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Development and characterization of yttria stabilized zirconia doped with erbia Verdon, Christopher 2015

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DEVELOPMENT AND CHARACTERIZATION OF YTTRIA STABILIZED ZIRCONIA DOPED WITH ERBIA   by  Christopher Verdon  B.A.Sc., The University of British Columbia, 2012  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF  THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF APPLIED SCIENCE  in  THE COLLEGE OF GRADUATE STUDIES   (Mechanical Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA   (Okanagan)   June 2015     © Christopher Verdon, 2015    ii  Abstract Zirconia ceramics have high thermal resistance, chemical inertness and irradiation stability. One application where zirconia composites may find use is in supercritical water nuclear reactors (SCWR), which are expected to have improved thermal efficiency in comparison to current nuclear reactors. Zirconia ceramics may be useful as insulator materials in SCWR pressure tubes where high thermal resistance, chemical stability, irradiation resistance and supercritical water (SCW) degradation resistance are necessary. Additionally, some preliminary tests have found that specific zirconia composites, such as yttria stabilized zirconia (YSZ) outperform pure zirconia in SCW environments. YSZ is a commonly used zirconia composite, and has an excellent thermal resistance and chemical inertness. Due to the presence of yttrium, the high temperature brittle failure caused by the tetragonal to monoclinic crystal structure transformation frequently observed in zirconia is avoided. However, currently available YSZ does not fully satisfy the SCW degradation targets for a SCWR, thus motivating research into the possibility of doping YSZ to further improve YSZ’s SCW degradation resistance. One possible dopant is erbium oxide (erbia), a rare earth oxide that is used as a technical ceramic and has been studied for use in nuclear applications due to its ability to absorb neutrons. Erbia has not been studied in depth as a dopant in the zirconia system or in YSZ. Thus, this research was conducted in order to determine the effect of erbia doping on the densification, hardness, grain size, crystal lattice parameter and SCW degradation resistance of YSZ. Erbia-YSZ composites of 8mol% YSZ with 5, 10 and 15mol% erbia were fabricated by spark plasma sintering (SPS), a novel powder metallurgy process that is known to produce high density composites, at 1200, 1300 and 1400°C with 30 and 60MPa pressures for 5min. Densification during SPS was characterized based on ram position during sintering. Grain size analysis of the as-sintered composites was performed on fractured surfaces. A preliminary study of the SCW degradation resistance of the erbia-YSZ composites was conducted by exposing them to SCW for 2.5h in pressure vessels. The results suggest that erbia-YSZ composites with >95% theoretical density (TD) were successfully fabricated using SPS. Erbia doping into YSZ was found to inhibit densification, but    iii  promoted grain growth and resulted in crystal lattice expansion. Additionally, weight loss during SCW degradation revealed that erbia doping improved the SCW degradation resistance of YSZ, but additional work is necessary to further improve the degradation resistance to meet SCWR targets.     iv  Table of Contents   Abstract .......................................................................................................................................... ii  Table of Contents .......................................................................................................................... iv  List of Tables ............................................................................................................................... vii  List of Figures ............................................................................................................................. viii  List of Abbreviations .................................................................................................................. xiii  List of Symbols ........................................................................................................................... xiv  Acknowledgements ..................................................................................................................... xvi  Dedication .................................................................................................................................. xvii  Chapter 1: Introduction .................................................................................................................. 1  Chapter 2: Literature Review ......................................................................................................... 4 2.1 Zirconia ................................................................................................................................. 4 2.1.1 Stabilization of ZrO2 by Y2O3........................................................................................ 7 2.2 Yttria Stabilized Zirconia ...................................................................................................... 8 2.2.1 Sintering of YSZ Ceramics ............................................................................................ 8  Conventional Sintering ........................................................................................... 8  Two Stage Sintering ................................................................................................ 9  Microwave Sintering ............................................................................................. 11  Spark Plasma Sintering ......................................................................................... 11 2.2.2 Effect of Sintering Parameters on the Density and Grain Size of YSZ ....................... 12 2.2.3 YSZ for Supercritical Water Applications ................................................................... 13  Properties of Supercritical Water .......................................................................... 13  Degradation Resistance of YSZ in Supercritical Water ....................................... 15 2.3 Erbia .................................................................................................................................... 17    v  2.3.1 Erbia Doped Zirconia Composites ............................................................................... 17 2.3.2 Erbia Doped YSZ Composites ..................................................................................... 19 2.4 Spark Plasma Sintering ....................................................................................................... 19 2.4.1 Densification Mechanisms ........................................................................................... 20 2.4.2 Material Fabrication by SPS ........................................................................................ 22 2.5 Summary ............................................................................................................................. 23  Chapter 3: Experimental Procedures ........................................................................................... 24 3.1 Precursor Powders .............................................................................................................. 24 3.2 Precursor Powder Blending ................................................................................................ 24 3.3 Spark Plasma Sintering ....................................................................................................... 25 3.4 Specific Gravity Measurements .......................................................................................... 30 3.5 Sample Polishing ................................................................................................................ 30 3.6 Microhardness ..................................................................................................................... 31 3.7 Microscopy, Chemical and XRD Analyses ........................................................................ 32 3.8 SCW Degradation Testing .................................................................................................. 33 3.8.1 Procedure ..................................................................................................................... 33 3.8.2 Degradation Analysis ................................................................................................... 34  Chapter 4: Results and Discussion ............................................................................................... 35 4.1 Powder Blending ................................................................................................................. 35 4.2 Spark Plasma Sintering ....................................................................................................... 39 4.2.1 Sintering Current .......................................................................................................... 42 4.2.2 Densification ................................................................................................................ 45  Effect of Sintering Temperature on Powder Densification ................................... 46  Effect of Sintering Pressure on Powder Densification ......................................... 51  Effect of Erbia Concentration on Powder Densification ...................................... 53    vi  4.2.3 Bulk Chemical Composition ........................................................................................ 58 4.2.4 Hardness ....................................................................................................................... 60  Effect of Sintering Temperature on Hardness ...................................................... 60  Effect of Sintering Pressure on Hardness ............................................................. 62  Effect of Erbia Concentration on Hardness .......................................................... 64 4.3 Microstructure Analysis ...................................................................................................... 64 4.3.1 General Observations ................................................................................................... 64 4.3.2 Grain Size..................................................................................................................... 69  Effect of Sintering Temperature on Grain Size .................................................... 73  Effect of Sintering Pressure on Grain Size ........................................................... 74  Effect of Erbia Concentration on Grain Size ........................................................ 76 4.3.3 XRD ............................................................................................................................. 78 4.3.4 Crystal Lattice Parameter ............................................................................................. 82 4.4 SCW Degradation Performance .......................................................................................... 84  Chapter 5: Conclusions and Future Work .................................................................................... 89 5.1 Future Work ........................................................................................................................ 90  References .................................................................................................................................... 91  Appendices ................................................................................................................................... 98 Appendix A: Software Procedure Used to Determine the Crystal Lattice Parameters ............. 98 Appendix B: SCW Degradation Experiment Calculations ....................................................... 99 Appendix C: Effect of Temperature on Powder Densification ............................................... 100 Appendix D: Effect of Erbia Concentration on Powder Densification ................................... 108 Appendix E: XRD Results for Precursor Powders and As-Sintered Composites .................. 114     vii  List of Tables Table 1: Zirconia’s thermo-physical properties [12] ...................................................................... 4 Table 2: Comparison of sintering techniques [25] .......................................................................... 9 Table 3: Chemical compositions of precursor powders. ............................................................... 24 Table 4: Theoretical chemical compositions of blended powders ................................................ 25 Table 5: Summary of sintering parameters ................................................................................... 27 Table 6: Polishing schedule .......................................................................................................... 31 Table 7: Bulk composition of precursor powders ......................................................................... 37 Table 8: Chemical composition of the regions labelled in Figure 20 ........................................... 39 Table 9: Photographs of SPS processed composites..................................................................... 40 Table 10: Integrity of SPS processed composites ......................................................................... 41 Table 11: Chemical composition of as-sintered YSZ and erbia-YSZ composites ....................... 59 Table 12: Chemical compositions of the regions labelled in Figure 47 ....................................... 65 Table 13: Chemical compositions of the regions labelled in Figure 49 ....................................... 67 Table 14: Grain structures of 0 and 5mol% erbia-YSZ composites ............................................. 70 Table 15: Grain structures of 10 and 15mol% erbia-YSZ composites ......................................... 71 Table 16: Summary of the erbia phase in 10 and 15mol% erbia-YSZ composites sintered at 1200°C .......................................................................................................................................... 81 Table 17: Crystal lattice parameters of sintered erbia-YSZ composites ....................................... 83 Table 18: Degradation rate of erbia-YSZ composites .................................................................. 84 Table 19: Composite properties before and after exposure to SCW ............................................ 85 Table 20: Bulk chemical composition of composites before and after exposure to SCW ........... 85       viii  List of Figures Figure 1: High efficiency channel pressure tube design [5] ........................................................... 2 Figure 2: Research overview .......................................................................................................... 3 Figure 3: Crystal structures of: a) cubic, b) tetragonal and c) monoclinic zirconia [13] ................ 5 Figure 4: Phase diagram for the zirconia-yttria system [14, 15]..................................................... 6 Figure 5: Two-stage sintering temperature profile [26] ................................................................ 10 Figure 6: A typical SPS chamber setup [32] ................................................................................. 12 Figure 7: Phase diagram for water [39] ........................................................................................ 