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Continuous electron beam melting technology of silicon powder by prefabricating a molten silicon pool
Vacuum 143 (2017) 336e343
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Continuous electron beam melting technology of silicon powder by prefabricating a molten silicon pool Tong Lu a, b, Yi Tan a, b, *, Shuang Shi a, b, Xiaoliang Guo b, Jiayan Li a, b, Dengke Wang b a b
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Laboratory for New Energy Material Energetic Beam Metallurgical Equipment Engineering of Liaoning Province, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2017 Received in revised form 23 June 2017 Accepted 24 June 2017 Available online 27 June 2017
We successfully achieved the continuous melting of silicon powder in a vacuum electron beam melting system. The characteristic of electron beam melting of silicon powder was revealed by comparing the three different approaches to prefabricate the molten silicon pool. The application of silicon block can be regarded as the most efficient method to prefabricate a stable molten silicon pool. Afterward, the silicon powder was added into the molten silicon pool at different rates by a spiral silicon powder feeding device. Finally, all of the silicon powder was melted completely. Although the P removal efficiency decreases with increasing powder feeding rate during the powder feeding process, an appropriate increase in the powder feeding rate is not only beneficial to improve the melting efficiency of silicon powder but also conductive to the improvement in uniformity of the obtained silicon ingot. Therefore, the powder feeding rate of 10 g/min was the best option in this study. Compared with the melting of silicon block, which has been wildly applied, this continuous powder feeding process can be beneficial to saving smelting time and the total cost. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Continuous electron beam melting Silicon powder Prefabricating molten silicon pool Removal efficiency Uniformity
1. Introduction In the recent years, the crystal silicon solar cells have rapidly developed because energy and environment problems, and polysilicon is the most wildly used materials for solar cells [1,2]. The electron beam melting purification process is an important part of the physical metallurgy method to produce solar energy polysilicon since 1980's [3]. Considering that the electron beam melting technology can provide a high temperature and high vacuum smelting environment, its combination with the melt agitation effect caused by the temperature gradient can help impurities diffuse directly to the melt surface, thus, this approach exerts an evident removal effect on volatile impurities, such as P, O and Al [4e7]. However, electron beam melting cannot achieve a continuous feeding of raw material at present because of the use of silicon blocks. Normally, raw material needs to be fed and subjected to a vacuum after repeated cooling to ambient temperature, which not only reduces production efficiency but also consumes considerable
* Corresponding author. School of Materials Science and Engineering, Dalian University of Technology, No.2 Linggong Road, Ganjingzi District, Dalian 116023, China. E-mail address: [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.vacuum.2017.06.037 0042-207X/© 2017 Elsevier Ltd. All rights reserved.
energy. Thus, another melting method should be introduced to improve the production efficiency [8]. Continuous powder melting is one of the best solutions to achieve this aim. Although J.C.S. Pires et al. have already chosen silicon powder as the raw material, they didn't consider the continuous powder feeding process [9,10]. Compared with silicon blocks, silicon powder, which possesses better fluidity, is easier to use for achieving continuous feeding of raw material. Moreover, silicon powder possesses a large specific surface area and fast melting speed, thus possibly improving the production efficiency substantially when a proper melting procedure is applied [11]. Silicon powder also is extensively sourced. For instance, in the broad application of the diamond wire saw technology for silicon wafer slicing, a large amount of silicon powder waste with the adhering oxidation film and volatile organic matter is difficult to be recycled [12e15]. Therefore, if the problem of continuous electron beam melting technology of silicon powder can be solved, this kind of silicon powder can completely be used as the silicon powder raw material for electron beam melting after a certain cleaning and drying process, which can improve the fluidity of silicon powder. This strategy can not only solve the problem of continuous production of electron beam melting at present stage but also solve the growing silicon powder waste recycling problem in the future, which is very important in reducing the overall cost
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reduction of the preparation of crystalline silicon solar cells. However, powder melting has always been one of the biggest issues in the industry. With the limitations of the special heating principle and the rigorous vacuum environment of the electron beam melting system under consideration, continuously melting silicon powder under such a harsh environment will be extremely difficult. Therefore, in this paper, we focused on the study of continuous electron beam melting of silicon powder. After trying three different prefabricating methods of molten silicon pool, we decided on the most efficient continuous melting process. Moreover, we designed a powder feeding device that can add silicon powder continuously at different rates. Furthermore, we also analyzed the feasibility of powder melting by prefabricating molten silicon pool in theory, illustrated the distribution of P in the silicon ingots after powder feeding, revealed the effect of the powder feeding rate on the removing efficiency of P and the homogeneity of resistivity during the powder feeding process briefly, and compared with the traditional melting of silicon block at present. 2. Experimental procedures 2.1. Raw materials and experimental apparatus High phosphorus silicon powder with 12.55 ppmw phosphorus was selected as the powder raw material (particle diameter varying between 150 and 170 mm, and the median diameter was 160 mm) and low phosphorus silicon block with 1 ppmw phosphorus was used for forming the molten silicon pool to analyze the removal efficiency of volatile impurities during the powder melting process and observe the distribution of volatile impurities in the silicon ingot samples after melting. The experiments were conducted in an electron beam melting furnace. The experimental apparatus primarily consisted of a vacuum chamber, an electron beam gun with an accelerating voltage of 30 kV, a water-cooled copper crucible, and our self-designed spiral silicon powder feeding device that can adjust the powder feeding rate by controlling the rotation rate of the motor. The specific schematic of the experimental apparatus are
337
shown in Fig. 1. 2.2. Melting process of silicon powder by electron beam Two processes are required to achieve continuous electron beam melting of silicon powder. The first process was the formation of a stable molten silicon pool, and the second process involved the continuous powder feeding melting. During the first process, three different methods were tested to form a stable molten silicon pool, and we selected the best method to do the continuous powder melting by considering various factors. 2.2.1. Prefabrication of a stable molten silicon pool 2.2.1.1. Direct melting of silicon powder. A 300-g portion of high P silicon powder raw material was directly placed into the watercooled Cu crucible after ultrasonic cleaning for 20 min and drying to constant weight. The bombardment voltage can be increased to 30 kV after the electron beam equipment was vacuumed to 2.4 102 Pa, and then electron beam was added gradually until the electron beam current reached to 100, 150, or 300 mA. 2.2.1.2. Mixture melting of silicon powder and silicon block. High P silicon powder (200 g) was placed into the water-cooled Cu crucible first, and then 100 g silicon blocks were putted on the surface of the silicon powder. The area of the silicon block accounted for about 60% of the whole surface. The whole experiment was under the condition of 200 mA electron beam current. First, the electron beam was focused on the silicon blocks to form some small molten silicon pools. After approximately 2 min, the position of the light spot of electron beam was moved to the margin of the small molten silicon pool to make the silicon powder melt gradually. Finally, a whole piece of molten silicon pool can be formed. 2.2.1.3. Direct melting of silicon block. 300 g low P silicon blocks were placed into the water-cooled Cu crucible. After the door of furnace was closed, the electron beam equipment was vacuumed to
Fig. 1. The experimental apparatus of electron beam melting of silicon powder.
