Teng-Cheong Ong Thesis

This is the author’s version of a work that was submitted/accepted for publication in the following source:

Ong, Teng-Cheong (2018) Research of the suppression effects of cooling rate on crystallization in ZBLAN glass. PhD thesis, Queensland University of Technology.

This ﬁle was downloaded from: https://eprints.qut.edu.au/116614/

Notice: Changes introduced as a result of publishing processes such as copy-editing and formatting may not be reﬂected in this document. For a deﬁnitive version of this work, please refer to the published source: https://doi.org/10.5204/thesis.eprints.116614 Research of the suppression effects of cooling rate on crystallization in ZBLAN glass

Teng-Cheong Ong

Bachelor of Engineering (Mechanical) (Hons)

Submitted in fulfilment of the requirements for the degree of

Doctor of Philosophy

School of Chemistry, Physics and Mechanical Engineering

Science & Engineering Faculty

Queensland University of Technology

2018

Abstract

ZBLAN glass is a heavy metal fluoride glass that has great potential in the application of long- haul telecommunication cables. However, during processing in the fibre-drawing temperature region, the material tends to undergo heavy devitrification, resulting in a crystalline fibre that is not usable for such purposes. There are many papers exploring various processing techniques in the aims of creating a test sample that can transmit with the theoretical minimum attenuation loss predicted for ZBLAN. As ZBLAN glass is cooled from its melt, crystallites form throughout the medium, their size and structure dependent on the rate of cooling and degree of undercooling. These crystallites act as scattering centres that degrade a signal that is propagated through the glass. This thesis seeks to explore processing techniques with different cooling rates in order to supress the process of crystallization.

Rapidly cooled ZBLAN test samples were analysed with a wide range of spectroscopy, diffraction and imaging techniques including SEM, TEM, XRD and ellipsometry. These techniques were used to characterize the structure of the crystallites in the ZBLAN glass and how they were affected by the various cooling rates. Ultimately, a critical cooling rate was empirically established to be between 900 ⁰C/min and 4000 ⁰C/min for a test volume size of

9.4 x 10-8 m3. This critical cooling rate yielded ZBLAN samples that were fully amorphous and completely free of nano-crystalline inclusions. Using an algorithm that incorporates classical nucleation formulae, a theoretical critical cooling rate was determined to be 1100 ⁰C/min, which is in agreement with the empirical results. Lastly, using Kramers-Kronig relations, the predicted attenuation loss for the fully amorphous ZBLAN test samples is 0.09 dB/Km at a

wavelength of 1800 nm, which although is not at the theoretical best, is still a marked improvement on the best attenuation loss achieved in ZBLAN glass fibres to date.

Recommendations are made for the next critical step for future work of this research, which is to create a fibre with this degree of amorphous “clarity”.

Keywords:

ZBLAN, fluoride glass, devitrification, crystallization, cooling rate, rapid quenching, amorphous, scanning electron microscopy, transmission electron microscopy, x-ray diffraction, ellipsometry, attenuation loss

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Acknowledgements

My PhD Journey has been a very unique one, not unlike other people’s experience in that it was taxing and gruelling at the best of times, but also very unpredictable and turbulent in many ways. However, the overcoming of great tribulations is an excellent teacher of fortitude and determination, where the completion of this thesis was only possible because of the immeasurable amount of help and guidance I received. For those people who helped me in my time of need, I am unbelievably grateful for.

Firstly, I would like to thank all of the technicians and staff who helped me with all of my laboratory and analysis work, without them I would have continued to run around like a headless chook. I would like to thank Lauren Butler for dealing with the ever temperamental glove box. The workshops boys in

O block, for being ever patient and always meeting all the deadlines, I hope that carton of beers made up for how demanding I was. The Chemstore boys for putting up with my demands and urgent deadlines. Graham Wright for providing me endless support with the electrical side of the project.

Leonora Newby, Dr. Peter Hines, Dr. James Riches, Dr. Tony Raftery, Donald McAuley and Rachel

Hancock for assisting with the analysis and spectroscopy work.

I would like to extend my heartfelt thanks to my fellow PhD students who helped me when they themselves were busy enough with their own work. Thank you Ralf Raud for not just helping me with my lab work, but being a friend whose been there with me for all the ups and down of this roller coaster ride. Thank you Owen for your matlab expertise, but mostly for the good chats, the moral support and the companionship out at the drop tower, we had some fun times out there.

I greatly appreciate all of the undergraduate engineers who provided their support to this project, especially when they weren’t being paid or getting credit for the work and just doing it out of the kindness of their own hearts. Special mention goes to Benjamin Forgarty, thank you for the great chats and the endless hours you dedicated to this project, building the rapid electro-thermal processing device

(or the REPD, pronounced “repid”) and the portable glove box. You are a man of many great talents and your contribution to this thesis has been monumental to say the least.

I would also like to express my deepest appreciation to my family and friends, most of whom did not have a direct contribution to this project, but it is through this personal support network that I was able to spend all of these years focused on this project. Special thanks goes to my mum, who although has

always questioned some of my decisions in life, has always been an unconditional supporter in all of my endeavours.

I would like to extend my gratitude to Dr. Mohammed Saad, senior scientist at THORLABS, for providing me the technical expertise I required, but most importantly for providing the ZBLAN material used in this experiment. The material was magnanimously provided for free, and without it this work would not have been possible.

I would like to thank Professor John Bell and Professor Esa Jaatinen, my associate supervisors. John provided me so much support, especially during the very difficult periods of my candidature. Esa, your expertise has been invaluable in this project, you have always been a great source of information and guidance when I needed it most.

Last but not least, Professor Ted Steinberg, the Principal Supervisor who inherited me and my project, you were a saviour to me, and so much more than just a supervisor/mentor to me, but a hero and source of motivation for me. Ted played the role of an “adopted surrogate” father, guiding me through the last

200 meters of this marathon run of a candidature with great encouragement and faith. I owe the completion of this thesis solely on your moral support, without you as a Principal Supervisor I wouldn’t have been able to muster the strength to restart this project from scratch and finish on time.

TABLE OF CONTENTS

Abstract...... ii

Acknowledgements ...... iv

TABLE OF CONTENTS ...... vi

LIST OF TABLES ...... xi

LIST OF FIGURES ...... xii

List of Abbreviations ...... xix

Statement of Original Authorship ...... xx

CHAPTER 1: Introduction ...... 1

1.1. ZBLAN Glass and its Applications ...... 1

1.2. Hypothesis ...... 1

1.3. Research Approach ...... 2

1.4. Project Objective ...... 2

1.5. Limitation of Study ...... 3

1.6. Thesis Chapter Summary ...... 4

CHAPTER 2: Background and Literature Review ...... 6

2.1. Background of ZBLAN Glass ...... 6

2.2. Material Properties of ZBLAN ...... 7

2.3. Crystallization ...... 9

2.3. Preform Fabrication ...... 12

2.4. Fibre Drawing ...... 16

2.5. Rapid Cooling ...... 18

2.6. Characterization of Attenuation Losses ...... 24

2.7. Literature Review Summary & Remarks ...... 26

CHAPTER 3: Methods and Research Plan ...... 28

3.1. Methodology ...... 28

3.1.1. Thermal Treatment Profile ...... 28

3.1.2. Test Matrix ...... 31

CHAPTER 4: Characterization of Experimental Equipment ...... 35

4.1. ZBLAN Glass ...... 35

4.2. Heating Equipment ...... 36

4.2.1. SEM (SA) Furnace ...... 36

4.2.2. Setaram Labsys DSC ...... 37

4.2.3. Rapid Electro-Thermal Processing Device (REPD) ...... 38

4.3. Platinum Crucible ...... 41

4.4. Bolt Container ...... 42

4.5. Analysis Equipment ...... 45

4.5.1. Imaging and Elemental Analysis ...... 45

4.5.2. Characterization of Crystalline Content of Test Samples ...... 53

4.5.3. Attenuation Loss Analysis ...... 55

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CHAPTER 5: Results and Discussion ...... 60

5.1. SEM Imaging and EDS Results ...... 60

5.1.1. ZBLAN Test Sample cooled at 5 ⁰C/min (cooled in DSC) ...... 60

5.1.2. ZBLAN Test Sample cooled at 20 ⁰C/min (cooled in DSC) ...... 72

5.1.3. ZBLAN Test Sample cooled at 100 ⁰C/min (cooled in Air) ...... 74

5.1.4. ZBLAN Test Sample cooled at 900 ⁰C/min, 4000 ⁰C/min & 8000 ⁰C/min ...... 78

5.1.5. SEM Results Summary ...... 79

5.2. XRD Results ...... 81

5.2.1. X-ray Diffractograms ...... 84

5.2.2. XRD Results Summary ...... 86

5.3. TEM Results ...... 86

5.3.1. ZBLAN Test Sample cooled at 100 ⁰C/min ...... 87

5.3.2. ZBLAN Test Sample cooled at 900 ⁰C/min ...... 91

5.3.3. ZBLAN Test Sample cooled at 4000 ⁰C/min & 8000 ⁰C/min ...... 95

5.3.4. TEM Results Summary ...... 98

5.4. Raman Analysis of 4000 ⁰C/min Test Samples ...... 100

5.5. Attenuation Loss Measurements ...... 102

5.5.1. Refractive Index Matching Liquids ...... 102

5.5.2. Ellipsometer ...... 102

5.5.3. Approximation of Attenuation Loss ...... 106

5.6. Algorithm to Theoretically Predict the Critical Cooling Rate ...... 108

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5.7. Error in Cooling Rate Measurements ...... 112

5.8. Discussion ...... 115

5.8.1. SEM Results ...... 115

5.8.2. XRD Results ...... 117

5.8.3. TEM Results ...... 118

5.8.4. Raman Analysis of 4000 ⁰C/min Test Sample ...... 121

5.8.5. Attenuation Loss Measurements ...... 123

5.8.6. Algorithm to Theoretically Predict the Critical Cooling Rate...... 124

5.8.7. Error in Cooling Measurements ...... 124

CHAPTER 6: Conclusion and Recommendations ...... 128

6.1. Conclusion ...... 128

Additional Findings ...... 131

6.2. Recommendations for Future Work ...... 132

REFERENCES ...... 134

APPENDIX A: Supplementary Literature Review ...... 142

A.1. Processing of ZBLAN in Microgravity ...... 142

A.2. Rapid Heating ...... 144

A.3. Concluding Remarks ...... 146

APPENDIX B: Attenuation Loss in Fibres ...... 147

B.1. Optical Fibre Loss...... 147

B.2. Extrinsic Loss mechanisms ...... 147

B.3. Intrinsic Loss Mechanisms ...... 148

B.4. Intrinsic Loss Measurements ...... 151

B.5. Extrinsic Loss Measurements (Absorption) ...... 151

B.6. Extrinsic Loss Measurements (Scattering) ...... 153

B.7. Experimental Loss Measurements in Fibres...... 153

APPENDIX C: Rapid Electro-thermal Processing Device (REPD) ...... 157

C.1. REPD Specifications ...... 157

C.1.1. General Set Up ...... 157

C.1.2. Programming Control ...... 158

C.1.3. Heating/Cooling Profile ...... 159

C.1.4. Safety Precautions ...... 160

C.2. REPD Wiring Diagram ...... 162

C.3. Processing Test Samples using the REPD ...... 164

APPENDIX D: Matlab Code for Algorithm for Theoretical Critical Cooling Rate ...... 166

APPENDIX E: Attenuation Loss Calculation Data ...... 167

E.1. Modelled Data generated from CompleteEASE Analysis of SE Data for 4000 ⁰C/min

Test Sample ...... 167

E.2. n, k and Attenuation Loss ...... 172

APPENDIX F: Processing with a Magnet ...... 176

LIST OF TABLES

Table 1. Experimental Test Matrix ...... 31

Table 2. Chemical Composition of ZBLAN as per EDS Analysis ...... 35

Table 3. EDS Analysis of “Feather” Morphology from Figure 35 ...... 70

Table 4. EDS Analysis of “Bow-Tie/V-Shaped” Morphology from Figure 35 ...... 71

Table 5. EDS Analysis of “Dark Region with Black Specs” Morphology from Figure 35 ...... 72

Table 6. EDS Analysis of Crystallites from Figure 42 ...... 77

Table 7. List of Peaks and Attributed Crystal Phases ...... 83

Table 8. Indexing of Diffraction Pattern from Figure 55 ...... 90

Table 9. Peak Parameters from Figure 56 ...... 91

Table 10. Crystal Phase Match for Diffraction Pattern in Figure 60...... 94

Table 11. Peak Deconvolution of Figure 67 [56] ...... 101

Table 12. Fitted Parameters for Layer 1 of SE Data ...... 104

Table 13. Parameters used for Nucleation Rate Formula, Eq. (5.5), as Summarized in an Article by Hopgood & Rosman [52] ...... 110

Table 14. Summary of Accuracy of Cooling Rate Measurements ...... 113

Table 15. Summary of Coefficient of Variation and Root Mean Square Error for all Cooling

Rates ...... 115

Table 16. Summary of Cooling Rate and its Effect on Crystal Size ...... 129

LIST OF FIGURES

Figure 1. Intrinsic losses, V-curves, showing the low attenuation losses theoretically predicted for ZBLAN compared to silica [8], calculated spectrum for an ultra-pure glass specimen ...... 6

Figure 2. TTT diagram of AgF doped heavy metal fluoride glass. Lines are drawn as guides for the eye, amorphous state is to the left side of the lines, and the crystalline state is to the right [23] ...... 19

Figure 3. Typical T-T-T diagram showing CR, the critical cooling path, as computed with Eq.

(2.1), and A, an arbitrary path with sub-critical rate [24] ...... 20

Figure 4. Images showing different cooling methods for contact with a heat sink (varying angles) on various substrates (stationary and in motion) [31], from left to right the set ups are capable of achieving cooling rates of 104 K/s for simply pouring, 108 K/s for laser surface glazing, 108 K/s for melt shooting, 106 K/s for a swinging wing ...... 23

Figure 5. Graph showing temperature curve of various processing regimes (Tm, Tx and Tg were averaged for ZBLAN, based on values taken from literature [12]), cooling rate of 100

⁰C/min in blue, cooling rate of 900 ⁰C/min in orange and cooling rate of 4000 ⁰C/min in red 29

Figure 6. Photograph of SEM (SA) Furnace ...... 36

Figure 7. (Left) Photograph of Setaram DSC, (right) layout of DSC rod ...... 37

Figure 8. Cross-sectional layout of the Setaram DSC ...... 38

Figure 9. Layout of REPD and connecting wires ...... 39

Figure 10. REPD apparatus with the inside compartment visible, where there is an 800 W transformer that provides the current supply, which is modulated by a solid-state relay (see

Appendix C for more information on the operation of the REPD) ...... 40

Figure 11. Crucible in position, locked in between mounting/cooling block and rod ...... 41

Figure 12. Photograph of CS Ceramic platinum crucible ...... 42

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Figure 13. Screenshot of Solidworks™ (left) wire model of bolt container parts, (right) full assembly ...... 43

Figure 14. Photograph of bolt container, (left) top and bottom hexagonal nuts removed, (right) assembly placed together ...... 44

Figure 15. Photograph of Zeiss Sigma FE-SEM ...... 46

Figure 16. Scanning Electron Microscope (SEM) image of a lower leaf surface (cucurbita maxima), showing stomates and tricomes [35] ...... 47

Figure 17. EDS Spectrum for ZBLAN ...... 48

Figure 18. This image shows (left) an electron diffraction pattern of a single-crystal

EuMnGe2O7 compound with zone axis (001) [36], (right) diffraction pattern of a film of amorphous SiN [37] ...... 50

Figure 19. TEM image and electron diffraction pattern of purified gold nanoparticles, (a) TEM image showing the morphology of the gold crystals, (b) poly-crystalline electron diffraction pattern from the GNPs showing the (111), (200), (220), (311), and (222) reflections of gold [38]

...... 50

Figure 20. Photograph of JEOL 2100 TEM ...... 51

Figure 21. XRD pattern of ZBLAN test samples from a study by L. Battezzati [19] ...... 54

Figure 22. Schematic showing layout of a XRD instrument, with the incident beam and measurement of diffracted x-rays [7] ...... 55

Figure 23. Becke line “moving” towards the liquid of 1.5 RI (test sample is the upper right region, liquid is the bottom left), which indicates the test sample has a lower refractive index than the medium it is submerged within ...... 57

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Figure 24. Change in polarization of light as it interacts with the sample, can be graphically represented as the shift in the components of the electric vector parallel (p-plane) and perpendicular (s-plane) to the incident plane [41] ...... 58

Figure 25. SEM image of test sample cooled at 5 ⁰C/min at low magnification ...... 61

Figure 26. SEM image of test sample cooled at 5 ⁰C/min at ...... 61

Figure 27. EDS Spectra taken at 6 different positions along the surface of the ZBLAN fragment cooled at 5 ⁰C/min. Regions with striated lines correlate to the ZBLAN matrix, regions with smoother texture correlate to higher Zr concentration ...... 62

Figure 28. EDS spectra taken from spectrum 1 site of a test sample cooled at 5 ⁰C/min, showing a typical ZBLAN molar composition (refer to Table 2 for expected molar ratio) ...... 63

Figure 29. EDS spectra taken from spectrum 5 site of test sample cooled at 5 ⁰C/min, showing a depletion of aluminium and sodium ...... 63

Figure 30. EDS element map of ZBLAN test sample cooled at 5 ⁰C/min, at the same sites as

Figure 27 ...... 64

Figure 31. EDS element maps for the various constituent elements of ZBLAN test sample cooled at 5 ⁰C/min ...... 65

Figure 32. VPSE image showing disparate regions of the 5 ⁰C/min ZBLAN test sample...... 66

Figure 33. SEM image showing a polished cross-section of the 5 ⁰C/min cooled ZBLAN test sample ...... 68

Figure 34. High magnification of 5 ⁰C/min ZBLAN test sample with three different crystal phases ...... 69

Figure 35. Backscatter image of 5 ⁰C/min ZBLAN test sample with three crystal phases ...... 70

Figure 36. SEM image of the “feather” morphology of 20 ⁰C/min ZBLAN test sample, which is smaller in scale than the 5 ⁰C/min test sample (compare to Figure 34) ...... 72

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Figure 37. Backscatter image of 20 ⁰C/min ZBLAN test sample at high magnification ...... 73

Figure 38. Backscatter image of 20 ⁰C/min ZBLAN test sample with three crystal phases ..... 73

Figure 39. SEM image of 20 ⁰C/min ZBLAN test sample, the left hand side of image is heavily crystalline, the right hand side is amorphous ...... 74

Figure 40. SEM image of 100 ⁰C/min cooled ZBLAN test sample, with AlF3 microcrystals highlighted in red...... 75

Figure 41. SEM image showing higher magnification of possible AlF3 crystallites, as highlighted in the red square of Figure 40 ...... 76

Figure 42. Backscatter image of possible AlF3 crystallites ...... 76

Figure 43. SEM image of 900 ⁰C/min cooled ZBLAN test sample ...... 78

Figure 44. Higher magnification SEM image of Figure 43, showing ZBLAN test sample cooled at 900 ⁰C/min ...... 79

Figure 45. Coloured regions showing various crystal phases for a sample cooled at 5 ⁰C/min

(left), coloured regions showing phases for sample cooled at 20 ⁰C/min (right). Both images are to equal scale...... 80

Figure 46. X-ray diffractogram for 20 ⁰C/min cooled ZBLAN test sample labelled with assigned crystal phases (shown in Table 7) as identified using the PDF-4+ database ...... 83

Figure 47. XRD pattern for a ZBLAN test sample cooled at 5 ⁰C/min ...... 84

Figure 48. XRD pattern for a ZBLAN test sample cooled at 20 ⁰C/min ...... 84

Figure 49. XRD pattern for a ZBLAN test sample cooled at 100 ⁰C/min ...... 85

Figure 50. XRD pattern for a ZBLAN test sample cooled at 900 ⁰C/min ...... 85

Figure 51. Overlay of various X-ray diffractograms for different cooling rate test samples ... 86

Figure 52. Bright field image of 100 ⁰C/min cooled ZBLAN test sample fragments spread out on a carbon grid...... 88

Figure 53. Higher magnification of a bright field image of a single ZBLAN fragment, crystallites are visible as dark speckles ...... 88

Figure 54. Higher magnification bright field image of another ZBLAN fragment with many crystallites...... 89

Figure 55. Diffraction pattern for a 100 ⁰C/min ZBLAN sample with assigned rings ...... 90

Figure 56. Graph showing peak intensities of diffraction pattern from Figure 55 ...... 91

Figure 57. Bright field image of 900 ⁰C/min ZBLAN test sample...... 92

Figure 58. Image of various 900 ⁰C/min ZBLAN fragments ...... 92

Figure 59. Higher magnification image of 900 ⁰C/min ZBLAN fragment, material is more homogeneous in structure, however, there are still “speckles” that could potentially be nano crystals ...... 93

Figure 60. Diffraction pattern of 900 ⁰C/min test sample, with spots belonging to three ring patterns ...... 94

Figure 61. Bright field image of 4000 ⁰C/min ZBLAN test sample fragments ...... 95

Figure 62. Bright field image of 4000 ⁰C/min ZBLAN test samples at higher magnification, darker area is ZBLAN fragment ...... 96

Figure 63. HRTEM image of disordered nature of 4000 ⁰C/min ZBLAN test sample ...... 97

Figure 64. Diffraction pattern of 4000 ⁰C/min test sample, showing an amorphous structure

...... 98

Figure 65. Bright field Image comparison of (above) single ZBLAN fragments of a 100 ⁰C/min cooled sample, and (below) a 4000 ⁰C/min cooled sample ...... 99

Figure 66. Diffraction pattern comparison between (left) a 100 ⁰C/min cooled sample, (middle) a 900 ⁰C/min cooled sample and (right) a 4000 ⁰C/min cooled sample ...... 100

Figure 67. Raman spectrum for 4000 ⁰C/min sample ...... 101

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Figure 68. Cross-sectional layout of ZBLAN test samples as prepared on soda lime glass slides

...... 103

Figure 69. Optical constants of ZBLAN test sample based off CompleteEASE ™ model ..... 105

Figure 70. Graph showing nucleation rate as calculated using matlab (above); corresponding to the upper limit curve of the nucleation rate as published in the study by Lu et al [51] .... 111

Figure 71. Raman spectra for vitreous and crystalline BaZrF6, with the vitreous sample having a matching dominant band (at 580 cm-1) with the 4000 ⁰C/min test sample [55] ...... 122

Figure 72. Attenuation loss for silica and theoretical ZBLAN [17], the attenuation loss for 4000

⁰C/min test sample has been added to the graph in purple, ZBLAN’s approximate theoretical attenuation loss is in black, silica fibre attenuation loss is in yellow ...... 130

Figure 73. Scanning electron micrograph of ZBLAN glass processed in unit gravity [16] ... 142

Figure 74. ZBLAN fibre processed in microgravity [16] ...... 143

Figure 75. Plot showing crystallization onset bypassing through a critical heating regime [69]

...... 145

Figure 76. Diagram showing structure of a cladded optical fibre [63] ...... 147

Figure 77. Graph showing a generalized V-curve [63] ...... 150

Figure 78. Chart summarizing all of the different sources of extrinsic and intrinsic losses [63]

...... 150

Figure 79. Infrared spectrum of silica aerogel [11] ...... 152

Figure 80. Diagram showing layout of a spectral loss measurement apparatus [63] ...... 154

Figure 81. Layout of a FTIR spectrometer system [63] ...... 155

Figure 82. Set up for a fibre calorimeter [63] ...... 155

Figure 83. Integrating sphere for scattering measurement [63] ...... 156

Figure 84. Photograph of Rapid Electro-thermal Processing Device ...... 157

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Figure 85. Live temperature reading of a crucible heated to 330 ⁰C, with the thermocouple attached to the bottom of the crucible ...... 159

Figure 86. Graph showing heating/cooling rate provided by REPD, this profile can be altered to have faster/slower rates ...... 159

Figure 87. REPD with front plate removed to reveal circuitry inside, (left) safety timer is located in the top left corner, (right) close up of safety timer with turn dial to set safety time

(currently set at 10 s) ...... 161

Figure 88. Safety timer reset button ...... 162

Figure 89. Wiring diagram for REPD, showing connections between the solid state relay,

Arduino board, timer and the transformer ...... 163

Figure 90. Rare earth magnetic lines positioning/orientation in relation to crucible (highlighted in blue) ...... 176

Figure 91. Diffraction pattern of 100 ⁰C/min ZBLAN test sample processed with a magnet, showing a fully amorphous glass matrix ...... 177

Figure 92. Lower magnification bright field image of 100 ⁰C/min ZBLAN test sample processed with a magnet ...... 178

Figure 93. Higher magnification bright field image of 100 ⁰C/min ZBLAN sample processed with a magnet ...... 178

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List of Abbreviations

DSC – Differential Scanning Calorimetry

EDX – Energy Dispersive X-Ray Analysis

EDS –Energy Dispersive Spectroscopy

HRTEM – High-Resolution Transmission Electron Microscopy

QUT- Queensland University of Technology

SEM – Scanning Electron Microscopy

TEM – Transmission Electron Microscopy

XRD – X-Ray Diffraction

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QUT Verified Signature

CHAPTER 1: Introduction

1.1. ZBLAN Glass and its Applications

Current telecommunication capabilities require long-haul oceanic cables to transmit large volumes of data intercontinentally. They are typically made from silica fibres, which although are effective as signal propagators, are limited by the attenuation losses inherent in the material. ZBLAN, a multi-component heavy metal fluoride glass, theoretically has the potential of propagating signals with an incredibly low degree of attenuation, as low as

0.01 dB/km [5], which is over a factor a hundred times less than silica. This would render the transmission of data for use in long-haul optics applications more efficient and effective.

