臺灣博碩士論文加值系統

本論文以傳統燒結爐和二氧化碳雷射之無壓力燒結技術，研究非計量式組成（TiO2過賸與BaO過賸）之陶瓷與添加A-位置和B-位置施體（donor）或受體（acceptor）之微結構發展與燒結體之導電度。結晶
相分析使用X-ray繞射（XRD）。光學顯微鏡（OM），掃描式電子顯微鏡（SEM）與穿透式電子顯微鏡（TEM）則用於微結構之分析。
研究結果發現非計量式組成鈦酸鋇利用等加熱速率（constant-heatingrate）之燒結法並不適用於決定燒結機制之活化能，而以等溫無壓力（isothermal-pressureless）燒結法求TiO2過賸組成鈦酸鋇所得之活化能值也在302至522kJ/mol之間，由高解析度穿透式電子顯微鏡之晶格影像分析發現雖然燒結溫度在BaTiO3-Ba6TiO40之共晶溫度（1332oC）以下，仍可見有玻璃相存在於晶界之間，因此燒結之緻密化機制會隨燒結溫度之升高而由固相燒結逐漸改變成液相燒結，使活化能值隨之改變。微結構分析顯示在TiO2過賸組成鈦酸鋇燒結體中均可發現BaO-TiO2系中之第二相Ba6Ti17O40，此相由微結構之特徵發現當燒結溫度低於共晶溫度時，即由粉體中之過賸TiO2 與BaTiO3經由固態反應生成共晶液相，此共晶液相在冷卻過程中凝固時，Ba6Ti17O40成核於BaTiO3晶粒表面上並有明顯之晶向關係，同時與其他TiO2過賸相長成層狀結構。若經長時間退火(annealing)，則層狀特徵消失，其它TiO2過賸相則再反應而生成Ba6Ti17O40相，實驗結果並顯示Ba6Ti17O40為1100oC以下最穩定之第二相。
施體添加於鈦酸鋇陶瓷之微結構觀察顯現，在添加施體La2O3與Y2O3之鈦酸鋇陶瓷燒結體中，分別發現第二相La2Ti2O7 和-Y2Ti2O7，因此當施體添加量高於GGIT(grain growth inhibition threshold) 值抑制晶粒成長之主要機制除因生成陽離子空缺而提高（緻密化/晶粒粗化）之比值外，也由於第二相之pinning作用。同時這些抑制晶粒成長機制在共晶液相出現後即失去其作用。由晶格常數、導電度與晶粒大小發現Y2O3添加於 TiO2過賸組成時，Y3+取代Ba2+位置而為施體之角色，但當添加於BaO過賸組成時， 則Y3+會自取代Ba2+位置逐漸轉換為取代Ti4+位置，當添加量達到0.65mol%時便完全轉換成受體之作用。二氧化碳雷射之鈦酸鋇陶瓷燒結體之導電度結果顯示添加施體之電荷補償機制與溫度有關，並可分成空缺補償混合補償與電子補償三個區域。
受體添加於鈦酸鋇陶瓷之微結構分析發現，MgTiO3在於添加MgO之鈦酸鋇陶瓷體之晶界上，而添加Na2O時，則出現Na2TiO4與Na2Ti2O9相。在BaTiO3晶粒中之Na2TiO4相由其SADP得到其晶向關係為[110]BT//[100]N4T，(111)BT//(010)N4T，(112)BT//(001)N4T。因為氧空缺的形成，受體添加於鈦酸鋇粉體中使高溫相h-BaTiO3以介穩態而保留至室溫。

Pressureless-sintering of non-stoichiometric barium titanate (BaTiO3) powder of TiO2- and BaO-excess compositions has been investigated using both conventional furnace and C02-laser. Both donor- and acceptor-doping are studied for their effect on sintering kinetics, microstructure and the resultant semiconductivity of the sintered ceramic. Crystalline phases are analysed by X-ray diffractometry (XRD). Attention has been paid to the analysis of the corresponding sintered microstructure by adopting optical microscopy (OM), scanning, and transmission electron microscopy (SEM and TEM).