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2.4 102 Pa, and then high voltage was started. When bombardment voltage increased to 30 kV, the electron beam was gradually increased until the electron beam current reached 300 mA. Silicon blocks were melted under this condition. The electron beam current was increased to 400 mA with 4.8 kW power after the silicon block was completely melted. 2.2.2. Continuous powder feeding melting of silicon powder High P silicon powder raw material was added into the continuous spiral powder feeding device after ultrasonic cleaning for 20 min and drying to constant weight. After the formation of the molten silicon pool, the liquid silicon was smelted with the electron beam current held constant for about 20 min to form a stable molten silicon pool. Meanwhile, the spiral powder feeding device was activated on the premise of continuous melting. According to different silicon powder adding rates, the corresponding rotational speeds of the motor were set (in this experiment, silicon powder adding rates were 5, 10, and 15 g/min respectively). The whole process of silicon powder feeding lasted for 10 min, and the electron beam gun was shut down when the powder feeding process was over. Finally, the silicon ingot was taken out after natural cooling for 30 min. 2.3. Composition detection ICP-MS was adopted to analyze the composition of the raw materials and the silicon ingots. This step was conducted mainly to study the distribution and the removal efficiency of volatile impurities (P) in silicon ingots after melting. To obtain the impurity distribution in the silicon ingots, we cut off four samples for composition analysis. The specific sampling method is shown in Fig. 2. The whole impurity content of silicon ingot can be regarded as the average of four samples. A KDY-1 four-point probe resistivity tester was used to analyze the distribution of resistivity of silicon ingot with respect to reflect the uniformity of the overall composition of the silicon ingot from the side. 3. Results and discussion 3.1. Melting results in samples subjected to different molten silicon pool prefabricating methods For direct melting of silicon powder, only trace melting occurred in the silicon powder after 3 min at the electron beam current of 100 mA because of the energy was too weak. When the current was increased to 150 mA, the amount of the melted silicon powder increased, but the silicon powder flew out of the water-cooled Cu crucible continuously. After approximately 2 min, only a small amount of silicon powder remained in the crucible, and a small silicon block can be observed. We can see the same phenomenon, in
which the silicon powder was splashed all over the furnace, when the electron beam current reached 200 mA. For determining the cause of this problem, the silicon powder was treated with 15% sodium hydroxide solution first until abundant bubbles can be observed. The following melting process shows that the pretreatment can effectively prevent the splashing phenomenon, and the silicon powder can be melted continually. The most plausible cause of the splashing phenomenon is the oxide layer on the surface of the silicon powder. The small silicon block which can be obtained after the electron beam melting of the untreated silicon powder may originate from the silicon powder whose oxide surface layer was punctured by the charge. Fig. 3 shows the specific direct melting process of silicon powder. The most likely explanation why we cannot directly melt silicon powder by the electron beam can be represented by Fig. 3 (b). Given that the surface of the silicon powder always coated with a thin film layer of non-conductive silicon dioxide, the negative charges can be enriched on the surface of each silicon powder as the electron beam continuously injected into the silicon powder. When a certain level is reached, the charges can produce a sufficiently large repulsion to make the silicon powder splash, which can be seen in Fig. 3 (c), and this process is not correlated to the electron beam current. Finally, this phenomenon can lead to the failure of the melting of silicon powder. Although we can solve this problem by alkali washing or other methods to destroy the silicon dioxide film, the additional process can substantially increase the cost of melting. Therefore, direct melting of silicon powder is not an appropriate option. As we know that, the silicon block is invariably selected as the raw material for melting in the industry. Thus, we mixed the silicon block and silicon powder together to achieve the melting of the silicon powder with the aim of preventing the splash of the silicon powder by focusing the electron beam on the silicon block first. Fig. 4 shows the specific situation during melting process, and we can see from Fig. 4 (c) that the silicon powder was completely melted into a silicon ingot. However, the melting rate of the silicon powder was extremely slow, with complete melting of the silicon powder requiring 36 min. Compared with the silicon block, silicon powder with the same mass possesses more interfaces, and the heat transfer between two silicon powder particles exerts a poor effect. Therefore, as shown in Fig. 4 (b), the whole melting rate depends on the infiltration rate of silicon fluid between the silicon powders, which is relatively slow. The whole operation process is more complicated, and the experiment is poorly repeatable. Therefore, this approach is a poor option, as well. However, from the previous sections, we learned that the formation of the molten silicon pool can be effective to prevent silicon powder splashing. Therefore, we decided to prefabricate a molten silicon pool with only the silicon block first, which requires approximately 10 min to form a molten silicon pool. Then, the silicon powder was added into the center of the molten silicon pool by
Fig. 2. The schematic diagram of sampling for composition analysis.
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Fig. 3. Direct melting process of silicon powder by electron beam melting. (a) Initial state of silicon powder, (b) Electron beam injection process, (c) Final state of silicon powder.