14 Figure 8: Water’s ionic product and density at high temperatures [38] ....................................... 15 Figure 9: Rate of degradation mass loss (mg/cm2/h) during exposure to SCW for 1000h [9] ..... 16 Figure 10: Phase diagram of the erbia-zirconia system [46] ........................................................ 18 Figure 11: Influence of sintering pressure on the temperature required for 95% TD in zirconia with corresponding grain size [35] .............................................................................. 20 Figure 12: Mechanism of neck formation during SPS [32] .......................................................... 21 Figure 13: Graphite die used for SPS ........................................................................................... 26 Figure 14: Temperature and pressure during sintering at 1200°C and 60MPa............................. 28 Figure 15: Spark plasma sintered samples: a) 0-1200-60 b) 5-1200-60 c) 10-1200-60  d) 15-1200-60 ........................................................................................................................ 29 Figure 16: Erbia-YSZ sample sectioned for hardness measurements .......................................... 32 Figure 17: SCW exposure test pressure vessel ............................................................................. 33 Figure 18: Precursor powders of: a) YSZ, b) erbia, c) 5mol% erbia-YSZ, d) 10mol% erbia- YSZ, d) 15mol% erbia-YSZ ........................................................................................ 36 Figure 19: Erbia particle ............................................................................................................... 37 Figure 20: BSE images of erbia-YSZ precursor powders with: a) 5mol%, b) 10mol% and c) 15mol% erbia ............................................................................................................... 38 Figure 21: DC current during SPS of composites sintered at 1200°C and: a) 30MPa and b) 60MPa .......................................................................................................................... 42 Figure 22: DC current during SPS of composites sintered at 1300°C and: a) 30MPa and b) 60MPa .......................................................................................................................... 43 Figure 23: DC current during SPS of composites sintered at 1400°C and: a) 30MPa and b) 60MPa .......................................................................................................................... 44    ix  Figure 24: Ram height during SPS of 15-1200-60 ....................................................................... 45 Figure 25: Densification rate of YSZ sintered at: a) 30MPa and b) 60MPa................................. 46 Figure 26: Densification rate of 5mol% erbia-YSZ composites sintered at: a) 30MPa and b) 60MPa .......................................................................................................................... 47 Figure 27: Densification rate of 10mol% erbia-YSZ composites sintered at: a) 30MPa and b) 60MPa .......................................................................................................................... 48 Figure 28: Densification rate of 15mol% erbia-YSZ composites sintered at: a) 30MPa and b) 60MPa .......................................................................................................................... 49 Figure 29: Average specific gravity of composites sintered at 30MPa ........................................ 50 Figure 30: Average specific gravity of composites sintered at 60MPa ........................................ 51 Figure 31: Average specific gravity of composites sintered at 1200°C........................................ 52 Figure 32: Average specific gravity of composites sintered at 1300°C........................................ 52 Figure 33: Average specific gravity of composites sintered at 1400°C........................................ 53 Figure 34: Densification rate of erbia-YSZ composites sintered at 1200°C and 30MPa ............. 53 Figure 35: Densification rate of erbia-YSZ composites sintered at 1200°C and 60MPa ............. 54 Figure 36: Densification rate of erbia-YSZ composites sintered at 1300°C and 30MPa ............. 54 Figure 37: Densification rate of erbia-YSZ composites sintered at 1300°C and 60MPa ............. 55 Figure 38: Densification rate of erbia-YSZ composites sintered at 1400°C and 30MPa ............. 55 Figure 39: Densification rate of erbia-YSZ composites sintered at 1400°C and 60MPa ............. 56 Figure 40: Summary of: a) average specific gravity and b) average porosity of composites sintered under every sintering condition ...................................................................... 57 Figure 41: Hardness of composites sintered at 30MPa ................................................................. 60 Figure 42: Hardness of composites sintered at 60MPa ................................................................. 61 Figure 43: Hardness of composites sintered at 1200°C ................................................................ 62 Figure 44: Hardness of composites sintered at 1300°C ................................................................ 62 Figure 45: Hardness of composites sintered at 1400°C ................................................................ 63 Figure 46: Summary of hardness of composites ........................................................................... 64 Figure 47: Glassy phase observed in the 5-1400-60 composite ................................................... 65 Figure 48: Cracks in the: a) 10-1400-30 and b) 15-1300-30 composites ..................................... 66 Figure 49: Erbia bands in the: a) 10-1200-60 and b) 15-1200-60 composites ............................. 67 Figure 50: Line scan of an erbia band in the 15-1200-60 composite............................................ 68    x  Figure 51: A pore containing grain in the 0-1300-30 composite .................................................. 69 Figure 52: Example microstructures of: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol%  erbia-YSZ composites .................................................................................................. 72 Figure 53: Grain sizes of composites sintered at 30MPa .............................................................. 73 Figure 54: Grain sizes of composites sintered at 60MPa .............................................................. 74 Figure 55: Grain sizes of composites sintered at 1200°C ............................................................. 75 Figure 56: Grain sizes of composites sintered at 1300°C ............................................................. 75 Figure 57: Grain sizes of composites sintered at 1400°C ............................................................. 76 Figure 58: Summary of grain sizes of composites ........................................................................ 77 Figure 59: XRD plot for the 0-1200-60 composite: a) before and b) after sintering .................... 78 Figure 60: XRD plot for the 10-1200-60 composite: a) before and b) after sintering .................. 79 Figure 61: XRD plots for the sintered: a) 10-1200-30 and b) 15-1200-30 composites ................ 80 Figure 62: XRD plots for the sintered: a) 10-1200-60 and c) 15-1200-60 composites ................ 80 Figure 63: Leftward shift in XRD plots for: a) 0mol%, b) 5mol%, c) 10mol% and d)  15mol% erbia-YSZ composites sintered at 1400°C and 60MPa ................................. 81 Figure 64: The 10mol% erbia-YSZ composite: a) before and b) after exposure to SCW ............ 86 Figure 65: Microstructure of the 5mol% erbia-YSZ composite: a) before and b) after  exposure to SCW ......................................................................................................... 87 Figure 66: Screenshot of the DICVOL 04 description from PANalytical’s X’Pert HighScore  Plus Software ............................................................................................................... 98 Figure 67: Densification of YSZ sintered at 30MPa with: a) specific gravity as a function of  time and b) specific gravity as a function of temperature .......................................... 100 Figure 68: Densification of YSZ sintered at 60MPa with: a) specific gravity as a function of  time and b) specific gravity as a function of temperature .......................................... 101 Figure 69: Densification of 5mol% erbia-YSZ sintered at 30MPa with: a) specific gravity as  a function of time and b) specific gravity as a function of temperature .................... 102 Figure 70: Densification of 5mol% erbia-YSZ sintered at 60MPa with: a) specific gravity as  a function of time and b) specific gravity as a function of temperature .................... 103 Figure 71: Densification of 10mol% erbia-YSZ sintered at 30MPa with: a) specific gravity  as a function of time and b) specific gravity as a function of temperature ................ 104    xi  Figure 72: Densification of 10mol% erbia-YSZ sintered at 60MPa with: a) specific gravity  as a function of time and b) specific gravity as a function of temperature ................ 105 Figure 73: Densification of 15mol% erbia-YSZ sintered at 30MPa with: a) specific gravity  as a function of time and b) specific gravity as a function of temperature ................ 106 Figure 74: Densification of 15mol% erbia-YSZ sintered at 60MPa with: a) specific gravity  as a function of time and b) specific gravity as a function of temperature ................ 107 Figure 75: Densification of composites sintered at 1200°C and 30MPa with: a) specific  gravity as a function of time and b) specific gravity as a function of temperature .... 108 Figure 76: Densification of composites sintered at 1200°C and 60MPa with: a) specific  gravity as a function of time and b) specific gravity as a function of temperature .... 109 Figure 77: Densification of composites sintered at 1300°C and 30MPa with: a) specific  gravity as a function of time and b) specific gravity as a function of temperature .... 110 Figure 78: Densification of composites sintered at 1300°C and 60MPa with: a) specific  gravity as a function of time and b) specific gravity as a function of temperature .... 111 Figure 79: Densification of composites sintered at 1400°C and 30MPa with: a) specific  gravity as a function of time and b) specific gravity as a function of temperature .... 112 Figure 80: Densification of composites sintered at 1400°C and 60MPa with: a) specific  gravity as a function of time and b) specific gravity as a function of temperature .... 113 Figure 81: XRD plots for precursor powders containing: a) 0mol%, b) 5mol%, c) 10mol%  and d) 15mol% erbia .................................................................................................. 114 Figure 82: XRD plots for composites sintered at 1200°C and 30MPa containing: a) 0mol%,  b) 5mol%, c) 10mol% and d) 15mol% erbia ............................................................. 115 Figure 83: XRD plots for composites sintered at 1200°C and 60MPa containing: a) 0mol%,  b) 5mol%, c) 10mol% and d) 15mol% erbia ............................................................. 116 Figure 84: XRD plots for composites sintered at 1300°C and 30MPa containing: a) 0mol%,  b) 5mol%, c) 10mol% and d) 15mol% erbia ............................................................. 117 Figure 85: XRD plots for composites sintered at 1300°C and 60MPa containing: a) 0mol%,  b) 5mol%, c) 10mol% and d) 15mol% erbia ............................................................. 118 Figure 86: XRD plots for composites sintered at 1400°C and 30MPa containing: a) 0mol%,  b) 5mol%, c) 10mol% and d) 15mol% erbia ............................................................. 119    xii  Figure 87: XRD plots for composites sintered at 1400°C and 60MPa containing: a) 0mol%,  b) 5mol%, c) 10mol% and d) 15mol% erbia ............................................................. 120     xiii  List of Abbreviations AECL   Atomic Energy Canada Ltd. BSE   Back Scattered Electron CRH   Conventional Ramp and Hold DC   Direct Current HEC   High Efficiency Channel MWS   Microwave Sintering SCW   Supercritical Water SCWR   Supercritical Water Reactor SEM   Scanning Electron Microscope SOFC   Solid Oxide Fuel Cell SPS   Spark Plasma Sintering TBC   Thermal Barrier Coating TD   Theoretical Density TSS   Two Stage Sintering XEDS   Energy Dispersive X-Ray Spectroscopy XRD   X-Ray Diffraction YSZ   Yttria Stabilized Zirconia      xiv  List of Symbols A   Initial Area Al2O3   Aluminum Oxide or Alumina CaO   Calcium Oxide or Calcia CeO2   Cerium Oxide or Ceria Cl   Chlorine Cu   Copper D   Instantaneous Density Df   Final Density Er   Erbium Er2O3   Erbium Oxide or Erbia H2O    Water H3O+   Hydronium Ion HCL   Hydrochloric Acid Hf   Hafnium  K   Unit Conversion Constant L    Instantaneous Sample Height La2O3   Lanthanum oxide Lf    Final Sample Height M   Molar mass MgAl2O4  Spinel MgO   Magnesium Oxide n   Number of moles    xv  Nd2O3   Neodymium Oxide O   Oxygen OH-   Hydroxide Ion P   Pressure r   Degradation Rate R   Particle Radius (or the gas constant in Appendix B) S.deviation  Standard Deviation t   Exposure Time T   Temperature TiO2   Titanium Oxide or Titania v   Volume Fraction V   Volume W   Sample Weight Loss Y   Yttrium Y2O3   Yttrium Oxide or Yttria Y3+   Yttrium ion z   Compressibility factor Zr   Zirconium  Zr4+   Zirconium ion ZrO2   Zirconium Oxide or Zirconia γ   Surface Energy ρ   Density      xvi  Acknowledgements I would like to thank my supervisor, Dr. Lukas Bichler for his continued support and guidance through the writing of this paper and for helping me to strive for excellence. I’ve learned much in my time working with you and I’m deeply grateful.  I offer thanks to my peers, whom I’ve had the pleasure of working with for these past two years. In particular, thank you Mathew Smith, Abhinav Karanam and Vishank Kumar for your insight, friendship and support.  I recognize David Arkinstall for his extensive assistance in the scanning electron microscope (SEM) lab. Thank you for helping to make my time in the SEM lab both straightforward and efficient.  I give thanks to my family, who has supported me through all my years of schooling. I’m especially grateful to my mother who has supported me financially and spiritually and pushed me toward relational excellence.    xvii  Dedication      To God my father, you give me the strength to press on and  your mercies toward me are new every morning. Thank you for making me the man I am today. 1  Chapter 1: Introduction This chapter provides an overview of supercritical water reactors and summarizes some candidate materials for use in these reactors. Also, a brief overview of the research completed in this thesis is provided. The demand for clean and sustainable energy sources has been increasing in recent years. As a result, various technologies such as wind, solar, hydro and nuclear power have been extensively investigated. Nuclear power, a well-known clean energy source, faces sustainability challenges due to the difficulties associated with disposing of hazardous waste products, which can require contained storage for hundreds of years. Atomic Energy Canada Limited (AECL) has proposed the development of a new type of nuclear reactor, which is more sustainable and more efficient than present reactors. The supercritical water reactor (SCWR) is designed to produce nuclear waste that decays in only decades and achieves thermal efficiencies between 45 and 50% [1, 2], nearly a 35% improvement over modern nuclear technologies [2]. The Canada Deuterium Uranium SCWRs are designed to be moderated by heavy water and cooled by supercritical water (SCW), allowing operating costs 30% lower than in present systems [3]. They employ a series of uranium fuel containing pressure tubes where light water (water that is not enriched in deuterium) enters at 350°C and 25MPa, just below water’s critical point, and exits at temperatures as high as 625°C, well above water’s critical point [4]. AECL’s most recent pressure tube design, the high efficiency channel (HEC), is depicted in Figure 1 [5].   