METALLURGICAL SILICON REFINING BY TRANSIENT DIRECTIONAL SOLIDIFICATION

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1 METALLURGICAL SILICON REFINING BY TRANSIENT DIRECTIONAL SOLIDIFICATION Moyses L. LIMA 1,2, Marcelo A. MARTORANO 2, João B. F. NETO 1 1 Laboratory of Metallurgical Processes - Institute for Technological Research IPT, Avenida Professor Almeida Prado 532, , São Paulo/SP, Brazil 2 Department of Metallurgical and Materials Engineering University of São Paulo, Avenida Professor Mello Moraes 2463, São Paulo/SP, Brazil Keywords: Metallurgical silicon, Silicon refining, Directional Solidification, Macrosegregation Abstract Directional solidification is an essential refining step to obtain solar grade silicon from metallurgical silicon. This step can be carried out in a Bridgman furnace, where nearly constant temperature gradients and solidification velocities are imposed on the solid-liquid interface. In the present work, this directional solidification was conducted in a static furnace, in which large temperature gradients and low solidification velocities were enforced to increase macrosegregation. The resulting ingots were analyzed regarding their macrostructures, microstructures and chemical composition. Using measured cooling curves in the ingot as boundary conditions, a mathematical model based on the concept of a stagnant liquid layer at the solid-liquid interface was implemented to predict the macrosegregation profiles. The chemical analyses of the ingots show macrosegregation of several impurities to the ingots top. The mathematical model indicates that liquid convection plays an important role in stabilizing the planar solid-liquid interface, increasing the macrosegregation of impurities. Introduction Most of silicon used for photovoltaic (PV) energy production is obtained by chemical processes, such as, Siemens and its derivations [1]. However, these chemical processes have drawbacks related to relatively high energy consumption and investment costs, and to safety and environmental problems. In order to overcome these difficulties and to fulfill the silicon demand for PV applications, several metallurgical routes are under development [2] to produce silicon of enough purity to PV applications (SoG-Si) from metallurgical grade silicon (MG-Si). Metallurgical routes involve different steps among which the directional solidification is pointed out as a fundamental one [1]. Directional solidification can cause macrosegregation of impurities to the last part of the ingot to solidify. The impurity segregation during solidification is related to the low solute partition coefficient (k 0 ) between liquid and solid silicon. Most of the impurities in MG-Si have k 0 << 1, except for boron, phosphorus and oxygen, which require specific steps of the metallurgical routes to be eliminated. The equilibrium partition coefficient and the solubility limits for the most important impurities in MG-Si are shown in table 1. Studies regarding the formation of macrosegregation during MG-Si directional solidification were done using methods like Czochralski [4], Bridgman [3], and electron beam furnaces [5]. In these studies, the influence of the solid-liquid interface velocity on the macrosegregation profile under constant thermal gradient was examined. The objective of the present work is to analyze 279

2 the macrosegregation profiles of metallic impurities in silicon ingots obtained by transient directional solidification using a static solidification apparatus. Different experimental conditions were tested to study the effect of the solid-liquid interface velocity and of the axial temperature gradient on the macrosegregation profiles. Table 1: Equilibrium partition coefficient (k 0 ), diffusion coefficient in the liquid (D l ), solute solubility in solid silicon (C sol ) and the concentration of the main metallic impurities in the MG-Si (C 0 ). Element k 0 * D l (m²/s)* C sol (ppmw)* C 0 (ppmw)** Fe 8.0 x x Ni 8.0 x x Ti 3.6 x x V 4.0 x x Mn 1.0 x x Cr 1.1 x x Cu 4.0 x x Al 2.0 x x *From reference [3];** Obtained by ICP-OES analyses Experimental Methodology A static electric resistance furnace was used to obtain silicon ingots by upward directional solidification. In this furnace there are two sets of resistive heaters (MoSi 2 ) with individual controls: one at the top of the internal chamber and another at the lateral wall. During melting, solidification, and cooling, an argon atmosphere was maintained over the melt surface to prevent excessive oxidation. Cylindrical ingots were obtained using 8kg of MG-Si (composition in table 1) melted in crucibles coated with silicon nitride. After melting, the furnace was held for 40min. at 1600 C to homogenize the liquid silicon temperature at approximately 1550 C. The solidification step began by positioning a copper water-cooled block in contact with the crucible base and cooling down the furnace at a controlled rate by adjusting the temperature of the heaters. The different conditions for each experiment are given in table 2, considering as a reference case. Table 2: Experimental conditions: crucible base material, ingot length(mm) and the use of controlled cooling during the solidification step. Experiment Crucible base material Ingot length (mm) Controlled cooling Graphite-clay 100 Yes _N Graphite-clay 100 No Graphite 100 Yes Graphite-clay 130 Yes The silicon temperature was measured during melting, solidification, and cooling using two type- B thermocouples (Pt-6 pct Rh / Pt-30 pct Rh) located along the ingot axis. The furnace heaters were controlled to cool down the furnace in the same way in all experiments, except for the GC- 100_N. In this experiment, the heaters were turned off at the beginning of the solidification step. 280

