Molten Salt Electrodeposition of Silicon in Cu-Si

by

Samira Sokhanvaran

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Departments of Materials Science and Engineering University of Toronto

© Copyright by Samira Sokhanvaran 2014

ii

Molten Salt Electrodeposition of Silicon in Cu-Si

Samira Sokhanvaran

Doctor of philosophy

Departments of Materials Science and Engineering

University of Toronto

2014

Abstract

Widespread use of solar energy has not been realized to date because its cost is not competitive

with conventional energy sources. The high price of solar grade silicon has been one of the

barriers against photovoltaic industry achieving its much anticipated growth. Therefore,

developing a method, which is energy efficient and will deliver inexpensive silicon feedstock

material is essential. The electrodeposition of Si from a cryolite-based melt was investigated in

the present work as a possible solution.

This study proposed electrowinning of Si in molten Cu-Si alloy, to decrease the working

temperature and increase the efficiency. Solvent refining can be used to recover Si from Cu-Si

and also as a second purification method. The physicochemical properties of the potential

electrolyte, cryolite–SiO2 melts, were studied in the first step of this work. The deposition

potential of Si on a graphite cathode was measured to determine the working potential and the

effect of SiO2 concentration on it. In the next step, the deposition potential of Si from cryolite–

SiO2 melt on Cu and Cu-Si cathodes was determined using cyclic voltammetry. Next, the

cathodic and the anodic current efficiencies of the process were measured. Continuous analysis

of the evolved gas enabled the instantaneous measurement of the current efficiency and the

kinetics of the deposition. Finally, the effectiveness of the process in delivering high purity Si

iii

was investigated. Si dendrites were precipitated out of the Cu-Si cathode and recovered to

determine the purity of the final product as the final step of this study. The produced Si was

separated from the alloy matrix by crushing and acid leaching and the purity was reported.

The findings of this research show that the proposed method has the potential to produce high

purity silicon with low B content. Further development is required to remove some metallic

impurities that are remained in Si.

iv

Acknowledgments

This research bears the imprint of many people who shared with me their knowledge and

experience. First and foremost, I wish to express my sincere gratitude to my supervisor, Dr.

Mansoor Barati, for his invaluable assistance, guidance and support through completion of this

research. I would like to thank him for teaching me how to solve the challenging and applied

problems and for being there whenever I needed him at all stages of this research.

Deep gratitude is also due to the members of the supervisory committee, Dr. R. Ravindran, Dr.

C. Jia and Dr. K. Lian whose assistance was elemental in successful completion of this study.

Also, I would like to take this opportunity to thank the Department of Materials Science and

Engineering for providing me the necessary facilities and warm environment to carry out the

research work. A special appreciation goes to Late Prof. T. Utigard, for the amenities and

support he provided on the characterization of electrolyte. I would also like to thank Sal Boccia

and Dan Grozea from the Department of Materials Science and Engineering and Mr. S. Salavati

the Department of Mechanical and Industrial Engineering for their advice and assistance in

sample preparation and SEM analysis. I also would like to thank RioTinto Alcan, NSERC and

MSE department for providing the financial support for this research.

I really appreciate the support of all members of the Sustainable Materials Processing Research

Group, specially S. Thomas, M. Li. I also thank a tireless man, K. Danaei, for his effort both in

the lab and in the office. Without his wise advice, invaluable help this research would not have

been possible.

I owe my deepest gratitude to my parents for their dedication and endless support and love

through the years. I gained so much drive under their watchful eyes. I also appreciate my two

sisters who always made me smile even on tough times when nothing worked well in the lab.

Lastly and most importantly, I would like to thank my life partner Mojtaba for standing beside

me through thick and thin. He was always there cheering me up on the dull days. His unwavering

love was undeniably the bedrock upon which the past twelve years of my life have been built and

I dedicate this dissertation to him.

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Table of Contents

Acknowledgments.......................................................................................................................... iv

Table of Contents .............................................................................................................................v

List of Symbols ............................................................................................................................. xii

List of Abbreviations ................................................................................................................... xiv

List of Tables .................................................................................................................................xv

List of Figures .............................................................................................................................. xvi

Chapter 1 Introduction .....................................................................................................................1

1.1 Motivation for the thesis ............................................................................................................1

1.2 Objectives of the study...............................................................................................................3

1.3 Organization of the thesis ..........................................................................................................4

Chapter 2 Literature Review ............................................................................................................5

2.1 Silicon ........................................................................................................................................5

2.1.1 Metallurgical grade silicon .....................................................................................................6

2.1.2 Semiconductor grade silicon ...................................................................................................7

2.1.3 Solar grade silicon...................................................................................................................8

2.2 SoG-Si production methods .......................................................................................................9

2.2.1 Refining of MG-Si ................................................................................................................10

2.2.1.1 Acid leaching .....................................................................................................................10

2.2.1.2 Reactive gas blowing .........................................................................................................11

2.2.1.3 Slagging .............................................................................................................................12

2.2.1.4 Solvent refining ..................................................................................................................12

2.2.1.5 Electrorefining ...................................................................................................................13

2.2.2 Silica reduction .....................................................................................................................16

vi

2.2.2.1 Carbothermal reduction of silica ........................................................................................16

2.2.2.2 Reduction by metals and compounds ................................................................................17

2.2.2.3 Electrodeposition ...............................................................................................................17

2.3 Electrowinning of silicon .........................................................................................................17

2.3.1 Electrowinning of solid Si ....................................................................................................18

2.3.1.1 Organic solvents.................................................................................................................18

2.3.1.2 Molten salts ........................................................................................................................19

2.3.1.2.1 Deposition from halide melts ......................................................................................... 19

2.3.1.2.2 Deposition from mixture of oxide without halides ........................................................ 20

2.3.1.2.3 Deposition from mixture of halides and silica ............................................................... 20

2.3.2 Electrowinning of molten Si .................................................................................................22

2.3.2.1 Above the melting temperature of Si .................................................................................22

2.3.2.2 Below the melting temperature of Si .................................................................................23

2.4 Cost considerations ..................................................................................................................24

2.5 Physicochemical properties .....................................................................................................25

2.5.1 Density ..................................................................................................................................25

2.5.1.1 Density measurement .........................................................................................................25

2.5.1.2 Density of molten cathode .................................................................................................26

2.5.1.3 Density of cryolite ..............................................................................................................26

2.5.1.4 Effect of silica on density of cryolite .................................................................................27

2.5.2 Electrical conductivity ..........................................................................................................27

2.5.2.1 Electrical conductivity measurement .................................................................................28

2.5.2.1.1 Cell Design..................................................................................................................... 28

2.5.2.1.2 Measurement techniques ................................................................................................ 29

vii

2.5.2.2 Conductivity of molten cryolite .........................................................................................31

2.5.2.3 Effect of silica on the conductivity of cryolite...................................................................32

2.5.3 Transference numbers ...........................................................................................................32

2.5.3.1 Transference number measurement ...................................................................................33

2.5.3.1.1 Faraday technique .......................................................................................................... 33

2.5.3.1.2 Stepped potential chronoamperometry .......................................................................... 34

2.5.3.2 Transference number in cryolite ........................................................................................35

2.5.4 Phase diagram .......................................................................................................................35

2.5.4.1 Determination of the phase diagram ..................................................................................35

2.5.4.1.1 Thermal analysis (TA) method ...................................................................................... 35

2.5.4.1.2 Differential thermal analysis method (DTA) ................................................................. 36

2.5.4.1.3 Quenching method ......................................................................................................... 37

2.5.4.1.4 Visual observation method ............................................................................................ 37

2.5.4.2 Systems containing Na3AlF6 ..............................................................................................38

2.5.4.2.1 Pure cryolite ................................................................................................................... 38

2.5.4.2.2 Na3AlF6- SiO2 system .................................................................................................... 38

2.5.4.2.3 Reactions between cryolite and SiO2 ............................................................................. 39

2.6 Decomposition and deposition potentials ................................................................................40