Fig. 4. Mixture melting process of silicon powder and silicon block by electron beam melting. (a) Initial state of the mixture, (b) Selective melting process of silicon block, (c) Final silicon ingot.
using the spiral silicon powder feeding device to achieve continuous melting of the silicon powder. Finally, we successfully achieved continuous silicon powder melting by this method, which is not only easy to operate but also offers cost saving. Fig. 5 shows the longitudinal section of the silicon ingots at different powder feeding rates. Evidently, all the silicon powder was completely melted.
3.2. Feasibility analysis of silicon powder melting by prefabricating molten silicon pool To simplify the calculation, we selected one silicon powder for the research and made several assumptions as follows, and the
mathematical model of melting process of single silicon powder is shown in Fig. 6: 1 The total melting process of silicon powder is under constant pressure 2 The falling resistance of silicon powder is zero in a vacuum environment 3 Silicon powders are monodisperse spherical particles, whose volume is conserved before melting 4 The temperature of molten silicon pool is constant 5 No temperature gradient occurs within the silicon powder 1) Calculation of the time required for completely melting silicon powder
Fig. 5. Morphology of the longitudinal section of the silicon ingot, with different powder feeding rate. (a) 5 g/min, (b) 10 g/min, (c) 15 g/min.
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Let us suppose that the mass of silicon powder is m and the radius is r, from Newton's second law, we can get:
mg kv rL gV ¼ m
dv dt
(10)
rL and rS are the densities of liquid silicon and solid silicon, respectively. With G ¼ g 1 rrL , we can get S
k dt ¼ m
Fig. 6. Mathematical model of melting process of single silicon powder.
Silicon powder melting occurs in two processes. First, silicon powder was heated from room temperature to the melting point. Then, the silicon powder undergoes an isothermal phase change process. Thus, the time needed for melting silicon powder was the sum of the duration of the two processes mentioned above. In the first heating stage of silicon powder, at constant pressure
dQ ¼ dH ¼ CP mdT
(1)
(2)
Therefore,
CP mdT ¼ hðTL TÞAdt
(3)
A is the surface area of silicon powder, and m is the mass of silicon powder.The general solution of the differential equation is
T ¼ TL ce
rr3hC t
(4)
S P
When t ¼ 0, T ¼ T0, therefore, rr3hC t
T ¼ TL ðTL T0 Þe
S P
(5)
The time required for heating silicon powder is as follows and can be recorded as t1:
rr C T Tm t1 ¼ S P ln L 3h TL T0
(6)
In the transformation stage of silicon powder:
mL ¼ qAt2
(7)
L is the phase transformation latent heat of silicon. Therefore, the time required for phase transformation is shown below, recorded as t2:
t2 ¼
mL r rS L ¼ qA 3hðTL Tm Þ
(8)
Consequently, the total time required to melt the silicon powder is
rr C T Tm r rS L t ¼ t1 þ t2 ¼ S P ln L þ 3h TL T0 3hðTL Tm Þ
(11)
k v Gm
General solutions:
v¼
m k G cem t k
(12)
m pffiffiffiffiffiffiffiffi k m G G 2gh em t k k
(13)
pffiffiffiffiffiffiffiffi Initial condition: when t ¼ 0, v ¼ 2gh:
v¼
The actual falling depth calculation of silicon powder is
Due to the thermal balance relationship
dQ ¼ qAdt
k v d Gm
(9)
2) The calculation of the actual falling depth of silicon powder in the molten silicon pool prior to complete melting
s¼
pffiffiffiffiffiffiffiffi k pffiffiffiffiffiffiffiffi m m m m m Gt þ G 2gh em t G 2gh k k k k k
(14)
Table 1 shows the basic parameters of the silicon powder. After substituting these basic parameters into the above equations, we can calculate several results from the calculations, which are shown in Table 2: In general, silicon blocks are selected as the raw material for electron beam smelting. First, the silicon blocks are melted using a low electron beam current to form a molten silicon pool, and then the silicon pool is smelted using a higher electron beam current. The highest surface temperature of the molten silicon pool can reach up to 2000 C. At this time, volatile impurities, such as P, can be removed from the surface of the silicon evaporation pool. While, the impurities within the silicon pool can only be removed by diffusing to the surface. This diffusion is a relatively slow process, and a large amount of energy is needed to maintain the stability of the molten silicon pool. However, for silicon powder melting, we can see that the melting time of silicon powder is extremely short that is 0.11 ms, because of the high specific surface area. After dropping into the molten silicon pool, the silicon powder can melt rapidly in the surface layer with the thickness is 0.31 mm. At this point, the impurities in the powder can quickly spread to the surface of the molten pool, and large amounts of volatile impurities can be removed during this process. Therefore, compared with the silicon block, this kind of powder feeding melting process could be beneficial to conserve the melting time.