However, ZBLAN glass has a tendency to devitrify when being cooled from the melt or being drawn into fibres, creating micro-crystals in the medium. This leads to a large increase in the attenuation of a light signal within the glass.

1.2. Hypothesis

The cause for why the low theoretical attenuation loss for ZBLAN glass has not been achieved to this date is due to the presence of micro-crystals in the glass [1, 2, 3], as manufactured by current methods (mostly analogous to melt-cast methods in metalworking applications). The hypothesis tested in this thesis was based on the concept that with a sufficiently high cooling rate, in magnitude approximately 10 - 100 times greater than the currently theoretically predicted critical cooling rates of 5 ⁰C - 8 ⁰C/min [4], the formation of micro-crystallites upon cooling from the parent molten phase of ZBLAN glass will be impeded entirely. With the

absence of micro and nano-sized crystallites, the ZBLAN glass matrix will have an amorphous structure that should produce a low attenuation loss.

1.3. Research Approach

The research approach for this project involved the characterization of the material relationships between processing (thermal treatment), structure (crystal phases and structure), properties (refractive index) and ultimately performance (attenuation loss) of

ZBLAN material, with a primary focus on the relationship between processing and structure.

Currently, many studies have been conducted to improve the casting and fibre drawing techniques for producing ZBLAN fibres, based on existing processes used for drawing silica fibres. However, there is an absence of in-depth investigations into the connection between crystallization and the heating/cooling processes performed on ZBLAN itself. This overall holistic approach will provide more knowledge between the manufacturing processes of stock

ZBLAN material to its end-product potential as a long-haul transmission medium.

1.4. Project Objective

The fundamental goal of this project was to investigate the effect of cooling rate on the suppression of crystallization of ZBLAN glass. This research intends to shed light on how the structure of ZBLAN glass changes when subjected to various cooling regimes, and how these changes impact the physical properties and quality of the glass. The main focus was to determine whether a sufficiently high “critical cooling rate” was capable of preventing formation of crystallites in the glass. By answering this question, it was intended to produce

“purely amorphous” test samples capable of ultra-low attenuation losses. Test samples of

ZBLAN were processed and subsequently analysed with a focus on studying volume crystallization. New analysis methods, previously never applied to ZBLAN glass, were used to quantitatively and qualitatively describe the change in crystal structure and the associated attenuation loss.

Ultimately, findings in relation to this project objective will shed more insight on the feasibility of practically achieving the theoretical attenuation loss by the eradication of crystallites. If the theoretical attenuation loss, or near to, is attainable by surpassing a critical cooling rate, this would provide a whole new manufacturing approach that could make ZBLAN glass a viable product for use in the long-haul telecommunications industry.

1.5. Limitation of Study

The limitations of the study are as follows:

· A large part of the quality of the sample, and hence the starting material properties of

ZBLAN, is determined on which manufacturer the material is sourced from. Different

suppliers produce ZBLAN differently, varying manufacturing methods impact the

initial performance capabilities of the sample and how homogeneous they are.

· The time and resource constraints of this project will provide only a preliminary look

into the effect a fully amorphous glass has on attenuation loss, ideally to be able to

create long fibres free of crystallites (as opposed to fragments processed in a crucible)

would provide a more accurate representation of what the practical attenuation losses

could be for long-haul applications.

· There are many factors that affect the suitability of ZBLAN for one of its intended

purposes as a long-haul telecommunication fiber, including losses associated with the

manufacturing the fiber, impurities in the starting material and the mechanical

strength of the fiber. These factors are incredibly vital to the performance of the fiber

itself, however are beyond the scope of this thesis and will be addressed in the

“Recommendations for Future Work” section of this thesis.

· ZBLAN glass has a high tendency to surface crystallize, however the scope of this

study was primarily based on crystallization phenomena within the bulk material of

ZBLAN. Further investigation into surface crystallization while fiber drawing a

sample must be conducted, and is included in the “Recommendations for Future

Work” section of this thesis.

1.6. Thesis Chapter Summary

Chapter 1 Introduction: This chapter covers the project objectives and the limitations of this study, as well as providing a context to why researching the potential of ZBLAN glass as a waveguide material is important.

Chapter 2 Background and Literature Review: This chapter provides a background to the discovery of ZBLAN glass, the research that has already been conducted in the field of processing and measuring the optical capabilities of fluoride glasses, and all other works related to this study.

Chapter 3 Methods and Research Plan: This chapter covers the methodology behind the experiments and summarizes all of the tests done in a test matrix. The parameters of this study are also outlined in this section.

Chapter 4 Characterization of Experimental Equipment: This chapter covers in detail the specifications of all the equipment used in the experiments from processing the samples, storage of the test samples and the analysis of them.

Chapter 5 Results and Discussion: This chapter presents the results of the experiment and the analysis work done on the test samples. Results from the various spectroscopic, diffraction analysis and imaging techniques are included here. A discussion section delves deeper into the overall impact of the results and how they relate to the original project objectives.

Chapter 6 Conclusions and Recommendations: This chapter summarizes all of the findings in chapter 5 and covers all of the conclusions that can be drawn from the results. This section answers the questions posed in chapters 1 and 2, and then provides suggestions for further work on the project.

CHAPTER 2: Background and Literature Review

2.1. Background of ZBLAN Glass

Fluoride glasses are a class of non-silica based optical glasses composed of heavy metal fluorides. They show great potential as signal transmitting media since they have been predicted to be capable of achieving low attenuation losses, however they are notorious for being difficult to handle and process. One of the forerunners in this group of heavy metal fluoride glasses is a compound named ZBLAN [6]. Varieties of ZBLAN glass are comprised of different ratios of the following five compounds; zirconium tetrafluoride, barium difluoride, lanthanum trifluoride, aluminium trifluoride and sodium fluoride (the starting letter of each compound’s elements makes up the acronym of ZBLAN). It is most promising as a material for manufacturing ultra-low loss attenuation solid fill fibres, which is attributed to its excellent internal reflection properties over a wide optical spectrum as shown in Figure 1

[7, 8].

Figure 1. Intrinsic losses, V-curves, showing the low attenuation losses theoretically predicted for ZBLAN compared to silica [8], calculated spectrum for an ultra-pure glass specimen

In 1974, ZBLAN was accidentally discovered by two researchers, Poulain and Lucas, at the

University of Rennes, in the northwest of France. Poulain's discovery of the fluorozirconate glasses was entirely a "surprise", when he was working on the synthesis and characterization of ZrF4-containing crystalline compounds doped with rare earths. It was a result of his attempt to add NaF to fill the voids in the crystal structure of a ZrF4-BaF2 mixture [9]. The mixture unexpectedly yielded a glass, and shortly after the first descriptions of 'fluorozirconate' glasses with additional vitreous compositions were published. Possibly the most important publication that jump started the search for an ultra-pure sample of ZBLAN was by Ohsawa et al [10], where it was first reported that ZBLAN glass of molar composition 53% ZrF4, 20%

BaF2, 4% LaF3, 3% AlF3 and 20% NaF would be the most stable glass practical for optical fibre applications.

The main application that ZBLAN is ideal for, and even touted as a revolutionary “holy grail”, is the telecommunications industry, specifically for fibre-optic communication systems. While conventional silica fibres have attained their theoretical minimum losses of 0.15 dB/km at

1600 nm wavelength, heavy metal fluoride glasses have the potential to only produce losses of 0.01 dB/km at 2200 nm wavelength [5]. This significant reduction in attenuation loss could lead to a drastic improvement in the data transmission capabilities.

2.2. Material Properties of ZBLAN

Thus far, there have been several papers published on the material properties and current synthesis processes used to make ZBLAN glass. Aasland and Grande had initially studied the crystallization process of ZBLAN in a journal article for the American Ceramic Society [12].

This study investigated the glass transition, onset of crystallization, solidus and liquidus

temperatures of ZBLAN through differential thermal analysis. The onset crystallization temperature was observed to be 335 °C for a powdered glass, while for a monolithic glass, prepared by in-situ quenching in a Pt capsule, the crystallization onset was observed to be

395 °C. Also, the solidus temperature was observed to be 452°C.

There are a number of other more recent papers that investigate the material properties of

ZBLAN glass, one that is of particular interest is a study performed by Ian Dunkley, where several important findings were made that redefined previously existing data [13]. A piezoelectric viscometer was constructed and employed to measure the viscosity of ZBLAN glass, this new and more precise method of interpolation of the temperature-viscosity gap yielded a set temperature range for optimal fibre drawing 20 °C lower than the range predicted by previously published data.

This is significant because it indicates that the degree to which the onset of crystallization must be suppressed is not as great as previously thought. Dunkley also provided a critical analysis of the many different theories explaining why devitrification is suppressed when ZBLAN glass is prepared in a microgravity environment [68]. An interesting hypothesis states that in a micro-gravity environment, the rate of diffusion would be lower than in comparison to unit gravity, even with an identical level of viscosity. This would mean the ZBLAN specimen is likely to remain diffusion limited and remain in the amorphous state to a greater temperature range.

A paper that delves further into an important fundamental property of ZBLAN is by L.G.

Hwa et al [14], which examines the elastic properties of the glass. The hydrostatic and uni- axial pressures, and the temperature-dependence of the elastic properties were determined by ultrasonic pulse-echo techniques. Their experimental results gave a complete set of the

components of the second order and third order elastic stiffness tensors for the glass.

Determining the temperature dependence of the viscosity of heavy metal fluoride glasses is vital for finding the most suitable processing temperatures for fibre drawing and understanding the crystallization behaviour in general. On top of that, various other material properties were determined including density, longitudinal and transverse sound velocities,

Young’s modulus, bulk modulus and Poisson's ratio.

2.3. Crystallization

Due to ZBLAN being a multi-component system, it has a higher probability of containing impurities, and therefore a higher level of heterogeneous nucleation sites. This may account for the formation of crystallites even at relatively “high” cooling rates (in the order of

102 ⁰C/min). Also, the complex nature of the multi-component system can tend towards phase precipitation of each component. This could be a possible explanation why ZBLAN has a high tendency to devitrify, there is a tendency for the system components to precipitate out and form crystal phases.

The transmission limitations of this material are due to the fact that ZBLAN readily crystallises, thus degrading the attenuation properties of the material [15]. The primary mechanism that drives crystallization during glass melting or the casting of the starting materials of ZBLAN is the presence of convection cells within the compound, which in turn are driven by gravity [16]. These convection cells cause the molecules within the compound to adhere with neighbouring molecules and thereby begin a process of nucleation and then crystal growth [17]. The fact that the glass transition temperature (260 ⁰C) of ZBLAN is close to that of the compound's crystallization temperature (335⁰C) makes thermal processing of

the glass without inducing crystallization incredibly challenging [15]. When the preform is heated prior to being drawn into a strand, it is difficult to maintain the compound at a temperature low enough that nucleation and subsequent crystal growth does not occur [17].

Other factors contributing to the formation of crystals can be explained by the contact between the ZBLAN material and the container walls it is processed in [15, 13]. At this interface, nucleation is more easily facilitated due to the lowering of surface energy. Previously, investigations have been undertaken to reduce these effects, including the use of gas levitation to eliminate the need of a container to hold the sample, as was done in a study conducted by

Castillo et al [18]. Through containerless processing of a ZBLAN test sample, it was intended to remove the formation of container nucleation sites, which was a proposed reason for the stress induced devitrification that resulted in crystallization. However, the majority of studies that have attempted containerless processing techniques have only yielded inconclusive results or data that have yet to provide any major developments in the goal to achieving ultra- low attenuation losses.

In a study by L. Battezatti et al, it was determined that for the crystalline structure of ZBLAN, the predominant phase is β-BaZrF6, also present is either β-Na2ZrF6 or Na7Zr6F3 [19]. In another study by S.F. Carter, a comprehensive list of phases were determined from crystallized samples including; NaZrF5, Na7Zr6F31, α-BaZr2F10, β-BaZr2F10, NaBaZr2F11, LaZr2F11 and a disordered form of β-BaZrF6 [20]. Carter also determined that the first phase to appear on cooling is heterogeneously nucleated LaF3. This is confirmed in another study by Li et al [2], where it was determined that the predominant micro-crystallite is LaF3. Li also stipulated that ionic bonding is the cause for glass instability and the tendency to devitrify.

Another source of heterogeneous nucleation occurs from impurities within the ZBLAN matrix that help promote nucleation and subsequent crystal growth. In a study by Moore et al. [73], it was shown that ZrO2 impurities acted as heterogeneous nucleation sites at the surface of the

ZBLAN material. They were also able to determine the surface crystal growth rates for several varieties of ZBLAN glasses, and found that the rates could be reduced by exposure of the glasses to a 1% F2/Ar atmosphere.

The study by Li et al also raised some interesting points in regards to the defects they found in ZBLAN glass, they were able to characterise the sources that cause signal scattering in the medium. The ZBLAN test sample was in their study cooled from 600 ⁰C to a mould that was already preheated to 270 ⁰C, and then subsequently allowed to anneal to room temperature.

They determined that in addition to LaF3, ZrF4 was also another predominant crystallite species found in ZBLAN. Along with microbubbles (ranging from approximately 2 μm to

8 μm in size) in the medium, these two crystallites were the main factors attributed as the primary sources of scattering. The LaF3 crystallites, with their typical hexagonal configuration, were more prolific in general, averaging at a size of 5 μm. It was stipulated that the LaF3 crystallites precipitated out due to inhomogenous mixing of the melt. This inhomogeneous mixing leads to local enrichment, which can be solved either by using less LaF3 or by melting

ZBLAN to a higher temperature (however this can cause selective vaporization of components).

There are papers published by J. Sestak that delve into crystallization kinetics and shed further light into the transition processes of glasses. In one study, phenomenological kinetics and the enthalpy versus temperature diagram were used (with its DTA derivative) to illustrate relaxation and glass transformation processes [21]. Thermophysical bases of vitrification and crystallization were discussed in terms of the enthalpy versus temperature diagram, where

particular focus was placed on examining the crystallization of amorphous states to form stable and metastable phases. The process focused on was the sequence of relaxation- nucleation-growth processes associated with the transition of a non-crystalline state to a crystalline one. These processes are accompanied by a change of enthalpy, which is detectable by thermometric measurements. An entire section is dedicated to showing easy ways of graphically illustrating these processes.

Furthermore, in another study by J. Sestak, an investigation of the crystallization kinetics of glasses has provided more insight into diffusion mechanisms and how they could potentially be a major driving factor in the process of crystallization [22]. Using Mn ferrite in glasses based on B2O3, two simple methods of evaluating kinetic data from a DTA run were investigated and compared. The activation energy was analysed for nucleation, crystal growth and diffusion, where the high energy value (above 100 kcal/mole) was found to be due to diffusion.

It was stipulated that the growth of nuclei was indeed controlled by diffusion, and this is the limiting factor since the nuclei themselves were already present in the quenched glass. What is important to note is all of these crystallization phenomena, from the formation of stable phases and diffusion mechanisms, can be influenced by particular processing methods such as cooling rate.

2.3. Preform Fabrication

There are a few conventional methods that have been used in the past to process ZBLAN from its starting compounds. The methods described here are taken from studies published in the fabrication of what is referred to as a ‘preform’, which is a bulk form of ZBLAN made by

melting all of the starting components under particular processing conditions and then casting them into a workable piece that is later drawn into a fibre.

The key to obtaining a bulk form of ZBLAN that can yield the purest fibres is to make certain all of the starting materials are extremely pure. Any inhomogeneities in the materials, especially in the form of contaminants, or impurities, can lead to heterogeneous nucleation or affect the material properties resulting in a higher attenuation loss. It is recommended that impurity levels only be a few ppb to make low loss fibres [27]. Typical purification techniques of the raw materials include ion exchange or sublimation to reduce transition metals and lanthanide impurities. Due to the material’s reactivity with moisture, most studies conducted with ZBLAN from its starting materials commonly require processing and handling of all related compounds in a dry, inert environment i.e. in a glovebox.

To melt all of the starting materials, it is preferable to melt the compounds in a platinum/gold crucible, and in certain cases vitreous carbon has been used (however, it cannot be used in air). Noble metals should be avoided as a crucible material since it causes scattering centres

[27]. In a study by J.M. Parker [27], ZBLAN glasses were prepared from pure fluorides at

850 ⁰C, to allow rapid solution of the more refractory components. However, care must be taken when processing the starting materials at high temperatures because ZrF4 itself sublimes at 600 ⁰C. In a study by J. Mathew and R. Doremus [42], it was determined that as soon as

ZBLAN reaches its melting point at approximately 450 ⁰C, there is already evidence of ZrF4 off-gassing. Fluoride gas is also released as low as 400 ⁰C, with off gassing levels increasing rapidly above the melting point.

The process of melting within the crucible itself is a challenge, as different melting regimes combined with different casting regimes can produce ZBLAN preforms of greatly varying

quality and material properties. Melts in crucibles experience convection (since the viscosity is less than one poise), which leads to variations in refractive index. It has been suggested that one method of melting/casting is to utilize rotational casting techniques to eliminate this problem. One way to control the refractive index profile is to dope the material that makes the core fibre with PdF2 or cladding with HfF4 or by replacing BaF2 with NaF [27].

In the study by S.F. Carter, ZBLAN Glass was processed according to a regime that is quite conventional and typical to many studies and manufacturing processes currently used to fabricate the preform. The starting materials were melted in a platinum/5% gold crucible under dry atmosphere (<5ppm H2O) according to the following regime; within 0 - 45 minutes the starting materials were heated to 400 ⁰C, then between the 45 – 180 minute mark, the melt was taken up to 800 ⁰C, and then between 180 - 225 minutes taken to a maximum temperature of 670 ⁰C. Then after the 225 minute mark, the perform was cast [20]. The melt was cast into a brass mould that was preheated close to the glass transition temperature to yield, after a period of annealing, a cylindrical rod 12 x 1 cm. Typically, resistance furnaces or any resistive heating mechanism is used to heat the starting materials [27].

In the study by Parker [27], Fluoro-indate glasses were processed in a method analogous to processing ZBLAN glass. Other potential heating methods suggested in the study include Rf induction heating or single crystal making furnaces. Ammonium bifluoride (NH4HF2) was used in the processing of the glass to reduce O2- and OH impurity levels, which has been proven to be exceptionally effective and commonly utilized in the processing of many fluoride glasses. However, it can leave black specs of reduced zirconium.

Casting the preform itself is the next challenge in the production process, where the main concern is to be able to cool the melt uniformly and without the creation of microbubbles in

the medium. It has been determined that for a 20 mm diameter preform, a ZBLAN melt has a maximum cooling rate of 3 ⁰C/s (theoretically estimated), which is determined by the temperature difference between the melt in contact with the mould wall and the centre of the melt [27]. Therefore the geometry of a cooled piece of ZBLAN affects the cooling rate experienced (for thinner geometries there is a more uniform cooling rate, for thicker geometries the central regions will cool more slowly). To avoid the formation of microbubbles, it is important that laminar flow is maintained when the preform is cast to allow gas bubbles to escape. Large volume contraction on cooling can also cause vacuum bubbles.

There are variations of the processing methods described previously, with different heating regimes, processing gas environments or different crucible materials being used. Different processing parameters have yielded different results. In a study by J. Lucas [60], BIGaZybT samples were successfully heated and cooled in a vitreous carbon container, with thicknesses of a few cm. In another study by J. Bei et al [61], rotational and suction casting methods were successfully utilized for fluoride glass preform fabrication, where it was then suggested to utilize billet extrusion or stacked extrusion for fibre drawing. In another study by C.R. Day et al [1], it has been recommended to use an ‘upset casting’ technique or rotational casting, which is a quite common method for processing fluoro-indate glasses.

In the study by J. Bei [61], batch/bulk glass was melted at 900 ⁰C for 3 hours, in a dry N2 atmosphere, where 30 - 70 g sized batches were prepared in a platinum/5% gold crucible.

Ammonium bifluoride (NH4HF2, 6.7 wt% of batch weight) was used to improve the quality of the preforms, which were cast into a pre-heated mould. In addition to ammonium fluoride, samples were treated in SF6, however the ammonium bifluoride treated samples yielded the highest quality preforms.

As described previously, heating for extended periods of time up to 900 ⁰C is quite a typical part of the heating regime of fluoride glasses. However, it has been recommended to limit the temperature and duration of heating, as this can lead to the volatilization of many components of the glass itself. As was discussed earlier in the study by J. Mathew and R. Doremus [42], fluorine gas and zirconium tetrafluoride gas already begins to evolve from ZBLAN glass at temperatures not much higher than its glass transition point.

In another study of note, Eley et al [34] processed ZBLAN glass into the form of microspheres, which were heated to 950 K by placing heated platinum coils just at the right distance. They observed that hydrous ZrF4 was converted to ZrOF2 and ZrO2 upon heating on air, these oxidation reactions creating impurities. To remove moisture, fibres were heated in a vacuum oven then stored in vacuum containers. There were plans made to place the entire fabrication set up in a glove box that limits water concentration to 1 ppm, to ensure a dry atmosphere to work in.

2.4. Fibre Drawing

The process of fibre drawing itself is possibly the biggest challenge as it can easily induce crystallites when heating the preform in the fibre-drawing temperature range. This is a major hindrance in that developing a process that can maintain the purity of the ZBLAN glass preform can become quite convoluted and much more difficult than drawing silica fibres.

It has been recommended that the viscosity ideal for traditional preform drawing is 104 Pas, and for crucible drawing it is 102 Pas [1]. A potential method that could be utilized is ‘reactive vapour transport’ (RVT), unfortunately typical modified chemical vapour deposition methods (commonly used for manufacturing silica fibres) cannot be used for ZBLAN.

Direct fibre drawing from the melt is not easy, as crystallization occurs readily. As a result the processing temperatures implemented for drawing the fibre must be sufficiently below the crystallization onset temperature. The purity of the sample is strongly dependent on the control of the furnace temperature, therefore the maximum recommended temperature for processing a ZBLAN preform is 310 ⁰C for a maximum of 5 minutes, otherwise heavy crystallization can occur [20]. Again, if the fibre drawing temperature is too high, not only can crystallization occur, but selective vaporization of the components can occur as well, particularly with fluorine off-gassing.

In the study by J. Bei [61], fluoro-indate glasses were successfully drawn into fibres with minimal crystallization and other defects. It has been recommended to clean the preform by submerging it in an isopropyl alcohol ultrasonic bath for 20 minutes prior to fibre drawing.

Graphite was used as a die material to extrude the preform into a fibre, and the preform was processed 70 ⁰C below the crystallization temperature, which is still above the glass transition point. The preform was also polished with Al2O3 powders, and then extruded at 310 ⁰C, 20 degrees above the glass transition point.

The extrusion process was then conducted at various temperatures, 317 ⁰C, 322 ⁰C and 330 ⁰C, requiring at least 8 hours to do so. For higher viscosity levels in such a process, a higher force was required to extrude the fibre (approximately 35 KN), which can cause the die to crack. 10

KN was required to extrude the fibre at 322 ⁰C and 7 - 8 KN at 330 ⁰C, which neither caused the die to crack. At 330 ⁰C however, the surface of the fibre can tend to crystallize, which suggests that even though the material is processed under its crystallization point, it can still devitrify if there is a long processing time under that temperature.

In a study by West & Hoffle [72], they also observed that intrinsic crystallisation occurs at each process stage during the manufacturing of fluoride glass fibers. The attenuation due to crystallisation was calculated using classical scattering theory, where the temperature of the core melt and subsequent fibre drawing conditions were strongly dependent on temperature.

They deduced that it is easier for melt casting to be temperature controlled to mitigate crystallization, however during the process of creating a fiber, it was found to be necessary to maximise the fibre tension and speed to reduce attenuation losses induced to a level below that of Rayleigh scattering.

To coat the fibres, polymeric coatings can be applied to ZBLAN fibres much in the same fashion as silica fibres. Hermetic coatings are essential to reduce attack by water, because

ZBLAN is highly sensitive to water contamination over time [27]. If decladding of fibres is necessary, to prevent water absorption and/or formation of an oxide layer on the fibre surface, use of plasma etching using an air ionized fluoride gas is recommended [34].

For less stable glasses, rapid quenching has been suggested by I.D. Aggarwal and G. Lu [62], similar to typical methods for rapid cooling of metals as mentioned in section 2.5. These methods would not result in a preform, but instead would require these rapidly cooled fragments of glass to be processed into a continuous length of fibre.

2.5. Rapid Cooling

The cooling rate for a material being undercooled from its melt is an important factor in processing, as the rate and the regime (whether or not it involves such practices as annealing) can significantly change the inherent material properties. The general expectation in the goal of achieving an amorphous state in any material, is the faster the cooling rate, the greater the

impedance of crystal growth will be. The minimum cooling rate required to completely impede the growth of any crystal phase in a material is termed the ‘critical cooling rate’.

For many glasses, and ZBLAN in particular, the critical cooling rate has been determined by referencing points on a T-T-T (Temperature Time Transformation) diagram, which in turn have been generated from DSC data. In a study by A. Janke and G.H. Frischat [23], a critical cooling rate to impede crystal growth in ZBLAN was achieved in this fashion. The rate was inferred by comparing the “nose” in the T-T-T diagram, or the outermost point of the boundary defining the change from an amorphous to crystalline state (refer to Figure 2).