2.6.1 Measurement techniques .......................................................................................................41

2.6.1.1 E-I method .........................................................................................................................41

2.6.1.2 Cyclic voltammetry ............................................................................................................42

2.6.2 Decomposition potential of SiO2 in cryolite melts................................................................43

Chapter 3 Experimental .................................................................................................................44

3.1 Phase I: Characterization of cryolite–SiO2 melts.....................................................................44

viii

3.1.1 Melt preparation ....................................................................................................................44

3.1.2 Density measurement ............................................................................................................45

3.1.3 Conductivity measurements ..................................................................................................46

3.1.3.1 Cell design .........................................................................................................................46

3.1.3.2 Cell constant.......................................................................................................................47

3.1.3.3 Calculation of the melt resistance ......................................................................................49

3.1.4 Transference number measurement ......................................................................................50

3.1.5 Phase diagram study .............................................................................................................51

3.1.5.1 Melt Preparation.................................................................................................................51

3.1.5.2 Melting and eutectic point .................................................................................................51

3.1.5.3 Characterization of the phases ...........................................................................................52

3.2 Phase II: Determination of deposition and decomposition potential .......................................53

3.2.1 Deposition potential ..............................................................................................................53

3.2.1.1 Instrumentation ..................................................................................................................55

3.2.1.2 Electrodes ...........................................................................................................................55

3.2.1.3 Experimental procedure .....................................................................................................56

3.2.2 Decomposition potential .......................................................................................................57

3.2.2.1 Instrumentation ..................................................................................................................57

3.2.2.2 Electrodes ...........................................................................................................................58

3.3 Phase III: Electrowinning and separation of Si........................................................................59

3.3.1 Determination of anodic and cathodic current efficiencies for silicon electrowinning on

Cu-Si Alloy ....................................................................................................................................59

3.3.1.1 Master alloy preparation ....................................................................................................60

3.3.1.2 Instrumentation ..................................................................................................................61

ix

3.3.1.3 Experimental procedure .....................................................................................................63

3.3.2 Combined electrolysis and solvent refining ..........................................................................64

3.3.2.1 Extended electrowinning ...................................................................................................64

3.3.2.2 Solvent refining ..................................................................................................................65

3.3.2.3 Acid leaching .....................................................................................................................66

Chapter 4 Results and Discussion ..................................................................................................68

4.1 Characterization of cryolite–SiO2 melts ..................................................................................68

4.1.1 Density measurements ..........................................................................................................68

4.1.1.1 Effect of temperature .........................................................................................................68

4.1.1.2 Effect of silica content .......................................................................................................68

4.1.2 Conductivity measurements ..................................................................................................71

4.1.2.1 Effect of temperature .........................................................................................................71

4.1.2.2 Effect of silica concentration .............................................................................................71

4.1.3 Transport number measurements ..........................................................................................75

4.1.3.1 Effect of temperature .........................................................................................................75

4.1.3.2 Effect of SiO2 concentration ..............................................................................................76

4.1.4 Phase diagram studies ...........................................................................................................77

4.1.4.1 Phase diagram ....................................................................................................................77

4.1.4.2 Characterization of the phases ...........................................................................................79

4.2 Determination of deposition and decomposition potentials.....................................................83

4.2.1 Deposition potential ..............................................................................................................83

4.2.1.1 Cyclic voltammetry in cryolite ..........................................................................................83

4.2.1.2 Cyclic voltammetry in cryolite– SiO2 melt ........................................................................85

4.2.1.2.1 Theoretical potential ...................................................................................................... 85

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4.2.1.2.2 Experimental potential ................................................................................................... 86

4.2.1.2.2.1 Effect of scan rate on cyclic voltammogram .............................................................. 91

4.2.1.2.2.2 SEM analysis .............................................................................................................. 94

4.2.2 Decomposition potential .......................................................................................................96

4.2.2.1 Voltammetry measurements on copper .............................................................................96

4.2.2.1.1 Cyclic voltammetry in cryolite ...................................................................................... 97

4.2.2.1.2 Cyclic voltammetry in cryolite– 6 wt% SiO2 melts ....................................................... 97

4.2.2.1.2.1 Effect of scan rate ....................................................................................................... 99

4.2.2.1.2.2 Effect of Si concentration ......................................................................................... 100

4.2.2.1.2.3 SEM analysis ............................................................................................................ 101

4.2.2.2 Voltammetry measurements on copper−8wt% Si alloy...................................................101

4.3 Electrowinning and separation of Si ......................................................................................102

4.3.1 Characterization of master alloy .........................................................................................102

4.3.2 Determination of anodic and cathodic current efficiencies ................................................104

4.3.2.1 Apparent cathodic current efficiency ...............................................................................104

4.3.2.2 Actual cathodic current efficiency ...................................................................................105

4.3.2.3 Anodic current efficiency ................................................................................................109

4.3.2.4 Effect of cell design on the efficiency .............................................................................113

4.3.3 Solvent refining ...................................................................................................................116

4.3.4 Acid leaching ......................................................................................................................118

Chapter 5 Summary and Conclusion ...........................................................................................122

Chapter 6 Future Work ................................................................................................................125

References ....................................................................................................................................126

Appendix I Error analysis of density measurement .....................................................................138

xi

Appendix II Molar conductivity calculation ................................................................................139

Appendix III Elemental mapping of cryolite- 1% SiO2 quenched at 990 °C ..............................140

Appendix IV Cu-Si phase diagram ..............................................................................................141

Appendix V Publications and presentations from this research ..................................................142

xii

List of Symbols

ajSiO2 activity of SiO2 in j (j= l (liquid), s (solid))

C* bulk concentrtion

Ci concentration of charge carriers

Cl concentration of impurities in liquid

Cs concentration of impurities in solid

D diffusion coefficient

Eo standard potential

theoretical decomposition potential

Ej potential (j= d (decomposition), pol (polarization), conc (concentration), p (peak), p/2(with half the peak current))

E activation energy

F Faraday’s constant

G geometry factor (cell constant)

ij conducted current (j=e (electronic), i (ionic))

ib background current

ip peak current

K rate constant of the reaction

mj mass (j=i (initial), f (final))

M mass

n no. of involved electrons

PO2 partial pressure of O2

R gas constant

Rj resistance (j=sol (ohmic resistance), pol (polarization resistance), meas (measured))

T temperature

Tm melting temperature

t time

tj transport no. (j=e (electronic), i (ionic))

V deposition potential

v volume

W weight of deposit

z valence no.

Zj impedance (j= L (inductance), C (capacitance))

Z real part of impedance

Z imaginary part of impedance

Epp peak separation

ΔG Gibb's free energy

Hf enthalpy of fusion

m actual mass deposited

xiii

mf Faradic mass deposited

Sf entropy of fusion

ΔT change in temperature

transfer coefficient

ρ density

segregation coefficient

efficiency

i mobility of carriers

oSiO2 standard potential of SiO2

jSiO2 standard potential of SiO2 in j (j= l (liquid), s (solid))

specific conductivity

j conductivity (j=e (electronic), i (ionic), t (total))

scan rate

frequency of AC current

xiv

List of Abbreviations

CE current efficiency

CR cryolite ratio

DC direct current

EDS energy-dispersive X-ray spectroscopy

EIS electrochemical impedance spectroscopy

EPMA electron probe microanalyzer

ICP inductively coupled plasma

MG-Si metallurgical grade silicon

ppb parts per billion atoms

ppm parts per million atoms

ppt parts per trillion atoms

PV photovoltaic

SEM scanning electron microscope

SeG-Si semiconductor grade silicon

SoG-Si solar grade silicon

XRD X-ray diffraction

XRF X-ray fluorescence

xv

List of Tables

Table 2-1. The acceptable level of impurity in MG-Si and SoG-Si [3]. ......................................... 8

Table 2-2. Effect of temperature on the electrical conductivity of cryolite. ................................. 31

Table 4-1. Equation of temperature dependency of density for different silica content. .............. 70

Table 4-2. Activation energy of electrical conductivity for different mixtures ............................ 74

Table 4-3. Ionic and electronic transport number for cryolite melt at different temperatures. ..... 75

Table 4-4. EPMA results of white phase of 6 wt% SiO2 sample quenched from 990 and 970°C.

