Transparent Ceramics: Magnesium Alumnate
[Engineering thesis posted with the permission of the author, Varun Dev V. In his words, he wanted to “pass the idea to the rest of the world.”]
A THESIS SUBMITTED TO THE UNIVERSITY OF KERLA
FOR THE DEGREE OF ENGINEERING
18 Th AUGUST 2008
BY
VARUN DEV V (05402053)
SREE CHITHIRATHIRUNAL COLLEGE OF ENGINEERING
TRIVANDRUM, UNIVERSITY OF KERLA
INDIA
PROJECT SUPERVISOR
PROF. CHITHRAKUMAR
SREE CHITHIRATHIRUNAL COLLEGE OF ENGINEERING
TRIVANDRUM, UNIVERSITY OF KERLA
WORD COUNT: 17,798
Acknowledgement
I express heartfelt gratitude to Dr HORMEESE (Mike) for finding me an appealing and real topic for research and also supporting me through out the project. His guidance, flavoured with precious advice and suggestions, helped shape the basis of my project. Also, his sincere and committed approach has amplified my enthusiasm which helped me come out with interesting results.
I am thankful to Mr CHTHRAKUMAR who always extracts time from his busy schedule to provide any guidance and support through out my research program. I will always remember him as a key mastery in Materials’ field (systematic approach) and wish him every success and prosperous growth in all attempts in his life. I would also like to acknowledge the wonderful time spent with Dr KIRAN, UMESH Sean, Eric, Pepe, Edward , Nima, Anuj Sood, Isaiah and also grateful for their words of motivation and support.
I am grateful to the entire academic and support staff at the Materials and Physics Department for their valuable enthusiastic support and co-operation. Special thanks to Dr. Mark Baxendale, Dr Rory Wilson, Dr. Zofia Luklinska, and Dr. James Busfield for their timely help, advice and hospitality which helped me gain momentum during my research work.
I am indebted to my Parents, my Sisters, Aunt and Uncle and God Almighty for gifting me this wonderful opportunity to educate myself at The Queen Mary, University of London. Finally, I would like to thank my oldest sister, Ash who has always supported and believed in me from ever since I can remember.
“This thesis is dedicated to the memory of my beloved grandmother who was always there to shower her blessings and inspired me to keep moving forward in every walk of my life.”
Declaration
I declare that this thesis is entirely based on the work carried out by myself, using the advanced facilities available within the Department of Physics – SCT COLLEGE OF ENGG, AND THE HELP FROM THE INDIAN ARMY.
Varun Dev V
Table of Contents
Acknowledgement 2
Declaration 3
Table of Contents 4
List of Figures 8
List of Tables 10
Abstract 11
PART: 1-Literature Survey on Transparent Ceramics 12
Chapter: 1 Introduction to Transparent Ceramics 13
1.1 Optical properties of transparent ceramics 13
1.1.1 Optical absorption 13
1.1.2 Colour 14
1.1.3 Reflection 14
1.1.4 Double refraction (bi refringence) 15
1.1.5 Transmission 15
1.2 Ceramic Candidates suitable for transparent applications 16
1.2.1 Introduction 16
1.2.2 Yttrium oxide 16
1.2.3 Aluminium OxiNitride (ALON) 18
1.2.4 Yttrium Aluminium Garnet 20
1.2.5 Poly crystalline alumina 22
1.2.6 Magnesium Aluminate – spinel 24
1.3 Application of Transparent Ceramics 24
1.3.1 Absorber tubes in solar field 24
1.3.2 Dental applications 25
1.3.3 Armour application 25
1.3.4 application in ir-Sensor 26
1.3.5 Laser applications 26
1.3.6 Decorative application 27
Chapter: 2 A Brief INTRODUCTION TO Magnesium Aluminate 28
2.1 Motivation 28
2.2 Requirements, Crystal structure and features of Magnesium Aluminate 29
2.3 Role of Sintering Aid on Densification of Magnesium Aluminate 31
2.4 Important conclusion from recent work on MgAl2O4 transparent ceramics 34
Chapter : 3 Processing of transparent ceramics: Spark Plasma sintering 37
3.1 Introduction 37
3.2 Historical Background 37
3.3 Process: Mechanism and Working 38
3.4 Unique Features of Spark Plasma Sintering: 40
3.5 Success stories of Spark Plasma Sintering in sintering Novel materials 42
Chapter : 4 Scattering sources of light in polycrystalline material 44
4.1 Introduction 44
4.2 Rough surface scattering 44
4.3 Scattering at pores 45
4.4 Grain boundary scattering 46
4.5 Scattering at secondary phases 47
PART: 2- CHAPTER: 5- Experiments, Results and Discussion 49
5.1 INTRODUCTION AND PARTICLE SIZE ANALYSIS OF POWDER 50
5.1.1 Magnesium Aluminate Powder 50
5.1.2. PARTICLE SIZE DISTRIBUTION OF MAGNESIUM ALUMINATE 52
5.1.3 LITHIUM FLUORIDE 52
5.2 SYSTEMATIC EXPERIMENTAL APPROACH: FROM OPAQUENESS TO TRANSPARENCY 54
5.2.1 SAMPLE: 1 54
5.2.2 SAMPLE: 2 57
5.2.3 SAMPLE: 3 60
5.2.4 SAMPLE: 4 63
5.3 INVESTIGATION ON THE NON-HOMOGENEOUS APPEARANCE OF SAMPLES AFTER SINTERING 67
5.3.1 X-RAY DIFFRACTION RESULT AND DISCUSSION 67
5.3.1.1 INTRODUCTION 67
5.3.1.2 DISCUSSION 67
5.3.1.3 CONCLUSION 68
5.3.2 SEM Images – Result and Discussion 69
5.3.2.1 INTRODUCTION 69
5.3.2.2 Discussion 69
5.3.2.3 CONCLUSION 70
5.4 STUDIES on the Annealing behaviour of Magnesium Aluminate
5.4.1 INTRODUCTION 71
5.4.2 EXPERIMENTAL 71
5.4.3 RESULTS AND DISCUSSION 73
5.4.3.1Optimising the annealing condition 73
5.4.3.2 Relation of annealing condition with the density and transmittance of sample 73
5.4.3.3 DIFFERENT SINTERING CONDITION SHOULD HAVE DIFFERENT ANNEALING TEMPERATURE 75
5.4.4 CONCLUSION 76
5.5 MEASUREMENT – DENSITY & OPTICAL TRANSMITTANCE 77
5.5.1 DENSITY MEASUREMENT 77
5.5.1.1 Introduction 77
5.5.1.2 PRINCIPLE AND EXPERIMENTAL PROCEDURE 77
5.5.1.3 RESULTS 78
5.5.1.4 DISCUSSION 78
5.5.1.5 CONCLUSION 80
5.5.2 TRANSMITTANCE MEASUREMENT 80
5.5.2.1 Introduction 80
5.5.2.2 PRINCIPLE AND EXPERIMENTAL PROCEDURE 80
5.5.2.3 RESULTS 81
5.6 Introduction to Sintering Aid- LiF 83
5.6.1 INTRODUCTION 83
5.6.2 EXPERIMENTAL PROCEDURE 83
5.6.2.1 POWDER MIXING 83
5.6.2.2 Preparation of Green Body 84
5.6.2.3 PROCESS: SPARK PLASMA SINTERING OF SAMPLE 84
5.6.2.4 PROCESS: REMOVING CARBON FROM SINTERED SAMPLE 85
5.6.3 Results 86
5.6.4 Discussion 87
5.6.5 CONCLUSION 89
PART: 3- CHAPTER: 6 – Conclusion and Recommendations for Future Work 90
6.1 Conclusion 91
6.2 Recommendations for Future Work 93
rEFEreNCE 95
List of Figures
Figure 1.1-[A] First translucent AlON disc produced by McCauley and Corbin [B] Commercial ALONTM products 18
Figure 1.2- Nd doped YAG Ceramics 21
Figure 1.3- An opaque tile and transparent window made from alumina 24
Figure 2.1 -Transmission spectra for Infra –Red Materials 31
Figure 3.1- SPS furnace (FCT Systeme, Germany)Nanoforce Technology Limited 40
Figure 3.2-Current flow in SPS Furnace 39
Figure 4.1- Schematics of Light scattering sources 44
Figure 5.1-SEM images of Magnesium Aluminate powder showing In-homogeneity in particle size 51
Figure 5.2- Sintering condition for sample 1 55
Figure 5.3- Annealing condition for sample 1 56
Figure 5.4- Sintering condition for sample 2 58
Figure 5.5- Annealing condition for sample 2 59
Figure 5.6- Sintering condition for sample 3 61
Figure 5.7- Annealing condition for sample 3 62
Figure 5.8- sample 3 (1800 o C/30 minutes). [1] Translucent, [2] Opaque 62
Figure 5.9- Sintering condition for sample 4 64
Figure 5.10- Annealing condition for sample 4 65
Figure 5.11-Progression of the transparency of the Spark Plasma Sintered MgAl2O4 discs 66
Figure 5.12-Sintered sample considered for XRD study due to non-uniform appearance 67
Figure 5.13-XRD pattern of Magnesium Aluminate powder 68
Figure 5.14- SEM image of the sample 2 after fracturing 69
Figure 5.15- Sintering condition for sample considered for Annealing study 72
Figure 5.16- Different annealing condition considered for study 72
Figure 5.17 -Photographs of annealed sample at different temperature 73
Figure 5.18 -Photographs of annealed sample at same conditions 75
Figure 5.19 -Scanning Electron Microscope image of sample showed 100 % density by Archimedes principle 79
Figure 5.20– Improvement in optical transmittance with change in optimising sintering condition and composition of starting powder 82
Figure 5.21- Sintering condition for sample with and without sintering aid 85
Figure 5.22- Annealing condition for sample with and without sintering aid 86
Figure 5.23 –Transmittance chart for MgAl204 samples with and without LiF 87
Figure 5.24- Samples sintered-[A] without sintering aid. [B] With 1 wt % LiF 88
Figure 5.25- Densification chart, piston movement against temperature 88
List of Tables
Table 2.1 -Comparison -Spinel (RCS Technologies) with the best transparent polycrystalline material, Aluminium Oxynitride (Raytheon Corporation) 29
Table 3.1- Diverse application of SPS Furnace in sintering novel materials 43
Table 5.1- Magnesium Aluminate Powder composition 50
Table 5.2- Data: Density and optical property for samples in annealing study 74
Table 5.3- The table shows that the different annealing conditions are required for sample sintered at different condition to in removing carbon 75
Table 5.4- Density of sample sintered at different conditions 78
Table 5.5- Optical transmittance of sample sintered at different conditions 81
Table 5.6- Density data for sample with and without sintering aid 87
Table 6.1- Summary of the experimental conditions and results in making transparent Magnesium Aluminate ceramics using SPS 92
Abstract
In the 1970’s, material engineers and scientists recognized the ability of transparent ceramics in armour systems compared to the conventional glass based materials. This opened a new path in exploring the application of transparent ceramics [29,129].Magnesium Aluminate (MgAl2O4) -spinel is of special interest due to its optical properties coupled with excellent mechanical strength. Hence it is a promising candidate to replace the currently available expensive and proprietary materials for commercial and defense transparent application [129,139]. Spark Plasma Sintering (SPS) technique is used widely these days in densification of ceramics. But research analysis on transparent ceramics is limited [28]. So the main aim of this work is to explore the adaptability of this machine in making transparent ceramics.
Magnesium Aluminate with a maximum transmittance of 80% in the visible wavelength of 695 nm in the electro magnetic spectrum is successfully fabricated using spark plasma sintering. The density of the sintered sample is measured using Archimedes method and its accuracy and feasibility of this technique in making transparent ceramics is also brought into light and proved with the help of SEM images along with the extend of scattering data by pores obtained from journals. The annealing condition is investigated and optimized to remove carbon successfully from the sintered sample without damaging the grain structure. SEM images are taken and microstructure is studied to find out the reason for opaqueness of sample due to cracking of grain at higher cooling rate and annealing temperatures. After optimizing the sintering and annealing condition, experiment is further carried out using sintering aid (Lithium Fluoride). The role of sintering aid in the improvement of densification of Magnesium Aluminate is discussed in great detail in the literature review and it confirms that the sintering aid improves the optical property of the sample by visual inspection and spectroscopy data collected for the sintered sample. A brief recommendation for future work is also included in the last few pages of this report.
Key words: Transparent Ceramics-Magnesium Aluminate, SPS, SEM, Sintering aid, OpticalTransmittance, Archimedes density measurement, Spectroscopy.
PART: 1
Chapters 1-4
Literature Survey on Transparent Ceramics
Chapter 1
Introduction to Transparent Ceramics
1.1 Optical properties of transparent ceramics
1.1.1 Optical absorption
Optical absorption takes place when the photons in the incident beam interacts and changes the energy state of electron within the solid. The absorption is stimulated by photons which results in lattice vibrations or excitation of electron between energy levels. The presence of impurity and lattice defects can also serve as source of absorption within the crystal. The absorption of light in the far infrared region is by lattice vibration and that in the UV region is by the excitation of electrons between energy bands. Absorption of light with characteristic wavelength is by impurities and lattice defects[149].
Excitation followed by absorption takes place only if the photon energy is greater that of the band gap (Eg ).
Energy Of a photon is give by the equation h γ= hc/λ.
