary segregation of impurities, Fe segregate at the grain boundary of TiSi2 grains, such that Fe eventually forms FeTiSi2 with TiSi2 at the grain boundary. The analysis of our study provides theoretical and experimental support for the purification of polycrystalline silicon from TieSi alloys, which further broadens the application of TieSi alloys and lays the foundation for comprehensive utilization of high-titanium blast furnace slag. © 2019 Elsevier B.V. All rights reserved.
Keywords: TieSi alloys Directional solidification Metal impurity removal Pulling rate Silicon refining
1. Introduction With the rapid development of the photovoltaic industry, several potential methods for recycling solar-grade silicon are currently under development. In the photovoltaic industry, Sibased solar cells have attracted significant attention due to their efficient performance, abundant raw materials, and long service life. The main raw material in the Si-based solar cells is solar-grade Si [1,2]. The impurity content of solar-grade Si has a significant impact on the photoelectric conversion efficiency of solar cells [3,4]. Fe, Al, and Ca are the most common metal impurities in Si and their existence affects the minority carrier lifetime of solar cells [4e6]. Additionally, Al and Ca also directly affect the resistivity of solar
* Corresponding author. ** Corresponding author. The National Engineering Laboratory for Vacuum Metallurgy, State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China. E-mail addresses: [email protected] (K. Wei), [email protected] (W. Ma).
cells, which affects their power generation efficiency [5,6], indicating that they must be removed to meet the performance requirements of solar cells. Scholars from around the world have conducted research on the removal of impurities from solar-grade Si using methods such as vacuum refining [7,8], directional solidification [9], slag refining [10,11], and alloy refining [12,13]. Among these methods, the preparation of solar-grade Si using alloy refining is one of the most popular solutions [13e15]. Research on alloy refining largely concentrates on AleSi alloys; however, the separation interface for AleSi alloy refining [14,15] is not clear. Additionally, primary Si is distributed in the Al matrix after refining, which necessitates the use of acid leaching to obtain primary and eutectic Si. The acid consumption for this process is significant with the presence of crystal boundary segregation of impurities in the obtained Si. Additionally, it is well known that directional solidification is suboptimal for removing nonmetallic impurities, such as B and P. Bai [16] and Lei [17] conducted studies on the addition of titanium (Ti) to remove B from AleSi alloys and obtained promising removal results. Microscopic morphology analysis revealed that the crystal
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Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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boundary segregation of impurities was also improved. Recently, Zhu et al. [18] proposed a novel TieSi alloy material for preparing Si using directional solidification. This material is prepared through the aluminothermic reduction (ATR) of titaniumbearing blast furnace slag. The advantage of this raw material is that TieSi alloys prepared from Ti slag through thermal reduction of metals do not contain B, P, or other nonmetallic impurities, thus, making this material conducive for the purification of primary Si [19e21]. Furthermore, the phase diagram of TieSi alloy [22] has a liquidus with a small slope on the high-Si side. During solidification, when a hypereutectic TieSi alloy is cooled from its liquid phase L1 at the eutectic temperature TM, the liquid phase initially decomposes into a pure solid Si phase and reaches the eutectic phase. As the melting point of Si is higher than that of the eutectic composition, TieSi alloy is preferentially nucleated at the tip of solidification, indicating that a large amount of Si can be precipitated when TieSi alloy is cooled within a certain temperature range. Zhu [18] separated Si from a Ti-85 wt% Si eutectic alloy through electromagnetically-induced directional solidification and studied the effects of electromagnetic stirring and pulling rate on the separation performance. In the absence of electromagnetic