Solvent Refining of Metallurgical Grade Silicon Using Iron

Shaghayegh Esfahani

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Materials Science and Engineering University of Toronto

Shaghayegh Esfahani

Master of Applied Science

Materials Science and Engineering University of Toronto

2010 Abstract

Purification of metallurgical grade silicon (MG- Si) by a combination of solvent refining and physical separation has been studied. MG-Si was alloyed with iron and solidified under different cooling rates to grow pure Si dendrites from the alloy. The Si dendrites and FeSi2, that were formed after solidification were then separated by a gravity-based method. The separation method relies on significantly different densities of Si and FeSi2, and uses a heavy liquid with specific gravity between the two phases to float the former on the surface of a heavy liquid, while the latter sinks to the bottom. The effect of particle size and cooling rate on the Si yield and separation efficiency of the Si phase was investigated. The floated Si particles were further purified by removing the physically adherent Fe-Si phase, using an acid leaching method. Analysis of the produced silicon indicates that several impurity elements including P and B can be efficiently removed using this simple and low-cost technique.

In the Name of God

To my parents who have sacrificed so much for their children and for being an endless source of inspiration and love in my life.

and also,

To my husband Farshid for being supportive and caring.

iii

Acknowledgments

First and foremost I offer my sincerest gratitude to my supervisor, Prof. Mansoor Barati, who has supported me throughout my thesis with his patience and knowledge. I attribute the achievement at this academic level to his encouragement and effort and without him this thesis, too, would not have been completed or written. One simply could not wish for a better or friendlier supervisor.

I am grateful of Dr. Hiroshi Soda for his the time he has offered me during this period and for the encouragement he has given me each day.

I would like to thank Sal Boccia of MSE Dept. (SEM), Geroge Kretschmann of Geology Dept. (XRD) and Dan Mathers of Chemistry Dept. (ICP) for their continues assistance and support and the useful skills they have taught me.

I am indebted to my colleagues Dr. Murray Johnston, Yasha Chaugule and Karim Danaie who helped me in laboratory work.

I owe my deepest gratitude to my father Prof. Mohammad Reza Esfahani and mother Mrs. Narjes Ebrahmi whose unwavering support from my childhood to present has made this undertaking possible. I also wish to thank my husband Mr. Farshid Bahrami for his moral support. His encouragement brightened the frustrating moments of my work and helped me move on.

I would like to thank the professors in Iran for the knowledge they have provided me before starting my Masters degree, with special thanks to Prof. Vahdati and Dr. Moayed.

I would like to thank the University of Toronto Open Fellowship, ARISE Technologies Corporation and NSERC for their financial support.

Table of Contents

Acknowledgments...... iv

Table of Contents ...... v

List of Tables ...... viii

List of Figures ...... ix

List of Abbreviations ...... xiii

Chapter 1: Introduction ...... 1

Chapter 2: Literature Review ...... 3

2.1 Energy and the Environment ...... 3

2.2 Solar Energy ...... 4

2.3 Silicon ...... 4

2.4 Grades of Silicon and their Specifications ...... 4

2.5 Metallurgical Grade Silicon (MG-Si) ...... 5

2.6 Carbothermal Reduction of Silicon ...... 5

2.7 Solar Grade Silicon (SoG- Si) ...... 7

2.8 Impurities in Solar Grade Silicon ...... 8

2.9 Refining Methods ...... 9

2.9.1 Conventional Method: Siemens Process ...... 10

2.9.2 Metallurgical Methods ...... 11

2.9.2.1 Directional Solidification (Zone Refining) ...... 11

2.9.2.2 Acid Leaching ...... 13

2.9.3 Plasma Process ...... 14

2.9.4 Vacuum Refining ...... 15

2.9.5 Slagging ...... 15

2.9.6 Reactive Gas Blowing ...... 16

2.9.7 Solvent Refining ...... 16

2.10 Segregation Coefficient ...... 17

2.11 Effect of Ti and Ca Addition on Efficiency of Solvent Refining ...... 24

2.12 Examples of Binary Systems of Silicon ...... 25

2.13 Properties of Iron in Silicon ...... 26

2.13.1 Solubility of Iron in Silicon ...... 30

2.13.2 Effect of the Distribution of Iron Silicide ...... 32

2.14 Summary...... 33

Chapter 3: Experiments and Materials ...... 35

3.1 Experimental Procedure-Part One: Quenching after Complete Solidification ...... 35

3.1.1 Alloy Preparation ...... 36

3.1.2 Melting and Solidification ...... 38

3.1.3 Crushing and Sieving ...... 40

3.1.4 Physical Separation ...... 41

3.1.5 Leaching Experiments ...... 43

3.2 Experimental Procedure- Part Two: Quenching above the Eutectic Temperature ...... 44

3.3 Characterization ...... 46

3.3.1 Scanning Electron Microscopy (SEM) and Electron Diffraction X-Ray (EDX): ...... 46

3.3.2 X-Ray Diffraction (XRD) ...... 46

3.3.3 Inductively Coupled Plasma- Atomic Electron Spectrometry (ICP-AES): ...... 47

3.3.4 Electron Probe Micro-Analyzer (EPMA): ...... 47

Chapter 4: Effectiveness of Physical Separation ...... 49

4.1 Characteristics of Solidified Alloys...... 49

4.2 Effect of Specific Gravity of Heavy Medium on Separation of Si...... 51

4.3 Effect of Cooling Rate and Optimum Particle Size...... 55

Chapter 5: Extent of Silicon Purification ...... 58

5.1 Quenching after Complete Solidification ...... 58

5.2 Quenching above the Eutectic Temperature...... 64

5.3 Effective Segregation Coefficient of P and B ...... 68

5.4 Effectiveness of Impurity Removal ...... 69

5.5 Other Alloys ...... 72

5.5.1 Zinc ...... 72

5.5.2 Calcium ...... 75

Chapter 6: Summary, Conclusion and Future Work ...... 80

6.1 Summary and Conclusion...... 80

6.2 Future Work...... 81

Bibliography ...... 82

Appendix A: ICP-AES Detection Limits ...... 88

Appendix B: Industrial Economic Evaluation of the Proposed Process ...... 91

Appendix C: Publications and Presentations from this Research ...... 93

vii

List of Tables

Table 1-Impurities of metallurgical and solar grade silicon [7]...... 6

Table 2-Segregation coefficient of some impurities in metallurgical grade silicon at melting point of silicon [16, 25]...... 12

Table 3-Purification of Si by leaching [26]...... 13

Table 4-Segregation coefficient of impurities between solid Si and liquid Si-Al and those between solid Si and liquid Si at silicon`s melting temperature [8]...... 18

Table 5-The impurity content of initial Si (ppmw) and final Si in solvent refining of Si and Al [49]...... 22

Table 6-Maximum solubility of some elements in solid silicon from Ref. [25] ...... 30

Table 7-Chemical analysis of MG-Si used in the experiments by ICP-AES...... 36

Table 8-Chemical analysis of iron powder (from the supplier)...... 37

Table 9-Charactristics of the used materials...... 37

Table 10–Impurity content of initial MG–Si and refined silicon...... 66

Table 11-Tolerable concentration (ppmw) of impurities in Si before directional solidification compared with the concentration achieved in current study...... 67

viii

List of Figures

Figure 1-Furnace used for reduction of silica with carbon [14]...... 5

Figure 2-Impurity concentration verses efficiency curves in p-type silicon :(1) semiconductor-, (2) solar-, and (3) metallurgical-grade silicon [14, 16]...... 7

Figure 3-Diffusion coefficients of some impurities in silicon [20]...... 9

Figure 4-Schematic diagram of Siemens process [23]...... 11

Figure 5-Schematic diagram of the purification system [30]...... 15

Figure 6-Al-Si phase diagram [56]...... 17

Figure 7-Schematic diagram of TGZM 1. SiC heating element; 2. thermocouple connected to PID controller; 3. gas outlet tube; 4. stainless steel holder; 5. single crystalline silicon; 6. aluminum foil on which red phosphorus powder was stuck or Al-(0.3-1.1) wt-% B foil; 7. mullite tube; 8. thermocouple for measuring temperature of molten zone; 9. porous alumina boat 10. gas inlet tube; 11. alumina plate; 12. sponge titanium [45]...... 21

Figure 8-Mechanism of continuous solidification of Si from Si-Al melt under induction heating [50]...... 23

Figure 9-Si dendrites agglomerated at the bottom of the sample [50]...... 23

Figure 10-Ellingham diagrams for stability of (a) borides and (b) phosphides...... 27

Figure 11-Phase diagram of Fe-Si [56]...... 28

Figure 12-Correlation between segregation coefficient and solid solubility of impurities in MG- Si [25, 57-63] ...... 29

Figure 13-Solubilities of representative interstitial transition metals in Si below the eutectic temperature [20]...... 31

Figure 14-Solid solubility in silicon [26]...... 32

Figure 15- Flowchart of the experimental procedure...... 35

Figure 16-Mullite crucible...... 38

Figure 17-Muffle (box) furnace...... 39

Figure 18-Cooling profile of the samples...... 39

Figure 19-Sample cut in half and taken out of the crucible...... 40

Figure 20-Image of solidified sample captured with digital camera...... 40

Figure 21-Backscattered SEM image of (a) as–solidified and (b) crushed alloy dispersed in dark background epoxy...... 41

Figure 22-Separation of the particles after one minute suspension in the heavy liquid...... 42

Figure 23-Useful apparatus for physical separation...... 43

Figure 24- Photograph of the leaching setup...... 44

Figure 25-Fe-Si binary system showing quenching temperatures...... 45

Figure 26- Alumina crucible used in the second set of experiments...... 46

Figure 27-XRD pattern of the solidified alloy...... 49

Figure 28-Cumulative wt% percentage verses the dendrite thickness...... 50

Figure 29-Dendrite thickness as a function of cooling rate...... 51

Figure 30-Recovery and grade in various S.G. of heavy liquid plotted for sample with cooling rate= 3C/min and particle size= 600–800 µm...... 52

Figure 31-Recovery verses grade plotted for samples with cooling rate (a) 0.5 C/min, (b) 1.5

C/min and (c) 3 C/min...... 54

Figure 32-Separation efficiency verses particle size for samples with different cooling rates. ... 56

Figure 33-Separation efficiency verses r= crushing size/ dendrite thickness...... 57

Figure 34-Impurity ratio in final Si/ MG–Si ...... 59

Figure 35-(a) Micrograph of the solidified alloy and (b) EPMA analysis for P across the line shown in (a)...... 61

Figure 36-(a) Micrograph of the solidified alloy and (b) EPMA analysis across the line shown in (a) ...... 62

Figure 37-Effect of particle size on (a) P and (b) B concentration in final Si product...... 63

Figure 38-Impurity ratio in final Si/ MG–Si ...... 65

Figure 39-Number of refining stages verses impurity ratio in final Si to initial Si...... 67

Figure 40-Solubility of P in Fe–Si melts at 1450 °C (reproduced from data in Ref. [71]) ...... 69

Figure 41-Phosphorus concentration profile of solidified sample based on Scheil equation a) complete solidification and b) about 50wt% of sample solidified...... 71

Figure 42-Zn-Si phase diagram [56]...... 73

Figure 43-Zn-P phase diagram [56]...... 73

Figure 44-Melting and solidification profile of Zn-Si experiments...... 74

Figure 45-Ca-Si binary phase diagram...... 76

Figure 46-Melting and solidification profile of Ca-Si experiments...... 76

Figure 47-SEM image of 10%Ca-Si sample...... 77

Figure 48-EDX pattern of 10%Ca-Si sample (area A in Figure 47)...... 77

Figure 49-EDX pattern of 10%Ca-Si sample (area A in Figure 47)...... 78

Figure 50-XRD of 10wt%Ca for Ca3P2 detection...... 78

Figure 51-Impurity ratio in CaSi2/ Si...... 79

xii

List of Abbreviations

MG-Si: Metallurgical Grade Silicon

SoG-Si: Solar Grade Silicon

HM: Heavy Medium

S.E: Separation Efficiency

G: Grade

R: Recovery

TGZM: Temperature Gradient Zone Melting

ICP-AES: Inductively Coupled Plasma- Atomic Electron Spectrometry

SEM: Scanning Electron Microscope

EDX: Electron Diffraction X-Ray

XRD: X-Ray Diffraction

EPMA: Electron Probe Micro-Analyzer

S.G: Specific Gravity

C.R: Cooling Rate r: particle size/ dendrite thickness

PR: Purification Ratio

P: phosphorus

B: boron

xiii 1

Chapter 1: Introduction

Silicon is the material of choice for manufacture of solar cells, accounting for over 90 percent of today's PV materials [1]. Traditionally, Solar Grade Silicon (SoG-Si) was supplied from the off- specification semiconductor grade silicon which is inherently more pure than the requirements for PV applications. Higher cost and insufficient supply of electronic grade silicon rejects together with a growing demand for SoG-Si, driven by need for clean energies, has created a substantial thrust for developing a dedicated method of SoG-Si production. One stream of research explores refining of Metallurgical Grade Silicon (MG-Si) to SoG-Si by inexpensive metallurgical refining routes.