....................................................................................................................................................... 82

Table 4-5. Effect of SiO2 concentration on theoretical deposition potential of Si. ...................... 85

Table 4-6. Anodic and cathodic potentials in 3 and 5wt% solutions at 20 and 50 mV.s-1

. .......... 91

Table 4-7. Cyclic voltammetry data of silicon reduction on graphite in Na3AlF6– 5% SiO2. ...... 92

Table 4-8. Diagnostic criteria for reversibility of B1 peak [58]. ................................................... 93

Table 4-9. Electrolysis with a Cu-Si cathode from cryolite-6 wt% SiO2, T= 1040 °C. ............. 104

Table 4-10. Mass balance calculation for the Cu mass loss in each experiment. ....................... 109

Table 4-11. Measured and calculated C consumption. ............................................................... 112

Table 4-12. Extended electrowinning experiments with different crucibles. ............................. 113

Table 4-13. EDS analysis of the phases presenting in Figure 4-55-c and d. .............................. 117

Table 4-14. EDS analysis of the phases presenting in Figure 4-57-b. ........................................ 119

Table 4-15. Concentration of impurities in the alloy and final Si in ppmw ............................... 121

Table 5-1. Energy consumption and carbon footprint of different Si production methods. ....... 124

xvi

List of Figures

Figure 1-1. Cost of electricity generated from different energy sources [1]. ................................. 1

Figure 1-2. Research plan. .............................................................................................................. 4

Figure 2-1. Relationship between the cost and the purity of various types of Si [15]. ................... 6

Figure 2-2. Schematic of the process for production of MG-Si [43]. ............................................. 6

Figure 2-3. Schematic of the Siemens process [44]. ....................................................................... 7

Figure 2-4- Effect of impurities on the performance of the p-type silicon. 1) semiconductor, 2)

solar and 3) metallurgical grade silicon[48]. .................................................................................. 9

Figure 2-5- Standard free energy of formation of impurity’s oxides [55]. ................................... 11

Figure 2-6. Comparison of segregation coefficient and electronegativity of impurities in MG-Si

[15, 75, 76] .................................................................................................................................... 14

Figure 2-7. Suggested cell design for dual refining of Si [35]. ..................................................... 15

Figure 2-8.Three layer technique for electrorefining of super pure Al [86]. ................................ 16

Figure 2-9. The cell design proposed for Si production [118]. ..................................................... 21

Figure 2-10. Schematic cell design for deposition of silicon above the melting temperature [13].

....................................................................................................................................................... 23

Figure 2-11. Schematic electrode designs for measuring electrical conductivity of liquids [143].

....................................................................................................................................................... 29

Figure 2-12.a) A typical Nyquist plot of a Randles cell, b) equivalent circuit of the Randles . ... 30

Figure 2-13. Schematic current response to square wave potential [157]. ................................... 34

Figure 2-14. a) Phase diagram of a hypothetical A-B system, b) The cooling curve corresponding

to phase diagram at different compositions [162]. ........................................................................ 36

Figure 2-15. Differential thermal analysis method a) classical apparatus b) heat flux c) DTA

curve for an endothermic reaction [163]. ...................................................................................... 37

Figure 2-16. Binary phase diagram of cryolite-SiO2 system at 1 atm. trd: tridymite, qz: quartz

[178]. ............................................................................................................................................. 39

Figure 2-17. a) Experimental setup for decomposition potential measurement, b) schematic of

the response current-voltage diagram [186]. ................................................................................ 42

file:///C:/Users/Mini/Dropbox/shared%20thesis/edited/final%20version.docx%23_Toc399249232

xvii

Figure 2-18. Potential- time wave in cyclic voltammetry, b) a typical cyclic voltammogram. .... 43

Figure 3-1- Schematic of the experimental setup for density measurement ................................. 45

Figure 3-2. a) Dimensions and arrangement of the electrode tips, b) electrode immersion depth

adjustment apparatus. .................................................................................................................... 47

Figure 3-3. Schematic of the experimental setup for measuring charge transport properties. ..... 48

Figure 3-4. Detection of the melt surface by chronomaperometry at 87 mV. .............................. 48

Figure 3-5. Nyquist diagram for different immersion depths in a 0.01D KCl solution. ............... 49

Figure 3-6. Cell constant as a function of immersion depth recorded in 0.1 and 0.01D KCl

standard solutions.......................................................................................................................... 49

Figure 3-7. The applied polarization wave in cryolite1 wt% SiO2 melt at 1000 C. ................. 50

Figure 3-8. The current response recorded during the polarization in cryolite1 wt% SiO2 melt at

1000 C. ........................................................................................................................................ 51

Figure 3-9. Schematic cell design used for deposition potential measurements. ......................... 54

Figure 3-10. a) the complete experimental setup, b) the graphite radiation shields. .................... 54

Figure 3-11. Open circuit potential of the cell containing cryolite as electrolyte. ........................ 56

Figure 3-12. Schematic of the experimental setup for decomposition potential measurements. . 58

Figure 3-13. Schematic of the experimental setup for preparing the master alloy. ...................... 61

Figure 3-14. The master alloy quenched from 1500 C. .............................................................. 61

Figure 3-15. The Al cap used for accommodation of the electrodes and sealing the furnace tube.

....................................................................................................................................................... 62

Figure 3-16. Schematic of the experimental setup for current efficiency measurements. ............ 63

Figure 3-17. Schematic drawing of the experimental setup for solvent refining. ......................... 65

Figure 3-18. Heating and cooling cycle utilized for solvent refining. .......................................... 66

Figure 4-1. Density of cryolite as a function of temperature; this study, Edwards [91]....... 68

Figure 4-2. Density of cryolite- silica for different concentrations of silica at 1000 °C. ............. 69

Figure 4-3. Effect of silica content on a) partial molar volume of cryolite, b) partial molar

volume silica, c) molar volume of solution at 1020 °C. ............................................................... 70

xviii

Figure 4-4. Effect of temperature on conductivity of cryolite; present study; Kalass [146];

Batashev [146]; Beljajew [195]; Vayna [194]; Edward [91]; Abramov [146]; Yim

[150]; Bajcsy [148]. .................................................................................................................. 71

Figure 4-5. Conductivity of Na3AlF6-SiO2 mixtures at 1000°C. present study; Grjotheim

[14]; Belyaev [149]. ................................................................................................................. 72

Figure 4-6. Comparison between the effect of SiO2 and Al2O3 [149] on conductivity at 1000°C.

....................................................................................................................................................... 73

Figure 4-7. Arrhenius plot of electrical conductivity for 0 and 3 wt% silica mixtures. ............... 74

Figure 4-8. Effect of silica concentration and temperature on molar conductance of mixture. ... 75

Figure 4-9. Effect of temperature on electronic, ionic and total conductivity of cryolite melts. .. 76

Figure 4-10. Effect of SiO2 concentration on electronic, ionic and total conductivities of

cryolite–SiO2 melt at 1020 °C ...................................................................................................... 77

Figure 4-11. Typical cooling curve of cryolite melt with larger magnification of the freezing

section. .......................................................................................................................................... 78

Figure 4-12. Liquidus and eutectic line for cryolite–SiO2 system from 0 to 6 wt% SiO2. (dashed

line shows the expected trend) ...................................................................................................... 79

Figure 4-13. SEM images of quenched samples. a) quenched liquid, 1 wt% SiO2 b) cryolite +

quenched liquid, 1 wt% SiO2 c) sodium aluminosilicate + quenched liquid, 6 wt% SiO2 d)

cryolite and sodium aluminosilicate particles after solidification. ............................................... 80

Figure 4-14. EDX spectrum of a cryolite1 wt% SiO2 sample. a) area A, b) area B in Figure

4-13- b. .......................................................................................................................................... 81