stirring, the mass transfer rate of Ti in the melt cannot keep up with the pulling rate, leading to incomplete separation of the silicon enrichment layer and the eutectic alloy. When electromagnetic stirring is performed, it enhances the flow of the TieSi melt, which improves the diffusion process in the melt. The resulting ingot separation interface is clear where the bottom is largely composed of Si phase that can be separated from the alloy using a diamond wire cutting machine, without the need for acid leaching, thus, reducing the environmental impact of the preparation of solargrade Si. As Ti-85 wt% Si alloy is not conducive to sampling analysis due to the thickness of the silicon enrichment layer following directional solidification, we performed experiments using electromagnetically-induced directional solidification of Ti-90 wt% Si alloys at different pulling rates. The microstructures of the ingots were observed using an electron probe microanalyzer (EPMA), energy-dispersive X-ray spectroscopy (EDS), and a wavelengthdispersive spectrometer (WDS). We also analyzed the impurity phase compositions and distributions of the ingots. The impurity contents (Ti, Fe, Al, Ca) at different heights in the silicon enrichment layer were measured using inductively coupled plasma optical emission spectrometry (ICP-OES). The composition of the impurity phase on the impurity enrichment region was analyzed using X-ray diffraction (XRD). 2. Experiments For industrial Si and Ti sponges with a mass ratio of 9:1 and weighing 40 g, the impurities introduced by the industrial silicon and titanium sponge were studied. The sample was placed in a graphite crucible with an inner diameter of 20 mm, an outer diameter of 28 mm, and a length of 120 mm, and subjected to a directional solidification rate of 1 mm/s, 3 mm/s, 5 mm/s, 7 mm/s, and 9 mm/s using an electromagnetically-induced furnace. Temperature was measured at the melt surface during solidification using a noncontact infrared thermometer, which was calibrated at the melting point of titanium with fluctuations of about ±15 C. Fig. 1a gives the schematic diagram of the experimental equipment. The parameters for directional solidification pull-down in this study were as follows: frequency of 15.3 kHz, vacuum of 10 Pa, current of 65 A, and power of 6.8 kW. The prepared Ti sponges and industrial Si were placed in a graphite crucible, pre-melted in an induction furnace, and kept at 1550 C for 10 min to ensure that the melts were sufficiently stirred
Fig. 1. Schematic diagram of the electromagnetic induction directional solidification furnace: 1. Gas outlet; 2. Gas inlet; 3. TieSi melt; 4. High-purity graphite crucible; 5. Mechanical pump; 6. Hydraulic elevator platform; 7. Supporting base; 8. Melt flowing; 9. Shell with cooling water; 10. Furnace chamber; 11. Cooling water; 12. Copper induction coil; 13. Viewport; 14. Furnace lid.
to attain uniformity. The Fe, Al, and Ca impurity contents in TieSi alloy ingots that were not subjected to directional solidification after pre-melting are listed in Table 1. After the pre-melting and warming stage, a temperature gradient of 0.5e2.5 C/mm was controlled using directional solidification at a pulling rate of 5 mm/s and a pulling down distance of 5.5 cm, which has been explained in our previous studies [18,21]. After each ingot was solidified and cooled, it was cut with a diamond wire cutter along its longitudinal cross-sectional center. Multi-point sampling was performed along the direction of solidification for each ingot and morphology analysis was carried out using EPMA. The distribution of Si, Ti, Fe, Al, and Ca in the ingots was analyzed via EPMA mapping. To analyze the presence of impurities, the impurity enrichment region of each ingot was analyzed using XRD for phase identification. The X-ray diffractometer used was equipped with a Cu Ka source that was operational at a wavelength of 0.1540 nm. The operating conditions were 40 kV and 30 mA. Scans were conducted at 5 /min with sampling interval of 0.03 . To study electromagnetic stirring and vacuum directional solidification coupling external field strengthening efficiency of impurity removal, three samples were collected along the growth direction of the ingot and the distribution of metal impurity content along the direction of solidification of the ingots were detected using ICP-OES.