2   Figure 1: High efficiency channel pressure tube design [5]  (© 2011 Elsevier, by permission) Challenges arise in selecting materials that can withstand the harsh environment of a SCWR. For example, the HEC insulator material needs strength, degradation resistance in SCW, thermal resistance, ability to withstand thermal cycling and irradiation stability [4]. A possible candidate material for these performance targets is yttria stabilized zirconia (YSZ).  YSZ is a ceramic that is important to many modern technological systems, such as solid oxide fuel cells (SOFCs), thermal barrier coatings (TBCs) and nuclear reactors [6]. This ceramic has been considered for SCWR applications, particularly for HEC pressure tubes because of its excellent thermal resistance, chemical inertness [7] and resistance to irradiation damage [4]. YSZ has been observed to resist degradation in SCW [8, 9], but additional dopants are being researched to further reduce SCW degradation rates to achieve the 30y design life of the HEC pressure tubes [10]. One of the candidate dopant materials is erbium oxide (erbia), which is insoluble in water and also used in nuclear applications.  The study of erbia-YSZ composites is a novel field, with little known about the fabrication and properties of these materials. Thus, the objectives of this research were to:  Fabricate novel erbia-YSZ ceramic composites by spark plasma sintering (SPS), a relatively new manufacturing process that can produce materials with nearly 100% theoretical maximum density (TD). 3   Characterize the effect of erbia on the YSZ system.  Examine the performance of erbia-YSZ composites when exposed to SCW.  This thesis is structured as follows:  Chapter 2 reviews the literature on the properties and applications of zirconia, YSZ and erbia, followed by an overview of the SPS technology.  Chapter 3 describes the experimental procedures used in conducting this research: fabrication of materials by SPS, material characterization and SCW degradation testing.  Chapter 4 summarizes the results of the experiments conducted in this research discussing how the properties of the erbia-YSZ composites varied with erbia concentration and sintering process parameters.  Chapter 5 presents the conclusions from this research.  Chapter 6 discusses ideas for future research.  Chapter 7 provides the references used in this thesis. The scope of this research is depicted schematically in Figure 2. Figure 2: Research overview  Spark Plasma Sintering (SPS)YSZ ceramicsCharacterizationDensification during SPSMicrostructure analysisSCW degradationHardness testingErbia-YSZ ceramics4  Chapter 2: Literature Review This chapter presents a review discussing the available literature pertaining to zirconia ceramics, YSZ composites, erbia dopants and spark plasma sintering. 2.1 Zirconia Zirconia (ZrO2) is a polymorphic ceramic with a high thermal resistance and toughness. It is often used in thermal barrier coatings (TBCs), solid oxide fuel cells (SOFCs) and oxygen sensors [11]. A few of zirconia’s thermos-physical properties are summarized in Table 1 [12]. Table 1: Zirconia’s thermo-physical properties [12]  Minimum Value Maximum Value Units Compressive Strength 1200 5200 MPa Tensile Strength 115 711 MPa Hardness 5.5 15 GPa Thermal Conductivity 1.7 2.7 W/m/K  Depending on the temperature and pressure, zirconia exhibits three crystal structures: monoclinic, tetragonal and cubic, as seen in Figure 3 [13], where the large spheres represent oxygen and the small spheres represent zirconium. Monoclinic structure is stable up to ~1170°C, tetragonal structure is stable between ~1170-2370°C and the cubic structure is stable above ~2370°C.    5      Figure 3: Crystal structures of: a) cubic, b) tetragonal and c) monoclinic zirconia [13] (© 2011 The Royal Society of Chemistry, by permission) Tetragonal and cubic zirconia can be stabilized to room temperature by the addition of oxide dopants such as yttria (Y2O3), calcia (CaO) and ceria (CeO2), where yttria is the most commonly used zirconia dopant. Figure 4 shows an equilibrium phase diagram for YSZ with regions of the different phases as a function of temperature and composition [14, 15]. Large spheres represent oxygen and small spheres represent zirconium 6   Figure 4: Phase diagram for the zirconia-yttria system [14, 15] (© 2007 John Wiley & Sons, Inc. and © 1975 Springer, by permission) In Figure 4, it can be seen that cubic or tetragonal zirconia undergoes a phase change from the tetragonal phase to the monoclinic phase upon cooling. This phase change is associated with a volume change (~4%) in the crystal lattice and often causes cracking of the ceramic [14, 16].  This has important practical implications, since if this material is exposed to elevated temperatures, the phase transformation induced cracking can result in a component failure [17]. Therefore, stabilizing the tetragonal or cubic phases in zirconia down to room temperature reduces the risk of brittle failure caused by the tetragonal to monoclinic phase transformation.  7  2.1.1 Stabilization of ZrO2 by Y2O3 Yttria (Y2O3) is commonly used to stabilize zirconia’s tetragonal and cubic crystal structures. The tetragonal structure has high hardness and strength [16], while the cubic structure has good ionic conductivity at high temperatures [18]. Stabilization of the tetragonal and cubic zirconia phases is achieved by the rearrangement of ions in the zirconia crystal lattice [19].  When yttrium ions, Y3+, are incorporated into the zirconia lattice, they replace zirconium ions, Zr4+ [20]. Since yttria has one less valence electron than zirconium, oxygen vacancies are created in the matrix to maintain neutral electronegativity, at a ratio of one oxygen vacancy for two substituted Y3+  [21]. These oxygen vacancies associate with zirconium ions and distort the matrix, allowing for the coexistence of 8 coordination number Y3+ with 7 coordination number Zr4+ in a cubic fluorite structure. Since 7 is zirconium’s stable coordination number at room temperature, the cubic form of zirconia becomes stable at room temperature [21].  The degree of stabilization depends on the concentration of yttria in the system. With 3-8mol%, the zirconia matrix becomes partially stabilized tetragonal, while at 8+mol%, a fully stabilized cubic structure forms [21]. Partially stabilized tetragonal zirconia is also called metastable tetragonal zirconia, because it can be destabilized with an annealing treatment [16]. The destabilizing effect has been studied by a number of research groups, because of its potential to initiate cracking of the material. In a study by Ilavsky and Stalick [22], plasma sprayed 8wt.% YSZ was annealed for 1h at temperatures 1100-1400°C. Rietveld analysis of neutron diffraction data determined that annealing caused the migration of yttrium from the tetragonal phase to the cubic phase and higher annealing temperatures enhanced this effect. The authors observed no increase in the weight fraction of the monoclinic phase; however, an increase in the monoclinic phase was suggested to occur with longer annealing times. The research by Witz, Shklover and Steurer [14] studied the phase evolution in 7.8wt.% YSZ before and after annealing at 1100-1400°C for up to 1000h. Similar to Ilavsky and Stalick’s [22] result, the authors observed that yttrium migrated from the tetragonal phase to the cubic phase. Further, longer annealing times enabled the transformation of the tetragonal phase to the monoclinic phase, confirming Ilavsky and Stalick’s [22] speculations. 8  Witz, Shklover and Steurer [14] also found that the transformed YSZ had high enough microstrains (up to 1%) to cause cracks that could lead to the coating’s failure. 2.2 Yttria Stabilized Zirconia YSZ is a ceramic that has been used extensively in TBC and SOFC applications, due to its high thermal resistance, toughness and ionic conductivity at elevated temperatures [11]. In the following text, the sintering methods and typical properties of YSZ are reviewed, as well as its potential application in SCW. 2.2.1 Sintering of YSZ Ceramics Sintering is a powder metallurgy technique that uses heat to consolidate powders into a bulk material at temperatures below the material’s melting temperature. This technique is particularly useful for fabricating materials which melt at high temperatures (e.g., YSZ melts at 2700°C [23]). Several sintering techniques can be used to fabricate YSZ ceramics, including conventional sintering, two-stage sintering (TSS), microwave sintering (MWS) and SPS, which are reviewed next.  Conventional Sintering Conventional sintering, also called conventional ramp and hold (CRH), is the most basic of the sintering techniques. CRH typically involves compacting the powder into a desired shape, heating the compact in air to the sintering temperature, dwelling at the sintering temperature for a set time to promote densification and subsequently cooling the composite [24]. In comparison with other sintering techniques, CRH generally produces parts with poor properties, because the process requires long sintering times for densification, leading to substantial grain growth [25].  Rajeswari et al. [25] carried out a fundamental study of four sintering techniques CRH, TSS, MWS and SPS. They sintered 8mol% YSZ using each technique and varied the sintering parameters in order to create dense (~99% TD) samples. This methodology allowed them to compare the effectiveness of each sintering technique on the basis of processing time, density and final grain structure. Their results are summarized in Table 2 [25].   9  Table 2: Comparison of sintering techniques [25]   As seen in Table 2 [25], CRH produced high density samples with 98.5 to 99.5% TD, but a high sintering temperature (1500-1550°C) and a long sintering time (2h) were required, resulting in a significant grain growth. Additionally, long sintering times and temperatures would likely result in increased costs associated with fabricating the YSZ ceramics.  Two Stage Sintering TSS uses a modified temperature profile that includes two distinct dwell periods: a short dwell at a high temperature followed by a long dwell at a low temperature, to reduce grain growth and promote material densification. An example TSS temperature profile can be seen in Figure 5 [26].  10   Figure 5: Two-stage sintering temperature profile [26] (© 2007 Elsevier, by permission) The TSS profile in Figure 5 [25] was designed to promote rapid densification at the first sintering temperature, T1, and further densification with limited grain growth at the lower ‘soak’ temperature, T2 [25]. Researchers [27] have postulated that densification without significant grain growth occurs during the soak period, because of the difference between the activation energies for grain boundary diffusion and grain boundary growth.  Compared to CRH, as seen in Table 2 [25], TSS produced a finer grain structure by ~50%, while maintaining a similarly high level of densification. The main drawback of TSS was the long sintering time (4h) required for densification. Thus, TSS resulted in improved material properties due to the reduced grain size, but was more expensive to perform due to the extended sintering duration. 11   Microwave Sintering MWS uses microwaves 2.45-60GHz [28] and the tendency of dielectric materials to couple with microwaves [25] to cause internal heating and densification. The internal heating results in significantly faster processing times compared to more conventional sintering techniques.   As seen in Table 2 [25], Rajeswari et al. used MWS with a 15min sintering time to make dense YSZ. This was achieved much faster than CRH or TSS, but produced samples with slightly reduced density (99.15% TD) compared to CRH and TSS. Also, the grain size increased compared to TSS. Thus, MWS provided fast production times with a slight decrease of the overall material properties.  Spark Plasma Sintering SPS uses the simultaneous application of pressure and pulsed electric current to compress and heat a powder into a bulk material [29]. In the case of fabricating YSZ ceramics, high densities (99% TD) were reported with SPS, even with processing at short sintering times and at low temperatures [30, 31]. In a typical SPS process, a material powder is inserted into a graphite die and placed in a water cooled SPS chamber. The chamber is evacuated and filled with inert gas or left under a vacuum. Uniaxial pressure is applied to the die by an upper and a lower ram. Then, a pulsed DC (direct current) is directed through the rams and the die, subjecting the powder to a predetermined pressure and temperature, which are maintained for a predetermined duration to allow for material densification. After sintering, the pressure and temperature are relieved [32]. A typical SPS chamber setup is depicted in Figure 6 [32]. 12   Figure 6: A typical SPS chamber setup [32] Rajeswari et al. [25] sintered YSZ at 1325°C for 5min using SPS to produce samples with high density (99.5% TD) and small grains, 1.16µm. In comparison to CRH, TSS and MWS from Table 2 [25], SPS required lower sintering times and temperatures, made denser or equally dense samples and produced finer grain structures. Therefore, it can be concluded from Rajeswari et al.’s [25] work that SPS provided unique processing possibilities for YSZ and resulted in a ceramic with improved material properties.  A more in-depth review of the SPS process is conducted in section 2.4. 2.2.2 Effect of Sintering Parameters on the Density and Grain Size of YSZ The sintering process parameters can have a significant effect on YSZ’s properties including bending strength [33], ionic conductivity [31], density and grain size. Density and grain size are of particular interest, because of their contribution to the overall material performance.  As was mentioned previously, Rajeswari et al. [34] sintered 8mol% YSZ samples at 1325 and 1425°C for 5min using SPS. They found that for dense YSZ (>99% TD), increasing the sintering temperature from 1325-1425°C increased the grain size from 1.16-13  8.8µm. This result showed the strong dependence of grain size on sintering temperature, since a 100°C increase in SPS sintering temperature produced nearly an eight fold increase in grain size.  Anselmi-Tamburini et al. [35] studied the effect of elevated sintering pressures (up to 1GPa) on the fabrication of dense fully stabilized YSZ. They were able to produce samples with 98% TD and ~15nm grains by sintering at 850°C and 530MPa for 5min, even though the reported SPS processing temperatures for YSZ typically range between 1300-1500°C. Their result illustrated the strong effect of pressure on the densification and the grain growth of YSZ. Additionally, high pressure sintering allowed for the formation of an ultra-fine grain structure (~15nm), which is very difficult to obtain in a conventional SPS machine.  