Separation and Recovery of Refined Si from Al–Si Melt by Modified

materials

Article Separation and Recovery of Reﬁned Si from Al–Si Melt by Modiﬁed Czochralski Method

Jingwei Li 1,2,3, Juncheng Li 4, Yinhe Lin 5,*, Jian Shi 1, Boyuan Ban 1, Guicheng Liu 6 , Woochul Yang 6 and Jian Chen 1,* 1 Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China; [email protected] (J.L.); [email protected] (J.S.); [email protected] (B.B.) 2 State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 3 Department of Materials Science and Engineering, University of Toronto, Toronto, ON M5S 3E4, Canada 4 School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China; [email protected] 5 School of Chemistry Engineering, Yangtze Normal University, Chongqing 408000, China 6 Department of Physics, Dongguk University, Seoul 04620, Korea; [email protected] (G.L.); [email protected] (W.Y.) * Correspondence: [email protected] (Y.L.); [email protected] (J.C.)

Received: 24 December 2019; Accepted: 18 February 2020; Published: 23 February 2020

Abstract: Separation of refined silicon from Al–Si melt is still a puzzle for the solvent refining process, resulting in considerable waste of acid and silicon powder. A novel modified Czochralski method within the Al–Si alloy is proposed. After the modified Czochralski process, a large amount of refined Si particles was enriched around the seed crystalline Si and separated from the Al–Si melt. As for the Al–28%Si with the pulling rate of 0.001 mm/min, the recovery of refined Si in the pulled-up alloy (PUA) sample is 21.5%, an improvement of 22% compared with the theoretical value, which is much larger 1.99 times than that in the remained alloy (RA) sample. The content of impurities in the PUA is much less than that in the RA sample, which indicates that the modified Czochralski method is effective to improve the removal fraction of impurities. The apparent segregation coefficients of boron (B) and phosphorus (P) in the PUA and RA samples were evaluated. These results demonstrate that the modified Czochralski method for the alloy system is an effective way to enrich and separate refined silicon from the Al–Si melt, which provide a potential and clean production of solar grade silicon (SoG-Si) for the future industrial application.

Keywords: modiﬁed Czochralski method; reﬁned Si separation; Al–Si alloy; boron and phosphorus removals; distribution mechanism

1. Introduction With the aggravation of environmental pollution and energy shortage, the development of clean and green energy, especially solar energy, is being increasingly emphasized all over the world. The main material for solar cells is crystalline silicon, which accounts for more than 90% of the total cost [1,2]. However, impurities in solar grade silicon (SoG-Si) can seriously decrease the efficiency of photovoltaic conversation [3,4]. It is necessary to remove the impurities from the metallurgical grade silicon (MG-Si) and obtain high purity silicon for higher photovoltaic conversion efficiency. At present, approximately 90% of Si-based solar cells globally are produced by the Siemens method [2]. The Siemens method as the main technology is not environmentally friendly, resulting in considerable pollution. In order to obtain low energy consumption and clean production, and improve the removal efficiencies of impurities, a series of purification methods of SoG-Si from MG-Si, such as solvent refining [5,6],

Materials 2020, 13, 996; doi:10.3390/ma13040996 www.mdpi.com/journal/materials Materials 2020, 13, 996 2 of 11 directional solidification [7–9], slag refining [10,11], electron beam melting treatment [12], and plasma treatment [13] etc. have been investigated. The solvent-refining method is an effective way to eliminate the impurities, such as boron (B) and phosphorus (P), which selects metal as the solvent to form the Si–metal alloy system. The principle is that impurities in MG-Si have the tendency to segregate into the alloy melt to obtain the high-purity Si because of small segregation coefficients. The Al–Si solvent-refining method [14–17] has the advantage of low melting point, without intermediate phase, low-energy consumption, which has been investigated by many researchers. However, the density difference between solid Si (2.3 g/cm3) and Al–Si melt (~2.4 g/cm3) is small [18], which inevitably generates much adhesive Al on the surface of the Si dendrites and consumes lots of acid solution to remove it, generating considerable waste of acid solution and Si powder. Therefore, the enrichment and separation of the refined silicon dendrites from the Al–Si matrix is still a puzzle. For separation of the refined silicon dendrites from the Al–Si matrix, agglomeration and enrichment methods of refined silicon grains from the Al–Si melt have been extensively investigated. Some physical fields, such as the electromagnetic field [19], super gravity [20] et al. have been introduced into separate the refined silicon from the Al–Si melt. Yu et al. [21], Zou et al. [22] and Xue et al. [23] applied electromagnetic force to agglomerate the refined silicon at the bottom of the Al–Si hypereutectic melt. Li et al. [24] proposed super gravity to realize the enrichment of refined silicon at the bottom of the crucible. Ban et al. [14] applied a rotating electromagnetic field to accelerate the enrichment of refined Si. However, it still needs to cut the agglomeration of refined Si from Al–Si alloy and consume a lot of the acid solution. A cleaner and lower-energy consumption was desired to separate and purify Si from the Al–Si solvent-refining process. The Czochralski (Cz) method [25] is an effective way to obtain high purity crystalline silicon from pure Si melt at 1698K. The principle is that the crystalline silicon seed is grown and pulled up from the pure silicon melt [26]. During this process, a large bulk crystalline silicon is generated and separated from the silicon melt, which can realize a continuous growth and separation from the silicon melt and obtain high-purity silicon. This is an effective way to obtain high purity silicon. However, this method is mainly applied to obtain large size of mono-crystalline Si rod from the pure silicon melt [27,28]. But the separation of the refined silicon from the Si-based alloy melt by the Czochralski method has not been investigated in detail. Compared with the pure silicon melt system, the Si-based alloy melt system has the advantage of lower melt temperature (less than 1473 K), and high refining efficiency, which is beneficial for the separation and purification of refined Si from the alloy system. If the refined Si is grown around the seed of crystalline Si and changed into bulk Si from the melt initially, the bulk Si maybe meet the impurities requirement thereby avoiding acid leaching directly. Therefore, it is a meaningful exploration to separate and obtain the enrichment bulk refined silicon, and reduce the acid consumption and environmental influence. In this work, a novel separation method of the refined silicon from the Si-based alloy system by a modified Czochralski process is proposed. The Al–Si alloy system is selected as the melt system, and the seed is a polycrystalline silicon piece. The seed was pulled up from the Al–Si alloy melt at a certain rate. The microstructures of refined silicon within different areas were investigated by optical microscopy. The contents of impurities at different locations of the samples were detected by inductively coupled plasma optical emission spectroscopy (ICP-OES). Apparent segregation coefficients of boron and phosphorus in the Al–Si alloy system were evaluated. The distribution mechanism of refined Si was elucidated. The recovery of the refined silicon was analyzed.