Figure 2. TTT diagram of AgF doped heavy metal fluoride glass. Lines are drawn as guides for the eye, amorphous state is to the left side of the lines, and the crystalline state is to the right [23]

Based on the T-T-T diagram in Figure 2, the critical cooling rate, ΔT/Δt, was determined by

௱் ሺ்௠ି்௡ሻ (2.1) ൌ , ௱௧ ௧௡

where Tm is the liquidus temperature (taken as 450 ⁰C), Tn is the nose temperature and tn is the time required to reach the “nose” temperature. The critical cooling rate was calculated to be 240 K/min, when melted in a crucible.

Figure 3. Typical T-T-T diagram showing CR, the critical cooling path, as computed with Eq. (2.1), and A, an arbitrary path with sub-critical rate [24]

Other studies have yielded different critical cooling rates, depending on different factors and processing regimes. In a study by Mitachi and Tick [25], a critical cooling rate of 3 - 15 ⁰C/min was calculated. In a study by F. Smektala and M. Matecki [26], using data from a DTA, the critical cooling rate for undoped ZBLAN was calculated to be 2.5 - 3.5 ⁰C/min. In another study by L. Busse et al [4], the critical cooling rate was determined to be 8 ⁰C/min. In a study by

Parker [27], the observed nose on a T-T-T plot was found to be at 380 ⁰C, which yielded an estimated critical cooling rate of 1.6 ⁰C/min. It was suggested in this study, that a cooling rate of 0.2 ⁰C/s is needed, which limits the thickness of any cast sample to be 25 mm to ensure even cooling.

Overall, a range of critical cooling rates between 2.5 - 240 ⁰C/min have been quoted by various studies. The 240 ⁰C/min cooling rate is in an order of magnitude much smaller compared to critical cooling rates of many other materials such as polymers or metals. There have also been other studies that have shown certain compositions of silicate glasses can have much higher cooling rates with an order of magnitude greater than this. In a study by Havermans et al.

[70], they determined the critical cooling rates of several glasses in an alkali silicate systems, some of which had extraordinarily high critical cooling rates depending on their composition.

Many materials have been processed by cooling much greater than their prescribed critical cooling rates. There are a number of currently popular rapid cooling techniques designed to process many different materials from a liquid to solid state, in particular to improve the glassy/amorphous structure in the material. The main premise of utilizing such techniques is to produce non-equilibrium effects in a cooling solid to limit the amount of transformation.

There are three main categories; the first is quenching, which involves changing from one set of conditions of temperature and pressure to another, rapidly enough to limit the amount of transformation to new molecular structural conditions [28]. It includes conventional quenching through the liquid to solid phase transformation entirely to the solid state. The second category includes molecular deposition, which involves a deposited layer of a material through thermal evaporation, sputtering or chemical reaction, or from salt solution by electro- less displacement or by electrolysis. The third category includes external action, methods such as deformation, irradiation or chemical attack.

The appropriate category of techniques for this particular project, and the most practical for the preparation of glassy materials, is the first category, quenching. It is a widely used and simplest technique because the preparation and handling procedures are relatively easy and

fast. The first metallic glass prepared by quenching from a melt was reported as recently as

1960, where a glass alloy of Au75Si25 was quenched using the so-called “splat-quenching” or

“gun” technique [29]. The quenching rate was estimated to be as high as 106 K/s, which was achieved by spreading liquefied metal into thin layers (approximately 10 μm) on thermally conducting substrates such as metal or sapphire. There have been extensive developments into the various ways of processing metallic glasses into wires and sheets, using the surface area of a rotating drum [29]. These melt-spinning techniques are quite common place and capable of high production rates with ribbons of considerable size and length.

The key advantage of processing materials with this method is the high cooling rate, which promotes sufficient supercooling to allow formation of new non-equilibrium phases. Such techniques are employed in the processing of metallic glasses to inhibit the rate of crystal growth. The main reason for this is the rate of crystal growth is strongly dependent on the amount of supercooling and the rate of cooling, which produces a high enough solidification front velocity to exceed the expected rate of crystal growth [28].

One study conducted by N. Mattern [30], delves into an experimental investigation on the structural behaviour of metallic glasses and the close relationship between the structure and corresponding under-cooled melts. Ribbons of a Ni-Nb-Y alloy were prepared by means of rapid quenching from the melt using a single-roller melt-spinner under argon atmosphere.

Electron diffraction patterns were analysed, confirming the amorphous structure of the cooled sample. The examined microstructure revealed that the heterogeneous two-phase metallic glass corresponds to the frozen-in structure of the rapidly quenched melt. This indicates that with a fast enough cooling rate, the composition and structure of a material in its molten state can be preserved as the material solidifies. This is the key to retaining the vitreous nature of a

glass forming material when it is rapidly quenched from its molten state, a state when it is fully amorphous/vitreous.

In another interesting study [31], Sestak discusses thermal annealing during processing of metallic glasses. This paper covers methods of rapid cooling techniques that are commonly employed for the acceleration of cooling rates when dealing with metallic glasses. Types of relaxation processes induced by thermal treatment were also analysed and flash annealing by heat pulses was discussed. The use of possible T-T-T diagrams were shown for some characteristic cases of temperature-controlled crystallization using possible courses of thermal treatment. One of particular interest is a three stage cooling method that employs the rapid cooling of the sample through heat conduction into a heat sink.

Faster cooling rates impede nucleation because “molecules need sufficient time to organize and align with their neighbours to form stable nuclei,” as quoted in a study by S. Martini et al

[67]. In this study, the effect of cooling rate was investigated in relation to the nucleation behaviour of milk fat-sunflower oil blends. The initial crystals were photographed, their thermal and polymorphic behaviours, and chemical composition were characterized using an

array of analysis techniques including calorimetry, x-ray diffraction and capillary gas chromatography. They observed that “in rapidly cooled systems, molecular organization took place at a lower temperature than in slowly cooled systems, in which molecular organization took place as the sample was cooling to TC (set low temperature point).” It was observed that for a large undercooling interval, cooling rate didn’t have a great effect, but at a smaller interval, a faster cooling rate significantly impeded crystallization.

The findings of this study suggest, and as stipulated in various other studies in relation to the impediment of the crystallization process [22], that crystal growth and nucleation are primarily impeded when molecular diffusion is restricted. The mobility and the time required for the molecules to arrange into their crystal lattices are directly affected by the cooling rate the material is subjected to.

2.6. Characterization of Attenuation Losses

The attenuation loss characteristics covered primarily in literature relate to intrinsic loss mechanisms. These loss mechanisms are related to the innate properties of the chemical composition of the fibre itself. The intrinsic scattering sources include Rayleigh, Raman and

Brillouin scattering as the signal wave interacts with the molecular particles of the material.

Intrinsic absorption phenomena is due to a variety of structural aspects such as electronic transitions and multi-phonon absorption.

In a study by L.G. Hwa and C.K. Shu [32], a structural investigation of ZBLAN was carried out by vibrational spectroscopy with regards to multi-phonon edge absorption, infrared reflectivity and polarized Raman scattering. From this study, a better understanding of the fundamental vibrational characteristics of the glass was determined. The IR edge was found

to occur at 6~7 μm, and was determined through the average optical phonon fundamental frequency, 528.6 cm-1, which was taken from reflectivity measurements. It was also determined that this type of glass shows two-to-three phonon absorption behaviour.

However, the position of the IR edge is dependent on the thickness or length of a test sample.

This has been shown in a paper by Kubat et al. [71], where ZBLAN fibers in the lengths of meters (sourced from the manufacturer FiberLabs) had an IR edge that occurred at 4.5 μm.

All of this provides a clearer view of the primary mechanisms influencing the infrared transparency and the structural factors that contribute to its low attenuation losses.

The most extensive empirical measurements of intrinsic scattering in fluorozirconate glasses were made by Shroeder et al [33]. This was done by examining the power scattered at 90 degrees from the beam of an argon ion laser operating at 488 nm passing through a ZBLAN fibre sample.

The overall intrinsic loss in a ZBLAN glass fibre was determined by Eq. (2.2),

଴Ǥ଻ଶ ି଻ଵǤ଺ସ ߙ ൌ ൅ܥቀ ቁ݀ܤȀ݇݉, (2.2) ூே ఒర ఒ where αIN is the total intrinsic loss, C is a material constant and λ is the wavelength.

For the multi-phonon aspect of the V-curve, a complete determination of the IR edge was made by combining data on bulk glasses with results from optical fibres. As a result, Eq. (2.2) defined a V-curve that was consistent with theoretical estimates for fluorozirconate glasses

(for more information on how the equation parameters were determined in the study by

Schroeder et al [33], and loss mechanisms in general, see Appendix B).

There are many reasons that have been stipulated as the cause for the attenuation losses in fibres, not only due to intrinsic loss mechanisms but also extrinsic loss factors in the material.

Crystallite formation in the medium act as primary scattering centres, along with other impurities and inhomogeneity (refractive and material disparities) in the bulk medium. In a study by J. Lucas, absorbing impurities were attributed to contaminants such as first row transition metal ions, lanthanide ions, OH-, complex anions, NH4+ and dissolved gases [20]. In another study by Eley et al [34], it was stated that at 2.9 μm, absorption losses were primarily due to the fundamental OH- mode.

As previously mentioned, the theoretically predicted minimum attenuation loss for ZBLAN glass is 0.01 dB/km, where the probable achievable absorption loss at 2.55 μm has been estimated to be 0.04 dB/km [1]. This represents a five-fold improvement, presently for long- haul telecommunication cables repeaters are set 80 - 100 km apart, but with this improvement they could be set 500 km apart. In a paper by J.M. Parker [27], it has been stipulated that if the absolute theoretical minimum were achieved, there would be no need to boost a signal for over a 1000 km, therefore it could be possible to have only a dozen repeaters to send a signal intercontinentally. To date however, only losses of 0.7 dB/km at 2.55 μm for short ZBLAN fibres have been achieved [27].

2.7. Literature Review Summary & Remarks

ZBLAN of the following composition 53% ZrF4, 20% BaF2, 4% LaF3, 3% AlF3 and 20% NaF has been suggested as the most stable for glass formation. Extrinsic losses are the main factor attributed to why ZBLAN fibres currently have not reached their predicted attenuation loss capabilities, due to crystallite and micro-bubble scattering centres. The lowest probable achievable attenuation loss is 0.04 dB/km at 2.55 μm wavelength.

The temperature range for fibre drawing must be strictly controlled and sufficiently below the crystallization onset to prevent surface crystallization. Even when processed beneath the crystallization onset temperature, fluoride glasses are susceptible to time dependent crystallization phenomena, therefore the time required for fibre-drawing processes needs to be kept to a minimum. Most crystallization occurs at the surface during fibre-drawing.

The theoretical estimations quote a relatively low required critical cooling rate to bypass all crystallization phenomena (between 2.5 - 240 ⁰C/min). There is a possibility that although crystallization events detected in the DSC curves suggest a low critical cooling rate, the formation of crystallites on a micro and nano-scale may go undetected. Currently manufactured ZBLAN preforms have a high enough density of crystallites to cause scattering that does not yield “pure” enough fibres.

LaF3 particles are the first predominant species of crystallites that form upon cooling from the melt, and it has been stipulated that they precipitate out due to inhomogenous mixing of the melt. In regards to the suppression of crystallite formation through manipulating the cooling process, a higher cooling rate has been shown to decrease growth and the impediment of nucleation. This has been evident in studies with various materials including glass forming metal alloys (metallic glasses) and milk fat-sun flower oil blends.

It has been suggested that molecular diffusion is the main factor being hindered when these materials are processed with a rapid cooling rate. Several methods have been presented to achieve rapid cooling rates, with quenching techniques deemed most suitable for this project.

CHAPTER 3: Methods and Research Plan

3.1. Methodology

The experiments for this project were organized using a test matrix that coordinates various heating/cooling regimes (using multiple processing methods) with various analysis techniques, with the ultimate goal of investigating what critical cooling rate can fully suppress crystallization. The processed samples were analysed with specific microscopy and spectrometry techniques to both qualitatively and quantitatively evaluate the microstructure of the glass samples. The main approach was to characterise how amorphous/crystalline the processed samples were with respect to increasing cooling rates. Then once test samples were completely free of any crystal content, after being subjected to a critical cooling rate, the attenuation loss of these fully amorphous test samples were characterised.

3.1.1. Thermal Treatment Profile

When ZBLAN is thermally processed, its material properties are influenced by the heating and cooling rates. This suggests that (similar to processing many different types of alloy steels) developing a particular thermal treatment regime can significantly alter many properties of the glass. Therefore by manipulating these rates, it should reveal the relevant aspects of the processing regime which will result in the improvement of a particular material property.

The following points in Figure 5 are the different processing parameters that a typical heating and cooling profile is comprised of, and forms the basis of the thermal treatment regimens taken to conduct these experiments.

Figure 5. Graph showing temperature curve of various processing regimes (Tm, Tx and Tg were averaged for ZBLAN, based on values taken from literature [12]), cooling rate of 100 ⁰C/min in blue, cooling rate of 900 ⁰C/min in orange and cooling rate of 4000 ⁰C/min in red

1. Heating Profile/Maximum Temperature- The section in red encompasses the heating

curve of the process. This relates to the rate at which the sample is heated, and is taken

as the temperature difference between T2 (room temperature), and T1 (maximum

temperature) divided by the time difference between both points (time interval

denoted by ht). T1 is a temperature above the melting point and renders the test

sample to its molten state.

2. Dwell Time- The duration (denoted by dt) in which the sample is held at its maximum

temperature, T1, above the melting point.

3. Cooling Profile- The section in blue relates to the cooling curve of the process. T2 is

the undercooled temperature and must be below the crystallization temperature in

order to supress crystallization. For this experiment, T2 is room temperature, all

samples were cooled to the ambient temperature of the laboratory.

3.1.2. Test Matrix

The test matrix in Table 1 outlines the heating and cooling regimes used to process the samples:

Table 1. Experimental Test Matrix

Thermal Heating Maximum Method Cooling Method Analysis Profile Rate Temperature of Rate of Techniques* (⁰C/min) (⁰C) Heating (⁰C/min) Cooling

1 20 500 DSC 5 DSC XRD, SEM, Heated Cooled EDS 2 20 500 DSC 20 DSC XRD, SEM, Heated Cooled EDS 3 20 500 Furnace 100 Air- XRD, SEM, Heated Cooled TEM, EDS

4 20 500 Furnace 900 Quenched XRD, SEM, Heated in soapy TEM, EDS water

5 20 500 REPD 4000 REPD XRD, SEM, Heated Cooled TEM, EDS, Ellipsometry 6 20 500 Furnace 8000 Quenched SEM, TEM, Heated in water EDS, Ellipsometry

*Analysis techniques acronyms: XRD- X-ray Diffraction, SEM- Scanning Electron Microscopy, TEM- Transmission Electron Microscopy, EDS- Energy Dispersive Spectroscopy, REPD- Rapid Electro-thermal Processing Device

The heating rate, maximum temperature and dwell time were kept the same for all test samples, since they were control parameters, while the cooling rate was a changing variable because it was the primary objective of this study. A heating rate of 20 ⁰C/min was applied for all samples, since the recorded temperature for the glass transition temperature (Tg) was determined in a DSC using the same heating rate [12]. The dwell time was kept to a minimum to reduce the amount of fluorine and ZrF4 off-gassing. As soon as the maximum target 31

temperature of 500 ⁰C was reached (T1 in Fig. 5), the samples were cooled immediately. A maximum temperature only slightly above the melting temperature of 452 ⁰C [12] for ZBLAN was chosen because it has been shown that the amount of ZrF4 that off-gasses increases as the temperature increases above the melting point [42].

The test samples were prepared by taking fragments from a bulk piece of ZBLAN material.

These fragments were then ground and placed in a platinum crucible (typically used for DSC applications) with dimensions 5mm dia x 8 mm. Each sample was measured to 130 mg (this amount filled the crucible midway).

The samples were heated in various ways, depending on which cooling method was used. In total, there were six different cooling rates investigated, ranging from 5 ⁰C/min for the slowest rate to 8000 ⁰C/min for the fastest. The 5 ⁰C/min and 20 ⁰C/min cooling rates required a controlled heating environment, therefore these test samples were processed entirely in a DSC

(Differential Scanning Calorimeter), which could be cooled at a slow and steady rate. The majority of the other test samples were heated in a conventional furnace, and then either air cooled or quenched in different media to achieve the required cooling rates. Lastly, test samples were processed in a device that was given the name “REPD” (Rapid Electro Thermal

Processing Device, see Appendix C for more information), which was built to rapidly heat and cool the samples. All of the test samples were cooled to just above room temperature. The temperatures were all recorded using a typical K-type thermocouple purchased from Jaycar™ electronics in conjunction with an Omron™ data logger.

All cooling rates, CR, were taken as

்௠ି்௚ ܥܴ ൌ , (3.1) ௱௧೘ష೒ where Tm is the melting temperature, 450 ⁰C, Tg is the glass transition temperature, 260 ⁰C, and Δtm-g is the time interval it took for the sample to cool between the two temperatures (in seconds). The error in this method of measuring the cooling rates in regards to the accuracy of the measuring equipment and the repeatability of the experiments is covered in detail in section 5.7. Once cooled, the ZBLAN test sample solidified into a cylinder approximately 4 x 4 mm as a single solid piece. The sample was extracted from the crucible by breaking apart the solid piece into shards using a pick and forceps. Only shards that were in contact with the crucible wall were selected for analysis, this was to ensure the extracted sample fragments would have been cooled as close to the measured cooling rate as possible (shards from the centre of the melt would have cooled more slowly).

For the characterization of the structure and atomic composition of the glass, XRD (X-ray

Diffraction), EDS (Energy Dispersive Spectroscopy) and TEM (Transmission Electron

Microscopy) analysis were used. For characterizing the crystallites in the micro sized region

(and larger), EDS and XRD analysis were useful, since the peaks in the spectra and diffractograms can be matched with various crystal phases. For crystallites in the sub-micron range, especially the nano-sized particles (as observed in the test samples of the faster cooling rates), TEM was particularly useful. The diffraction patterns were useful to identify the composition of the nano and micro-crystalline phases.

SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) imaging techniques were used to image the crystals themselves, where using backscatter imaging

(SEM) and dark field/bright field imaging (TEM) allowed for easy identification of the different crystal phases and individual crystallites.

The x-ray diffractograms were primarily used to observe the change in degree of crystal/amorphous content in each sample. Ellipsometry was used to examine the fully amorphous test samples to determine their optical constants and associated attenuation losses, at wavelength range 240 nm (Middle UV) to 1700 nm (near infrared).

CHAPTER 4: Characterization of Experimental Equipment

This chapter covers in detail the various equipment and materials used in the experimental procedure, and also the analysis techniques required to examine the test samples.

4.1. ZBLAN Glass

The ZBLAN material used in this experiment was provided courtesy of THORLABS, an optical equipment company based in New Jersey, United States of America. The chemical composition of the material is outlined in Table 2, which was determined using Energy-

Dispersive Spectroscopy (EDS).

Table 2. Chemical Composition of ZBLAN as per EDS Analysis

Component Formula Composition (mol%) Composition (Wt%)

Zirconium Tetrafluoride ZrF4 52 54

Barium Difluoride BaF2 21.12 33

Lanthanum Trifluoride LaF3 4.43 7

Aluminium Trifluoride AlF3 3.25 1

Sodium Fluoride NaF 19.1 5

4.2. Heating Equipment

Various equipment were used to heat the ZBLAN material, and were chosen on how effectively they could control the heating and cooling profile of the test samples for each of the different cooling rates.

4.2.1. SEM (SA) Furnace

The SEM (SA) furnace is a typical resistive heating furnace that utilizes an electric element, and is lined with lightweight ceramic fibre insulation. It was used to heat the majority of the test samples, which upon reaching the required maximum temperature were taken out of the heating chamber and quenched using different mediums to achieve the various cooling rates.

The furnace is programmable with ramping time (dictates heating rate) and target temperature, with a relatively high accuracy (confirmed with an external K-type thermocouple which showed readings that closely correlated with the set target temperature of the furnace itself).

Figure 6. Photograph of SEM (SA) Furnace

4.2.2. Setaram Labsys DSC

A Setaram Labsys DSC (Differential Scanning Calorimeter) was used to heat the test samples and to provide controlled cooling at the much slower rates of 5 ⁰C/min and 20 ⁰C/min. The housing structure of the DSC is built around a metal element heated furnace with a temperature range that extends as high as 1600°C. The equipment has plate type DSC rods that provide useful quantitative measurements with low variation in sensitivity, and is comprised of a metal frame with two housings for the crucibles.

The following images in Figures 7 and 8 illustrate the appearance, schematics and overall layout of the Labsys Setaram.

Figure 7. (Left) Photograph of Setaram DSC, (right) layout of DSC rod

Figure 8. Cross-sectional layout of the Setaram DSC

4.2.3. Rapid Electro-Thermal Processing Device (REPD)

The Rapid Electro-Thermal Processing Device (REPD) is a heating device that was built and developed by an undergraduate mechatronics student, Ben Fogarty. This device is a compact heating and cooling unit capable of processing the test samples at extremely fast rates (in the order of 1000s of ⁰C/min), without the need for an inert processing environment. It was used to process the test samples with a cooling rate of 4000 ⁰C/min.

4.2.3.1. Heating Design

This device operates on a transformer that is rated at approximately 1100 - 1200 W (for the primary winding) as specified by the microwave model that it was salvaged from. In practical reality, the power rating is more likely to be 600 W to 800 W due to efficiency losses, and with 38

a modification to the secondary winding, this provides the rest of the device with a supply of

2 volts.

A completed circuit is made when one terminal from the secondary winding is connected to

an aluminium frame (highlighted in green in Figure 9) and rod (highlighted in blue in Figure

9), and a conductive crucible (made from platinum) is clamped in between (the crucible itself

closes the circuit).

Rod

Arduino/ Display Screen Connecting Crucible wires to transformer

Solid State Relay

Aluminium Frame Transformer Figure 9. Layout of REPD and connecting wires

Figure 10. REPD apparatus with the inside compartment visible, where there is an 800 W transformer that provides the current supply, which is modulated by a solid-state relay (see Appendix C for more information on the operation of the REPD)

As can be seen in Figure 9, the apparatus forms a closed circuit through the connection of the transformer to the aluminium rod (sections highlighted in blue), which is then connected to the crucible, and then to the aluminium frame (sections highlighted in green) that is connected back to the transformer. Due to the small volume of the crucible itself, the flow of current bottlenecks at this point, and cannot dispense the flow of current as effectively, resulting in massive power losses in the form of heat generation in the crucible. The crucible heats significantly faster than any other part of the apparatus (while the crucible itself can reach temperatures of a 1000 ⁰C, the rest of the apparatus only reaches a maximum of 60 ⁰C). This set up is capable of heating up a crucible at 2000 K/s, meanwhile the rest of the rod and frame construct remains cool because of the heat dissipation throughout the entire aluminium

framework. The entire construct itself acts as a huge heat sink and is capable of cooling the crucible at rates as high as 18,000 K/min.

Rod

Crucible

Mounting Block

Figure 11. Crucible in position, locked in between mounting/cooling block and rod

For a more detailed description of the operation of the REPD, see Appendix C.

4.3. Platinum Crucible

The ZBLAN test samples were processed directly in platinum crucibles. Borosilicate vials and other storage equipment were used to store the samples, but platinum crucibles were used for processing because of how chemically inert they are and their ability to conduct heat away from the sample. The platinum crucibles were used in all the various heating devices, and were stable under all the high operating temperatures. The only drawback with using platinum crucibles is although they did an excellent job of containing the test samples and not reacting with them, the crucibles were too ductile and malleable. This would result in the crucibles warping out of shape easily due to the mechanical stress applied when handling them. 41

For these experiments, the crucibles were supplied by CS Ceramic.

Specifications for CS Ceramic Platinum Crucibles:

Model number: CS-0001 Dimensions: 5 mm x 8 mm

Figure 12. Photograph of CS Ceramic platinum crucible

4.4. Bolt Container

To heat the crucible in the SEM (SA) furnace, a special container was required to seal in the argon atmosphere required for the ZBLAN to be processed. The DSC was capable of containing an inert environment, and running the samples in the REPD at rapid heating and cooling rates bypassed the need for an inert environment (when the REPD was processed at slower rates, this was done in a glovebox). The SEM furnace however operated in ambient air and was heated to high temperatures, therefore not only was an airtight container necessary to seal the argon gas within it, but also a container that could withstand the high temperatures.

The following design was made by Ralf Raud, a fellow PhD candidate. This container was dubbed a “Bolt Container” because it was essentially constructed out of bolt components. The 42

container was comprised of two hexagonal M6 bolts, one that acted as a base, the other as a lid. They both screwed into the bottom and top, respectively, of a hexagonal M6 coupling nut, which is a longer version of a typical hexagonal nut. The hexagonal bolts were sawn off in the threaded section to shorten the length of the entire bolt. When fastened to either end of the coupling nut, there was sufficient space inside of the coupling nut to house the platinum crucible holding the sample. The general assembly of the bolt container can be seen in Figure

13.

M6 Bolt

Crucible

M6 Coupling Nut

M6 Bolt

Figure 13. Screenshot of Solidworks™ (left) wire model of bolt container parts, (right) full assembly

Figure 14. Photograph of bolt container, (left) top and bottom hexagonal nuts removed, (right) assembly placed together

The entire assembly of the bolt container, when fastened tightly (with a wrench and held in a vice) was air tight; it was able to contain the argon gas while the whole construct was heated in a furnace.