3. Results and discussion 3.1. Morphology analysis The structures of the final TieSi alloys were found to be similar for all the five different directional solidification pulling rate Table 1 Initial elemental impurity contents in alloy ingots. Element Content/10
4
wt%
Fe
Al
Ca
2800
1700
260
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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conditions. Fig. 2(a) shows the typical structure of a Ti-90 wt% Si alloy ingot with a pulling rate of 7 mm/s. It can be seen that the TieSi alloy is clearly divided into three layers after directional solidification. From the bottom to the top, the layers are silicon enrichment layer, TieSi eutectic alloy, and impurity enrichment region. Unlike in the case of directional solidification of AleSi alloys [16,17], in this case, as can be seen from Fig. 2(a), there is a clear silicon enrichment layer at the bottom, allowing Si with higher purity to be obtained without acid washing. The black and green dotted lines are the dividing lines, the area above the black dotted line is the impurity enrichment region, the area between the black and green dotted lines is the TieSi eutectic alloy layer, and the area below the green dotted line is the silicon enrichment layer. There are clear cracks between the silicon enrichment layer and TieSi eutectic alloy layer, which aids in the separation of the silicon enrichment layer and TieSi eutectic alloy layer. The ingot was cut using a diamond wire cutter to obtain block L1~L4, R1, and R2 samples, where the height of L1~L3 was 7 mm and the height of L4 and R2 was 5 mm, as shown in Fig. 2(a). After the process of sanding and polishing, the microstructures of samples R1 and R2 was analyzed by EPMA. In addition, the distribution of Si, Ti, Fe, Al, and Ca was analyzed by using wavelength-dispersive spectrometer mapping. The samples L1~L4 were ground into powder after homogeneous mixing; the L1~L3 powder samples were selected for ICP detection to analyze the longitudinal distribution of impurities in the silicon enrichment layer. XRD analysis was carried out on the powder samples of L4 to identify the phase formed by the impurities. In our previous research [21], we investigated the influence of pulling rate on the height of the silicon enrichment layer; and found that the height exhibited an increasing trend with a decrease in the pulling rate. Fig. 3 shows the height and purity of the silicon enrichment layer measured in this experiment. The blue value indicates the average purity of silicon in the silicon enrichment layer and the black value indicates the height of the silicon enrichment
Fig. 2. Structure of TieSi alloy: (a) macro view of TieSi alloy ingot center cross section; (b) SEM ingot diagrams of directional solidification of TieSi alloys: (A-1)~(E 1) under 200 times magnification; (A-2)~(E 2) under 500 times magnification.
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layer. It can be found that the height ratio of the silicon enrichment layer gradually decreases at first, and then, tends to flatten. Moreover, there is a minimum limit on the height ratio. At this point, the purity of silicon does not rise. This is because the pulling rate is too rapid and the rate of advancement upwards of the liquid-solid interface is less than the pulling rate, causing the formation of a mushy zone between the liquid-solid interface and the melt. The appearance of the mushy zone greatly affects the growth of silicon crystals and the mass transfer rate of impurities during impurity segregation at the liquid-solid interface. The faster the pulling rate, the greater is the thickness of the mushy zone followed by a greater effect on the growth of silicon crystals and the mass transfer rate of impurities. Furthermore, the higher the height of the silicon enrichment layer, the more impurities remain in the silicon enrichment layer followed by lower purity of silicon in this layer. The lowering of the pulling rate reduces the thickness of the mushy zone and facilitates the upward migration of impurities, thereby, facilitating the purification of silicon in the silicon enrichment layer. When the pulling rate is reduced to 5 mm/s, the rate of upward advances of the liquid-solid interface was basically equal to the pulling rate. When the pulling rate continues to decrease, the upward rate of the liquid-solid interface can only be equal to this value, but cannot exceed it. Eventually, the height of the mushy zone is reduced to the lower limit and the pulling rate has minimal effect on the growth of silicon crystals and the mass transfer rate of impurities. Finally, the pulling rate is no longer the main factor for impurity removal, the height ratio of the silicon enrichment layer is no longer reduced, and the purity of silicon