Where:
H= planks constant =4.13Χ10 -15 eVs
γ= frequency of incident light
λ= wave length of incident light
For absorption to take place
hc / λ > Eg
Minimum wave length for visible light is .4 µm and maximum wavelength is .7 µm[18].
So from the above equation we can get the corresponding photon energy as 1.8 eV and 3.1 e V. So from this we can conclude that if the band gap energy is greater than 3.1 eV no absorption takes place and the crystal structure will be transparent if no other scattering encounters with the crystal structure. The solid will be opaque if the band energy is less than 1.8eV and translucent if it comes in between the above mention range. This may be the reason why diamond, Al2O3, SiO3 with energy band gap 5.3eV, 7.4eV and 8 eV respectively appears white. So ceramics with band gap greater than 3.1eV can be ideal candidate for transparent applications [18].
1.1.2 Colour
Transparent Material appears coloured due to selective absorption of light. If the absorption is uniform then also the materials appears colourless as in high purity in organic glasses, single crystal diamond and sapphire. Selective absorption is due to the excitation of electron between different energy bands within the crystal. Selective absorption takes place for visible spectrum if the band gap energy is between 1.8eV and 3.1eV[18].
For example, the band gap of Cadmium sulphide (Cd S) is 2.4eV and it undergoes selective absorption when exposed to visible spectrum. CdS crystal absorbs light in the violet blue region of visible spectrum and the non absorbed beam of light has a colour composition of yellow – orange. So the CdS appears in yellowish orange[18].
1.1.3 Reflection
When light travels from one medium to another scattering of light takes place at the interface between the medium due to difference in refractive index(n). The reflectivity which is the ratio of intensities of incident and reflected beams gives a measure of reflection at the surface.When light is transmitted from air or vacuum into the solid, then the reflectivity is given by
R = ( ns - 1)2
(ns +1)2
Where ns =refractive index of solid. Refractive index of air or vacuum is taken as 1[18].
From above equation we can see that the for higher refractive index of solid the reflectivity will be higher. So reflection losses on the surface of solid has to be minimised to make high transparent ceramics .A thin layer coating of dielectric material such as magnesium fluoride (MgF2) is used in lenses and optical instruments to reduce the reflection of light at the interface of the solid [18].This information matches with the work done by (Villalobos et al., 2005) [139] in making transparent spinel.
1.1.4 Double refraction (bi refringence)
The change in direction of light beam when it is transmitted through a material is called refraction. Crystal with cubic structure have refractive index same in all direction and is optically isotropic. For non cubic crystal structure the light beam falling on it undergoes double refraction (bi refringence) producing two beams with different velocities and plane of vibration. But in the case of mono axial crystal such as hexagonal or tetragonal crystal structure bi refringence phenomenon does not take place if the light is projected parallel to the c-axis. The biaxial crystal for e.g. monoclinic, triclinic or orthorhombic crystals does not exhibit bi refringence if the light is parallel to the axis of symmetry of the crystal[149]
This is one main of the reason why aluminium oxy nitride(ALON) has very good transmittance can be seen in literature[46,104].Since Spinel (MgAl2O4) has cubic crystal structure, it is optically isotropic with refractive index same in all directions and can be an ideal candidate for making transparent ceramics[108].
1.1.5 Transmission
When light falls on a transparent sintered body it undergoes many phenomenon such as reflection, refraction, absorption due to non homogeneity and finally gets transmitted though the other side of the sintered body. According to Lambert beer rule intensity of transmitted light is given by
I =I 0 (1-R) 2 exp (-µx)
Where
I 0 = intensity of incident beam
R= reflectivity of the material
µ = absorption co efficient
x= thickness of sintered body [149]
From the above equation we can see that the intensity of transmitted beam depends on reflectivity of the material, absorption co efficient, thickness of sintered body. So when to make transparent ceramics we have to consider and control all factors mentioned above which reduces transmittance by scattering the light.
1.2 Ceramic Candidates suitable for transparent applications
1.2.1 Introduction
This chapter gives a brief introduction to materials of current interest in making transparent ceramics for various applications. The materials discussed below are capable of exhibiting good optical property (good transmittance) along with mechanical strength if the processing conditions are apt to provide non porous samples, with narrower and cleaner grain boundary to prevent scattering of light within the sample [16,76].Cubic crystal structure such as Aluminium Oxy nitride (ALON), Yttrium Aluminum Garnet (YAG) and Yttrium oxide (Y2O3) are considered in the discussion. The Poly crystalline alumina, a capable candidate for energy saving application [69]is also discussed in detail.
1.2.2 Yttrium oxide
The study on Yttrium oxide (Y203) is not yet explored in depth when compared with the other transparent ceramics such as aluminium oxy nitride, Magnesium Aluminate, etc[76].The field of application and interest of polycrystalline yttrium (PCY) includes making crucibles in the casting industry, missile dome, optical windows, laser etc [32,44,68,84,95]. Transparent yttrium has a broad range of optical transparency in the visible and infrared region along with a high melting point of 2430 0C[46,92,108]. Apart from the optical properties, Yttrium (Y2O3), also offers good corrosion resistance and also good plasma erosion property compared to alumina and quartz and will be most suitable for the next generation candidates in the making of equipments used for the manufacture of semiconductors[17,92,146].
Yttrium (Y2O3) has a lower emissivity at high temperatures than the sapphires and AlON; the materials of current interest [27,46]. So yttrium (Y2O3) is an ideal candidate for making transparent windows and domes if we can increase the mechanical property of this material to the required efficiency. Since the hardness of ceramic is inversely related to the grain size and thus possible to enhance the mechanical property by doping with La2O3[27,46]. The doping process can hinder the grain growth during sintering but will effect the heat capacity and hence badly reduce the resistance of the material to thermal shock[27]. Hence, the best option is to sinter yttrium using nano powder[27]and prevent grain growth during sintering by using advance sintering technique such as spark plasma sintering which can prevent grain growth due to the shorter sintering time, faster heating and cooling.
(Mingsheng et a1.,2006) fabricated transparent yttrium using combination mode, spark plasma sintering (SPS) by Hot Isostatic Pressing (HIP) without any additives and studied the effect of sintering process on the microstructure and optical properties of the yttrium ceramics[92]
(Eilers,2007) fabricated Transparent yttrium with starting powder of nano size, ceramic green body is prepared by dry pressing with out any binder and densification is obtained by Hot Isostatic Pressing (HIP) to obtain an optical transmittance of the sintered sample after polishing of about 60 % in the near IR region with a significant hardness rating[27].
(Casolco et al., 2008) worked on yttrium stabilized zirconium to fabricate multifunctional transparent / translucent materials with measured fracture toughness of 3.0 and 8.1 MPa m1/2 for the fully stabilized and partially stabilized zirconia which is comparably higher than that of the fracture toughness of a single crystal of sapphire which is 1.2 MPa m1/2[23].
(Mouzon et al., 2008) fabricated transparent yttrium using vacuum sintering followed by hot isostatic process with an in line transmittance of nearly 80% in the visible region, the adverse effect of secondary phase which can come into play due to the silicon dioxide from the glass capsule is also discussed in detail[95].
(Ivanov et al., 2008) fabricated transparent yttrium is doped with Nd ceramics with the help of magnetic pulse compaction technique followed by sintering and the effects of sintering process on the thickness and acoustic impedance of the grain boundaries in the ceramics was studied [59]. The main conclusion of (Ivanov et al., 2008) from their study is that the average grain-boundary parameters depend significantly on the sintering condition[59].
1.2.3 Aluminium Oxy Nitride (ALON)
Figure 1.1 – [A] First translucent AlON disc produced by McCauley and Corbin (1976)[87].[B] Commercial ALONTM products; Courtesy: LeeMGoldman, President
ALON Products Group, Surmet Corporation, Burlington, Massachusetts, USA[96].
The journey towards the best transparent ceramics available now (Aluminium Oxy Nitride) began in 1959 when researchers reported the presence of spinel phase in Al2O3–AlN system[96,148]. But later researches[1,79,80,81,54]proved that spinel phase is present in the system and structure of Aluminium Oxy Nitride can be explained as “cubic γ Al2O3 (spinel structure) with 5 wt % of nitrogen”[76].
In 1973 there was significant amount of interest shown in the new armour materials with good optical, mechanical, thermal properties [96]. In 1976, (McCauley and corbin,1976) made the first translucent Aluminium Oxy Nitride and is shown in figure above [A][87,96].Restrictions on processing aluminium oxy nitride came in to act in 1980, when McCauley et al. Patented the process of making it[88]. Raytheon Corporation, the main supplier of ALON powder started of with the making of transparent aluminium oxy nitride in collaboration with Dr. Rick Gentilman et al. in the year 1977[42,96].
From then till now vast researches are going on inorder to find the applications of Aluminium Oxy Nitride and finally had come up with interesting results. Some of the applications identified includes “military aircraft and missile domes, transparent armor, IR windows, hyper-hemispherical domes, laser windows, military aircraft lenses, semi-conductor processing applications, and scanner windows”[96].
The composition of Aluminium Oxy Nitride is Al23O27N5 (9Al2O3.5AlN) with a theoretical density of 3.69 g/cm3 [16]. Aluminium Oxy Nitride has a wide range of transmittance in the visible and Near infrared region ranging from 0.2 µm to 6.0µm[16]. Other than the optical properties Aluminium Oxy Nitride (ALON) has a very good hardness value of HV10 ≈15 GPa when compared to other transparent ceramics, but is low when compared to transparent sub micron Al2O3[76].(Patel et al.,2006) noticed from their experiment that AlON was ~25%harder than spinel, the second best candidate of interest[104].
So the optical properties combined with good strength make ALON the best suitable candidate for making transparent armour[16,45].The cubic crystal structure of Aluminium Oxy Nitride[16]adds additional advantage in making transparent ceramics as it limits birefringence light scattering at the grain boundaries and has isotropic thermal properties[144].
Now Aluminium Oxy Nitride of about 0.5µ m powder is manufactured by Surmet corporation and is not available for commercial application and the final product grain size is between 250-300 µm[76]. The figure [B] above shows the transparent ceramics made by Surmet Corporation using ALON. Now a days wide range of researches are going on in finding new materials to replace the currently proprietary material.
1.2.4 Yttrium Aluminium GARNET (YAG)
Yttrium Aluminium Garnet is an alumina-rich cubic with garnet having the molecular formula Y3Al5O12[76]. In 1984, the first translucent yttrium aluminium garnet was fabricated using a solid state reaction method and until then vast researches were going on to develop transparent YAG by carefully selecting the starting powder and optimizing the processing condition for various application conditions[24,47,111].
Neodymium doped yttrium aluminium garnet (Nd:YAG) single Crystal was used in solid state laser in a range of fields from medicine to military and researches over the past four decades [125]. Czochralski (CZ) method, which is normally used in making doped yttrium aluminium garnet single crystal is expensive and less effective in terms of achieving high dopent concentrations [58,112]
In 1960 as a result of in-depth research work, the first poly crystalline laser material based on fluoride materials was introduced [14,85]. In the year 1995, Ikesue et al. introduced doped Yttrium Aluminium Garnet in laser application [56,111].Out of the three stable phases of Y2O3–Al2O3 system, polycrystalline YAG was later developed to replace the single crystals which are used in laser devices[76].Low thermal distortion of the laser beam coupled with mechanical strength make Neodymium doped Yttrium Aluminium Garnet (Nd:YAG) the ideal candidate to replace the lanthanide doped single crystal in laser application [10,76].
The ploy crystalline sintered YAG exhibits low light scattering loses due to its cubic crystal structure (no birefringence) and also gives a option for doping as per requirement compared to that of single crystalline YAG. [10,76].Sintered YAG also possesses good chemical and thermal property with a sintering temperature which makes it easy to fabricate compared to its single crystal[82,83,153].
The grain growth rate of Nd:YAG is about 1 mm/hour in another disadvantage in making single crystals which can be overcome by using the most advance sintering technique, Spark Plasma Sintering[10]. The ceramics used for laser application is commercially produced by Konoshima Chemical Co. Ltd. ,Japan [10,76].
(Appiagyei et al., 2007) successfully sintered transparent ceramics with a transmittance greater than 80% in the range between from 340 nm to 840 nm, which is comparable to the single crystal[38]. (Appiagyei et al., 2007) noticed good optical property in Well-dispersed Newtonian suspensions and concluded that degree of slurry dispersion has a direct impact on transmittance of the final product[38].
(Rabinovitch et al., 2008) fabricated transparent yttrium aluminium garnet doped with neodymium from calcinated freeze dried precursors obtained followed by Vacuum sintering and hot static pressing treatment[111]. (Rabinovitch et al., 2008) sintered transparent YAG disk is of 1.6 mm thick and is shown in figure below[111].
(H. Zhang et al., 2007) sintered transparent Nd:YAG with an optical transmittance of 45% in the visible light wavelength and 58% in the near-infrared wavelength by vacuum sintering[153].(H. Zhang et al., 2007) also concluded that transmittance increases with increase in equivalent sphere diameter of grains and also derived that 20 µ m diameter can reach the theoretical transmittance of single crystal [153].
. 