Table 1 The basic parameters of silicon powder. Parameter
Symbol
Value
Initial temperature (K) Melting point (K) Phase change latent heat (kJ/mol) Isobaric heat capacity of solid (J/(mol$K)) Radius (m) Solid density (g/cm3) Liquid density (g/cm3) Falling height (m) Viscosity of molten silicon (mm2/s)
T0 Tm L CP r
298 1683 44.44 927 0.00008 2.35 2.53 0.3 0.000756
rS rL H
h
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Table 2 The calculated results of the melting process of silicon powder. Calculation results
Value
The time required for melting silicon powder (s) The actually falling depth of silicon powder in the molten silicon pool (m)
0.00011 0.00031
3.3. Analysis of the distribution and the removal effectiveness of phosphorus in silicon ingots after melting As shown in Fig. 7, the P content in other parts of the silicon ingot increased with the powder feeding rate, however, the P content at the bottom (Sample 4) almost remained the same. Because when the silicon blocks were completely melted into a molten silicon pool, a temperature gradient will occur between the high temperature surface and the low temperature water-cooled Cu crucible, the bottom of the crucible can form a solid silicon layer with a certain thickness as a result of the low temperature. And solid phase diffusion of P is a relative slow process. Therefore, the P content at the bottom was less affected by the subsequent powder melting process. The highest P content was found near the dividing line of solidification (Sample 2) because the solubility of P in liquid was higher than that in solid. As the surface of the silicon pool radiates heat to the environment and the bottom of the silicon pool transmits heat to the water-cooled Cu crucible, the silicon molten pool will experience a directional solidification process from the outside surface to the inside after the electron beam gun is shut off. The area near the dividing line of solidification is the final solidification region, and thus contains the highest P content in the silicon ingot. Notably, although the silicon powder was completely melted in the surface layer and most of P can be removed in this phase, the P content in the middle of the molten silicon pool (Sample 3) can still increase, because of the strong stirring effect of electron beam melting. This part of P can only be removed by diffusion to the surface of the molten silicon pool, which is a relatively slow process. So, the P content in sample 3 is slightly higher than the sample 1. Therefore, combined with the calculated results of the melting process of silicon powder, the total melting and P impurity removing process of silicon powder by prefabricating molten silicon pool method can be represented by Fig. 8. The removal efficiency of phosphorus (h(P)) with different powder feeding rate was calculated using Eq. (15):
hðPÞ ¼ 1
(15)
where Ci(P) (ppmw), CB(P) (ppmw), and CP(P) (ppmw) are the contents of P in the final silicon ingot, silicon block used for prefabricating molten silicon pool and silicon powder, respectively. mB (g) is the mass of the silicon block, and vP (g/min) and t (min) are the powder feeding rate and duration. Fig. 9 shows that the removal efficiency of P decreased with the increase in powder feeding rate, which decreased sharply to 38.6%, at the powder feeding rate of 15 g/min. This finding suggests that the excess amounts of P impurity in the surface layer of the molten silicon pool were transferred to the inside before removal by evaporation on the surface, thus resulting in lower P removal efficiency. Owing to the limitation of the evaporation of P, reduction of the removal efficiency of volatile impurities can occur although the silicon powder can be melted completely with a higher powder feeding rate. Therefore, the powder feeding rate must be reasonably controlled to maximize the advantage of volatile impurity removal offered by powder melting. In this study, comparing with that at the powder feeding rate of 5 g/min, the removal efficiency of P only decreased by 0.5% when the powder feeding rate reached to10 g/min during the powder feeding process. Moreover, we can see from Fig. 10 that the powder feeding rate can affect the physical properties of material, such as resistivity homogeneity. The feeding rate of 10 g/min results in the best uniformity, which means that appropriate powder feeding can make the silicon powder spread out evenly along the center of the molten silicon pool, facilitating the improvement of the overall composition uniformity of the silicon ingot. Thus, we may select 10 g/min as the actual powder feeding rate, because of higher product efficiency, relatively high impurity removal rate, and superior physical properties. According to the previous work in our laboratory [16], when the silicon block was selected as the raw material, we can determine that the overall mass transfer coefficient of phosphorus KT(P) is 7.45 106 m/s at 4.8 kW and obtain the relationship between P content and the melting time, which is shown in Fig. 11, by using Eq. (16):
ln
Fig. 7. The distribution of P in the silicon ingots after melting.
CiðPÞ ðmB þ vp tÞ CBðPÞ mB þ CPðPÞ vp t
ct S ¼ KTðPÞ t V c0
(16)
where Ct is the P content at t s and C0 is the content at 0 s. S is the melt surface, and V is the volume of the molten silicon pool, which can be identified as 153 cm2 and 150 cm3 in this calculation, respectively. When the P content decreased from 3.88 ppmw to 2.09 ppmw, the time needed for continuous melting of silicon powder was 600 s less than the 813 s melting time for the silicon block. Thus, this approach can shorten the melting time and save the cost. The final P content in the silicon ingot was 2.09 ppmw, which failed to meet the solar degree standards, after the powder was fed for 600 s. However, the subsequent purification process after the ceasing of powder feeding can refer to the electron beam melting process of silicon block, which has the same p content. The optimum melting parameters and energy consumption can be
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Fig. 8. The schematic diagram of the P impurity removal during continuous powder feeding electron beam melting process.
Fig. 9. The removal efficiency of P after melting with different powder feeding rate. Fig. 11. Comparison of P removal rate between continuous melting of silicon powder (10 g/min) and traditional melting of silicon block at 4.8 kW.
determined by another work in our laboratory [17].
4. Conclusions We achieved continuous electron beam melting of silicon powder by prefabricating molten silicon pool with silicon block as the precursor. As determined from the molten silicon pool prefabricating process, the electron beam cannot melt silicon powder directly, possibly because of the surface oxide layer on the silicon powder. The feasibility analysis in theory showed that the silicon powder mainly melted in the surface layer of the molten silicon
Fig. 10. The distribution of resistivity of silicon ingot with different powder feeding rate. (a) 5 g/min; (b) 10 g/min; (c) 15 g/min.
pool, which aids in the removal of volatile impurities, such as P, in silicon powder. The actual removal efficiency can reach 46.5% even during the powder feeding process without subsequent purification, and the removal efficiency will decrease with the increasing of powder feeding rate. However, an appropriate increase in the powder feeding rate is not only beneficial to improve the melting efficiency of silicon powder, but is also facilitates improved uniformity of the obtained silicon ingot. Thus, the powder feeding rate of 10 g/min with the removal efficiency of 46% was the best
T. Lu et al. / Vacuum 143 (2017) 336e343
selection in this study. Comparing with the melting of metallurgical-grade silicon block, which has been largely used at present, this continuous powder feeding melting method can conserve melting time to a certain degree during the powder feeding process, which means indirect cost savings. The electron beam melting of silicon powder is a complex system, and the solution can be divided into a thin stratosphere on the surface and a troposphere beneath the stratosphere. The falling depth of the silicon powder may markedly influence the effects of impurity removal, which will be the focus in our subsequent study.