Figure 4-15. EDX spectrum of a cryolite6 wt% SiO2 sample. a) area A, b) area B in Figure

4-13-c. ........................................................................................................................................... 81

Figure 4-16. XRD pattern of the sample containing 1 wt% quenched from 970°C. .................... 81

Figure 4-17. XRD pattern of the sample containing 6 wt% quenched from 970°C. .................... 82

Figure 4-18. Cyclic voltammetry in molten SiO2–free cryolite; scan rate 50 mV.s-1. ................ 84

Figure 4-19. Cyclic voltammetry on graphite electrode at 1040°C; scan rate 50 mV.s-1. ........... 86

Figure 4-20. Cyclic voltammetry in cryolite– 3 % SiO2 melt at 1040 °C; scan rate 50 mV.s-1. . 87

Figure 4-21. Cyclic volatmmogram showing the effect of SiO2 concentration on the deposition

potential of Si; scan rate 50 mV.s-1. ............................................................................................. 88

xix

Figure 4-22. Volatmmograms showing the effect of SiO2 concentration on the deposition

potential of Si; scan rate 20 mV.s-1 .............................................................................................. 89

Figure 4-23. Schematic of the expected concentration profiles of Si4+ and Si2+ ions at the

electrode surface for 3 and 5wt% SiO2. ....................................................................................... 89

Figure 4-24. Cyclic voltammogram in cryolite–5 wt% SiO2 melt; scan rates 5, 10, 20, 50 and

100 mV.s-1. ................................................................................................................................... 92

Figure 4-25. Plot of cathodic peak current versus square root of sweep rate. (The background

current was subtracted from the peak current for the corrected data). .......................................... 94

Figure 4-26. SEM image of electrode tip. ..................................................................................... 95

Figure 4-27. EDS analysis of matrix (A). ..................................................................................... 95

Figure 4-28. EDS analysis of the smooth, bright phase (B). ........................................................ 96

Figure 4-29. EDS analysis of the dark phase (C).......................................................................... 96

Figure 4-30. a)Voltammogram in molten SiO2–free cryolite; 20 mV.s-1, b) Voltammogram in

molten SiO2–free cryolite; 50 mV.s-1. ......................................................................................... 97

Figure 4-31. Voltammogram for deposition of Si on copper in molten cryolite– 6 wt% SiO2; 10

mV.s-1. .......................................................................................................................................... 98

Figure 4-32. Effect of scan rate on the recorded voltammogram in cryolite–6 wt% SiO2 melt,

scan rates 20, 50, 80, 100 mV.s-1. ................................................................................................ 99

Figure 4-33. Recorded voltammogram in cryolite–6 wt% SiO2 melts at scan rates of 5 and 10

mV.s-1. ........................................................................................................................................ 100

Figure 4-34. Cyclic voltammogram recorded in cryolite–6 wt% SiO2 after two deposition steps

of 20 min; a) scan rate 20 mV.s-1, b) scan rate 50 mV.s-1 ........................................................ 101

Figure 4-35. Elemental mapping of cathode after 40 min electrolysis at -1.2 V. ....................... 101

Figure 4-36. Voltammogram for Cu-8 wt% Si in molten cryolite–6 wt% SiO2; 50 mV.s-1. .... 102

Figure 4-37. SEM image and the elemental mapping of the master alloy. ................................. 103

Figure 4-38. EDS analysis of the master alloy sample. .............................................................. 103

Figure 4-39. The XRD analysis of Cu-Si master alloy. .............................................................. 103

Figure 4-40. Dependence of the apparent cathodic CE on the current. ...................................... 105

Figure 4-41. Cathodic CE calculated from the mass change and mass balance. ........................ 106

xx

Figure 4-42. Solidified electrolytes after electrolysis under 1.1 A. ............................................ 106

Figure 4-43. Pictures of anode and cathode after 5 h electrolysis under different currents. ...... 106

Figure 4-44. Distribution of Si across the cathode after electrolysis under a) 1.5 A, b) 1.69 A. 107

Figure 4-45. Depletion of Si in a thin surface layer (bright area) on top section of the cathode

after electrolysis under 1.1 A. ..................................................................................................... 107

Figure 4-46. XRD pattern of solidified electrolyte after electrolysis with 1.5A. ....................... 108

Figure 4-47. Anodic CE calculated from gas analysis and actual CE calculated from EDS results.

..................................................................................................................................................... 110

Figure 4-48. Mass of Si reported to the alloy cathode and Si reduced in the cell ...................... 110

Figure 4-49. Instantaneous anodic current density. .................................................................... 111

Figure 4-50. Relationship between the cathodic current efficiency and CO2 concentration. .... 112

Figure 4-51. Anode and cathode after electrolysis in experiment # 4. ....................................... 114

Figure 4-52. Crucibles after Experiments a) # 4, b) # 5. ............................................................ 114

Figure 4-53. SEM observation and EDS analysis of the cathode from experiment # 4. ............ 115

Figure 4-54. SEM observation and EDS analysis of the cathode from experiment # 5. ............ 115

Figure 4-55. SEM images of the cathode a, b) after extended electrowinning and c, d) after

solvent refining. .......................................................................................................................... 117

Figure 4-56. a) Micrograph of solvent refined alloy, b) EDS analysis across the line shown in (a).

..................................................................................................................................................... 118

Figure 4-57. SEM image of the Si phase after acid leaching; a) X100, b) X998. ...................... 119

Figure 4-58. XRD pattern of the Si phase after acid leaching. ................................................... 120

Figure 5-1. Overall process flow chart. ...................................................................................... 124

1

Chapter 1 Introduction

1.1 Motivation for the thesis

The global increase in energy consumption, limited reserves of fossil fuels, and the

environmental pollutions associated with them are the drivers for a drastic change in our fuel–

based energy towards alternative and renewable sources such as solar, wind, and biomass. Solar

energy in the form of heat or photovoltaic electricity is the most abundant renewable energy

form. Although it is not yet clear what portion of our energy will eventually be supplied by solar

power, it is well known that its potential can far exceed the total energy demand of the globe.

This potential together with the need for green energy sources have provided a unique

opportunity for the photovoltaic industry to grow at a rate that is not imaginable for many other

industries. Despite the high potential and well established technology, solar energy is not a main

contributor to today’s energy basket. One of the current hurdles facing the widespread use of

solar energy is the high cost. Figure 1-1 compares the cost of electricity from different sources in

2013, confirming that solar power is not competitive with other sources.

Figure 1-1. Cost of electricity generated from different energy sources [1].

Today, over 90% of the photovoltaic materials in commercial products are silicon based [2]. Si

being the dominant photovoltaic material, accounts for 25–50% of the cost of solar arrays [3]. To

reduce the cost of solar cells, it is therefore necessary either to decrease the cost of Si or shrink

the Si consumption in the fabrication process. Traditionally, the majority of solar silicon was

2

from overcapacity, scraps, and rejects of the semiconductor industry, costing around $80/kg.

Since 2003, demand for polysilicon has exceeded the supply due to the skyrocketing growth in

the PV industry [4, 5]. As the purity requirement of silicon for solar application can be less than

that of the semiconductor grade silicon (6N compared to 9N [3]), a new type of silicon known as

solar grade silicon (SoG–Si) was introduced, for which the target price is $10–15 /kg. The price

has in recent months been around $20–25/kg, down from over $200/kg in 2008. This price is

believed to be unstable as it is dictated by the oversupply.

The existing method of SoG-Si production involves a combination of crude Si–making process,

which produces metallurgical grade Si (MG–Si), and a refining process known as Siemens,

through which MG–Si is upgraded to SoG–Si. This technique consumes 120–200 kWh/kg of

energy and produces approximately 90 tonnes of CO2 per every tonnes of Si [3] as well as toxic

gases. About 95% of required SoG–Si by the PV industry is produced by this method. However,

due to the large operating cost and energy consumption, and low productivity, an alternative

process is highly sought after. The feasibility of developing such a process has been studied by

various researchers in the past two decades. The investigations have generally followed one of

the following two approaches: direct production of SoG-Si from ultrapure feedstock or refining

of MG–Si. Reduction by carbon [6, 7] or metals and compounds [8, 9] as well as

electrodeposition [10-14] are the methods which directly produce high purity Si. The refining

techniques such as acid leaching [15-17], reactive gas blowing [7, 18], slagging [19, 20], and

solvent refining [21-25], purify MG–Si into SoG-Si. Up to now, these achievements are limited

to laboratory and pilot scale and not fully implemented in commercial scale.