Constant-heating rate (CHR) sintering technique appears unsuitable for determining the activation enthalpy (AH) of densification in BaTiO3. A considerable span of DH = 392 - 522 kJ-mol-1 is also found from isothermal pressureless-sintering of TiO2-excess compositions. High-resolution TEM reveals that SiO2-containing glass formed due to the trace impurities associated with the initial powders at T <1332oC (lowest eutectic temperature of BaTiO3-Ba6Til7O40) is responsible for the scattering of AH values. Its formation has changed the sintering mechanism from solid-state to liquid phase.
Second-phase of Ba6Til7O40 has been identified ambiguously in sintered TiO2-excess compositions. It is formed at low temperatures (of <1332oC) by reacting the excessive TiO2 or exsolved TiO2 with BaTiO3 in the solid-state upon heating. Characteristic microstructural feature of the solid state-reacted Ba6Til7O40 has been identified. Eutectic liquid is subsequently generated at T>1332oC from BaTiO3-Ba6Til7O40- Upon cooling, the eutectic liquid solidifies again to Ba6Til7O40 which nucleating on BaTiO3 grain surface to develop characteristic lamellar structure and crystallographic orientation relationships. The lamellar microstructural characteristic disappears upon long-term annealing of laser-sintered discs when polytitanates have also homogenized to become Ba6Til7O40. It is evdient that Ba6Til7O40 is the most stable polytitanate phase at room temperature.
"Third-phases" of La2Ti2O7 and Y2Ti2O7 have also been identified by selected-area diffraction patterns (SADP) in La2O3- and Y203-doped compositions, respectively. It is indicative that for donor-concentration exceeding the grain-growth inhibition threshold (GGIT), apart from the increased densification/coarsening (dr/dG) ratio, grain growth is also hindered by a second-phase pinning mechanism. The effect of these inhibition mechanisms is largely diminished once when eutectic liquid of BaTiO3-Ba6Til7O40 is formed. De-sintering associated with eutectic liquid formation is deferred to higher temperatures of T >13320C since the eutectic compositions have been altered to BaTiO3-Ba6Til7O40-(third-phase). Electrical conductivity measurement also suggests that the substitution of A- site byy3+-cation occurs at low Y203-doping levels, but progressively changes to B-site at doping levels exceeding 0.65mol%. Charge compensation mechanism depending upon doping level and temperature can be divided into three schemes of, (1) vacancy, (2) mixed, and (3) electron regime as suggested by semiconductivity of quenched samples from laser-sintering.
Third-phases of MgTiO3, and Na2TiO4 and Na2Ti2O9 have also been identified in MgO- and Na2O-acceptor-doped compositions, respectively. Crystallographic orientation relationships Of [110]BT//[100]N4T, (11l)BT/ / (010)N4T, (112)BT/ / (001)N4T are also determined for Na2TiO4 located intragranularly in BaTiO3 grains. It is also indicative that oxygen vacancies generated by acceptor-doping has resulted to the metastable retention of high temperature hexagonal-BaTiO3 to room temperature.