Additionally, the container itself was surprisingly efficient at conducting heat away, due to the bolt components being made from zinc plated steel. When the entire construct was quenched in a large volume of water, cooling rates as high as 8000 ⁰C/min were achieved. The bolt container was also very robust and able to be reused in the furnace repeatedly without degrading.

4.5. Analysis Equipment

In this section, the analysis equipment used to examine the test samples are described with a brief overall summary of how they operate and the method/configuration in which they were used.

4.5.1. Imaging and Elemental Analysis

Qualitative examination of the various cooled test samples were imaged using both Scanning

Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques, both in the micrometre and nanometre scales respectively. In conjunction with imaging using the

SEM, Energy-Dispersive Spectroscopy (EDS) was used to determine the atomic makeup of the different crystal phases of the samples.

4.5.1.1. SEM/EDS

For the SEM and EDS analysis, a Zeiss Sigma FE-SEM was used because this piece of equipment had both capabilities.

Figure 15. Photograph of Zeiss Sigma FE-SEM

The Zeiss Sigma was used with the following configuration:

Accelerating Voltage: 15 kV

Working Distance: 8 mm

Specimen Tilt: 0⁰

Energy Range: 20 KeV

Energy per Channel: 10 KeV

Scanning Electron Microscopy (SEM)

In Scanning Electron Microscopy, an electron microscope generates images by focusing a beam of electrons onto a sample, where the interaction between the incident electrons and the sample is detected by an electron detector and used to construct a resulting image. It is particularly useful as a topographical tool that can scan a surface of a sample and generate

images based on the uppermost layer. Depending on the modality of the imaging requirements, either secondary electrons “reflected” from the sample or backscattered electrons are used to form two different types of images.

Images generated from the detection of secondary electrons (secondary imaging mode), which are caused by the incident electrons knocking an electron out of the test sample’s atomic shell, are used most frequently. The detection of these secondary electrons can provide high level detail and highly magnified images that would never be capturable in an ordinary light microscope. Images generated from backscattered electrons (backscattered imaging mode) provide a better indication of the variations in density of the sample, since the phenomena of backscattering is related to the collision of the incident electrons with the nuclei of the atoms in the sample.

Figure 16. Scanning Electron Microscope (SEM) image of a lower leaf surface (cucurbita maxima), showing stomates and tricomes [35]

Energy Dispersive Spectroscopy (EDS)

This form of spectroscopy utilizes x-ray excitation of a sample, in order to analyse the atomic structure of a material. When a beam of highly charged particles or x-rays interact with the atoms of a sample, the incident beam creates electron holes in the inner shells, which then become occupied as electrons from outer, higher energy shells, shift to the inner shells. As these electrons transition they release discrete amounts of energy in the form of x-rays. The number and energy levels of the x-rays emitted are detected by an energy dispersive spectrometer, which constructs a graph based on this data. Based on the unique set of x-ray emission spectra it is possible to characterize the relative molar proportion of the molecular constituents and the types and quantity of each atom within a sample.

Figure 17. EDS Spectrum for ZBLAN

To view the samples properly in the SEM, the samples were set in a polymer resin block and then polished to reveal a flat surface for inspection. The polished test samples tended to fluoresce, therefore they were carbon coated before being loaded into the SEM for analysis. 48

4.5.1.2. TEM

In transmission electron microscopy (TEM), a focused beam of electrons is passed through an ultra-thin sample, where the interaction of the incident electrons on the sample as they are transmitted influences the image that is generated. Similar to the image generation by a light microscope, the electrons that are transmitted through a specimen are focused by an objective lens onto a phosphor screen or a charge coupled device (CCD) camera. As the transmitted electrons come into contact with the phosphor screen or the CCD camera, they generate light that can be seen on the phosphor screen or become converted into a digital signal to be viewed on a monitor. In both cases, the lighter regions of a specimen indicate areas where electrons were able to transmit through the sample more readily, and vice versa for the darker regions.

This kind of imaging provides an almost “translucent” view of the sample, where different phases, inhomogeneities and variations in the density of a sample are easily apparent.

One useful aspect of TEM is the diffraction patterns that can be generated from the electrons as they pass through the specimen. After leaving the specimen and through the objective lens, a diffraction pattern is formed on the back focal plane of the microscope. By adjusting the configuration of the microscope, it is possible to project this diffraction pattern onto the phosphor screen.

The diffraction pattern itself is a result of the diffraction grating phenomenon created by the periodic structure of a crystalline solid. Depending on the crystal structure of the sample, the incident beam of electrons will scatter in a particular orientation that results in a diffraction pattern, it is possible to work backwards and deduce the original crystal structure. More highly ordered structures (single crystal) tend to form spots or lines on a diffraction pattern, whereas micro or nano-crystalline (poly-crystalline) structures tend to form distinct ring

formations. Amorphous materials do not have long-range order in their atomic lattice, consequently they produce diffuse rings with maximum intensity, and do not have discrete reflections.

Figure 18. This image shows (left) an electron diffraction pattern of a single-crystal EuMnGe2O7 compound with zone axis (001) [36], (right) diffraction pattern of a film of amorphous SiN [37]

One aspect of TEM analysis that is particularly useful is its sensitivity to detecting crystalline particles that are in the nano and micrometre range. Typically, other techniques such as XRD

are able to detect the presence of micro-crystalline structures, however at the nanoscale

(smaller than 3 nm) it is more difficult. TEM is capable of detecting ordered structures at the sub micrometre range.

Test samples were prepared by crushing the sample into a fine powder in a mortar and pestle, then submerging the particulates in a small quantity of 30% ethanol. The liquid and test sample mix were then pipetted onto carbon grids, and the ethanol was allowed to evaporate.

The carbon grid (now carrying tiny fragments of the test sample) was then loaded onto the sample holder of the TEM.

The TEM equipment used for this experiment was a JEOL 2100.

Figure 20. Photograph of JEOL 2100 TEM

The following list summarizes the configuration of the JEOL 2100 for high resolution imaging:

General Setup Objective Lens

Acceleration Voltage: 200 kV Focal length: 2.3 mm

Minimum step: 1.5 nm Spherical Aberration: 1.0 mm

Spot Size (diameter): 20 - 200 nm Chromatic Aberration: 1.4 mm

Resolution

Point: 0.23 nm

Lattice: 0.14 nm

When subjected to the strength of the electron beam, it was found that some of the smaller test sample fragments would be affected and subsequently disintegrate because of the bombardment of energy. A few methods were employed when such instances occurred including applying a low-dose technique, reducing the incident intensity and increasing the spot size. The low-dose technique involved avoiding the pre-irradiation of the region of interest, by focusing the image on a larger fragment nearby and then deflecting the incident beam onto the smaller fragment of interest. This minimized the amount of time the smaller fragment was subjected to the electron beam, providing sufficient time for image acquisition if it was taken immediately. Reducing the incident-beam current was an effective way to prevent damage, even if it increased the length required to record an image. The most effective method to mediate beam damage, was by using a larger spot size, especially at higher magnification, which reduced the current density and provided a much larger time frame to take images (this only had a slight impact on the resolution of the images).

4.5.2. Characterization of Crystalline Content of Test Samples

To support the EDS analysis of the crystal phases of the samples, X-Ray Diffraction (XRD) analysis was performed. This information was used not only to corroborate the findings of the EDS analysis, but also to provide a more quantitative perspective on the change of crystal content in the samples as the cooling rate increased.

4.5.2.1. XRD

X-ray diffraction (XRD) utilizes x-rays to quantify and characterize the crystalline composition of materials by measuring the diffraction of the incident x-rays due to the planes of atoms of the material. It is possible to determine the type and relative positions of atoms and provide a length scale for which the crystalline structures have grown to. The exact nature of the crystalline content can be characterized in great detail, including identifying phases present and even the preferential ordering and epitaxial growth of crystallites.

The peaks of an x-ray diffractogram are due to the concentration of number of d-spacings related to the lattice ordering of a crystal structure. The higher the concentration of a particular d-spacing, the greater the diffraction signal becomes for that particular spacing, reinforcing to form a peak. Depending on the intensity and position of peaks in a diffractogram, the prevalence of any crystal phase can be determined.

Figure 21. XRD pattern of ZBLAN test samples from a study by L. Battezzati [19]

For the purposes of this experiment, based on the relative intensity of specific peaks, a fully crystalline material will have well-defined intensities, whereas a fully amorphous material will only have a ‘broad hump’. Furthermore, as the crystal content in the material diminishes from test sample to test sample in correlation to the increasing cooling rate, the peak intensities themselves will also diminish from one diffractogram to the other.

Test samples were prepared for analysis by being ground into finely divided powders, alternatively they could have also been analysed using a flat surface of the test sample, as long as it had a low surface roughness.

Figure 22. Schematic showing layout of a XRD instrument, with the incident beam and measurement of diffracted x-rays [7]

The XRD equipment used in this experiment was a Panalytical MPD that utilizes Co Kα radiation, and was configured with the following settings:

Generator Voltage: 40 kV

Tube Current: 40 mA

Scan Speed: 3 ⁰/min

Scan Range: 4 ⁰ - 90 ⁰ (2Ө)

Scan Increments: 0.0167

4.5.3. Attenuation Loss Analysis

The attenuation loss of the test samples was the primary point of interest in terms of the performance capabilities of the processed test samples. The attenuation losses were calculated by utilizing Kramers-Kronig analysis in conjunction with material properties of ZBLAN glass and the refractive index of the test samples. To determine the refractive index of the test samples, an ellipsometer was used in addition with RI matching liquids.

4.5.3.1. Refractive index matching liquids

For this experiment, the refractive index matching liquids used were supplied by Cargille

Laboratories. Refractive index matching liquids are used for many applications, from mineralogical identification to quality control of materials, and are used in a wide variety of fields such as chemicals, engineering, medical, forensic, optics and instrumentation [39].

The liquids themselves are composed of substances with known refractive indices, and based on the proportion of each substance, a set of matching liquids can be made with varying refractive indices. Each set covers an interval range of indices, depending on how accurately the sample’s RI needs to be identified.

For this experiment, the set that was used was designated Cat # 18091. A-1, which is comprised of 91 liquids that are matched with refractive indices between 1.460 - 1.640, at intervals of

0.002. This provided a sufficiently high enough accuracy for identifying the refractive index to three decimal places.

A fragment of the processed test sample was placed on a slide, and then submerged in any one of the matching liquids (a liquid with 1.5 RI was chosen as a starting point). Then, under polychromatic light of a petrographic microscope, the test samples were viewed. A moving

Becke line is formed when there is a disparity between the refractive index of the surrounding matching liquid and the test sample itself. At the outer edges of the test sample, when the stage of the microscope is lowered (the focal distance is increased), a dim light will form (the

Becke line) and move in a particular direction either inwards or outwards from the edge.

Depending on the direction, this indicates whether or not the sample matches the RI matching

liquid. The light moves towards the medium that has a higher refractive index. The goal is to keep submerging the sample in a variety of liquids, until the Becke line no longer moves in either direction. This match was able to provide the refractive index of the processed test samples, based on which liquid yielded a Becke line that didn’t shift in either direction.

ZBLAN Fragment

Becke line moving towards liquid

RI Matching Liquid

4.5.3.2. Ellipsometer

An ellipsometer is a device that utilizes an optical technique for determining the dielectric properties of a material, usually prepared as thin films. It is capable of characterising material properties such as the complex refractive index, the composition, surface roughness, thicknesses, crystalline nature, doping concentration and electrical conductivity of a material

[40].

This technique measures the change in polarization of a linearly polarized light beam as it reflects from/or transmits through the thin film. Depending on the properties and thickness of the material, the linearly polarized light will shift to an elliptically polarized light beam.

This change in polarization can be represented as a relative amplitude change, ψ, and a relative phase change, Δ [41].

The measured data of ψ and Δ, cannot be converted directly into optical constants. It is typical that a model analysis must be applied to this SE data in order to determine the optical constants and other material properties. Due to this indirect method, the only drawback with ellipsometry is the way the model analysis is performed can significantly affect the optical constants that are derived from it. Models can either be physically based on energy transitions or use free parameters to fit the data, and depending on how low the generated MSE (mean squared error) is of the modelling, the derived optical constants can be deemed usable or not.

However, there are intricacies in whether or not a low MSE actually indicates an appropriate model match, or whether or not the fitted optical constants are suitable or realistic. A large proportion of the modelling process is up to the discretion of the person performing the analysis.

Therefore, for the purposes of this experiment, the data collected from the ellipsometer was used in conjunction with the refractive matching index liquids, since the latter method is robust and can be used to verify the SE data modelling.

CHAPTER 5: Results and Discussion

This chapter will present the results obtained from the experiments and their subsequent analysis, for each of the 6 cooling rates evaluated:

1. 5 ⁰C/min

2. 20 ⁰C/min

3. 100 ⁰C/min

4. 900 ⁰C/min

5. 4000 ⁰C/min

6. 8000 ⁰C/min

All test samples of each cooling rate had consistent results and the cooling rates had a high reproducibility (the lower cooling rates were highly repeatable, the faster cooling rates were less repeatable, refer to section 5.7 for error analysis).

5.1. SEM Imaging and EDS Results

5.1.1. ZBLAN Test Sample cooled at 5 ⁰C/min (cooled in DSC)

5.1.1.1. Topographical Analysis (Test Sample that has not been polished)

For this analysis, the surface topography of a sample cooled at 5 ⁰C/min was studied. This is for a test sample taken directly from the platinum crucible and not polished. The samples are usually prepared for SEM analysis by polishing to a 1 micrometre finish, but not in this particular instance.

The surface topography shows heavy cavitation, which is typical of a vast amount of off- gassing [42], as can be seen in Figures 25 and 26.

Figure 25. SEM image of test sample cooled at 5 ⁰C/min at low magnification

Figure 26. SEM image of test sample cooled at 5 ⁰C/min at

high magnification

The surface topography shows different regions of the ZBLAN material have distinct texture changes that represent different phase regions (striated lines correlate to ZBLAN matrix, whereas smoother areas correlate to regions higher in relative Zr concentration).

Figure 27 displays the sites inspected along the surface of the sample, where an EDS spectrum was taken at each site. Accompanying the image is the EDS spectra for sites 1 and 5, which are both clearly different, indicating the inhomogeneity in this sample.

Figure 28. EDS spectra taken from spectrum 1 site of a test sample cooled at 5 ⁰C/min, showing a typical ZBLAN molar composition (refer to Table 2 for expected molar ratio)

Figure 29. EDS spectra taken from spectrum 5 site of test sample cooled at 5 ⁰C/min, showing a depletion of aluminium and sodium

Sites 1, 2 and 3 had a normal molar composition reflective of the expected ZBLAN matrix

(refer to Table 2 for expected ZBLAN molar ratio). Site 5 however has a different composition,

with a depletion of aluminium and sodium, resulting in a much higher relative level of zirconium, lanthanum and barium than the expected molar ratio for ZBLAN. Sites 4, 5 and 6 have higher concentrations of these elements relative to the other components and there is much less fluorine. This can be seen in the EDS element maps of Figure 31, where the different colour zones illustrate the different distributions of the various elements.

Figure 30. EDS element map of ZBLAN test sample cooled at 5 ⁰C/min, at the same sites as Figure 27

Figure 31. EDS element maps for the various constituent elements of ZBLAN test sample cooled at 5 ⁰C/min

The spectra taken at sites 4, 5 and 6 correlate to areas on the EDS maps where there is a lower concentration of fluorine (red figure in Figure 31). As for the other species concentration figures, it is harder to discern a difference, but there was indeed a lower concentration of Na and Al in sites 4, 5 and 6. These three elements (Zr, Ba and La) had a higher molar ratio in sites

4, 5 and 6 because of the relative lower concentration of Na and Al. This is more evident when the region is scanned using VPSE (Variable Pressure Secondary Electron) imaging as can be seen in Figure 32.

Figure 32. VPSE image showing disparate regions of the 5 ⁰C/min ZBLAN test sample

In Figure 32, the darker regions indicate areas with a depletion of Al and Na; whereas the lighter areas indicate regions that have a typical ZBLAN molar composition. This indicates that under a slow cooling regime, when the ZBLAN material solidifies from the molten state,

the different components tend to aggregate into particular regions, resulting in phase separation and inhomogeneities in the test sample. Additionally, there is discernible evidence of cavitation throughout the test sample as indicated by the “potholed” surface topography, and in conjunction with the spectral analysis, indicates that the fluorine is off-gassing. This is expected as the longer the ZBLAN material remains above its glass transition point, it will continue to release fluorine, in effect leaching the test sample of this element.

5.1.1.2. SEM Imaging and EDS Analysis (Test Sample that has been polished)

The same test sample cooled at 5 ⁰C/min was evaluated, however, it was prepared by polishing a section to a surface roughness of 1 micrometre. Polishing the test sample revealed the internal structure of the ZBLAN sample, and ultimately obtained a better view of the crystal phases present. Figure 33 presents an image of the crystallization pattern formed by the various crystal phases at low magnification.

Figure 33. SEM image showing a polished cross-section of the 5 ⁰C/min cooled ZBLAN test sample

As shown in Figure 33, this cross section of the fragment of ZBLAN sample shows the entire sample is heavily crystalline. There are crystal phases of distinct morphology spreading throughout the entire sample (only a small proportion of the fragment remains in an amorphous glassy state).

Figure 34 shows a higher magnification SEM image with three distinct morphologies,

“feathery type” and “bow tie/V-shaped” type crystals [43], surrounded by a regions of darker coloration with “black specs” throughout it.

Dark regions with black specs

Bow -Tie

Light grey area, feather morphology

Figure 34. High magnification of 5 ⁰C/min ZBLAN test sample with three different crystal phases

Figure 35 presents a backscatter image highlighting these three distinct regions/morphologies, where the different crystal phases are indicated by different shades of grey depending on the average atomic density of each region. The contrast in shade indicates the difference in atomic number of each phase, heavier elements (with high atomic number) backscatter electrons more than lighter elements (with low atomic number), and therefore appear brighter in the image. “Whiter” or “lighter coloured” regions in a backscatter image are composed of elements with a higher average atomic number. “Darker coloured” regions are comprised of elements that have a lower average atomic number [44].

Bow Tie

Dark regions with black specs

Light grey area, feather morphology

Figure 35. Backscatter image of 5 ⁰C/min ZBLAN test sample with three crystal phases

The light grey areas, which correspond to the feather morphology, have high levels of zirconium and barium, with a deficiency in sodium, and are most likely to be a zirconium barium crystal matrix, Zr2BaF13 (refer to Table 3). The atoms of this crystal phase have a much larger nucleus, therefore appear lighter in colour in the backscatter image.

Table 3. EDS Analysis of “Feather” Morphology from Figure 35

Element Wt %

Zr 37

Ba 23

F 40

Molar ratio Zr 2: Ba 1: F 13

The regions with a “bow-tie/V- shaped” morphology have a relatively typical ZBLAN composition, which is reflected in the medium grey coloration in the backscatter image. Table

4 shows the EDS Analysis for this crystal phase.

Table 4. EDS Analysis of “Bow-Tie/V-Shaped” Morphology from Figure 35

Element Wt %

Zr 49.3

Ba 40.5

La 6.9

Al 0.1

Na 3.2

Molar ratio Zr 52: Ba 29: La 5: Al 1: Na 13

The darkest regions with black specs have an average weight composition of several measured EDS sites that correlates to a zirconium sodium fluoride crystal phase. However, its exact composition is difficult to discern because of the many possible phase compositions of a zirconium sodium fluoride crystal that can coexist within the ZBLAN glass matrix, including NaZr F5, β-Na2ZrF6 and Na7Zr6F31 [19, 20]. The average composition taken from several EDS sites yielded a weight ratio of Na 1: Zr 1: F 6. The average of the nuclei density of this phase is lower than the other two crystal phases, which is reflected in how darkly coloured this region is in the backscatter image.

Table 5. EDS Analysis of “Dark Region with Black Specs” Morphology from Figure 35

Element Wt %

Zr 40

Na 11

F 49

Molar Ratio Na 1: Zr 1: F 6

5.1.2. ZBLAN Test Sample cooled at 20 ⁰C/min (cooled in DSC)

Figure 36 shows a polished cross section of a fragment of ZBLAN cooled at 20 ⁰C/min. The light coloured feathery morphology seen in Figure 34 (5 ⁰C/min cooled test sample) is clearly visible, however the crystal structures in Figure 36 are much smaller in scale.

Figure 36. SEM image of the “feather” morphology of 20 ⁰C/min ZBLAN test sample, which is smaller in scale than the 5 ⁰C/min test sample (compare to Figure 34)

Figures 37 and 38 shows a backscatter images with the same three morphologies of the

5 ⁰C/min cooled sample (refer to Figure 35), except, again each phase is smaller in scale.

Figure 37. Backscatter image of 20 ⁰C/min ZBLAN test sample at high magnification

Bow Tie

Light grey area, feather Dark regions with black morphology specs

Figure 38. Backscatter image of 20 ⁰C/min ZBLAN test sample with three crystal phases

Figure 39 shows how various parts of the sample have now maintained an amorphous state when cooled at 20 ⁰C/min. On the right hand side of the image, where there are no light colour shapes, is amorphous. On the left hand side of the image, the “feathery” crystal phase is visible, however this phase does not extend throughout the entire sample.

Amorphous Region

“Feather” Crystal Phase

100μm

Figure 39. SEM image of 20 ⁰C/min ZBLAN test sample, the left hand side of image is heavily crystalline, the right hand side is amorphous

5.1.3. ZBLAN Test Sample cooled at 100 ⁰C/min (cooled in Air)

The ZBLAN samples cooled at a 100 ⁰C/min were mostly amorphous, with these amorphous regions appearing in the SEM images as uniform grey areas with no distinct shapes or

features. However, scattered throughout the glassy matrix are crystallites, which on average are approximately 2 - 3 μm in diameter.

Figure 40 shows a polished cross section of the test sample fragment, where the uniform grey area is the amorphous part of the ZBLAN glass matrix. The dark area in the bottom left corner is the epoxy resin that the test sample fragments are embedded in prior to polishing. In the bottom left area at the boundary between the sample and the epoxy resin are some dark grey specs, which are the crystallites.

Figure 40. SEM image of 100 ⁰C/min cooled ZBLAN test sample, with AlF3 microcrystals highlighted in red

Figure 41 is a higher magnification of the crystallites as highlighted in the red square of Figure

40. As can be seen by the scale, these crystallites can range anywhere between 1 μm to 4 μm in length.

Figure 41. SEM image showing higher magnification of possible AlF3 crystallites, as highlighted in the red square of Figure 40

Figure 42 presents a backscatter image of the same section of Figure 40.

Figure 42. Backscatter image of possible AlF3 crystallites

The crystallites are more visible in the backscatter image as darker colored regions, which reflect their atomic makeup. Based on the darker shade of grey, this indicates the average atomic nuclei density of these crystals is lower than the surrounding ZBLAN glass region.

An EDS analysis of the crystallites from Figure 42 reveals that the crystals have high levels of aluminium and low levels of lanthanum and in particular barium, see Table 6.

Table 6. EDS Analysis of Crystallites from Figure 42

Element Wt% of Crystal Region Expected ZBLAN Wt%

Zr 46 54

Ba 14 33

La 3 7

Al 32 1

Na 5 5

Molar ratio Zr 25: Ba 5: La 1: Al 59: Na 10

The crystallites observed in Figure 42 are likely to be aluminium trifluoride crystals, AlF3, because of the high concentration of aluminium and this species of crystallite was identified in the other test samples (refer to Table 10).

5.1.4. ZBLAN Test Sample cooled at 900 ⁰C/min, 4000 ⁰C/min & 8000 ⁰C/min

The SEM images of the 900 ⁰C/min test samples as shown in Figure 43 present a polished cross section of the test sample fragments that is uniformly grey in appearance with no discernible features. This indicates the entire sample is amorphous, however the 900 ⁰C/min samples still reveal evidence of nano-crystal inclusions. These nano-crystals are not visible under SEM, but are detectable using TEM analysis. Figure 43 shows what an amorphous region appears like under SEM, where the grey coloured area is the ZBLAN glass matrix, and the darker area in the bottom left corner is the epoxy resin the glass is embedded in for polishing.

Amorphous ZBLAN

Epoxy Resin

Figure 43. SEM image of 900 ⁰C/min cooled ZBLAN test sample

Figure 44 is a higher magnification of Figure 43, and shows an SEM image with no discernible features, indicating an amorphous phase extends throughout the entire sample. EDS analysis 78

also confirms that the test sample itself has maintained a typical and consistent ZBLAN glass composition after processing at these higher cooling rates.

Figure 44. Higher magnification SEM image of Figure 43, showing ZBLAN test sample cooled at 900 ⁰C/min

SEM images for the 4,000 ⁰C/min and the 8,000 ⁰C/min appear exactly the same, where there are no discernible features. These results clearly demonstrate that at a cooling rate equal to or greater than 900 ⁰C/min, crystallites or any crystal phases are no longer visible using SEM analysis.

5.1.5. SEM Results Summary

There were three distinct crystal phases observed in the slower cooled samples. Figure 45 provides a side by side comparison of how the reduction in size of these crystal occur between the 5 ⁰C/min cooled and 20 ⁰C/min cooled test samples. The three phases have been

highlighted with the light feathery morphology in green, dark regions in red and v-shaped phase in blue. Take note, the side by side images are to equal magnification, the highlighted regions demonstrate a true scale of the actual size difference.