is no longer improved. Fig. 2(b) shows the magnified microstructure of the ingot after directional solidification of the TieSi alloy. The microstructure of region A in Fig. 2(a) corresponds to Figs. 2(A-1) and Fig. 2(A-2) in Fig. 2(b), and the corresponding rule of the B ~ E region is the same as region A. Figs. 2(A-1) - 2(E 1) show the microstructure with a magnification of 100 times, and Fig. 2(A-2) - 2(E 2) show the microstructure with a magnification of 500 times. It can be seen from Figs. 2(A-1) - 2(E 1) that the impurity has a tendency to decrease significantly in the longitudinal direction, and a highpurity Si phase region exists at the bottom of the silicon enrichment layer as shown in Figs. 2(B-1). However, large amounts of impurities are also found at the bottom of the ingot, that is, at the corners of the side wall and bottom of the crucible as shown in Figs. 2(A-1). This is because the polishing of graphite crucibles in these areas is not perfect, leading to a large amount of graphite debris. Moreover, as the energy of nucleation of impurities on these debris is low, it easily leads to heterogeneous nucleation that grows upward, thus, causing large amounts of impurity enrichment in these regions, eventually forming a small range of impurity phase enrichment regions. In Fig. 2(C-2), one can see that TiSi2 is largely dispersed on the Si matrix in the form of dendrites or rods with impurities coated onto the grain boundaries of some of this TiSi2. In the longitudinal direction in Fig. 2(B-2) - 2(E 2) from top to bottom, it can be seen that the TiSi2 content is significantly reduced. Additionally, the impurity phase tends to decrease gradually. In Fig. 2(D-2), one can see that TiSi2 is largely distributed in a strip shape or needle shape in the Si matrix with no obvious impurity phase. In Fig. 2(E 2), one can see that TiSi2 grains without impurities are uniformly distributed on the Si matrix in the form of needles or rods. TiSi2 grains with impurity enrichment at the grain boundaries are distributed irregularly in the Si matrix in the form of rods or spheres. One can see that the impurities are largely segregated and concentrated on the grain boundaries without any significant crystal segregation, indicating that the impurities concentrated in the grain boundaries can be effectively removed via pickling. In general, the ingots of TieSi are different from the ingots of AleSi alloy after directional solidification [16,17]. When TieSi
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Fig. 3. The height and purity of silicon enrichment layer in an ingot after directional solidification of TieSi alloy
alloy undergoes directional solidification, impurities are mainly enriched at the top of the ingot, and rarely precipitate in the silicon enrichment layer. There is a relatively pure silicon phase area in the silicon enrichment layer, therefore, this layer can be separated from the TieSi eutectic alloy layer with the use of a diamond wire cutting machine to obtain relatively highly pure silicon.
3.2. Impurity phase analysis As the impurity content in the silicon enrichment layers is low, the impurity phase cannot be analyzed accurately using XRD. However, the impurity enrichment regions at the top of the ingots contain a large amount of Fe, which facilitates characterization of the phase formed by Fe. Therefore, the impurity phase in the impurity enrichment region of an alloy and the impurity phase in the silicon enrichment layer were compared and analyzed. Fig. 4 presents the results of the EPMA surface analysis of the impurity phase of the silicon enrichment layer and impurity enrichment region. The results reveal that Fe, Al, and Ca are largely distributed on the grain boundaries of TiSi2 and not inside. Furthermore, the SieTieFe impurity phase is dominant. Compared to the directional solidification of AleSi alloys [16,17] and metallurgical grade Si [23], the impurities are clearly more concentrated on the grain boundaries of TiSi2 without obvious crystal segregation, which is beneficial for the purification of directionally solidified silicon enrichment layers via pickling to obtain highly purity silicon. Fig. 5 and Fig. 6 present the energy spectra of the impurity phase of the silicon enrichment layer and the impurity enrichment region, respectively. From Fig. 5, one can see that, after precipitation, TiSi2 gradually grows into a crystal phase (point 3; Ti:Si z 1:2). Thereafter, the grain boundaries form a FeeTieSi phase. From the inside to the outside, the Fe content gradually increases until it reaches a maximum of approximately 6.76 at.%. In Fig. 6, one can see that Fe content gradually increases in the alloy as well until it reaches a maximum value of approximately 7.72 at.%. This difference of