Among the studied methods, electrochemical approach is considered to be a promising route for

silicon production due to the analogy of Si with Al that is produced by electrodeposition in the

well established Hall-Heroult process. A detailed review of previous works [13, 26] reveals the

great potential of the method in generating SoG–Si with low cost and energy consumption. For

example, the production cost of Al is ~ 2$/kg, with the major operating cost being the pre–

processing involved in isolating alumina from its ore, while the widespread availability of

inexpensive high purity silica ($0.02 /kg [27]) can contribute to generating low cost Si. Although

this method has been shown to be capable of delivering a very high purity Si (99.999%) [11]

which can be turned into SoG–Si after one step of melting and directional solidification [28], it

3

has not been commercialized primarily due to one major problem: the high melting point of Si

(1412°C).

Deposition of Si below the melting point (in the solid state) is slow [11] and results in the

formation of powdery Si dispersed in the melt which is hard to recover afterward [29-31]. On the

other hand, electrodeposition above the melting point enhances the production rate [31] but still

suffers from high working temperature and low current efficiency. To overcome these

limitations, electrodeposition of different Si alloys, molten at 1000°C, has been studied. Initial

investigations on Cu–Si [12] and Al–Si [14, 32, 33] have shown that this approach is promising

to produce Si with acceptable range of impurities (P, B) in the alloy. However, separation of Si

from the alloy has remained a challenge. Electrorefining of the molten alloy to recover Si from

the alloy has been attempted [11, 12, 34-37] but the problem of solid deposition in cahode

remains a hurdle.

Improving the efficiency of the electrolytic process together with overcoming the separation

challenge were the primary motivations for this study. The proposed technique combines

electrowinning in a molten alloy to improve the efficiency of deposition and solvent refining to

separate the Si dendrite from the alloy matrix.

1.2 Objectives of the study

The overall objective of this research was to develop an inexpensive and sustainable process for

producing high purity silicon. The detailed objectives are:

1- To characterize the cryolite-SiO2 system as a possible electrolyte by measuring its

physicochemical and charge transport properties.

2- To deposit Si from cryolite-SiO2 melt into a molten cathode and study the effect of applied

potential and current on the purity of the final product.

3- To measure the deposition and the decomposition potential on the surface of candidate

cathodes: graphite, copper and copper-silicon alloy.

4- To determine the current efficiency of the process and the electrodeposition rate.

4

1.3 Organization of the thesis

The thesis has been structured into 6 Chapters. Chapter 1 provides an introduction with respect to

background, motivations behind the work, and objectives. Chapter 2 summarizes the knowledge

in the literature pertaining to the objectives of the research. Chapter 3 presents the details of

experimental work including the materials used, equipment, and the procedures for conducting

the experiments and assessing the results. Chapter 4 provides the results of this research and an

explanation of the findings. Chapter 5 summarizes the main achievements of each phase of this

study and provides the conclusions. Finally, Chapter 6 presents the author’s suggestions for

future studies that will complement this work.

The details of calculation of error analysis for density measurement are shown in Appendix I.

Appendix II shows how the molar conductivity values were calculated in this thesis. Appendix

III demonstrates the elemental mapping of cryolite−SiO2 melt after quenching. Appendix IV

contains the Cu-Si binary phase diagram and a list of publications from this Ph.D. work is

provided in Appendix V.

A flowchart presented in Figure 1-2 provides an overview of the scope of this research.

Proposed Method:

Deposition of Si below its melting temperature

through alloying and separation of Si from

alloy matrix through solvent refining

Main Objective:

Developing an inexpensive and environmental

friendly process for producing SoG-Si

Research Plan:

Three different phases

Charactrization of cryolite- SiO2 melt

1- Density measurment

2- Electrical conductivity measurment

3- Tranport number measurement

4- Phase equibliria development

Determination of the required potential

1- Deposition potential on graphite

2- Decomposition potential on Cu

3- Decomposition potential on Cu-Si

Electrowinning and separation of Si

1- Anodic and cathodic current efficiency measurement

2- Extended electrowinning

3- Solvent refining

4- Reportig final purity

Figure 1-2. Research plan.

5

Chapter 2 Literature Review

2.1 Silicon

Numerous photovoltaic materials have been investigated as possible candidates for high

efficiency and low cost conversion of Sun’s energy to electricity, including silicon, cadmium

telluride, copper indium deselenide and gallium arsenide. However, Si remains as the most

widely used photovoltaic material due to, abundance of quartz as its source material, proven

effectiveness of Si in solar cells, established low-risk technologies around Si, and minimum

health and environmental issues for its use or end-of-life disposal/recycling. [38].

Silicon is the second element in group IVA of the periodic table of elements. It has a band gap of

1.12 eV at 25°C which is very close to the ideal band gap of 1.4 eV for harvesting sun’s energy

[2]. Silicon is the second most available element in mass after oxygen [39]. In nature, it exists in

the form of oxides and silicates, the main constituents of the Earth’s crust. Silicon reaction with

oxygen is very fast and forms a thin film of silica less than 100 Å that protects the bulk of silicon

from further oxidation. Silicon has a semi polymeric behavior that can cause the formation of Si-

Si, - (SiH2)p- or -(SiF2)p- chains in the structure. The characteristics of these chains are very

similar to hydrocarbons and fluorocarbons chains.

Silicon has a wide range of applications in various forms such as semiconductor in

microelectronic and PV devices, silicone-based polymers, ceramics (oxide, nitride, and carbide

of Si), alloys, and chemicals.

There are three main categories of silicon metal available in the market depending on their level

of purity: 1) Metallurgical grade silicon (MG–Si) with a purity of 98-99%, 2) Semiconductor

grade silicon (SeG–Si), with the purity in the range of ppb− ppt, and 3) Solar grade silicon,

SoG–Si, with the impurity content in the range of ppm. Figure 2-1 shows the price of these Si

types as a function of their impurity content. Due to the specific purity requirement for SoG-Si,

there is an opportunity to produce this material at a cost of $10- 15/kg, which enables widespread

use of solar power by reaching grid parity with the traditional electricity sources. Since 2008, the

6

market price of SoG-Si has been fluctuating between $20 and $200/kg [40] depending on the

supply-and-demand situation.

Figure 2-1. Relationship between the cost and the purity of various types of Si [15].

2.1.1 Metallurgical grade silicon

Metallurgical grade silicon also known as silicon metal is produced in submerged arc furnaces.

The furnace is fed with quartz and carbon, where the carbothermic reduction of silica at 2000 C

generates liquid Si. The metal is tapped from the bottom of the furnace (Figure 2-2). The main

impurities in this type of Si are Fe, Al, Ca, Ti, and C. Approximately, one million tons of MG-Si

is produced each year that is used in making Al and steel alloys, deoxidation of steel, synthesis

of silicon, and production of photovoltaic materials for the solar and electronic industries [15, 41,

42].

Figure 2-2. Schematic of the process for production of MG-Si [43].

7

2.1.2 Semiconductor grade silicon

To achieve the required semiconductor properties, the impurity level should be as low as 1 ppb

in this type of Si. SeG-Si is produced by purification of MG-Si through a vapor deposition

process known as Siemens. MG-Si is first reacted with hydrogen chloride gas at 300 C to form

chlorosilane or trichlorosilane:

Si (impure) + 4HCl (g) → SiCl4 (g) + 2H2(g) (2-1)

Si (impure) + 4HCl (g) → SiHCl3 (g) + H2(g)+1/2 Cl2(g) (2-2)

The impurities are separated by fractional distillation based on the difference in their boiling

temperature. The vaporized chlorosilane is then decomposed to Si and HCl in the presence of

hydrogen. The high purity Si grows on the surface of the seed, which is a heated Si rod at

1150°C according to Reaction ( 2-3).