Page
Abstract 1
Contents v
List of Tables xiv
List of Figures xvi
Chapter 1 Introduction 1
1.1 obective Of research 5
Chapter 2 Review of relevant literature 9
2.1 Crystal structure of BaTiO3 9
2.2 Polymorphic phase transformations in BaTiO3 16
2.2.1 Lattice dynamic of theory - the soft modes 16
2.2.2 Phenomenological observations 20
2.2.3 Origin of ferroelectiicity 24
2.2.4 order of phase transformation 29
2.3 Equilibrium phase diagram of the BaO-TiO2 system 35
2.3.1 Polytitanate phases 37
2.4 Defect chemistry of BaTiO3 46
2.4.1 Intrinsic defects 46
2.4.2 Extrinsic defects 48
2.4.2.1 Donor-doped compositions 49
2.4.2.2 Recent modifications 68
2.4.3 Defect reactions for donor-doping 72
2.4.4 Defect reactions for acceptor-doping 76
2.4.5 Principal charge compensating mechanism upon
donor-doping and the rate-determining species of sintering 78
2.4.6 Defect equilibrium diagram - the Brouwer diagram 81
2.5 Sintering of BaTiO3 ceramics 86
2.5.1 Fundamentals of sintering 86
2.5.1.1 Thermodynamic driving force for sintering 87
2.5.1.2 Sintering kinetic equations 89
2.5.2 Fast-firing technique 92
2.5.2.1 Fast-firing of BaTiO3 96
2.6 Activation enthalpy 96
2.6.1 Derivation of DH from Arrhenius plot 97
2.6.2 Sintering kinetics of BaTiO3 99
2.6.3 Grain growth -coarsening 101
2.6.3.1 Pore-boundary interaction 106
2.6.3.2 Controlled grain growth 110
2.6.4 Grain-growth inhibition threshold (GGlT) 113
2.7 The (111) twins and faceted planes 116
2.7.1 Wuff theorem and the g-plot 119
2.8 Ferroelectiic domains and twinning 131
2.8.1 Types of ferroelectric domains 132
2.8.2 Domain boundary fringes 133
2.8.2.1 a-boundaries 138
2.8.2.2 Freidel''s law 144
2.8.2.3 a-boundaries 145
2.8.3 Observations of domains and domain boundaries 147
2.8.3.1 Domains and boundaries of the 90o-type 149
2.8.3.2 Domains and boundaries of the 180o-type 153
2.8.4 Thermodynamics 155
2.8.5 Experimental observations of domain size 156
Chapter 3 Experimental procedures 157
3.1 Starting powders 157
3.2 Sample preparation 159
3.2.1 Pressureless sintering 163
3.2.2 Fast firing by using CO2 laser 164
3.3 Sintering kinetics 166
3.3.1 Density measurement 166
3.3.2 Sintering kinetic curves 167
3.4 X-ray diffractometry 167
3.5 Semiconductivity measurement 167
3.6 Microstructure observations 167
3.6.1 Scanning electron microscopy 168
3.6.2 Grain size determination 168
3.63 Transmission electron microscopy 169
Chapter 4 Experimental results (I) - Undoped compositions 171
4.1 Pressureless-sintering in convention at furnace 171
4.1.1 Sintering kinetics for undoped TiO2-excess 0.997 powders 171
4.1.1.1 Isothermal sintering 171
4.1.1.2 Constant heating-rate sintering 175
4.1.2 Microstructural development of sintered TiO2-excess composition 178
4.1.2.1 Nucleation density of the plate-like grains 186
4.1.2.2 Liquid-phase- sintering microstructure 189
4.1.23 Lattice-fringe imaging of the BaTiO3- Ba6Ti17O40 grain-boundary 199
4.1.3 Faceting in sintered TiO2-excess composition and the g-plot 214
4.1.4 Grain-boundary phases 219
4.1.4.1 Coherent grain-boundary 230
4.1.4.2 Orientation relationships between BaTiO3 and Ba6Til7O40 236
4.1.43 Planar defects in triple-grain junctions 242
4.1.5 Sintering kinetics for undoped BaO-excess 1.013 powders 253
4.1.6 Microstructuraf development of sintered BaO-exces composition 253
4.1.6.1 Second-phases 259
4.2 Activation enthalpy 273
4.2.1Activation enthalpy for conventional pressureless-sintering 273
4.3 Fast firing using CO2 laser 276
4.3.1 Sintering behaviour 276
4.3.2 Sintered density reduction 276
4.3.3 Microstructural analysis of laser-sintered BaTiO3 284
4.3.3.1 TiO2-excess composition 284
4.3.3.2 BaO-excess composition 289
4.4 Microstructural development and heating-rate fo TiO2-excess composition 291