In another study, Anselmi-Tamburini et al. [36, 37] spark plasma sintered 8mol% YSZ at 600-1400°C and 15-106MPa for 0-25min using 50-300°C/min heating rates. The authors observed that varying the heating rate and the sintering duration (5-16min) did not affect the final density. The final density increased with increased sintering pressure and temperature in the range of 900-1400°C; below 900°C densification only occurred by particle rearrangement. Additionally, the grain size was found to increase with increasing sintering temperature and sintering time. The authors also performed Vickers hardness measurements and found that the samples sintered at 1200°C and 106MPa for 5min had the highest hardness of ~16GPa. 2.2.3 YSZ for Supercritical Water Applications As introduced in Chapter 1, YSZ is an attractive candidate material for use in SCWRs, where a material’s low thermal conductivity, high strength and a high degradation resistance in SCW are important [4]. The properties of SCW and the degradation resistance of YSZ in SCW are discussed in the following sections.  Properties of Supercritical Water When water reaches temperatures and pressures above the critical point, 374°C and 22MPa, it is considered “supercritical”, exhibiting both vapour-like and liquid-like properties, high diffusivity and good heat transfer properties [38]. The properties of SCW can be adjusted to be more similar to the gas or liquid states of water by changing the pressure and temperature. 14  SCW also exhibits a single phase above the critical point. A pressure-temperature phase diagram of water is provided in Figure 7, with the critical point labelled.  Figure 7: Phase diagram for water [39] The ionic product, defined as the concentrations of hydronium and hydroxide ions multiplied together, also changes at the critical point [40]. The concentration of hydronium and hydroxide ions depends on water’s temperature dependent dissociation, given by Equation 1:  2H2O ⇆ OH-+H3O+ Equation 1 As the temperature increases, the reaction in Equation 1 promotes the formation of hydronium and hydroxide ions, thus the ionic product increases. This trend changes abruptly above water’s critical point, as shown in Figure 8 [38].  15   Figure 8: Water’s ionic product and density at high temperatures [38] (© 2004 Elsevier, by permission) Above the critical temperature, marked by vertical lines in Figure 8 [38], water’s ionic product decreases with increasing temperature. This reduction in ionic product corresponds to a decrease in water’s dielectric constant, from ~80 at room temperature to 2 at 450°C and causes water to become non-polar [41]. As a result, SCW is a poor polar solvent and a strong non-polar solvent that has complete solvency for organics and thus is an aggressive oxidizing environment.  Degradation Resistance of YSZ in Supercritical Water Literature on the degradation resistance of YSZ when exposed to SCW is limited to a few select studies. Barrett et al. [9] performed SCW degradation experiments on alumina (Al2O3), 7mol% YSZ and titania (TiO2) doped 7mol% YSZ, by exposure to SCW for 940-1000h. SCW testing was performed in autoclaves at 500-540°C with 22.4-27.8MPa pressures. The mass of the samples was measured before and after SCW exposure and the resulting degradation rates were plotted in Figure 9 [9]. 16   Figure 9: Rate of degradation mass loss (mg/cm2/h) during exposure to SCW for 1000h [9] (© 2013 Springer, by permission)  Figure 9 suggests that in comparison to alumina, a commonly used protective ceramic in SCW environments, 7mol% YSZ had a significantly lower weight loss, 0.12µg/cm2/h compared to alumina’s 1.66µg/cm2/h. The titania doped YSZ showed similar degradation resistance. These results suggest that YSZ could be a suitable material for use in SCWRs.  In another study, Boukis et al. [8] evaluated the response of various high performance ceramics to SCW containing 0.44mol/kg oxygen and 0.05mol/kg HCl at 465°C and 25MPa. In particular, they compared the degradation response of several zirconia ceramics: YSZ with varying concentrations of yttria, YSZ doped with MgO and YSZ doped with MgO and spinel (MgAl2O4). The authors found that the YSZ samples rapidly disintegrated, while the YSZ-MgO-spinel mixtures had a lower mass loss with the 3.5mol% MgO doped zirconia having the lowest mass loss (<0.45µg/cm2/h). The rapid degradation observed in the plain YSZ samples was attributed to cracking and subsequent crumbling that resulted from the tetragonal to monoclinic phase change. Before exposure to SCW, the MgO-YSZ sample contained 7% monoclinic phase, 30% tetragonal phase and 63% cubic phase. After SCW exposure, the sample contained 37% monoclinic phase and 63% cubic phase. 17   Work by Siebert-Timmer [10] studied the effect of CeO2 and Nd2O3 dopants on YSZ’s degradation resistance in SCW. 8mol% YSZ as well as 8mol% YSZ containing varying concentrations of CeO2 and Nd2O3 were exposed to SCW for 2.5h at 400°C and 31MPa. The YSZ samples degraded at ~1100mm/y with two out of the three samples disintegrating completely. CeO2 and Nd2O3 doped YSZ had a significantly reduced material loss as low as ~14 and ~12mm/y, respectively. In summary, the addition of oxide dopants, such as MgO, CeO2 and Nd2O3 to YSZ was seen to improve YSZ’s degradation resistance in SCW. Thus, this research focused on investigating the SCW degradation resistance of YSZ doped with erbia. 2.3 Erbia The literature available on erbia (Er2O3) composites is limited, with a few publications describing potential optical, electrical, nuclear and chemical applications [42-45]. Erbia has been considered as a stabilizing dopant for zirconia [24, 46, 47], due to its high chemical stability and creep resistance, which are desirable properties for SCWR application [42, 48].  2.3.1 Erbia Doped Zirconia Composites Duran [46] characterized the effect of erbia concentration on zirconia’s crystal lattice parameters and lattice symmetry. Erbia-zirconia powder blends were prepared by pelletizing and subsequent firing at 1900°C for 4h and 1100°C for 48h, respectively. The erbia-zirconia powders were then pressed and sintered at 1450-2000°C for 10-30h in a furnace. XRD analysis revealed that the lattice parameter increased from 5.12 to 5.20Å when erbia concentration increased from 5 to 35mol%. A mixture of the monoclinic and cubic structures was observed between ~1 and ~7mol% erbia, while the system was fully cubic above ~7mol% erbia. The author developed the erbia-zirconia phase diagram shown in Figure 10 [46] to summarize his results. 18   Figure 10: Phase diagram of the erbia-zirconia system [46] (© 1977 John Wiley & Sons, Inc., by permission)  A study by Maschio et al. [24] found that zirconia’s tetragonal lattice could be stabilized with erbia doping between 1.5-3mol% [24]. Erbia-zirconia powders were prepared by co-precipitation method then isostatically pressed at 300MPa and sintered at 1350°C for 1h. XRD analysis revealed that 1.5mol% erbia-zirconia contained a mixture of monoclinic and tetragonal phase, while 2-3mol% erbia-zirconia contained only tetragonal phase. Thus, 19  the phase diagram in Figure 10 [46] should be corrected to include formation of a tetragonal phase below 500°C, between 1 and 3mol% erbia and possibly up to 7mol% erbia. 2.3.2 Erbia Doped YSZ Composites There is a paucity of research published on erbia doped YSZ, but of the four papers that were found, two were relevant to the current topic of research. Aktas et al. [49] studied the effect of erbia on YSZ’s electrical conductivity. 8mol% YSZ powders were doped with 1-15wt.% erbia and ball milled, then fabricated by CRH sintering at 1500°C. Microstructure analysis revealed that erbia precipitated as an intergranular phase that hindered grain growth (the grain size decreased from 5.33-3.11µm for 0-15wt.% erbia). It was also found via alternating current impedance spectroscopy, that high concentrations of erbia reduced the electrical conductivity of YSZ, but the addition of 1wt.% erbia caused an increase in electrical conductivity. In a study to reduce the thermal conductivity of TBCs Nicholls et al. [50] compared the thermal conductivity of various YSZ composites, including erbia-YSZ composites. Coatings containing 7mol% YSZ and 2-8mol% erbia were deposited by electron beam physical vapour deposition. An addition of 8mol% erbia was found to reduce YSZ’s thermal conductivity by 32%. 2.4 Spark Plasma Sintering SPS is a powder metallurgy process that was patented by Kiyoshi Inoue in 1966 and uses the simultaneous application of pressure and pulsed DC current to rapidly densify material powders at relatively low temperatures [25]. In comparison to other sintering methods such as CRH and MWS, SPS can produce denser materials with smaller grain sizes, while using lower sintering temperatures and times [25]. The primary challenge in SPS processing is selecting the appropriate sintering parameters for the desired properties of the as-sintered ceramic [30]. The effect of temperature and pressure on the grain size of SPS processed zirconia is illustrated in Figure 11 [35]. 20   Figure 11: Influence of sintering pressure on the temperature required for 95% TD in zirconia with corresponding grain size [35] (© 2006 Elsevier, by permission)  Figure 11 [35] indicates that sintering at high pressure and low temperature would be ideal for minimizing grain growth. However, if the sintering temperature is too low, then material densification may not occur [25]. Therefore, in order to fabricate dense materials with small grains, SPS pressure and temperature need to be optimized. 2.4.1 Densification Mechanisms In SPS, heat is applied to a material by a pulsating DC electric current via the ram electrodes and the graphite die. The DC current switching on and off causes spark discharges in the material, which create high temperature regions (on the order of 10,000°C) and causes localized melting and vapourization [32]. The spark discharge and localized melting purifies particles by eliminating adsorbed gases and also causes a phenomenon referred to as “necking”. “Necks” are plastically deformed by applied uniaxial pressure and result in joining of powder particles. This process of necking and densification is illustrated in Figure 12 [32].         Temperature ∙∙∙∙    Grain size   21   Figure 12: Mechanism of neck formation during SPS [32]  The uniaxial pressure applied during sintering enhances densification by causing particle rearrangement and break-down of agglomerates, particularly for larger particles [29]. It can also cause superplastic flow via grain boundary sliding in ceramics [51]. Anselimi-Tamburini et al. [36] described the effect of pressure on densification using Equation 2, which gives the driving force for initial densification.  𝑫𝒓𝒊𝒗𝒊𝒏𝒈 𝑭𝒐𝒓𝒄𝒆 =  𝛄 +𝐏𝐫𝛑 Equation 2 Where: γ is the surface energy, P is the applied pressure and r is the particle radius. When pressure is increased, the driving force for densification increases as well; however, Skandan et al. [52] experimentally found that pressure only promoted densification when the pressure effect was greater than the surface energy effect. They found that for 6 and 12nm zirconia powders, 35 and 10MPa pressures were required to enhance densification, 22  respectively. Thus, for smaller particle sizes, higher pressures were required to enhance densification. 2.4.2 Material Fabrication by SPS  Researchers have used SPS to fabricate various materials, including metals and ceramics. Zhang et al. [53] studied the SPS processing of copper and how SPS parameters affected density and grain size. They varied the initial pressure, sintering pressure, temperature, hold time and heating rate. They found that the optimal sintering parameters for high density (>96%) and low grain size (2.2µm) copper samples were: initial pressure of 1MPa, sintering pressure of 50MPa, temperature of 750°C, 6min hold time and 80°C/min heating rate. Of the various properties tested, the authors found that pressure and temperature were the key factors in determining copper’s density and grain size. Increasing the sintering temperature from 400 to 800°C increased the relative density and the grain size from 77 to 99% and 2 to 6.5µm, respectively. Increasing the sintering pressure from 0 to 50MPa increased the relative density from 74 to 99%.  Shen et al. [54] used SPS to fabricate alumina ceramic and examined the effect of temperature, hold time, pressure, heating rate and pulse sequence (time ratio for DC current on:off) on the properties of as-sintered alumina. They found that compared to hot pressing, SPS allowed for densification to occur at much lower temperatures (1125°C for SPS compared to 1400°C for hot pressing). They observed that grain growth was minimal below 1250°C, but increased almost exponentially above this temperature. It was also found that when the sintering temperature was low (1125°C), the density could be increased to 100% relative density by increasing the hold time or pressure. Heating rate did not affect densification below 330°C/min, but did affect the grain size. The 50 and 200°C/min heating rates resulted in 9.6 and 3.9µm grain sizes, respectively, for samples sintered at 1400°C. The pulse sequence did not have a significant effect on alumina’s sintered properties.  Anselmi-Tamburini et al. [36] investigated the effect of SPS parameters: sintering temperature (600-1400°C), sintering pressure (15-106MPa), hold time (0-25min), heating rate (50-300°C/min) and pulse sequence (2:2-48:2) on the density and the grain size of 8mol% YSZ ceramics. They found that increasing the sintering temperature from 900-1400°C at 106MPa caused both the density and the grain size to increase: from ~50 to ~98% 23  relative density and from ~50 to ~700nm, respectively. Increasing the pressure from 20 to 141MPa at 1200°C increased the relative density from ~60 to ~100% with no significant effect on the grain size. Hold time (>2min), heating rate and pulse sequence did not significantly affect the density or the grain size. 2.5 Summary Zirconia ceramics are attractive for use in various applications because of their excellent material properties. They are also known to undergo brittle failure caused by the tetragonal to monoclinic phase change at elevated temperatures [14]. This drawback can be reduced with dopants such as yttria, which stabilize the tetragonal or cubic crystal structures. YSZ materials showed potential for use in SCWRs, but required further enhancement of degradation resistance in SCW, which may be achieved by addition of oxide dopants. A potential dopant in the YSZ system, erbia, was proposed, but very little literature is currently available on erbia-YSZ composites.   24  Chapter 3: Experimental Procedures This chapter provides an overview of the experimental procedures used in this thesis to fabricate, characterize and expose erbia-YSZ composites to a SCW environment. 3.1 Precursor Powders Precursor powders of 8mol% fully stabilized cubic YSZ and erbia were obtained from Inframat Advanced Materials (USA) and City Chemical (USA), respectively.  The YSZ powder was manufactured via the wet chemical co-precipitation method to 99.9% purity metal basis, excluding Hf. The erbia powder also had a 99.9% purity. The composition of both powders, measured using energy-dispersive X-ray spectroscopy (XEDS), is provided in Table 3. In the case of the YSZ powder, the Cl and Hf impurities were likely present as trace residues from the powder manufacturing method. Table 3: Chemical compositions of precursor powders. Precursor powder Element wt.%  Zr O Y Er Hf Cl YSZ 54.9 35.2 8.1 - 1.3 0.5 Erbia - 15.1 - 84.9 - -  Due to instrumentation and data collection challenges, additional repeat samples containing a replacement erbia powder from Sigma-Aldrich were fabricated to collect additional DC current data. Testing and analysis of the results confirmed that the erbia powder from Sigma-Aldrich behaved similarly to that from Inframat Advanced Materials. 