approximately 1% can be attributed to different concentration fluctuations of Fe in the melts. As Fe has a small segregation coefficient in Si, directional solidification progresses to be largely distributed in the liquid phase. As the solid-liquid interface moves upward, the melt gets reduced and the concentration of Fe gradually increases. Due to lower energy impurity segregation, metal impurities segregate at the grain boundary of TiSi2, and Fe eventually combines with TiSi2 and forms FeTiSi2 at the grain boundary. Therefore, in the longitudinal direction of the ingot, TiSi2 is enriched in Fe, giving an increasing trend in terms of concentration. In addition, it can be noted from the energy spectrum results that the atomic ratios of Fe in the impurity phase of the silicon enrichment layer and impurity enrichment region are similar at approximately 4e8 at.%. Additionally, the phase compositions of the two samples are consistent. The powder in the impurity enrichment region at the top of the alloy was also subjected to XRD analysis. The results are shown in Fig. 7. The results demonstrate that the composition of the alloy is largely divided into four phases, namely a, b, g, and ε in the XRD pattern, which correspond to Si, TiSi2, FeTiSi2, and FeAl2.7Si2.3, respectively. Following full-spectrum fitting and finishing, the contents of the phases were found to be 47.4%, 22.8%, 34.2%, and 0.6%, respectively. The Si, Ti, Fe, and Al ratios were 66.92%, 20.73%, 12.12%, and 0.23%, respectively. In the Xray scanning region, Fe exhibited clear enrichment in the alloy with content as high as 12.12 wt%. The main composition phase is FeTiSi2 with a small amount of FeAl2.7Si2.3. Fig. 8 presents the migration mechanism of the main metal element in the Si melt during the pull-down process. Firstly, at high temperatures, impurities are uniformly distributed in the Si melt because of electromagnetic stirring [18,23]. After the initiation of pull-down, a small amount of TiSi2 crystals precipitate at the bottom of the crucible based on the corresponding temperature drop. This phenomenon occurs because the impurities undergo heterogeneous nucleation at the bottom of the silicon enriched layer, which is mainly due to poor polishing of the graphite at the corners between the bottom and side walls of the crucible. It is well known
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Fig. 4. Impurity distributions in Si measured via EPMA-mapping: (a) the upper part of silicon enrichment layer near the TieSi eutectic alloy and (b) the impurity enrichment region
Fig. 5. EDS results for residual impurities in silicon enrichment layer
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Fig. 6. Results of electron probe dotting in the impurity enrichment region.
Fig. 7. XRD results for the impurity enrichment region.
that the energy of heterogeneous nucleation is much lower than the required energy for homogeneous nucleation [24]. Therefore, the solidification of metals and alloys always occurs through heterogeneous nucleation. Based on the high temperature during the solidification stage, the precipitated TiSi2 crystals act as the impurity in the Si melt outside the Si element, which means that they will first heterogeneously nucleate in the melt, and then, grow excessively at the bottom, leading to a small amount of impurities at the bottom of the final ingot. As the segregation coefficients of metal impurities are generally low during the process of directional solidification, impurities will be redistributed at the solid-liquid interface, eventually leading to their upward migration. During this process, impurities will be evenly distributed by electromagnetic stirring. When directional solidification is almost complete, the impurity concentration reaches the partial condensation limit,
thus, leading to the impurities being precipitated together with the TieSi eutectic alloy at the top. Due to the low energy of impurity segregation, metal impurities segregate at the grain boundary of TiSi2, forming an ingot similar to that shown in Fig. 2(a). 3.3. Impurity removal in Si The electromagnetic stirring [18,21] mainly forms from the Lorentz force acting on the melt, combining the eddy current in the melt and the penetration depth of the magnetic induction. The Lorentz force is applied to the impurity atoms or clusters in the melt to promote their enrichment toward the solidification front. As the segregation coefficients [25] of metal impurities (Ti: 2 10 6, Fe: 6.4 10 6, Al: 8.0 10 2, Ca: 1.6 10 3) are smaller than 1, the impurities will segregate into the melt during directional
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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Fig. 8. Mechanism of impurity removal in electromagnetically-induced directional solidification of TieSi alloys.