4.5 Ferroelectric domains 300
4.5.1 Observations with optical and scanning electron microscopy 300
4.5.2 Observations with transmission electron microscopy 300
4.5.2.1 Unconventional domain boundaries 303
4.5.3 Determination of displacement vectors 314
Chapter 5 Experimental results (II) - Donor-doped compositions 319
5.1 TiO2-excess 0.997 BaTiO3 composition 319
5.1.1 Sintering behaviour of doped compositions for A-site donor-oxides 319
5.1.2 Sintered microstructures 326
5.1.2.1 La2O3-doped samples 326
5.1.2.2 Y2O3-dopedsampla 341
5.1.23 Grain growth inhibition threshold 357
5.1.2.4 Microstructure at de-sintering temperature 358
5.2 B-site donor-oxides 358
5.3 Determination of lattice constants 372
5.3.1 La2O3-doping 372
53.2 Y2O3-doping 377
5.4 Semiconductivity 377
5.5 BaO-excess 1.013 BaTiO3 composition 394
5.5.1Sintering behaviour and microstructure of theA-site-donor-doped
compositions 394
5.5.1.1Grain growth inhibition and the conductivity
of Y2O3-doped compositions 410
5.5.1.2 Lattice constants of Y2O3-doped compositions 416
5.5.2 Sintering behaviour and microstructure of the
B-site-donor-doped compositions 416
5.5.2-1 Grain growth inhibition by B-site donor-oxides 425
5.6 Estimation of activation enthalpy 432
Chapter 6 Expetimental results (III) - Acceptor-doped compositions 441
6.1 TiO2-excess 0.997 BaTtO3 compositions 441
6.1.1 Microstructure observations of MgO-doped TiO2-excess samples 448
6.1.2 Microstructural observations of Na2O-doped TiO2-excess samples 475
6.2 BaO-excess 1.013 BaTiO3 compositions 487
6.2.1 Microstructure observations of MgO-doped BaO-excess compositions 492
Chapter 7 Discussion Of result 502
7.1 Density reduction - de-sintering 502
7.1.1 Origin of CO2 gas 503
7.1.2 Entrapped of CO2gas in liquid phase 504
7.13 Doped TiO2-excess compositions 508
7.1.4 Summary 511
7.2 Polytitanates in sintered BaTiO3 ceramics 511
7.2.1 Polytitanates and their formation by solid-state reaction 512
7.2.2 Polytitanates and eutectic solidification 518
7.2-3 Change of the lowest eutectic temperature in BaO-TiO2 system 532
7.2.4 Retention of hexagonaf-BaTiO3 and polytitanate second-phases from
eutectic solidification 533
7.2.5 Crystallographic orientation relationships of h- and c-BaTiO3 540
7.2.6 Summary of microstructure development in triple-grain junctions 540
7.2.7 Diffraction patterns of second-phase Ba6Ti17O40 and other polytitanates 543
7.2.8 Summary 545
7.3 Second-phases in sintered BaO-excess compositions 546
7.4 Liquid-phase-sintering in TiO2-excess compositions 547
7.5 Plate-like (111) twin grains in TiO2-excess compositions 551
7.5.1 Growth of the (111) twin grains 553
7.5.2 Evaporation-condensation mechanism 557
7.5.3 Summary 558
7.6 Rate-determining mechanism for the sintering of BaTiO3 compositions 559
7.6.1 Undoped compositions 559
7.6.2 Doped compositions 562
7.6.3 Principal charge compensation mechanism and room-temperature
conductivity 563
7.6.3.1 Donor-oxide La2O3 in TiO2-excess compositions 563
7.6.3.2 Free energy diagram for charge compensation defects 575
7.6.3.3 Donor-oxidde Y2O3 in TiO2-excess compositions 578
7.6.4 Addition with Y2O3-donor oxide in BaO-excess 1.013 compositions 581
7.6.5 Brouwer defect equilibrium diagram - interpretationOf conductivity and
grain-growth inhibition 587
7.7 Rate-determining mechanism for densification:species and step 593
7.8 Acceptor-doping and the retention of high-temperature polymorphs 596
7.8.1 Grain growth inhibition in acceptor-doped compositions 599
Chapter 8 Conclusions 601
Chapter 9 Suggestions to future work 604
Appendices 607
Appendix A-1 607
Appendix A-2 612
Appendix A-3 613
References
614
Biographical Sketch 635

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