Figure 45. Coloured regions showing various crystal phases for a sample cooled at 5 ⁰C/min (left), coloured regions showing phases for sample cooled at 20 ⁰C/min (right). Both images are to equal scale.

Summary of change in size of crystal phases:

Light Feather Morphology (high levels of zirconium and barium, with deficiency in sodium):

x 5 ⁰C/min sample formed crystals of average length of 100 - 500 μm x 20 ⁰C/min sample formed crystals of average length 20 to 30μm x This correlates to a 90% decrease in size

Dark region with black specs Morphology (high levels of zirconium and sodium, deficiency in barium):

x 5 ⁰C/min sample formed crystals of an average length of 50 μm x 20 ⁰C/min sample formed crystals of an average length of 10 - 15 μm x This correlates to a 70% decrease in size

V shaped (bow tie) Morphology:

x 5 ⁰C/min sample formed crystals of an average length of 40 μm x 20 ⁰C/min sample formed crystals of an average length of 2 μm x This correlates to a 95% decrease in size

At a cooling rate of 100 ⁰C/min, these three morphologies disappear entirely, and only small crystallites in the order of micrometres in size were detected.

At a cooling rate of 900 ⁰C/min, sporadic crystallites exist in the sub-micron range, however they are not observable in the SEM Images, but were detectable using TEM.

5.2. XRD Results

To obtain an analysis of the chemical composition and amount of crystals formed with each of the cooling rates, a small portion of ZBLAN was taken from tests samples made at each cooling rate (minimum of four samples were done for each cooling rate) to ensure statistical accuracy. These test sample portions were ground and combined into one large sample, which was then analysed by XRD. One x-ray diffractogram was then obtained for each cooling rate.

This process was then repeated so that an x-ray diffractogram was obtained for every cooling rate, however in this results section, only the results of the first four cooling rates are presented. This is because after a certain cooling rate, the XRD is no longer able to detect the presence of crystallites, even if present. This detection limit is due to XRD not having a sufficient sensitivity for signals generated by crystals in the nanoscale region or at the lower end of the micro-scale region. Volume fraction effects plays a significant role in the peak intensities, since the intensities are directly proportional to the d-spacings apparent in the crystallographic planes present in the crystalline constituents of the ZBLAN matrix. Therefore an absence of a peak does not necessarily imply that there are no crystals, only a small concentration of them. As a result, the patterns for the cooling rates equal to or faster than 900

⁰C/min were all identical, with a typical “broad amorphous hump.”

XRD is a useful technique in providing a quantitative examination at the level of order when the crystallites are in the larger micro-scale region, forming large crystals and crystal phase regions that extend throughout the test sample.

The first predominant phase observed in the x-ray diffractograms is a zirconium and barium rich phase, while the other is a zirconium and sodium rich phase. Their exact molecular makeup, however, can be quite varying as many possible phases with these elements can be formed with similar molar ratios. Other crystal phases that have been observed include β-

Zr2BaF6 [19], α-Zr2BaF10, LaZr2F11 [20] and β-2ZrF4BaF2 [45]. For this analysis, the peaks that were observed in the XRD patterns were the same as those observed in a paper by Battezzati et al [19], refer to Figure 21. The patterns have a few variations, but the primary peaks occur at the same angles and have the same relative intensities so they are easily identifiable. The main species of crystal phases that the peaks can be attributed to are β-ZrBaF6 and ZrNaF5.

Some of the larger peaks have been attributed to a few other crystal phases including

NaBaZr2F11 and Na7Zr6F31. The spectra were analysed using HighScore Plus™ and the patterns that were referenced were taken from the program, PDF-4+™, which has an extensive materials database.

The primary peaks and the phases they relate to can be seen in Figure 46, and are outlined in detail in Table 7.

Figure 46. X-ray diffractogram for 20 ⁰C/min cooled ZBLAN test sample labelled with assigned crystal phases (shown in Table 7) as identified using the PDF-4+ database

Table 7. List of Peaks and Attributed Crystal Phases

Phase Symbol on Figure 46 2Ө D-spacing (Å)

ZrNaF5 23.52 5.57

26.53 3.86

54.51 1.99

β-ZrBaF6 33.13 3.13

51.13 2.08

55.58 1.97

Na Ba Zr2 F11 62.74 1.72

Na7 Zr6 F31 66.27 1.63

As noted, aside from the two main phases, there is a multitude of other crystal phases and crystallites that are formed in the ZBLAN glass. However, the assignment of these particular x-ray reflections becomes a convoluted and difficult process because of the high number of possible phases [19]. The ZBLAN matrix is a quinary system, and with the possibility of the

overlap in peaks generated by the reflections of these different phases, it is challenging to pinpoint the entire array of crystal species that are possible.

5.2.1. X-ray Diffractograms

The following Figures 47 - 50 display the diffractograms for each cooling rate, by increasing cooling rate.

5 ⁰C/min Cooling Rate

Figure 47. XRD pattern for a ZBLAN test sample cooled at 5 ⁰C/min

20 ⁰C/min Cooling Rate

Figure 48. XRD pattern for a ZBLAN test sample cooled at 20 ⁰C/min 84

100 ⁰C/min Cooling Rate

Figure 49. XRD pattern for a ZBLAN test sample cooled at 100 ⁰C/min

900 ⁰C/min Cooling Rate

Figure 50. XRD pattern for a ZBLAN test sample cooled at 900 ⁰C/min

5.2.2. XRD Results Summary

The following graph in Figure 51 shows an overlay of three spectra for each of the following cooling rates; 5 ⁰C/min (green), 20 ⁰C/min (blue), 100 ⁰C/min (red). It is more apparent when the diffractograms are set side by side, how the peaks diminish proportionately to an increasing cooling rate. This reflects and further substantiates the decrease in size of the crystal phases due to cooling rate, as was observed in the SEM images.

Figure 51. Overlay of various X-ray diffractograms for different cooling rate test samples

5.3. TEM Results

In the “bright field imaging mode” of the TEM, the difference in light and dark represent regions of varying density and capacity to transmit electrons. The lighter regions are able to transmit electrons more readily, while the darker regions do not. A more ‘speckled’ appearance indicates the ZBLAN material contains disparate regions that are not homogeneous in their constituent molecular makeup. This is indicative of the presence of crystals in the medium of the ZBLAN matrix, where different crystal phases will have

different densities and therefore transmit electrons differently. This is quite clear to see in the following results, where the TEM images illustrate how the “texture” of the samples change in their overall appearance with increasing cooling rate and become more amorphous.

The fragments of the test samples were ground into a fine powder and suspended in 30% ethanol. This mixture was then pipetted onto a carbon grid and the ethanol allowed to evaporate before the carbon grid was loaded in the TEM for inspection. The fragments are thin enough so that the electrons are able to transmit through the thin layer and reveal whether or not there are any crystals or other inhomogeneities present.

5.3.1. ZBLAN Test Sample cooled at 100 ⁰C/min

Figures 52 - 54 display fragments from the samples cooled at 100 ⁰C/min, which contain microcrystals as seen in the SEM images. Under TEM analysis, with bright field imaging, the fragments appear “grainy” and “speckled”, Figure 54 displays at this cooling rate the material is still quite inhomogeneous and the ZBLAN matrix is scattered with a plethora of micro and nano sized crystals.

The crystallites can be seen in the difference in shade, and the different textures of the various fragments are indication of how irregular the fragments are in composition.

ZBLAN Fragment

Carbon Grid

Figure 52. Bright field image of 100 ⁰C/min cooled ZBLAN test sample fragments spread out on a carbon grid

ZBLAN Fragment Carbon Grid

Figure 53. Higher magnification of a bright field image of a single ZBLAN fragment, crystallites are visible as dark speckles

Carbon Grid

ZBLAN Fragment

200 nm

Figure 54. Higher magnification bright field image of another ZBLAN fragment with many crystallites

Diffraction patterns were taken from ten different fragments that were randomly chosen throughout the carbon grid. Figure 55 shows a diffraction pattern taken from a single fragment of the ZBLAN test sample. There are multiple rings, each one relating to the d- spacing of a particular lattice in the structure of the crystallites. The rings of this pattern were indexed and matched to a barium fluoride crystallite, using CSpot™ software to evaluate the peak positions of the rings and PDF-4+™ to identify the indexed rings.

dBaF2 (110)= 2.126Å dBaF2 (202)= 1.532Å dBaF2 (212)= 1.243Å

dBaF2 (312)= 0.958Å

dBaF2 (402)= 0.873Å

Figure 55. Diffraction pattern for a 100 ⁰C/min ZBLAN sample with assigned rings

Table 8. Indexing of Diffraction Pattern from Figure 55

Ring Designation Crystal Species D-Spacing (Å) (hkl)

1st Ba F2 2.126 110

2nd Ba F2 1.532 202

3rd Ba F2 1.243 212

4th Ba F2 0.958 312

5th Ba F2 0.873 402

Figure 56. Graph showing peak intensities of diffraction pattern from Figure 55

Table 9. Peak Parameters from Figure 56

Peak Number I (a.u.) K(1/Å) d=1/k(Å)

1 99.39 0.4649 2.1510

2 64.99 0.6567 1.5227

3 12.35 0.8031 1.2451

4 5.57 1.0366 0.9646

5 3.7 1.1353 0.8809

5.3.2. ZBLAN Test Sample cooled at 900 ⁰C/min

Figures 57 - 59 display fragments from the samples cooled at 900 ⁰C/min, displaying how the texture of the fragments have changed to a more even one and the glass matrix itself has a more uniform appearance. However, the material still contains traces of nano-crystals, as is evidenced in the diffraction pattern in Figure 60, which shows low level crystalline diffraction.

Carbon Grid

ZBLAN Fragment

1 μm

Figure 57. Bright field image of 900 ⁰C/min ZBLAN test sample

ZBLAN Fragment Carbon Grid

Figure 58. Image of various 900 ⁰C/min ZBLAN fragments 92

ZBLAN Fragment

Carbon Grid

Figure 59. Higher magnification image of 900 ⁰C/min ZBLAN fragment, material is more homogeneous in structure, however, there are still “speckles” that could potentially be nano crystals

Figure 60 shows a diffraction pattern taken from one of the fragments, the pattern has changed significantly, where the rings seen in the diffraction patterns of the 100 ⁰C/min cooled sample have now disappeared (see Figure 55 for comparison).

(012) (11-3)

(024)

(012)

Figure 60. Diffraction pattern of 900 ⁰C/min test sample, with spots belonging to three ring patterns

The spots in Figure 60 correlate to a ring position that have a d-spacing of length 3.64 Å, 2.21

Å and 1.87 Å (going from the inner to the outer ring). Based on the possible matches found using the PDF-4+™ software, these d-spacings correlated with AlF3.

Table 10. Crystal Phase Match for Diffraction Pattern in Figure 60

Ring Designation Crystal Species D-Spacing (Å) (hkl)

1st AlF3 3.517 012

2nd AlF3 2.118 11-3

3rd AlF3 1.800 024

The presence of the distinctive (012) spot in the top left corner of the intense band of Figure 60 indicates that there is a sizable single crystal, however, the other distinctive spot to the bottom

right also aligns with the ring pattern, which indicates there is another single crystal but with a different orientation.

5.3.3. ZBLAN Test Sample cooled at 4000 ⁰C/min & 8000 ⁰C/min

Figures 61 and 62 are images taken of fragments from the test samples cooled at the faster cooling rates. These test samples are completely homogenous in appearance, with no speckles or different shaded regions. This uniformity in appearance is indicative of a regular glass matrix that is entirely amorphous.

ZBLAN Fragment

Carbon Grid

Figure 61. Bright field image of 4000 ⁰C/min ZBLAN test sample fragments

Figure 62 was taken at a much higher magnification, where it is possible to see how uniform the fragments actually are, that even at a high magnification there are no signs of any irregularities.

ZBLAN Fragment

Figure 62. Bright field image of 4000 ⁰C/min ZBLAN test samples at higher magnification, darker area is ZBLAN fragment

Figure 63 presents a HRTEM (High Resolution TEM) image, which shows how highly disordered the structure of the sample is when cooled rapidly. There are no signs of nano- crystals or ordered clusters, which confirms the sample’s fully glassy nature as seen in the uniformity in the appearance of the other bright field images (Figures 61 and 62).

Figure 63. HRTEM image of disordered nature of 4000 ⁰C/min ZBLAN test sample

Figure 64 shows a diffraction pattern taken from the test sample fragments. The diffraction pattern shows a typical diffusion halo with an intense band signature of an amorphous material. The diffraction pattern indicates the absence of precipitates larger than 3 nm, confirming the absence of micro and nano-crystal inclusions when processed at this cooling rate.

Figure 64. Diffraction pattern of 4000 ⁰C/min test sample, showing an amorphous structure

5.3.4. TEM Results Summary

The main distinction that can be made from the TEM imaging is the consistency of what the fragments appear like, and the change in the diffraction patterns. As can be seen, the ZBLAN material can appear ‘speckly’ or inhomogenous in the samples that were cooled slowly. By comparison, the ZBLAN material that was rapidly cooled, appear much more “smooth” and homogenous.

Carbon Grid

ZBLAN Fragment

Carbon Grid

Figure 65. Bright field Image comparison of (above) single ZBLAN fragments of a 100 ⁰C/min cooled sample, and (below) a 4000 ⁰C/min cooled sample

The diffraction pattern of the slower cooled (100 ⁰C/min) test samples reveals the presence of crystals, where there are many rings correlating to the d-spacings of the lattice structure of these crystals. These rings disappear entirely in the diffraction patterns taken from the rapidly cooled samples (including and above 4,000 ⁰C/min), which suggests the complete elimination of micro and nano-crystal inclusions. This also suggests a critical cooling rate must be somewhere between 900 ⁰C/min (diffraction pattern still shows traces of nanocrystals) and

4,000 ⁰C/min.

Figure 66. Diffraction pattern comparison between (left) a 100 ⁰C/min cooled sample, (middle) a 900 ⁰C/min cooled sample and (right) a 4000 ⁰C/min cooled sample

5.4. Raman Analysis of 4000 ⁰C/min Test Samples

Raman spectroscopy is a technique used to observe vibrational, rotational and other low frequency modes of the molecule under study. It utilizes inelastic Raman scattering of monochromatic light to interact with molecular vibrations, phonons or other excitations in the sample to generate shifts in energy of the vibrational states. By correlating the shifts in energy states to known Raman scattering spectra, it is possible to identify molecular and phase structures in a material.

Figure 67 and Table 11 display the Raman spectra, peak deconvolution and their corresponding structural vibrational modes.

100

ZrF Zr-Fb-Zr

Zr-Fb δ(Fnb-Zr- Fnb) Zr-Fnb

Figure 67. Raman spectrum for 4000 ⁰C/min sample

Table 11. Peak Deconvolution of Figure 67 [56]

Peak Number Raman Shift Position Band Identification (cm-1)

1st 583 Total symmetric stretching vibration of ZrF bonds

2nd 482 Antisymmetric vibrational modes of Zr- Fnb (non-bridging fluorine atoms)

3rd 401 Antisymmetric vibrational mode of Zr- Fb (bridging fluorine atoms)

4th 339 Bending modes of δ(Fnb-Zr- Fnb)

5th 204 ‘B’ band of the Zr-Fb-Zr in-chain bending

The 4000 ⁰C/min samples were analysed via Raman spectroscopy, and their spectra were deconvoluted using the program GRAMS™. By studying the spectra generated from these

101

samples, it is possible to gain a better understanding of the short-range structuring and coordination between the anions and cations of the ZBLAN molecules.

5.5. Attenuation Loss Measurements

5.5.1. Refractive Index Matching Liquids

The first step in determining the attenuation loss of the samples is to determine the refractive index. Using a petrographic microscope with polarized polychromatic light, the refractive index was determined for a wavelength range of approximately 400 - 700 nm. The refractive index matching liquids were particularly useful in that they were incredibly easy to utilize and the process of identifying which liquid matched the sample was conveniently quick.

The refractive index for the samples cooled at 4000 ⁰C/min (completely free of crystallites) was determined to be 1.497. This is in line with previously published data on the refractive index of ZBLAN, where the glass has been quoted to have a RI of 1.5 in the visible wavelength range

[66].

5.5.2. Ellipsometer

The refractive index matching liquids were used to identify the refractive index within the visible spectrum, however in order to determine the attenuation loss of the test samples, the optical properties needed to be determined for as wide a possible range of wavelengths. The ellipsometer was able to determine the n (refractive index) and k (extinction coefficient) values of the samples from 240 nm (Middle UV) all the way to 1700 nm (near infrared).

102

To obtain measurements from the ellipsometer, the ZBLAN test sample fragments extracted from the crucibles had to be mounted on soda lime glass slides with a particular set up (refer to the Figure 68).

ZBLAN Glass Fragments 135μm Epofix Epoxy

Renlam Resin= 5μm

Soda Lime Glass Slide

Figure 68. Cross-sectional layout of ZBLAN test samples as prepared on soda lime glass slides

The top layer consisted of ZBLAN test sample fragments (irregularly shaped but mostly cylindrical) embedded in epoxy. This top layer was glued onto the soda lime glass slide using renlam resin. The top layer was approximately 135 micrometres in thickness. The renlam layer was thin but on average its thickness was approximated to be about 5 micrometres. When the beam illuminated the ZBLAN fragments from above, the fragments were wide enough that the beam only interacted with the fragments and not the epoxy (since the beam was incredibly narrow).

The SE (spectroscopic ellipsometry) data generated from the ellipsometer was then analysed and modelled using a program called CompleteEASE ™. Initially SE data was taken from a part of the sample slide that did not contain ZBLAN fragments, thus this section of the slide only contained epoxy and the renlam resin as a top layer, and the soda lime glass slide as the bottom layer. This initial data was modelled using a “Cauchy Substrate” model, with reference data for soda lime glass taken from [47]. This model was then dubbed “resin area”. 103

This “resin area” model was then used to analyse a part of the sample slide that did actually contain the ZBLAN fragments.

Table 12 provides parameter information on how the layers of the analysis was set up for the part of the slide containing ZBLAN. The top layer (which represents the ZBLAN test sample fragments) was modelled with a “Cauchy” model and designated as “Layer 1”. The surface roughness, layer thickness, A, B, C, k Amplitude and exponent parameters were all fitted (refer to

Table 12 for values used for the fit). The substrate layer was modelled using the “resin area” model, based off the SE data of the slide set up without the ZBLAN fragments. The epoxy, renlam resin and the soda lime glass all have identical optical properties, therefore their optical properties were taken as one unified substrate layer in the modelling.

Table 12. Fitted Parameters for Layer 1 of SE Data

Fitted Parameters Parameter Value

Surface Roughness 7.71 nm

Thickness 139,805.33 nm

A 1.500

B -0.00060912

C 0.00020762

K Amplitude 0.00531

Exponent 0.0

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This analysis yielded an MSE (mean squared error) of 3.617, which although is not as low as it could possibly be, yielded more realistic results than other models that were used to analyse the data and yielded lower MSEs. This analysis generated the following optical properties in

Figure 69 for “Layer 1”, which represents the ZBLAN fragments.

Figure 69. Optical constants of ZBLAN test sample based off CompleteEASE ™ model

The refractive index data from this analysis matched the values measured using the matching liquids, further substantiating the validity of the results of this CompleteEase™ model. These optical constants were then taken for further examination using an approach to evaluate the k values (extinction coefficient), in order to approximate an attenuation loss for the

4000 ⁰C/min ZBLAN test samples.

105

5.5.3. Approximation of Attenuation Loss

The optical data yielded from the CompleteEASE analysis of the SE data, provided both n and k values for the ZBLAN Samples. The n values matched the data retrieved from using the refractive index matching liquids, thus these values have been verified within the visible spectrum range. However, the k values themselves have not been substantiated, and upon further examination, it has been determined that the constant k values of 0.00284 (refer to

Appendix E) is a reflection of the instrument noise floor of the ellipsometer.

Therefore, in order to estimate the appropriate k values to approximate an attenuation loss value for the ZBLAN samples, an approach as performed in a study by Forouhi et al [48] was conducted. In this study by Forouhi et al, using an expression that utilizes material properties of the sample in question, and in conjunction with kramers-kronig analysis, they were able to generate theoretical n and k values for the complex index of refraction. They successfully generated theoretical optical data that were in excellent agreement with published values for a whole range of dielectrics and semiconductors.

The k values form the imaginary part of the complex index of refraction, which in this approach by Forouhi et al was determined from an expression that is a function of photon energy, E,

(5.1) ,

where A, B and C are positive, non-zero, constants generated using formulae that utilize physical characteristics of the sample medium, with the following condition holding,

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The constant Eg is the optical band gap energy, which is the threshold for photons to be absorbed, or the corresponding energy to which the minimal amount of absorption occurs.

For the purposes of analysing the ZBLAN samples, the value used for Eg was derived from a publication by Gan [49], and is approximately 9.94x10-20 J (0.62eV).

The n values, or the real part of the complex index of refraction, was determined by Eq. (5.2), which was derived using Kramers-Kronig relation (dispersion relations that connect the imaginary and real parts of the complex index of refraction [48]),

(5.2) ,

where Bo and Co are constants comprised of the aforementioned constants A, B, C and Eg. n(∞) is a constant greater than 1, and is dependent on the particular material being analysed, in this case for ZBLAN it was taken as 1.43.

With these expressions for approximating the theoretical values for n and k, the material properties of ZBLAN were used to generate a new data series of n and k values. The theoretical values using this approach were compared to the optical data generated from the

CompleteEASE analysis of the Ellipsometer data. The theoretical n values closely matched the empirical n values from the Ellipsometer data (there was a 0.11% error margin between the experimental and theoretical data points), which in turn also closely lined up with the refractive index matching liquids data (see Appendix E for list of n values). This implies that the theoretical k values can therefore be used appropriately in substitution of the k values from the ellipsometer data.

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To obtain an approximated attenuation loss value at various wavelengths, the theoretical generated k values were converted to Attenuation Loss in dB/km using the following relation of Eq. (5.3),

݀ܤ ߨ ͳͲଵଷ (5.3) ൬ ൰ൌ݇ൈͶ൬ ൰ ቆ ቇ ݏݏ݋ܮݐݐ݁݊ݑܽݐ݅݋݊ܣ ݓܽݒ݈݁݁݊݃ݐ݄ ͳͲ ݉ܭ

The attenuation loss for the 4000 ⁰C/min (fully amorphous) cooled samples were calculated to be:

-At 1500 nm wavelength, the loss is 1.02 dB/Km.

-At 1600 nm wavelength, the loss is 0.532 dB/Km.

-At 1800 nm wavelength, the attenuation loss is 0.0917 dB/Km.

Refer to Appendix E.2. and Figure 72 for attenuation loss for the entire wavelength range

200 – 1800 nm

5.6. Algorithm to Theoretically Predict the Critical Cooling Rate

This section covers a theoretical approach for determining a critical cooling rate to quench the

ZBLAN material, resulting in a sample completely free of all micro and nano-crystal inclusions. This approach is based off two studies, one by Lysenko et al [50], and another by

Lu et al [51].

In the study by Lysenko et al, homogenous nucleation rate function was incorporated as part of a greater algorithm that would yield a critical cooling rate that should hinder the formation of even a single nuclei. This model is based off the condition that in order to achieve the

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complete suppression of crystallization, when the sample is cooled from a melt (starting at a temperature above the melting temperature, Tm) to below the glass transition temperature

(Tg), there must be zero nuclei formation per unit volume. This condition is represented by the following relation,

ଵ ்௠ (ሺܶሻ݀ܶ ൏ ͳ, (5.4ܫ ׬ ௚் כ௩

where v* is the Critical Cooling Rate, Tm is the Melting Temperature, Tg is the Glass Transition

Temperature and I(T) is the function for homogeneous nucleation rate.

The function for the homogeneous nucleation rate, I(T), was simply derived from classical nucleation theory, and for the purposes of their application, the effect of non-stationary size distribution of heterophase fluctuations were taken into account. Lysenko was able to utilize this condition to approximate the critical cooling rate for various materials with different glass forming abilities, such as aluminium and nickel. What they concluded based off this approach was that to completely prevent crystallization in pure metal melts, the critical cooling rate they calculated was much higher than can be achieved in practical reality. They suggested that the high critical cooling rates were directly correlated to the high nucleation abilities of the materials in their study.

For the purposes of this thesis, the same approach will be taken as stated in the Lysenko study, however a different function will be used for I(T), the nucleation rate. In the study by Lu et al

[51], of the Naval Research Laboratory (NRL), they investigated the nucleation and crystal growth rate of ZBLAN glass both theoretically and empirically. By using a series of equations derived from classical nucleation theory, they were able to estimate the nucleation rate and

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crystal growth rate of the glass at various temperatures. According to their results, their method of theoretically predicting these rates closely aligned with their empirical data, using fitted parameters.

The equation they used to predict the nucleation rate was the following formula [52],

(5.5) ,

where a0, a1, a2, a3 and a4 are constants chosen to provide the best fit to their experimental data. a0 is an analogous parameter of the pre-exponent factor, a1 and a2 are analogous parameters of the activation barrier and Boltsmann constant, a3 was taken as “0” and a4 was an arbitrarily chosen temperature to fit the functions to the empirical data.

All of the parameters to satisfy Eq. (5.5) were chosen to fit their experimental data, and two curves were generated to represent both the upper and lower limit of their highly scattered data. For this analysis, the parameters for the upper limit of their data were used because this would represent the worst case scenario in which the nucleation rate would be at its maximum. These parameters were cited and presented in another paper by Hopgood &

Rosman [52].