Figure 1.2- Nd doped YAG Ceramics .Reproduced from [111]
1.2.5 Poly crystalline alumina (Al2O3)
Poly crystalline alumina has good response to high temperature and also to corrosive environment and hence is suitable to use as lamp envelope in high pressure sodium lamps [3, 36]. European research project named Starelight commented that metal halide lamps with transparent and strong Al2O3 lamp envelopes can replace energy-wasting halogen lamps on a large scale [69]. The mechanical strength is important criteria for the energy saving application due to high pressure encountered during operation [69]. The sintering at high temperature for long hours can increase the grain size of poly crystalline alumina and hence dramatically decreases the mechanical properties [75,142]. Poly crystalline alumina with sub micron grain size offer good mechanical resistance under severe operating conditions [36]. According to (Krell et al., 2003), the sub micrometer microstructure and the optical properties can be preserved above 1100 0C by shift the sintering temperature by using dopants [69].
The optical un -symmetry (birefringence) of Poly crystalline alumina due to crystal structure causes scattering of light at the grain boundary and hence reduces transmittance [3,8,30,106,113]. The optical property of the crystal can be improved in terms of grain boundary by increasing the grain size, as larger grains imparts fewer grain boundaries[3]. Hence fewer grain boundaries can be achieved by sintering it at a higher temperature for long time[75,142]. In poly crystalline alumina it is possible to reduce the overall cost by near net shape forming by using appropriate sintering methods[63].From the literature we can see that researchers have fabricated polycrystalline alumina with good optical property (transmittance) and mechanical property (toughness) which is compared to that of sapphire[3,36,70].
Ø (Kim et al., 2007)sintered fine grain transparent alumina by spark plasma sintering method and achieved a inline transmission of 47% with a residual porosity of 0.03%.(Kim et al., 2007) noticed a lower heating rate is required to produce transparent alumina at a sintering temperature of 1150 OC[75].
Ø (Jiang et al. ,2008) sintered transparent alumina in the mid – infrared region and suggested that slower heating rate, longer sintering time, annealing after sintering can eliminate the residual porosity and thus can increase the transmittance. They observed an excellent transmittance of 85% in the sample sintered at 1300 0C for 5 min. Jiang et al. also discuss about the influence of residual stress/strain at grain boundaries and oxygen vacancy on the optical properties of SPS-sintered alumina[63].
Ø (Apetz and van Bruggen,2003) developed a model based on Rayleigh-Gans-Debye light-scattering theory to explain the transmittance of polycrystalline ceramics. This model clearly defines the reason for remarkable transparency of poly crystalline alumina sample with grain size less than 2 µm. (Apetz and van Bruggen) prepared fine-grained alumina by colloidal processing and they exhibited a good match with the scattering theory with a real in line transmission of 71% at a wavelength of 620nm. They noticed that the scattering theory break completely below 5 µm grain size as the samples becomes fully transparent[3].
Ø (Godlinski et al., 2002) made transparent ceramics using sub – micrometer and narrow particle size distribution. Godlinski et al. used control consolidation and drying by float packing process and was able to sintered fully dense transparent alumina at lower temperature 12750C without any additives. The sintered sample has the same in-line transmittance as the commercially available Poly crystalline alumina tubes for lighting purpose[36].
Ø (Krell et al., 2003) sintered transparent alumina using hot isostatic pressing process with an in-line transmission of (55%-65%) along with good hardness and bending strength [69].
The figure below shows an advance ceramic armour tile of alumina and a transparent window made of alumina (Fraunhofer Institute for Ceramic Technologies and Systems,Germany). Both have the same purity, young’s modulus, bending strength and hardness. The only difference between the materials is in the residual porosity which made one sample transparent and other opaque[74]. Low in-line transmission of Polycrystalline Alumina is [3] one of the reason for translucency of the material. To increasing the transmission, the porosity within the poly crystalline sample has to be less than 0.05%[69,119]. During the quantitative assessment about the homogeneity of particle coordination by investigating the pore size in densification of transparent alumina, (Krell and Klimke, 2006) noticed that the percentage of pore size was the same in all stages of sintering process [74].
Figure 1.3- An opaque tile and transparent window made from alumina.(Fraunhofer Institute for Ceramic Technologies and Systems, Germany)-Reproduced from [74].
1.2.6 Magnesium Aluminate – spinel
Magensium Aluminate has good optical property with a transmittance range from 0.2 µm to 5.5 µm which is comparable with the best transparent ceramics ALON[94,108].Magnesium Aluminate exhibits excellent mechanical strength compared to bullet proof glass[139] with a wide range of deformation load range where the material deforms before failure compare to aluminium oxy nitride[104]. Other than mechanical and optical properties Magnesium Aluminate is highly chemically and thermally stable[7,25,40,89,110]which widens the area of application. The Cubic crystal structure enhances the optical property by reducing the optical scattering at the grain boundary[108].The experiments, results and discussions presented in this report are about the sintering of Magensium Aluminate and hence a brief discussion about this cubic structured ceramics is included in the next chapter.
1.3 Application of Transparent Ceramics
1.3.1 Absorber tubes in solar field
Transparent ceramics can be used in absorber tubes in solar field [35].The current design includes an outer tube and an inner tube[35,76].The inner tube is made of metal through which the working fluid circulates requires special coating to increase high absorbance for the incoming light through the outer glass tube[35]. Design of this coating becomes complicated when we consider stability of coating at high temperature, exposed environment, oxide formation on outer glass tube etc[76].So if is possible to replace the inner tube through which the working fluid circulates with transparent ceramics then the operating temperature can be increased, and also direct radiation heating is possible which can change the future of solar field[35].
1.3.2 Dental applications
Translucent ceramics can be used for making clear matching brackets which is used in orthodontic treatment[65]. For making brackets requires materials with lower transmission but should posses’ excellent strength[76].The material now used in making brackets includes stainless steel, titanium alloy since it possesses good strength, biocompatibility, corrosion resistance etc[2,26,37,67,115].The outlook that these type of materials offers are often taken into consideration by patients who are concerned about aesthetic appearances[2].
Translucent ceramics with good hardness, strength can provide a novel solution for this problem by providing matching brackets with natural tooth colour [2,76,114].
1.3.3 Armour application
Ceramics which can provide good optical property coupled with mechanical features such as strength, impact resistance, hardness etc can be serve the requirement of advance armour[76] .Armour application includes transparent armour in light vehicles, windows and domes for high speed missiles , goggles and face shield in infantry helmets are a very few to mention[108,139].
The main ceramics used for this application include aluminium oxynitride(ALON), sapphire[28,129,139].Now research has proved that Magnesium aluminate spinel is an able candidate to replace the expensive and proprietary materials currently used [139]. Fine grained Spinel has good ballistic strength than sapphire and bullet proof glass (hard core 7.62 x 51 AP ammunition 850 m/s, 1/4 in. of spinel offers the same resistance as 2.5 in. of bullet-proof glass)[76,139].
1.3.4 application in IR -Sensor
Infrared sensors in devices have to be protected with a skin which allows IR rays to pass through[63].This protection is vital in equipments which have to withstand aerodynamic heating in corrosive and harsh environmental condition for e.g. in heat seeking missiles[63]. For this application requires material with good transmittance in the short wave length range or the long wavelength of the IR bands is required[46,63].Transparent poly crystalline materials such as Aluminium oxy nitride, Magnesium Aluminate etc with cubic crystal structure can serve this requirement as it offers good transmittance in the near infrared region which is really important for seeker and electro-optic imaging systems [108].In the case of sensor protection systems a wide range of ceramics with cubic structure can be taken into consideration as the priority of mechanical property is secondary when compared with that of the armour application.
1.3.5 Laser applications
The wavelength required varies according to the application, For e.g. the wave length required for lasers used in medicine is different from that used for optical amplification in optical fibre data transmission [39].
The ceramic material cubic crystal structure with good thermal conductivity, high damage threshold, Chemical stability, mechanical strengths, low thermal distortion of the laser beam, etc and optical property can be good candidate for laser application[10,39].Doped cubic sesquioxides, especially scandium oxide (Sc2O3), doped Yttrium Aluminium Garnet (YAG) transparent ceramics are used for laser application[10,11,39].In polycrystalline ceramics the grain boundary offers space to introduce new atoms into the crystal lattice[76]. This kind of doping also reduces the porosity and scattering at the pores can be reduced and hence transmittance can be increased[76].So polycrystalline ceramics are good candidates for doped applications. The new available powder processing techniques takes the away the problem of growing crystal to large size as per requirements at high processing temperature[39,57,86].
1.3.6 Decorative applications
As we know the natural gemstones such as sapphire, spinal etc is expensive, colours are limited [76]. Hence limits its application on designer’s dress, bags, sandals etc. In the case of poly crystalline materials doping is easy as the grain boundary offers enough space to accommodate the doped species[76] .Another advantage of introduction of doped species is that it covers the pores within the polycrystalline material, which is one of the main scattering sources of light [76].Now cubic zirconia (ZrO2)are widely used in jewellery as a substitute for diamond , chromium doped coloured single crystal Al2O3 is also used in mechanical watch wear resistant components such as impulse jewel ,bearings etc[76]. So if it is possible to make artificial gems stones cheaply then it is possible to crack the gem industry by giving solutions by opening a new design era in making attractive products. This is possible using most advance sintering machines now available [72] which offers high temperature over a short time which the natural crystals experiences underneath the earth crust for decades.
Chapter 2
A Brief Introduction to Magnesium Aluminate (MgAl2O4)
2.1 Motivation
Although studies on Magnesium Aluminate is carried out from 1960’s[105,107,139,140], it is yet impossible to produce high transparency comparable to single crystal sapphire or polycrystalline Aluminium oxinitride (ALON). This is because the densification dynamics of spinel is not studied properly [139]. The Commercial requirements of transparent ceramics is not explored yet as the manufacturers have not experienced an increase in demand for large transparent ceramics except in scanners and fluorescent lighting [129].
In 1970’s the material engineers and scientist recognized the ability of transparent ceramics in armours system compared to the conventional glass based materials[29,129].This opened a new path in exploring the diverse applications of transparent ceramics.In military applications, the scientist are now interested in incorporating transparent armour (wind shield) in light vehicles[108,139] ,seeking window, domes in high speed missiles, fighter aircraft etc [108], strike plate and face shield on infantry helmets[139] used by soldiers. Due to the high thermal resistance combined with its mechanical properties, it can be used to make safety personnel equipments to protect fire fighters and factory workers from high temperature hazards. Other than military applications, transparent ceramics can find place in IR sensors, jewellery, and energy saving applications and is explained in chapter 1.
The potential candidates for transparent application include Aluminium oxynitride, Magnesium-Aluminate spinel, Silicate glasses and Sapphire.[28,129,139]. Sapphire and Aluminium oxy nitride got its advantages and disadvantages as discussed below. The Sapphire, a non-cubic crystal structure is not processed as a polycrystalline material [28], so is not suitable to make complex shapes. More over, sapphire is not recommended for high-volume applications in terms of cost and processing time[108].Aluminium oxy nitride – the best transparent ceramics is expensive and the powder is proprietary as it is owned and patented by Raytheon Company, U.S[28,123]. Aluminium oxy nitride (ALON) is not available for commercial applications as the company supplies the powder to Surmet Corporation, U.S where they make end product [123]. Silicate glasses is suitable for normal windows but is not suitable for transparent armour or domes in high speed missile as it is too week and will out transmit beyond 2 µ m wavelength [139].
Magnesium Aluminate (MgAl2O4) can be purchased from commercial suppliers and their synthesis is relatively simple as it can be processed as a poly crystalline material to make complex shapes [28]. Magnesium Alumiante also possess good mechanical property along with optical property makes it superior enough to replace the expensive and proprietary materials now in use.
Table 2.1 -Comparison -Spinel (RCS Technologies) with the best transparent polycrystalline material, Aluminium Oxynitride (Raytheon Corporation). Reproduced from [118].
2.2 Requirements, Crystal structure and features of Magnesium Aluminate
Requirements
The requirement in terms of property of transparent ceramics depends on the application, whether military or commercial. Good transmittance in the visible region along with good strength is a pre requisite for all application. For military application of transparent ceramics, apart from the good optical property other requirements are as follows: inexpensive enough for single use munitions [108], erosion resistant in high speed environment[108], lightweight [102,108], zero depletion of optical property after multiple localized hits [102,108], transmittance in the near infrared region[139]etc.
Structure
Magnesium Aluminate has a spinel structure with a stiochiometry AB204 where A and B are divalent and trivalent cations respectively. A unit cell of spinal contains eight FCC oxygen sub lattice in a cubic array with 32 oxygen ions, 16 octahedral cations, and 8 tetrahedral cations.In MgAl2O4 crystal structure oxygen anions forms the ABCABC packed face centered cubic sub lattice where Mg2+ and Al3+ cations are then introduced into the tetragonal and octahedral void in the FCC crystal lattice respectively. 50% of the octahedral sites and 12.5% of tetrahedral sites are filled in spinel crystal structure. To satisfy Pauling second rule, each oxygen anion in the FCC has to be coordinated by 3 octahedral cations and one tetrahedral cation and hence MgAl2O4 has a 4, 6, 4 coordination[15,108].
Features of Magnesium Aluminate (MgAl2O4)
Ø Magnesium Aluminate (MgAl2O4) can be processed as a polycrystalline ceramic material[139], so it is possible to fabricate complex shapes easily.
Ø Magnesium aluminate spinel has excellent optical property from 0.2 to 5.5 μm (in the visible- and IR-wavelength ranges)[94].