MG-Si is a relatively low purity source of Si (~98% Si). However, its availability in large quantities and low price favors the use of MG-Si as the primary feedstock for SoG-Si production. A conventional method called Siemens is already in use by many manufacturers but the production cost is rather high due to energy consumption. Corrosive chemicals which are used in this process are another drawback for this technology.

It is believed that a single metallurgical refining process is not sufficient to lower the impurity level of MG-Si to SoG-Si specifications, due to presence of numerous impurities with different chemical properties. A successful refining process should likely combine several refining stages, each responsible for lowering a certain number of impurities to meet the purity requirements. However, it is worth bearing in mind that metallurgical purification processes such as oxidation, slagging, and controlled solidification are relatively cheap. Thus a multi-stage process could still provide an economically attractive option for upgrading MG-Si to SoG-Si.

Solvent refining process is used for the purification of MG-Si by alloying with a solvent metal and growing purified Si dendrites in the matrix. This process essentially mimics the crystal growth techniques (such as zone refining – float zone – or Czokralski), but is in a much smaller scale. Unlike similar float zone techniques in which the impurity elements are rejected to the solid-liquid Si interface during solidification, in the solvent refining process impurities are rejected to a second phase which has higher affinity for impurities.

In the current research, the solvent refining of MG-Si using iron as the impurity getter combined with physical separation has been investigated.

Chapter 2 of this thesis provides a review of the previous literature pertinent to this study. In Chapter 3, the experimental procedures are discussed. Chapters 4 and 5 present the findings including the effectiveness of the physical separation and the extent of silicon purification using the technique. A summary of the research is presented in Chapter 6 and the conclusions are drawn.

Chapter 2: Literature Review

2.1 Energy and the Environment

Energy is considered a main factor in the generation of wealth and economic development. The historical data verifies a strong relationship between the availability of energy and economic activities.

In the last few decades there have been two main issues regarding energy, making it one of the critical problems of the new millennium. On one hand, energy demand has increased due to the population growth, as well as higher consumption. On the other hand, the ability of the earth to sustain this rate of growth and consumption is fast decreasing. For finding solutions to the environmental problems that humanity faces today, long term potential actions for sustainable development is needed. Considering these facts, exploiting renewable energy resources such as solar energy appears to be one of the most efficient solutions.

The concept of sustainable development is developing the economy while protecting the environment or avoiding exhaustion of the earth limited resources. World Commission of Environmental Development (WCED) has defined sustainable development as: “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. There are many factors that contribute to achieving sustainable development. Today, one of the main factors that must be considered in discussions of sustainable development is energy, and one of the most important issues is the requirement of energy that is fully sustainable [2, 3].

Fossil fuel energy may be inexpensive and plentiful in nature but the problem lies within the emission of greenhouse gas that it causes. Despite the well known consequences of fossil fuel combustion on the environment such as global warming, ozone depletion and acid rain, it is expected that by the year 2025 the world’s oil consumption be increased by 62% reaching 123 million barrels per day [4].

There are two alternatives to avoid the impact of these phonamena, one is to improve the fossil fuels quality, so that it would cause less harm on the environment, the other is to subtitute it with other sources of energy which are clean and renewable. Among these sources, solar energy is one of the best choices due to its abundance and even ditribution in nature than any other renewable energy type such as wind, geothermal, hydro, wave, and tidal energies [5].

2.2 Solar Energy

Solar energy is one of the most promising energy supply sources of the future. This continuous energy source spreads dramatically over the earth and can be collected, concentrated and stored to be converted to other energy sources such as heat or electricity.

Harnessing of solar energy dates back to 1960’s when “Solar Cells” provided electricity for several space vehicles [6]. The energy crisis of 70’s motivated the terrestrial use of solar power. Since then many investigations have been done on the conversion of solar energy to electricity.

2.3 Silicon

The essential component of a solar panel is a silicon wafer photovoltaic cell. Silicon is mostly found in the form of silica and silicates in nature and makes up 25.7% of the earth’s crust [7]. The main issue in the PV industry is to convert this highly available material to pure silicon. In fact, the production of silicon in a metallic form is even more difficult than extracting gold. The reason for this is the high affinity of silicon to combine with other elements, especially with oxygen so it is not easy to remove impurities by oxidation refining, which is a common purification technique in the production of metals such as steel and copper.

2.4 Grades of Silicon and their Specifications

Different grades of silicon are categorized in three ranges based on the use and the purity level. Metallurgical grade silicon (MG-Si), which has purity of 98%- 99%, solar grade silicon (SoG-Si) with purity of 6N and finally semiconductor grade (SEG-Si), which has purity of 9N to 11N [9]. As expected, the material of lower purity is priced less. For example, metallurgical grade silicon can cost US$ 1- 1.5 per kg, while semiconductor grade polysilicon can cost about 35 to 200 times higher depending on the market conditions [10].

2.5 Metallurgical Grade Silicon (MG-Si)

Most of the world production of raw Si still goes to the steel and silicone industries and only a small part is used as feedstock for the semiconductor industry. MG-Si is produced through carbothermal reduction of quartz at a capacity of 900,000 ton/yr [11]. In September 2008, the price of metallurgical grade silicon was about $1.45 per pound (3.20/ kg) [12] up from $0.77 per pound ($1.70/kg) in 2005 [13].

Metallurgical grade silicon is a cost effective alternative for the production of solar cells, the reason for this will be discussed in Section 2.7. The commercial method to convert SiO2 to metallurgical grade silicon is carbothermal reduction which will be discussed in the next section.

2.6 Carbothermal Reduction of Silicon

Silicon is commercially prepared by reducing it with carbon in an electric arc furnace (Figure 1) according to the overall reaction which takes place at temperatures over 1900oC:

automatic vertical regulator

input of quartz and carbon

electrode holder

electric contact

carbon electrode

trapping electrode

Figure 1-Furnace used for reduction of silica with carbon [14].

SiO2 (s) + 2C(s) = Si (l) + 2CO (g) Eq. [1]

Liquid silicon is collected at the bottom of the furnace, and is drained and cooled.

The purity level of this silicon (MG-Si) is about 98-99%. The major source of impurities are the raw materials used (specially the reductant mixture consisting of lignite, petroleum, coke, charcoal and wood chips) [15]. A typical analysis of commercial MG- Si is given in Table 1.

Table 1-Impurities of metallurgical and solar grade silicon [7].

Element Impurity content, ppm

Metallurgical grade Solar grade

98- 99% 99.5%

Al 1000- 4000 50- 600 <0.1

Fe 1500- 6000 100- 1200 <0.1

Ca 250- 2200 100-300 <1

Mg 100- 400 50-70 <1

Mn 100-400 50-100 <<1

Cr 30-300 20-50 <<1

Ti 30-300 10-50 <<1

V 50- 250 <10 <<1

Zr 20-40 <10 <<1

Cu 20-40 <10 <1

B 10-50 10- 15 1-1.5

P 20- 40 10- 20 0.1-1

C 10000- 3000 50- 100 0.5- 5

2.7 Solar Grade Silicon (SoG- Si)

In the 1980’s, the beginning of the PV industry, the waste from microelectronic industry was used as the starting material for the PV applications. This material provided a low cost feedstock that answered the still small PV market. However, with the growth of the PV industry and the rise in the demand for solar silicon, finding a new method to produce large quantities of solar grade silicon has became an important issue. The new method has to be cost effective so that the cost of the electricity produced is comparable with that from fossil fuels or nuclear energy.

As mentioned before, the purity in metallurgical grade silicon is around 98%. It has been well recognized that the high impurity concentrations prevent the use of MG-Si directly for photovoltaic applications. To reach the requirements of solar grade, the purity level should reach 6–7N. Therefore, it is necessary to remove most of the impurities of MG–Si. Figure 2 shows the degradation thresholds of solar cells conversion efficiency (the percentage of power converted from absorbed light to electricity when the cell is connected to electrical circuit) as a function of the concentration of various elements in silicon.

Figure 2-Impurity concentration verses efficiency curves in p-type silicon :(1) semiconductor-, (2) solar-, and (3) metallurgical-grade silicon [14, 16].

2.8 Impurities in Solar Grade Silicon

Metallic impurities in commercial multicrystalline silicon are reported to be in the range of 1014 to 1016 cm-3[17]. These impurities can exist in different forms in a solar cell such as point- like defects (interstitial or substitutional), or as precipitates. The precipitates are often formed in the grain boundaries. These impurities decrease the efficiency of silicon in a variety of ways.

In solar cell, recombination centers are energy levels in the middle of the band gap that will enhance carrier (electrons and holes) recombination and reduce the total current. Diffusion length is defined as the effective path length of the free carriers before recombination. Impurities and precipitates form recombination centers in the band gap and thus degrade the performance of solar cell by reducing diffusion length and device short circuit current [18].

Most transition metal impurities exist as interstitial impurity in silicon when they are below the solid- solubility limit. The chemical state of impurities is related to their rate of diffusion in silicon. Ni, Cu and Co have high diffusion coefficient (Figure 3) and almost always completely precipitate out at surfaces or internal defects during cooling after crystal growth. Small fraction of these impurities may remain as substitutional form, but if they do, they are believed to have very little effect on photovoltaic silicon materials [19].

Impurities which have moderate diffusivities such as Fe, Cr and Mn tend to co- precipitate at the same sites of Ni and Cu and form silicides [19].

Ti, Mo and V are examples of very slow diffusing impurities which are uniformly distributed during crystal growth, unless initially present as undissolved inclusions in the melt. Most of these impurities tend to be in interstitial form. Their low diffusivity also results in being very resistant to gettering at temperatures and times which are normally used for processing silicon devices.

As mentioned before, the distribution and chemical state of impurities differ and consequently, their influence on solar cell efficiency will be different. Thus, the distribution and chemical state of impurities should be taken into consideration rather than just their average concentration.

Generally, point-like impurities such as Ti, Mo and V have higher influence on decreasing the cell efficiency since they have greater recombination strength “per atom” than metal impurities which exist as precipitated silicides such as Ni, Cu, Co, Fe, Mn and Cr [19].

Figure 3-Diffusion coefficients of some impurities in silicon [20].

2.9 Refining Methods

Refining of MG-Si is feasible by two approaches: chemical and metallurgical. The former being a more conventional and common method and the latter somewhat new and in research phase. Metallurgical methods are believed to be five times more energy efficient than the conventional Siemens process that uses about 120-200 kWh /kg [21].

Usually a combination of processes in a certain sequence is used to remove impurities from metallurgical grade silicon. These processes include: Acid Leaching (hydrometallurgical refining), Slagging, Gas Blowing, Vacuum Refining, Directional Solidification, etc. Each of these steps reduces the concentration of a number of impurities by about one order of magnitude. The purification efficiency depends on the physicochemical properties of impurities specially their segregation coefficients. In the following, some main and relevant techniques to this study for silicon refining will be discussed.