3.2 Precursor Powder Blending In order to prepare blends of YSZ and erbia powders, appropriate amounts of the powders were measured by electronic balance and combined in weights corresponding to 5, 10 and 15mol% erbia in YSZ. The theoretical compositions of the powder blends are summarized in Table 4.   25  Table 4: Theoretical chemical compositions of blended powders Erbia content in YSZ (mol%) Element wt.%  Er Zr Y O 5 11.6 55.4 9.4 23.6 10 21.4 48.3 8.2 22.2 15 29.7 42.2 7.2 21.0  50g of each powder blend were mixed in 40ml of ethyl alcohol for 5 hours. Mixing was performed by a magnetic stir stick on a hot plate. During mixing, the hot plate was heated to 150°C to accelerate alcohol evaporation. Resulting pastes were air dried and manually ground using a mortar and pestle. This process was followed for the 5, 10 and 15mol% erbia-YSZ composites. 3.3 Spark Plasma Sintering Thermal Technology’s 10-3 spark plasma sintering machine was used to sinter 19mm diameter discs. 7g of each powder blend were packed into 50mm cylindrical graphite dies lined with grafoil sheets to protect the dies. The graphite dies had a hole drilled into the side wall to allow for pyrometer measurements near the center of the die, as shown in Figure 13.  26   Figure 13: Graphite die used for SPS Sintering was carried out at 1200, 1300 and 1400°C dwell temperatures and at 30 and 60MPa sintering pressures. These temperature and pressure values were chosen based on similar sintering conditions used by other researchers [10, 36, 54]. All materials were sintered for 5min with an on-off DC pulse sequence of 20:2ms. The heating rate was 100°C/min and the samples were cooled at the maximum obtainable cooling rate for the SPS machine (~100-250°C/min). A summary of the sintering parameters used is provided in Table 5. A representative plot of the sintering path, showing the temperature and pressure variation for a composite processed at 1200°C and 60MPa is shown in Figure 14.   Pyrometer hole 27  Table 5: Summary of sintering parameters     Sintering parameters  Sample identification Erbia content (mol%) Temperature (°C ) Pressure  (MPa) Time (min) 0-1200-30 0 1200 30 5 0-1200-60 0 1200 60 5 0-1300-30 0 1300 30 5 0-1300-60 0 1300 60 5 0-1400-30 0 1400 30 5 0-1400-60 0 1400 60 5 5-1200-30 5 1200 30 5 5-1200-60 5 1200 60 5 5-1300-30 5 1300 30 5 5-1300-60 5 1300 60 5 5-1400-30 5 1400 30 5 5-1400-60 5 1400 60 5 10-1200-30 10 1200 30 5 10-1200-60 10 1200 60 5 10-1300-30 10 1300 30 5 10-1300-60 10 1300 60 5 10-1400-30 10 1400 30 5 10-1400-60 10 1400 60 5 15-1200-30 15 1200 30 5 15-1200-60 15 1200 60 5 15-1300-30 15 1300 30 5 15-1300-60 15 1300 60 5 15-1400-30 15 1400 30 5 15-1400-60 15 1400 60 5  28   Figure 14: Temperature and pressure during sintering at 1200°C and 60MPa During the heating of the sample, a 10s dwell at 600°C was imposed to allow time for the SPS pyrometer to stabilize and begin measurements, since the pyrometer was calibrated for temperature measurements above 600°C. The heating rate was irregular from 0-3min because the SPS system couldn’t measure the heating rate until the pyrometer was able to make temperature measurements. After holding at the sintering temperature, the composites were cooled at the maximum obtainable rate for the SPS system (~100-250°C/min). After sintering, the discs were removed from the sintering die using a bench-top hydraulic press. Any residual grafoil sheet adhering to the as-sintered discs was removed with a knife. For some combinations of SPS process parameters, the discs fractured upon ejection from the graphite die, as seen in Figure 15, which shows pictures of sintered 0, 5, 10 and 15mol% erbia-YSZ discs.  015304560759010502004006008001000120014000 5 10 15 20Pressure (MPa)Temperature (°C)Time (min)SinteringTemperatureSinteringPressure29   Figure 15: Spark plasma sintered samples: a) 0-1200-60 b) 5-1200-60 c) 10-1200-60  d) 15-1200-60 Sintering experiments were also carried out with empty graphite dies at 1200, 1300 and 1400°C with 30 and 60MPa pressures. These experiments were used to calibrate the ram displacement data and the real-time sample height measurements during sintering. This calibration was necessary to enable subsequent instantaneous density calculation, which was made using Equation 3 [55].  𝑫 = 𝑳𝒇𝑳∗ 𝑫𝒇 Equation 3  Where: D was the instantaneous density, Lf was the final sample height, L was the instantaneous sample height and Df was the final density. The L parameter was recorded during the SPS experiment by the machine’s data acquisition unit. a) b) c) d) 30  3.4 Specific Gravity Measurements The specific gravity of every sample was measured via Archimedes’ principle. The weight of each composite was measured in air and in water using an electronic balance with an accuracy of 0.0001g. The specific gravity of the as-sintered composites was calculated according to Equation 4.  𝑺𝒑𝒆𝒄𝒊𝒇𝒊𝒄 𝒈𝒓𝒂𝒗𝒊𝒕𝒚 =  𝒘𝒆𝒊𝒈𝒉𝒕 𝒊𝒏 𝒂𝒊𝒓𝒘𝒆𝒊𝒈𝒉𝒕 𝒊𝒏 𝒂𝒊𝒓 −  𝒘𝒆𝒊𝒈𝒉𝒕 𝒊𝒏 𝒘𝒂𝒕𝒆𝒓 Equation 4  The theoretical maximum density of each composite was calculated (to approximate porosity) using the rule of mixtures (provided in Equation 5) and the theoretical maximum densities of YSZ and erbia, which were 6.1g/cm3 [56] and 8.64g/cm3 [57], respectively.                                        𝝆𝑻 = 𝝆𝑬 ∗ 𝒗𝑬 + 𝝆𝒀 ∗ 𝒗𝒀 Equation 5   Where: ρT, ρE, and ρY represent the theoretical maximum densities of the composite, erbia and YSZ, respectively and vE and vY represent the volume fractions of erbia and YSZ, respectively. 3.5 Sample Polishing Upon sintering, material testing including SCW degradation, hardness tests and microstructure characterization were carried out. Representative samples were first cut on a Leco VC50 diamond saw then mounted in Metlab Fast Cure Acrylic. 5-1200-60, 10-1200-60 and 15-1200-60 samples were prepared for both SCW degradation experiments and microstructure analysis since these SPS conditions yielded high integrity discs. For all composites, the polishing procedure outlined in Table 6 was followed.   31  Table 6: Polishing schedule Stage Grit Paper Time (min) Intended Sample Characterization 1 120 SiC until flat Degradation experiments (2 polishing stages) 2 400 SiC 1-3 3 800 aluminum oxide 2-3 Hardness tests (3 polishing stages) 3 3/0 emery 2-3 Degradation microstructure (6 polishing stages)** 4 6µm diamond paste* 2 5 3µm diamond paste* 2 6 1µm diamond paste* 2 *on Metlab 8" micro polishing cloth **800 grit aluminum oxide paper was not used on samples prepared for microstructure analysis Grain size analysis was performed on fractured surfaces since etching of chemically inert YSZ ceramics was not successful. 3.6 Microhardness Microhardness measurements were made using a Wilson VH3100 automatic Vickers hardness tester, which was used to indent every composite with a diamond shaped indenter and a 1kg load applied for 10s. The 5-1300-30, 10-1300-30 and 15-1400-60 samples were not characterized for hardness because they fractured upon ejection from the sintering die. A minimum of 6 hardness measurements were taken per sample with indentation dimensions of 30-60µm. The VH3100 automatically calculated the sample hardness based on the indentation dimensions (a smaller indentation indicates a harder sample). Hardness measurements were taken from each sample’s cross section in the region indicated in Figure 16. 32   Figure 16: Erbia-YSZ sample sectioned for hardness measurements 3.7 Microscopy, Chemical and XRD Analyses Every powder and sintered disc were imaged with a Tescan MIRA 3 XMU Field Emission Scanning Electron Microscope. Secondary and back scattered electron images were taken at a 20kV beam current. SEM images were used to characterize the microstructure of as-sintered discs and SCW degraded samples. Grain size analysis was quantified with the Buehler Omnimet image analysis software, where 100 grains per sample were measured to ensure statistical relevance. Chemical analysis was performed with an Oxford AZtec X-Max EDS system. All samples were coated with 10nm of platinum-palladium alloy in a Cressington 208HR Sputter Coater to prevent charge build up during SEM operation. A Bruker D8 Advance X-ray powder diffractometer was used to study the crystal structure of the sintered composites and powders. Cu-Kα radiation was used with a detector operating from 10-100° with a step size of 0.04° and 146s per step. Crystallographic structures of sintered samples were characterized using the DICVOL 04 indexing method within PANalytical’s X’Pert HighScore Plus software. DICVOL 04 was used to enable identification of monoclinic lattice, expected to be present in the YSZ lattice. A brief explanation of the software procedures used to determine the crystal lattice parameters is provided in Appendix A. 33  3.8  SCW Degradation Testing SCW exposure degradation tests were carried out on the 5, 10 and 15mol% erbia-YSZ discs sintered at 1200°C and 60MPa. The author performed degradation testing of erbia composites, while degradation testing of YSZ was performed by a colleague [10]. 3.8.1 Procedure SCW degradation testing methodology followed procedures specified by the ASTM G31-72 Standard Practice for Laboratory Immersion Corrosion Testing of Metals [58]. In this thesis, sintered ceramic composites were exposed to a 400°C temperature and 31MPa pressure, which is above water’s critical point of 374.2°C and 22.1MPa [10, 59]. The calculations used to calculate the temperature and pressure inside the pressure vessels are provided in Appendix B. 4x4x4mm3 erbia-YSZ cubes were cut from the region marked in Figure 16 with a diamond saw and polished with 400 grit silicon carbide paper. Samples were placed into pressure vessels with 20ml of distilled water (assumed pH of 7). The pressure vessels were made from 1” (25.4mm) 316L stainless steel tube with a 0.12” (3.05mm) wall thickness. The stainless steel tubes were cut to a 10” (254mm) length, as seen in Figure 17, and sealed with 316 SS-1610-C end caps purchased from Swagelok®. Prior to conducting an experiment, the pressure vessels were passivated in 0.5vol% nitric acid for 30min to inhibit corrosion of the pressure vessels.  Figure 17: SCW exposure test pressure vessel The loaded pressure vessels were placed horizontally in a CF 190 Euclid kiln, heated to 404°C at 800°C/h and held at the temperature for 2.5h, followed by furnace cooling. The furnace was heated to 404°C to prevent temperature fluctuations from causing the temperature in the pressure vessels to drop below 400°C, ensuring the supercritical state was maintained throughout the test. 34  3.8.2 Degradation Analysis As-sintered and post degradation samples were ultrasonically cleaned, for 5min in a Branson 2800 ultrasonic cleaner with alcohol and weighed. Equation 5 [58] was used to calculate the degradation rate of the samples exposed to SCW in units of “mm/y.” The samples degraded volumetrically, but Equation 5 was used to convert the volumetric degradation into an equivalent average unidirectional degradation rate.  𝒓 =𝑲 ∙ 𝑾𝑨 ∙ 𝒕 ∙ 𝝆 Equation 6 Where: K was 8.76x104 in order for the final units of r to be in “mm/y” [58], W was the sample weight loss (g) due to dissolution or crumbling, A was the initial surface area (cm2), t was the exposure time (h) and ρ was the final sample density (g/cm3). An example calculation of a sample’s degradation rate is provided in Appendix B.   35  Chapter 4: Results and Discussion This chapter provides the results of powder blending, SPS processing of composites and subsequent degradation testing in SCW. All sections also include microscopic characterization of the composites during the various stages of fabrication and testing. 4.1 Powder Blending SEM micrographs of as-received powders and blended erbia-YSZ powders are provided in Figure 18. As can be seen from Figure 18, the powder particles had sharp features. Further, powder particles varying in both shape and size were also observed, as seen in Figure 19 in detail. It was also noted that the erbia particles were larger than the YSZ agglomerations, as seen in: a) of Figure 18. Powder particles which are not spherical, but have sharp corners can cause local “hot spots” in the powder during sintering, as current density increases with a decrease in particle radius. The resulting thermal gradients can induce internal stresses in the powder particles that may contribute to sample fracturing upon ejection from the sintering die. Also, the high current densities effectively create temperatures in excess of the desired sintering temperature. If the sintering temperatures are sufficiently high, localized melting of powder particles could be observed (as will be discussed in following sections).  36     Figure 18: Precursor powders of: a) YSZ, b) erbia, c) 5mol% erbia-YSZ, d) 10mol% erbia-YSZ, d) 15mol% erbia-YSZ a) b) c) d) e) 37   Figure 19: Erbia particle In addition to the powder morphology, the bulk composition of the precursor powders was analyzed using XEDS and the results are provided in Table 7. Table 7: Bulk composition of precursor powders         Element wt.%     Composition   Zr O Y Er Hf Cl YSZ Theoretical 63.9 25.3 10.8 - - - Actual 54.9 35.2 8.1 - 1.3 0.5  Erbia Theoretical - 12.5 - 87.5 - - Actual - 15.1 - 84.9 - -  5mol% erbia- Theoretical 55.4 23.6 9.4 11.6 - - YSZ Actual 54.4 30.2 8.1 5.4 1.5 0.5 10mol% erbia- Theoretical 48.3 22.2 8.2 21.4 - - YSZ Actual 48.6 31.7 7.1 11.5 0.7 0.4 15mol% erbia- Theoretical 42.2 21.0 7.2 29.7 - - YSZ Actual 45.9 25.9 6.7 19.8 1.3 0.4   As can be seen in Table 7, the oxygen content was higher than the theoretical value (calculated based stoichiometric compositions) and the yttrium content was lower than the theoretical value in every precursor powder. This was attributed to the variations of composition in the supplied powders from their expected stoichiometric concentrations. Similar results were also seen for the average composition of each powder blend after sintering. 38  The erbium content of the erbia-YSZ powders increased with increasing doping level, as expected. However, the erbia concentrations were lower than the theoretical values. This phenomenon was possibly related to powder inhomogeneity. As seen in the back scattered electron (BSE) images in Figure 20, the blended powders contained erbia agglomerates, which are labelled “A”, “B” and “C”, with chemical compositions provided in Table 8.   Figure 20: BSE images of erbia-YSZ precursor powders with: a) 5mol%, b) 10mol% and c) 15mol% erbia A C B a) b) c) 39  Table 8: Chemical composition of the regions labelled in Figure 20         Element wt.%     Composition   Zr O Y Er Hf Cl  5mol% erbia- Theoretical 55.4 23.6 9.4 11.6 - - YSZ A 21.7 19.0 2.6 55.9 0.6 0.2 10mol% erbia- Theoretical 48.3 22.2 8.2 21.4 - - YSZ B 20.5 15.1 3.1 60.9 0.2 0.2 15mol% erbia- Theoretical 42.2 21.0 7.2 29.7 - - YSZ C 10.2 18.2 1.2 70.2 0.1 0.0   Due to the inhomogeneity of the powder blends, settling of the denser erbia particles may have occurred during or after the powder blending process, resulting in non-representative powder samples with reduced erbia content. It is important to note that despite the inhomogeneity of the powders, the bulk compositions of the sintered erbia-YSZ composites had near theoretical erbium contents, as will be discussed in section 4.2.3. 4.2 Spark Plasma Sintering With the exception of the 0-1300-30, 5-1200-60, 10-1200-30, 10-1200-60, 15-1200-30, 15-1200-60 composites, all other composites fractured upon ejection from the sintering die.  The YSZ composites were dark grey in colour, while the erbia doped composites exhibited pink and dark grey colourations. The dark grey colourations were attributed to oxygen depletion due to the highly reducing hot graphite tooling, which was in contact with the powders during SPS processing. Similar observations were made by other researchers [60, 61]. Photographs of the as-sintered composites are provided in Table 9.     