Ø Ballistic testing proved that Magnesium aluminate spinel has excellent mechanical properties; 1/4 in. of spinel offers the same resistance as 2.5-inch of bulletproof glass [139].
Ø Magnesium aluminate possess good mechanical property and optical property, so it is an ideal candidate for armour application [139].
Ø Magnesium Aluminate is thermally stable and exhibits good mechanical property at normal and high temperature that is useful in many applications[7,25,40].
Ø Magnesium aluminate is chemically stable with chemical inertness to strong acids and alkali solutions[25,40,89,110].
Ø From the transmission spectra for infrared materials, we can see the excellent transmittance of spinel in the wavelength 4.5 to 5.5 μm. The transmittance of spinel in this wave length makes it most suitable for seeker and electro optic imaging application[108].
Ø Magnesium Aluminate has cubic crystal structure and hence scattering of light at the grain boundary is almost zero due to optical isotropy compared to other non cubic crystals[108].
Ø (Patel et al., 2006) concluded from their experiment “Magnesium Aluminate has a wide load range (1–3.8 N) where deformation rather than fracture is dominant in the area immediately around the indentations” [104].
Figure 2.1 -Transmission spectra for Infra –Red Materials. Reproduced from [108].
2.3 Role of Sintering Aid on Densification of Magnesium Aluminate
Introduction
The production of high quality MgAl2O4 (spinel) failed due to lack of understanding clearly its densification dynamics [108,139]. CaO[6,41] , LiF [140] and B2O3 [138] were used by material engineers in experiments in the attempt to improve the quality of MgAl2O4 (spinel) [94].Out of these mentioned above, the most commonly used sintering aid is Lithium Fluoride. Even though the presence of lithium fluoride within MgAl2O4 (spinel) mixture during processing makes the system complex, without the presence of LiF the final sintered sample seems to be grey in colour with reduced transmittance and high scattering due to the more pores compared to undoped specimen can be found in literature [28,94,108,139].
(Frage at al., 2007)[28] found that addition of 1wt% LiF to spinel manufacture by SPS have almost the same elastic modulus, hardness and density value. From the experiment of Frage et al. (2007)[28]main difference between doped and un-doped is in its appearance (grey colour) and micro structure which in turn reflected in the optical properties with a reduced transmittance difference of 25%. The specimen doped with 1 wt % LiF by (Frage at al.,2007) also showed a improved transmittance in the visible light wave length of 400 nm when compared to that in un doped spinel prepared by (K. Morita et al., 2008) by spark plasma sintering. So to make transparent spinal it is important to understand the influence of LiF in the densification process of spinal.
Powder Mixing
Ultrasonic mixing apparatus at 40KW for 30 sec[110] or dry tumbling of powder in a polyethylene bottle using alumina balls[20]can be used for mixing the powder before making the green body. In the recent study (Reimanis et al.,2008)[110] used Methyl alcohol in the ratio 2:1 with MgAl2O4 spinel powder for enhancing the mixing process. Deionised water is not used as it can absorb LiF upto 0.375g/ml [110,143]. After mixing process, drying the mixture of spinel, Lithium Fluoride and methyl alcohol can cause evaporation of the latter give us the homogenous mixture of spinel and Lithium Fluoride.
To make high quality transparent ceramics, we have to make sure that the powder is mixed properly with the pre decided weight percentage of sintering aid. (Reimanis et al., 2008)[110] used, Methyl alcohol in the ratio 2:1 with MgAl2O4 spinel powder to mix the lithium fluoride. It is not feasible to produce homogenous mixture by this method as Lithium Fluoride (2.64g/cm3) and Magnesium Aluminate (3.6 g/cm3) have different densities and the particles will not float in the solution during ultra-sonification as the quantity of methyl alcohol is less. One of the Proper mixing techniques is used for performing my experiment and is discussed in detailed.
Mechanism
Using master sintering curve (MSC) theory[110,117], (Reimanis et al.,2008) found that the effective sintering activation energy with 1 wt % LiF sample sintered at 33 MPa 1500 degree temperature using HP is 250 KJ/mol and that without LiF is about 500 KJ/mol. They also found that all samples doped with LiF doped have sintering activation energy is less than 300KJ/mol. According to (Reimanis et al.,2008) a part of this reduction in sintering energy is due to advantages of (a)traditional liquid phase sintering advantage and rest due to (b) oxygen vacancies.
(a) Traditional liquid phase sintering
When LiF melts, the density of the of LiF and MgAl2O4 mixture increases due to rearrangement of particles. Once the reaction between LiF and spinel starts then fluoride in the mix encourages a solution re precipitation process and thus new fractured spinel grains starts growing in the presence of oxygen vacancy as mentioned below[110].
(b) Oxygen vacancies
From the TEM observations [109,110], for mixture with less than 5 wt% LiF a reaction takes place between LiF and MgAl2O4 according to equation below[110].
This reaction results in the formation of oxygen vacancy. When the oxygen vacancy increases; there will be enough room for enhanced diffusion. Hence diffusion takes place at lower energy and sintering energy is reduced. They also concluded that one of they main factors that limits the densification of spinel is the lack of oxygen vacancy and it is provided by lithium fluoride. [110].
Energy dispersive spectroscopy (EDS) analysis by (Villalobos et al., 2005)[139] on a non fully dense sample MgAl2O4 spinel doped with LiF suggests that the small grains are made up of an MgO-rich phase. Magnesium oxide (MgO) has a considerably higher melting temperature than spinel and will not density at lower sintering temperature. More over at 1000 °C, according to (Reimanis et al., 2008) [110], the LiF evaporates and hence raising the required sintering activation energy for further densification.
Removal of LiF
The melting point of LiF is 845 OC. So as reaction precedes the vapour pressure above the fluoride melt increases. At above 1000 OC LiF begins to vaporize and leaves the mix[110].
2.4 Important conclusion from recent work on MgAl2O4 transparent ceramics
Ø (Frage et al.,2005) fabricated transparent Magnesium Aluminate by spark plasma sintering technique. Two types of sample, with and without sintering aid is considered for study and the optical, mechanical and general properties are investigated in detail. Under the same sintering condition, the doped and undoped sample showed an optical transmittance of 75% and 50% respectively. According to (Frage et al., 2005), the mechanical hardness, elastic moduli and density are the same in both the sample but the main difference is in the microstructure. They concluded that Spark Plasma Sintering is a promising technique to produce 100% dense Magnesium Aluminate transparent ceramics. The presence of sintering aid modifies the spinel microstructure by eliminating the residual boundary phases and helps in formation of large well defined grains[28].
Ø (Morita et al.,2005) fabricated ZrO2–spinel composite using Spark Plasma Sintering. By preventing the grain growth and hence preserving the nano structure (Morita et al.,2005) noticed an increase in maximum strength by a factor of 2.5. They noticed a massive increase in flexural strength from 935 MPa to 1900MPa when the grain size changes from 300 nm to 96nm and also concluded that high energy ball milling process is an ideal method to produce ZrO2–spinel composite of less than 10 nm from submicron size[90].
Ø (Villalobos et al.,2005) at NRL studied the densification dynamics of Magnesium Aluminate. They developed transparent Magnesium Aluminate using a new particle-coating technique with a transparency of above 85% in the range of wavelength from 300nm to 5500 nm (i.e.: – in the visible and near infrared region). In this technique, sintering aid LiF is mixed uniformly with Magnesium Aluminate[139].
Ø (Reimanis et al., 2005) prepared Magnesium Aluminate transparent ceramics by controlling the particle size, size distribution, purity and stochiometry of the starting powder. They studied the influence of LiF in densification of Magnesium Aluminate and conclude that the transparency is highly sensible to the microstructure, percentage addition of sintering aid and purity of the starting powder. They noticed a dramatic increase in transparency from nearly 40% to above 85% in the visible and near infrared region when they replaced ordinary powder with ultra high purity powder[108].
Ø (Dericioglu and Kagawa,2002) fabricated transparent spinel with a maximum transmittance of ~ 60% in the UV visible region and ~ 70% in the infrared region. The effect of microstructure on the optical property of light is discussed in detail. The study focused on cracking at grain boundary and its role in reduction of transmittance of the sample[25].
Ø (Rozenburg et al.,2008) studied the sintering kinetics of Magnesium Alumiante with sintering aid LiF. The fabrication was carried out using vacuum hot press and the sintering kinetics study using the master sintering curve technique. From the study they concluded that densification of Magnesium Alumiante spinel is limited by oxygen vacancy and it is provided by adding LiF[110].
Ø (Sands et al.,2008) worked on static and dynamic modelling approach to evaluate the armour design based on a transparent magnesium aluminates spinel ply backed by polycarbonate. Using this model (Sands et al.,2008) characterize the influence of defects on the failure of laminates and gives a overview about the defect tolerance of spinel ceramics subjected to ballistic loading[129].
Ø (Ganesh et al.,2008) conducted a brief study on the fully dense stoichiometric and non-stoichiometric magnesium aluminate (MgAl2O4) spinel prepared by double-stage firing process. The study was focussed on influence of processing parameters on the densification behaviour of the crystal .The results are based on the bulk density, apparent Porosity, and water absorption capacity, and micro structural observations of the sintered sample[40].
Ø (Villalobos et al., 2005) conducted a study on the reaction taking place between sintering aid (LiF) and Magnesium Aluminate (spinel). The sample fabricated by hot pressing with different wt% of LiF. This report discusses in detail on the optical property and LiF degradation during the sintering process of the samples. From the study of (Villalobos et al., 2008) it is clear that Lithium Fluoride is a important requirement to make fully dense transparent spinel, but excess percentage of LiF along with poor sintering process can cause new reaction zones between Al2O3 and LiF resulting in Magnesium rich area, and thus making the sample opaque[140].
Ø (Morita et al., 2008) fabricated sintered transparent Magnesium aluminate without sintering aid using Spark Plasma Sintering technique. According to them the high temperature provide by SPS is good enough to cause the densification of the spinal powder without sintering aid. They studied the optical property and mechanical property at different heating rate. the achieved a maximum in-line transmission of 47% for a visible-wavelength of 550 nm and a fracture strength of 500 MPa.from the investigation (Morita et al., 2008) concluded the slower heating rate can produce high quality transparent ceramics by reducing the percentage of residual pores [94].
Chapter 3
Processing of transparent ceramics: Spark Plasma sintering
3.1 Introduction
Spark Plasma Sintering (SPS) is the most advanced technique which can apply high temperature and axial pressure simultaneously, enabling the densification of materials within very short period of time [75,91,128]. This technique allows the sintering of ceramics with melting points even above 2000oC, as well as consolidation of polymers and joining of metals etc. [98]. This advance sintering technique is known by various names, including: Spark Plasma System (SPS) [98], plasma activated sintering (PAS) [55] can be seen in literature. According to (Omori, 2000), Spark Plasma System is an apt term to define the capability of the machine as the term includes Spark Plasma Sintering (SPS), Spark Plasma Consolidation (SPC), Spark Plasma Joining (SPJ), Spark Plasma Growth (SPG) and Spark Plasma Reaction(SPR) [98]. The main advantage of this machine over a conventional sintering technique includes faster heating and cooling rate and a shorter dwell time allowing us to sinter powder to 100% dense product[4,61]. Apart from the advantages mentioned, this sintering technique has the potential to retain the Nano crystalline structure by preventing the grain growth due to the shorter sintering time[91,99,135,128,136,141].
3.2 Historical Background
Sintering of materials is considered as a highly skilled art from Mesopotamia civilization[91]to current copper age, From bricks made out of clay to the most advance spark plugs for automobiles. With the production of porcelain in China and Europe, exploring new ceramic material, sintering gained a position as a branch of science [91].
Frenkel, Kuczynski, Lenel, Coble and many others have identified mass transport as the phenomenon behind sintering [64,77,91]. This invention opened a new gateway for materials personnel to do more work on sintering techniques. Investigations were either focused on reducing the sintering energy; enhance the mass transport [91]or improving the sintering techniques to produce products of better quality. In the 1930’s and 1940’s Cremer (1944) and Taylor(1933) patented the process of using electric discharge or current for sintering and sinter joining of powder and metal[12,130,131]. In 1950, Lenel[78,91] at Rensselaer Polytechnic Institute, USA invented the new spark sintering technique to densification the materials. This concept was later developed in the 1960’s and 1970’s by material scientist in Lockheed Missile and space company in California [5,31,91]as well as by Inoue in Japan[52,53,91].
Investigations continued for the development of techniques using electric current continued for many years. Some of the amazing outcomes from the research within field-assisted sintering techniques (FAST) include Plasma-Assisted Sintering (PAS) [116], Pulsed Electric Current Sintering (PECS) [145], Electric Pulse Assisted Consolidation (EPAC) [33], and Spark Plasma Sintering (SPS) [91,132]. The electric discharge sintering machine and its apparatus for electrically sintering discharged bodies was patented by Kiyoshi Inoue in year 1966[50,51]. However, the patent had expired in the 1980[98]which allowed companies to work more on the sintering techniques to bring up the most advance, fast, densification technique up to date: the Spark Plasma Sintering (SPS).