2.9.1 Conventional Method: Siemens Process

The dominant chemical method for purification of silicon is known as Siemens process. The basis of this process is that when silicon combines with hydrogen and halogen elements, it forms compounds with very low boiling points that are purified by distillation and later decomposed to high purity silicon. The starting material is MG-Si and the product is very high purity silicon (>11N). This process is described as follows:

First, Si is converted to SiHCl3 in a fluid bed reactor via the reaction shown below:

Si (s) + 3HCl (g)  SiHCl (g) + H (g) 3 2 Eq. 1

This reaction, assisted by a catalyst, takes place at around 300oC.

The gaseous product of this reaction is volatile SiHCl3 (Trichlorosilane or TSC) with boiling point of 31.8°C which is more pure than Si but still needs to be purified.

The impure TCS gas is distilled to remove impurities, based on their different boiling points, resulting in much more pure TCS. The pure gas is converted to Si through Eq. 2, in the presence of hydrogen. The decomposition of TCS takes place on an inverse U–shaped silicon rod that is heated electrically to 1000-1100 oC.

SiHCl3 (g) + H2g Si (s) + 3HCl(g) Eq. 2

It is believed that P and B are removed during this process. The Si product is very fine-grained and of high purity.

There are two main problems associated with this method. Firstly, it involves the production of chlorosilanes and reactions with hydrochloric acid. In addition to being toxic, these compounds are corrosive, causing irritations on the skin and mucous membranes [22]. The second is the high energy consumption since it needs high temperature for long periods of operation.

Despite these problems, the Siemens process is currently the dominant method of producing ultra-pure Si for electronic industry. A schematic diagram of the process flow is demonstrated in Figure 4.

Figure 4-Schematic diagram of Siemens process [23]. 2.9.2 Metallurgical Methods 2.9.2.1 Directional Solidification (Zone Refining)

Since silicon has high affinity for oxygen, it is difficult to remove impurities by oxidation. However, silicon has an interesting and helpful property which is having higher solubility for most impurities in liquid state than in solid state. This characteristic makes it possible to remove impurities to a great extent by directional solidification. One example of this method is zone refining in which impurities are moved to one end of a charge by a series of molten zones all passed in the same direction. Impurities that lower the freezing point of the solvent, travel with the zones and accumulate at the end of the charge; impurities that raise the melting point travel opposite to the zones and accumulate at the beginning of the charge [24]. In other words impurities which were distributed uniformly are redistributed during the solidification as a result

12 of phase equilibrium such that most impurities, but not all of them, are left in the liquid phase and the remaining will be purified solid silicon. The amount of removal of a specific element depends on its segregation coefficient which is the ratio of the concentration of an element in silicon at its liquid state to its concentration in silicon at solid state. Since many elements such as Cu, Fe, Al, Ni etc. have segregation coefficients ranging from 10-6 to 10-1 (Table 2) by repeated directional solidifications, impurity levels can be reduced to a great extent. However, while most of the impurities have low segregation coefficients, P and B have segregation coefficients close to unity (0.35 and 0.8 respectively) so it is not possible to remove them effectively by directional solidification. Therefore, other purification methods have to be used along this method in order to remove P and B efficiently. Some approaches to this problem will be discussed later.

Table 2-Segregation coefficient of some impurities in metallurgical grade silicon at melting point of silicon [16, 25]. Impurity Segregation Coefficient Cu 4× 10-4 Zn 1× 10-5 B 0.8 P 0.35 Ga 8.0× 10-3 In 4× 10-4 Al 2.0× 10-3 S 10-5 Mn 10-5 Fe 8× 10-6 Co 8× 10-6 Ni 3.2× 10-5 Sb 0.023 Au 2.5× 10-5

The advantage of the directional solidification process is that it is an easy process and does not use any chemical reactions. However silicon losses occur since the portion of silicon which solidifies at the last stage should be disposed of because all impurities are concentrated in this section. Another disadvantage is that elements such as phosphorus and boron cannot be removed and an additional process has to be applied in order to make PV-grade silicon.

2.9.2.2 Acid Leaching

Acid leaching is a hydrometallurgical method of refining silicon and the removal of metallic impurities. The major advantage of acid leaching is that it can be practiced in low operating temperatures and thus being a low energy consuming method.

As mentioned before, impurities except P and B have very low segregation coefficients in silicon, indicating that they have low solubility in solid silicon compared to its liquid state. This will result in the concentration of impurities in grain boundaries during solidification. If the solidified metal is crushed, cracks are most likely to form through these grain boundaries, as grain boundaries are usually concentrated from the impurities and are thus more brittle. Silicon crystals have high corrosion resistance against most acids except the combination of hydrofluoric acid in the presence of an oxidant. When solidified samples are pulverized to small particle sizes the impurities in the grain boundaries will dissolve and be removed by acid leaching. The acids used can be one or a combination of hydrofluoric acid, hydrochloric acid, aqua regia and sulfuric acid.

The efficiency of leaching depends on three main parameters: particle size, time of leaching, and leaching temperature. In a study by Dietl [26], these parameters have been studied and high purity silicon with impurity amounts shown in Table 3 was achieved. In order to achieve this level of purity very fine grinding up to 20 μm is required which is not favorable in terms of materials handling.

Table 3-Purification of Si by leaching [26].

Fe Ca Mn Ti Al

Before leaching 1250 1050 400 290 100

After Leaching <1 <2 <1 <0.3 <1

Although leaching has showed success in removing some impurities, not all elements could be removed by this method. Impurities that exist in solid solution or impurities that are trapped as separated phases within the silicon crystals will not be removed. In other words, acid leaching is

14 an effective method to remove the impurities that have already been separated during the solidification.

The efficiency of the acid leaching can be improved by the addition of calcium. In a study by Schei [27] in Elkem, silicon containing few percents of calcium was cast, cooled slowly and crushed into lumps around 5cm. A CaSi2 phase formed at the grain boundaries contained most of the impurities. Exposing the silicon lumps to hydrochloric acid and ferric chloride, disintegrated them into silicon crystals of below 2mm. These crystals were further purified by hydrofluoric acid in combination with an oxidizing agent.

2.9.3 Plasma Process

The plasma process for the purification of silicon has been known for more than 10 years. The method is believed to be effective in removing those impurities that are more volatile than silicon. Morvan et al. [28] used argon plasma with added oxygen as the reactive gas. Ikeda et. al.[29] have developed a purification process combining electron beam, arc plasma, and directional solidifications. An electron beam was utilized to remove phosphorus, while plasma treatment used water to remove boron. This process uses MG-Si as its feedstock but it is not cost effective because of the high energy consumption.

In a work done by Delannoy [30], the three main steps of a plasma refining process have been described as following: 1) transport of the impurities in the liquid silicon to the free surface of the melt 2) evaporation or chemical reaction of impurities to transfer to the gas phase 3) blowing the gas away from the surface by plasma torch. In order for these steps to take place, the silicon should be maintained in liquid state throughout the process, the shape of the surface should be controlled and the liquid should be electromagnetically stirred for the rapid transport of impurities from the bulk to the surface. The plasma torch is composed of a mixture of water and argon and the velocity of the flow was (10–40 m/s). A schematic diagram of the purification system is presented in Figure 5.

Figure 5-Schematic diagram of the purification system [30].

The most effective method to remove B is oxidation refining utilizing plasma. Since B forms more stable species than SiO2 such as B2O3 at temperatures higher than 1623 K, by oxidation refining utilizing plasma the concentration can be lowered to 0.1 ppm [31].

2.9.4 Vacuum Refining

Since elements such as P, Fe and Ca have high vapor pressure in molten silicon, they can be removed by applying vacuum to the melt. In a study by Yuge et al. [32] vacuum refining under atmospheres of 8.0×10-3~3.6×10-2 Pa at 1722-1915 K was applied. The removing rates of these elements are controlled by the diffusion in molten silicon and evaporation from the silicon surface.

2.9.5 Slagging

Slag refining is based on liquid-liquid extraction. It consists of reduction and oxidation reactions of impurities in the interface of slag-liquid. These reactions are based on relative thermodynamic stability of the formed oxide compounds. The oxides can either float on top or sink to the bottom of the liquid. The slag which is used for extraction of impurities should have these main properties [33]:

- The oxide should not dissolve a significant amount of silicon

- The MG-Si which is going to be purified should be low in elements which their oxides are less stable than the slag constituents

- The slag should not contaminate MG-Si

- Fluidity of slag should be high so that the mass transport can readily take place

- There should be a density difference between the slag and the liquid for easier separation. 2.9.6 Reactive Gas Blowing

In this method, inert or reactive gas is purged into melt. Reactive gas can easily react with impurities and form volatile compounds thus removing impurities in molten MG-Si. These reactive gases include Cl2, O2, SiCl4, wet hydrogen and CO2 or combinations of these gases. As an example, Cl2 can form volatile chlorides which have low melting point and boiling point. Another example is wet hydrogen which can react with B and form hydride which is also volatile at melting point of Si. It has been mentioned that by this method impurities such as C, Ca, Mg, Al, P, and B can be removed effectively due to their thermodynamic properties [7].

Inert gas can also be used to promote stirring and to increase the reaction rate of reactive gas with impurities [22].

2.9.7 Solvent Refining

“Solvent refining is a purification process in which recrystallization takes place from the supersaturated melt depending on the segregation behavior of different elements”, J.Dietl [34].

Solvent refining is based on a physic phenomenon called gettering. Gettering has different definitions with regards to silicon technology. The definition which is relevant to solvent refining is that when silicon is alloyed with an element such as aluminum, metal impurities will have higher solubility in the alloy melt if the temperature exceeds the eutectic temperature of that binary system. The binary system should be in such a way that the element to be alloyed with silicon should have no (or little) solubility in silicon. The binary system of Si and Al have a unique characteristic and that is no intermetallic compound is formed thus the two phases that are formed, are only eutectic and purified Si dendrites. The eutectic phase which contains most of

17 the impurities and can be dissolved in acid will be removed easily. For better understanding of this binary system the Al-Si phase diagram is presented in Figure 6.

Figure 6-Al-Si phase diagram [56]. 2.10 Segregation Coefficient

During solidification, a metal impurity (M) in silicon will be distributed between the solid silicon (dendrites) and the melt. This distribution is described by the equation bellow:

Eq. 3 where and are the contents of M in solid and liquid silicon respectively and is the segregation coefficient. Equilibrium values can in principle be read from the phase diagram of M-Si, but often special measurements give more accurate results [35]. This value is fairly constant with temperature, when the concentration of the element in silicon is low.

The smaller this value is for a specific impurity, the more it will be distributed in the liquid phase compared to the solid phase. Therefore, the amount of removal of a specific element in solvent refining depends on its segregation coefficient. Since many elements such as Cu, Fe, Al, Ni etc. have small segregation coefficients, (Table 2) their levels can be reduced substantially by

18 repeated directional solidification. However, as mentioned before, while most of the impurities have low segregation coefficients, P and B have relatively large segregation coefficients that render them unresponsive to directional solidification. However, since P has high vapor pressure in silicon, removal of P can be achieved to a great extent by methods such as vacuum treatment

[36]. B can also be removed by oxidizing plasma treatments with Ar/H2O gas. Both of these methods, however; are very energy consuming since they need high operating temperatures at long periods of time and thus are not considered economical. Therefore, other purification methods have to be used along directional solidification in order to remove P and B efficiently.

When silicon is alloyed with a getter element such as aluminum, metal impurities will have higher solubilities in the alloy melt compared to solid silicon melt. The segregation coefficient in this ternary Si-getter-impurity system is defined as the concentration of impurities in solid silicon to the concentration of impurity in the Si-getter alloy melt. This segregation coefficient has been proven to be lower than the segregation coefficient of non-alloyed silicon. An example for this is the work done by Morita et al. [8] with Al being the getter element. In Table 4 these two segregation coefficients for some impurities have been presented from their work.