SiHCl3 (pure)+ H2 (g) → Si (pure) + 3HCl (g) (2-3)

A schematic diagram of the Siemens process is shown in Figure 2-3. This technique is very

energy intensive (120-200 kWh/kg) and produces approximately 90 tonnes of CO2 per every

tonnes of Si [3]. Currently, the bulk of Si for solar industry is produced by this method.

Figure 2-3. Schematic of the Siemens process [44].

8

2.1.3 Solar grade silicon

Silicon used in solar cells is typically cut from the ingot of SoG-Si. A method with lower

production cost and dedicated to meet the more relaxed purity requirement of SoG-Si is desired.

The level of impurities in MG-Si is compared with SoG–Si in Table 2-1. The requisite level of

purity can be achieved by the conventional metallurgical purification methods for most of the

impurities except P and B.

Table 2-1. The acceptable level of impurity in MG-Si and SoG-Si [3].

The effect of each impurity on the efficiency of p-type silicon is shown in Figure 2-4, based on

simulation results. The combined effect of several impurities is complex and not simply

cumulative. Many researchers [45-47] have tried to draw a guideline for SoG–Si specifications,

although a consensus has not been reached to date.

In the past two decades, many researches have been conducted to produce SoG-Si in a low-cost

and environmentally friendly process. These methods are either metallurgical or chemical

processes and will be discussed in the next section.

impurity MG-Si

98-99% pure (ppm)

allowed concentration in raw

SoG- Si (ppm)

Al 1000-4000

9

Figure 2-4- Effect of impurities on the performance of the p-type silicon. 1) semiconductor,

2) solar and 3) metallurgical grade silicon[48].

2.2 SoG-Si production methods

Generation of SoG-Si involves multiple refining processes, although repeapted directional

solidification could deliver Si as pure as desired from MG-Si, but at a large cost. In directional

solidification, solid Si is grown from liquid to leave the impurities behind. The distribution of

impurities between liquid and solid is known as segregation coefficient, , which is defined by

= Cs/CL (2-4)

where Cs and CL are the equilibrium concentrations of impurities in solid and liquid,

respectively. According to Equation ( 2-4), the lower the , the higher the extent of impurity

removal from solid.

In practice, directional solidification is reazlised by crystal growth or zone refining. The former

method, known commercially as Czogralski technique, involves pulling of a Si crystal from melt.

This is achieved by mounting a seed of Si single crystal on a rod, dipping the seed in Si, and

gradually pulling the rod away so that a large, single-crystal forms from the melt [49]. In zone

refining, a Si rod is melted from one end using a heating element, then the heating zone moves

along the rod. The melted section of the rod resolidifies, pushing the impurities into the liquid

front. The rate of Si production in both of these techniques is very low and in zone refining, the

rod size is limited to 10 cm in diameter. While most impurities can be removed to acceptable

levels within one or a few passes, those with high segregation coefficient require numerous

10

repeats, hence prohibitively large refining cost. These elements include P and B with segregation

coefficients of 0.35 and 0.8, respectively [50]. Their removal is preferentially carried out by

other more effective techniques such as Siemens, while other methods with lower cost and

higher productivity are desired, as explained earlier.

The researches in the recent years have been focused on low-cost purification of Si through two

different approaches. The first involves the pyrometallurgical refining of MG-Si, the second uses

high purity feedstock (such as carbon black, quartz or silicon fluoride compounds) and clean

processing to deliver silicon with low impurity content. The following section will review the

various techniques in each category.

2.2.1 Refining of MG-Si

MG-Si is an inexpensive source of silicon with an average price of $1-2 /kg and the purity about

95-99% while the required purity for solar application should be 99.99999%. The methods

presented in the literature for the upgrading MG-Si are discussed here briefly.

2.2.1.1 Acid leaching

Acid leaching is a primary treatment for upgrading MG-Si due to its low cost and simplicity. The

principle of acid leaching is based on low segregation coefficient of impurities in silicon. As the

silicon metal is solidified, impurities with small segregation coefficient are rejected to the

solid/liquid interface and eventually end up in the grain boundaries or at the interstitial positions

in polycrystalline silicon. The metal is ground to small particles (50-70 m) to expose the grain

boundaries to an acid solution that dissolves the impurity-rich fractions.

In a few studies on acid leaching process [16, 17, 30, 51-53], the effect of temperature, particle

size and acid composition were investigated. HCl, HF, H2SO4, HNO3 and their combinations

have been used for selective dissolution of impurities. Leaching MG-Si with aqua regia for long

period of time has shown the most promising results [52]. These investigations were successful

in removing some metallic impurities such as iron, aluminum, calcium and magnesium.

However, it was believed that this process is less effective for interstitial and substitutional

impurities such as boron, phosphorous, carbon and oxygen [16, 17, 51, 52]. Therefore, this

process is not solely sufficient to upgrade MG-Si to SoG-Si and should be followed by a

pyrometallurgical process to provide SoG-Si with acceptable range of B and P. Addition of Ca

11

to Si before acid treatment improves P removal, down to 5 ppm in concentration [53]. The

maximum achieved purity by acid leaching is 99.99% [54].

2.2.1.2 Reactive gas blowing

Reactive gas blowing is one of the main pyrometallurgical techniques for upgrading MG-Si. In

this process a reactive gas, which is diluted by an inert gas is bubbled through molten MG-Si or

acid leached MG-Si. The gas reacts with the dissolved impurities and forms volatile compounds

which are removed from the surface by the gas flow [3, 15].

Figure 2-5- Standardfreeenergyofformationofimpurity’soxides[55].

The type of gas is important in the success of this process. Cl2, O2, SiCl4, wet H2 and CO2 have

been used [3, 15, 19, 56]. Cl2 is widely used for purification because it selectively reacts with

impurities and forms volatile chloride compound. Among different compounds, chlorides of Al,

Mg, Mn and B have very low boiling points; therefore, it is easy to transport and remove them

from silicon [56]. The elements more active than silicon can also be removed as oxides, using

oxygen. Figure 2-5 shows the free energy of formation for different oxides. According to this

figure, Ca, Mg, Al, and Ti oxides have lower Gibbs energy of formation, thus can be removed

from Si without substantial Si loss [15]. However, Oxygen is not successful in removing P and B

12

from the silicon. The free energy of formation of these two impurity oxides are higher than that

of silica, also their concentration is several orders of magnitude smaller than Si, thus significant

Si oxidation occurs before their effective removal. Under oxidizing environment, different boron

oxides form, among which BO is the most volatile compound [19, 57]. Lynch [58] and

Nordstrand [59] showed that H2-H2O environment can effectively remove B by formation of

HBO volatile compounds. CO, CO2 and SiF4 have been used to remove the less active elements.

CO2 is a suitable gas for P and C removal while SiF4 reacts with B, Cu, Ca and Mn and forms

fluorides which are transported to the gas [3].

2.2.1.3 Slagging

This method employs molten slag containing oxides (and sometimes chloride or fluoride) of Ca,

Si, Mg, Al, Na, Ba, etc. to absorb impurities from the metal. The impurities get oxidized and

dissolve in the slag phase. The slag should have specific characteristics for optimal impurity pick

up, including: 1) the impurity oxide should have high solubility in slag, 2) the slag should not

cause severe oxidation and loss of silicon and 3) the slag and silicon should be easily separatable

(i.e. different density). The amount of slag in metal refining is typically 1-5 wt% of silicon [15].