40  Table 9: Photographs of SPS processed composites Sintering Temperature and Pressure 1400°C 60MPa     30MPa            1300°C 60MPa     30MPa     1200°C 60MPa     30MPa      Erbia content 0mol%   5mol%   10mol%   15mol%   Table 9: Photographs of SPS processed composites 41  There were no notable macroscopic differences between fractured and whole composites that could explain the varying disc integrities summarized in Table 10. Therefore, possible causes of sample fracturing could be related to the shape of the powder particles, as previously discussed in section 4.1, the glass like phase observed in SEM micrographs, as will be discussed in section 4.3.1, and internal stresses that may have developed during SPS processing. Internal stresses can result from uneven powder compaction and thermal gradients that can develop during SPS. Table 10: Integrity of SPS processed composites Sample (mol% Erbia-Temperature-Pressure) Disc Integrity 0-1200-30 fractured 0-1200-60 fractured 0-1300-30 good 0-1300-60 fractured 0-1400-30 fractured 0-1400-60 fractured 5-1200-30 fractured 5-1200-60 good 5-1300-30 fractured 5-1300-60 fractured 5-1400-30 fractured 5-1400-60 fractured 10-1200-30 good 10-1200-60 good 10-1300-30 fractured 10-1300-60 fractured 10-1400-30 fractured 10-1400-60 fractured 15-1200-30 good 15-1200-60 good 15-1300-30 fractured 15-1300-60 fractured 15-1400-30 fractured 15-1400-60 fractured The results indicate that 1200°C and 60MPa were the best sintering conditions for fabricating high integrity erbia-YSZ composites. 42  4.2.1 Sintering Current The evolution of the DC current was recorded during the SPS experiments. Figures 21, 22 and 23 show the trends of sintering current as a function of erbia content for the 1200, 1300 and 1400°C sintering conditions, respectively.  Figure 21: DC current during SPS of composites sintered at 1200°C and: a) 30MPa and b) 60MPa a) b) 43   Figure 22: DC current during SPS of composites sintered at 1300°C and: a) 30MPa and b) 60MPa a) b) 44   Figure 23: DC current during SPS of composites sintered at 1400°C and: a) 30MPa and b) 60MPa DC current during sintering varied with erbia concentration, but the trend was not consistent between sintering conditions. During the temperature dwell from ~9-14min, it was generally seen that the sintering current increased with increasing erbia content. Although this evidence wasn’t conclusive, this may have been a result of erbia causing an increase in electrical conductivity, since other research indicates that erbia reduces the thermal conductivity of YSZ [50, 62]. An increase in the electrical conductivity of the powder would cause the SPS machine to produce more current in order to maintain the desired sintering temperature. Additionally, the increased current may indicate that more heat was absorbed by the composite powders during densification, causing increased grain growth, as will be discussed in section 4.3.2.3. a) b) 45  Some other trends observed in Figures 21, 22 and 23 were abrupt changes in the DC current at ~3 and ~9min after the SPS experiment began. These changes in the DC current corresponded to the two changes in set-point temperature during sintering: 1) after dwelling at 600°C for pyrometer calibration and 2) after dwelling at the sintering temperature, respectively. Also, the steady increase in DC current from ~3-9min was a result of the increasing SPS temperature. It was noted that the difference in sintering current decreased erbia compositions processed at higher sintering pressures. This can be clearly seen in Figure 21 between ~9-15min for composites sintered at 1200°C. This trend was likely a result of increased contact pressure between particles, which would reduce the electrical contact resistance caused by air voids between the powder particles. 4.2.2 Densification The ram position was recorded during SPS processing and was used to calculate the instantaneous specific gravity of the compacts during sintering. An example of the ram height data recorded during sintering is provided in Figure 24 for both a loaded and an empty graphite die. Empty die experiments were conducted to determine the effect of die shrinkage/expansion on the ram position.  Figure 24: Ram height during SPS of 15-1200-60 46   The ram position data was used to develop the graphs of specific gravity versus time and temperature. All graphs are presented in Appendices C and D. A detailed examination of the underlying densification trends is presented in the following sections.  Effect of Sintering Temperature on Powder Densification The densification rate profiles presented in Figures 25-28 were used to examine the influence of the sintering temperature on the densification of the composites.  Figure 25: Densification rate of YSZ sintered at: a) 30MPa and b) 60MPa a) b) 47   Figure 26: Densification rate of 5mol% erbia-YSZ composites sintered at: a) 30MPa and b) 60MPa a) b) 48   Figure 27: Densification rate of 10mol% erbia-YSZ composites sintered at: a) 30MPa and b) 60MPa a) b) 49   Figure 28: Densification rate of 15mol% erbia-YSZ composites sintered at: a) 30MPa and b) 60MPa From the densification rate graphs in Figures 25-28, it was seen that for 0 and 5mol% erbia-YSZ composites, the sintering temperature did not have a consistent impact on the time to reach the maximum densification rate. The observed variations were possibly related to uneven powder compaction due to powder inhomogeneity. However, it was found that the area under the densification curves increased with increasing sintering temperature, indicating an overall increase in final density of the composites. The one exception was the 5-1200-30 composite, which showed a higher densification than the remaining 5mol% erbia-YSZ composites sintered at higher temperatures. The trend of area under the curve increasing with increasing temperature was also seen in the 10 and 15mol% erbia-YSZ composites. a) b) 50  For the 10 and 15mol% erbia-YSZ composites, it was seen that there were two densification peaks at ~5 and ~8min. The first peak is generally associated with particle rearrangement during sintering. The second densification peak was related to densification by grain growth. As has been reported in the literature, densification of YSZ involves two stages: lattice diffusion of cations followed by grain boundary sliding [51]. Comparison of densification data with the grain size results in section 4.3.2 revealed that composites which had a reduced second densification peak lacked grain growth. Therefore, densification by grain growth in erbia-YSZ composites required elevated temperatures to complete: at least 1300°C for 10mol% erbia and at least 1400°C for 15mol% erbia. The one exception was the 15-1300-30 composite, which exhibited a large second densification peak, but only limited grain growth. It was also noted that the first densification peak (associated with particle rearrangement) was larger for higher sintering pressures. The first densification peak reached maximums of ~1/min when sintering at 60MPa, but reached maximums of ~0.75/min when sintering at 30MPa. With a higher applied pressure, a more extensive powder rearrangement was achieved. Average specific gravity was plotted in Figures 29 and 30 as a function of sintering temperature for composites sintered at 30 and 60MPa.  Figure 29: Average specific gravity of composites sintered at 30MPa 51   Figure 30: Average specific gravity of composites sintered at 60MPa With the exception of the 5-1200-30 composite, the specific gravity increased with sintering temperature for every composition sintered at 30 and 60MPa. These results are in agreement with the expected trend of density increasing with sintering temperature as reported in literature [25, 36, 53, 54]. Increased sintering temperature enhances densification by enhancing “necking”, diffusion and grain growth. The 5-1200-30 composite’s high specific gravity was considered to be an outlier caused by uneven powder compaction due to powder inhomogeneity.  Effect of Sintering Pressure on Powder Densification  The average specific gravities of each sintered composite are presented as a function of sintering pressure for the 1200, 1300 and 1400°C sintering temperatures in Figures 31-33. 52   Figure 31: Average specific gravity of composites sintered at 1200°C  Figure 32: Average specific gravity of composites sintered at 1300°C 53   Figure 33: Average specific gravity of composites sintered at 1400°C  With the exception of the 5-1200-30 and 10-1400-30 composites, specific gravity increased with increasing sintering pressure for every composite. These results were in agreement with those of other SPS processed materials, where material density increased with pressure due to increased efficiency in powder packing, breakdown of agglomerates and plastic deformation of “necks” formed from localized melting [36, 53, 54] during SPS processing.   Effect of Erbia Concentration on Powder Densification The effect of erbia content on the densification rate is presented in Figures 34-39.  Figure 34: Densification rate of erbia-YSZ composites sintered at 1200°C and 30MPa 54   Figure 35: Densification rate of erbia-YSZ composites sintered at 1200°C and 60MPa  Figure 36: Densification rate of erbia-YSZ composites sintered at 1300°C and 30MPa  55   Figure 37: Densification rate of erbia-YSZ composites sintered at 1300°C and 60MPa   Figure 38: Densification rate of erbia-YSZ composites sintered at 1400°C and 30MPa 56   Figure 39: Densification rate of erbia-YSZ composites sintered at 1400°C and 60MPa It was found that YSZ composites had one densification peak during sintering, while erbia-YSZ composites had two densification peaks with the second peak being larger when sintered at high temperatures, as was discussed in section 4.2.2.1. The first peak was related to particle rearrangement and diffusion of cations and showed minor variations between erbia concentrations. The second densification peak was associated with grain growth and came at later times and higher temperatures during sintering for higher erbia concentrations, suggesting that erbia doping increased the temperature required for grain growth to occur, leading to a delayed densification. Additionally, the second densification peak decreased with increasing erbia concentration. This appeared to contradict the grain size results that will be discussed in section 4.3.2.3, where grain size increased with increasing erbia concentration, but the amount of powder that exhibited no grain growth remaining in sintered compacts increased with erbia concentration as well, as will be discussed in section 4.3.2.   The specific gravity data and porosity, calculated based on volume averaged theoretical densities, are summarized in Figure 40 as a function of erbia content for every sintering condition used.   57    Figure 40: Summary of: a) average specific gravity and b) average porosity of composites sintered under every sintering condition The specific gravity generally increased with erbia concentration for composites that exhibited grain growth. This result was expected due to the higher density of erbia (8.64g/cm3 [57]) compared to YSZ (6.1g/cm3 [56]). Exceptions to this trend were the 10-1300-30 and 10-1400-30 composites, which were likely outliers caused by uneven powder compaction and inhomogeneous powder morphology. The composite porosity (calculated based on the approximated maximum theoretical densities) generally increased with increasing erbia concentration. These results suggest that as denser erbia was added to the a) b) 58  YSZ matrix, the composite’s density increased. Simultaneously, however, densification during SPS was hindered by erbia, resulting in an increase of as-sintered composite porosity. Also, it was observed that the porosity of the 10-1200-30 and 15-1200-30 composites was higher than the porosity of the other composites by ~5-10%. 4.2.3 Bulk Chemical Composition The as-sintered bulk matrix compositions of YSZ and erbia-YSZ composites are provided in Table 11.    59  Table 11: Chemical composition of as-sintered YSZ and erbia-YSZ composites   Composition Sample Element wt.% Zr O Y Er Hf Cl     YSZ     Theoretical 63.9 25.3 10.8 - - - 0-1200-30 55.2 35.3 8.2 - 1.2 0.1 0-1200-60 55.9 33.9 8.3 - 1.2 0.7 0-1300-30 57.1 32.9 8.4 - 1.5 0.1 0-1300-60 63.5 24.2 9.2 - 2.7 0.3 0-1400-30 57.1 33.3 8.3 - 1.3 0.0 0-1400-60 58.9 30.9 8.4 - 1.6 0.2 YSZ Average 58.0 31.8 8.5 - 1.6 0.2 Std. deviation 3.0 4.0 0.4 - 0.6 0.2    5mol% erbia- YSZ     Theoretical 55.4 23.6 9.4 11.6 - - 5-1200-30 51.6 26.4 7.6 12.4 1.6 0.4 5-1200-60 48.3 32.4 7.5 9.8 1.1 0.9 5-1300-30 51.3 27.1 7.7 12.5 1.2 0.2 5-1300-60 53.8 25.8 7.7 11.6 1.1 0.0 5-1400-30 47.7 33.6 7.0 10.5 0.7 0.4 5-1400-60 50.4 29.6 7.1 11.4 1.3 0.3 5mol% Average 50.5 29.2 7.4 11.4 1.2 0.4 Std. deviation 2.2 3.3 0.3 1.1 0.3 0.3    10mol% erbia- YSZ     Theoretical 48.3 22.2 8.2 21.4 - - 10-1200-30 41.1 29.1 6.0 22.3 0.9 0.5 10-1200-60 46.6 20.8 7.2 24.2 1.0 0.2 10-1300-30 40.3 27.1 6.0 24.6 1.4 0.6 10-1300-60 49.2 20.5 7.2 22.0 1.1 0.1 10-1400-30 41.2 29.6 6.4 21.4 1.2 0.3 10-1400-60 42.6 26.1 6.2 23.5 1.4 0.3 10mol% Average 43.5 25.5 6.5 23.0 1.1 0.3 Std. deviation 3.6 4.0 0.6 1.3 0.2 0.2    15mol% erbia- YSZ     Theoretical 42.2 21.0 7.2 29.7 - - 15-1200-30 34.5 28.3 5.2 31.0 0.7 0.3 15-1200-60 40.8 20.9 6.2 30.9 0.8 0.4 15-1300-30 39.5 18.6 6.1 33.3 1.7 0.8 15-1300-60 39.4 21.0 5.7 32.3 1.2 0.3 15-1400-30 35.6 29.2 5.3 28.5 0.9 0.5 15-1400-60 36.0 23.5 5.6 32.8 1.2 0.9 15mol% Average 37.6 23.6 5.7 31.5 1.1 0.5 Std. deviation 2.6 4.3 0.4 1.7 0.3 0.3  60  In comparison to the theoretical stoichiometric values, the average compositions for the composites were: deficiency of Y and excess of O, which was also seen in the analysis of blended powders (section 4.1s). The average Er content of every erbia-YSZ composition was close to theoretical values, differing by 2wt.% or less from the theoretical values. The Hf and Cl impurities together accounted for ~2% of each composite’s mass and contributed to the variations from stoichiometric values. These results, in combination with the XRD results in section 4.3.3, confirm that powder blending and subsequent SPS processing were successful in fabricating new solid solution ceramic composites. 4.2.4 Hardness The hardness of every composite was measured at the cross-section and the results are reported in the following text. With an indentation diameter of 30-60µm, the hardness represents the average hardness of ~12 grains, based on an average grain size of 3.7µm.  Effect of Sintering Temperature on Hardness The hardness values of erbia-YSZ composites sintered at 30 and 60MPa are presented with respect to the sintering temperature in Figures 41 and 42.  Figure 41: Hardness of composites sintered at 30MPa 61   Figure 42: Hardness of composites sintered at 60MPa  The hardness of every composite sintered under 30 and 60MPa increased with increasing sintering temperature from 1200 to 1300°C, then decreased with increasing sintering temperature from 1300 to 1400°C.   The observed relationship between hardness values and sintering temperature was likely a result of changes in porosity [63, 64] and grain size. The densification results in section 4.2.2.1 revealed that composite specific gravity increased (porosity decreased) with increasing sintering temperature, except for the 5-1200-30 composite. The grain size results in section 4.3.2.1 suggest that grain size increased with increasing sintering temperature, except for the 5-1200-30 composite. Thus, the increase in hardness from 1200-1300°C may have been related to the porosity. Between 1300 and 1400°C, grain growth resulted in a weaker material, as per the classical Hall-Petch effect.  