solidification. At the same time, due to the obvious temperature gradient at the solidification front, the front end of the liquid-solid phase of the melt forms a thermoelectric potential. This leads to the formation of the thermal electromagnetic force in the axial and radial direction of the ingot. This promotes the agitation of the melt in the axial direction and the reflow of the melt in the radial direction at the solidification front, both of which improve the mass transfer characteristics of the atoms in the melt. And then this facilitates the migration and enrichment of metal impurity atoms or clusters toward the solidification front and eventually segregation into the liquid phase. In addition, controlling the rational directional solidification parameters is beneficial to reduce the impurity phase in the silicon enrichment layer. The decrease in the pulling rate increases the mass transfer time of impurities in the melt, such that metal impurity atoms or clusters have more time to migrate to the solidification front and accumulate, thereby, redistributing at the liquid-solid interface and finally, segregating into the liquid phase. Therefore, coupling to the electromagnetic field and directional solidification can improve the migration of impurities in the melt, thereby, achieving efficient removal of impurities in the silicon enrichment layer. The concentrations of impurities (Fe, Al, Ca, and Ti) at different heights in the silicon enrichment layer were measured using ICPOES. The results are shown in Fig. 9. The contents of each impurity element at different heights in the silicon enrichment layer exhibit trends similar to the Ti concentrations with different directional solidification pulling rates. When the pulling rate is 9 mm/s or 7 mm/s, due to the mushy zone, the microscopic migration rate of Ti is smaller than the advancing rate of the solid - liquid interface. This means that a large amount of Ti remains in the upper portion of the silicon enrichment layer. Additionally, Fe, Al, and Ca exhibit a similar distribution to Ti. Metal impurity contents above a pulling rate of 5 mm/s are typically of the same order of magnitude. Based on the analysis in Section 3.2, this is because Ti largely exists in the form of TiSi2 in Si and the impurities segregate on the grain boundaries of TiSi2. At a low pulling rate, the Fe content can be reduced from 2800 10 4 wt% to 9 10 4 wt%, the Al content can be reduced from 1700 10 4 wt% to 21 10 4 wt%; and the Ca content can be reduced from 200 10 4 wt% to 10 10 4 wt%.
These changes represent removal rates of 99.68%, 98.76%, and 95%, respectively. As shown in Fig. 9, in the range of 0e7 mm, the content of Ti decreases initially, then, begins to increase and a minimum value of 350 ppmw to exist, which is related to the heterogeneous nucleation illustrated in Figs. 2 and 8. Ti is the most concentrated element in the melt, apart from Si. At the bottom of the silicon enrichment layer, some Ti and Si combine to form TiSi2, which act as centers for heterogeneous nucleation. However, because of directional solidification, the segregation coefficient of Ti is much smaller than one, which means that most of the Ti will migrate upward. Following nucleation and excessive growth, TiSi2 is immediately coated with Si, which means that the bottom of the silicon enrichment layer has a Ti concentration lower limit (350 ppmw). So when the thickness of the silicon enrichment layer is in the range of 0e7 mm, heterogeneous nucleation is present making the content of Ti relatively high. The content of Ti intercepted in the upper area of the silicon enrichment layer is related to the pulling rate during directional solidification. The greater the pulling rate, the more Ti is intercepted. To reduce the time required for directional solidification and remove as much Ti as possible from the silicon enrichment layer, a pulling rate of 5 mm/s is appropriate. This is also consistent with the analysis of the height and purity of the silicon enrichment layer discussed in the previous section. At this pulling rate, the average Ti content remaining in the silicon enrichment layer is 583 ppmw. For a TieSi alloy with an initial Ti content of 10 wt%, the removal rate of Ti in the silicon enrichment layer is 99.4%. Fig. 10 shows the average removal rate of Fe, Al, and Ca in the silicon enrichment layer after directional solidification of a TieSi alloy. One can see that as the pulling rate decreases, the average removal rate of impurities increases and the gradually becomes flat. However, for Ca, excessively low pulling rates are not conducive to the removal of impurities; the specific reasons are still under further study. Under the experimental conditions in this study, the optimal pulling rate was 5 mm/s. At this rate, the removal rates of Fe and Al approach their limits and the removal rate of Ca shows significant improvement. Using an optimal rate can reduce directional solidification time and result in greater removal efficiency.