2. Materials and Methods Figure 1 shows the schematic diagram of experimental equipment. A lifting device was controlled by an electromotor and placed in the electronic resistance furnace. The seed crystal was a small polycrystalline silicon with a purity of 99.9999% and was pulled up by a motor. Contents of impurities both in the MG‐Si and the Al are shown in Table 1. Two different processes were conducted. The Al‐25 wt.% Si sample (consisting of Si‐A and high purity Al) was heated to 1273 K at a rate of 10 K/min in Al2O3 crucible (inside diameter = 54 mm, height = 120 mm), then held for 2 h, and the Al–Si melt was stirred every half an hour by a quartz rod. Then, the sample was cooled down to 973 K at 10 K/min and held for 4 h. At the same time, the seed crystal (diameter = 22 mm) was lowered into the Al–Si melt by the motor with the depth of ~1.5 mm. Then, the crystal seed was pulled up at 0.1 mm/min to leave the melt at 973 K. After that, the Al–Si melt was cooled down to room temperature by turning off the power. The Al‐28 wt.%Si sample (consisting of Si‐B and high purity Al) was heated to 1073 K at a rate of 10 K/min in the crucible (inside diameter = 85 mm, height = 60 mm), then held for 3 h, and the Al–Si melt was stirred every half an hour by a quartz rod. Then, the sample was cooled down to 953 K at 5 K/min and held for 1 h. At the same time, the seed crystal (20 mm × 20 mm) was lowered into the Al–Si melt by the motor with the depth of ~1 mm. Then, the crystal seed was pulled up at 0.001 mm/min to leave the melt at the cooling rate of 0.01 K/min. When the temperature reached 873 K, the crystal seed was pull out of the melt thoroughly and the Al–Si melt was cooled down to the room temperature by turning off the power. During this process, the sample enrichment on the seed crystal was defined as the pulled‐up alloy (PUA), and the sample remained in the crucible was defined as the remained alloy (RA). Both PUA and RA samples were divided in two halves along the axis direction. Part of each of the halves was polished and then investigated by an optical microscope, and the average length of refined silicon particles was calculated. On the other half acid leaching by HCl solution was conducted to dissolve the aluminum matrix. After that, the refined silicon particles were filtrated with deionized water, and dried in an oven. Then the contents of impurities, such as boron, phosphorus et al. in the primary silicon particles, which are defined as refined silicon, were analyzed by ICP‐OES, and recovery of the refined silicon was calculated. The distribution mechanism of refined Si was elucidated, and the apparent segregation coefficients of boron and phosphorus in the

MaterialsAl–Si 2020system, 13, 996were calculated. 3 of 11

Figure 1. Schematic diagram of experimental equipment.

Table 1.FigureContents 1. Schematic of impurities diagram in MG-Si of experimental and high purity equipment. Al (ppmw).