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Acknowledgment [11]
The authors gratefully acknowledge financial support from the National Natural Science Funds of China (Grant No. 51574057). References [1] A.R.A. Dezfoli, W.-S. Hwang, J. Augusto, A.K. Shukur, S. Tzeng, Modeling of poly-crystalline silicon ingot crystallization during casting and theoretical suggestion for ingot quality improvement, Mater. Sci. Semicond. Process 53 (2016) 36e46. [2] B. Gao, K. Kakimoto, Three-dimensional analysis of dislocation multiplication in single-crystal silicon under accurate control of cooling history of temperature, J. Cryst. Growth 396 (2014) 7e13. [3] D. Casenave, R. Gauthier, P. Pinard, A study of purification process during the elaboration by electron bombardment of polysiliocn ribbons designed for photovoltaic conversion, Sol. Energy Mater. Sol. cells 5 (1981) 417e423. [4] Qiang Wang, Wei Dong, Yi Tan, Dachuan Jiang, Cong Zhang, Xu Peng, Impurities evaporation from metallurgical- grade silicon in electron beam melting process, Rare Met. 30 (2011) 274e277. [5] H.M. Noor ul Huda Khan Asghar, Yi Tan, Shuang Shi, Dachuan Jiang,
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Continuous electron beam melting technology of silicon powder by prefabricating a molten silicon pool Tong Lu a, b, Yi Tan a, b, *, Shuang Shi a, b, Xiaoliang Guo b, Jiayan Li a, b, Dengke Wang b a b
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Laboratory for New Energy Material Energetic Beam Metallurgical Equipment Engineering of Liaoning Province, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2017 Received in revised form 23 June 2017 Accepted 24 June 2017 Available online 27 June 2017
We successfully achieved the continuous melting of silicon powder in a vacuum electron beam melting system. The characteristic of electron beam melting of silicon powder was revealed by comparing the three different approaches to prefabricate the molten silicon pool. The application of silicon block can be regarded as the most efficient method to prefabricate a stable molten silicon pool. Afterward, the silicon powder was added into the molten silicon pool at different rates by a spiral silicon powder feeding device. Finally, all of the silicon powder was melted completely. Although the P removal efficiency decreases with increasing powder feeding rate during the powder feeding process, an appropriate increase in the powder feeding rate is not only beneficial to improve the melting efficiency of silicon powder but also conductive to the improvement in uniformity of the obtained silicon ingot. Therefore, the powder feeding rate of 10 g/min was the best option in this study. Compared with the melting of silicon block, which has been wildly applied, this continuous powder feeding process can be beneficial to saving smelting time and the total cost. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Continuous electron beam melting Silicon powder Prefabricating molten silicon pool Removal efficiency Uniformity
1. Introduction In the recent years, the crystal silicon solar cells have rapidly developed because energy and environment problems, and polysilicon is the most wildly used materials for solar cells [1,2]. The electron beam melting purification process is an important part of the physical metallurgy method to produce solar energy polysilicon since 1980's [3]. Considering that the electron beam melting technology can provide a high temperature and high vacuum smelting environment, its combination with the melt agitation effect caused by the temperature gradient can help impurities diffuse directly to the melt surface, thus, this approach exerts an evident removal effect on volatile impurities, such as P, O and Al [4e7]. However, electron beam melting cannot achieve a continuous feeding of raw material at present because of the use of silicon blocks. Normally, raw material needs to be fed and subjected to a vacuum after repeated cooling to ambient temperature, which not only reduces production efficiency but also consumes considerable
* Corresponding author. School of Materials Science and Engineering, Dalian University of Technology, No.2 Linggong Road, Ganjingzi District, Dalian 116023, China. E-mail address: [email protected] (Y. Tan). http://dx.doi.org/10.1016/j.vacuum.2017.06.037 0042-207X/© 2017 Elsevier Ltd. All rights reserved.
energy. Thus, another melting method should be introduced to improve the production efficiency [8]. Continuous powder melting is one of the best solutions to achieve this aim. Although J.C.S. Pires et al. have already chosen silicon powder as the raw material, they didn't consider the continuous powder feeding process [9,10]. Compared with silicon blocks, silicon powder, which possesses better fluidity, is easier to use for achieving continuous feeding of raw material. Moreover, silicon powder possesses a large specific surface area and fast melting speed, thus possibly improving the production efficiency substantially when a proper melting procedure is applied [11]. Silicon powder also is extensively sourced. For instance, in the broad application of the diamond wire saw technology for silicon wafer slicing, a large amount of silicon powder waste with the adhering oxidation film and volatile organic matter is difficult to be recycled [12e15]. Therefore, if the problem of continuous electron beam melting technology of silicon powder can be solved, this kind of silicon powder can completely be used as the silicon powder raw material for electron beam melting after a certain cleaning and drying process, which can improve the fluidity of silicon powder. This strategy can not only solve the problem of continuous production of electron beam melting at present stage but also solve the growing silicon powder waste recycling problem in the future, which is very important in reducing the overall cost
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reduction of the preparation of crystalline silicon solar cells. However, powder melting has always been one of the biggest issues in the industry. With the limitations of the special heating principle and the rigorous vacuum environment of the electron beam melting system under consideration, continuously melting silicon powder under such a harsh environment will be extremely difficult. Therefore, in this paper, we focused on the study of continuous electron beam melting of silicon powder. After trying three different prefabricating methods of molten silicon pool, we decided on the most efficient continuous melting process. Moreover, we designed a powder feeding device that can add silicon powder continuously at different rates. Furthermore, we also analyzed the feasibility of powder melting by prefabricating molten silicon pool in theory, illustrated the distribution of P in the silicon ingots after powder feeding, revealed the effect of the powder feeding rate on the removing efficiency of P and the homogeneity of resistivity during the powder feeding process briefly, and compared with the traditional melting of silicon block at present. 2. Experimental procedures 2.1. Raw materials and experimental apparatus High phosphorus silicon powder with 12.55 ppmw phosphorus was selected as the powder raw material (particle diameter varying between 150 and 170 mm, and the median diameter was 160 mm) and low phosphorus silicon block with 1 ppmw phosphorus was used for forming the molten silicon pool to analyze the removal efficiency of volatile impurities during the powder melting process and observe the distribution of volatile impurities in the silicon ingot samples after melting. The experiments were conducted in an electron beam melting furnace. The experimental apparatus primarily consisted of a vacuum chamber, an electron beam gun with an accelerating voltage of 30 kV, a water-cooled copper crucible, and our self-designed spiral silicon powder feeding device that can adjust the powder feeding rate by controlling the rotation rate of the motor. The specific schematic of the experimental apparatus are
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shown in Fig. 1. 2.2. Melting process of silicon powder by electron beam Two processes are required to achieve continuous electron beam melting of silicon powder. The first process was the formation of a stable molten silicon pool, and the second process involved the continuous powder feeding melting. During the first process, three different methods were tested to form a stable molten silicon pool, and we selected the best method to do the continuous powder melting by considering various factors. 2.2.1. Prefabrication of a stable molten silicon pool 2.2.1.1. Direct melting of silicon powder. A 300-g portion of high P silicon powder raw material was directly placed into the watercooled Cu crucible after ultrasonic cleaning for 20 min and drying to constant weight. The bombardment voltage can be increased to 30 kV after the electron beam equipment was vacuumed to 2.4 102 Pa, and then electron beam was added gradually until the electron beam current reached to 100, 150, or 300 mA. 2.2.1.2. Mixture melting of silicon powder and silicon block. High P silicon powder (200 g) was placed into the water-cooled Cu crucible first, and then 100 g silicon blocks were putted on the surface of the silicon powder. The area of the silicon block accounted for about 60% of the whole surface. The whole experiment was under the condition of 200 mA electron beam current. First, the electron beam was focused on the silicon blocks to form some small molten silicon pools. After approximately 2 min, the position of the light spot of electron beam was moved to the margin of the small molten silicon pool to make the silicon powder melt gradually. Finally, a whole piece of molten silicon pool can be formed. 2.2.1.3. Direct melting of silicon block. 300 g low P silicon blocks were placed into the water-cooled Cu crucible. After the door of furnace was closed, the electron beam equipment was vacuumed to
Fig. 1. The experimental apparatus of electron beam melting of silicon powder.