Table 4-Segregation coefficient of impurities between solid Si and liquid Si -Al and those between solid Si and liquid Si at silicon`s melting temperature [8]. Segregation ratio between Si-Al Segregation ratios between solid/liquid Element melt and solid Si at 1000°C silicon at melting point of silicon 1410°C

P 8.5×10-2 3.5×10-1

B 1.5×10-1 8.0×10-1

Al 4.9×10-4 2.8×10-3

Fe 5.8×10-9 6.4×10-6

Ti 5.7×10-7 2.0×10-6

Cu 4.9×10-6 4.0×10-4

Ag 8.2×10-7 5.0×10-5

Pb 2.9×10-4 2.0×10-3

The different methods in solvent refining vary by two main factors: the alloying getter-element and the method of separating the dendrites. In the following paragraphs a brief review on the history of solvent refining is presented.

The earliest metallurgical application of solvent refining is desilvering of lead. Zinc is added to silver-lead melt as the getter. Since silver is more soluble in zinc than in lead, by lowering the temperature a layer of solid silver-zinc floats on top and skimmed off. The zinc is later distilled at high temperature leaving pure silver behind.

One of the earliest researches concerning the use of this technique for purification of silicon is the work by Wakefield et al. in 1976 [37]. They developed a method for refining silicon by introducing silicon in a tin–lead alloy at about 1000°C. At this temperature the impurities in silicon were believed to dissolve in the alloy. On cooling, the silicon dendrites were formed from the alloy, leaving the impurities in the melt. The dendrites were then recovered by floating to the surface, due to density difference, and were adhered to a crystal of Si that was pulled out from the melt at 800°C as a single crystal. For performing this, a single crystal seed of silicon was kept in contact with the surface of the melt and was pulled out as the purified crystal. The impurities were believed to precipitate out as insoluble slicides that drop to the bottom of the melt.

In another study, Kotvel et al. [38] used aluminum as the getter metal. Silicon dendrites were precipitated and separated from the melt by filtration. The dendrites were cleaned from the adherent eutectic phase by 4-37% HCl acid leaching. It has been mentioned that while mechanical or other means of mixing is optional during alloying silicon with aluminum; it may be not desirable to stir the alloy during cooling. Stirring would break the silicon dendrites that will consequently make their recovery difficult. In this process, iron and aluminum contents were reduced from 0.48 and 1.26 wt% in the starting silicon to respectively 1000–1500 ppm and 20– 50 ppm in the refined product. Since Al has relatively high segregation coefficient in silicon (2.8×10-3), some amounts might stay in the silicon product.

The solvent refining step can be followed by a slag oxidation refining process to remove the microscopic inclusions of aluminum eutectic that are retained in the silicon dendrites. This step

20 involves mixing and melting of silicon dendrites in contact with high purity slag. The slag removes aluminum from the silicon by oxidation forming Al2O3.

Kramer et al. [39] added Ti to form TiB2 with the boron present in MG–Si. It has been mentioned the Ti amount should not exceed 0.2wt% and preferably 0.1wt%.

Gumaste et al. [40] studied the effect of cooling rate and holding time on the purity of the silicon dendrites precipitated from Al-35% Si melt. They found that higher cooling rates are less effective for removing impurities since faster cooling restricts the rejection of the impurities to the liquid, hence, the solid composition approaches that of the liquid melt. Among three different cooling rates of 20, 40, 60oC/ hr, 20 °C/hr resulted in the highest purity of the dendrites. Holding the melt temperature about 100 °C above the eutectic temperature resulted in larger dendrites and higher recovery of silicon. However, the purity of silicon decreased by increasing the holding time. They discussed that this could be because of the increase in the concentration of impurity elements in the residual alloy which is present in the interdendritic spacing during the last stage of dendrite growth.

Some transition metal impurities are removed effectively through this method. However, with regards to elements such as Al, Fe, P, Cu, Ti there seems to be no significant change and has been suggested to remove these impurities further by slag refining, vacuum boiling and blowing moist argon.

As mentioned earlier, in metallurgical refining of Si for solar cell production, removal of B and P are most challenging due to their high segregation coefficient. The latest work done on solvent refining is by Morita and his co-workers [8, 41-49]. They have studied the effect of alloying silicon with aluminum on the removal of impurities specially P and B. Their work is based on thermodynamic calculations as well as experiments. Their main goal was to find out if the segregation coefficient of silicon impurities, specially P and B, in Si-Al melt is lower than the segregation coefficient in pure silicon. In other word if Si-Al has more solubility for impurities than pure silicon.

As a result of their studies, the activity coefficient of B and P and the interaction parameter between Al and B in solid Si in solid silicon is also found. All this has been achieved by

Temperature Gradient Zone Melting (TGZM) method which is described briefly in the following paragraphs.

The basis of this method is an experimental apparatus which is shown in Figure 7. An Al–(0.3- 1.1) wt-% B foil of 100μm thick is placed between two single crystalline silicon plates in a temperature gradient zone in the furnace and is held in an argon atmosphere for 24 hours. The instrument is designed to form equilibrium between Si-Al melt and solid Si so the distribution of P or B between these two phases can be studied in such equilibrium conditions. The distribution of P or B between these two phases (at 900-1100 °C temperature range) will result in finding the segregation coefficient of P or B i.e. the ratio between the solubility of P or B in solid Si and Si- Al melt at their infinite dilution. This ratio is then compared to the segregation coefficient of solid/liquid silicon at its melting point.

Their results show that segregation coefficients of P and B were reduced from 0.35 and 0.8 to 0.08 and 0.15 respectively at 1000 °C. The reduction for other impurities was shown earlier in Table 4 . This means that Si-Al melt has the tendency to dissolve such impurities and can be effective in purification of MG-Si by the means of solvent refining.

After the decrease in the segregation coefficient as a result of alloying Si with Al is proven, it is necessary to find an appropriate method for separating the bulk Si grains from Si-Al melt. Acid leaching is an effective way for the removal of the inter-grains composed of Si-Al eutectics between Si grains after the solidification of the alloyed melt. In order to improve the acid leaching one other method is necessary to be applied prior to leaching to collect the Si grains and produce a higher density of the grains. Therefore, two methods were investigated by Morita’s group. One was separation by gravity force and the other electromagnetic force during solidification which will be explained in the following paragraphs.

In regards to the latter it was found out that separation of the solidified Si grains is not possible during solidification, since after melting and quenching the Si grains were found to be dispersed uniformly in the sample. The reason for this, mentioned by the authors, is the high viscosity of the melt in which particles dispersed. The other reason could be the low density difference between the eutectic melt and the Si grains which is approximately 0.1 g/cm3. It was thus concluded that the use of gravity force was not an effective separation method. The other method was continuous solidification of Si from Si-Al melt under induction heating. In this method, the separation of Si dendrites was investigated by solidification under fixed alternating magnetic field (Figure 8). The Si dendrites are agglomerated at the bottom of the sample although the density of these dendrites is lower than the eutectic phase Figure 9. After performing refining test, the effective removal of impurities such as Fe, Ti, B, P and Al by this method was confirmed.

Table 5-The impurity content of initial Si (ppmw) and final Si in solvent refining of Si and Al [49].

Fe Ti Al B P

Amount in initial Si 4500 690 1280 56 36

Amount in final product 13 5.2 599 0.81 0.93

Induction coil B B

Si-Al melt Solidified Si

Figure 8-Mechanism of continuous solidification of Si from Si-Al melt under induction heating [50].

Figure 9-Si dendrites agglomerated at the bottom of the sample [50].

2.11 Effect of Ti and Ca Addition on Efficiency of Solvent Refining . Effect of Ti Addition

Through Morita and Yoshikawa’s work [51], B removal by titanium addition in solidification refining of silicon with Si-Al Melt was also studied. This process involves B removal by solidification refining of Si using a Si-Al melt with Ti addition in order to effectively remove B from Si. Since Ti forms TiB2, a strongly stable compound, it can be added to the Si-Al melt prior to the precipitation of the Si dendrites. To investigate the effect of Ti addition on B removal, two separate experiments were carried out. First, the solubility of TiB2 in Si-Al melt was investigated. This was done by equilibrating Si-Al melt with TiB2. In order to achieve this equilibrium, excess Ti was added to the Si-Al melt which contained B and the melt was saturated with TiB2. This melt was held for more than 6 hours in Ar-H2 atmosphere. After equilibration, the samples were cooled in argon gas flow. The composition of the formed precipitates were analyzed and confirmed to be TiB2. TiB2 solubility in Si-Al melts was determined by measuring the equilibrium concentrations of B and Ti in the presence of TiB2 precipitates. The small solubility of TiB2 in the Si-Al melt indicates the effective removal of B from the Si-Al melt by Ti addition [52].

The second set of experiments was to investigate the removal of B from Si with solidification of Si-Al melt containing B by Ti addition. This was studied by melting Si-Al-B alloy and holding the sample close to the eutectic temperature of the Si-Al alloy in an induction furnace. The solidification was performed under electromagnetic force. The part of the sample with high density of Si dendrites was crushed into particles less than 840 µm. The adherent eutectic phase to the particles was leached in aqua regia containing H2SO4 . In addition to TiB2, (Al, Si)3Ti may also form. Since these precipitates are soluble in aqua regia, they can be removed and will not affect the purity of the obtained solidified silicon. The final results reveal that Ti addition can reduce the B content of MG-Si to a few ppma with no Ti contamination in the final product.

. Effect of Ca Addition

Min and Sano [53] have investigated the formation of Ca3P2. The reaction of Ca and P has been expressed by the reaction bellow:

3Ca(l)+ P2(g)= Ca3P2(s) [5]

In their study, it has been reported that Ca3P2 is a stable compound and Ca and P show attractive interaction in molten silver. The stability of this compound is also shown in the phosphide ellingham diagram that is plotted with Factsage software [54] shown in Figure 10. Hence; it was expected that Ca has great affinity for P and can reduce the activity coefficient of P in molten silicon. In other words it can reduce the segregation coefficient of P in Si. To confirm the affinity and also study the affinity of Ca for B, Morita [36] has conducted some experiments and by some thermodynamic analysis, it was found that although the interaction parameter between Ca and B was slightly negative, that between Ca and P showed a large negative value. This compound was found to segregate in the grain boundaries of silicon and was removed by acid leaching. When 5.92 at% of Ca was added to molten silicon, the removal fraction of P was reported to be 80.4%, while this fraction for B appeared to be 40% when 6.2 at% of Ca was added.

On the other hand, as mentioned before in Section 2.9.2.2., Ca will also improve the leaching of impurities such as Fe and Ti.

2.12 Examples of Binary Systems of Silicon

So far only aluminum, lead-tin and copper [55] have been used as the getter in solvent refining of silicon. Some other candidates could be Fe, Zn, Sn and Ni. One of the important properties for the solvent is to have low solid solubility in silicon. The other is to have high affinity for impurities in silicon. The segregation coefficient of solvent in silicon could also be important since the lower the segregation coefficient; the easier it could be removed later in directional solidification refining process. The kind of the eutectic phase formed after solidification is also important. Some elements such as Al, Zn and Sn form a eutectic phase of M-Si but elements such as Fe, Ni, Mn, Mg, and Cu form an intermetallic compound with Si during the eutectic solidification.The easier eutectic phase can be leached by acids in the leaching process, the better it can be removed from the purified silicon dendrites. Some precipitates such as FeSi2 can only be dissolved in hydrofluoric acid whereas; eutectic phase in Si-Al system, Si-Zn and Si-Sn can be dissolved at lower cost and with less hazardous acids. In this study the separation of silicon particles from the eutectic phase is achieved by gravity separation which is based on the

26 difference in the density of the formed eutectic phase and silicon. Thus, the higher the density of the solvent, the easier it can be removed.

2.13 Properties of Iron in Silicon

In the current study iron has been chosen as the solvent element. The main reasons are as follows

. Low solubility in Si (the maximum solid solubility has been reported to be 1.1ppmw [25]).

. High affinity for impurity elements, particularly phosphorus and boron that are difficult to remove by other methods such as directional solidification. Ellingham diagrams for formation of phosphides and borides of several elements are generated using FactSage [54] and presented in Figure 10. As seen, Fe has a higher tendency than Si to form compounds with both P and B.

. Higher density of Fe (~7.9 g/cm3) compared to Si (2.3 g/cm3) allows for gravity separation of the Si dendrites from the matrix alloy.

. The very low segregation coefficient of iron in Si (8× 10–6) [19, 25] allows the removal of dissolved iron through subsequent directional solidification.