B P Fe Ti Si-A 28 46 2818 437 Si-B 207 217 1696 263 High purity Al - - 7.00 1.00

Two different processes were conducted. The Al-25 wt.% Si sample (consisting of Si-A and high purity Al) was heated to 1273 K at a rate of 10 K/min in Al2O3 crucible (inside diameter = 54 mm, height = 120 mm), then held for 2 h, and the Al–Si melt was stirred every half an hour by a quartz rod. Then, the sample was cooled down to 973 K at 10 K/min and held for 4 h. At the same time, the seed crystal (diameter = 22 mm) was lowered into the Al–Si melt by the motor with the depth of ~1.5 mm. Then, the crystal seed was pulled up at 0.1 mm/min to leave the melt at 973 K. After that, the Al–Si melt was cooled down to room temperature by turning off the power. The Al-28 wt.%Si sample (consisting of Si-B and high purity Al) was heated to 1073 K at a rate of 10 K/min in the crucible (inside diameter = 85 mm, height = 60 mm), then held for 3 h, and the Al–Si melt was stirred every half an hour by a quartz rod. Then, the sample was cooled down to 953 K at 5 K/min and held for 1 h. At the same time, the seed crystal (20 mm 20 mm) was lowered into the Al–Si melt by the motor with the depth × of ~1 mm. Then, the crystal seed was pulled up at 0.001 mm/min to leave the melt at the cooling rate of 0.01 K/min. When the temperature reached 873 K, the crystal seed was pull out of the melt thoroughly and the Al–Si melt was cooled down to the room temperature by turning off the power. During this process, the sample enrichment on the seed crystal was defined as the pulled-up alloy (PUA), and the sample remained in the crucible was defined as the remained alloy (RA). Both PUA and RA samples were divided in two halves along the axis direction. Part of each of the halves was polished and then investigated by an optical microscope, and the average length of refined silicon particles was calculated. On the other half acid leaching by HCl solution was conducted to dissolve the aluminum matrix. After that, the refined silicon particles were filtrated with deionized water, and dried in an oven. Then the contents of impurities, such as boron, phosphorus et al. in the primary silicon particles, which are defined as refined silicon, were analyzed by ICP-OES, and recovery of the refined silicon was calculated. The distribution mechanism of refined Si was elucidated, and the apparent segregation coefficients of boron and phosphorus in the Al–Si system were calculated. Materials 2020, 13, x FOR PEER REVIEW 4 of 11

Table 1. Contents of impurities in MG‐Si and high purity Al (ppmw).

B P Fe Ti Si‐A 28 46 2818 437 Materials 2020, 13, 996 Si‐B 207 217 1696 263 4 of 11 High purity Al ‐ ‐ 7.00 1.00

3.3. Results Results and and Discussion Discussion 3.1. Morphologies and Distribution of the Refined Si 3.1. Morphologies and Distribution of the Refined Si Figure2 shows the macrostructures of the samples both pulled-up alloy (PUA) grown on the seed Figure 2 shows the macrostructures of the samples both pulled‐up alloy (PUA) grown on the crystal and remained alloy (RA) in the crucible. In Figure2a, several tiny needle-like primary particles seed crystal and remained alloy (RA) in the crucible. In Figure 2a, several tiny needle‐like primary were agglomerated around the seed crystal when the seed was pulled up from the Al–25%Si melt at particles were agglomerated around the seed crystal when the seed was pulled up from the Al– the pull rate of 0.1 mm/min. The average length of primary Si in the PUA sample is 1.7 mm. Moreover, 25%Si melt at the pull rate of 0.1 mm/min. The average length of primary Si in the PUA sample is 1.7 there were some cracks in the seed, which maybe caused the stress concentration when the seed was mm. Moreover, there were some cracks in the seed, which maybe caused the stress concentration lowered into the melt. Figure2b shows the cross-section of the RA sample remained in the crucible. when the seed was lowered into the melt. Figure 2b shows the cross‐section of the RA sample The size distribution of the refined silicon has an obvious gradient distribution. The primary Si particles remained in the crucible. The size distribution of the refined silicon has an obvious gradient mainly distributed at the bottom and the wall. No large refined silicon particles were detected in distribution. The primary Si particles mainly distributed at the bottom and the wall. No large refined the center of the RA sample. This macrostructure of the RA sample is similar to the structure of Al–Si silicon particles were detected in the center of the RA sample. This macrostructure of the RA sample hypereutectic alloy solidified under a rotating electromagnetic field [14]. But there is an obvious is similar to the structure of Al–Si hypereutectic alloy solidified under a rotating electromagnetic difference on the top surface of the Al–Si hypereutectic alloy by two different separation methods. field [14]. But there is an obvious difference on the top surface of the Al–Si hypereutectic alloy by Under the rotating electromagnetic field, the primary Si particles were distributed around the entire two different separation methods. Under the rotating electromagnetic field, the primary Si particles surface, including the top surface. Compared with Al–25%Si, the size of primary Si particles is much were distributed around the entire surface, including the top surface. Compared with Al–25%Si, the larger in the Al–28%Si alloy with the pulling rate of 0.001 mm/min shown in Figure2c,d. Several size of primary Si particles is much larger in the Al–28%Si alloy with the pulling rate of 0.001 large primary Si particles also agglomerated around the seed crystal. The average length of primary mm/min shown in Figure 2c and d. Several large primary Si particles also agglomerated around the Si particles in the PUA sample in Figure2c is 5 mm. The distribution and number of primary Si seed crystal. The average length of primary Si particles in the PUA sample in Figure 2c is 5 mm. The particles in the RA sample shown in Figure2d are not di fferent from the sample in Figure2b. Because distribution and number of primary Si particles in the RA sample shown in Figure 2d are not of the different pulling rates of the seed crystal, the pulling rate of Al–28%Si alloy was 0.001 mm/min, different from the sample in Figure 2b. Because of the different pulling rates of the seed crystal, the which is much smaller by 100 times than that in Al–25%Si alloy. The cooling rate of PUA sample was pulling rate of Al–28%Si alloy was 0.001 mm/min, which is much smaller by 100 times than that in affected directly by the pulling rate. Therefore, the average length of primary Si particles in Al–28%Si Al–25%Si alloy. The cooling rate of PUA sample was affected directly by the pulling rate. Therefore, was 2.9 times larger than that in Al–25%Si alloy. By a comparison of the distribution difference the average length of primary Si particles in Al–28%Si was 2.9 times larger than that in Al–25%Si of the refined silicon particles both in the PUA sample and RA sample, it can be concluded that alloy. By a comparison of the distribution difference of the refined silicon particles both in the PUA the modified Czochralski method can realize the enrichment and separation of the refined silicon sample and RA sample, it can be concluded that the modified Czochralski method can realize the particles from the Al–Si melt in the crucible. enrichment and separation of the refined silicon particles from the Al–Si melt in the crucible.