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2.4 102 Pa, and then high voltage was started. When bombardment voltage increased to 30 kV, the electron beam was gradually increased until the electron beam current reached 300 mA. Silicon blocks were melted under this condition. The electron beam current was increased to 400 mA with 4.8 kW power after the silicon block was completely melted. 2.2.2. Continuous powder feeding melting of silicon powder High P silicon powder raw material was added into the continuous spiral powder feeding device after ultrasonic cleaning for 20 min and drying to constant weight. After the formation of the molten silicon pool, the liquid silicon was smelted with the electron beam current held constant for about 20 min to form a stable molten silicon pool. Meanwhile, the spiral powder feeding device was activated on the premise of continuous melting. According to different silicon powder adding rates, the corresponding rotational speeds of the motor were set (in this experiment, silicon powder adding rates were 5, 10, and 15 g/min respectively). The whole process of silicon powder feeding lasted for 10 min, and the electron beam gun was shut down when the powder feeding process was over. Finally, the silicon ingot was taken out after natural cooling for 30 min. 2.3. Composition detection ICP-MS was adopted to analyze the composition of the raw materials and the silicon ingots. This step was conducted mainly to study the distribution and the removal efficiency of volatile impurities (P) in silicon ingots after melting. To obtain the impurity distribution in the silicon ingots, we cut off four samples for composition analysis. The specific sampling method is shown in Fig. 2. The whole impurity content of silicon ingot can be regarded as the average of four samples. A KDY-1 four-point probe resistivity tester was used to analyze the distribution of resistivity of silicon ingot with respect to reflect the uniformity of the overall composition of the silicon ingot from the side. 3. Results and discussion 3.1. Melting results in samples subjected to different molten silicon pool prefabricating methods For direct melting of silicon powder, only trace melting occurred in the silicon powder after 3 min at the electron beam current of 100 mA because of the energy was too weak. When the current was increased to 150 mA, the amount of the melted silicon powder increased, but the silicon powder flew out of the water-cooled Cu crucible continuously. After approximately 2 min, only a small amount of silicon powder remained in the crucible, and a small silicon block can be observed. We can see the same phenomenon, in
which the silicon powder was splashed all over the furnace, when the electron beam current reached 200 mA. For determining the cause of this problem, the silicon powder was treated with 15% sodium hydroxide solution first until abundant bubbles can be observed. The following melting process shows that the pretreatment can effectively prevent the splashing phenomenon, and the silicon powder can be melted continually. The most plausible cause of the splashing phenomenon is the oxide layer on the surface of the silicon powder. The small silicon block which can be obtained after the electron beam melting of the untreated silicon powder may originate from the silicon powder whose oxide surface layer was punctured by the charge. Fig. 3 shows the specific direct melting process of silicon powder. The most likely explanation why we cannot directly melt silicon powder by the electron beam can be represented by Fig. 3 (b). Given that the surface of the silicon powder always coated with a thin film layer of non-conductive silicon dioxide, the negative charges can be enriched on the surface of each silicon powder as the electron beam continuously injected into the silicon powder. When a certain level is reached, the charges can produce a sufficiently large repulsion to make the silicon powder splash, which can be seen in Fig. 3 (c), and this process is not correlated to the electron beam current. Finally, this phenomenon can lead to the failure of the melting of silicon powder. Although we can solve this problem by alkali washing or other methods to destroy the silicon dioxide film, the additional process can substantially increase the cost of melting. Therefore, direct melting of silicon powder is not an appropriate option. As we know that, the silicon block is invariably selected as the raw material for melting in the industry. Thus, we mixed the silicon block and silicon powder together to achieve the melting of the silicon powder with the aim of preventing the splash of the silicon powder by focusing the electron beam on the silicon block first. Fig. 4 shows the specific situation during melting process, and we can see from Fig. 4 (c) that the silicon powder was completely melted into a silicon ingot. However, the melting rate of the silicon powder was extremely slow, with complete melting of the silicon powder requiring 36 min. Compared with the silicon block, silicon powder with the same mass possesses more interfaces, and the heat transfer between two silicon powder particles exerts a poor effect. Therefore, as shown in Fig. 4 (b), the whole melting rate depends on the infiltration rate of silicon fluid between the silicon powders, which is relatively slow. The whole operation process is more complicated, and the experiment is poorly repeatable. Therefore, this approach is a poor option, as well. However, from the previous sections, we learned that the formation of the molten silicon pool can be effective to prevent silicon powder splashing. Therefore, we decided to prefabricate a molten silicon pool with only the silicon block first, which requires approximately 10 min to form a molten silicon pool. Then, the silicon powder was added into the center of the molten silicon pool by
Fig. 2. The schematic diagram of sampling for composition analysis.