It was also noted that the hardness of the 10-1200-30 and 15-1200-30 composites was lower than every other composite, this was thought to be a result of the low sintering temperature causing high porosity in these samples. This phenomenon will be further discussed in section 4.2.4.2. 62   Effect of Sintering Pressure on Hardness The hardness of erbia-YSZ composites sintered at 1200-1400°C is presented with respect to sintering pressure in Figures 43-45.  Figure 43: Hardness of composites sintered at 1200°C  Figure 44: Hardness of composites sintered at 1300°C 63   Figure 45: Hardness of composites sintered at 1400°C The hardness of erbia-YSZ composites generally decreased with increased sintering pressure, though in some cases this decrease was marginal, such as the 5mol% erbia-YSZ composites sintered at 1400°C. The main exceptions to this trend were the 10 and 15mol% erbia-YSZ composites sintered at 1200°C, which showed an increase in hardness with increased sintering pressure. The reason for this anomaly is unknown, but was thought to be related to high sample porosity, and requires further investigation. The grain size results in section 4.3.2.1 show that grain size increased with increasing sintering pressure, except for the 5-1200-30 composite. Thus, the general decrease in hardness that occurred when sintering pressure was increased from 30-60MPa may have been caused by the grain growth that accompanied the pressure change. For the 10 and 15mol% erbia-YSZ composites sintered at 1200°C, the observed increase in hardness with increased sintering pressure was likely caused by reduced porosity, as was previously discussed in section 4.2.2.3. For both compositions, an 11% reduction in porosity was observed when sintered at 60MPa, compared to 30MPa. The change in hardness for the 5mol% composites sintered at 1200°C may be explained by the increased porosity with increase of pressure from 30 to 60MPa. 64   Effect of Erbia Concentration on Hardness Hardness measurements are presented in Figure 46 for every composite, showing the effect of the erbia concentration on the material hardness.  Figure 46: Summary of hardness of composites The hardness of the erbia-YSZ composites decreased with increasing erbia content. A number of factors may have influenced this trend. Increasing erbia concentration was observed to generally increase porosity, as was discussed in section 4.2.2.3, increase grain size if grain growth occurred, as will be discussed in section 4.3.2.3 and expand the crystal lattice, as will be discussed in section 4.3.4. Each of these changes in the sintered properties may have contributed to the reduced hardness of erbia doped composites because of the presence of voids, the classical Hall-Petch effect and/or weakened interatomic bonds, respectively. 4.3 Microstructure Analysis The microstructure of sintered erbia-YSZ composites was characterized by SEM imaging, XRD and XEDS chemical analysis methods. 4.3.1 General Observations The general microstructure of the 0-15mol% erbia-YSZ composites is presented in Figures 47-52.  65   Regions containing a glassy phase (transparent, amorphous regions with no apparent grain structure) were often found in the sintered composites, as seen in Figure 47. The chemical compositions of representative regions of the microstructure are provided in Table 12.  Figure 47: Glassy phase observed in the 5-1400-60 composite Table 12: Chemical compositions of the regions labelled in Figure 47         Element wt.%      Composition   Zr O Y Er C Hf Cl 5mol% erbia- YSZ Theoretical 55.4 23.6 9.4 11.6 - - - A 7.6 6.9 4.3 1.7 76.4 0.2 2.8 B 40.4 24.8 5.9 9.0 18.4 1.3 0.3   A glassy phase was observed in every sintered composite, similar to the one seen in region “A” of Figure 47. These regions were rich in carbon, yttrium and chlorine. According to Roesler et al. [65], the formation of a glassy phase in ceramics can occur at the grain boundaries during sintering due to the presence of sintering aids that produce a liquid to A B 66  facilitate sintering. These glassy regions can contribute to crack propagation along the grain boundaries. Although sintering aids weren’t used in the current research, cracks were seen to propagate along the matrix grain boundaries, as seen in of Figure 48a, and in some cases the glassy phase itself may have instigated cracking, as seen in of Figure 48b. Additionally, the glassy phase was rich in yttrium and chloride, which may have been the result of yttrium and chlorine reacting to form yttrium chloride, which has a melting temperature of 721°C [66]. Thus, an yttrium chloride phase may have acted similarly to a sintering aid and contributed to composites fracturing upon ejection from the sintering die. The mechanism responsible for the formation of the glassy phase remains unclear. However, it is believed that localized melting of the matrix powder particles may be a factor. This would be facilitated by the sharp edges and features of the YSZ particles, as seen in Figure 19. Further work on this is necessary.    Figure 48: Cracks in the: a) 10-1400-30 and b) 15-1300-30 composites  Bands or agglomerations of erbia were found in the matrix of select sintered composites, as shown in Figure 49, with the chemical compositions of select regions provided in Table 13. a) b) 67    Figure 49: Erbia bands in the: a) 10-1200-60 and b) 15-1200-60 composites Table 13: Chemical compositions of the regions labelled in Figure 49         Element wt.%     Composition   Zr O Y Er Hf Cl 10mol% erbia- YSZ Theoretical 48.3 22.2 8.2 21.4 - - A 3.9 14.6 0.4 80.6 0.2 0.3 B 50.4 19.9 9.1 19.1 1.3 0.2 15mol% erbia- YSZ Theoretical 42.2 21.0 7.2 29.7 - - C 3.3 13.4 0.4 82.5 0.0 0.5 D 39.1 15.9 4.5 39.3 0.9 0.3  BSE imaging revealed the presence of bands of erbia (white bands in Figure 49) in the microstructures of 10 and 15mol% erbia-YSZ composites sintered at 1200°C. The volume fraction of these bands increased with increasing erbia concentration. These bands were caused by incomplete diffusion of erbium into the zirconia matrix, as seen by the presence of a 1µm erbium diffusion layer in Figure 50, and as will be discussed in section 4.3.3. Also, insufficient mixing of the precursor powders prior to sintering contributed to the presence of the erbia bands.  A B D C a) b) 68   Figure 50: Line scan of an erbia band in the 15-1200-60 composite 69   It was also observed that the matrix contained grains with pores, as seen in Figure 51.  Figure 51: A pore containing grain in the 0-1300-30 composite Chen et al. [67] suggested that pores within YSZ grains were more common in SPS processed composites compared to conventionally sintered composites. One key difference between SPS and conventional sintering is the heating/cooling rate used. The pores inside the grains may have resulted from the rapid cooling rate used during SPS (~100-250°C/min) preventing diffused gases from escaping by rapidly reducing the solubility of the gases in the grains. 4.3.2 Grain Size SEM micrographs taken at 5000x magnification of the grain structures of erbia-YSZ composites are presented in Tables 14 and 15. All micrographs presented are at the same magnification of 5kx. It was observed that 10mol% erbia-YSZ composites sintered below 1300°C and 15mol% erbia-YSZ composites sintered below 1400°C did not exhibit any visible grain growth, but maintained the original size of the precursor powders. Grain growth occurred in the sintered composites that had grain sizes larger than the size of the precursor powders. In composites that did not exhibit grain growth, a magnification greater than 5000x was required to clearly see the grains. Pore 70  Table 14: Grain structures of 0 and 5mol% erbia-YSZ composites  Sintering Temperature and Pressure Sintering 1400°C 60MPa   30MPa          1300°C 60MPa   30MPa   1200°C 60MPa   30MPa    Erbia content 0mol%     5mol%     71  Table 15: Grain structures of 10 and 15mol% erbia-YSZ composites   Sintering Temperature and Pressure Sintering 1400°C 60MPa   30MPa           1300°C 60MPa   30MPa   1200°C 60MPa   30MPa    Erbia content 10mol%     15mol%     72  Qualitative analysis of the microstructure also revealed that the powder content in the sintered composites generally increased with increasing erbia concentration, as is illustrated in Figure 52 for composites sintered under various sintering parameters. Regions where grain growth occurred are labelled as “A” and powder regions are labelled as “B”.    Regions where grain growth occurred are labelled as “A” and powder regions are labelled as “B”. Figure 52: Example microstructures of: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol% erbia-YSZ composites a) b) c) d) A A A A B B B 73   It was also noted that the grains were polygonal in shape, as has been commonly observed in the literature for YSZ composites [10, 26, 27, 68].  Effect of Sintering Temperature on Grain Size The grain size measurement results are presented as a function of the sintering temperature in Figures 53 and 54. Grain size data is not provided for 10mol% erbia-YSZ composites sintered below 1300°C and 15mol% erbia-YSZ composites sintered below 1400°C due to the absence of grain growth in these composites. In these ceramic composites, the grain size was comparable to the particle size of the raw powders.  Figure 53: Grain sizes of composites sintered at 30MPa 74   Figure 54: Grain sizes of composites sintered at 60MPa  In general, the grain size increased with increasing sintering temperature for composites sintered at 30 and 60MPa. These results were supported by the findings of other researchers who observed that the grain size of other materials increased with increased sintering temperature during SPS [25, 36, 53, 54].  Grain growth and recrystallization are thermally activated processes. Once the critical temperature for grain growth is reached, grain growth increases exponentially with temperature. In this research, the critical temperature for grain growth could not be determined for 0 and 5mol% erbia-YSZ composites. For 10mol% erbia-YSZ composites, the critical temperature for grain growth was between 1200 and 1300°C and for 15mol% erbia-YSZ composites it was between 1300 and 1400°C.  Effect of Sintering Pressure on Grain Size The grain sizes of the sintered erbia-YSZ composites are presented as a function of the sintering pressure in Figures 55-57. 75   Figure 55: Grain sizes of composites sintered at 1200°C  Figure 56: Grain sizes of composites sintered at 1300°C 76   Figure 57: Grain sizes of composites sintered at 1400°C In general, the grain size increased with increasing sintering pressure for composites sintered between 1200-1400°C. These results were supported by the findings of other researchers who observed that grain size increased with increased sintering pressure during SPS [54, 69, 70]. An increase in the sintering pressure is associated with increased contact pressure between neighbouring grains, which promotes the migration of grain boundaries and the fusion of adjacent grains. Additionally, the improved contact facilitates enhanced heat transfer leading to faster grain growth. A possible competing mechanism introduced by the increased pressure is an increased contribution of any grain pinning effects. However, this factor must have had a lesser impact on the grain size than the thermally activated factors.   Effect of Erbia Concentration on Grain Size The grain sizes of the sintered composites are presented as a function of the erbia concentration in Figure 58. 77   Figure 58: Summary of grain sizes of composites Grain size was found to increase with increasing erbia concentration for composites sintered under every condition. This result differed from those of other researchers who found that doping YSZ with erbia [49] or other rare earth oxides such as Nd2O3, La2O3 and CeO2 reduced grain growth [10, 68]. One primary difference between the mentioned studies and the current research was that the other rare earth oxide precursor powders had particle sizes smaller than the YSZ powder. The particle size of the erbia powder used in this study was larger than the YSZ matrix powder, as was previously discussed in section 4.1. Thus, the large size of the erbia powder may have contributed to the grain growth in erbia-YSZ composites. Another contributing factor may have been the powder homogeneity. In the previous studies, rare earth oxide banding or intergranular phases were seen in sintered composites, indicating poor powder homogeneity, while in this study erbia banding was only seen in a few composites. Lastly, as was previously discussed in section 4.2.1, the DC current during the sintering temperature dwell was higher for higher erbia concentrations. This may be a result of changes to the composite powder properties (such as increased electrical conductivity), or it may indicate increased heat absorption, which would cause more growth of the matrix grains. 78  4.3.3 XRD The XRD plots for 0 and 10mol% erbia-YSZ before and after sintering are provided in Figures 59 and 60. These two composites were taken as representative plots for the other composites, but not for the erbia-YSZ composites sintered at 1200°C, which will be discussed later in this section. Additional XRD plots for each precursor powder and sintered composite are provided in Appendix E.  Figure 59: XRD plot for the 0-1200-60 composite: a) before and b) after sintering   C Cubic YSZ M Monoclinic YSZ   Intensity (A.U.) a) b) 79   Figure 60: XRD plot for the 10-1200-60 composite: a) before and b) after sintering Comparison of the XRD peaks before and after sintering in Figures 59 and 60 revealed two trends. First, both the monoclinic and cubic YSZ phases were present in YSZ and erbia-YSZ precursor powders, but the deletrious monoclinic phase was absent in the as-sintered composites because it may have transformed into the cubic crystal structure. This result was supported by the findings of Siebert-Timmer [10], who also found that the monoclinic phase was present before sintering, but not after sintering for the same 8mol% YSZ powder. The second observation was that the erbia-YSZ powders had distinct YSZ and erbia phases, but only the cubic YSZ phase was detected after sintering for composites sintered at 1300-1400°C. The absence of the erbia phase after sintering indicated that erbia was incorporated into the zirconia matrix. This result was supported by the absence of erbia bands in erbia-YSZ composites sintered at 1300-1400°C, as was previously discussed in section 4.3.1, as well as the increasing crystal lattice parameter, which will be discussed in section 4.3.4.  With the exception of the 10-1200-60 composite, the erbia-YSZ composites sintered at 1200°C had XRD plots that differed from the plot shown for the 10mol% erbia-YSZ composite in Figure 60. In these composites, a retained erbia phase was detected, as shown in Figures 61 and 62. C Cubic YSZ M Monoclinic YSZ E Erbia Intensity (A.U.) a) b) 80   Figure 61: XRD plots for the sintered: a) 10-1200-30 and b) 15-1200-30 composites  Figure 62: XRD plots for the sintered: a) 10-1200-60 and c) 15-1200-60 composites The retained erbia phase indicated that the diffusion of erbium into the zirconia matrix in the 10-1200-30, 15-1200-30 and 15-1200-60 composites was incomplete. These results were confirmed by the presence of erbia bands seen in the sintered microstructures, as was previously discussed in section 4.3.1. Bands were also sparsely seen in the 10-1200-60 composite, but the volume fraction of these bands was low and likely below the detection limit of the XRD equipment. A summary of the composites containing a separate erbia phase C Cubic YSZ E Erbia   C Cubic YSZ E Erbia   Intensity (A.U.) a) b) Intensity (A.U.) a) b) 81  is provided in Table 16. A separate erbia phase was not detected or observed in any other composite. Table 16: Summary of the erbia phase in 10 and 15mol% erbia-YSZ composites sintered at 1200°C Sample (mol% Erbia-Temperature-Pressure) Erbia detected Banding observed 10-1200-30 Yes Yes 10-1200-60 No Yes 15-1200-30 Yes Yes 15-1200-60 Yes Yes  Additionally, a leftward peak shift was observed in the sintered composites. This peak shift was associated with the incorporation of erbia into the YSZ matrix with increasing erbia concentration. An example of this peak shift is illustrated in Figure 63.  