3 After cooling down the system, the ingots were cut in the longitudinal section. The central part was cut for macrostructure, microstructure, and chemical analysis by inductively coupled plasma mass spectrometry (ICP-MS). Mathematical Model Description A mathematical model was proposed and implemented to predict the formation of macrosegregation of the impurity elements during directional solidification of the ingots. To derive the model equations, the volume averaging method [6] and the stagnant liquid layer concept were used. The stagnant liquid layer concept was proposed by Burton et al. [7] to approximately consider the effect of liquid convection in the mass transport during solidification by a planar solid-liquid interface. In this concept, a liquid layer is assumed adjacent to the interface in which the solute transport takes place only by diffusion. The concentration of impurities in the remainder liquid is considered uniform. To correctly satisfy mass conservation at the interface between the stagnant liquid layer and homogeneous liquid, an equation proposed by Martorano et al. [3] was used. In the model, the following simplifying hypotheses were assumed: (1) unidirectional heat and mass transfer; (2) binary alloy (diluted solution); (3) constant thickness of stagnant liquid layer (δ BPS ); (4) the solute transport in the mushy zone and stagnant liquid layer takes place only by diffusion; (5) no solute diffusion in the solid; (6) the density, heat capacity and diffusion coefficient are constant; (6) local thermodynamic equilibrium at any solid-liquid interface; (7) the liquidus and solidus lines are straight. The equations, written for a rectangular coordinate system fixed at the ingot base, the initial and boundary conditions are summarized in table 3. Table 3: Equations, initial and boundary conditions of the mathematical model. Description Equation Initial condition Boundary conditions Constitutive relation - - Heat transport ( ) ( ) Solute conservation in the liquid Solute conservation in the solid Solute conservation in the stagnant liquid layer ( ) ( ) ( ) ( ) - ( ) - ( ) 281

4 The symbols in the equations of table 3 are: ε s and ε l are the volume fractions of solid and liquid, respectively; ρ is the silicon density; C 0 is the initial solute concentration; C l is the solute concentration in the liquid; C l is the solute concentration in the homogeneous liquid; C S is the solute concentration in the solid; D L is the solute diffusivity in the liquid ; is the solid-liquid interface position; ΔH f is the silicon latent heat; T B and T T were the cooling curves measured at the ingot base and top, respectively, which were adopted as boundary conditions of the first kind; k is the solute partition coefficient; K is the silicon thermal conductivity; C p is the silicon heat capacity; L is in the ingot length; is the position of the end of the stagnant liquid layer; δ BPS is the stagnant liquid layer thickness. The equations in table 3 were coupled by the equation of liquidus line where is the liquidus temperature, is the liquidus line slope, and is the MG-Si liquidus temperature. The equations in table 3 were solved numerically using the implicit finite volume method using a mesh of at least 1000 volumes and a time step of 0.01s. The model was tested using several analytical models [8, 9] for solute redistribution, showing excellent agreement. Results and discussion Cooling Curves Figure 1 shows the cooling curves measured in the silicon during cooling and solidification for experiments and, with the curves for the experiment. The initials points of the curves correspond to the time when the copper block was positioned at the crucible base. It was not possible to measure the temperature in experiment _N, because the thermocouple protection was disrupted after turning off the furnace. Figure 1-(b) shows that the cooling curves for experiments and are similar. Since the distance between the measuring points in experiment is 15mm greater than that in the, the lower temperature gradient (G) in experiment is evident Temperature ( C) x = 0mm (base) x = 90mm (top) Temperature ( C) x = 90mm (top) x = 0mm (base) x = 105mm (top) ,0 2,5 5,0 7,5 10,0 Cooling time (h) (a) (b) Figure 1 Cooling curves measured in silicon during solidification in experiments: (a) and G- 100 and (b) and. Macro and Microstructures The macrostructures of the longitudinal section of the ingots are shown in figure 2. It can be seen that the macrostructures are essentially formed by columnar grains parallel to the axial direction (solidification direction), as generally expected in directionally solidified ingots. In the GC- 100_N experiment, columnar grains show side branches, while sharp boundaries are observed in the grains from other experiments. A progressive increase of grain size exists in the ,0 2,5 5,0 7,5 10,0 Cooling time (h) 282