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Fig. 3. Direct melting process of silicon powder by electron beam melting. (a) Initial state of silicon powder, (b) Electron beam injection process, (c) Final state of silicon powder.
Fig. 4. Mixture melting process of silicon powder and silicon block by electron beam melting. (a) Initial state of the mixture, (b) Selective melting process of silicon block, (c) Final silicon ingot.
using the spiral silicon powder feeding device to achieve continuous melting of the silicon powder. Finally, we successfully achieved continuous silicon powder melting by this method, which is not only easy to operate but also offers cost saving. Fig. 5 shows the longitudinal section of the silicon ingots at different powder feeding rates. Evidently, all the silicon powder was completely melted.
3.2. Feasibility analysis of silicon powder melting by prefabricating molten silicon pool To simplify the calculation, we selected one silicon powder for the research and made several assumptions as follows, and the
mathematical model of melting process of single silicon powder is shown in Fig. 6: 1 The total melting process of silicon powder is under constant pressure 2 The falling resistance of silicon powder is zero in a vacuum environment 3 Silicon powders are monodisperse spherical particles, whose volume is conserved before melting 4 The temperature of molten silicon pool is constant 5 No temperature gradient occurs within the silicon powder 1) Calculation of the time required for completely melting silicon powder
Fig. 5. Morphology of the longitudinal section of the silicon ingot, with different powder feeding rate. (a) 5 g/min, (b) 10 g/min, (c) 15 g/min.
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Let us suppose that the mass of silicon powder is m and the radius is r, from Newton's second law, we can get:
mg kv rL gV ¼ m
dv dt
(10)
rL and rS are the densities of liquid silicon and solid silicon, respectively. With G ¼ g 1 rrL , we can get S
k dt ¼ m
Fig. 6. Mathematical model of melting process of single silicon powder.
Silicon powder melting occurs in two processes. First, silicon powder was heated from room temperature to the melting point. Then, the silicon powder undergoes an isothermal phase change process. Thus, the time needed for melting silicon powder was the sum of the duration of the two processes mentioned above. In the first heating stage of silicon powder, at constant pressure
dQ ¼ dH ¼ CP mdT
(1)
(2)
Therefore,
CP mdT ¼ hðTL TÞAdt
(3)
A is the surface area of silicon powder, and m is the mass of silicon powder.The general solution of the differential equation is
T ¼ TL ce
rr3hC t
(4)
S P
When t ¼ 0, T ¼ T0, therefore, rr3hC t
T ¼ TL ðTL T0 Þe
S P
(5)
The time required for heating silicon powder is as follows and can be recorded as t1:
rr C T Tm t1 ¼ S P ln L 3h TL T0
(6)
In the transformation stage of silicon powder:
mL ¼ qAt2
(7)
L is the phase transformation latent heat of silicon. Therefore, the time required for phase transformation is shown below, recorded as t2:
t2 ¼
mL r rS L ¼ qA 3hðTL Tm Þ
(8)
Consequently, the total time required to melt the silicon powder is
rr C T Tm r rS L t ¼ t1 þ t2 ¼ S P ln L þ 3h TL T0 3hðTL Tm Þ
(11)
k v Gm
General solutions:
v¼
m k G cem t k
(12)
m pffiffiffiffiffiffiffiffi k m G G 2gh em t k k
(13)
pffiffiffiffiffiffiffiffi Initial condition: when t ¼ 0, v ¼ 2gh:
v¼
The actual falling depth calculation of silicon powder is
Due to the thermal balance relationship
dQ ¼ qAdt
k v d Gm
(9)
2) The calculation of the actual falling depth of silicon powder in the molten silicon pool prior to complete melting
s¼
pffiffiffiffiffiffiffiffi k pffiffiffiffiffiffiffiffi m m m m m Gt þ G 2gh em t G 2gh k k k k k
(14)
Table 1 shows the basic parameters of the silicon powder. After substituting these basic parameters into the above equations, we can calculate several results from the calculations, which are shown in Table 2: In general, silicon blocks are selected as the raw material for electron beam smelting. First, the silicon blocks are melted using a low electron beam current to form a molten silicon pool, and then the silicon pool is smelted using a higher electron beam current. The highest surface temperature of the molten silicon pool can reach up to 2000 C. At this time, volatile impurities, such as P, can be removed from the surface of the silicon evaporation pool. While, the impurities within the silicon pool can only be removed by diffusing to the surface. This diffusion is a relatively slow process, and a large amount of energy is needed to maintain the stability of the molten silicon pool. However, for silicon powder melting, we can see that the melting time of silicon powder is extremely short that is 0.11 ms, because of the high specific surface area. After dropping into the molten silicon pool, the silicon powder can melt rapidly in the surface layer with the thickness is 0.31 mm. At this point, the impurities in the powder can quickly spread to the surface of the molten pool, and large amounts of volatile impurities can be removed during this process. Therefore, compared with the silicon block, this kind of powder feeding melting process could be beneficial to conserve the melting time.