3.3 Process: Mechanism and Working
Mechanism
The effect of electric field in sintering of materials is clearly mentioned in the literature [91].The dc pulse supply produces spark and joule heating points between the particles within the graphite die. The impurities and gases at the surface of the particles encourage the introduction of spark. The spark produces a high temperature of 10,000 0C which is high enough to melt the particle surface and also evaporates the impurities. Due to the phenomenon of flow of electrons during ON pulse and vacuum during the OFF pulse of the supply, the particles are drawn to form necks. The radiant Joule heating accompanied with high-pressure results in the development of necks resulting in densification of powder[4].
Working
Sintering condition Recipe is discussed and prepared. The heating rate, holding time, cooling rate and pressure to be applied during each segment of sintering is fed into the machine computer control unit. The powder is measured and poured into graphite dies and cold pressed manually to prepare the green body. The die set is kept in the SPS chamber. The lead of the thermocouple is kept in place to measure the sintering temperature. Then the metallurgical recipe is send to the machine from the control computer to execute densification of the green body as per the program.
Figure 3.2-Current flow in SPS Furnace. Reproduced from[4]
3.4 Unique Features of Spark Plasma Sintering:
Figure 3.1- SPS furnace (FCT Systeme, Germany) at Nanoforce Technology Limited.
Ø In Spark Plasma Sintering the particle is under high pressure and temperature, this will enhance the high spark assisted diffusion process to takes place [4]. The diffusion rate (mass transport) by the rearrangement of particles under the influence of high temperature and pressure in SPS will be more when compared to conventional sintering, where particles are only subjected to temperature[91].
Ø During sintering, the spark produces a high temperature of 10,000 oC which evaporates the impurities and also prevents the pre-existing oxidation which gives us sintered products of high purity, quality and excellent properties due to stronger bonding between particles due to absence of impurity during sintering [4].
Ø The sintering time is very small compared to conventional sintering [61].During this short time period the heat is mainly concentrated on the surface of particles, thus by preventing the grain growth it is possible to preserve the nano crystalline structures without changing the particle characteristics[4,91,134,137].
Ø Spark Plasma Sintering can be used to density of powder with a wide range of particle size but now the interest is in sintering Nano powder[4]which is not explore yet due to limitations of conventional sintering due to grain growth and its destruction[91,134,137]. This novel technique helps the materials engineers to explore and find new application of Nanostructure materials.
Ø In Conventional sintering, specimen fails due to internal defects due to non-uniform heating which is relative low in SPS due to elimination of internal stress by the speed of process and the consistent heating[4].
Ø In this age of industrial revolution, all mass production processes are automated which immensely reduces labour costs. The working of spark plasma sintering machine provide is simple and provide us with the option of robotic interface and can be successfully connected to automated systems [4].
Ø Operating cost of Spark Plasma Sintering is up to 80% less than in conventional sintering[4] .The overall sintering time is less due to faster heating and cooling rate[4,61]. Hence conventional sintering machine has to work for long hours which require more electrical energy, which has a direct impact on operating cost.
Ø Spark Plasma Sintering provide solution in joining dissimilar advance layers material (functionally graded material), it is possible to maintain homogeneity in terms of composition, density and shape by properly mixing the dissimilar powder followed by spark plasma sintering [4].
Ø Most of the conventional sintering requires special additives to enhances the process which can effects the wear, mechanical property etc of the sintered sample [4].Using binder free material can provide amazing improvements in properties of ceramic fuel cell, ceramic optics etc [4].
Ø The sample sintered using SPS machine have a better properties such as electrochemical properties, corrosion resistance etc when we compare the one sintered using the conventional machine[150].
3.5 Success stories of Spark Plasma Sintering in sintering Novel materials
Cleaner and less segregation of impurity at grain boundary, which is one of the key requirements to prevent scattering of light [16] in making transparent ceramics can be achieved by processing materials using spark plasma sintering can be seen in literature [19,91].Using spark plasma sintering, (Shen et al., 2003) was successful in preparing Si3N4 based ceramics with good super plasticity with very little glassy phase and can be used for high temperature application[91,122]. (Yue et al., 2003) found that process a promising method for the production of magnets with ideal overall performance after conducting an investigation on Nd-Fe-B composition [91,150]. (Yan et al., 2006) noticed that ferroelectric and piezoelectric properties can exist at very low temperature after investigating the relaxor behaviour in Aurivillius phase of BaBi2Nb2O9 ceramics prepared by SPS[151]. (Inam et al., 2007) was successful in producing electrically conductive, 100% dense CNT –ceramic composites using Spark Plasma Sintering which has wide range of application in the coming decades [60].
The use of spark plasma sintering in making novel materials is vast. So just for an overview of the success of spark plasma sintering in the densification of novel material is sketched in the table below [91]
Table 3.1- Diverse applications of SPS Furnace in sintering novel materials.
Chapter 4
Scattering sources of light in polycrystalline material
4.1 Introduction
Grain boundary, rough surfaces, pores and the secondary phase inclusions are the main sources of scattering and absorption of light in polycrystalline material [3].Absorption coefficient of a material is given by µ= α +S im+ Sop—- (1), Where α, S im, Sop are absorption and scattering terms for pores, secondary phases and optical anisotropy respectively. Since α is characteristic of material so in order to make polycrystalline transparent ceramic material it is always good to choose whose single crystal is transparent [149].
So for making ceramics material with good transmittance we have to prevent the scattering of light by the above mentioned scattering sources.
Figure 4.1- Schematics of Light scattering sources in a polycrystalline material. Reproduced from [3].
4.2 Rough surface scattering
When light falls on a rough surface, specular reflection and diffuse scattering of light takes place [3]. This phenomenon reduces the transmittance of light when we consider Lambert Beer equation for intensity of transmitted light through a sintered body [149]. The specimen is polished to1 μm or below using diamond polishing media after sintering of polycrystalline material such as Magnesium Aluminate (MgAl2O3) or Alumina (Al203) for enhancing the surface finish and thereby increasing transmittance can be seen in literature[63,94,108].Thus it is clear that to make high transparent polycrystalline materials it is necessary to reduce scattering effect at rough surface by making the sample surface polished to high degree. According to (Apetz and van Bruggen, 2003)[3], if the surface is smooth then diffuse scattering is ignored and it is necessary to consider only the specular reflection. If Apetz et al. above mentioned concept is true, then an anti reflection coating on the surface of the specimen can prevent specular reflection [149]. More over (Villalobos, Sanghera, and Aggarwal ,2005) [139]work on transparent Magnesium Aluminate- spinel highlighted the same concept that a antireflection coating can increase the transmission above 99%.
4.3 Scattering at pores
When light enters the pores in the sample from the material then scattering of light take place due to difference in refractive index[3]. From the literature[3,75,94] , its clear that presence of pores has a adverse effect on transmittance property of the material.
(Kim et al., 2007)[75], noticed a change in transmittance from 47 %( transparent) to 0.2 % (opaque) when the porosity increased from 0.03% to 0.59% using Spark Plasma Sintering for making transparent alumina. (Kim et al., 2007) also noticed that heating rate has an effect on the porosity of the material, which in turn affects the transmittance of the sample. Hence we can conclude that heating rate has to be controlled to make non-porous transparent Ceramics.
Scattering term for secondary phases (Sim) in absorption coefficient (µ) depend on porosity of the material (equation 1). Porosity can be controlled by controlling the heating rate and the grain growth during sintering .Grain growth can be controlled by adding grain growth inhibitor during sintering[149]. (Jiang et al., 2008)[63] Produced alumina with about 85% transmittance by controlling the heating rate and adding 0 .2 wt % of MgO. From the (Jiang, 2008) work on transparent ceramics, by choosing the correct additives and sintering conditions it is possible to make non-porous materials.
Presences of pores influence the elastic properties and strength. Then modulus of elasticity and porosity fraction is related by
E=E0 (1–1.9P+ .9P2)
E = modulus of elasticity
E0 = modulus of elasticity of non porous material
P = volume fraction of porosity
From the above equation it is clear that modulus of elasticity reduces with increase in pore fraction. Presence of pores has a negative effect on flexural strength of material because pores reduce the bearing area when a load is applied and also acts as stress concentrators. For e.g. 10 volumes % porosity will decrease flexural strength by 50% of the value of 100% dense sample. Flexural strength (σ fs) is related to volume fraction porosity (P) as
σ fs= σ 0 exp(-n P),Where σ 0 and n are experimental constants [18].
Coble and Kingery (1956) [13] found that their experimental value for sintered alumina holds good with the equations mentioned for both flexural strength and also elastic modulus above.
From the application point of view for transparent ceramics, other than transmittance of light at different wavelength another important requirement is its mechanical stability so we have to make pores free, fine grain material to gain good mechanical property [18].
4.4 Grain boundary scattering
Grain boundary is one of the main scattering sources in poly crystalline non-cubic crystals [3, 8, and 30,106,113]. According to Apetz and van Bruggen (2003)[3]due to Birefringence there is discontinuity in refractive index at grain boundary, if the crystallographic orientations of the neighbouring grains are not the same then leads to reflection and refraction of light at the grain.
If the grain size is large then grain boundary per unit area is less than that of fine grain [3]. Hence logically, amount of scattering in coarse grain boundary is less compared to that of fine grain [3]. This concept matches when we compare with the transmittance and the grain size of aluminium oxy nitride [76] but is against that mentioned in the literature by (Peelen and Metselaar, 1974). According to (Peelen and Metselaar, 1974), the scattering at grain boundary can be ignored if the size of the particles is of the order 1-2 µm since it is comparable to the wavelength of light [3,100]
Scattering at Grain boundary can be negligible if the crystal has a cubic structure since it has no birefringence and the crystal is optically isotropic [3,101].This may be one of the reasons for high transmittance of Aluminium Oxy Nitride (ALON), the best poly crystalline transparent ceramic material known. Aluminium Oxy Nitride of about 0.5µ m powder is manufactured by Surmet Corporation and is not available for commercial application and the final product grain size is between 250-300 µ m [76]. So the number of grain boundary is less per unit area and hence very high transmittance can be achieved. The MgAl2O4 spinel has a similar structure (cubic) as aluminium oxy nitride and its optical properties can be compared to that of ALON and sapphire [20,108]. So when we consider the grain boundary phenomenon, cubic crystal is one of the best choices to replace the high proprietary ALON [76,139]. The features of cubic crystal structure can be exploited only if it can be processed properly by controlling the over grain growth to prevent grain distortion [76].
4.5 Scattering at secondary phases
The presence of impurity has massive impact on the transmittance of the sample. This reduction in transmittance is due to optical absorption and also due to scattering at the secondary phases within in the sample [149].
The mechanism of impurity can be explained by refraction and absorption of light [149]. The secondary phase or impurity and the ceramic material have different refractive index[3]. So when light passes through the material into the secondary phase within the sample, due to difference in refractive index scattering and absorption phenomenon will take place.
Alpha Optical company and Coors Ceramics stopped production of Magnesium Aluminate -spinel domes for stinger post missile in 1993 due to lack of poor yield of spinal part, one of the main reason mentioned was due to inconsistent quality of staring raw Magnesium Aluminate powder[20].So high purity starting powder and processing of green body in a clean environment is of vital importance and have to do with great care.
(Reimanis et al., 2004)[108] , noticed a dramatic change in transmittance value from 42 to 85% when replaced the normal powder of purity with ultra high purity Magnesium aluminate powder using hot Iso -static process. From the SEM image study (Reimanis et al., 2004) also concluded that micro structure of sample prepared by normal powder is different from that of ultra high purity powder. A change in microstructure of sample also changes the mechanical property of the sample [18], which is an important requirement for all application. So for making better transparent ceramics with high transmittance value and good mechanical properties, ultra high purity starting powder is a vital requirement [108]. Apart from ultra high purity powder other requirements include particle size, particle size distribution; surface area can be found in literature [20].
PART: 2
Chapter 5
Experiments, Results and Discussion
5.1 Introduction and Particle Size Analysis of Powder
5.1.1 Magnesium Aluminate Powder
* Information obtained from American Elements material safety data sheet.
For the spark plasma sintering of transparent ceramics, the Magnesium Aluminate (MgO.Al2O3) pre-synthesized powder was purchased from American Elements, California. The particle size of the powder is 325 mesh with a purity of 99.8%. The white odourless powder has a density of 3.6g/cm3 at 20 oC and has a melting point of 2135 oC. According to the certificate of analysis, the purchased powder contains the following composition explained in the table below: The PH of the powder is 9.34.
Table 5.1- Magnesium Aluminate Powder composition
As per the scale (0-4) of Hazardous Materials Identification System (HMIS), the powder is inflammable and is non reactive but has acute health effects with a scale degree of 1. While the sintering of Magnesium Aluminate powder, special care need to be taken and the use of breathing equipments are highly recommended when handling high concentrations as mentioned in the product material safety data sheet.
Figure 5.1-SEM images of Magnesium Aluminate powder showing.In-homogeneity in particle size [A] Nano size particles [B] Micron size particles.
The particle size of powder according to the powder data sheet is 325 mesh. From the SEM images of magnesium aluminate powder, I have noticed that the particles are not of the same size. From the SEM image of powder in figure [B] above, it can be confirmed that most of the particles are below 44µm. There are just few particles above the limit also. On further analysis of powder at 60,000 times magnification it was clear that some particles are of nano size. To confirm the distribution, I decided to do the particle size distribution of magnesium aluminate powder.
5.1.2Particle size distribution of Magnesium Aluminate
The particle size of the Magnesium Aluminate powder is measured using the Zetasizer Nano series machine. This experiment is conducted to study and confirm the homogeneity in grain size of Magnesium Aluminate powder.