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
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J. Hu et al. / Journal of Alloys and Compounds xxx (xxxx) xxx
Fig. 9. Impurity content in the silicon enrichment layer
Ca in the silicon enrichment layer were 99.5%, 98.765%, and 85.83%, respectively. In addition, during the early stages of directional solidification, based on heterogeneous nucleation, impurities also exhibited a small amount of enrichment at the bottom of the silicon enrichment layer. Acknowledgments This research was supported by the National Natural Science Foundation of China project (No.U1702251), Special Funds of State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization (Grant No. CNMRCUTS1604), the Reserve Talents of Young and Middle-aged Academic and Technical Leaders in Yunnan Province (2018HB009), and the Program for Innovative Research Team in University of Ministry of Education of China (No. IRT17R48). Fig. 10. Average removal rate of impurities in the silicon enrichment layer at different pulling rates
The average removal rates of Fe, Al, and Ca in this study were 98.431%, 99.262%, and 85.833%, respectively. 4. Conclusions The mechanism of impurity removal in the silicon enrichment layer during directional solidification of Ti-90 wt%Si alloy was analyzed. Due to heterogeneous nucleation, TiSi2 first precipitated in the melt during directional solidification. Next, TiSi2 formed the FeTiSi2 phase with Fe, which was enriched on the grain boundaries of TiSi2. Following the directional solidification of TieSi alloys, impurities were found to be concentrated on the grain boundaries of TiSi2 without any significant crystal segregation. Furthermore, under the experimental conditions in this study, the optimal pulling rate was identified to be 5 mm/s. At this pulling rate, directional solidification time can be reduced and excellent removal efficiency can be achieved. The average impurity removal rates of Fe, Al, and
References [1] Y. Lei, L.E. Sun, W.H. Ma, K.X. Wei, K. Morita, Enhancing B removal from Si with small amounts of Ti in electromagnetic solidification refining with AleSi alloy, J. Alloy. Comp. 666 (2016) 406e411. [2] H. Chen, Y. Wang, W.J. Zheng, Q.C. Li, X.Z. Yuan, K. Morita, Model implementation of boron removal using CaCl2-CaO-SiO2 slag system for solar-grade silicon, Metall. Mater. Trans. B 48 (2017) 3219e3227. [3] A.A. Istratov, H. Hieslmair, E.R. Weber, Iron and its complexes in silicon, Appl. Phys. A-Mater. 69 (1) (1999) 13e44. [4] B. Gao, S. Nakano, K. Kakimoto, Influence of back-diffusion of iron impurity on lifetime distribution near the seed-crystal interface in seed cast-grown monocrystalline silicon by numerical modeling, Cryst. Growth Des. 12 (2011) 522e525. [5] X. Peng, W. Dong, Y. Tan, D.C. Jiang, Q. Wang, G.B. Li, Evaporation behavior of calcium in metallurgical silicon by an electron beam melting method, J. Funct. Mater. 41 (2010) 117e120. [6] S.P. Huang, W.H. Ma, K.X. Wei, S.Y. Li, K. Morita, A model for distribution of aluminum in Si refined by vacuum directional solidification, Vacuum 96 (2013) 12e17. [7] J. Safarian, M. Tangstad, Vacuum refining of molten silicon, Metall. Mater. Trans. B 43 (2012) 1427e1445. [8] Y. Tan, S.Q. Ren, S. Shi, S.T. Wen, D.C. Jiang, W. Dong, M. Ji, S.H. Sun, Removal of aluminum and calcium in multicrystalline silicon by vacuum induction melting and directional solidification, Vacuum 99 (2014) 272e276.