Table 1 The basic parameters of silicon powder. Parameter
Symbol
Value
Initial temperature (K) Melting point (K) Phase change latent heat (kJ/mol) Isobaric heat capacity of solid (J/(mol$K)) Radius (m) Solid density (g/cm3) Liquid density (g/cm3) Falling height (m) Viscosity of molten silicon (mm2/s)
T0 Tm L CP r
298 1683 44.44 927 0.00008 2.35 2.53 0.3 0.000756
rS rL H
h
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Table 2 The calculated results of the melting process of silicon powder. Calculation results
Value
The time required for melting silicon powder (s) The actually falling depth of silicon powder in the molten silicon pool (m)
0.00011 0.00031
3.3. Analysis of the distribution and the removal effectiveness of phosphorus in silicon ingots after melting As shown in Fig. 7, the P content in other parts of the silicon ingot increased with the powder feeding rate, however, the P content at the bottom (Sample 4) almost remained the same. Because when the silicon blocks were completely melted into a molten silicon pool, a temperature gradient will occur between the high temperature surface and the low temperature water-cooled Cu crucible, the bottom of the crucible can form a solid silicon layer with a certain thickness as a result of the low temperature. And solid phase diffusion of P is a relative slow process. Therefore, the P content at the bottom was less affected by the subsequent powder melting process. The highest P content was found near the dividing line of solidification (Sample 2) because the solubility of P in liquid was higher than that in solid. As the surface of the silicon pool radiates heat to the environment and the bottom of the silicon pool transmits heat to the water-cooled Cu crucible, the silicon molten pool will experience a directional solidification process from the outside surface to the inside after the electron beam gun is shut off. The area near the dividing line of solidification is the final solidification region, and thus contains the highest P content in the silicon ingot. Notably, although the silicon powder was completely melted in the surface layer and most of P can be removed in this phase, the P content in the middle of the molten silicon pool (Sample 3) can still increase, because of the strong stirring effect of electron beam melting. This part of P can only be removed by diffusion to the surface of the molten silicon pool, which is a relatively slow process. So, the P content in sample 3 is slightly higher than the sample 1. Therefore, combined with the calculated results of the melting process of silicon powder, the total melting and P impurity removing process of silicon powder by prefabricating molten silicon pool method can be represented by Fig. 8. The removal efficiency of phosphorus (h(P)) with different powder feeding rate was calculated using Eq. (15):
hðPÞ ¼ 1
(15)
where Ci(P) (ppmw), CB(P) (ppmw), and CP(P) (ppmw) are the contents of P in the final silicon ingot, silicon block used for prefabricating molten silicon pool and silicon powder, respectively. mB (g) is the mass of the silicon block, and vP (g/min) and t (min) are the powder feeding rate and duration. Fig. 9 shows that the removal efficiency of P decreased with the increase in powder feeding rate, which decreased sharply to 38.6%, at the powder feeding rate of 15 g/min. This finding suggests that the excess amounts of P impurity in the surface layer of the molten silicon pool were transferred to the inside before removal by evaporation on the surface, thus resulting in lower P removal efficiency. Owing to the limitation of the evaporation of P, reduction of the removal efficiency of volatile impurities can occur although the silicon powder can be melted completely with a higher powder feeding rate. Therefore, the powder feeding rate must be reasonably controlled to maximize the advantage of volatile impurity removal offered by powder melting. In this study, comparing with that at the powder feeding rate of 5 g/min, the removal efficiency of P only decreased by 0.5% when the powder feeding rate reached to10 g/min during the powder feeding process. Moreover, we can see from Fig. 10 that the powder feeding rate can affect the physical properties of material, such as resistivity homogeneity. The feeding rate of 10 g/min results in the best uniformity, which means that appropriate powder feeding can make the silicon powder spread out evenly along the center of the molten silicon pool, facilitating the improvement of the overall composition uniformity of the silicon ingot. Thus, we may select 10 g/min as the actual powder feeding rate, because of higher product efficiency, relatively high impurity removal rate, and superior physical properties. According to the previous work in our laboratory [16], when the silicon block was selected as the raw material, we can determine that the overall mass transfer coefficient of phosphorus KT(P) is 7.45 106 m/s at 4.8 kW and obtain the relationship between P content and the melting time, which is shown in Fig. 11, by using Eq. (16):
ln
Fig. 7. The distribution of P in the silicon ingots after melting.
CiðPÞ ðmB þ vp tÞ CBðPÞ mB þ CPðPÞ vp t
ct S ¼ KTðPÞ t V c0
(16)
where Ct is the P content at t s and C0 is the content at 0 s. S is the melt surface, and V is the volume of the molten silicon pool, which can be identified as 153 cm2 and 150 cm3 in this calculation, respectively. When the P content decreased from 3.88 ppmw to 2.09 ppmw, the time needed for continuous melting of silicon powder was 600 s less than the 813 s melting time for the silicon block. Thus, this approach can shorten the melting time and save the cost. The final P content in the silicon ingot was 2.09 ppmw, which failed to meet the solar degree standards, after the powder was fed for 600 s. However, the subsequent purification process after the ceasing of powder feeding can refer to the electron beam melting process of silicon block, which has the same p content. The optimum melting parameters and energy consumption can be
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Fig. 8. The schematic diagram of the P impurity removal during continuous powder feeding electron beam melting process.
Fig. 9. The removal efficiency of P after melting with different powder feeding rate. Fig. 11. Comparison of P removal rate between continuous melting of silicon powder (10 g/min) and traditional melting of silicon block at 4.8 kW.
determined by another work in our laboratory [17].
4. Conclusions We achieved continuous electron beam melting of silicon powder by prefabricating molten silicon pool with silicon block as the precursor. As determined from the molten silicon pool prefabricating process, the electron beam cannot melt silicon powder directly, possibly because of the surface oxide layer on the silicon powder. The feasibility analysis in theory showed that the silicon powder mainly melted in the surface layer of the molten silicon
Fig. 10. The distribution of resistivity of silicon ingot with different powder feeding rate. (a) 5 g/min; (b) 10 g/min; (c) 15 g/min.
pool, which aids in the removal of volatile impurities, such as P, in silicon powder. The actual removal efficiency can reach 46.5% even during the powder feeding process without subsequent purification, and the removal efficiency will decrease with the increasing of powder feeding rate. However, an appropriate increase in the powder feeding rate is not only beneficial to improve the melting efficiency of silicon powder, but is also facilitates improved uniformity of the obtained silicon ingot. Thus, the powder feeding rate of 10 g/min with the removal efficiency of 46% was the best
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selection in this study. Comparing with the melting of metallurgical-grade silicon block, which has been largely used at present, this continuous powder feeding melting method can conserve melting time to a certain degree during the powder feeding process, which means indirect cost savings. The electron beam melting of silicon powder is a complex system, and the solution can be divided into a thin stratosphere on the surface and a troposphere beneath the stratosphere. The falling depth of the silicon powder may markedly influence the effects of impurity removal, which will be the focus in our subsequent study.
[6]
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[8]
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[10]
Acknowledgment [11]
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