Procedure
A new cuvette is used for experiment to prevent cross contamination. It is filled with acetone and a small amount of Magnesium Aluminate (MgAl2O4) which is dispersed in the solution. The cuvette is then closed and kept for 10 minutes in an Ultra Sonicator to make the Magnesium Aluminate powder dispersed fully in the acetone solution. Then the cuvette is kept safely into the testing chamber of the Zetasizer Nano series machine. The standard program is selected to measure the particle size distribution and the result is directly read from the graph.
Results and Conclusion
From the experiment, I have got a Z-Average of 1571 d.nm. So the large particles observed in the SEM images can be agglomerates of Magnesium Aluminate, which is dispersed efficiently in the acetone solution during Ultra Sonification.
5.1.3 Lithium Fluoride
* Information obtained from Sigma Aldrich material safety data sheet for LiF
Lithum Fluoride (LiF) is purchased from Sigma – Aldrich which is used as a sintering aid in the fabrication of Magnesium Alumiante transparent ceramics. Lithium fluoride used is precipitated with 99.995% purity. According to the material safety data sheet received with the powder, this chemical emits toxic gas. Additional precautions need to be taken and it is recommended that we wear necessary protective equipments such as respirators, chemically compatible resistance gloves for hand protection and chemical safety goggles for eye protection. This powder has a density of 2.64 g/cm3 with a melting point of 845 oC. The solubility of Lithium Fluoride in water is 0.01 M at 20oC and there is chance for the formation of hazardous composition product, Hydrogen Fluoride on reaction.
5.2 Systematic Experimental Approach: From opaqueness to transparency
5.2.1 Sample: 1
Aim
Ø To conduct a trail sintering for new Magnesium Aluminate powder in making transparent ceramics.
Ø To find out the densification temperature. This allows us to do design sintering conditions to obtain a 100 % dense sample.
Sintering Condition
For the first sample, 6 grams of the Magnesium Aluminate powder is used to make the green body of 20 diameter disk. The first sample is sintered at a temperature of 1900oC. This sintering temperature was chosen after considering the melting point of powder of Magnesium Aluminate (MgAl2O4), which is 2135oC according to the materials data sheet obtained from American elements, L.A. The maximum pressure determined to be applied is 85 MPa. I have decided to apply this pressure after taking into consideration the melting point of the powder, holding time and creep that can encounter at a temperature of 1900oC from the past experiences with the SPS machine (FCT Systeme, Germany). The holding time is preset to 10 minutes at the temperature of 1900 o C, along with a pressure of 85 MPa. The heating rate is set at 423oC per minute from 450oC to 1000oC and 100oC per minute thereafter until the maximum temperature of 1900oC is achieved. After holding the sample at the maximum temperature and pressure, I have decided to cool the sample at a rate of 100oC per minute to a temperature of 450oC from 1900oC. The Temperature (oC), Pressure (KN) and Time (minutes) taken for each segments of the sintering process is explained in the figure below.
Figure 5.2- Sintering condition for sample 1
Annealing Condition
The annealing condition for the first sample is sketched below. The specimen is annealed at 1600oC for 2 hours in the furnace (Carbolite HTF 18/8). The sample is annealed at this temperature because the normal annealing temperature is 200oC to 300oC below the sintering temperature. The heating rate was decided to be 2 o C per minute. At high cooling rates of furnace cooling, there is a risk of cracking the grain. This can then result in an irregular grain shape, hence reducing the transmittance of the sample. So, for this sample, a cooling rate of 2 o C per minute is preferred to the furnace cooling, and thus to prevent the cracking of the grain due to thermal shock.
Figure 5.3- Annealing condition for sample 1
Inference
During the sintering of this sample at 1900oC, there was some discharge observed through the observation window of the SPS machine. Due to this, I couldn’t hold the sample as per our SPS recipe shown above as it can melt the sample even some more. The sample was thick with no sign of cracking. Optical property (transmittance) of the sample was also very poor. After annealing, the sample was white with small black spots on the surface which could be removed by diamond polishing. Diamond polishing is preferred to silicon carbide paper to prevent contamination and also to make the sample of uniform thickness which is difficult to obtain by the manual operation of grinding wheel. After diamond polishing also there was no change in the optical property and the sample was 100 % opaque by visual inspection. I also noticed that the densification temperature of Magnesium Aluminate from the data was not clear but doing a thorough close shrinkage analysis, I have concluded that it is between 1400oC and 1600oC.
Discussion and modelling of next sintering conditions
The discharge that occurred during sintering was due to the melting of powder. The size distributions of the powder particles were uneven, and this may be the reason for the melting of powder during sintering. Non uniform particle size distribution can cause melting of the power during sintering at well below the actual melting temperature. As per the product (Magnesium Aluminates) data sheet obtained from American Elements, the particle size of powder is 324 mesh (i.e.:- 44 microns) with a melting point of 2135oC. Even though the melting point is 2135oC, the heat of sintering at 1900oC is strong enough to melt the particles of Nano grain size. Therefore, I have decided to do the particle size distribution of the powder and also to sinter the next sample at 1700oC, to avoid melting of powder during sintering. The sample was too thick when we choose 6 grams powder to make a 20mm diameter disc. Subsequently, I have decided to reduce the quantity of the powder for the next sample to make it thinner.
5.2.2 Sample: 2
Aim
Ø To make a thin sample compared to the previous one.
Ø To hold the sample during sintering process, allowing grain growth in order to remove the pores which is the main source of scattering of light.
Sintering Condition
For this sample, the green body is prepared using 1 gram powder of Magnesium Aluminate. To prevent the melting of powder as observed in the previous sample, the sintering temperature is reduced to 1700oC from 1900oC. The heating rate from temperature 450 o C to 1000 o C is 423 o C per minute and thereafter up until 1700oC is 200oC per minute. The constant pressure of 5KN is applied up at 1000oC. The pressure varies from 5 KN to 31.4KN (100MPa) as the temperature changes from 1000oC to 1700oC, the maximum sintering temperature. The sample is held for 10 minutes at this maximum temperature and pressure of 1700oC and 100MPa respectively. This holding time is provided in the sintering program with the aim to over come the small pores by allowing the grain growth and hence helps to achieve the development of grains. The cooling rate in this sample was programmed to 357 o C per minute (i.e.: – from 1700 o C to 450 o C in 3.5 minutes).
Figure 5.4- Sintering condition for sample 2
Annealing Condition
The sintered sample is annealed at 1600 for 2 hours. The heating rate was 2oC per minute. At 1600 the dwell time is set to be 2 hours. At the end of the holding time the sample is allowed to cool at a rate of 2oC per minute. Overall, the time taken for whole annealing process is about 28 hours.
Figure 5.5- Annealing condition for sample 2
Inference
The sample removed from graphite die after sintering was very thin and was broken into pieces. There was hope for transparency at the edge of the sample. However, the centres of the pieces were opaque and covered with carbon particles from the dies during the densification process.Analysing the sample; I have noticed that the optical properties and appearance of the sample were not uniform throughout. There was non-uniformity in appearance between the edges and the centre. The edges of the sample were translucent with a slight grey shade and the other areas looked whitish. Even though the edge is translucent it is slightly greyish in colour.
Discussion and modelling of next sample sintering conditions
The sample broke due to the thermal shock due to the high cooling rate of 357 o C per minute. So for the next sample I have decided to use lower cooling rate of 100 o C per minute. Another reason could be that the sample was very thin as I used 1 gram powder for this sample. This shows that in sintering the next sample, I would need to use 2 grams of powder to increase the thickness. The difference in the homogeneity of the edge and middle portion may be due to different reasons; presence of new phase, different in grain structure, or due to holding time – which is not sufficient enough to provide uniform grain growth. To confirm whether any secondary phase is present, I have decide to X-ray diffraction and compare between the middle and the edge portion with another sample and also with the XRD results of powder used. I am also eager to see whether any considerable difference in grain size between these two surfaces. So I decided to do the take the Scanning Electron Micro (SEM) images and compare these two areas (middle and edge) in the samples.
In the next sample I have decided to reduce the cooling rate to prevent the failure of the sample during sintering by thermal shock. The edges with grey colour in appearance are noted in the entire specimen sintered before, without the sintering aid. I am not clear about the phenomenon behind this, as researchers have different opinions. Ultimately, if it is possible to achieving the same amount of translucency all over is good because once we have the result we can add sintering aid to make it fully transparent.
5.2.3 Sample: 3
Aim
To tackle these problems in the second sample sintered at 1700 oC for a holding time of 10 minutes. The main problem with the second sample was the following:
Ø The specimen broke into pieces.
Ø The optical property and appearance of the specimen was not uniform.
Sintering Condition
For this sample, the quantity of starting powder is increased from 1 gram to 2 grams in order to increase the thickness of the sample. I have also decided to go to 1800 oC per minute for this sample. To obtain large and fully developed grains and hence to achieve less grain boundary per unit area after densification, I have decided to increase the dwell time as well. In this sample the dwell time was set to 30 minutes compared to the 10 minutes in the previous sample. The pressure is reduced to 70 MPa from 100 MPa to prevent failure of the die which can occur during the holding time of 30 min at a temperature of 1800 oC. The SPS machine is programmed to provide a hating rate of 300 degree Celsius per minute from 450 to 1200 o C and thereafter 181 o C /min to raise the temperature to 1800 oC.
Since the holding time is high, the pressure was reduced to prevent breakage of the sample. The Spark Plasma Sintering machine is programmed to provide a force of 5KN to 22 KN as the temperature ascends from 400 oC to 1800 oC. Slow cooling rate of 100 oC / min is preferred to prevent thermal cracking during cooling, which may be one of the reasons for the failure of the second sample.
Figure 5.6- Sintering condition for sample 3
Annealing Condition
This sample is annealed at 1450 oC for 3 hours. The heating rate for this sample was 2.5 oC per minute up to 1450 oC and the dwell time at the maximum temperature is set for 3 hours. After holding for 3 hours, the cooling of this sample is controlled by the furnace.
Figure 5.7- Annealing condition for sample 3
Inference
The resulting sintered sample looked black with the presence of carbon particles over and within it. Even with the presence of carbon, however, the sample sintered was translucent. The sample even came out as a single piece without any cracks observed, and the optical property of the sample seemed uniform throughout the sample. The translucent sample after sintering is shown in figure below.
After annealing at 1450 oC for 3 hours the sample turned white as shown in figure below. After annealing also the sample was uniform throughout in appearance with no visible cracks.
Figure 5.8- sample 3 (1800 o C/30 minutes).
[1] Translucent – after Sintering.
[2] Opaque – after annealing.
Discussion and modelling of sintering conditions for next sample
Comparing this sample with the previous one, we can note the difference in increasing the holding time and reduction of pressure to 70MPa from 100MPa. Increasing the duration of dwell time allows the grain growth. During this time, the grain will grow and develop with cleaner and lesser grain boundaries because of large number of grains per unit area. The development of grain during holding time also removes the small pores that may be present in the sample. From the results, it is clearly understood that the holding time has a direct relation with the transparency of sample. This was the reason why I decided to focus on the influence of the holding time on the optical property of the magnesium aluminate.
The reason the sample turned white may be due to high annealing temperature, heating rate or cooling rate, since the heating rate used for this sample is 2.5 oC per minute. Therefore it can be due to either fast furnace cooling or due to high annealing temperature. Overall, I have decided to remove the carbon from the sample at a lower annealing temperature and to cool slowly after dwell period in annealing process.
5.2.4 Sample: 4
Aim
The previous sample was transparent after sintering but turned white when it was annealed at 1450 o C for 3 hours. So I decided to change the annealing condition to obtain a transparent sample.
Sintering condition
The sintering condition is similar to that of the previous sample. The sintering temperature was 1800 oC and the maximum force applied is 22 KN (70 MPa). The holding time was 30 minutes. The heating rate was 181.8 oC per minute to raise the temperature of the sintering chamber to 1800 oC from 1200 oC. At the end of the holding time the sample is allowed to cool at a rate of 100 oC per minute.
Figure 5.9- Sintering condition for sample 4
Annealing condition
The previous sample turned white may be due to the following annealing conditions:
Ø Due to high annealing temperature of the sample, in the previous case it was1450 o C.
Ø Due to faster cooling rate after holding time, in the previous case it was furnace cooling.
So in this attempt I have decided to reduce the annealing temperature from 1450 oC to1000 oC in order to remove the carbon from the annealed sample. To raise the temperature to 1000 o C from room temperature, the heating rate was 2 o C per minute was preferred. After holding at this temperature for 3 hours at 1000 oC, then the sample is allowed to cool slowly at rate of 2 oC per minute. This slow cooling rate is preferred to prevent destruction of grain due to sudden thermal shock. The annealing condition used for this sample is sketched in figure below.
Figure 5.10- Annealing condition for sample 4
Inference
The sample after sintering was translucent with the presence of carbon particles.The sample after sintering was obtained in one single piece and there were no visible cracks. There is a sure sign of transparency and the optical property seems uniform throughout the sample. The sample annealing at 1000 oC for 3 hours was transparent as shown in figure 5.11.