Table 13. Parameters used for Nucleation Rate Formula, Eq. (5.5), as Summarized in an Article by Hopgood & Rosman [52]

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The program Matlab™ was used to reproduce the nucleation data in the Lu et al study and the following graph in Figure 70 was generated, which aligns quite closely with various other studies, such as the nucleation rate curve presented in a study by Leede et al [53] (the peak values of the nucleation rate may differ slightly, but the temperature range and overall shape of the peak is quite similar). Appendix D covers in greater detail the process of implementing the parameters as part of the nucleation rate formula, with the matlab code fully included.

Figure 70. Graph showing nucleation rate as calculated using matlab (above); corresponding to the upper limit curve of the nucleation rate as published in the study by Lu et al [51]

By using this reproduced data as the data set for the I(T) function in the condition as stated in the study by Lysenko et al, the parameter v* (the critical cooling rate) could then be determined. Solving for v* and then applying it per sample volume of ZBLAN used in the experiments for this thesis, which was 9.4 x 10-8 m3 (the volume of sample that fit in the crucible), yielded a critical cooling rate of 1100 ⁰C/min.

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5.7. Error in Cooling Rate Measurements

The cooling rate measurements were taken primarily with two methods. The first method, for the slower cooling rates of 5 ⁰C/min and 20 ⁰C/min, measured the rates using the internal instrumentation of the DSC. The second method (which was applied to the other 4 cooling rates), took measurements using a k-type thermocouple connected to an OMRON™ data logger.

An error analysis was carried out to determine the accuracy of the measuring methods and the repeatability of achieving the target cooling rates.

For the accuracy of the measurements, an error margin in the measurements was determined using the root sum square method. The independent error sources are comprised of the measurement of the temperatures, T, and the time intervals, t, used in Eq. (3.1) from Chapter 3.

The root sum square method uses the following equation to compute the error,

ο் ଶ ο௧ ଶ (5.6) οܴ ൌ ට൬ቀ ቁ ൅ ቀ ቁ ൰ ൈܴ ் ௧ ,

where ΔR is the absolute error in units of ⁰C/min, R is the cooling rate in ⁰C/min, ΔT is the accuracy of the temperature measurement instrument, T is the temperature measurement value, Δt is the accuracy of the time measurement instrument and t is the overall time interval measured.

For an example calculation, for the 100 ⁰C/min cooling rate, the accuracy of a k-type thermocouple is +/- 2.2 ⁰C, which was used as the value for parameter ΔT in Eq. (5.6). The T parameter was taken as 450 ⁰C, as this was the maximum temperature used in Eq. (3.1) to determine the cooling rate. For Δt, the data logger was set to record with an accuracy of +/-

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250 ms. For t, the total time taken to reach the glass transition temperature (260 ⁰C) from the melting temperature (450 ⁰C) was used, which was 120 s for the 100 ⁰C/min rate. For R, the cooling rate for this calculation was 100 ⁰C/min.

By inputting the aforementioned parameters into Eq. (5.6), a percentage error was computed to be 0.53 %, which gives an absolute error value, ΔR, of +/- 0.5 ⁰C/min, for the 100 ⁰C/min cooling rate. The percentage error and absolute error values for each cooling rate is summarized in Table 14.

Table 14. Summary of Accuracy of Cooling Rate Measurements

Cooling Rate % Error Absolute Error (⁰C/min)

5 ⁰C/min 0.031 +/- 0.002

20 ⁰C/min 0.13 +/- 0.03

100 ⁰C/min 0.53 +/- 0.5

900 ⁰C/min 2.0 +/- 20

4000 ⁰C/min 6.3 +/- 300

8000 ⁰C/min 35 +/- 3000

A third source of error comes from the temperature gradient between the outer wall of the crucible containing the test sample, and the test sample itself. This temperature gradient was considered to be negligible since the thickness of the crucible wall is incredibly thin, 0.1 mm in thickness. Calculations for the Biot number were done to substantiate this, where the Biot number, Bi, is defined as,

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௛௅೎ (5.7) ܤ݅ ൌ , ௞್

where h is heat transfer coefficient, Lc is the characteristic length (computed as the volume of the crucible divided by its surface area) and kb is the thermal conductivity of platinum. This calculation was performed using the heat transfer coefficient of air in free convection.

Inputting the relevant parameters into Eq. (5.7) yielded a Biot number of 0.0024. Since this number is less than 0.1, it indicates that the heat conduction inside the crucible wall is faster than the heat convection away from the wall, therefore it can be assumed the temperature is constant throughout the wall.

For examining the repeatability of the methods used to achieve the various cooling rates, a coefficient of variation approach was used to analyse the distribution in the recorded cooling rates.

The coefficient of variation, CV, was determined by calculating the ratio of the root mean squared error, RMSE, to the mean of the recorded cooling rates, using the following equations,

Eq. (5.8) and Eq. (5.9),

(5.8) ଵ ே ൌ ට σሺݔ െݔො ሻଶ ܧܵܯܴ ே ௜ୀଵ ௜ ௜ ,

,where N is the number of observations, ݔ௜ is the observed cooling rate for the ith observation

and ݔො௜ is the expected cooling rate. Dividing the RMSE by the mean value of the observed cooling rates, yields the following,

ܴܯܵܧ (5.9) ܥܸ ൌ ݔҧ 114

where CV is the coefficient of variation, RMSE is the root mean squared error and ݔҧ is the mean of the observed cooling rates.

For the cooling rates observed for the test samples to be cooled at 100 ⁰C/min, the spread of the observed rates was quite low with an RMSE of 6 ⁰C/min, which corresponds to a CV of

6%. The RMSE and CV of all the cooling rates are summarized in Table 15.

Table 15. Summary of Coefficient of Variation and Root Mean Square Error for all Cooling Rates

Cooling Rate RMSE (⁰C/min) CV (%)

5 ⁰C/min 0.01 0.2

20 ⁰C/min 0.36 2

100 ⁰C/min 6.0 6

900 ⁰C/min 68 8

4000 ⁰C/min 310 10

8000 ⁰C/min 2205 30

5.8. Discussion

5.8.1. SEM Results

The SEM images from Figure 45 (showing side by side comparison of 5 ⁰C/min and 20 ⁰C/min cooled test samples) clearly demonstrate that an increasing cooling rate impedes the growth of a crystal phase. As the cooling rate increases, the images display each crystal phase 115

decreasing in size. The most noticeable difference in size is the light “feathery” morphology.

In the test sample cooled at 5 ⁰C/min, the “feathery streaks” grew to an average length between

100 to 500 μm. For the test sample cooled at 20 ⁰C/min, these crystals only grew to an average length of 20 to 30 μm.

The Bow-Tie/V shaped crystal phase also shows a large decrease in size between the 5 ⁰C/min and 20 ⁰C/min samples, from an average width (measured from the top point of either side of the “V”) of 40 μm to 2 μm. The dark regions with “black specs”, diminish in size from 50 μm to 10 - 15 μm in length.

The suppressive effect cooling rate has on crystallization can be seen most evidently in

Figure 39 (test sample cooled at 20 ⁰C/min), where the growth of the “feather” crystal phase stretches through the left side of the SEM image, but the right side remains fully amorphous.

This image shows that the growth of this crystal phase extends dendritically, akin to snowflake formation.

This is common in many melts that are undercooled, where dendritic growth occurs when the interface between the liquid and solid moves into a super cooled liquid region with a temperature that falls in advance of the interface. The heat of fusion being released at this interface provides conditions where the temperature at the interface is greater than both the solid and liquid regions on either side, creating a temperature inversion. Under these conditions, any small perturbations can cause cells to grow from the interface into the liquid.

Secondary branches form on these primary cells, triggering the first stage of dendritic growth.

The latent heat raises the temperature of the liquid adjacent to developing primary cells, retarding the growth of cells on the general interface. Along with the crystallographic

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directional growth, this leads branches in the dendrites to grow in very complex, ordered patterns.

However, the growth velocity is limited, and if the surrounding liquid region solidifies sufficiently rapidly, the dendrites will cease to grow. This is evident in Figure 39, where the growth of the dendrites can be seen to halt abruptly, leaving large portions of the ZBLAN matrix still in an amorphous state.

5.8.2. XRD Results

The x-ray diffractogram, as shown in Figure 47, for the 5 ⁰C/min test sample illustrates that the sample is heavily crystalline when cooled at a slow cooling rate. Multiple strong distinct peaks can be seen with high intensities, which indicates a multi-phase crystalline structure in the test sample.

As the cooling rate increases, these peaks diminish in intensity, but the number of peaks and their location remain more or less the same (see Figure 51). This level of consistency corroborates what was seen in the SEM images and the EDS spectra, and matches phases identified in literature with papers by Barrico et al [19], Carter et al [20], Gerard De Leede [43].

The peaks primarily relate to two typical crystal phases, one phase is rich in zirconium and sodium, the other is rich in zirconium and barium. As the cooling rate increases, these crystal phases do not have sufficient time to grow, nevertheless, their morphology still remains the same. This explains the overall peak positions for each spectrum remaining similar with increasing cooling rate, only the intensity of the peaks themselves decrease.

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The test samples cooled 900 ⁰C/min or faster produced diffractograms without defined peaks, which would suggest there are no longer crystal phases present. However, this is not entirely the case. The peaks and their intensity relate to the concentration of d-spacings related to a particular crystal plane configuration. The lack of peaks only suggests that the concentration is minute, therefore it is no longer detectable by this analysis method. This is why other microscopy techniques such as TEM were necessary, because they can probe for crystal structures on much smaller scales and at a lower level of ordering.

5.8.3. TEM Results

Both the SEM images and XRD patterns indicate that there are no crystal phases present for the test samples cooled at 900 ⁰C/min. However, these results were due to a detection limit for these techniques. Under TEM analysis, these samples were shown to still contain nano- crystals. By examining the bright field images and corresponding diffraction patterns, it was possible to probe for crystal structures in the sub-micrometre range.

For bright field imaging, typically when a test sample is amorphous, the sample will have a consistent appearance without any irregularities in light or shade, the specimen should have a uniform “texture”. For the test samples cooled at 100 ⁰C/min, the “grainy” and “speckled” appearance of the sample fragments were strong indicators of the presence of crystallites. For the test samples cooled at 900 ⁰C/min, the bright field images showed fragments that were still

“grainy” in appearance, but with a more uniform texture in their overall appearance.

For the 4000 ⁰C/min and 8000 ⁰C/min cooled test samples, it was visible how exceptionally uniform the fragments were, even at a high magnification there were no signs of irregularities.

This was further substantiated by the high resolution, HRTEM, image (refer to Figure 63), 118

which presented a view of the disordered molecular structure of the 4000 ⁰C/min ZBLAN test sample.

As informative as the bright field images were, the accompanying diffraction patterns were crucial to gaining more information about the structure of the crystallites. In general, the diffraction patterns appeared consistent from fragment to fragment, indicating that in terms of the crystal/amorphous structure, the fragments within a test sample were consistent.

For the 100 ⁰C/min test samples, the rings of the diffraction pattern seen in Figure 55 indicate that the crystal grains are fine and the material is polycrystalline. This is expected because observing the bright field imaging (Figures 52 - 54), an abundance of crystallites in randomised directions were visible throughout the ZBLAN glass matrix. The rings were matched to a BaF2 crystal structure which is expected, since this constituent is found in the many crystal species that first crystallize when the sample is cooled slowly [2].

For the 900 ⁰C/min test samples, the fragments were primarily amorphous, as can be seen by the characteristic appearance of the “intense band” and diffuse outer ring of the corresponding diffraction pattern in Figure 60. However, there are still distinct spots in the band and the outer ring. These spots are part of a ring pattern, but the lack of a full ring indicates that the concentration of crystallites in this sample is reduced significantly. The spots in this pattern are much broader than the widths of the rings observed in the 100 ⁰C/min diffraction pattern (refer to Figure 55), which indicates that the grain size of these crystallites is smaller. Larger grains result in narrow rings, while smaller grains result in broad rings or spots as observed in the 900 ⁰C/min diffraction pattern of Figure 60. This diffraction pattern corroborates the fact that as the more rapid the cooling rate becomes, there is less time for the crystal grains to grow in size, resulting in the suppression of crystallization. The crystal

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species in this diffraction pattern AlF3 is expected, as was reported in the study by Li et al. [2], where alongside LaF3 and ZrF4 were some of the crystal species observed to crystallize first out of the melt when cooled. The other samples in this thesis were identified to contain many different crystal species, which is an indicator of the plethora of variant species that can be found in the ZBLAN matrix. This is not surprising since ZBLAN is a multi-component system that has the potential to form many different crystals composed of a very complex variety.

In conjunction with the amorphous broad hump seen in the corresponding 900 ⁰C/min XRD pattern in Figure 50, it can be deduced that although there are micro-crystallites in the

900 ⁰C/min cooled test samples, they are either in a low concentration (if there are micro- crystals, they are not in a concentration able to form discernible peaks) or in the nano- crystalline range (since XRD is not sensitive to detecting crystallites in the sub micrometre range).

For the test samples at a higher cooling rate (4000 ⁰C/min and above), the diffraction patterns present what is considered a typical “amorphous halo” and intense band (see Figure 18 and

64), which indicates the ZBLAN test samples processed at this rate have a disordered molecular composition (which was corroborated by the HRTEM image in Figure 63). At this level of molecular disorder, the presence of the long range order from the previously observed crystal phases are completely absent. There is still a level of ordering in the 4000 ⁰C/min test sample, but only at a short range.

Typically, amorphous phases can be interpreted as structures composed of ultrafine random nano-clustered structures with characteristic cluster size of 10 – 30 atoms only [54]. In regards to crystalline structuring, the 4000 ⁰C/min test sample can be considered “fully amorphous” without any ordered structures larger than 3 nm. Any ordered structure in this test sample

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would associated with the polyhedral arrangements of the 5 heavy metal constituents of

ZBLAN and their fluoride counterparts.

5.8.4. Raman Analysis of 4000 ⁰C/min Test Sample

Raman Analysis was conducted on the 4000 ⁰C/min test sample to determine the short range order of the ZBLAN glass matrix. The Raman spectra consisted mainly of broad bands and a relatively strong continuous background due to Rayleigh scattering, which is typical of glassy materials [55]. The Raman spectrum in Figure 67 appears similar to the low-temperature processed test samples (which were assumed to still be in a glassy/amorphous state) in the study done by Qin et al [56]. The completely polarized dominant band at 583 cm-1 has a full width at half maximum of 68 cm-1 (see Figure 67 and Table 11), and this particular band has already been studied extensively to determine the coordination number of the Zr cations and their associated F anions.

In a study by Almeida and Mackenzie, they analysed a series of binary fluorozirconate glasses

(ZrF4-BaF2 System) and proposed that the basic structure consists of zig-zag chains of ZrF6 octahedra, cross-linked by Ba-F ionic bonds [57]. In a study by Kawamato and Sakaguchi [55], they performed a series of Raman analysis on a number of different fluorozirconate glasses, generating spectra for test samples that were both in a crystalline state and a vitreous state. In one of the spectra (see Figure 71), taken from a vitreous sample of BaZrF6, they found a dominant band that matches the one seen at 583 cm-1 in Figure 67.

Similar to the Raman spectrum for the 4000 ⁰C/min test sample in Figure 67, the spectrum for the vitreous BaZrF6 only has a strong and prominent peak for the polarized band, while the other bands are broad and low in intensity. According to the study by Kawamato and 121

Sakaguchi, this dominant band correlates to a singular zirconium atom coordinated with 8 fluorine atoms. It has been proposed that the basic structure of the vitreous compounds are chains of ZrF8 dodecahedra.

Figure 71. Raman spectra for vitreous and crystalline BaZrF6, with the vitreous sample having a matching dominant band (at 580 cm-1) with the 4000 ⁰C/min test sample [55]

As can be seen from Figure 71, Raman spectra were collected for various samples, some crystalline and one that was claimed to be vitreous. The bands (non-dominant) in the spectra from the crystalline samples are no longer apparent in the spectra from the vitreous sample.

The similarities in the spectra between the 4000 ⁰C/min sample and vitreous BaZrF6 (which is both 8 coordinated in both its vitreous and crystalline phases), suggest that the dominant peak in the 4000 ⁰C/min spectrum is due to the short range order of the 8 -coordinated zirconium fluoride chained molecules.

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5.8.5. Attenuation Loss Measurements

The attenuation loss for the 4000 ⁰C/min test samples were low, approaching close to the best theoretical attenuation loss predicted up to the 1600 nm wavelength. Although the attenuation loss is not as low as the theoretical best, it is still a considerable result because even at this level of loss, the degradation of a signal through a ZBLAN fibre with this attenuation would be greatly minimized.

The attenuation loss of 0.532 dB/Km at 1600 nm and 0.0917 dB/Km at 1800 nm, follows the theoretical trend as shown in Figure 72. There is a possibility that if further data could be collected further into the deep IR wavelengths, the attenuation loss for this test sample could have reached as low as 0.05 dB/km at 2200 nm. This attenuation loss is close to the value estimated to be the probable achievable loss, 0.04 dB/km, as suggested in an article by Day et al [1].

Using the analysis approach outlined in the study by Forouhi et al and the data from the ellipsometer, it is important to note that the calculations for these attenuation loss values are based off known and expected characteristics of the UV absorption of optical materials. The k values generated by this approach are less reliable when the wavelength goes further into the mid and far IR region, meaning the reliability of the attenuation loss values can only be taken into consideration within the near infrared range. To be able to generate results that are more representative of the mid and far infrared range (in the 1800 - 2700 nm wavelength range) of the 4000 ⁰C/min test samples, more data needs to be collected.

Additionally, finding an analysis approach to determine the attenuation loss for the crystallized test samples should be done in the future. This would provide a greater understanding of how the crystallites directly affect the attenuation loss in the test samples.

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5.8.6. Algorithm to Theoretically Predict the Critical Cooling Rate

This critical cooling rate of 1100 ⁰C/min is a reasonable value, and using this approach has (as expected) yielded a slightly higher cooling rate than 900 ⁰C/min (the 900 ⁰C/min samples were almost fully amorphous, but still contained traces of nano and micro-crystals). This value matches well with the experimental observations of this thesis, since the 4,000 ⁰C/min samples were fully amorphous, the theoretical critical cooling rate was expected to be in the order of magnitude of 103 ⁰C/min.

The result that this theoretical approach has yielded a critical cooling rate that corroborates the findings of the experimental observations is not surprising, considering the original model data in the study by Lu et al was also fitted empirically to their experimental findings. The theoretical critical cooling rate yielded by this approach appears reliable and for any future work should be used as the baseline cooling rate for processing ZBLAN.

5.8.7. Error in Cooling Measurements

For the accuracy of the cooling rate measurements, as is evident in Table 14, the slower cooling rates of 5, 20 and 100 ⁰C/min have a low percentage error, below 1%. For the 900 and

4000 ⁰C/min cooling rates, the percentage error is still relatively low. However, for the fastest cooling rate of 8000 ⁰C/min, the percentage error increases significantly. This is an acceptable inaccuracy, since the error margin indicates that the possible recorded value at the lowest end of the margin (8000 – 3000 = 5000 ⁰C/min) is still higher than the cooling rate below it

(4000 ⁰C/min). The importance of the 8000 ⁰C/min test samples was to determine if test samples cooled at a rate faster than the 4000 ⁰C/min no longer showed any changes and a critical cooling rate had indeed been achieved.

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For the repeatability of the cooling rates, the RMSE (root mean square error) is an indicator of the standard deviation of the cooling rates achieved. As is evident in Table 15, the RMSE is low for the slow cooling rates, however there is a higher variation for the recorded higher cooling rates. The CV (coefficient of variation) value reflects this increase in the distribution, where for the fastest rate, the 8000 ⁰C/min test samples, the actual cooling rates achieved varied as high as 30% from their target cooling rate of 8000 ⁰C/min. This CV result was expected for the fastest cooling rate, because the methods of achieving exceptionally high cooling rates are difficult to control.

The test samples processed in the DSC are in a controlled environment, and slower cooling rates are much easier to maintain (this was the highest precise method of cooling). However, as can be seen from the 8000 ⁰C/min cooling rate CV results, quenching the test samples in water is not a precise method and has a low reproducibility in achieving a consistent cooling rate. For these test samples, the mean observed cooling rate was 8000 ⁰C/min, however some of the test samples for this run (samples were quenched in water) were recorded to have a cooling rate as low as 7000 ⁰C/min, whilst others (cooled with exactly the same method) were as high as 11,000 ⁰C/min. This lack of repeatability is most likely due to a combination in the error margin of the data logger (the time recordings should have been recorded at a higher resolution) combined with using water as a quenching media (the water volatized immediately on contact, therefore making the contact between the water and bolt container inconsistent).

Nevertheless, as was mentioned previously, for the test samples intended to be cooled at 8000

⁰C/min, the main purpose of investigating this particular cooling rate was to determine whether or not there was a change in the samples cooled faster than 4000 ⁰C/min. Therefore

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this large spread in the 8000 ⁰C/min cooling rates is not relevant, since all of these test samples showed no differences under TEM analysis, all of them appeared identical.

The 4000 ⁰C/min cooling rates had a relatively lower spread within a tolerable limit, which indicates processing the test samples in the REPD (Rapid Electro-thermal Processing Device, refer to Appendix C for more information) is a sufficiently precise method of cooling the test samples.

Concluding Remarks

The results of these various microscopy and diffraction analysis techniques indicate a strong correlation between the impediment of the crystallization process and the cooling rate of the test samples. As the samples are cooled faster, the rate of diffusion of the molecules (which are in in a disordered arrangement in the molten state) decreases as well, therefore there is not sufficient time for the molecules to rearrange to the most energetically stable state (the crystalline state for ZBLAN).

The growth of the crystal phases are strongly dependent on the diffusion rates, the rate of cooling and the availability of nucleation sites for crystallization. When the test samples were cooled slowly, with a high presence of nucleation sites (mostly heterogeneous), many small grains proliferated, were able to grow and agglomerated into large crystals. This growth mechanism is common to most highly crystal forming materials [67, 74].

With an increase in the cooling rate, and a high presence of nucleation sites, instead of large crystals forming, the ZBLAN material formed a large number of small grains, resulting in micro-crystals. With a sufficiently fast cooling rate, the diffusion rate is restrictive to the point that the nuclei are unable to grow, effectively resulting in the suppression of crystallization. 126

There have been various studies in literature examining the importance of diffusion mechanisms in the phenomenon of crystallization, most noticeably in a study by Sestak [22].

With only the sparse formation of nuclei, this creates a “glassy” matrix that can be considered fully amorphous or “nano-crystalline” (the delineation between the two however can be difficult to substantiate). Ultimately, if it were possible to “eliminate” nucleation sites entirely, a test sample could be cooled slowly and still result in a “true glass”.

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CHAPTER 6: Conclusion and Recommendations

6.1. Conclusion

With the collation of all the results, a holistic picture of the heat treatment process and its effect on the material properties and performance capabilities of ZBLAN glass has been achieved.

The various analysis methods used to characterize the test samples, and conducting the experiments has culminated in a much better understanding of some key important factors to processing ZBLAN suitable for fibre optic applications.

First and foremost, in response to the primary hypothesis in question, in relation to the suppression effects of cooling rate on the crystallization of ZBLAN, a critical cooling rate does indeed exist that is capable of yielding samples that are entirely free of crystallites.

Theoretically, based on an algorithm combining work from two studies by Lysenko and Lu et al [50, 51] that utilizes classical nucleation theory, a theoretical critical cooling rate of approximately 1100 ⁰C/min was determined for a sample volume size of 9.4 x 10-8 m3 (the volume size of ZBLAN test samples processed in the crucibles).

Empirically, the critical cooling rate was determined to be somewhere between 900 ⁰C/min and 4000 ⁰C/min, which is corroborated by the theoretical critical cooling rate. There were still traces of nano-crystals in the 900 ⁰C/min test samples but no traces of crystallites in the

4000 ⁰C/min test samples. Table 14 summarizes the crystal sizes observed for each cooling rate.

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Table 16. Summary of Cooling Rate and its Effect on Crystal Size

Cooling Rate Crystallization Crystal Size Observed?

5 ⁰C/min Yes Large crystals in excess of 100 μm

20 ⁰C/min Yes Medium crystals in range of 2- 30 μm

100 ⁰C/min Yes Micro-crystals ˂ 10 μm

900 ⁰C/min Yes Nano-crystals

4000 ⁰C/min No Fully amorphous structure

8000 ⁰C/min No Fully amorphous structure

As can be seen in Table 16, the greater the cooling rate (with the same level of undercooling from the melt temperature), the more crystal growth rate is impeded. In the order of magnitude of 101 ⁰C/min cooling rate, the crystals grow to a length in the hundreds of micrometres. In the order of magnitude of 102 ⁰C/min cooling rate, the crystals are significantly impeded and only form crystallites in the single digit range of micrometres. It is only in the order of magnitude of 103, in the thousands of ⁰C/min, that the test samples become fully amorphous with the exclusion of even nano-crystals.

The optical constants for the fully amorphous ZBLAN samples were then characterized using a combination of refractive index matching liquids, ellipsometer data and an analysis approach by Forouhi et al [48]. It was estimated that for the fully amorphous ZBLAN test samples (cooled at 4000 ⁰C/min), they have an attenuation loss of 0.532 dB/Km at 1600 nm, which although is not at the theoretical best, is still a marked improvement on the attenuation loss of ZBLAN fibres manufactured to date. The lowest attenuation loss based on this data is 129

0.092 dB/Km at 1800 nm, however there is a possibility that if further data could be collected further into the mid and deep IR wavelengths, the attenuation loss for this test sample could have reached as low as 0.05 dB/km at 2200 nm.