. Abundance and low cost of iron.

. The relatively high diffusion coefficient which results in easier precipitation of intermetallic compound (of the solvent element with silicon) in the grain boundaries instead of remaining in substitutional or interstitial form.

. The remaining alloy (FeSi2) being ferrosilicon can be recycled and used in metallurgical operations.

The phase diagram of Fe-Si is presented in Figure 11. As seen, the eutectic temperature is o 1207 C. The eutectic structure is either β-FeSi2 or α-FeSi2 depending on the cooling conditions.

Low solubility and segregation coefficient of iron are evident in Figure 12. This figure demonstrates that from these aspects, Fe, Zn, Mn, and Co are superior to Cu and Al, as the getter elements.

(a)

(b) Figure 10-Ellingham diagrams for stability of (a) borides and (b) phosphides.

Figure 11-Phase diagram of Fe-Si [56].

100000 As

P 10000 Sn Sb B Ga 1000 Al Mg Bi 100 Ca Ni Cu (ppmw) Au O 10 Zn Mn Solubility / Concentration 1 Fe Co

Cr 0.1 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 Segregation Coefficient

Figure 12-Correlation between segregation coefficient and solid solubility of impurities in MG-Si [25, 57-63]

2.13.1 Solubility of Iron in Silicon

In order to get the minimum contamination of the solvent into silicon dendrites, the solvent should have a very low solid solubility in silicon. The solubility of iron in silicon compared to other elements is shown in Figure 13 and Table 6. It is reported that iron has extremely low solubility in silicon [64]. This content is given as 4.6× 1014 atoms/cm3 at 1000oC and 1.5× 1016 atoms/cm3 at 1207oC, corresponding to 9.2× 10-7 and 3.0× 10-5 at.% Fe respectively.

In another reference [35] the maximum solid solubility of iron in silicon has been reported to be 1.1 ppmw which is much lower than the solubility of most elements in Table 6.

Table 6-Maximum solubility of some elements in solid silicon from Ref. [25]

Element Solubility Element Solubility Element Solubility Element Solubility (ppmw) (ppmw) (ppmw) (ppmw)

S 0.8 Au 17 Li 312 Sb 6 001

Co 1.0 O 22 Al 384 P 26 527

Fe 1.1 Cu 59 Ga 1 977 As 97 129

Mn 1.3 Bi 123 B 4 437

Zn 2.8 Ca 171 Sn 5 335

Figure 13-Solubilities of representative interstitial transition metals in Si below the eutectic temperature [20].

Figure 14-Solid solubility in silicon [26].

2.13.2 Effect of the Distribution of Iron Silicide

The distribution of metallic silicides strongly affects solar cell performance. Metallic silicides are considered as recombination-active nono-precipitates, are homogeneously distributed within Si, and are separated by only a few microns. Therefore, their effect on minority-carrier diffusion length is much greater than micron-sized precipitates that are separated by several hundreds of microns. Thus, iron silicide nano-precipitates have greater direct impact on cell performance while larger, micron sized precipitates have a small indirect impact [17].

This behaviour means that during solidification of Fe with silicon even if there are some percipitates formed that cannot be separated later during leaching, as long as they are micron sized, they will not affect the cell performance.

2.14 Summary

In order to convert MG-Si to SoG-Si a combination of methods should be used because various impurities behave differently under different processes. The main challenge in these methods is the removal of P and B which because of their properties in silicon, are much more difficult to remove.

Directional solidification has proven to be a simple and effective way to reduce impurities but elements such as B and P which have high segregation coefficient cannot be removed to the levels that are needed for PV industry. Some other elements can only be removed to the desired amount if this process is applied several times but due to the energy consumption it will not be cost effective.

Plasma and vacuum refining are methods that can be used to remove P and B to desired levels but these two processes are applied in very high temperatures and for long periods of time and hence are not economically favored.

Acid leaching is another process which is used but can remove impurities that have been separated during the solidification meaning that this method needs to be combined with other methods to be effective for the removal of different impurities that exist in silicon.

Solvent refining appears to be an effective way to remove impurities when is used in combination with acid leaching. This method involves alloying Si with another element such as aluminum and solidifying it. When the binary system solidifies, impurities will be dissolved in the eutectic phase and pure silicon dendrites will grow. These dendrites can be collected above the eutectic temperature, while the getter phase is still liquid. One other method to collect these dendrites is to solidify the whole alloy and pulverize it. The impurities which tend to segregate in the grain boundaries and are adherent to these particles can be removed by acid leaching. Addition of some elements such as Ca and Ti are effective for the removal of P and B since they will form stable compounds such as Ca3P2 and TiB2 which also prefer to precipitate in grain boundaries and can be removed through acid leaching.

Based on this review, it can be concluded that a combination of solvent refining, acid leaching and solidification refining is a suitable choice for upgrading metallurgical grade silicon to solar

34 grade silicon. A review of the properties of iron showed that it could be an effective getter metal. Its usefulness in purification of silicon is the focus of the present work and will be discussed in the subsequent chapters.

Chapter 3: Experiments and Materials

In this chapter the experimental approach of the investigation is presented together with the materials used. This chapter is presented in two main sections since two sets of cooling conditions were examined.

3.1 Experimental Procedure-Part One: Quenching after Complete Solidification

The overall procedure of the proposed process is shown in Figure 15. Metallurgical grade silicon was crushed and pulverized then mixed with iron powder. The mixed powder is then melted in a muffle furnace, then solidified under different cooling rates for growing dendrites with different sizes. The solid samples which contain Si dendrites and alloy matrix (FeSi2) are then crushed and sieved to different particle sizes and are later separated by a heavy liquid. The separated Si particles, having much lower density, float to the top of the liquid. The adherent FeSi2 to the Si particles is later leached by hydrofluoric acid which does not dissolve Si. The Si particles can be further purified by directional solidification.

Powder Mixing MG-Si+Fe

Alloy Melting

Solidification

Crushing and Grinding

Physical Separation

Leaching

Directional solidification Figure 15- Flowchart of the experimental procedure.

3.1.1 Alloy Preparation

The chemical composition of the MG-Si used in the process was analyzed by ICP-AES and is shown in Table 7. The MG-Si lumps were crushed in a jaw crusher and later pulverized in a ball- mill for 20 minutes. Pulverized MG-Si was blended with Fe powder of 99.5% purity to form a mixture of Si-28 wt% Fe. Additional P and B, 0.3 wt% each, in the form of red phosphorous and boron powders were added to the batch to increase the concentration of these impurities for improved detection in the subsequent analysis. The alloy composition was selected at 28% Fe to yield a microstructure that consists of 50 vol% of each Si and FeSi2, according to lever rule and using the phase diagram. The equal volume percent for each phase was decided as a trade–off between the amount of Si produced and the amount of impurities retained in the matrix (FeSi2).

Larger vol% for FeSi2 improves the impurity removal but at the expense of producing small amounts of silicon.

About 200 g of the powder mix was placed in a mullite crucible (height=10 cm, larger diameter=5cm) sealed with a mullite lid covering the crucible and attached to it by ceramic paste (Figure 16).

Table 7-Chemical analysis of MG-Si used in the experiments by ICP-AES.

Impurity MG–Si (ppm) Impurity MG–Si (ppm)

Al 980 Li 3

B 27 Ni 118

Mn 158 Zn 23

K 39 Be 1

V 86 Cr 5

Cd 38 Mg 57

Ba 12 P 68

Fe 3108 Total 4723

Table 8-Chemical analysis of iron powder (from the supplier).

Impurity Content (ppm) Impurity Content (ppm)

As 5 Pb 20

Cu 100 Zn 50

Mn 1000 Cl 20

Ni 5000 S 10

Table 9-Charactristics of the used materials.

Material Description Supplier

ARISE technology Corporation (from MG-Si lumps Becancour Silicon)

Iron Powder Assay: min 99%, <212μm, Sigma-Aldrich

Red Phosphorus Assay: min 97%, max 0.2% Fe Riedel DeHaen

Boron Assay: min 95-97%, Amorphous Powder, Sigma-Aldrich

High Temperature Ceramic Maximum temperature: 1650°C, main Aremco Adhesive constituent: alumina

Figure 16-Mullite crucible.

3.1.2 Melting and Solidification

The crucibles were placed in a muffle furnace (Figure 17), then heated to 1550oC at the rate of 10oC/min in argon atmosphere, then held for 4 hours for homogenization of the liquid after which they were cooled under various cooling rates (Figure 18) of 0.5, 1.5, and 3oC/min (S1, S2, and S3 respectively). The samples were quenched in water once the temperature dropped to 1007oC which is 200oC bellow the eutectic temperature of the Fe-Si binary system (Figure 11).

The solidified samples containing Si dendrites and FeSi2 phase were then cut in half (Figure 19). The dendrite size ranges were analyzed. For this purpose 20 dendrites were analyzed in each sample (an example of these images is shown in Figure 20). Four widths along each dendrite were measured; therefore, 80 data points for each sample was collected overall. These measurements were performed by Image J software and the images were taken by a digital camera.

Figure 17-Muffle (box) furnace.

Figure 18-Cooling profile of the samples.

Figure 19-Sample cut in half and taken out of the crucible.

5 mm

Figure 20-Image of solidified sample captured with digital camera.

3.1.3 Crushing and Sieving

Each alloy sample in the first set of experiments, weighing about 200 g, was divided into four 50 g batches for further processing. Each batch was ground to a target particle size range in an agate mortar. The four target size ranges were A (600-800 µm), B (454-600 µm), C (212-454 µm), D (106-212 µm).

3.1.4 Physical Separation

Heavy media used in this study is a neutral liquid also called LST heavy liquid manufactured by Central Chemical Consulting in Australia. It is a low toxicity, dense liquid containing lithium heteropolytungstates (80-85wt%) in water commonly used for float-sink particle separations. The density of the heavy medium (HM) is 2.85-2.9 g/cm3, which is between the density of the silicon 3 3 dendrites (2.3 g/cm ) and FeSi2 matrix (4.74 g/cm ). The effectiveness of the physical separation was first examined by a semi-empirical analysis. From each batch, over 100 randomly selected particles were studied by image analysis to quantify total area and fractions of Si and FeSi2 in each particle. Backscattered SEM images (Figure 21) provided adequate contrast between the two phases for quantitative analysis of their area. The area fractions were then translated to the aggregate density of the particle, using the density of each phase and assuming that the volume of the phase is proportional to its exposed area. In a “perfect” separation process, all particles with specific gravity higher than the HM sink while the remaining particles report to the floats. Such perfect separation requires that (a) sufficient time is allowed for settling/floating of particles with various densities and (b) physical entrapment of the particles in the “wrong” fraction (i.e. floats/sinks) is eliminated.

Si FeSi2

(a) (b)

Figure 21-Backscattered SEM image of (a) as–solidified and (b) crushed alloy dispersed in dark background epoxy.

The above analysis allowed to quantify the degree of separation of each phase, based on the number of particles floated or (sank), and the composition of the particle (%Si or FeSi2). In addition to this semi–empirical treatment, the actual (effective) separation of Si was also investigated. 17 g of each of the four samples were suspended in 10 ml of HM in glass tubes (Figure 22) then agitated manually and left for 24 hours. The lighter Si particles float on top while the heavier FeSi2 particles sink to the bottom. The floats were scooped out and the sinks were separated by filtering the remaining liquid. Some tools that were found suitable for removing the floats are shown in (Figure 23), one being a plastic tube with a conical tip cut along its vertical axis with a hole in the bottom. The tube is inserted in the floats and taken out once filled with particles. This will limit the loss of heavy media.

The heavy liquid was cleaned for recycling by passing through several filters. Both the Si and

FeSi2 particles were rinsed with DI-water for a few times.

Figure 22-Separation of the particles after one minute suspension in the heavy liquid.

Figure 23-Useful apparatus for physical separation.