Discussion
Compared to the previous sample which turned white this one was transparent. Thus we can conclude that the annealing condition is responsible for the whitening of sample. So a brief study on the effect of annealing condition in removing the carbon particles from sintered samples is necessary .For the annealing study, I have decided to optimise annealing conditions to remove the carbon particles on and within the sample. In the annealing study, I have also decided to check whether annealing condition required for samples changes with sintering conditions. Diamond polishing increased the appearance and optical property of the sample. This can be due to preventing the scattering of light by the rough surface. To study the whitening and confirm the phenomenon behind that, I have decided to do XRD analysis and also check the SEM images of the samples to find the uniformity in microstructure structure.
Figure 5.11 -Progression of the transparency of the Spark Plasma Sintered MgAl2O4 discs.
[1]1900oC/0min/85MPa
[2]1700oC/10min/100MPa
[3]1800oC/30min/70MPa
[4]1800oC/30min/70MPa
5.3 Investigation on the non-homogeneous appearance of samples after sintering
5.3.1 X-Ray Diffraction Result and Discussion
5.3.1.1 Introduction
Data collected from X-ray diffraction can be used for identification of new phases in the sintered sample. The sample 2 shown in figure below has a non-homogenous appearance with outer region translucent and the middle portion white. So I have decided to use XRD technique to find any new crystalline phase is present in sample 2, when comparing with the homogeneous translucent sample 4 shown below.
Figure 5.12-Sintered sample considered for XRD study due to non-uniform appearance.
Sample 2 was sintered at 1700 oC for 10 min with a maximum pressure of 100MPa, while sample 4 was sintered at 1800 0C for 30 min with a maximum pressure of 70 MPa. The Sample 2 is annealed at 1600 oC for 2 hours and the sample 4 is annealed at 1000 o C for 3 hours. The XRD pattern below shows the X-ray diffraction pattern of the powder used and samples mentioned above. The peaks show clear evidence that the Magnesium Aluminate MgAl2O4 powder used is highly crystalline. All the peaks are identical in both the sample and also in the powder. This in turn gives evidence that there is no additional phase present in the sample. Further studies have to be done to find out the reason for the whitening of the sample.
Figure 5.13-XRD pattern of Magnesium Aluminate powder, a sample sintered at 1700/10min/100MPa and sample sintered at 1800/30min/70MPa. The same powder without any additives is used.
5.3.1.3 Conclusion
The powder is highly crystalline. There is no sign of another evident phase being present. The whitening phenomenon noticed must be due to the grain cracking from thermal shock and not because of the introduction of a new phase during sintering. I believe this as I noticed a well-defined crack in one of the samples while conducting a brief study on optimising the annealing conditions so as to remove the carbon particles without destructing the grain.
Thermal shock, in the case of processing transparent ceramics can come into play due to rapid cooling during sintering and annealing or due to high annealing temperature. This can be studied using the Scanning Electron Microscope (SEM) images of the sintered sample by checking the homogeneity of grain structure. If the sample has experienced a thermal shock during sintering then there is chance for the cracking of grains.
5.3.2 SEM Images – Result and Discussion
5.3.2.1 Introduction
From the XRD data it is clear that no additional phase is present in sample 2. The whitening of sample can be due to grain cracking as a clear crack is noticed for a sample annealed at 1200 oC for 3 hours. SEM imaging of the samples can confirm whether this whitening phenomenon is due to grain cracking by comparing the microstructure at the edge and middle portion that can be encountered at high sintering or annealing temperature or due to thermal shock during cooling. 
.
Figure 5.14- SEM image of the sample 2 after fracturing. [1] Over view [2] Edge-Translucent area of sample 2 [3] Middle portion- white area of sample 2.
5.3.2.2 Discussion
The figure (1) shows the SEM image of the sample 2 after fracturing the surface. From that surface it is clear that a combination of inter granular and Trans granular fracture is taking place in Magnesium Aluminate. The grain boundary is not visible and there were some signs of pores also.
The figure (2) and (3) above shows the SEM image of the fractured surface of sample 2. The Images 2 and 3 are taken from the edge and from the centre of the sample respectively. From the Scanning Electron Microscope images we can see that there is a massive difference in microstructure at the middle portion and edges, which is translucent. Both the images are of same magnification (10000X), it is clear that the size of grain has grown very big at the edge and that in the centre has grown big enough and finally collapsed due to high cooling rate of the sample of 357 oC per minute.
5.3.2.3 Conclusion
From the XRD result it is clear that the whitening phenomenon is not due to a new phase and from the SEM image analysis, it can be confirmed that it is due to grain cracking during sintering or annealing which makes the sample appear white. Further studies have to be conducted to find out more precise details on annealing conditions and cooling rates after holding time in sintering so as to prevent thermal shock which turns transparent samples white.
5.4 studies On Annealing behaviour of Magnesium Aluminate
5.4.1 Introduction
In spark plasma sintering process dies used are made of graphite. So during sintering the carbon particles from the die will be entering within and also on the surface of the sample. Even though carbon will start oxidize at above temperature 600 oC but in SPS due to vacuum in sintering chamber there is no chance for the carbon to oxidize. So we have to do annealing in a furnace after sintering to remove the carbon particles on and within the sample. This brief study on annealing behaviour is done for a sample sintered at 1800 oC for a holding time of one-hour in spark plasma sintering furnace. This investigation focus on the optimisation of the annealing condition. The optical property relation with various annealing temperature and also origin of cracking behaviour is considered in this study.
5.4.2 Experimental
Sintering condition
The figure 5.15 shows the sintering condition of sample considered for study. The sample is sintered at 1800 o C for 1 hour with a maximum pressure of 70 MPa .The heating rate to reach maximum temperature from 1200 oC to 1800 oC is 180 oC/minute. After holding it is allowed to cool down at a rate if 100 oC /minute .After sintering the sample is cut into four symmetric pieces using a Automatic precisionCut-off machine (Struers Accutom-5) to study the annealing behaviour and influence on optical property.
Figure 5.15- Sintering condition for sample considered for Annealing study.
Annealing condition
The sample is annealed at three different temperature 1000 0C, 1200 0 C and 1400 0C. The holding time for all the three samples is the same.The heating rate and cooling rate during annealing was 2 0C per minute with a holding time of 3 hours. Annealing condition considered for the study for the three samples is sketched below.
Figure 5.16- Different annealing condition considered for study.
5.4.3 Results and Discussion
5.4.3.1Optimising the annealing condition
Figure 5.17 -Photographs of annealed sample at different temperature.
[A] 1000 0 C for 3 hours [B] 1200 0 C for 3 hours [C] 1400 0 C for 3 hours
The figure above shows the sample annealed at different temperature (10000 C, 1200 0 C, 1400 0 C). From the image, there is noticeable difference in appearance of the sample. The first sample [A] is annealed at 1000 0C for 3 hours look dark. This is because the temperature is not high enough to oxidise the carbon entrapped in the pores within the sample. So the next piece was annealed at 1400 0C for 3 hours. The appearance of this sample was milky white as shown in figure [C] .the sample does show any carbon particle and was 100% opaque. The reason for this change in appearance can be explained by the same phenomenon discussed in the previous chapter, cracking of grain at high temperature and producing new surfaces which scatter light. So to find out this transformation temperature to white, I have decided to sinter a sample at 1200 0C for 3 hours. This is shown in figure5.17 [B] and was translucent .The edge of the sample was white showing the indication that the whitening phenomenon started around 1150 0C. From this we can conclude that optimise annealing temperature has to be between 1000 0C and 1200 0C to remove carbon from the sintered sample. The same sample showed excellent transparency without the trace of carbon using the annealing condition showed in figure 5.16[D]
5.4.3.2 Relation of annealing condition with the density and transmittance of sample
The density of the sample was measured using Archimedes principle. All the samples annealed at different temperature showed 100 % density. Archimedes principle method is not suitable for measuring micro pores in sample which can scattering light, but unfortunately it is the only method available as other method such as gas pycometry and mercury intrusion method has it own limitation to find the density of Magnesium Aluminate.
In poly crystalline alumina (Kim et al., 2007) noticed a change in transmittance from 47 % to 2% (Opaque) with a change in porosity from 0.035%to 0.59%.The annealing after sintering can cover the small pores present in the sample which can increase the transmittance of sample. Even though the density is 100 % the sample not fully transparent .This is a clear indication of presence of either porosity which can’t be detected by Archimedes principle and also the presence of other scattering sources within the sample such as impurity or irregular gain boundary or new surfaces created by cracking of grains.
Table 5.2- Data: Density and optical property for samples in annealing study.
5.4.3.3 Different sintering condition should have different annealing temperature
Discussion
Two samples sintered at different sintering condition, one at 1800 0C for a holding time of 30 minutes and other sintered at 1800 0C with a holding time of 1 hour is considered for the study. Then both these samples are kept together in a furnace for annealing at 1000 0C for 3 hours. From the sample observation it was clear that the one sintered at 1800 0C for a holding time of 30 min was translucent and the other one was dark. This is due to the difference in percentage of carbon trapped within the sample. Introduction and type of pores within the sample changes with sintering conditions. So to remove the carbon from the closed pores high heating temperature or high holding time is required. On further annealing of the dark sample at a high temperature also turned translucent by the complete removal of carbon.
Figure 5.18 -Photographs of annealed sample at same conditions.
Table 5.3- The table shows that the different annealing conditions are required for sample sintered at different condition to in removing carbon.
5.4.4 Conclusion
From they study conducted on annealing behaviour to efficiently remove carbon from the sintered sample, conclusion are as follows:
Ø Annealing condition required to remove carbon particle which is entrapped within the sintered sample varies with sintering condition.
Ø Whitening of sample is noticed due to grain cracking at higher annealing temperature.
Ø After removing all carbon particles the sample showed 100% density by Archimedes principle, but the sample is not 100 % transparent indicates the presence of other scattering sources within the sample.
5.5 Measurement – Density & Optical Transmittance
5.5.1 Density Measurement
5.5.1.1 Introduction
100 % density is an important requirement for making fully transparent ceramics. The porosity will serve as an important scattering source and is discussed in brief in the in the literature review of this report. To measure the density of spark plasma sintered sample Archimedes method is used. Archimedes principle is accepted in all application and used widely in measuring the density of materials for decades but its accuracy in measuring density of transparent ceramics is discussed in detailed. The feasibility of other technique Such as gas pycometry and mercury intrusion method in measuring density of Magnesium Aluminate -spinel is also discussed.
5.5.1.2 Principle and Experimental Procedure
Principle
According to Archimedes principle when an object is suspended in a liquid, then the buoyant force experienced by the object is directed vertically upward and is equal to the weight of the fluid displaced by the object. The mass of the object in air divided by the difference in mass of sample in air and water gives the density of the suspended object.
Procedure
The sample is cleaned using acetone to remove the impurity from the surface. The sample is then dried and the mass is calculated in a weighting machine. This gives the mass of sample in air (Ma). Then the sample is suspended in a glass beaker filled with water and the mass of sample in water (Mw) is taken. For measuring the mass in water Archimedes density-measuring arrangement is used. Then the mass in air and water is substituted in the equation Ma/(Ma-Mw) to find the density of the sample. The Relative theoretical density and also percentage porosity is also calculated.
5.5.1.3Results
Table 5.4- Density of sample sintered at different conditions
5.5.1.4 Discussion
We can see from the table above that the entire Magnesium Aluminate sample prepared by Spark Plasma sintering was around 100 % dense according to the Archimedes density measurement. The SEM image of a sample which showed 100 % dense by Archimedes method is shown below. we can see pores of small size within the sample which is ignored by the Archimedes method.(Kim et al., 2007) noticed a change in transmittance from 47% (transparent) to 0.2 % (opaque) when the porosity increased from 0.03% to 0.59% using Spark Plasma Sintering while making transparent poly crystalline alumina. So this method is not appropriate for measuring the density of transparent ceramics, as we required precise data about density to find role of pores as scattering source. Unfortunately this is the only technique now available as the other method such as gas pycometry, mercury displacement methods etc are not feasible to find density of Magnesium Aluminate. Mercury Intrusion Porosimetry is not advisable for my sample (MgAl2O4), because very high pressure is required to pump mercury into the sample. All these methods give wrong results and concepts if a closed pore is present within the sample, which is good enough to scatter light and make the sample opaque.
An informal discussion with Dr. Nina Orlovskaya from University of Central Florida about a conference related to transparent ceramics that she had attended I learned that the Japanese researchers are working on microscopic method to measure the pores percentage in the sample.
Figure 5.19 -Scanning Electron Microscope image of sample showed 100 % density by Archimedes principle.
5.5.1.5 Conclusion
From the density measurement data using Archimedes principle and from the SEM images of 100 % dense sample, conclusions are as follows:
Ø Successfully sintered 100 % dense Magnesium Aluminate samples using spark plasma sintering furnace according to Archimedes density measuring technique.
Ø From the SEM images, Archimedes method is not suitable to find the density of transparent ceramics as this method is not sensible to closed pores and also pores ranging from micro to nano size.
Ø Other methods such as gas pycometry and mercury intrusion method are not advisable in MgAl2O4 samples.
Ø Scanning electron microscopic images is the only source to have an understating of extent of porosity present in transparent sample.
5.5.2 Transmittance Measurement
5.5.2.1 Introduction
The transmittance of the Magnesium aluminate sample sintered using Spark Plasma Sintering is measured using sample UV/Visible Spectrometer (Perkin Elmer Lambda 950).This spectrometer has a wave length in the visible and infrared range of 175 nm to 3300 nm with a photometric accuracy of + 0.006A.