Figure 2. Cross section of the samples within diﬀerent part: (a) pulled-up alloy (PUA) in Al–25%Si alloy; (b) remained alloy (RA) in Al–25%Si alloy; (c) PUA in Al–28%Si alloy; (d) RA in Al–28%Si alloy.

Figure 2. Cross section of the samples within different part: (a) pulled‐up alloy (PUA) in Al–25%Si alloy; (b) remained alloy (RA) in Al–25%Si alloy; (c) PUA in Al–28%Si alloy; (d) RA in Al–28%Si alloy.

Figure 3 shows the microstructure of the interface between the seed and the agglomerated alloy in the Al–25%Si. There is an interspace between the seed and the alloy at the bottom in the PUA Materialssample 2020as ,shown13, 996 in Figure 3a. Large refined silicon grains adhered to the boundary. There 5is of no 11 obvious interspace on the side boundary shown in Figure 3b. The alloy was mainly made up of Al–Si obviouseutectics interspace phase and on no the large side refined boundary Si shownparticles in were Figure distributed.3b. The alloy As was for mainly the reason made of up interspace of Al–Si eutecticsgeneration, phase during and the no alloying large refined process, Si particles part of liquid were distributed. Al reacted with As for oxygen the reason to form of interspacealuminum generation,oxide, and the during aluminum the alloying oxide process,floated on part the of surface liquid Alduring reacted the with stirring oxygen process. to form Once aluminum the seed oxide,contacted and with the aluminumthe surface oxideof the melt, floated the on interface the surface of the during boundary the stirringat the bottom process. of the Once seed the will seed be contactedcovered by with the the aluminum surface of oxide, the melt, which the prevents interface ofthe the growth boundary of the at therefined bottom silicon of the grains seed willon the be coveredsurface of by the the seed, aluminum resulting oxide, in an which interspace prevents at the the growth bottom of of the the refined seed. As silicon for the grains lateral on theside surface of the ofseed, the the seed, aluminum resulting oxide in an on interspace the surface at will the bottombe destroyed of the towards seed. As the for edge the lateralof the crucible, side of the because seed, theof the aluminum shear stress oxide of on the the seed surface during will the be destroyedlower sinking towards process. the edgeTherefore, of the crucible,there is no because obvious of theinterspace shear stress on the of the boundary seed during at the the lateral lower sinking side of process. the seed. Therefore, From Figure there is 3c,d, no obvious there is interspace a white onskeletal the boundary‐shape phase at the shown lateral in side the of PUA. the seed. The result From of Figure EDS3 c,d,analyses there spectra is a white indicated skeletal-shape that it mainly phase showncontains in Al, the Si PUA. and The Fe. resultFrom ofthe EDS literature analyses [29–31], spectra combined indicated with that itthe mainly weight contains percent Al, of Si each and Fe.element, From the literaturestructure [of29 –impurity31], combined phase with is α‐ theAl8SiFe weight2 intermetallic percent of eachwith element, a characteristic the structure skeletal of shape. impurity phase is α-Al8SiFe2 intermetallic with a characteristic skeletal shape.

Figure 3. Microstructures of interface in the PUA sample in Al–25%Si alloy (a) boundary at the bottom; (Figureb) boundary 3. Microstructures at the lateral side; of interface (c) impurities in the in PUA the PUA sample sample; in Al–25%Si (d) EDS analyses alloy (a spectra) boundary of impurity at the phasebottom; in ( theb) boundary red circle. at the lateral side; (c) impurities in the PUA sample; (d) EDS analyses spectra of impurity phase in the red circle. 3.2. Recovery of Refined Si by Modified Czochralski Method 3.2. Recovery of Refined Si by Modified Czochralski Method Figure4 shows the recovery of refined Si with PUA and RA samples. As can be seen, the recovery of refinedFigure Si 4 in shows the PUA the is recovery larger than of thatrefined in the Si RAwith samples. PUA and As RA for thesamples. Al–25%Si As can with be the seen, pulling the raterecovery of 0.1 of mm refined/min, theSi in recovery the PUA of is refined larger Sithan in thethat PUA in the is RA 14.9%, samples. which As is much for the larger, Al–25%Si by 1.71 with times, the thanpulling that rate in the of 0.1 RA mm/min, sample. As the for recovery the Al–28%Si of refined with Si the in pulling the PUA rate is of14.9%, 0.001 which mm/min, is much the recovery larger, by of refined1.71 times, Si in than the PUAthat in is the 21.5%, RA ansample. improvement As for the of Al–28%Si 22% compared with the with pulling the theoretical rate of 0.001 value, mm/min, which isthe much recovery larger, of by refined 1.99 times, Si in than the thatPUA in is the 21.5%, RA sample. an improvement These results of indicate22% compared that the modifiedwith the Czochralskitheoretical value, method which within is Al–Si much alloy larger, system by 1.99 can realizetimes, thethan enrichment that in the and RA separation sample. These of the refinedresults siliconindicate grains that the from modified the Al–Si Czochralski alloy, which method is beneficial within Al–Si for the alloy separation system andcan realize purification the enrichment of MG-Si from the Al–Si melt.