3.1.5 Leaching Experiments

A leaching step was required to remove the adherent Fe–Si from the Si in order to analyze the purified Si alloy. Si can be only dissolved in the presence of an oxidizing agent such as nitric acid [68] but Fe–Si dissolves in HF readily. The Fe–Si phase was thus dissolved away using a 9.5% HF solution at 75°C for four hours. 20vol% acetic acid was also added to enhance the wetting of the fine particles. The Si residue after this stage of leaching was then rinsed thoroughly with de–ionized water. 0.25 g of the Si product was digested in a 20 ml HF–HNO3–

H2O solution (volume ratio of 1:2:2) for 30 minutes and at 50°C, to prepare the ICP samples for analysis.

Figure 24- Photograph of the leaching setup.

3.2 Experimental Procedure- Part Two: Quenching above the Eutectic Temperature

The first set of experiments was modified in the second stage. Since the second set of experiments is very similar to the first, only the changes are outlined here.

. The main change was the quenching temperature of the samples from bellow eutectic to above eutectic temperature.

. The iron powder was changed to electrolytic iron with 99.99% purity from Alfa Aeser.

. The powder mix was not doped with P or B.

. The total amount of powder mix was 20 g instead of 200 g.

. The crucible in which the powder mix was solidified was cylindrical alumina Figure 26 in much smaller size (D=40, H=50 mm) than the previous set of experiments.

. The melting and solidification were performed in a horizontal tube furnace and under argon atmosphere. First the sample was heated to 1600°C at the rate of 10°C/min, left at this temperature for 4 hours then cooled at the rate of 0.5°C/min to 1220°C.

. The quenching temperature which was changed from 1007°C to 1220°C (from B to A in Figure 25) in which the two phases of liquid Fe-Si and solid Si exist.

. The composition was changed from 72wt% Si to 83wt% Si which will result in the formation of about 50vol% of Si after solidification at the new quenching temperature.

. The sample was not sieved since the main aim of these experiments was to study the extent of purification and investigation of the physical separation was not of interest.

Figure 25-Fe-Si binary system showing quenching temperatures.

Figure 26- Alumina crucible used in the second set of experiments.

3.3 Characterization 3.3.1 Scanning Electron Microscopy (SEM) and Electron Diffraction X-Ray (EDX):

The Hitachi S570 scanning electron microscope was used for observing the structure and distribution of the phases in the solidified samples and also for image analysis of the crushed particles. The samples were mounted in epoxy and carbon coated to cover the non-conductive surface with a conductive layer. As described earlier, backscattered SEM images provided adequate contrast between the two phases for quantitative analysis of their area. The scanning electron microscope was operated at 20kV and with a 15 mm working distance.

EDX was also used to confirm the composition of the two Si and FeSi2 phases.

3.3.2 X-Ray Diffraction (XRD)

XRD was used to distinguish the structure of FeSi2 phase (α-FeSi2 or β-FeSi2) and also to confirm that all of the FeSi2 phase has dissolved during the leaching and before ICP-AES analysis of Si.

For this purpose Philips Diffractometer (model PW 3710) with X’PERT graphics software package was employed. A rectangular aluminum-glass composite sample holder (2cm×1cm×0.2

47 cm) was filled with the powdered sample. The samples were analyzed using a CuKα radiation (λ= 1.54056Å) with nickel filter. Bragg’s angel (2theta) range of 10-50° and a scan speed of 0.72 degree per minute with a step-size of 0.015° were used. The Philips diffractometer was operated at 40 kV and 40 mA. The structural pattern was recorded and analyzed with X’PERT HighScore TM software.

3.3.3 Inductively Coupled Plasma- Atomic Electron Spectrometry (ICP-AES):

Optima 7300 ICP-AES was used to analyze the impurity content of initial MG-Si and the final purified Si. For this purpose 0.25 g of Si was digested into 20 ml of HF: HNO3: H2O=1:2:2. The concentration of the acids used was 70wt% and 48wt% nitric acid and hydrofluoric acid respectively. The digestion was done in Teflon beaker covered with a Teflon lid for about half an hour at 50°C. The digested sample was transferred to a 50 ml falcon tube and made up to 50 ml with DI-water. A 50 ml blank sample was also prepared with the exact same ratios of the acids used for digestion. The calculation for converting the concentration of elements in solution to the amount of them in the solid sample is as follows.

C (solid Si) in ppm= C* (solution) × prep volume × 1000/ wt of Si digested

Where C* is the difference of the concentration of element in solution and in the blank sample.

It should be noted that when the amount of an element is reported as a negative value the detection limit of the element is used instead of C* in the equation above.

3.3.4 Electron Probe Micro-Analyzer (EPMA):

In order to analyze the profile of impurities, specially phosphorus, across Si-FeSi2 interface EPMA analysis was carried out. Cameca SX50 electron microprobe, with 3 spectrometers was used with operating conditions of 40 degrees takeoff angle, and beam energy of 20 keV. The beam current was 50 nA, and the beam diameter was 1 microns.

The detection limit of phosphorus was about 100 ppm. The as-solidified samples were mounted and carbon coated then placed in the probe chamber. The lines of interest were chosen. About 10-20 points per interface were analyzed on each line giving the profile of elements in both

48 phases. The distance between the points varied from 7-70 microns. The main element of interest was phosphorus. It should be pointed out that EPMA was not capable of measuring boron.

Chapter 4: Effectiveness of Physical Separation

4.1 Characteristics of Solidified Alloys

Analysis of the solidification products (Figure 27) by XRD together with elemental analysis using SEM–EDS confirm that the matrix surrounding Si dendrites is iron silicide (FeSi2). it was expected that quenching the sample from 1000°C would yield –FeSi2 as the stable phase which is stable form below 962 °C [59]. The XRD pattern shown in Figure 27 verifies the same phase.

Figure 27-XRD pattern of the solidified alloy.

Variations of the width of the Si primary dendrites with cooling rate are presented in Figure 28 and Figure 29. As expected, faster cooling yields finer Si crystals. Based on the principles of segregation, slower cooling during growth of Si crystals allows for more rejection of impurities with small segregation coefficient to the liquid, thus increases the Si purity. Slow cooling also results in growing thicker dendrites, that are easier to separate from the alloy as (a) in order to liberate Si from the matrix, fine crushing is not required, and (b) larger particles sink/float faster,

50 thus gravity separation is accelerated. However, these advantages may be offset by the longer solidification time and more consumption of energy associated with it. Therefore, it is critical to identify the optimum separation conditions for each cooling rate.

Figure 28-Cumulative wt% percentage verses the dendrite thickness.

Figure 29-Dendrite thickness as a function of cooling rate.

4.2 Effect of Specific Gravity of Heavy Medium on Separation of Si

Two widely used quantities of separation processes, i.e. recovery (percent by weight of Si in the feed that reports to the floats) and grade (%Si in the floats) are considered to evaluate the effectiveness of the physical separation stage. The grade (G) weight % and recovery (R) of Si in the floats fraction of the material are calculated as following, based on the relative density of each particle and the fluid, which decide whether a particle will report to the floats or the sinks.

Eq. 4

Eq. 5

In these equations, mi and fi are the total mass of particle and mass fraction of Si in particle i, respectively, t is the total number of particles studied (sample population) and n is the number of particles with density greater than that of the heavy medium.

An example of the recovery and grade variations with the specific gravity of the liquid is displayed in Figure 30. As seen, an increase in the SG of the liquid gives rise to higher recovery, while grade drops. This is expected, as by an increase in the liquid density, larger fraction of the feed can float, which in turn increases the recovery. On the other hand, since this increase is obtained at the cost of floating more heavy particles (that naturally contain more FeSi2), the iron content of the floats increases, lowering the grade.

Figure 30-Recovery and grade in various S.G. of heavy liquid plotted for sample with cooling rate= 3C/min and particle size= 600–800 µm.

The results in Figure 30 may also be interpreted as a drop in the recovery by increasing the grade that implies an inverse relationship between G and R. This is the typical dependence of recovery and grade, as the two quantities are not independent. Increasing the grade would require that only highly pure Si particles (i.e. with less adherent FeSi2 phase) float, which inherently decrease the recovery of Si. Therefore, in a perfect separation process, R and G are decided by how well the

Si crystals are liberated from the matrix. Consequently, maximizing both parameters is not possible and selecting optimum conditions will depend on the particular operation and involves a trade–off between grade and recovery. On the other hand, it is possible to increase both recovery and grade simultaneously, by increasing the degree of liberation of Si particles. For example, by grinding the alloy to fine particles, the amount of the composite Si-FeSi2 particles is reduced, thus both higher grade and recovery can be achieved.

Figure 31 shows the recovery-grade relationship for the three samples cooled under various rates. As seen, in each case smaller particles yield higher grade and recovery, although the inverse relationship between the two parameters still holds. Although beneficial to extent of separation, the finer particles are generally associated with more difficult handling, higher grinding costs, and slower separation. Therefore, grinding should be limited to the largest size that also provides acceptable grade and recovery.

(a)

(b)

(c) Figure 31-Recovery verses grade plotted for samples with cooling rate (a) 0.5 C/min, (b) 1.5 C/min and (c) 3 C/min.

4.3 Effect of Cooling Rate and Optimum Particle Size

A comparison of the G–R curves for the three cooling rates in Figure 31 reveals that generally, slower cooling yields higher grade and recovery for a given particle size. The role of cooling rate can be explained through its effect on the alloy microstructure. It may be speculated that for an alloy consisting of thinner dendrites, grinding to smaller particles is required to liberate Si from the matrix alloy. Therefore, it is the relative size of dendrites to particles that dictates the degree of liberation, and consequently, the extent of separation. Hence, a quantity such as particle size divided by dendrite thickness could be used as the single effective parameter that influences the separation extent. Furthermore, it was discussed that maximizing both R and G is not possible for a physical separation process. Conventionally, in ore benefication processes, Separation Efficiency (SE), as defined in Eq. (6) has been used to evaluate the performance of the process. SE essentially integrates the R and G, to establish a unified “process efficiency” measure [69], based on which the optimum point on the G–R curves can be found.

Eq. 6

In this equation, R denotes recovery of the desired phase (Si) and R represents the Si FeSi 2 recovery of the FeSi2, both to the product fraction (floats).

For a series of tests performed using heavy medium with SG of 2.90, the separation efficiency was calculated based on the actual recoveries of Si and FeSi2. The results are plotted in Figure 32 against particle size for samples with different cooling rates i.e. different dendrite thickness. From this figure it can be found that with the decrease of particle size SE will also decrease. Moreover, SE is higher for lower cooling rates or larger dendrite sizes.

Figure 32-Separation efficiency verses particle size for samples with different cooling rates.

According to the justification made earlier, it is logical that more important than the dendrite thickness or the particle size, it is the ratio of the two that controls the SE. In Figure 33, SE is plotted against r= (particle size)/(dendrite thickness). As shown, the results of all experimental conditions with various cooling rate and dendrite width fit well on a single curve that shows an increase in SE as r is decreased. The SE however reaches a maximum and plateaued when r drops below unity. This indicates that the optimum size to which the alloy has to be crushed before the physical separation is equal to the average thickness of the primary Si dendrites. In other words, grinding the alloy to a critical size, that is equal to the dendrite thickness, increases the liberation of Si. Any further size reduction beyond this critical value will not improve the separation efficiency.

Figure 33-Separation efficiency verses r= crushing size/ dendrite thickness.

It may be concluded from Figure 33 that slower cooling rate for formation of larger dendrites is favorable as fine grinding is not required and the comminution costs are lowered. Also, the physical separation process is accelerated for larger particles; resulting is higher throughput. However, determination of the optimum cooling, grinding, and phase separation conditions for solvent refining of Si require additional considerations. For high purity requirements such as SoG–Si, dependence of the purity of the Si crystals on the growth conditions has to be investigated. This will be discussed in Chapter 5.

Chapter 5: Extent of Silicon Purification

The ultimate objective of this research was purification of MG–Si. As discussed previously, the effect of cooling conditions including cooling rate and also the quench temperature were investigated on the Si purity.