Figure 72. Attenuation loss for silica and theoretical ZBLAN [17], the attenuation loss for 4000 ⁰C/min test sample has been added to the graph in purple, ZBLAN’s approximate theoretical attenuation loss is in black, silica fibre attenuation loss is in yellow

Although there may be many complications and difficulties with processing ZBLAN, with the conclusions drawn from this project, turning ZBLAN into a viable material as an optical waveguide for fibre optic technology, especially long haul applications, is certainly achievable. The results of this thesis provide strong evidence that ZBLAN has the potential to reach an attenuation loss better than silica fibres, however more testing needs to be done to strengthen the validity of this conclusion. Even more testing needs to be done to determine whether or not it is possible to manufacture a long fibre to this degree of amorphous “clarity”.

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It is one feat to process ZBLAN samples within a crucible, however translating the same process to drawing fully amorphous fibres will in itself be a great challenge.

Additional Findings

There have been a number of other important conclusions that can be drawn from the results of this experiment, they include:

-Test samples can be powdered if processed in the REPD. If ZBLAN is powdered and processed in a crucible under a slower heating and cooling regime, the sample produces a large quantity of bubbles that become entrained in the sample as it cools. However, when processed rapidly, even when in powder form, the samples moltenize and resolidify quickly enough that there is a great decrease in the formation of entrained bubbles

-Magnets stabilize vitrification of material under rapid cooling and impede crystallization.

One of the more important findings, applying a rare earth magnet whilst processing the test samples allows the sample to remain vitreous even at a slower cooling rate. When in the presence of a magnet, test samples cooled at 100 ⁰C/min were fully amorphous (See Appendix

F).

-Magnets improve stability of material. Test samples processed with a magnet exhibit a greater uniformity in the matrix of the glass, whereas test samples without a magnet are more likely to have phase inhomogeneity and precipitates forming.

-ZBLAN material shows inhomogeneities in general. ZBLAN itself has varying material properties, whether it is phase inhomogeneity or precipitation of certain components in the matrix. This suggests that throughout the glass matrix, many of the material properties are 131

not consistent throughout. An example of this is the refractive index. When studied with the use of the refractive matching index liquids, various sections of the test sample had different refractive indices within a range of +/- 0.004.

6.2. Recommendations for Future Work

The findings of this thesis are very promising, with results that strongly support a critical cooling rate does indeed exist in which all crystallite formation in ZBLAN can be suppressed.

This critical cooling rate is practically achievable and is in the order of magnitude 103 (with a minimum of 1100 ⁰C/min as established from the theoretical approach) for a sample size of 9.4 x 10-8 m3. For future experimental and manufacturing work, the cooling rate itself will need to be scaled accordingly to the dimensions of the sample or fibre being processed.

There are a few additional processing factors that must be taken into consideration in any future work including; experimenting with annealing processes to remove microbubbles, the test samples or fibres need to be processed in an absolutely clean environment to prevent any impurities contaminating it, and all processing should be done in conjunction with a magnet.

If test samples/fibres are processed rapidly enough in the future (rapid cooling/heating rates achieved in the REPD), they will not require an inert environment, which would alter the processing procedure to be more convenient and easier. Note must also be taken that ZBLAN has a time dependent tendency to crystallize whenever it is above the glass transition temperature, therefore it is even more imperative to have a rapid thermal processing regime whenever it is processed.

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The next important step in this research would be to determine a processing method that utilizes a rapid cooling rate to draw a fully amorphous ZBLAN fibre. This fibre should then be analysed to determine its attenuation loss for long haul fibre optic applications. This would be easier to achieve as opposed to the approach applied in this thesis, since there are already techniques in existence to measure the attenuation loss directly from fibres (see Appendix

B.7).

Investigation of impurities in the starting material in the manufacturing of ZBLAN needs to be conducted to study their effects on attenuation loss. Further investigation into surface crystallization while fiber drawing a sample must be conducted, since within the scope of this thesis doesn’t cover surface nucleation/crystallization during a fiber drawing process.

Additionally, further research into other factors such as improving the mechanical strength of the ZBLAN fiber must be carried out to ensure its utility in outdoor environments.

With more rigorous testing and experimentation, there may be a way for ZBLAN to be manufactured in a cost effective and convenient method that can indeed transmit to near its theoretical attenuation loss. It will be an important question to ask whether or not utilizing

ZBLAN glass for this purpose will be worthwhile, if its improved performance over silica is enough to justify the complexity and the costs of manufacturing/processing it. This would require a great deal of further research work, however based on the findings of this thesis, there is indeed great potential for ZBLAN glass to be the “holy grail” it was touted to be for the telecommunications industry.

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APPENDIX A: Supplementary Literature Review

A.1. Processing of ZBLAN in Microgravity

A compelling qualitative study published in 2004 was conducted by Tucker et al., in which

ZBLAN was processed in a unit gravity environment and then a reduced gravity environment

[16]. Based on the findings of the study, ZBLAN glass can be processed in microgravity without crystallizing. In addition, a more detailed theory based on shear thinning phenomena was proposed as an explanation for the results obtained. Unfortunately, there is no quantitative data from the observed results obtained by Tucker et al.

Figure 73. Scanning electron micrograph of ZBLAN glass processed in unit gravity [16]

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Figure 74. ZBLAN fibre processed in microgravity [16]

Ultimately, with all the proposed theories and possibilities of synthesizing an ultrapure

ZBLAN sample, the most promising method appeared to be processing it in a microgravity environment. This is further confirmed by two other studies conducted on rocket experiments. The first study by S. Varma et al [5], saw a test rig processing doped and undoped samples of ZBLAN (of the L4 and L8 variety) aboard the CSAR-II rocket. The samples were re-heated to temperatures approximately 310 ⁰C, which is the conventional fibre drawing temperature, during the rocket’s suborbital flight. This yielded samples that showed reduced optical degradation than those processed in unit gravity. High gravity exposure of the samples during the rockets re-entry caused extensive crystallization in the samples, which remained at a temperature close to the onset of crystallization. This led to the conclusion that samples should either be rapidly cooled or quenched after processing. Another study by Gary

Workman et al [58], involved processing ZBLAN on NASA’s KC-135 reduced gravity aircraft and on a payload aboard the Conquest I rocket. Both studies have also provided definitive

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results that microgravity does indeed have a substantial effect on the suppression of crystallization.

In a study by A. Torres et al, it was stipulated that convection was the main factor being suppressed in microgravity, which in turn supresses crystallization [59]. This was determined through the analysis of the diffusion and convection terms in their contribution of overall mobility in the mechanism of crystal growth. Crystal growth is impeded because in microgravity, the suppression of convection starves developing crystallites, however only to a certain temperature. Any higher than 400 ⁰C results in the suppression of convection being ineffective. It has been recommended that within the fibre drawing range, microgravity processing provides the ideal environment to produce ZBLAN fibres with minimal defects due to a widened working temperature range.

A.2. Rapid Heating

Rapid heating of a material, particularly a glass forming one, is an important factor when it comes to processing a sample that is crystallite free. Such an example of the importance of applying a critical heating rate can be seen in a study conducted by Johnson et al [69], where crystallization in glass forming metals has been suppressed due to a rapid heating and cooling procedure. For a metallic glass that is continuously heated, the glass transition temperature

Tg and onset of crystallization Tx are heating rate-dependent, where the temperature interval between the two defines an available process window for the undercooled liquid [69].

The larger the temperature interval is, the higher the metastability of the cooled liquid is with respect to crystallization. In the study, it was found that at a conventional heating rate of 20

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K/min, the temperature interval is 85 K. However, above a critical heating rate of 200 K/s, crystallization was avoided entirely during heating. Such a process completely bypasses the onset of crystallization on heating at such a high rate. It is believed that the cause for this is that at conventional heating rates, 1 K/s, the undercooled liquid transitions through a narrow temperature range, in which crystallization becomes kinetically favourable.

Figure 75. Plot showing crystallization onset bypassing through a critical heating regime [69]

Using a heating rate on the order of 106 K/s, the undercooled liquid is accessible at any temperature above the glass transition point, through the melting point and above. In this study, the rapid heating method used was a rapid capacitive discharge e, in which a capacitor bank with capacitance C = 0.264 F and charging voltage of up to Vo = 200 V was used to store energies up to 5280 joules. Electrical pulses were used to then raise the temperature of the metallic glass at the critical heating rate. The most important achievement of this study was

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that by applying this heating regime, it was possible to 'beat' the intervening crystallization onset and successfully process even marginal glass-forming alloys that have limited stability against crystallization and are not usually processable by conventional means.

It is interesting to note that a similar phenomenon has been observed in ZBLAN glass. In the study by Battezzati [19], when ZBLAN samples were run through a DSC, it was determined that heating up a sample greater than 5 ⁰C/min merges the crystallization and melting points to the same point. Hence there is a possibility, that with a fast enough heating rate, the crystallization onset can be bypassed and a stable undercooled liquid can be accessed much more easily for fibre drawing.

A.3. Concluding Remarks

It has been demonstrated that in glass forming metal alloys, the crystallization onset can be entirely bypassed by a high enough heating rate. Currently there are several factors that have been attributed to being able to improve the extrinsic attenuation losses in ZBLAN; impurities need to be reduced to impede heterogeneous nucleation; ZBLAN needs to be heated high enough and long enough to encourage component homogeneous mixing, but low enough to avoid vaporization; it needs to be processed in such a way to avoid bubbling; and microgravity significantly improves the vitrification stability. As important as these factors are however, the factor of cooling rate could potentially be the most significant contributing factor, and may be the easiest method and cost effective way of processing ZBLAN.

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APPENDIX B: Attenuation Loss in Fibres

B.1. Optical Fibre Loss

The theoretical losses in the transmission of a signal can be attributed to several properties and aspects of the design and composition of a fluoride fibre. The following section briefly describes the loss mechanisms and components of a fibre’s structure that leads to such losses.

Below, in Figure 76, is a schematic of the structure of an optical fibre, illustrating the nature of the intrinsic and extrinsic loss mechanisms.

Figure 76. Diagram showing structure of a cladded optical fibre [63]

B.2. Extrinsic Loss mechanisms

Extrinsic properties of the fibre material can contribute to attenuation losses, most of these properties are related to the manufacturing process of the fibre. They include but are not limited to signal scattering sources due to bubbles, crystals, inclusions, the core/clad interface and geometrical defects. Other impurities introduced into the material during manufacture

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include transition metals such as iron and copper, rare earth metals and various other

molecular species. These impurities are sources of absorption for the signal. All of these

contributing factors can be controlled depending on the environment and procedural

standards put in place during the fabrication process. These factors dominate the losses

displayed by the attenuation spectrum observed in most fluoride fibres.

B.3. Intrinsic Loss Mechanisms

These loss mechanisms are related to the innate properties of the chemical composition of the

fibre itself. Intrinsic scattering sources include Rayleigh, Raman, Brillouin scattering as the

signal wave interacts with the particles of the material. Intrinsic Absorption includes sources

such as electronic transitions and multi-phonon absorption.

The generalized V-curve of the theoretical intrinsic attenuation or loss curve for a transparent

solid is a result of three aforementioned sources: the first part of the curve is due to electronic

transitions, the second is due to Rayleigh scattering and the last segment is due to IR edge

absorption, also known as the multi-phonon edge.

The V-curve can be approximated with the following formula,

௔ ஻ ௖ (ݔ݌ሺെ ሻ, (B.1݁ܥݔ݌ቀ ቁ൅ ൅݁ܣߙ ൌ ௧ ఒ ఒర ఒ

where αt is the total intrinsic loss, a, c, A, B, and C are material constants and λ is the

wavelength. At shorter wavelengths, optical absorption increases because the energy of a

photon of light causes electrons to jump from the valence to the conduction band, essentially

148

freeing the electron from its binding atom and releasing it into the atomic lattice as a

‘delocalized electron’.

Rayleigh scattering dictates the bottom part of the curvature and is the limiting factor in the minimum attainable losses for a fibre material. This phenomenon results from the localized fluctuations in the refractive index of the material, which is caused by the variations in density or composition of the glassy material as it is cooled from a molten state.

At longer wavelengths the attenuation losses are dictated by an infrared vibrational or ‘multi- phonon’ edge. This is due to photons of light, which have a frequency that matches the fundamental vibrational frequency of the cation-anion pairs of a substance, being absorbed and increasing the amplitude of the inter-atomic vibrations. The atoms in a structure have many degrees of freedom, which ultimately means that the substance itself can absorb energy at wavelengths longer than that of the fundamental. Combinations of these modes with the fundamental, in combination with overtones of the fundamental itself, produce a continuous absorption spectrum that comprises this infrared vibrational or multi-phonon edge.

149

Figure 77. Graph showing a generalized V-curve [63]

Figure 78. Chart summarizing all of the different sources of extrinsic and intrinsic losses [63]

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B.4. Intrinsic Loss Measurements

The most extensive measurements of intrinsic scattering in fluorozirconate glasses were made by Shroeder et al [33]. This was done by examining the power scattered at 90 degrees from the beam of an argon ion laser operating at 488 nm passing through the sample. Using this set up, values for B (coefficient in Eq. (B.1)) have been recorded between the range of 0.14 - 0.3. France et al [64] have applied a more precise technique by using an integrating sphere to collect light scattered out from fibres transmitting a krypton ion laser. In general, values of B ranging between 0.6 - 1.34 have been obtained through various measurement techniques, which is consistent with theoretical estimates for fluorozirconate glasses.

For the multi-phonon aspect of the V-curve, a complete determination of the IR edge has been made by combining data on bulk glasses with results from optical fibres.

The overall intrinsic loss in a ZBLAN glass fibre has been determined to follow the following parameters:

଴Ǥ଻ଶ ି଻ଵǤ଺ସ ߙ ൌ ൅ܥቀ ቁ݀ܤȀ݇݉, (B.2) ூே ఒర ఒ where αIN is the total intrinsic loss, C is a material constant and λ is the wavelength.

B.5. Extrinsic Loss Measurements (Absorption)

In most cases an IR Spectrometer can be used to measure transmission spectra, where percentage transmission T is plotted as a function of wavenumber. This can be seen in Figure

79, where transmittance for a silica aero gel sample cuts off near 300 nm and around 2700 –

3200 nm.

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Figure 79. Infrared spectrum of silica aerogel [11]

Essentially, the intensity of a beam of radiation directed normally to the surface of a glass sample is compared to the emergent beam. This comparison is recorded as a percentage proportion of the initially transmitted beam, where the absorption is assumed to be given by the Lambert-Beer law.

Samples of bulk glass are first cut and polished before measuring in the spectrometer. Special preparation techniques must be employed when preparing fluoride glass samples, since their properties differ greatly from traditional oxide materials. Depending on how a sample is prepared can correspond to anomalous, large peaks in the spectra. These peaks can be indicators of contaminants on the surface and within the matrix of the test sample.

Particular care should be taken with the conditions of the spectrometer, to obtain clear bulk data, including the sample chamber being flushed with N2 to remove CO2 noise areas at 4.3

μm and 2.7 μm. Samples should be washed with CCl4 and handled with gloves to avoid surface C-H absorptions at 3.5 μm. 152

B.6. Extrinsic Loss Measurements (Scattering)

Scattering centres found in fibres can be apparent to the eye when illuminated by light from a HeNe laser, and they require a microscope to view them. Large crystals, gas bubbles and sub-micron centres (nucleated centres) are identifiable as discrete points of red light and distinguishable from their surrounding medium by the change in refractive index of the scattering centres.

The scattering theory is covered by the following relation,

௔ ଶ ߙൌͳͲ݈݋݃ ߝܳܯ ቀ ቁ , (B.3) ଵ଴ ௥ where ε is the scattering coefficient, M is the number of scattering centres per km of length, a is the radius of the scattering centres, r is the core radius and Q is a parameter that is dependent on the relative refractive index and relative particle sizes of the medium and scattering centres.

B.7. Experimental Loss Measurements in Fibres

An optical fibre is normally characterised by measuring its total attenuation, which is the proportion of the input power remaining after propagation through a unit length of the material.

Broadband sources such as quartz-halogen and platinum lamps are adequate for measurements on fluoride fibres out to 4 μm. Lasers are more preferable because scattering and absorption measurements often require high intensity sources, such as Ar/Kr ion lasers,

Nd:YAG lasers, tunable lead salt diodes and colour centre lasers.

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Total loss measurements commonly use the cutback method. The transmission of the entire fibre length is first recorded as a function of wavelength, using an appropriate source and detector. Fourier transform infrared instruments (FTIR) or grating monochromators have been used.

Optical time domain reflectometry (OTDR) is a commonly used studying technique, which sends a pulse of laser light into the fibre and the intensity of reflected light is followed as a function of time or distance into the fibre. It is useful in pinpointing breaks or scattering defects.

Figure 80. Diagram showing layout of a spectral loss measurement apparatus [63]

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Figure 81. Layout of a FTIR spectrometer system [63]

Absorption losses can be measured utilizing calorimetric methods, where an illuminated fibre sample is placed through a calibrated calorimeter cell and the temperature increase is measured. As the absorbed energy in the sample leads to degradation by non-radiative thermal decay, this leads to an increase in temperature. White et al [65] constructed a cell using alcohol to transfer the heat to an adjacent thermocouple.

Figure 82. Set up for a fibre calorimeter [63]

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Scattering measurements can be taken using the integrating sphere or the integrating cube detector. In the integrating sphere, the fibre passes through a hollow sphere with an interior surface coated to allow diffuse reflectance of the scattered light. A fixed proportion of this light falls on a perpendicularly mounted detector.

Figure 83. Integrating sphere for scattering measurement [63]

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APPENDIX C: Rapid Electro-thermal Processing Device (REPD)

C.1. REPD Specifications

This section contains a description and wiring diagram that describes how the internal

circuitry of the REPD (Rapid Electro-thermal Processing Device) has been set up between the

transformer, Arduino, the safety timer and the solid state relay.

C.1.1. General Set Up

The general set up of the REPD can be seen in the following photograph (Figure 84). At the

front of device are several buttons used to change the input settings of the program that runs

the device, a screen that shows a live temperature reading of the apparatus and a reset button

for the safety switch.

Display Screen

Pin and Mounting/Cooling Block Assembly

User Input Control Buttons Removable Front Plate Safety Timer Reset

Figure 84. Photograph of Rapid Electro-thermal Processing Device

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C.1.2. Programming Control

The device is programmed using an Arduino board and is designed to interface with a serial communication port which can be read by Matlab. The program itself is started either by a red push button on the front plate of the device, or using a Matlab input from an attached laptop (laptop must be attached at all times for recording data and interfacing with the

Arduino).

The Arduino interfaces with a solid state relay, and utilizing pulse width modulation, controls the duration or intervals of power transfer from the transformer to the apparatus. By specifying the intervals in which the transformer is transferring power, it is possible to control how much current is flowing through the crucible, and therefore the temperature rise of the crucible itself (refer to Appendix C.2. for wiring diagram). By determining the length of these intervals and their frequency, the crucible temperature is raised in a continuous stream of controlled ‘heating steps’.

A thermocouple is attached at the bottom of the crucible and sends a signal back to the

Arduino, which reads inputs at a speed of 50 ms (20 Hz frequency). The Matlab program receives information from the Arduino at intervals of 100 ms, this in effect means the thermocouple sensor is scanned twice before each packet is sent to Matlab and the program decides to execute the next heating step. The program continues these heating steps with each iterative cycle, until it reads that the target temperature has been reached by the thermocouple. Then it will no longer input any further heating steps and deactivate.

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Figure 85. Live temperature reading of a crucible heated to 330 ⁰C, with the thermocouple attached to the bottom of the crucible

C.1.3. Heating/Cooling Profile

The following heating/cooling profile has been recorded using the Matlab™ program and has a high reproducibility with the device itself, indicating that this form of control is precise.

HR CR Key Tm HR: Heating Rate CR: Cooling Rate Tm: Melting Temp Tg: Glass Transition

Figure 86. Graph showing heating/cooling rate provided by REPD, this profile can be altered to have faster/slower rates 159

Heating Rate (HR)

The REPD when set at the maximum heating rate, is capable of heating the crucible from 30 ⁰C

(room temperature) to 450 ⁰C (melting point of ZBLAN) in 0.95 seconds.

This equates to a heating rate of approximately 440 K/s or 26,400 K/min.

Cooling Rate (CR)

The REPD is capable of cooling the crucible from natural convection and conduction through the aluminium frame from 450 ⁰C (melting point of ZBLAN) to 260 ⁰C (glass transition of

ZBLAN) in 0.633 seconds.

This equates to a cooling rate of approximately 300 K/s or 18,000 K/min (this maximum cooling rate was achieved with an empty crucible).

C.1.4. Safety Precautions

The apparatus itself is conductive during the operation process, however although the current flow is high, the operating voltage is only 2 volts. The human body does not have a high enough permeability for current to cause damage unless the voltage is higher than 50 VAC

(voltage high enough to cause current flow through the human body). Without enough voltage, the current flow cannot penetrate the human body due to its extremely high innate resistance. In addition to this, when being operated within the glove box, neoprene gloves were used, which in turn have a high resistance and are not conductive.

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Within the programming parameters itself, the Arduino reads the thermocouple sensor twice before the next heating step is applied, which acts as a built in redundancy, making sure the temperature will never exceed the target temperature. The temperature probe itself is analysed faster than the output of the transformer, which guarantees the equipment will mechanically act within the bounds of the program.

As an additional fail safe, a safety switch that is an independent part to the rest of the apparatus, acts as an automatic shutdown system.

Safety Timer

Figure 87. REPD with front plate removed to reveal circuitry inside, (left) safety timer is located in the top left corner, (right) close up of safety timer with turn dial to set safety time (currently set at 10 s)

The user specifies the maximum length of each run, which by default has been set to 10 seconds. When the apparatus is run and 10 seconds has elapsed, this timer will cut off power to the apparatus. Once the safety timer has activated, the entire apparatus cannot be run anymore. In order to run the next test sample, the entire apparatus needs to be either switched

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back off and then on (as per the Standard Operating Procedure protocol) or the safety timer has to be reactivated with a press of a button located at the front of the device. This acts as an extra monitoring step in the protocol, as a result the whole apparatus in effect needs to be resetted before every run.

Black button for resetting the safety timer, only press if mains power is not switched off after every run

Figure 88. Safety timer reset button

C.2. REPD Wiring Diagram

Figure 89 shows the wiring diagram for the entire apparatus.

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Figure 89. Wiring diagram for REPD, showing connections between the solid state relay, Arduino board, timer and the transformer 163

C.3. Processing Test Samples using the REPD

Using the REPD to process the test samples provided some very interesting results. The first and foremost important processing factor was the REPD allowed the test samples to be rapidly processed. Not only was this equipment capable of cooling the samples at extraordinarily fast rates (maximum cooling rate of 4000 ⁰C/min for processing ZBLAN), but it was able to heat the test samples at an even faster rate (maximum heating rate of

26,000 ⁰C/min actually achieved with ZBLAN).

This device shortens what would ordinarily take a thermal treatment process in the time scale of hours/minutes and reduces it to a process that only takes seconds. Not only would this result in a dramatically reduced processing time in the overall manufacturing method of

ZBLAN glass but this device also produces high quality glass free from crystallites.

One of the most significant results from using such a rapid heating and cooling process, is the

REPD was capable of processing ZBLAN in ambient air. Typically, ZBLAN must be processed in an oxygen free atmosphere, either under a high purity nitrogen or argon atmosphere.

ZBLAN tends to oxidize readily when processed at high temperatures in the presence of oxygen, thus most forms of heat treatment presently require a contained inert environment.

However, when the ZBLAN test samples were processed rapidly in the REPD outside of the glovebox (which is sealed with an argon environment) it was usually operated in, the test samples surprisingly showed no signs of oxidation (confirmed with an EDS analysis) and contained no crystallites. The REPD was operated under the fume hood of the laboratory, which would have been exposed to ambient atmosphere. Ultimately the samples created in this fashion were identical to the test samples processed with the slower heating regimes when the REPD was operated inside of the glovebox.

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It has been hypothesized that when the samples are processed at such a fast heating rate, the glass itself only remains in a molten state no longer than 500 milliseconds, which is not long enough for the glass to react with the ambient oxygen molecules. This indicates that so long as the processing method used to treat ZBLAN is rapid enough, it will circumvent the need for an inert processing environment, which would make manufacturing quality ZBLAN much easier and cost effective.

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APPENDIX D: Matlab Code for Algorithm for Theoretical

Critical Cooling Rate

This section covers the matlab coding used to recreate the data from the study by Lu et al [51]. All of the parameters are quoted as summarized in the article by Hopgood & Rosman [52].