Materials 2020, 13, x FOR PEER REVIEW 6 of 11

Materials 2020, 13, x FOR PEER REVIEW 6 of 11 and separation of the refined silicon grains from the Al–Si alloy, which is beneficial for the separation and purification of MG‐Si from the Al–Si melt. Materialsand separation2020, 13, 996 of the refined silicon grains from the Al–Si alloy, which is beneficial for6 ofthe 11 separation and purification of MG‐Si from the Al–Si melt.

Figure 4. Recovery of refined Si with different Al–Si alloys. FigureFigure 4.4. Recovery of refinedrefined SiSi withwith didifferentfferent Al–SiAl–Si alloys.alloys. 3.3. Removal Fraction of Impurities at Different Locations 3.3. Removal Fraction of Impurities at Different Locations 3.3. RemovalFigure 5 Fractionshows the of Impuritiescontents of at impurities Different Locations in the RA and the PUA samples in the Al–28%Si alloy. As canFigureFigure be seen5 5 shows shows in Figure the the contents contents5a, content of of impurities impuritiesof B in the in inRA the the sample RA RA and and is the 10.87the PUAPUA ppmw, samplessamples with inin the thethe removal Al–28%SiAl–28%Si fraction alloy.alloy. ofAsAs 94.75%. cancan be be seen seenContent in in Figure Figure of B5 a,in5a, content thecontent PUA of of Bsample inB in the the RAis RA 7.51 sample sample ppmw, is 10.87is 10.87with ppmw, ppmw,the removal with with the thefraction removal removal of fraction 96.37%.fraction of Content94.75%.of 94.75%. Contentof PContent in the of BRA of in thesampleB in PUA the is sample PUA39.06 sampleppmw, is 7.51 ppmw, iswith 7.51 the withppmw, removal the removalwith fraction the fraction removal of 82.00%. of fraction 96.37%. Content Contentof 96.37%.of P in of thePContent in PUA the RA sampleof sampleP in theis 26.00 is RA 39.06 sampleppmw, ppmw, iswith with39.06 the the ppmw, removal removal with fraction fraction the removal of of 88.02%. 82.00%. fraction Fe Content and of Ti 82.00%. of has P in similar the Content PUA variation, sample of P in andisthe 26.00 PUAthe ppmw,content sample with of is Fe26.00 the and removal ppmw, Ti in fractionthewith RA the sample of removal 88.02%. is fraction31.92 Fe and ppmw Tiof has88.02%. (98.12% similar Fe variation,andremoval Ti has fraction) and similar the content variation,and 2.72 of ppmwFeand and the (98.97% Ti content in the RAremoval of sampleFe and fraction), isTi 31.92 in the ppmwrespectively. RA sample (98.12% Contentis removal 31.92 ofppmw fraction) Fe and (98.12% andTi in 2.72 removalthe ppmw PUA fraction) (98.97%sample and removalis 28.86 2.72 ppmwfraction),ppmw (98.30%(98.97% respectively. removal Content fraction),fraction) of Fe andrespectively.and Ti1.74 in the ppmw PUA Content sample(99.34% of isFe 28.86removal and ppmwTi infraction), the (98.30% PUA respectively. removal sample fraction)is 28.86 By comparisonandppmw 1.74 (98.30% ppmw of the (99.34%removal RA and removal PUAfraction) samples, fraction), and the1.74 respectively. content ppmw of (99.34%impurities By comparison removal in the of PUAfraction), the RAis much and respectively. PUAless than samples, that By inthecomparison the content RA sample, of of impurities the which RA and indicates in PUA the PUAsamples, that is the much the modified content less than Czochralski of thatimpurities in the method RAin the sample, PUAis effective is which much to indicates lessimprove than thatthatthe removalthein the modified RA fraction sample, Czochralski of whichimpurities. methodindicates is that effective the modified to improve Czochralski the removal method fraction is effective of impurities. to improve the removal fraction of impurities.