5.5.2.2 Principle and Experimental procedure
The UV win lab software is used in UV/Visible Spectrometer to find the transmittance of the sample. The sample is cleaned using acetone and is carefully kept in specard sample holder of circular aperture of 10 mm diameter. The scattered transmittance with integrating sphere method is used to carry out the experiment.
The spectrometer is programmed to measure the transmittance in the wavelength range from 300 nm to 800 nm to study the transmittance in the visible wavelength of the electro magnetic spectrum. The machine is calibrated automatically by performing auto zero. After doing all preliminary settings the sample is placed in position and transmittance data is collected.
5.5.2.3 Results
Table 5.5- Optical transmittance of sample sintered at different conditions
Figure 5.20– Improvement in optical transmittance with change in optimising sintering condition and composition of starting powder.
5.6 Introduction to Sintering Aid- LiF
5.6.1 Introduction
After optimising the sintering condition and annealing condition I have decided to study the effect of sintering aid in the optical property (transmittance) of sample.
As a beginner in adding sintering aid in Magnesium Aluminate – spinel, I decided to use Lithium Fluoride LiF, the most commonly used sintering aid for the densification of Magnesium Aluminate.
5.6.2 Experimental Procedure
5.6.2.1 Powder Mixing
The Lithium Fluoride (LiF) powder of 99.995% purity (Sigma Aldrich ,U.K) and Magnesium Aluminate(MgAl3O4) of purity 99.8% (American Elements, California.) is used in this experiment. I have noticed from the LiF property data sheet mentions about the solubility in water. According to the material data sheet the solubility is (0.01 M in H2O) at 20 oC. So for mixing process, I have decided to use methyl alcohol to prevent the solubility of lithium fluoride.
For this sample Magnesium Aluminate is mixed with 1 wt % of Lithium Fluoride. Magnesium Aluminate (5 grams) and 0.05 grams of Lithium Fluoride is weighed and placed in a glass beaker. Then 200 ml of methanol is poured into the beaker 1:40 ratio of powder to methanol is selected in order to prevent the powder to settle down due to different density and also to disperse the powder fully in the solution to obtain good result in mixing. Then the solution is stirred manually for 30 min to make the agglomerates to break up and disperse in the methyl alcohol solution. After that the glass beaker is covered to prevent evaporation of methyl alcohol and also to prevent the contamination from the surrounding during ultrasonifcation process. After that the mixture containing glass beaker is kept in an ultrasonic mixing apparatus (Engi Sonic Plus, U.K) for 2.5 hours for homogenous mixing. This long time duration for mixing is selected because the quantity of Lithum fluoride (0.05 grams) is very small, so it requires longer time for uniform mixing in 5 grams of Magnesium Aluminate.
After ultra sonificaton the solution is poured in a mild steel dish. According to the materials data sheet, the melting point of methyl alcohol (CH3OH) used has a boiling point of 64.7 ºC (337.8 K). So to evaporates the methyl alcohol it is heated in a Hot Plate (Camlab Limited,U.K) at 150 ºC which took nearly 30- 40 minutes. During heating the solution is stirred manually to obtain homogenisation by preventing the formation of agglomerates and to prevent the escape of powder along with the evaporation of methyl alcohol. This may occur when powder settle down due to different in densities. Then the mild steel plate is kept in a oven (Elite thermal systems ,U.K)set at 100ºC for 24 hours to extract a homogenous mixture of Magnesium Aluminate with 1 wt % LiF. The homogeneous mixture is then carefully sieved (250 micron) to break the agglomerates into fine particles. All the above process is carefully carried out in a clean environment to preventing contamination of the powder.
5.6.2.2 Preparation of Green Body
Two grams of powder is weighed from the batch of homogenous mixture of Lithium Fluoride and Magnesium Alumiante prepared by ultrasonic mixing method. The graphite die and graphite punch used for making the green body is cleaned thoroughly. The assembled die and punch system is pressed manually and then it is ready to go to Spark Plasma Sintering furnace chamber for densification.
5.6.2.3 Process: Spark Plasma Sintering of Sample
Two samples were sintered using the best sintering and annealing condition obtained from the study explained in the previous chapters. The sintering condition of the samples is explained below.
Figure 5.21- Sintering condition for sample with and without sintering aid.
The heating rate, cooling rate, holding time and pressure that has to be applied during each segment is programmed into the machine control computer and is mentioned by the above schematic diagram.
The heating rate is 300 0C per minute from 450 0C to 1200 0C. From 1200 0C to maximum sintering temperature of 1800 0C the heating rate was 181 0C. After holding at the maximum temperature and pressure, the sample is allowed to cool down at a rate of 100 0C from 1800 0C to 450 0C. The pressure is varied from 5 KN to maximum pressure of 22 KN (70 MPa) as the temperature ascends from 450 0C to 1800 0C. The sintering condition for the sample with and without sintering aid is the same as that mentioned above.
5.6.2.4 Process: Removing Carbon from sintered sample
A brief study on annealing behaviour is conducted and explained in the previous chapter of this report. From the study I have noted that different sintering conditions require different annealing condition to successfully remove the carbon without damaging the grain by cracking. From the annealing study the best condition to remove carbon for a sample sintered at 1800 /30 min is 1000 oC for 3 hours. So I decided to remove carbon from the sintered sample by annealing it at1000 oC for 3 hours. The figure below explain the annealing condition used in this study with Lithium Fluoride. Both the sample with and without sintering aid is annealed at the same conditions. The heating rate and cooling rate adopted is 2 oC /min. The slow heating rate is preferred to prevent cracking of grain due to thermal shock.
![clip_image042[1]](http://world.intellectualprops.com/wp-content/uploads/2010/06/clip_image04211.jpg "clip_image042[1]")
Figure 5.22- Annealing condition for sample with and without sintering aid.
Density measurement
The density of the sample is calculated using Archimedes method. The density along with sintering condition and annealing condition for the sample with and without sintering aid is captured in the table below.There is still pores observed in the sample. Hence this method is not appropriate to find the density of transparent ceramics as the presence of small pores is good enough to scatter the light and make it opaque.
Table 5.6- Density data for sample with and without sintering aid.
Spectroscopy
The transmittance of the sample is measured using UV/ Vis spectrometer and data is sketched below. Data is collected only for wavelength from 400 nm to 750 nm to study the performance of transparent Magnesium Aluminate in the visible region of Electro magnetic spectrum. The sample with 1 wt % lithium fluoride showed a maximum transmittance of 95% at a wave length of 725 nm. The sample without lithium fluoride reached a maximum transmittance was 48% in the red region of the visible spectrum where as with sintering aid it was around 80%.
Figure 5.23 –Transmittance chart for MgAl204 samples with and without Lithium Fluoride.
5.6.4 Discussion
The sample with sintering aid has a better optical property with a transmittance of 80% in the visible region. This is clearly the effect of sintering aid as both the sample was processed in the same condition and the only difference was in the composition of the powder. Even though both the samples showed 100% density by Archimedes method the increase in transmittance in the sample with sintering aid could be explained further by the SEM images (micro structure) and from the densification chart plotted from the SPS data collected during sintering.
Figure 5.24- Samples sintered-[A] without sintering aid. [B] With 1 wt % Lithium Fluoride.
Densification chart
Figure 5.25- Densification chart, piston movement against temperature
This piston movement against temperature graph is plotted from the spark plasma-sintering machine. The aim behind this is to understand the densification behaviour of LiF in the Magnesium Aluminate powder. A shift is observed between the two curves due to difference in manual compaction during preparation of green body from the powder. The graph is plotted only from 1200 oC as there will not be any densification at lower temperature. In both the samples, with and without Lithium Fluoride we can see that there is no fluctuation in the graph during holding time. This is a clear indication that both the samples are fully dense.
From the graph we can see that the slope of the graph is different which is clear indication of difference in shrinkage rate or densification rate in the sample with LiF compared to that without any sintering aid.
The shrinkage rate of Magnesium Alumiante with LiF is 0.0015-mm/ oC and that without Lithium Fluoride is 0.0022-mm/ oC. Faster densification rate can cause destructive grain growth and can explain the reduction in optical property of the sample. Further study has to be done to understand the role of sintering aid in the densification of Magnesium Aluminate.
5.6.5 Conclusion
Two samples are sintered; one with 1 wt %Li F and other without LiF using the same sintering and annealing conditions is used for the study. The conclusion from the study is as follows:
Ø The optical appearance of sample with sintering is different with that with out LiF.
Ø The sample with sintering aid reached a maximum transmittance of 80% in the visible region which is 32% higher than magnesium aluminate without sintering aid.
Ø The shrinkage rate of Magnesium Alumiante with LiF is 0.0015 mm/ oC and that without Lithium Fluoride is 0.0022 mm/ oC. Thus the presence of LiF decreases the densification rate which allows the grain to grow properly with out turbulence.
Ø Further study has to be done to understand the influence LiF (more weight %) in the densification of Magnesium Aluminate, micro structure and optical property.
PART: 3
Chapter 6
Conclusion and Recommendations for Future Work
6.1 Conclusion
From the experiment, results and discussion in making transparent ceramics using Spark Plasma Sintering Furnace the conclusion are as follows:
Ø Transparent Magnesium Aluminate with a maximum transparency of 80% in the 695 nm visible wavelength is successfully fabricated using Spark plasma sintering technique. So SPS is a promising method to sintering transparent ceramics in future.
Ø From the XRD result it is clear that the whitening phenomenon is not due to a new phase and from the SEM image analysis, it can be confirmed that in- homogeneity is due to grain cracking (thermal Shock) during sintering or annealing which makes the sample appear white. Thermal Shock comes into play at higher annealing temperature or during sudden cooling in sintering and annealing process.
Ø The use of graphite die imparts impurities within the sample after sintering .Annealing condition required to remove carbon particle which is entrapped within the sintered sample varies with sintering condition.
Ø After removing all carbon particles the sample showed 100% density by Archimedes principle, but the sample is not 100 % transparent indicates the presence of other scattering sources within the sample.
Ø From the SEM images, Archimedes method is not suitable to find the density of transparent ceramics as this method is not sensible to closed pores and also pores ranging from micro to nano size. Other methods such as gas pycometry and mercury intrusion method are not advisable in MgAl2O4 samples and hence SEM images are the only source to have an understating of extent of porosity present in transparent sample.
Ø The optical appearance of sample with sintering aid is different with that with out LiF.
Ø The sample with sintering aid reached a maximum transmittance of 80 % in the visible region which is 32 % higher than magnesium aluminate without sintering aid.
Ø The shrinkage rate of Magnesium Alumiante with LiF is0 .0015 mm/ oC and that without Lithium Fluoride is 0.0022 mm/ oC .Thus the presence of LiF decreases the densification rate which allows the grain to grow properly with out turbulence .
Table 6.1- Summary of the experimental conditions and results in making transparent Magnesium Aluminate ceramics using SPS
6.2 Recommendations for Future Work
Ø The purity of the Magnesium Aluminate powder used is 99.8% with small traces of other elements which can efficiently scatter light and reduce the transmittance .So if it is possible to replace this powder with ultra high purity starting powder of uniform particle size distribution can prevent the scattering of light introduced by secondary phases.
Ø The use of graphite die imparts impurities with the sample which requires further optimise annealing condition to remove the carbon trapped in the pores without causing defect to the grain structure. So if it is possible to use ceramic dies instead of that it can enhance the optical property without imparting impurities.
Ø The experiment showed that Annealing condition required to remove carbon particle which is entrapped within the sintered sample varies with sintering condition .so further study has to be conducted to derive the relation between sintering condition and required annealing condition for the same.
Ø Scanning electron microscopy (SEM) images and XRD results confirmed that there is no other secondary intermediate phases present in the sintered sample. Further study has to be conducted using Transmission electron microscopy (TEM) or Energy dispersive spectroscopy (EDS) technique can give us a better picture regarding the same.
Ø From the experiment conducted using sintering aid Lithium Fluoride came up with interesting results compared to the sample without sintering aid. Further experiment and research has to be conducted by increasing the weight percentage of lithium fluoride and also have to investigate new sintering aid which can enhance the optical properties of Magnesium Aluminate .
Ø For mixing Magnesium Aluminate with sintering aid, ultrasonification method is used in this study. Further study ahs to be conducted using ball milling machining for long hours as the percentage weight of sintering aid used is very low to obtain a homogenous mixture .while using ball milling technique we have to use new grinding jar and balls used to prevent cross contamination.
Ø In future special care has to be taken to conduct experiment in 100% contamination free environment to prevent introduction of impurities in the sintered sample which can reduce the optical properties.
Ø Mechanical characterizations are essential for these transparent ceramics. So work needs to be conducted to SPS fully dense Nano Magnesium Aluminate to obtain good mechanical strength coupled with optical properties.
Ø Further work has to done to find the influence of microstructure, porosity, defects, grain size, etc on the optical property of Magnesium Aluminate.
Ø Since Archimedes method is not suitable to find the density of transparent ceramics due to the reasons discussed in this work, we have to research and come up with a solution to identify and quantify the percentage of pores within the sample to study the extend of scattering .
Ø Diamond Polishing is done to prevent the scattering at the rough surface of the sample. Since all surface finishing leaves some flaws or irregularities on the surface we should carefully clean the Diamond polishing medium and select the most appropriate Liquid Diamond / Diamond Slurry products. This prevent cross contamination and produces good specimens.
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