MATLAB SCRIPT: clear all close all clc

% NRL Upper parameters a_0 = exp(261); % cm^-3 s^-1 K^-1 a_1 = 1.05 * 10^5; % K a_2 = 3.7 * 10^9; % K^3 a_3 = 0; % K a_4 = 850; % K

% integrate over temperature range (in Kelvin) T_max = 800; T_min = 200;

% Pre-allocate T and I vectors T = zeros(T_max - T_min + 1, 1); I = zeros(T_max - T_min + 1, 1); for ii = 1:(T_max - T_min + 1) T(ii) = ii + T_min - 1; % K I(ii) = a_0 * T(ii) * exp(-a_1/(T(ii)-a_3)) * exp(-a_2/(T(ii)*(T(ii)- a_4)^2)); end trapz(T,I) % integration

% graph limits (in K) x_max = 700; x_min = 400; figure(1) line(T-273.15, I, 'color', 'k') xlabel('Temperature (°C) ') ylabel('Nucleation Rate (cm^-3 s^-1)') ax1 = gca; set(ax1, 'Xcolor', 'k', 'YColor', 'k', 'xlim', [(x_min-273.15) (x_max- 273.15)]) figure(2) line(T, I, 'color', 'k') xlabel('Temperature (K) ') ylabel('Nucleation Rate (cm^-3 s^-1)') ax1 = gca; set(ax1, 'Xcolor', 'k', 'YColor', 'k', 'xlim', [x_min x_max])

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APPENDIX E: Attenuation Loss Calculation Data E.1. Modelled Data generated from CompleteEASE Analysis of SE Data for 4000 ⁰C/min Test Sample

λ (nm) n k λ (nm) n k

246.136200 1.535257 0.002834 390.811005 1.507303 0.002834

250.904343 1.533557 0.002834 400.351135 1.506413 0.002834

260.441010 1.530417 0.002834 409.891113 1.505584 0.002834

269.978241 1.527587 0.002834 421.020844 1.504685 0.002834

279.515961 1.525028 0.002834 430.560394 1.503969 0.002834

290.643860 1.522341 0.002834 438.509827 1.503406 0.002834

300.182556 1.520262 0.002834 440.099701 1.503298 0.002834

309.721527 1.518365 0.002834 441.689545 1.503190 0.002834

319.260864 1.516629 0.002834 451.228455 1.502567 0.002834

330.390411 1.514785 0.002834 460.766998 1.501981 0.002834

339.930237 1.513342 0.002834 470.305115 1.501431 0.002834

351.060242 1.511800 0.002834 481.432312 1.500829 0.002834

360.600403 1.510588 0.002834 490.969391 1.500344 0.002834

370.140564 1.509465 0.002834 500.505920 1.499887 0.002834

381.270813 1.508258 0.002834 510.041809 1.499455 0.002834

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λ (nm) n k λ (nm) n k

521.166199 1.498980 0.002834 691.047668 1.494315 0.002834

530.700623 1.498596 0.002834 700.561829 1.494148 0.002834

540.234253 1.498232 0.002834 710.074341 1.493988 0.002834

551.355896 1.497831 0.002834 721.170288 1.493810 0.002834

560.887817 1.497506 0.002834 730.679321 1.493663 0.002834

570.418823 1.497197 0.002834 740.186646 1.493522 0.002834

581.537048 1.496855 0.002834 749.692200 1.493386 0.002834

591.065979 1.496578 0.002834 760.779663 1.493234 0.002834

600.593811 1.496313 0.002834 770.281250 1.493108 0.002834

610.120544 1.496060 0.002834 781.364014 1.492968 0.002834

621.233704 1.495780 0.002834 790.861450 1.492852 0.002834

630.757935 1.495551 0.002834 800.356812 1.492741 0.002834

640.280945 1.495333 0.002834 811.432129 1.492615 0.002834

651.389526 1.495090 0.002834 820.923035 1.492512 0.002834

660.909729 1.494891 0.002834 830.411743 1.492412 0.002834

670.428528 1.494701 0.002834 841.479187 1.492300 0.002834

681.531982 1.494489 0.002834 850.963074 1.492208 0.002834

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λ (nm) n k λ (nm) n k

860.444702 1.492118 0.002834 1021.815430 1.490953 0.002834

871.503601 1.492017 0.002834 1042.152100 1.490843 0.002834

880.980103 1.491934 0.002834 1062.504761 1.490740 0.002834

890.454224 1.491853 0.002834 1072.687256 1.490690 0.002834

899.925781 1.491775 0.002834 1082.873657 1.490642 0.002834

901.504089 1.491762 0.002834 1089.666870 1.490610 0.002834

910.972656 1.491686 0.002834 1099.860107 1.490564 0.002834

920.438660 1.491613 0.002834 1103.258667 1.490549 0.002834

931.479126 1.491531 0.002834 1110.057251 1.490519 0.002834

940.939331 1.491462 0.002834 1120.258545 1.490475 0.002834

950.396851 1.491396 0.002834 1130.463745 1.490433 0.002834

959.851685 1.491332 0.002834 1140.673096 1.490391 0.002834

970.878601 1.491259 0.002834 1150.886353 1.490351 0.002834

980.327271 1.491199 0.002834 1161.103760 1.490312 0.002834

989.772827 1.491140 0.002834 1171.324951 1.490273 0.002834

999.215637 1.491083 0.002834 1181.550537 1.490236 0.002834

1011.653137 1.491010 0.002834 1191.779907 1.490200 0.002834

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λ (nm) n k λ (nm) n k

1195.190674 1.490188 0.002834 1352.567139 1.489732 0.002834

1202.013428 1.490164 0.002834 1362.863770 1.489707 0.002834

1212.250854 1.490130 0.002834 1373.164307 1.489683 0.002834

1222.492432 1.490096 0.002834 1380.033691 1.489668 0.002834

1232.737915 1.490063 0.002834 1390.341064 1.489645 0.002834

1239.570557 1.490041 0.002834 1400.652344 1.489622 0.002834

1249.822754 1.490010 0.002834 1410.967773 1.489600 0.002834

1260.079102 1.489979 0.002834 1421.287231 1.489578 0.002834

1263.498779 1.489969 0.002834 1431.610596 1.489557 0.002834

1270.339478 1.489949 0.002834 1441.938232 1.489537 0.002834

1280.603760 1.489920 0.002834 1452.269775 1.489516 0.002834

1290.872192 1.489891 0.002834 1462.605225 1.489496 0.002834

1301.144653 1.489863 0.002834 1472.944824 1.489477 0.002834

1311.421021 1.489835 0.002834 1483.288452 1.489458 0.002834

1321.701538 1.489809 0.002834 1490.186401 1.489445 0.002834

1331.986084 1.489782 0.002834 1500.536743 1.489427 0.002834

1342.274536 1.489757 0.002834 1510.891113 1.489409 0.002834

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λ (nm) n k λ (nm) n k

1521.249512 1.489391 0.002834 1611.190918 1.489252 0.002834

1531.611938 1.489374 0.002834 1621.588257 1.489237 0.002834

1541.978271 1.489357 0.002834 1631.989624 1.489223 0.002834

1552.348877 1.489340 0.002834 1642.395142 1.489208 0.002834

1562.723389 1.489324 0.002834 1652.804443 1.489195 0.002834

1573.101807 1.489308 0.002834 1659.746338 1.489186 0.002834

1580.023193 1.489297 0.002834 1670.162476 1.489172 0.002834

1590.408447 1.489282 0.002834 1680.582642 1.489159 0.002834

1600.797607 1.489267 0.002834 1687.531616 1.489150 0.002834

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E.2. n, k and Attenuation Loss Values Generated using Approach by Forouhi et al [48]

λ (nm) n k dB/Km λ (nm) n k dB/Km 200 1.57 1.20E-07 3.27E+04 405 1.51 6.85E-09 9.23E+02 205 1.57 1.05E-07 2.79E+04 410 1.51 6.57E-09 8.75E+02 210 1.56 9.25E-08 2.40E+04 415 1.51 6.31E-09 8.29E+02 215 1.56 8.21E-08 2.08E+04 420 1.51 6.06E-09 7.87E+02 220 1.55 7.33E-08 1.82E+04 425 1.51 5.82E-09 7.47E+02 225 1.55 6.59E-08 1.60E+04 430 1.51 5.60E-09 7.10E+02 230 1.55 5.94E-08 1.41E+04 435 1.51 5.38E-09 6.75E+02 235 1.54 5.39E-08 1.25E+04 440 1.50 5.18E-09 6.43E+02 240 1.54 4.91E-08 1.12E+04 445 1.50 4.99E-09 6.12E+02 245 1.54 4.48E-08 9.98E+03 450 1.50 4.80E-09 5.83E+02 250 1.54 4.11E-08 8.97E+03 455 1.50 4.63E-09 5.55E+02 255 1.53 3.78E-08 8.09E+03 460 1.50 4.46E-09 5.30E+02 260 1.53 3.49E-08 7.32E+03 465 1.50 4.31E-09 5.05E+02 265 1.53 3.22E-08 6.64E+03 470 1.50 4.16E-09 4.83E+02 270 1.53 2.99E-08 6.04E+03 475 1.50 4.01E-09 4.61E+02 275 1.53 2.78E-08 5.51E+03 480 1.50 3.87E-09 4.40E+02 280 1.53 2.59E-08 5.04E+03 485 1.50 3.74E-09 4.21E+02 285 1.53 2.42E-08 4.63E+03 490 1.50 3.62E-09 4.03E+02 290 1.52 2.26E-08 4.25E+03 495 1.50 3.50E-09 3.86E+02 295 1.52 2.12E-08 3.91E+03 500 1.50 3.38E-09 3.69E+02 300 1.52 1.99E-08 3.61E+03 505 1.50 3.27E-09 3.54E+02 305 1.52 1.87E-08 3.34E+03 510 1.50 3.17E-09 3.39E+02 310 1.52 1.76E-08 3.09E+03 515 1.50 3.07E-09 3.25E+02 315 1.52 1.66E-08 2.87E+03 520 1.50 2.97E-09 3.11E+02 320 1.52 1.56E-08 2.67E+03 525 1.50 2.87E-09 2.99E+02 325 1.52 1.48E-08 2.48E+03 530 1.50 2.79E-09 2.87E+02 330 1.52 1.40E-08 2.31E+03 535 1.50 2.70E-09 2.75E+02 335 1.52 1.32E-08 2.16E+03 540 1.50 2.62E-09 2.64E+02 340 1.51 1.26E-08 2.02E+03 545 1.50 2.54E-09 2.54E+02 345 1.51 1.19E-08 1.89E+03 550 1.50 2.46E-09 2.44E+02 350 1.51 1.13E-08 1.77E+03 555 1.50 2.39E-09 2.35E+02 355 1.51 1.08E-08 1.66E+03 560 1.50 2.32E-09 2.26E+02 360 1.51 1.03E-08 1.56E+03 565 1.50 2.25E-09 2.17E+02 365 1.51 9.79E-09 1.46E+03 570 1.50 2.18E-09 2.09E+02 370 1.51 9.34E-09 1.38E+03 575 1.50 2.12E-09 2.01E+02 375 1.51 8.91E-09 1.30E+03 580 1.50 2.06E-09 1.94E+02 380 1.51 8.51E-09 1.22E+03 585 1.50 2.00E-09 1.87E+02 385 1.51 8.14E-09 1.15E+03 590 1.50 1.94E-09 1.80E+02 390 1.51 7.79E-09 1.09E+03 595 1.50 1.89E-09 1.73E+02 395 1.51 7.46E-09 1.03E+03 600 1.50 1.84E-09 1.67E+02 400 1.51 7.14E-09 9.75E+02 605 1.50 1.79E-09 1.61E+02

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λ (nm) n k dB/Km λ (nm) n k dB/Km 610 1.50 1.74E-09 1.55E+02 835 1.49 5.70E-10 3.73E+01 615 1.50 1.69E-09 1.50E+02 840 1.49 5.57E-10 3.62E+01 620 1.50 1.64E-09 1.45E+02 845 1.49 5.45E-10 3.52E+01 625 1.50 1.60E-09 1.40E+02 850 1.49 5.33E-10 3.42E+01 630 1.50 1.56E-09 1.35E+02 855 1.49 5.21E-10 3.33E+01 635 1.50 1.52E-09 1.30E+02 860 1.49 5.09E-10 3.23E+01 640 1.50 1.48E-09 1.26E+02 865 1.49 4.98E-10 3.14E+01 645 1.50 1.44E-09 1.22E+02 870 1.49 4.87E-10 3.06E+01 650 1.50 1.40E-09 1.17E+02 875 1.49 4.77E-10 2.97E+01 655 1.50 1.36E-09 1.14E+02 880 1.49 4.66E-10 2.89E+01 660 1.50 1.33E-09 1.10E+02 885 1.49 4.56E-10 2.81E+01 665 1.50 1.29E-09 1.06E+02 890 1.49 4.46E-10 2.73E+01 670 1.50 1.26E-09 1.03E+02 895 1.49 4.36E-10 2.66E+01 675 1.50 1.23E-09 9.93E+01 900 1.49 4.27E-10 2.59E+01 680 1.50 1.20E-09 9.61E+01 905 1.49 4.17E-10 2.52E+01 685 1.50 1.17E-09 9.30E+01 910 1.49 4.08E-10 2.45E+01 690 1.50 1.14E-09 9.00E+01 915 1.49 4.00E-10 2.38E+01 695 1.50 1.11E-09 8.71E+01 920 1.49 3.91E-10 2.32E+01 700 1.50 1.08E-09 8.44E+01 925 1.49 3.82E-10 2.26E+01 705 1.50 1.06E-09 8.17E+01 930 1.49 3.74E-10 2.20E+01 710 1.50 1.03E-09 7.91E+01 935 1.49 3.66E-10 2.14E+01 715 1.50 1.00E-09 7.66E+01 940 1.49 3.58E-10 2.08E+01 720 1.49 9.80E-10 7.43E+01 945 1.49 3.51E-10 2.03E+01 725 1.49 9.56E-10 7.20E+01 950 1.49 3.43E-10 1.97E+01 730 1.49 9.33E-10 6.97E+01 955 1.49 3.36E-10 1.92E+01 735 1.49 9.10E-10 6.76E+01 960 1.49 3.29E-10 1.87E+01 740 1.49 8.89E-10 6.55E+01 965 1.49 3.22E-10 1.82E+01 745 1.49 8.67E-10 6.35E+01 970 1.49 3.15E-10 1.77E+01 750 1.49 8.47E-10 6.16E+01 975 1.49 3.08E-10 1.72E+01 755 1.49 8.27E-10 5.98E+01 980 1.49 3.02E-10 1.68E+01 760 1.49 8.07E-10 5.80E+01 985 1.49 2.95E-10 1.64E+01 765 1.49 7.88E-10 5.62E+01 990 1.49 2.89E-10 1.59E+01 770 1.49 7.70E-10 5.46E+01 995 1.49 2.83E-10 1.55E+01 775 1.49 7.52E-10 5.30E+01 1000 1.49 2.77E-10 1.51E+01 780 1.49 7.35E-10 5.14E+01 1005 1.49 2.71E-10 1.47E+01 785 1.49 7.18E-10 4.99E+01 1010 1.49 2.65E-10 1.43E+01 790 1.49 7.01E-10 4.84E+01 1015 1.49 2.60E-10 1.40E+01 795 1.49 6.85E-10 4.70E+01 1020 1.49 2.54E-10 1.36E+01 800 1.49 6.69E-10 4.57E+01 1025 1.49 2.49E-10 1.32E+01 805 1.49 6.54E-10 4.43E+01 1030 1.49 2.43E-10 1.29E+01 810 1.49 6.39E-10 4.31E+01 1035 1.49 2.38E-10 1.26E+01 815 1.49 6.25E-10 4.18E+01 1040 1.49 2.33E-10 1.22E+01 820 1.49 6.10E-10 4.06E+01 1045 1.49 2.28E-10 1.19E+01 825 1.49 5.97E-10 3.95E+01 1050 1.49 2.23E-10 1.16E+01 830 1.49 5.83E-10 3.84E+01 1055 1.49 2.19E-10 1.13E+01

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λ (nm) n k dB/Km λ (nm) n k dB/Km 1060 1.49 2.14E-10 1.10E+01 1285 1.49 8.05E-11 3.42E+00 1065 1.49 2.10E-10 1.07E+01 1290 1.49 7.87E-11 3.33E+00 1070 1.49 2.05E-10 1.05E+01 1295 1.49 7.70E-11 3.24E+00 1075 1.49 2.01E-10 1.02E+01 1300 1.49 7.52E-11 3.16E+00 1080 1.49 1.97E-10 9.94E+00 1305 1.49 7.35E-11 3.07E+00 1085 1.49 1.92E-10 9.68E+00 1310 1.49 7.19E-11 2.99E+00 1090 1.49 1.88E-10 9.43E+00 1315 1.49 7.02E-11 2.91E+00 1095 1.49 1.84E-10 9.19E+00 1320 1.49 6.86E-11 2.84E+00 1100 1.49 1.81E-10 8.96E+00 1325 1.49 6.71E-11 2.76E+00 1105 1.49 1.77E-10 8.73E+00 1330 1.49 6.55E-11 2.69E+00 1110 1.49 1.73E-10 8.51E+00 1335 1.49 6.40E-11 2.62E+00 1115 1.49 1.69E-10 8.29E+00 1340 1.49 6.25E-11 2.55E+00 1120 1.49 1.66E-10 8.08E+00 1345 1.49 6.11E-11 2.48E+00 1125 1.49 1.62E-10 7.87E+00 1350 1.49 5.97E-11 2.41E+00 1130 1.49 1.59E-10 7.67E+00 1355 1.49 5.83E-11 2.35E+00 1135 1.49 1.55E-10 7.47E+00 1360 1.49 5.69E-11 2.28E+00 1140 1.49 1.52E-10 7.28E+00 1365 1.49 5.56E-11 2.22E+00 1145 1.49 1.49E-10 7.10E+00 1370 1.49 5.43E-11 2.16E+00 1150 1.49 1.46E-10 6.92E+00 1375 1.49 5.30E-11 2.10E+00 1155 1.49 1.43E-10 6.74E+00 1380 1.49 5.17E-11 2.05E+00 1160 1.49 1.40E-10 6.57E+00 1385 1.49 5.05E-11 1.99E+00 1165 1.49 1.37E-10 6.40E+00 1390 1.49 4.93E-11 1.94E+00 1170 1.49 1.34E-10 6.24E+00 1395 1.49 4.81E-11 1.88E+00 1175 1.49 1.31E-10 6.08E+00 1400 1.49 4.69E-11 1.83E+00 1180 1.49 1.28E-10 5.92E+00 1405 1.49 4.58E-11 1.78E+00 1185 1.49 1.25E-10 5.77E+00 1410 1.49 4.47E-11 1.73E+00 1190 1.49 1.23E-10 5.62E+00 1415 1.49 4.36E-11 1.68E+00 1195 1.49 1.20E-10 5.48E+00 1420 1.49 4.25E-11 1.63E+00 1200 1.49 1.17E-10 5.34E+00 1425 1.49 4.15E-11 1.59E+00 1205 1.49 1.15E-10 5.20E+00 1430 1.49 4.04E-11 1.54E+00 1210 1.49 1.12E-10 5.07E+00 1435 1.49 3.94E-11 1.50E+00 1215 1.49 1.10E-10 4.94E+00 1440 1.49 3.84E-11 1.46E+00 1220 1.49 1.08E-10 4.81E+00 1445 1.49 3.75E-11 1.42E+00 1225 1.49 1.05E-10 4.69E+00 1450 1.49 3.65E-11 1.37E+00 1230 1.49 1.03E-10 4.57E+00 1455 1.49 3.56E-11 1.34E+00 1235 1.49 1.01E-10 4.45E+00 1460 1.49 3.47E-11 1.30E+00 1240 1.49 9.85E-11 4.33E+00 1465 1.49 3.38E-11 1.26E+00 1245 1.49 9.63E-11 4.22E+00 1470 1.49 3.29E-11 1.22E+00 1250 1.49 9.42E-11 4.11E+00 1475 1.49 3.21E-11 1.19E+00 1255 1.49 9.21E-11 4.01E+00 1480 1.49 3.12E-11 1.15E+00 1260 1.49 9.01E-11 3.90E+00 1485 1.49 3.04E-11 1.12E+00 1265 1.49 8.81E-11 3.80E+00 1490 1.49 2.96E-11 1.08E+00 1270 1.49 8.62E-11 3.70E+00 1495 1.49 2.88E-11 1.05E+00 1275 1.49 8.42E-11 3.61E+00 1500 1.49 2.80E-11 1.02E+00 1280 1.49 8.24E-11 3.51E+00 1505 1.49 2.73E-11 9.89E-01

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λ (nm) n k dB/Km λ (nm) n k dB/Km 1510 1.49 2.65E-11 9.59E-01 1735 1.49 5.74E-12 1.81E-01 1515 1.49 2.58E-11 9.30E-01 1740 1.49 5.50E-12 1.72E-01 1520 1.49 2.51E-11 9.01E-01 1745 1.49 5.25E-12 1.64E-01 1525 1.49 2.44E-11 8.73E-01 1750 1.49 5.02E-12 1.57E-01 1530 1.49 2.37E-11 8.46E-01 1755 1.49 4.79E-12 1.49E-01 1535 1.49 2.31E-11 8.20E-01 1760 1.49 4.57E-12 1.42E-01 1540 1.49 2.24E-11 7.94E-01 1765 1.49 4.35E-12 1.35E-01 1545 1.49 2.18E-11 7.69E-01 1770 1.49 4.15E-12 1.28E-01 1550 1.49 2.11E-11 7.44E-01 1775 1.49 3.94E-12 1.21E-01 1555 1.49 2.05E-11 7.21E-01 1780 1.49 3.75E-12 1.15E-01 1560 1.49 1.99E-11 6.97E-01 1785 1.49 3.56E-12 1.09E-01 1565 1.49 1.93E-11 6.75E-01 1790 1.49 3.37E-12 1.03E-01 1570 1.49 1.88E-11 6.53E-01 1795 1.49 3.20E-12 9.71E-02 1575 1.49 1.82E-11 6.31E-01 1800 1.49 3.02E-12 9.17E-02 1580 1.49 1.77E-11 6.10E-01 1585 1.49 1.71E-11 5.90E-01 1590 1.49 1.66E-11 5.70E-01 1595 1.49 1.61E-11 5.51E-01 1600 1.49 1.56E-11 5.32E-01 1605 1.49 1.51E-11 5.14E-01 1610 1.49 1.46E-11 4.96E-01 1615 1.49 1.42E-11 4.78E-01 1620 1.49 1.37E-11 4.61E-01 1625 1.49 1.33E-11 4.45E-01 1630 1.49 1.28E-11 4.29E-01

1635 1.49 1.24E-11 4.14E-01 1640 1.49 1.20E-11 3.98E-01 1645 1.49 1.16E-11 3.84E-01 1650 1.49 1.12E-11 3.69E-01 1655 1.49 1.08E-11 3.55E-01 1660 1.49 1.04E-11 3.42E-01 1665 1.49 1.00E-11 3.29E-01 1670 1.49 9.67E-12 3.16E-01

1675 1.49 9.32E-12 3.04E-01 1680 1.49 8.98E-12 2.92E-01 1685 1.49 8.64E-12 2.80E-01 1690 1.49 8.32E-12 2.69E-01 1695 1.49 8.00E-12 2.58E-01 1700 1.49 7.69E-12 2.47E-01 1705 1.49 7.39E-12 2.37E-01 1710 1.49 7.10E-12 2.27E-01

1715 1.49 6.81E-12 2.17E-01 1720 1.49 6.53E-12 2.07E-01 1725 1.49 6.26E-12 1.98E-01 1730 1.49 6.00E-12 1.89E-01

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APPENDIX F: Processing with a Magnet

The entire experiment was repeated with the use of rare earth magnets, which were positioned just below the crucible when cooled via the different cooling methods and at different rates.

The orientation of the crucible in relation to the magnet can be seen in Figure 90.

Figure 90. Rare earth magnetic lines positioning/orientation in relation to crucible (highlighted in blue)

The results were intriguing, where adding a rare earth magnet provided significant advantages in the thermal processing of the ZBLAN test specimens:

x ZBLAN in the molten state displays paramagnetic properties, where it can be

manipulated in the presence of a magnet (the melt is attracted to the applied magnetic

field).

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x When cooled the test samples were able to retain their fully amorphous structure at

higher temperature ranges (at and above the crystallization onset temperature), which

would usually result in heavy crystallization

x Most importantly, when processed at a slow cooling rate of 100 ⁰C/min, the

magnetized test sample were fully amorphous

A possible explanation for why the 100 ⁰C/min test samples were fully amorphous is because in its molten state, the ZBLAN particle lined up with the magnetic field lines of the magnet.

The magnetic field lines applied to the test sample branch out like a water fountain, as seen in

Figure 90, which could potentially contribute to the particles being even more disordered.

When the particles moved along these field lines, it caused the particles to be unable to aggregate or diffuse to create the lattice structure required for crystallites to form. Therefore even with a slow cooling rate, the ZBLAN particles were unable to nucleate or grow into crystals in the presence of a magnetic field.

Figure 91. Diffraction pattern of 100 ⁰C/min ZBLAN test sample processed with a magnet, showing a fully amorphous glass matrix

177

ZBLAN Fragment

Figure 92. Lower magnification bright field image of 100 ⁰C/min ZBLAN test sample processed with a magnet

ZBLAN Fragment

Figure 93. Higher magnification bright field image of 100 ⁰C/min ZBLAN sample processed with a magnet

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Currently, the use of an external magnetic field to manipulate the behaviour of melts as they solidify is becoming a more widespread technique in the metals and semiconductors industry

[46]. This technique has already been successfully implemented to improve in process performance and better quality control in the fabrication of many semi-conductor materials.

As opposed to mechanical forces applied to a melt, a field can be configured in numerous ways to provide an effective means of melt stirring to obtain a desired mixing pattern. This in turn can dictate the way in which crystal growth can occur in the solidification process, with proven results for obtaining grain refinement during metal casting. A strong turbulent shear flow can be produced in the melt by Lorentz forces, ultimately providing a means of shaping or controlling the flow of the melt in any desired way, even completely eliminating the formation of crystallites.

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