FigureFigure 5. 5. ContentContent of of impurities impurities in in the the RA RA and and PUA PUA samples samples in Al–28%Si in Al–28%Si alloy: alloy: (a) B ( aand) B andP; (b P;) Fe (b and) Fe Ti.andFigure Ti. 5. Content of impurities in the RA and PUA samples in Al–28%Si alloy: (a) B and P; (b) Fe and Ti. 3.4.3.4. Apparent Apparent Segregation Segregation Coefficients Coefficients of Impurities 3.4. ApparentSegregationSegregation Segregation coefficient coefficient Coefficients is is defined defined of Impuritiesas as the the ratio ratio of ofimpurity impurity content content in the in the refined refined Si to Si that to that in the in the initial sample. In order to evaluate the segregation ability of impurities between refined silicon and initialSegregation sample. In ordercoefficient to evaluate is defined the segregationas the ratio ofability impurity of impurities content in between the refined refined Si to silicon that in and the initial sample. In order to evaluate the segregation ability of impurities between refined silicon and

Materials 2020, 13, 996 7 of 11

Materials 2020, 13, x FOR PEER REVIEW 7 of 11 the Al–Si melt, an apparent segregation coefficient (kapp) was proposed by Li et al. [32]. A kapp equation is derived from the Scheil equation: the Al–Si melt, an apparent segregation coefficient (kapp) was proposed by Li et al. [32]. A kapp equation is derived from the Scheil equation: kapp 1 CS = kappCo(1 fS) − (1) − 𝐶 𝑘𝐶1 𝑓 (1) where Cs is the content of impurity in the refined silicon; Co is initial content of impurity element in where Cs is the content of impurity in the refined silicon; Co is initial content of impurity element in the melt; fs is solid fraction of Al–Si alloy. the melt; fs is solid fraction of Al–Si alloy. According to the mass conservation law of the impurity element, the formula is obtained by According to the mass conservation law of the impurity element, the formula is obtained by the the integration of Equation (1): integration of Equation (1): Z f S kapp 1 kappCo(1 x)−dx = C f (2) 𝑘𝐶 1 − 𝑥 𝑑𝑥S 𝐶 𝑓S (2) o    C𝐶Sf𝑓S 𝑘 =𝑙𝑜𝑔 1  (3) kapp log1 fS1  (3) −  − C𝐶O  wherewhere CCs sis theis the content content of impurity of impurity in the refinedin the silicon;refinedk appsilicon;is the apparentkapp is the segregation apparent coesegregationfficient of coefficientimpurity. Thus,of impurity.kapp of impurityThus, kapp canof impurity be obtained can bybe obtained Equation by (3). Equation (3). ForFor Al–28%Si, Al–28%Si, ffs sisis 0.18 0.18 based based on on the the Al–Si Al–Si phase phase diagram. diagram. Because Because it it is is a a solidification solidification process, process, thethe average average solidification solidification temperature temperature was was selected selected as asthe the median median temperature. temperature. The The kapp ofkapp boronof boron and phosphorusand phosphorus can be can calculated be calculated from Equation from Equation (3) in this (3) work, in this and work, the results and the are results shown are in shownFigure 6. in DuringFigure6 the. During solidification the solidification process, the process, solidification the solidification temperature temperature is a temperature is a temperature range, thus, range, the averagethus, the temperature average temperature point was point selected was selected as the as abscissa the abscissa point. point. As Ascan can be beseen, seen, the the apparent apparent segregationsegregation coefficients coefficients of of boron boron in in the the PUA PUA and and RA RA samples samples are are 0.119 0.119 (at (at 913 913 ± 4040 K) K) and and 0.173 0.173 (at (at ± 973973 ± 100100 K), K), respectively. respectively. The The apparent apparent segregation segregation coefficients coefficients of of phosphorus phosphorus in in the the PUA PUA and and RA RA ± samplessamples are are 0.405 0.405 (at (at 913 913 ± 4040 K) K) and and 0.620 0.620 (at (at 973 973 ± 100100 K), K), respectively. respectively. The The apparent apparent segregation segregation ± ± coefficientscoefficients of of boron boron and and phosphorus phosphorus in in the the PUA PUA sample sample are are much much smaller smaller than than in in the the RA RA sample. sample. TheseThese results results demonstrate that the segregationsegregation abilityability ofof boron boron and and phosphorus phosphorus in in the the PUA PUA sample sample is ismuch much better better than than in the in RAthe sample. RA sample. Compared Compared with the with theoretical the theoretical segregation segregation coefficient coefficient of boron and of boronphosphorus and phosphorus in the Al–Si–B in the or Al–Si–B Al–Si–P or system Al–Si–P [33 system[33,34],,34], the apparent the apparent segregation segregation coefficients coefficients of boron ofand boron phosphorus and phosphorus are larger, are which larger, indicates which that indicates the segregation that the segregation behaviors of behaviors boron and of phosphorus boron and phosphorusare not suffi cient.are not In sufficient. order to achieve In order the to thermodynamic achieve the thermodynamic equilibrium, theequilibrium, solidification the processsolidification needs processto be optimized. needs to be optimized.

Figure 6. Relationship between apparent segregation coeﬃcients of B, P vs. temperature. Figure 6. Relationship between apparent segregation coefficients of B, P vs. temperature.