This method reduces the concentration of some elements such as Al, Mn, V and Ti by about one

order of magnitude but is not successful in the case of B and P since these elements are more

noble than Si [60]. CaO-SiO2 based slags are most common for boron removal [61-63]. Johnston

and Barati found a linear relationship between P removal and CaO:SiO2 ratio [20]. They have

reported that due to the relatively small distribution of B and P between slag and metal, this

method requires large amounts of slag, making it unattractive from the operational and the cost

points of view. A more recent study by the same group on slag treatment of Cu-Si alloy using

Na2O-containing slags has shown effective B removal [64], and the success has been attributed

to the effect of Cu on activity coefficient of B. Phosphorus removal by slagging is however not

as effective as B removal [19].

2.2.1.4 Solvent refining

Solvent refining is a controlled solidification process, during which an alloy of a metallic agent

with silicon traps the impurities and leaves pure silicon dendrites behind. The process involves

melting of crude Si with the impurity getter alloying element, slow cooling of the alloy to allow

13

precipitation of pure Si crystals, and separation of the Si from the alloy. Depending on the

physical and chemical properties of the alloy, the Si product can be recovered by one or a

combination of liquid filtration, leaching or electrochemical dissolution of the solidified alloy,

physical separation of the two phases based on their different density, and electromagnetically

induced separation [19].

Different metals [65] have been used as the alloying elements such as Al [23-25, 66-68], Fe [21,

22], Ca [65], Cu [30, 69-71], Mg [30], Zn [42], Sn [42], Sb [42], and Ni [72]. The metal should

be inexpensive and readily dissolve silicon in liquid state while posses small solubility in solid

state and higher affinity for impurities than silicon.

Morita et al. [23-25, 73] performed a comprehensive study on Si-Al system. According to their

findings, this method is more efficient in removing metallic impurities namely Fe, Ti, Cu, Mn

and Ni rather than P and B. They also studied the effect of Ti and Ca addition to Al-Si system,

and found that Ti improves B removal [66] and Ca is effective for P removal [53]. Solvent

refining of Cu-Si followed by acid leaching was studied by Mukherejee [30]. According to this

study, Mn, Mg, Cr, Ni, Al and Ca were partially removed by solvent refining and further

treatment by aqua regia removed more Cr and Ca. A second leach in HF removed B, Fe and Mn

substantially. Recently Mitrašinović and Utigard [70, 71] conducted solvent refining of Si using

Cu. Based on their results, this technique is effective in removing the metallic impurities namely,

Au, Co, Cr, Cu, Fe, Mn, Mo, Ni, Ti, and Zr, but not B and P. Iron was the other solvent that was

used by Esfahani and Barati [21, 74]. Their findings also shows the effectiveness of iron as a

getter in removing over 90% of almost all impurity elements but B and P. Khajavi [22] also

studied the thermodynamics of phosphorous distribution between Si and Fe-Si alloy and found

that higher P removal is expected at higher temperature, which means quenching from

temperatures above the eutectic temperature is favorable for P removal.

2.2.1.5 Electrorefining

Based on the electrorefining principal, if Si is placed in the anode of an electrorefining cell,

elements with more positive electronegativity than Si will not anodically dissolve, while those

with electronegativity lower than Si will anodically dissolve but will not be cathodically

deposited. Figure 2-6 maps various elements based on their segregation coefficient in Si and

electronegativity. It indicates that B and P, while particularly unresponsive to directional

14

solidification, can potentially be removed by electrorefining. These will remain in the anode

during electrorefining, while other elements such as V, Ti, and Mg that migrate to the electrolyte

from the anode will not deposit at the cathode.

Figure 2-6.Comparisonofsegregationcoefficientandelectronegativityofimpuritiesin

MG-Si [15, 75, 76]

The idea of electrorefining of Si was first introduced by Monnier [12, 35] in 1964 when he

investigated the possibility of electrorefining of either solid silicon or molten alloy (e.g. Cu-Si

alloy). He proposed a dual cell design for this purpose, as seen in Figure 2-7 [12]. In the first

cell, silica is reduced to Si by electrolysis of a cryolite−silica solution and Si is deposited into a

liquid Si-Cu alloy. In the second cell, the Si-Cu plays the role of an anode; Si migrates from the

anode and the refined solid Si deposits onto a graphite cathode. The method has been reported to

be capable of delivering 99.99% pure Si with around 75% current efficiency.

Later, Olson and Carleton [77] extended the idea of electrorefining of Cu-Si alloy to lower

working temperature (750 C). This alloy was then refined in a KF-LiF-K2SiF6 melt. This

technique was successful to deliver 99.999% pure silicon; this achievement is remarkable for P

and B removal: up to 80 and 96% respectively.

15

Figure 2-7. Suggested cell design for dual refining of Si [35].

In mid 1980s, Sharma and Mukherjee [78] investigated the feasibility of MG-Si purification in

molten KF−LiF−K2SiF6 electrolyte, avoiding the additional alloying step. This study showed

effective removal of Al, Fe and Ca as well as B (91% removal). Recently, Zou et al. [79] also

tried direct electrorefining of MG-Si in KCl-NaF solution at 800 C. This technique was not

effective in B removal (80%) compared to other impurities; still the total purity is higher than

that in Sharma’s study.

Before 2007, all of researches on electrorefining of MG-Si had been carried out in fluoride based

electrolytes. However, there are several industrial complications dealing with fluoride melts such

as toxic volatiles and corrosive environment of the melt. Kongstein et al. [80] and Cai et al. [81]

used a chloride based melt containing (CaCl2−NaCl−CaO), which is less corrosive and more

stable at the working temperature. Cai et al. [81] conducted the electrorefining of Cu-MG Si

alloy in this electrolyte at 850-950 C. Promising B and P removals (99 and 96% respectively)

were achieved, but Cu and Ca were retained in the product.

In 2010, a new electrorefining technique, called three layered refining process, was proposed for

upgrading MG-Si. This process is commercially used for production of super-pure aluminium

[82, 83]. Electrochemical cell in this process consists of three different molten layers. The

bottom layer is mostly anode, which is an alloy of impure metal with a noble metal such as Cu.

The second layer is the electrolyte, which is often a molten salt, and the last layer is the deposited

metal with the lowest density, which floats on top of the cell (Figure 2-8). This principal was

16

used by Oslen et al.[84] and Lai et al. [85]. The method has been reported to be capable of

removing most of the metallic impurities such as V, Ni, Mn, P, Fe, Ti, Al, Cu but is not

successful in B removal.

Figure 2-8.Three layer technique for electrorefining of super pure Al [86].

Despite these achievements in production of crystalline silicon, manufacturing of solar panels is

accompanied with significant silicon loss during cutting and wafering. To overcome this

problem, several studies have been conducted to electrodeposit Si on an appropriate substance,

namely monocrystalline Si [87, 88], Ag [88] and finally graphite [28]. Osen et al. [89]

investigated the electrodeposition of Si from KF-Li-K2SiF6 on different substances such as Ag,

Si, W, and glassy carbon. They discussed that deposition of Si is diffusion controlled and takes

place in two steps: Si (IV) Si (II) Si. The best results were achieved by electrodeposition on

Ag at 800 °C without any salt inclusion and nodular surface.

2.2.2 Silica reduction

The widespread availability of highly pure silica at a relatively low cost presents an opportunity

to produce high grade silicon in one step of the reduction. In the following, different routes for

production of SoG-Si from SiO2 are described.

2.2.2.1 Carbothermal reduction of silica

This method is based on reducing silica by carbon, which is the main technique for MG-Si

production. Principally this method can also be used to produce low cost SoG-Si if the impurity

introduction into the Si is well controlled by selecting high purity silica and carbon sources and

17

operating in a clean environment. The process was first practised by Dosaj et al.[6]. The main

reaction is:

SiO2+ C Si + CO2 (2-5)

Despite the low level of some impurities, the high concentration of carbon in the final product

warrants further refining, making the process expensive [3].