The effectiveness of the solvent refining process is expressed in terms of Purification Ratio (PR) here that is the ratio of concentration of an impurity in the final Si product to that in the starting metallurgical grade silicon (MG–Si). Based on this definition, elements with a PR smaller than unity are effectively removed from Si, the concentration of those with PR=1 remains unchanged, and elements with PR>1 favour Si over the Fe–Si phase and are segregated to Si. Clearly, smaller PR values indicate greater removal of the element from MG–Si. In the following sections, the extent of Si purification is discussed for the various cooling conditions.

5.1 Quenching after Complete Solidification

The PR values presented in Figure 38 show that the concentration of many impurities including Al, Mn, K, Ba, Li, Ni, Zn, and Mg is reduced over an order of magnitude, when the Fe–Si alloy is quenched from a temperature below the eutectic. Several other elements such as V and Cr are removed to a lower extent. However, the close to 1 PR values of P and B reveals that their concentration in the product is in effect equal to those in MG–Si. In fact, for B, the PR values were above 1, indicating that B favours Si over Fe–Si under the studied conditions. Similar trends were observed in another study [70] on purification of Si using nickel. The unsuccessful removal of P and B may at first be related to the low segregation coefficient of these elements in Si. It may be thought that addition of iron to Si has not lowered their segregation coefficient, thus is not beneficial in eliminating these impurities. However, as shown in the following equations, iron has a higher thermodynamic affinity for B and P than Si, thus a liquid Fe–rich phase should normally favour these impurities, more than Si. In other words, lower segregation coefficients are expected in the presence of iron.

Figure 34-Impurity ratio in final Si/ MG–Si

Eq. 7

Eq. 8

This surprising response of B and P to the solvent refining process was justified as follows, and based on the observations of impurity concentration dependence on the particle size of Si.

During the precipitation of the Si dendrites, the impurities including P and B are rejected to the growth front, creating a large concentration of the impurities at the interface. The transport of the impurities and their distribution in the liquid is limited because of either slow liquid–phase diffusion, or solidification of the eutectic liquid from the melt bulk towards the dendrites. Therefore, on complete solidification, an impurity–enriched layer is formed at the interface

60 between Si and Fe–Si matrix, as experimentally evidenced in Figure 35 and Figure 36. Although the liquid Fe–Si alloy favours impurities (P and B) over the solid Si, the same behavior may not hold when this liquid is solidified to the intermetallic phase FeSi2. Therefore, the tendency is reversed towards rejection of the impurities from the FeSi2 matrix to the Si. This process is inherently slow as it involves solid state diffusion. Nevertheless, the relatively slow cooling rates of 0.5–3.0 °C/min and the large difference between the eutectic and quenching temperatures (~ 200 °C) allow enough time (~1–7 hours) for back–diffusion of large quantities of the impurities to the Si. At the same time, solidification of the eutectic phase followed by quenching leaves significant residual stresses and microcracks at the Si/Fe–Si interface (see Figure 35(a) and Figure 36(a)). As a consequence, grinding generates a distribution of particles where majority of the small Si particles are from the edge of the dendrites. The larger particles, on the other hand, are more produced from the core of dendrites. The combined effects of the back–diffusion phenomenon and the breakage pattern of Si result in higher concentration of the impurities in the finer particles. This hypothetical mechanism is supported by some results such as the concentration variations seen in Figure 37.

FeSi2

(a)

(b) Figure 35-(a) Micrograph of the solidified alloy and (b) EPMA analysis for P across the line shown in (a)

(a)

(b) Figure 36-(a) Micrograph of the solidified alloy and (b) EPMA analysis across the line shown in (a)

(a)

(b) Figure 37-Effect of particle size on (a) P and (b) B concentration in final Si product.

Based on the above suggested mechanism for impurity re–distribution, it is expected that quenching the alloy just after precipitation of the dendrites and while the eutectic phase is still liquid, would retain the impurities in the matrix. This would prevent the reverse segregation of the impurities to Si, as no time is essentially given for the diffusion. Accordingly, the experimental procedure was modified to quench the alloy just above the eutectic temperature. The results are discussed in the following section.

5.2 Quenching above the Eutectic Temperature

The findings in Section 5.1 lead to the modification of the solidification conditions by quenching the alloy in the liquid–solid region i.e. before completion of solidification. The values of PR, obtained after analysis of Si product are provided in Figure 38 and compared to the previous solidification conditions. As seen, for almost all elements, the PR has decreased substantially, indicating improved removals. In particular, it is seen that for P and B, the ratio is significantly smaller than 1, pointing to the effective removal of these impurities that would otherwise remain in Si.

In Table 10 the composition of MG–Si and the final purified Si together with the removal percentages are presented for the major impurities. It is clear that for MG–Si, impurity reductions over 90% are realized for all elements except Cr and P that showed still substantial 80% and 57% removal percentages, respectively. Interestingly, the final iron content of the purified Si is about 1 ppmw, although large quantities of iron have been used in contact with Si, as the getter element. This is presumably because of the very small segregation coefficient of iron being 8×10–6[25].

Figure 38-Impurity ratio in final Si/ MG–Si

Table 10–Impurity content of initial MG–Si and refined silicon.

Impurity MG–Si (ppm) Final Product (ppm) Removal %

Al 980 10 99.0

B 27 2 92.6

Mn 158 3 98.1

K 39 1 97.4

V 86 <0.3 99.7

Cd 38 3 92.1

Ba 12 <0.1 99.2

Li 3 <0.2 93.3

Ni 118 3 97.5

Zn 23 <0.4 98.3

Be 1 <0.1 90.0

Cr 5 1 80.0

Mg 57 <0.1 99.8

P 68 29 57.4

Fe 3108 1 100.0

Total 4723 53 98.9

In order to investigate the suitability of the Si product as feedstock for SoG–Si manufacture, the tolerable content of impurities before directional solidification together with the amounts achieved in this investigation were compared (Table 11). As seen, the refined Si exceeds the requirements of directional solidification feedstock for all impurities except P. One approach to

67 achieve even higher purity levels is to repeat the above process, by using the purified Si product as the starting feed. Figure 39 shows the impurity ratio in final Si to initial Si against the number of refining stages. For phosphorous, a second time refining would decrease the concentration from initial 69 ppmw to 12.5 ppmw, that is below the acceptable level for directional solidification.

Table 11-Tolerable concentration (ppmw) of impurities in Si before directional solidification compared with the concentration achieved in current study.

Al B Mn K V Cd Ba Li Ni Zn Be Cr Mg P Fe

Current 10 2 3 1 <0.3 3 <0.1 <0.2 3 <0.4 <0.1 1 <0.1 29 1 Study

[18] 374 13.9 ------14 14.7 615

[19] 0.7 - 76 - 2.2 ------72 - 11.4 186

Figure 39-Number of refining stages verses impurity ratio in final Si to initial Si .

5.3 Effective Segregation Coefficient of P and B

Many elements such as Cu, Fe, and Ni with small segregation coefficients in the order of 10–5- 10–4 [25] are readily removed by directional solidification of Si, without a need for pre–treatment of Si. On the other hand, for P and B with large segregation coefficients of 0.35 and 0.8 respectively, only directional solidification is not adequate to yield a product with SoG–Si specifications. Using techniques such as solvent refining can improve the removal of these impurities by lowering their segregation coefficient. In the presence of getter metal M, the effective segregation coefficient of impurity i between Si and Si–M can be defined as the following:

Ci(Si) Eq. 9 kieff  Ci(SiM )

where Ci(Si) and Ci(SiM ) are the concentrations of impurity i in solid Si and liquid Si–M, respectively. The smaller this value is for a specific impurity, the more the element will be segregated to the liquid phase compared to solid silicon.

By using the concentration of the impurity in Si dendrites and Si–Fe matrix, as and respectively, it is possible to calculate the effective segregation coefficient for the Si–Fe system.

According to the result, ki–eff of phosphorous between Si–Fe melt and solid Si at 1220°C is 0.42 that is about the same value between solid and liquid Si (0.35). This implies that addition of iron has not been very effective on P removal. As discussed earlier, Figure 10(b) suggests higher affinity for P of iron than Si, thus one expects that in the presence of iron, the segregation coefficient of P in Si is lowered. However, the anticipation may not be valid, as previous studies on Fe-Si-P [71] show that the activity coefficient of P in Fe–Si melts increases by addition of iron up to about 50 wt% Fe (close to the eutectic composition). The increase causes a drop in the solubility of P in Fe–Si melt, as seen in Figure 40. In fact, the solubility is minimum, where our eutectic composition (matrix composition) is. Therefore, the higher affinity of Fe for formation of phosphates does not necessarily translate into more gettering of P, because of the effect of activity coefficient of dissolved phosphorous. In other words, although the activity of P is lowerd

69 by solidification of iron, the increase in the activity coefficient causes a decrease in the P content of Fe-Si.

Figure 40-Solubility of P in Fe–Si melts at 1450 °C (reproduced from data in Ref. [71])

In the case of boron, the segregation coefficient is reduced to 0.07 that is over one order of magnitude lower than that in Si (0.8). It may then be assumed that the activity coefficient of boron in Fe–Si melts is not increased as it does for phosphorous.

5.4 Effectiveness of Impurity Removal

In the previous section, the segregation coefficient of P between solid silicon and liquid silicon (0.35) was compared to the segregation coefficient between solid silicon and Fe-Si melt which was calculated to be 0.42 based on the experimental results. However, when the impurity removal of solvent refining is compared to that in directional solidification, it must be noted that in solvent refining, a substantial volume (or mass) of the impurity trapper phase (Fe in this case), is introduced. Therefore, for equal yield of silicon in both processes, the solvent refining should still remove a considerable amount of the impurities.

Therefore, in order to compare the effectiveness of impurity removal of the current process with directional solidification, the following was carried out. First, the impurity removal in directional solidification was calculated for a given fraction of recovered silicon (the remaining being sliced off as the impurity–laden part of the ingot). For phosphorus, the concentration profile of the solidified fraction of the sample in Figure 41 was plotted based on the Scheil equation given below.

Eq. 10

where is the segregation coefficient of P in Si (0.35), is the initial concentration of impurity (68 ppm), is the solidified fraction of liquid Si and is the phosphorus concentration in solid silicon. This equation is for solidification with homogeneous liquid and no diffusion in solid.

From the obtained profile, the average concentration of the impurity for a certain fraction of the ingot (fx) can be calculated by integrating the equation and dividing by the fraction recovered (fx). This fraction is decided to be equal to the yield of silicon in the solvent refining process, in order to make a reasonable comparison between the two techniques. In other word, it is attempted to answer the question that “how would the final concentration of P in solvent refining and directional solidification compare, if the amounts of Si recovered in both processes were equal?”.

(a)

(b) Figure 41-Phosphorus concentration profile of solidified sample based on Scheil equation a) complete solidification and b) about 50wt% of sample solidified.

As shown in Figure 41 (b), if 57wt% of a silicon ingot purified by directional solidification is recovered, its average P content will be 30.4 ppmw. The 57wt % represents the same recovery that may be obtained, if the a Si-17%wt Fe is cooled to above eutectic, quenched, and subjected to heavy media separation and acid leaching; In other words, from 100 g of Si-17% alloy, 59.3 g silicon will be precipitated, according to the lever rule. Based on the physical separation test results, it may be assumed that the Si recovery is 80% in the HM separation. Therefore, the recovered Si will be 0.8×59.3= 47.4 g. the overall recovery of Si will then be 47.4/0.83= 57%. The P content of the Si obtained from solvent refining is 30.4 ppmw, equal to that from the directional solidification (30.4 ppmw). Therefore, despite the larger segregation coefficient of P in Fe-Si system, the effective removal of P in the two techniques is comparable. This is because the negative effect of higher segregation coefficient is offset by the larger volume of liquid acting as trapper.

5.5 Other Alloys

In addition to iron, zinc and calcium which were thought to be appropriate alloying candidates were studied. In this section the properties of these elements that make them good candidates for solvent refining and the experiments that were done is briefly discussed.

5.5.1 Zinc

The properties that are believed to make zinc a good gettering element are listed below:

- Zinc has low maximum solid solubility (2.8 ppmw) at about 1320°C [26] in solid silicon and at the same time a very low segregation coefficient (1×10-5 [16]) (Figure 12).

- The binary system is such that solvent refining can be performed at lower temperatures Figure 42.