3.5. Distribution Mechanism of Refined Si According to the Al–Si phase diagram [35], the size of the refined Si particles from the Al–Si hypereutectic melt gradually increases when the temperature decreases along the liquidus shown in Figure 7. When the temperature is above the liquidus of the Al–Si melt, most of the impurity elements are in the form of atoms in stage I and the impurities, such as P, B, Ti, Fe et al. are

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3.5. Distribution Mechanism of Refined Si According to the Al–Si phase diagram [35], the size of the refined Si particles from the Al–Si hypereutecticMaterials 2020, 13, meltx FOR gradually PEER REVIEW increases when the temperature decreases along the liquidus shown8 of 11 in Figure7. When the temperature is above the liquidus of the Al–Si melt, most of the impurity elementsdistributed are uniformly in the form in of the atoms entire in stageSi–Al I alloy and the melt. impurities, Initially, such the asnucleation P, B, Ti, Fe of et the al. refined are distributed silicon uniformlyparticles was in thegenerated entire Si–Al when alloy the temperature melt. Initially, was the cooled nucleation down of below the refined the liquidus silicon line particles for Al–Si was generatedalloy, which when is shown the temperature in stage II. wasAt the cooled same down time, belowthe impurities the liquidus elements line for begin Al–Si to alloy,segregate which into is shownthe semi in‐ stagesolid II.melt At thefrom same the time, refined the impuritiessilicon particles elements because begin toof segregate the existence into the of semi-solidmuch smaller melt fromsegregation the refined coefficient silicon of particles impurity because between of the the refined existence silicon of much and smallerAl–Si semi segregation‐solid melt coe [17].fficient When of impuritythe temperature between continues the refined to silicon decrease, and the Al–Si size semi-solid of the refined melt [17 silicon]. When particles the temperature gradually continues becomes tolarger decrease, and the the size size of of the refinedrefined siliconsilicon particles particles gradually increases becomes along specific larger andcrystal the orientations size of the refined from siliconstage II particles to III. When increases the temperature along specific reaches crystal the orientations eutectic point, from the stage Al–Si II to eutectic III. When phase the temperatureis generated reachesshown in the the eutectic stage IV. point, the Al–Si eutectic phase is generated shown in the stage IV.

Figure 7. Segregation process of the impurities during Czochralski (Cz) process. Figure 7. Segregation process of the impurities during Czochralski (Cz) process. When the Al–Si alloy decreased to the temperature below the liquidus, some primary Si particles When the Al–Si alloy decreased to the temperature below the liquidus, some primary Si occurred nucleation and grown in the semi-solid alloy melt. The seed was sinked into the semi-solid particles occurred nucleation and grown in the semi‐solid alloy melt. The seed was sinked into the melt, and some tiny primary Si particles agglomerated on the surface of the seed, because of the existence semi‐solid melt, and some tiny primary Si particles agglomerated on the surface of the seed, because of the temperature gradient. Schematic drawing of the growth of the primary Si particles during of the existence of the temperature gradient. Schematic drawing of the growth of the primary Si the Cz process is shown in Figure8. The refined silicon grains were not uniformly distributed in particles during the Cz process is shown in Figure 8. The refined silicon grains were not uniformly the RA sample shown in Figure2b, and mainly distributed near the bottom and the lateral side of distributed in the RA sample shown in Figure 2b, and mainly distributed near the bottom and the the alloy, which is different from the Al–Si solidification alloy without the pulling-up process. During lateral side of the alloy, which is different from the Al–Si solidification alloy without the pulling‐up the pulling up process, the size of the refined silicon grains in the PUA sample became larger. When process. During the pulling up process, the size of the refined silicon grains in the PUA sample the temperature reaches the eutectic point (849 K), the Al–Si eutectic phase begins growth and some became larger. When the temperature reaches the eutectic point (849 K), the Al–Si eutectic phase small size Si particles distributed in the eutectic area. In this work, the ultimate goal of the modified begins growth and some small size Si particles distributed in the eutectic area. In this work, the Czochralski process is to obtain the large bulk refined Si from the melt and avoid the inclusion of ultimate goal of the modified Czochralski process is to obtain the large bulk refined Si from the melt the Al–Si eutectic phase by control the pulling up parameters shown in Figure8. From the experimental and avoid the inclusion of the Al–Si eutectic phase by control the pulling up parameters shown in result at present, the needle-like refined Si could realize the enrichment, which indicates that it is Figure 8. From the experimental result at present, the needle‐like refined Si could realize the a potentially effective way to obtain the large bulk refined Si by further research. enrichment, which indicates that it is a potentially effective way to obtain the large bulk refined Si by further research.