5 macrostructures of the ingots,, and, possibly due to a competitive growth mechanism. A type of grain structure transition occurs in the macrostructure at about 70mm from the ingot base, above which the directional growth is not evident. Analogous transitions also exist in the and ingot macrostructures. Top Bottom 1cm (a) (b) (c) (d) Figure 2 Macrographs of the longitudinal section of the ingots (central portion) obtained in experiments: (a), (b) _N, (c) and (d). Representative micrographs are shown in figure 3 for two different positions along the axial direction. In all ingots except GA-100_N, it is possible to observe the formation of a region free of intermetallic compounds. This type of region is an evidence of the reduction in the local impurity concentration [3]. These refined regions extend from the ingot base up to 70mm for, 40mm for, and throughout the ingot for. 75mm 55mm 75mm 55mm 5mm 10mm 5mm 5mm (a) (b) (c) (d) Figure 3 Micrographs at two different positions relative to the ingot base along the axial direction: (a), (b) _N, (c) and (d). Below each micrograph is indicated the distance from the ingot base. The arrows point to small intermetallic compounds in the microstructure. 283

6 Macrosegregation Profiles Profiles of Fe, Al, Mn, and Ti concentration relative to the average concentration (C imp /C 0 ) along the ingot axis as a function of the relative position in the axial direction (x/l) are given in figure 4. The quantitative limits () of the chemical analysis technique are indicated in the profiles. No evidence of important macrosegregation was seen in the micrographs of ingot _N, consequently, macrosegregation profiles were not measured. The concentration profiles show macrosegregation towards the ingot top (x/l = 1) for experiments and. The position in which the first intermetallic compounds were found in the microstructure of experiments and coincides with the points in which the concentration of impurities increases abruptly in the profile (Figure 4) and also coincides with the transition in grain structure observed in the macrographs (Figure 2(a) and (d)), indicating that the increase in impurity concentration, the formation of intermetallic compounds, and the change in grain morphology are all related. These observations are in agreement with the results of Yuge et al. [5], who related the macrostructure transition and intermetallic compounds to the interface morphology transition from planar to cellular/dendritic. For experiment, all concentrations in the profile are below the initial average concentration (C 0 ), which is consistent with the absence of intermetallic compounds in all micrographs. The impurities were segregated to the regions near the lateral surface, which was not analyzed. 1.E+02 1.E+00 1.E+02 1.E+00 C Fe / C 0 C Al / C x/l 1.E+02 1.E+00 (a) x/l 1.E+02 1.E+00 (b) C Mn / C 0 C Ti / C x/l x/l (c) (d) Figure 4 Profiles of relative concentration (C imp /C 0 ) measured by ICP-MS along the axial direction at the ingot center for (a) Fe, (b) Al, (c) Mn, and (d) Ti as a function of the relative position, x/l. Mathematical model In Figure 5(a) and (b), calculations with the present mathematical model show that the velocity of the solid-liquid interface (V) and the temperature gradient (G) at this interface were essentially equal (V < 5.0 x 10-6 m s -1 and G ~800 Km -1 ) in experiments and up to ~0.03m 284