2.2.2.2 Reduction by metals and compounds

This method involves reducing silicon halides by various metals or compounds. The source of

silicon can be SiO2, SiF4 and SiCl4 while two different types of reductants have been used:

compounds such as NH3 and CH4, and metals namely Na, K and Zn, which were used in large

scale production [7, 15]. Among different possible reactions, reduction of SiF4 by Na yields the

lowest concentration of impurities [90]. In this method, Both SiF4 gas and solid Na are charged

to a reactor at 400 C. They react according to:

SiF4+ 4Na Si+ 4NaF (2-6)

The reaction produces Si and NaF, also some Na2SiF6 is collected at the bottom of the reactor

from which Si can be recovered.

2.2.2.3 Electrodeposition

Electrodeposition of Si from various electrolytes has been studied from 1950s. Since the topic of

this research is on producing Si by this technique, it will be discussed in more details in the

following section.

2.3 Electrowinning of silicon

This technique has been studied by different researchers trying to develop a process similar to

the Hall-Héroult, for silicon [37, 91-96]. The overall reaction is decomposition of silica to silicon

and oxygen, using electricity, if an inert anode is used, which presents the opportunity of a

carbon-free process. It has been estimated that electrodeposition of Si requires 28.5% more

energy than Al due to higher ionic charge (IV for Si compared to III for Al) [13].

Monnier [37] gives the credit of the first attempt on silicon electrodeposition to DeVille who

electrodeposited silicon from a solution of KF/NaF containing SiO2 on a platinum cathode. The

cathode reacted with Si and formed platinum silicide. Gore [29] also claimed to have deposited

18

silicon from an aqueous solution of potassium monosilicate. This claim has not been confirmed

later. The electrodeposition of Si in elemental form was first reported by Ullik [97]. His

electrolyte was KF and he used K2SiF6 as the source of silicon. Minet [98] conducted the first

research on electrodeposition of Si in the form of Fe-Si and Al-Si alloy from molten cryolite,

NaCl, SiO2 and Al2O3.

In spite of the above scattered attemps, a systematic study of silicon electrodeposition was not

done until 1930’s when Dodero [99, 100] electrodeposited silicon from molten alkaline or

alkaline earth metal silicates, at the temperature range of 800-1250 C (benefiting from fluoride

additives). In this study, alkaline and alkaline earth metals were also deposited with Si because

of the large applied potential. The best result was 72% Si produced from a 5SiO2- 1Na2O-

0.2NaF melt at 1150 °C.

The electrodeposition can use MG–Si to produce a higher grade Si (i.e. electro–refining) or

produce Si from its compounds (i.e. electrowinning). The latter is a more attractive process as it

starts directly from Si containing species (silicates and fluorosilicates) and eliminates the reliance

on the availability of MG-Si. Several studies have been performed on electrowinning of Si,

trying different techniques and electrolytes to deposit highly pure SoG-Si. These works are

divided into two main categories based on the operating temperature: Electrowinning below the

melting point of silicon where deposited metal is solid and, electrowinning above the melting

temperature where the deposited silicon is liquid.

2.3.1 Electrowinning of solid Si

Different types of electrolytes have been proposed for this process, namely organic electrolytes

and molten salts.

2.3.1.1 Organic solvents

The advantage of using an organic electrolyte is low working temperature. Previous

investigations reveal that it is possible to deposit amorphous Si (a-Si) at temperature close to

ambient [13, 37, 101-103], resulting in low energy consumption. Several organic solvents have

been used such as propylene carbonate [101, 104], dimethylformamide (DMF) [101],

tetrahydrofuran [101] or its mixtures with toluene or benzene [102] and ethyl alcohol [103].

Silicon tetrachloride or triochlorosilane were used as the source of silicon.

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The extracted a-Si is mostly hydrogenated which possess interesting semiconductor properties.

Bucker and Amick [102] reported that the deposited a-Si is not stable against atmospheric attack

and need further heat treatment to remove the excess hydrogen. In order to increase the stability,

Kroger [103] tried to deposit the fluorinated a-Si. The main challenge for this type of electrolyte

is low efficiency of the deposited a-Si which restricts its application in solar cells; but the

technique for the deposition of a-Si is well established [13]. Deposition of Si in nano scale from

ionic liquid was reported by Al-Salman [105].

2.3.1.2 Molten salts

2.3.1.2.1 Deposition from halide melts

Fluoride melts offer advantages such as high conductivity, low viscosity, high decomposition

voltage and high solubility for metal oxides which is beneficial for cleaning the surface of

metallic electrodes [106]. The halide electrolytes include alkaline or alkaline earth fluorides,

cryolite and SiF4 [37]. The source of silicon used in combination with these electrolytes is often

an alkaline fluorosilicates (Me2SiF6) which is stable below the melting temperature of Si.

Therefore, the cathodic product of these electrolytes is either solid Si or molten alloy. Ullik [97]

was the first to deposit silicon from KF melts. Oltowski [107] also tried to electrodeposit Si from

a mixture of LiF, NaF and Na2SiF6 using Fe or Cu cathode; he produced a small quantity of

silicon and silicon intermetallics. Monnier and Giacometti [92] suggested liquid Cu-Si alloy as

the anode in a NaF, KF or LiF and K2SiF6 bath.

The Recent studies are mostly focused on electrodeposition of a Si layer on an appropriate

substance to directly produce Si wafer for solar cells. The deposited layer can be used as n-type

Si and prevent Si loss of the ingot slicing process. Cohen [108] pioneered electrodeposition of Si

film from a mixture of alkaline fluoride and K2SiF6 system for direct solar application. A single

crystal epitaxial layer was deposited from LiF-KF solution. A continuous film was also produced

from this melt using a dissolving Si as the anode.

Rao and Elwell [26, 28, 88, 109, 110] conducted an extensive investigation on electrodeposition

of Si from different electrolytes and Si sources and on a range of substrates. According to their

findings, deposition of Si from binary LiF/KF eutectic and ternary LiF/NaF/KF eutectic melts

and on a silver substrate have given the best results [110]. Silver is an expensive material to be

20

used in bulk deposition for solar cells so the possibility of deposition of a coherent layer of Si on

the surface of graphite was also studied [28]. This research could successfully deposit a dense

and continuous layer of Si at -0.75 0.05 V vs. Pt or Ag onto graphite from a K2SiF6/LiK-KF salt

at 745 5 C. The deposited Si was 99.999% pure.

Boen [111] studied the effect of electrolysis parameters on the deposition of Si from Na2SiF6 in

LiF/KF solution. This investigation shows that it is not possible to deposit Si on graphite or Si

electrode using non-purified electrolyte. They also found that using direct current does not allow

deposition of a thin and pure silicon layer; instead, pulsed current was suggested for this purpose.

The level of purity obtained from two different anodes, insoluble graphite and soluble MG-Si,

demonstrates that the level of most of the impurities is lower in case of graphite anode.

All of the above studies were conducted in a molten fluoride electrolyte. The research conducted

by Devyatkin [112] is the first that used K2SiF6 in a chloride melt (NaCl-KCl). This study

confirms the possibility of Si deposition from the chloroflouride melts. Andrikko [113] used a

mixture of fluoride- chrolide melt as K2SiF6/KCl-KF solution. SiCl4 gas is another source of Si

that was used for deposition of Si in LiCl-KCl melt at 450C [114].

2.3.1.2.2 Deposition from mixture of oxide without halides

The main source of Si in non-halide electrolyte systems is SiO2, which is added to a mixture of

different oxides to improve the conductivity and lower the melting point. A few studies on this

system have been carried out in the past noticeably the research by Andrieux [115], who

deposited a-Si from a mixture of Na2O and SiO2. Jorgensen [116] studied deposition of a−Si on

polycrystalline silicon by electrolysis of silica. Finally, Lyakovich [37] deposited a layer of Si on

iron cathode from molten Na2O and SiO2 mixture between 1050 and 1150°C. He aimed to

protect the iron by formation of intermetallic compound of Fe–Si. This electrolyte has not been

used for extraction of pure Si.

2.3.1.2.3 Deposition from mixture of halides and silica

Cryolite has been used for electrodeposition of silicon because of its ability to dissolve silica and

its established use in the extraction of aluminum. The process must be carried out at higher

tempe