Materials 2020, 13, 996 9 of 11 Materials 2020, 13, x FOR PEER REVIEW 9 of 11

Figure 8. Diagrammatic drawing of the growth of refined silicon during the Czochralski process. Figure 8. Diagrammatic drawing of the growth of refined silicon during the Czochralski process. 4. Conclusions 4. Conclusions A novel method of the separation and purification of MG-Si from Al–Si melt by the modified CzochralskiA novel process method was of proposed.the separation After and the purification modified Czochralski of MG‐Si process,from Al–Si large melt amounts by the of modified refined siliconCzochralski particles process were was enriched proposed. around After the the seed. modified The refined Czochralski silicon particles process, in large remained amounts alloy of samplerefined generatedsilicon particles an obvious were gradient enriched distribution. around the As seed. for the The Al–28%Si refined with silicon the pullingparticles rate in of remained 0.001 mm /alloymin, thesample recovery generated of refined an obvious Si in the gradient PUA is 21.5%, distribution. an improvement As for the of Al–28%Si 22% compared with withthe pulling the theoretical rate of value,0.001 mm/min, which is muchthe recovery larger 1.99 of refined times than Si in that the in PUA the RAis 21.5%, sample. an The improvement content of Bof and 22% P compared in the RA samplewith the is theoretical 10.87 ppmw value, and which 39.06 is ppmw, much larger respectively. 1.99 times Content than that of in B the and RA P insample. the PUA The samplecontent of is 7.51B and ppmw P in andthe RA 26.00 sample ppmw, is respectively. 10.87 ppmw The and content 39.06 ppmw, of impurities respectively. in the PUA Content is much of B less and than P in that the inPUA the sample RA sample, is 7.51 which ppmw indicates and 26.00 that ppmw, the modified respectively. Czochralski The content method of isimpurities effective in at the improving PUA is themuch removal less than fraction. that in The the apparent RA sample, segregation which coeindicatesfficients that of Bthe and modified P in the Czochralski PUA and RA method samples is areeffective 0.119 at (at improving 913 40 K)the and removal 0.173 fraction. (at 973 The100 apparent K), respectively. segregation These coefficients results demonstrate of B and P in that the ± ± thePUA modified and RA Czochralskisamples are method0.119 (at within 913 ± 40 the K) alloy and system 0.173 (at is 973 an e ±ff ective100 K), way respectively. to enrich andThese separate results refineddemonstrate silicon that from the the modified Al–Si melt, Czochralski which provides method potential within the clean alloy production system is ofan SoG-Si effective for way future to industrialenrich and applications. separate refined silicon from the Al–Si melt, which provides potential clean production of SoG‐Si for future industrial applications. Author Contributions: Conceptualization, J.L. (Jingwei Li); methodology, J.L. (Juncheng Li) and J.S.; validation, B.B.;Author formal Contributions: analysis, B.B.; Conceptualization, investigation, B.B.;J.L. (Jingwei Project Li); administration, methodology, J.C.; J.L. resources,(Juncheng Li) J.S.; and Supervision, J.S.; validation, J.C.; writing—originalB.B.; formal analysis, draft B.B.; preparation, investigation, J.L. (Jingwei B.B.; Project Li); writing—review administration, and J.C.; editing, resources, G.L., J.L.J.S.; (Juncheng Supervision, Li), W.Y.J.C.; and Y.L.; funding acquisition, J.L. (Jingwei Li), Y.L. and B.B. All authors have read and agreed to the published versionwriting—original of the manuscript. draft preparation, J.L. (Jingwei Li); writing—review and editing, G.L., J.L. (Juncheng Li), W.Y. and Y.L.; funding acquisition, J.L. (Jingwei Li), Y.L. and B.B. All authors have read and agreed to the published Funding:version ofThis the manuscript. work was financially supported by National Natural Science Foundation of China (No. 51874272, No. 51804294 and No. U1960101); State Key Laboratory of Refractories and Metallurgy, Wuhan University of ScienceFunding: and This Technology work was (G201910); financially Science supported and by Technology National Natural Major Project Science of Foundation Sichuan Province of China (2019YFSY0029); (No. 51874272, AnhuiNo. 51804294 Provincial and NaturalNo. U1960101); Science State Foundation Key Laboratory (No. 1808085ME121); of Refractories and Key Metallurgy, Laboratory Wuhan of Photovoltaic University and of Energy Conservation Materials, Chinese Academy of Sciences (No. PECL2018QN002, PECL2019ZD005 and PECL2019KF003);Science and Technology CASHIPS (G201910); Director’s Science Funds and (Grant Technology No. YYJJ201624), Major Project State of Key Sichuan Laboratory Province of Pollution (2019YFSY0029); Control andAnhui Resource Provincial Reuse Natural Foundation, Science (NO. Foundation PCRRF18017), (No. 1808085ME121); Ministry of Science Key andLaboratory ICT through of Photovoltaic the National and Research Energy FoundationConservation of KoreaMaterials, (No. 2019H1D3A2A02100593),Chinese Academy of NationalSciences Research(No. PECL2018QN002, Foundation of Korea PECL2019ZD005 (NRF) grant funded and byPECL2019KF003); the Korea government CASHIPS (MSIT) Director’s (No. 2019R1C1C1006310), Funds (Grant No. and YYJJ201624), iCET by China State Scholarship Key Laboratory Council. of Pollution ConflictsControl and of Interest:ResourceThe Reuse authors Foundation, declare (NO. no conflict PCRRF18017), of interest. Ministry of Science and ICT through the National Research Foundation of Korea (No. 2019H1D3A2A02100593), National Research Foundation of Korea (NRF) Referencesgrant funded by the Korea government (MSIT) (No. 2019R1C1C1006310), and iCET by China Scholarship Council. 1. Chigondo, F. From Metallurgical-Grade to Solar-Grade Silicon: An Overview. 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