7 from the ingot base. However, both the velocity and temperature gradient in experiment were larger (V~ 1.5 x 10-5 m s -1 ). These results are consistent with the cooling curves presented in figure 1 and are probably related to the larger thermal diffusivity of graphite (7.3 x 10-5 ms -2 ), which is the material of the crucible base in experiment, in comparison with the conductivity of graphite-clay (1.8 x 10-6 ms -2 ), the material of the crucible base for the and experiments. After solidification of this part of the ingots (~0,03m), the interface velocity for experiment becomes larger and the temperature gradient lower than those for. These results are probably related to the longer ingot length in experiment (Table 2). The decrease in temperature gradient and the increase in interface velocity for experiment might have destabilized the planar solid-liquid interface, which might have become cellular/dendritic at about 0.04m from the ingot base, causing the precipitation of intermetallic compounds and the increase in impurity concentrations observed at this position. The combination of larger solid-liquid interface velocities and temperature gradients in the ingots of experiment might be responsible for the severe macrosegregation and larger purification effect in comparison with the other experiments. 2.0E E+03 Interface velocity - v (m/s) 1.0E-05 G (K/m) 4.0E E E E E Position (m) 0.0E Position (m) (a) (b) Figure 5 Mathematical model results: (a) solid-liquid interface velocity (v) and (b) temperature gradient (G) as a function of the distance relative to the ingot base along the axial direction. Figure 6 shows the concentration profiles of Fe calculated with the mathematical model (with and without convection) and that measured in experiments and. The calculated profiles show an abrupt increase in concentration similar to that seen in the measured profiles. Nevertheless, this abrupt increase occurs earlier (nearer the ingot base) for the calculations without convection, i.e., only diffusive solute transport in the liquid. When convection is considered by adopting a stagnant liquid layer of thickness 45 x 10-3 m for and 3.7 x 10-3 m for, the calculated and measured abrupt concentration increase occurs at approximately the same ingot position. For, the stagnant layer was an order of magnitude thinner than that in, possibly as a result of more vigorous convection in GC Note that it is not possible to analyze the quality of the model results when the measured concentrations are below the quantification limit (), representing most of the measured profile in experiment. The calculated profile with convection in experiment coincides with that given by the Scheil model up to ~ 0.06m, indicating that convection was very efficient in homogenizing the liquid composition. In all, the results in figure 6 indicate the fundamental role played by convection in the formation of macrosegregation during the directional solidification experiments in the present work. During the upwards directional solidification, convective currents might have originated from radial temperature gradients [10]. 285

8 1.E+00 Model with convection 1.0E+00 C Fe (wt%) Model without convection Scheil Experiment C Fe (wt%) 1.0E E-04 Model with convection Model without convection Scheil Experimental Position (m) (a) (b) Figure 6 Concentration profiles of Fe calculated using the mathematical model (with and without convection), measured in the experiments, and calculated with Scheil model: (a) ; (b). Conclusions Directionally solidified ingots were obtained from metallurgical grade silicon under transient conditions using a static electric resistance furnace with special control of heaters. The furnace heaters imposed relatively low solid-liquid interface velocities and relatively high temperature gradients. The grain macrostructure in most of the ingots consisted of columnar grains and the microstructure showed intermetallic compounds at ingot positions with larger impurity concentrations. An abrupt increase in the profile of impurity concentration coincided with the appearance of intermetallic compounds and a change in the columnar grain morphology. This transition seems to be related to a probable change in the solid-liquid interface morphology from planar to cellular / dendritic. The profiles of impurity concentrations showed that macrosegregation was more intense for the experiment with the lowest solid-liquid interface velocity and the largest temperature gradient. A mathematical model indicates that convective transport of solute played a fundamental role in increasing macrosegregation and in extending the region of a stable planar solid-liquid interface in the present work experiments. Acknowledgements 1.0E Position (m) Authors thank the support from Companhia Ferro-Ligas Minas Gerais (Minas Ligas) and from the National Bank for Social and Economic Development BNDES. References 1. J. Safarian, G. Tranell and M. Tangstad, Energy Procedia, vol. 20, pp , S. Pizzini, Solar Energy Materials & Solar Cells, vol. 94, pp , M. A. Martorano et al., Metallurgical and Materials Transactions A, vol. 42A, E. Kuroda and T. Saitoh, Journal of Crystal Growth, vol. 47, pp , N. Yuge, K. Hanazawa and Y. Kato, Materials Transactions, vol. 45, pp , J. Ni and C. Beckermann, Metallurgical Transactions B, vol. 22B, pp , June J. A. Burton, R. C. Prim and W. P. Slitcher, J. of Chem. Phys., vol. 21, pp , J. J. Favier, Acta Metall., vol. 29, pp , M. C. Flemings, Solidification Processing, McGraw-Hill, C. Beckermann, International Materials Reviews, vol. 47, pp ,