- As shown from the Si-Zn phase diagram, the binary system has a simple eutectic with no intermetallic compounds, indicating that the leaching of the product will be simple.

- Zinc has much higher density than silicon (7.14 g/cm3compared to 2.33 g/cm3), allowing for gravity separation of the two produced phases.

- Phosphorus has liquid solubility in zinc at above 420°C (Figure 43).

- Zinc phosphide and boride are more stable than silicon boride and phosphide (see ellingham diagram plotted for phosphides and borides in Figure 10).

Figure 42-Zn-Si phase diagram [56].

Figure 43-Zn-P phase diagram [56].

. Experiments:

An alloy with the composition of 75wt% Zn powder with MG-Si was prepared. 200 g of the mixed powder was placed in alumina crucibles (Figure 26) with a covering lid that was attached by ceramic paste. The crucible was heated in a vertical tube furnace and under argon atmosphere. The heating profile of the sample is shown in Figure 44.

Figure 44-Melting and solidification profile of Zn-Si experiments.

After quenching the sample at 300°C, it was found that most of the zinc had evaporated from the crucible. This was expected from the very low boiling point of zinc being 907°C and the resulting high vapor pressure inside the crucible.

This experiment was repeated with better sealing of the crucible but the result was the same. In all cases microcracks were visible on the crucible sealing, where zinc was presumably escaped from.

Unfortunately the available furnaces did not allow for further investigation of this process.

5.5.2 Calcium

As discussed in Section 2.11, calcium can have two purification effects when alloyed with MG-

Si. First, it will improve leaching of impurities from MG-Si by forming CaSi2 at the grain boundaries and acting as a collector phase for the impurities. It can then be easily dissolved using aqua regia. The other is that it can form strongly stable phases with P and B, specially with P.

The stability of Ca3P2 can be seen in the ellingham diagrams (Figure 10).

In the study that was done on solvent refining using iron, it was thought that after the iron- alloying the purified Si product can be further purified by adding calcium since in the proposed process the removal of phosphorus was found to be less successful compared to boron.

. Experiments:

10wt% granular calcium from provided from Sigma- Aldrich (99% purity and 6 mesh) was added to MG-Si, doped with 0.3wt% P and 0.3wt% B and placed in ceramic crucibles same as Zn-Si experiments and heated in a horizontal tube furnace. The heating profile of the sample was as shown in Figure 46. As expected from the phase diagram, the SEM images show the two phases of Ca and CaSi2 (Figure 47). The EDS results showed the darkest phase was Si, the lightest was CaSi2 rich in Fe and the light gray phase was CaSi2. The iron in the Fe rich phase is from the iron in the MG-Si (more than 0.3wt%).

Figure 45-Ca-Si binary phase diagram.

Figure 46-Melting and solidification profile of Ca-Si experiments.

Figure 47-SEM image of 10%Ca-Si sample.

Element wt% at%

Si 48 62

Ca 21 19

Fe 31 19

Figure 48-EDX pattern of 10%Ca-Si sample (area A in Figure 47).

Element wt% at%

Ca 42 66

Si 58 34

Figure 49-EDX pattern of 10%Ca-Si sample (area A in Figure 47).

The XRD pattern results show the formation of Ca3P2 phase that are not able to be detected by SEM.

Figure 50-XRD of 10wt%Ca for Ca3P2 detection.

The sample was crushed to >106 µm. 1g of the sample was leached for 4 hrs at 75°C in 20 ml of

HNO3: HCL: H2O with ratio of 1:3:6 respectively. 0.25 g of the remaining solid was then analyzed by ICP-AES. The ICP analysis is shown in Figure 51. In this figure the distribution of impurities in CaSi2 and Si is presented. The columns show the ratio of impurities in CaSi2 to Si meaning that when this ratio is higher than 1, CaSi2 phase has higher affinity for impurities than Si and is successfully removed. Therefore, calcium treatment can be used as an alternative refining to repeated Fe alloying, after Si was purified with Fe solvent refining.

Figure 51-Impurity ratio in CaSi2/ Si.

Chapter 6: Summary, Conclusion and Future Work

6.1 Summary and Conclusion

A combination of solvent refining technique and physical separation was employed to grow pure silicon crystals. Metallurgical grade silicon and iron were melted together and solidified under various cooling rates to form dendrites of silicon. The Si was separated from the matrix by grinding the alloy and floating the light Si on a heavy fluid, while the FeSi2 phase was settled. It was found that recovery and grade of Si in the product (floats) follow an inverse relationship. Grinding the alloy to a smaller particle size yielded higher recovery and grade. For a given particle size, faster cooling had a negative impact on the separation, because of forming a fine microstructure (smaller dendrites) that are more difficult to liberate from the matrix. The ratio of particle size to dendrite thickness, r, appeared to integrate the effects of both microstructure and particle size into a single parameter. To identify the optimum trade–off between grade and recovery, Separation Efficiency was used. It was shown that SE is a strong function of r. By decreasing r, SE first increases and then becomes stable when r drops below unity. The optimum grinding size is therefore, dictated by the cooling conditions and is equal to the thickness of the primary dendrites.

The results of the dendrite purification indicate that when samples are quenched above the eutectic temperature i.e. within the solid Si and Fe–Si melt region, the removal of impurities is significantly improved compared to the case when quenched from fully solid state. This was attributed to back–diffusion of impurities to Si, during the slow cooling period of the latter conditions. By alloying Si with Fe, the effective segregation coefficient of B was reduced from 0.8 (between solid Si and Si melt) to 0.07 (between solid Si and Fe–Si melt). Not such decrease was observed for P, because of an increase in the activity coefficient of P in Si, in the presence of iron. It was found that the solvent refining of MG–Si with Fe as the getter can reduce the concentration of the impurities significantly, to levels below the tolerable levels in directional solidification. The total concentration of the major impurities was reduced from 4700 to 53

81 ppmw by one stage of solvent refining. It is believed that the process may be repeated to achieve substantially higher purity.

6.2 Future Work

The following investigations are suggested for future work:

. The results show that the proposed method has less effect on the removal of phosphorus compared to that of boron. Based on literature review and the preliminary experiments that were done on calcium, this element is believed to have a significant effect on the

removal of phosphorus from MG-Si by forming Ca3P2 and also improving the leachability after solvent refining. For further studies, iron and calcium can be used together or subsequently in MG-Si purification.

. It is suggested that this process be repeated several times to further purifiy the silicon. A model that takes into account the yield of silicon can be established to estimate the overall operating cost of the process.

. Thermodynamic evaluations can be done on the activity coefficient of P and B in Si-Fe. This can help justification of the removal of these impurities from Si to liquid Si-Fe.

. The effectiveness of other getter such as Zn in impurity gettering will be an interesting study, due to the favorable properties of zinc.

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Appendix A: ICP-AES Detection Limits

Element (Primary Line) QDL (Radial) ug/mL QDL (axial) ug/mL

Ag 328.068 0.1 0.01

Al 396.153 0.3 0.03

As 188.979 2 0.2

Au 267.595 0.4 0.04

B 249.772 0.1 0.01

Ba 233.527 0.01 0.001

Be 313.107 0.01 0.001

Bi 223.061 2 0.2

Ca 317.933 0.002 0.0002

Cd 228.803 0.1 0.01

Ce 413.764 0.5 0.05

Co 228.616 0.1 0.01

Cr 267.716 0.2 0.02

Cs 455.531 1 0.1

Cu 327.393 0.04 0.004

Dy 353.170 0.2 0.02

Er 337.271 0.1 0.01

Eu 381.967 0.01 0.001

Fe 238.204 0.2 0.02

Ga 417.206 0.4 0.04

Gd 342.247 0.1 0.01

Ge 209.426 2 0.2

Hf 277.336 0.4 0.04

Hg 253.652 0.1 0.01

Ho 345.600 0.04 0.004

In 230.606 0.9 0.09

Ir 205.222 0.5 0.05

K 766.490 2 0.02

La 408.672 0.1 0.01

Li 670.784 0.03 0.003

Lu 261.542 0.02 0.002

Mg 285.213 0.01 0.001

Mn 257.610 0.04 0.004

Mo 202.031 0.3 0.03

Na 589.592 0.3 0.03

Nb 309.418 0.5 0.05

Nd 406.109 0.1 0.01

Ni 231.604 0.5 0.05

Os 228.226 0.04 0.004

P 178.221 3 0.3

Pb 220.353 1 0.1

Pd 340.458 0.3 0.03

Pr 390.844 0.1 0.01

Pt 265.945 1 0.1

Rb 780.023 3 0.3

Re 197.248 0.5 0.05

Ru 240.272 0.6 0.06

S 180.669 3 0.3

Sb 206.836 1 0.1

Sc 361.383 0.03 0.003

Se 196.026 5 0.5

Si 251.611 0.4 0.04

Sm 359.260 0.2 0.02

Sn 189.927 3 0.3

Sr 407.771 0.006 0.0006

Ta 226.230 1.5 0.15

Tb 350.917 0.2 0.02

Te 214.281 1 0.1

Th 283.730 5 0.5

Ti 334.940 0.05 0.005

Tl 190.801 3 0.3

Tm 313.126 0.05 0.005

U 385.956 1.5 0.15

V 310.230 0.05 0.005

W 207.912 0.8 0.08

Y 371.029 0.03 0.003

Yb 328.937 0.03 0.003

Zn 206.200 0.1 0.01

Zr 343.823 0.08 0.008

Appendix B: Industrial Economic Evaluation of the Proposed Process

In order to have an estimate of the economics cost of the overall process a brief evaluation of the major cost items is presented. These calculations are based on producing 1 kg of purified Si as the final product:

1) Energy . Heating/Melting: The energy needed to melt 1kg of alloy is about 5 MJ=1.4 kWh Considering a thermal efficiency of 50%, the total energy required is: 1.4×2=2.8kWh (heat loss) . Grinding: According to the Bond’s equation the energy needed to grind 1 ton of ferrosilicon from is:

Where is the Bond work index for ferrosilicon, P is the diameter in microns that 80% of the product passes and F is the size in which 80% of the feed passes [69]. From this equation: the energy needed to grind 1 inch ferrosilicon lumps to 200μm particles is about 0.007 kWh/kg.

Therefore, the total energy required is 2.8+0.007= 2.8 kWh/kg alloy.

1 kg of 83wt%Si will result in about 600g of Si dendrites. If 80% of the dendrites are recovered in the physical separation the total amount of purified Si will be 480g. Therefore, the energy required per kg of Si is 6 kWh/kg Si that is significantly below the energy consumption for the Siemens process (120-160 kWh)

The cost of energy, if all energy is electricity, would be 6×$0.06=$0.36

2) Leaching

1 kg of Si product will consume 2 lit of hydrofluoric acid which will cost about $1.2.

3) Physical Separation In industrial scale, the gravity separation can be done at much lower cost than the heavy medium used in laboratory scale of this study. For example in upgrading coal where low- ash coal is separated from the heavier high-ash coal, cheap heavy media such as high density slurries are used [69]. 4) Labor Work and other

Total cost:

From the above calculations it is estimated that the cost for producing 1 kg of purified Si in industrial scale will be about $2 based on $0.5/kg as labor and physical separation.

Appendix C: Publications and Presentations from this Research

Journal Articles

 S. Esfahani, M. Barati, “Purification of MG-Si using Iron as Impurity Getter, Part 1: Growth and Separation of Si”, Submitted to Metals and Materials International, (2010).

 S. Esfahani, M. Barati, “Purification of MG-Si using Iron as Impurity Getter, Part 2: Extent of Si Purification”, Submitted to Metals and Materials International, (2010).

 Z. Yin, A. Oliazadeh, S. Esfahani, M.D. Johnston, M. Barati, “Solvent Refining of Silicon using Nickel as Impurity Getter”, Submited to Canadian Metallurgical Quarterly, (2010)

Conference Articles

 S. Esfahani, M. Barati, “A Novel Purification Method for Production of Solar Grade Silicon”, Materials Challenges in Alternative & Renewable Energy Conference Proceedings, 2010, Amrican Ceramic Society, Florida, USA, 2010.