Figure 63: Leftward shift in XRD plots for: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol% erbia-YSZ composites sintered at 1400°C and 60MPa Intensity (A.U.) a) b) c) d) 82  The observed leftward peak shift was a result of crystal lattice expansion. This was verified by the calculated crystal lattice parameters, as will be discussed in the next section. Internal macroscopic stress was likely not a factor during XRD analysis of the peak shift because the analyzed samples were fractured. It was also noted that the full width at half maximum varied between composites, which may be related to lattice strains caused by crystal lattice expansion. 4.3.4 Crystal Lattice Parameter Crystal lattice parameters were calculated from the XRD data for each composite using the DICVOL 04 method. The sintered composites had cubic crystal structures and the lattice parameters are listed in Table 17.   83  Table 17: Crystal lattice parameters of sintered erbia-YSZ composites Sample (mol% Erbia-Temperature-Pressure) Lattice Parameter (Å) 0-1200-30 5.139 0-1200-60 5.138 0-1300-30 5.139 0-1300-60 5.141 0-1400-30 5.141 0-1400-60 5.140 5-1200-30 5.149 5-1200-60 5.148 5-1300-30 5.149 5-1300-60 5.149 5-1400-30 5.150 5-1400-60 5.149 10-1200-30 5.157 10-1200-60 5.163 10-1300-30 5.160 10-1300-60 5.161 10-1400-30 5.161 10-1400-60 5.159 15-1200-30 5.160 15-1200-60 5.172 15-1300-30 5.174 15-1300-60 5.178 15-1400-30 5.173 15-1400-60 5.172   The average crystal lattice parameter of 5.14Å for YSZ corresponded to the values reported in the literature for 8mol% YSZ [71, 72]. The lattice parameter was found to increase with increasing erbia concentration, ~5.14, 5.15, 5.16 and 5.17Å for 0-15mol% erbia-YSZ composites, respectively, but did not show any correlation to the sintering parameters. Literature suggests that erbia causes crystal lattice expansion in zirconia [46]. Other rare earth oxides (Nd2O3 and CeO2) have also been found to cause crystal lattice expansion in 8mol% YSZ [10].  84  Erbium has a larger ionic radius (0.89Å) [73] in comparison to zirconium (0.78Å) [74]. Thus, it was hypothesized that erbia doping involved a similar mechanism to yttria doping [20], where erbium ions replaced zirconium ions in the zirconia matrix and erbium’s large ionic radius causes crystal lattice expansion. It was noted that the 15-1200-30 composite had a lattice parameter that was smaller than the other 15mol% erbia-YSZ composites by ~0.1Å, which was likely a result of the erbia not being fully incorporated into the zirconia matrix. This result was verified by the presence of erbia agglomerations in the sintered microstructures, as previously discussed in section 4.3.1, and the presence of a separate erbia phase detected by XRD, as previously discussed in section 4.3.3. 4.4 SCW Degradation Performance The 5-1200-60, 10-1200-60 and 15-1200-60 composites were exposed to SCW for 2.5h and their degradation performance was observed. These experiments were preliminary, as only one sample per composition was examined. The results of degradation testing are summarized in Table 18; as was mentioned previously, the electronic balance had an accuracy of 0.0001g. Table 18: Degradation rate of erbia-YSZ composites   Sample mass (g)  Mass loss (g) Degradation  rate (mm/y) Sample As-Sintered Post-Degradation 5-1200-60 0.3284 0.3255 0.0029 21.796 10-1200-60 0.3447 0.3404 0.0043 29.460 15-1200-60 0.3408 0.3399 0.0009 5.780   All composites exhibited mass loss after exposure to SCW. These results were compared to those of Siebert-Timmer [10], who used the same SCW degradation experiment methodology to study the SCW degradation resistance of YSZ, CeO2 doped YSZ and Nd2O3 doped YSZ. YSZ had poor degradation resistance, with two of the three YSZ test composites disintegrating and the third degrading at 1100mm/y. 5mol% CeO2 doped YSZ composites degraded at 16-107mm/y and 5mol% Nd2O3 doped YSZ composites degraded at 12-15mm/y, 85  while composites with greater concentrations of CeO2 and Nd2O3 in YSZ generally had higher degradation rates.  The lowest degradation rate found for erbia-YSZ composites (5.8mm/y) was less than half of the lowest degradation rate observed in CeO2 and Nd2O3 doped YSZ composites. Thus, erbia-YSZ composites showed improved SCW degradation resistance when compared with CeO2 and Nd2O3 doped YSZ. The density, grain size, hardness, lattice parameters and chemical compositions of erbia-YSZ composites before and after exposure to SCW are presented in Tables 19 and 20. Table 19: Composite properties before and after exposure to SCW      Specific gravity     Lattice Parameter (Å) Sample    Grain size (μm) Hardness (BHN) 5-1200-60   Sintered 5.46 3.16 919.13 5.148 Post-Degradation 5.47 3.26 901.90 5.148 10-1200-60   Sintered 5.45 N/A 712.76 5.163 Post-Degradation 5.53 N/A 675.92 5.163 15-1200-60   Sintered 6.10 N/A 632.58 5.172 Post-Degradation 6.19 N/A 705.47 5.173  Table 20: Bulk chemical composition of composites before and after exposure to SCW     Element wt.%  Sample     Zr O Y Er Hf Cl 5-1200-60   Sintered 52.3 26.3 7.7 11.9 1.2 0.5 Post-Degradation 53.2 24.8 7.4 12.9 1.6 0.0 10-1200-60   Sintered 48.6 22.7 7.4 20.3 0.7 0.2 Post-Degradation 48.4 21.8 7.2 21.4 1.1 0.0 15-1200-60   Sintered 40.8 20.9 6.2 30.9 0.8 0.4 Post-Degradation 41.5 19.5 6.5 31.4 1.0 0.0   No statistically significant changes in density, grain size, hardness, or lattice parameter were observed (ie. in excess of one standard deviation); however, a change was observed in the chemical composition. Before degradation, chlorine was detected throughout the composites, but after degradation bulk area scans did not detect any chlorine. Thus, 86  depletion of chlorine occurred during the SCW degradation experiments. The composite weight losses were 0.3-1.2wt.% and the chlorine content was 0.2-0.5wt.% before SCW degradation, which indicates that chlorine may have been a primary constituent of the degraded material. Unfortunately, mass analysis of the dissolved solids in the test fluid could not be performed due to budget limitations.  In Siebert-Timmer’s [10] experiments, it was found that YSZ degraded because of oxygen vacancy annihilation and yttrium dissolution, which destabilized the matrix and allowed for the formation of the monoclinic phase. It is unclear if these degradation mechanisms occurred in the present experiments, but any observed changes in the yttrium content of the composites were minor and no transformations of the cubic phase were observed. Thus, the zirconia matrix was considered to be stable during SCW exposure.  The erbia-YSZ composites changed colour after SCW exposure, but no spallation or pitting was observed, as illustrated in Figure 64.   Figure 64: The 10mol% erbia-YSZ composite: a) before and b) after exposure to SCW  No changes were observed in the microstructures of composites before and after SCW exposure. Micrographs of the 5mol% erbia-YSZ composite before and after exposure to SCW are provided in Figure 65. a) b) 87    Figure 65: Microstructure of the 5mol% erbia-YSZ composite: a) before and b) after exposure to SCW Given the 30y design life of a SCWR pressure tube [4], an erbia-YSZ composite used as an insulator material would require at least 174mm of sacrificial material in addition to the required insulator thickness. This is not feasible because these pressure tubes are expected to have an inner diameter of 120mm [4]. Therefore, based on the results of this preliminary study, the current erbia-YSZ composites require further optimization for use as thermal insulators in SCWR pressure tubes. However, there were several factors that may have resulted in elevated degradation rate in the current erbia-YSZ composites. The actual duration of SCW exposure was likely longer than 2.5h since the composites were heated above the critical point and the heating and cooling times would have increased the duration of SCW exposure. The test duration used in this work was too short because the degradation rates were low and according to ASTM G31-72 [58], the test duration for low degradation rates should be calculated with Equation 6.                  𝒉𝒐𝒖𝒓𝒔 = 𝟐𝟎𝟎𝟎/(𝒄𝒐𝒓𝒓𝒐𝒔𝒊𝒐𝒏 𝒓𝒂𝒕𝒆 𝒊𝒏𝒎𝒎𝒚) Equation 7 a) b) 88  Equation 6 gives a test duration of 70-330h for the degradation rates calculated for erbia-YSZ composites. Lastly, depletion of chlorine was considered a substantial portion of the observed weight loss and a longer test may have resulted in a reduced degradation rate after the chlorine impurity was fully depleted.  89  Chapter 5: Conclusions and Future Work This chapter presents the conclusions drawn from the fabrication and characterization of erbia-YSZ composites and some suggestions for future work pertaining to the fabrication and SCWR performance of these materials. The work conducted in this research is one of the first to report on the development of erbia doped YSZ ceramics via the SPS manufacturing route. The most important observations are as follows:  Erbia-YSZ composites may fracture after SPS processing, possibly due to the formation of a glass like phase and internal stresses.  Elevated sintering temperatures enable increased densification, but result in grain growth.  Elevated sintering pressures enable increased densification, but result in grain growth and reduced hardness.  Doping of YSZ with erbia generally increased the sintering current required to maintain the sintering temperature.  Porosity generally increased with increasing erbia concentration.  Addition of erbia delayed densification by grain growth of the YSZ matrix.  Erbia doping increased the sintering temperature required for grain growth to occur.  Erbia doping reduced the hardness of the sintered composites.  The hardness of erbia-YSZ composites was not linearly dependent on sintering temperature, suggesting that optimum SPS processing conditions must be determined.  The sintered composites maintained a cubic crystal structure.  Erbia was successfully incorporated into the zirconia matrix and caused crystal lattice expansion.  Addition of erbia to YSZ improved the SCW degradation resistance of the ceramic.  The matrix of erbia-YSZ composites remained stable after SCW exposure, but depletion of chlorine was observed. 90  5.1 Future Work  Optimize SPS processing parameters for fabricating high integrity erbia-YSZ composite discs.  Study and optimize the fabrication of erbia-YSZ composites by commercial coating processes, such as plasma spraying.  Perform long term (>300h) SCW degradation experiments on erbia-YSZ composites, with multiple trials to accurately determine the degradation rates.  Perform SCW degradation experiments under conditions that more closely match the SCWR pressure tube environment, with a similar temperature, pressure and radiation.  Perform an in-depth analysis of the SCW degradation mechanisms present in erbia-YSZ composites.  91  References [1] D. 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measurements.   99  Appendix B: SCW Degradation Experiment Calculations Calculation of the pressure inside the pressure vessel: 𝑃 = 𝑧 ∗ 𝑛 ∗ 𝑅 ∗𝑇𝑉 𝑧 = 0.37  determined iteratively based on the compressibility chart for water 𝑛 =𝑉𝑤 ∗ 𝜌𝑤𝑀𝑤=20𝑚𝑙10002∗1000𝑘𝑔𝑚318.015𝑘𝑔𝑘𝑚𝑜𝑙= 0.00111𝑘𝑚𝑜𝑙 𝑅 =8.3145𝐾𝐽𝐾𝑚𝑜𝑙 ∗ 𝐾 𝑇 = 273.15 + 400 = 673.15𝐾 Free volume (volume of pressure vessel minus volume of sample): 𝑉 = 𝑉𝑣 − 𝑉𝑠 = 𝑙 ∗ 𝜋(𝑑 − 2𝑡2)2 − 43 = 254 ∗ 𝜋 ∗ (25.4 − 2 ∗ 3.0482)2− 64 = 74275.26𝑚𝑚3 𝑉 =74275.2610003𝑚3 = 7.427526𝑥10−5𝑚3  𝑃 = .37 ∗ .00111 ∗ 8.3145 ∗673.157.427526𝑥10−5= 30900𝑘𝑃𝑎 𝑃 =309001000𝑀𝑃𝑎 = 30.9𝑀𝑃𝑎 Degradation rate calculation (the 10mol% erbia-YSZ composite was used as an example): The sample dimensions were 3.97x3.88x4.02mm and the density was 5.445g/cm3. 𝑟 =𝐾 ∙ 𝑊𝐴 ∙ 𝑡 ∙ 𝜌=8.76 ∗ 104 ∗ (.3447 − .3404)2 ∗ (3.97 ∗ 3.88 + 3.97 ∗ 4.02 + 3.88 ∗ 4.02)100 ∗ 2.5 ∗ 5.445=29.460𝑚𝑚𝑦   100  Appendix C: Effect of Temperature on Powder Densification  Figure 67: Densification of YSZ sintered at 30MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature a) b) 101   Figure 68: Densification of YSZ sintered at 60MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature  a) b) 102   Figure 69: Densification of 5mol% erbia-YSZ sintered at 30MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature  a) b) 103   Figure 70: Densification of 5mol% erbia-YSZ sintered at 60MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature  a) b) 104   Figure 71: Densification of 10mol% erbia-YSZ sintered at 30MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature  a) b) 105    Figure 72: Densification of 10mol% erbia-YSZ sintered at 60MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature  a) b) 106   Figure 73: Densification of 15mol% erbia-YSZ sintered at 30MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature  a) b) 107   Figure 74: Densification of 15mol% erbia-YSZ sintered at 60MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature   a) b) 108  Appendix D: Effect of Erbia Concentration on Powder Densification  Figure 75: Densification of composites sintered at 1200°C and 30MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature a) b) 109   Figure 76: Densification of composites sintered at 1200°C and 60MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature a) b) 110   Figure 77: Densification of composites sintered at 1300°C and 30MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature a) b) 111   Figure 78: Densification of composites sintered at 1300°C and 60MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature  a) b) 112   Figure 79: Densification of composites sintered at 1400°C and 30MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature  a) b) 113   Figure 80: Densification of composites sintered at 1400°C and 60MPa with: a) specific gravity as a function of time and b) specific gravity as a function of temperature   a) b) 114  Appendix E: XRD Results for Precursor Powders and As-Sintered Composites  Figure 81: XRD plots for precursor powders containing: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol% erbia    C Cubic YSZ M Monoclinic YSZ E Erbia Intensity (A.U.) a) b) c) d) 115   Figure 82: XRD plots for composites sintered at 1200°C and 30MPa containing: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol% erbia   C Cubic YSZ E Erbia Intensity (A.U.) a) b) c) d) 116   Figure 83: XRD plots for composites sintered at 1200°C and 60MPa containing: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol% erbia   C Cubic YSZ E Erbia Intensity (A.U.) a) b) c) d) 117   Figure 84: XRD plots for composites sintered at 1300°C and 30MPa containing: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol% erbia   C Cubic YSZ     Intensity (A.U.) a) b) c) d) 118   Figure 85: XRD plots for composites sintered at 1300°C and 60MPa containing: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol% erbia   C Cubic YSZ     Intensity (A.U.) a) b) c) d) 119   Figure 86: XRD plots for composites sintered at 1400°C and 30MPa containing: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol% erbia   C Cubic YSZ     Intensity (A.U.) a) b) c) d) 120   Figure 87: XRD plots for composites sintered at 1400°C and 60MPa containing: a) 0mol%, b) 5mol%, c) 10mol% and d) 15mol% erbia  C Cubic YSZ     Intensity (A.U.) a) b) c) d) 

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