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621
J. Hu et al. / Journal of Alloys and Compounds xxx (xxxx) xxx [9] T.F. Ciszek, G.H. Schwuttke, K.H. Yang, Solar-grade silicon by directional solidification in carbon crucibles, IBM J. Res. Dev. 23 (2010) 270e277. [10] Y. Wang, K. Morita, Measurement of the phase diagram of the SiO2-CaCl2 system and liquid area study of the SiO2-CaO-CaCl2 system, Metall. Mater. Trans. B 47 (2016) 1542e1547. [11] H. Chen, X.Z. Yuan, K. Morita, Y.J. Zhong, X.D. Ma, Z.Y. Chen, Y. Wang, Reaction mechanism and kinetics of boron removal from molten silicon via CaO-SiO2CaCl2 slag treatment and ammonia injection, Metall. Mater. Trans. B 50 (2019) 2088e2094. [12] X. D Ma, T. Yoshikawa, K. Morita, Si growth by directional solidification of SieSn alloys to produce solar-grade Si, J. Cryst. Growth 377 (2013) 192e196. [13] W.Z. Yu, W.H. Ma, Z. Zheng, Y. Lei, W.Y. Jiang, J. Li, Si purification by removal of entrapped Al during electromagnetic solidification refining of Si-Al alloy, Metall. Mater. Trans. B 48 (2017) 2804e2811. [14] S.D. Hu, Y.C. Dai, A. Gagnoud, Y. Fautrelle, R. Moreau, Z.M. Ren, K. Deng, C.J. Li, X. Li, Effect of a magnetic field on macro segregation of the primary silicon phase in hypereutectic Al-Si alloy during directional solidification, J. Alloy. Comp. 722 (2017) 108e115. [15] W.Z. Yu, Y. Xue, J. Mei, X.Z. Zhou, M.L. Xiong, S.F. Zhang, Segregation and removal of transition metal impurities during the directional solidification refining of silicon with Al-Si solvent, J. Alloy. Comp. 805 (2019) 198e204. [16] X.L. Bai, B.Y. Ban, J.W. Li, Z.Q. Fu, Z.J. Peng, C.B. Wang, J. Chen, Effect of Ti addition on B removal during Si refining in Al-30% Si alloy directional solidification, Separ. Purif. Technol. 174 (2017) 345e351. [17] Y. Lei, L.E. Sun, W.H. Ma, K.X. Wei, K. Morita, Enhancing B removal from Si
[18]
[19]
[20]
[21]
[22]
[23] [24] [25]
9
with small amounts of Ti in electromagnetic solidification refining with AleSi alloy, J. Alloy. Comp. 666 (2016) 406e411. K.S. Zhu, J.F. Hu, W.H. Ma, K.X. Wei, T.L. Lv, Y.N. Dai, Effects of solidification parameters and magnetic field on separation of primary Si from hypereutectic Tie85 wt.% Si melt, J. Cryst. Growth 522 (2019) 78e95. Y. Lei, L.E. Sun, W.H. Ma, X.D. Ma, J.J. Wu, S.Y. Li, K. Morita, An approach to employ titanium-bearing blast-furnace slag to prepare Ti and AleSi alloys, J. Alloy. Comp. 769 (2018) 983e990. Q.Y. Huang, X.W. Lv, R. Huang, J.J. Song, Preparation of TieSieAl alloy by aluminothermic reduction of TiO2 bearing blast furnace slag, Can. Metall. Q. 52 (2013) 413e421. K.S. Zhu, W.H. Ma, K.X. Wei, Y. Lei, J.F. Hu, T.L. Lv, Y.N. Dai, Separation mechanism of TiSi2 crystals from a Ti-Si eutectic alloy via directional solidification, J. Alloy. Comp. 750 (2018) 102e110. Scientific Group Thermodata Europe (SGTE), Binary system Si-Ti, in: Ternary Steel Systems: Phase Diagrams and Phase Transition Data, Springer, Berlin, Heidelberg, 2012, 59-59. M.A. Martorano, J.B.F. Neto, T.S. Oliveira, T.O. Tsubaki, Refining of metallurgical silicon by directional solidification, Mater. Sci. Eng. B 176 (2011) 217e226. D. Turnbull, Kinetics of heterogeneous nucleation, J. Chem. Phys. 18 (1950) 198e203. Y. Lei, C. Wang, W.H. Ma, J.J. Wu, K.X. Wei, S.Y. Li, G.Q. Lv, K. Morita, A novel approach to prepare high-purity Si and Si/TiSi2 materials simultaneously using Ti-bearing blast furnace slag, J. Alloy. Comp. 798 (2019) 333e341.
Please cite this article as: J. Hu et al., Effects of pulling rate on metal impurity removal during Si refining in Ti-90 wt.% Si alloy directional solidification, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152621