本文第一部分，將用氧化鈷(Co1-xO)和氧化鎂(MgO)粉末分別以9:1和1:9的莫耳比混和，在1600oC熱處理5小時及接續的低溫長時間時效處理，分別生成缺陷集合(defect cluster)和析出物。在氧化鎂植入鈷離子的鹽岩結構內，並沒有順晶(paracrystal)出現，而有規則的波浪狀影像及其它複雜的缺陷。而在氧化鈷植入鎂離子的鹽岩結構內也沒有順晶的出現，但尖晶石(spinel)結構內有順晶形成。尖晶石顆粒同時溶入二價鈷離子、三價鈷離子與二價鎂離子，因此出現體積補償及電荷補償效應，造成點缺陷缺陷集合，並在內部形成缺陷聚集的順晶分佈，其週期為尖晶石晶胞參數之4.5倍，比起純氧化鈷尖晶石中約晶胞參數5倍距離之順晶週期小，顯見植入二價鎂離子使尖晶石缺陷濃度增加。在鹽岩結構與尖晶石界面發現有平行{111}平面的缺陷，此現象意味陽離子空缺可能沿著氧原子最密堆積的(111)平面聚集，並且缺陷集合之型式可能與閃鋅礦型結構類似。

第二部分，用氧化鎂和二氧化鈦(TiO2)以9：1莫爾比例混合，在1600oC反應燒結成固溶程度低之MgO/Mg2TiO4陶瓷複合材料，並且進一步在900oC退火使之析出整合良好之G.P. zone及奈米結晶。結果發現四價的鈦離子在冷卻過程中從氧化鎂基材排出，並藉由燒結應力沿著特定 (001)面產生版狀的G.P. zone，緊接著析出Mg2TiO4尖晶石。至於晶粒間微米級Mg2TiO4尖晶石顆粒則沿著 方向呈現暈狀繞射，可歸因於短程序化亦或分解初期所產生的現象。

第三部分，用氧化鈷和二氧化鈦以9：1莫爾比例混合，在1450oC反應燒結成Co1-xO/Co2TiO4陶瓷複合材料，並且利用X光繞射與分析式電子顯微鏡觀察微觀組織與形成奈米Co2TiO4尖晶石顆粒的機制。結果亦發現四價的鈦離子在冷卻過程中從Co1-xO基材排出，導致奈米Co2TiO4尖晶石析出，但是未發現G.P. zone。而由反應燒結形成的Co2TiO4尖晶石顆粒，能夠脫離Co1-xO晶界，並且藉由布朗運動與Co1-xO基材達到平行磊晶關係。至於由鹽岩結構氧化成尖晶石結構的氧化鈷，常在試片表面，或Co1-xO/Co2TiO4的界面處形成。

第四部分，將含二氧化鋯(ZrO2)八莫爾百分比的二氧化鈦中，於大氣爐1600oC燒結24小時，及接續在900oC時效處理2至200小時。結果顯示含二氧化鋯的二氧化鈦具有金紅石結構，且在此基材中發現G.P. zone、超晶格與公度(commensurate)和非公度(incommensurate)超結構，這些現象可能是ZrTi2O6(斯里蘭卡石)析出的先驅物。推敲原因，可能是由於四價鋯離子取代四價鈦離子造成體積補償作用，而產生缺陷集合，進而造成鋯離子與鈦離子的序化與聚集形成公度和非公度超結構，並在金紅石的繞射圖形呈現沿著[001]方向的暈狀繞射。

第五部分，加入不同量的二氧化鋯(20莫爾百分比)於二氧化鈦中，在1600oC反應燒結成金紅石與具有a- PbO2結構的ZrTiO4陶瓷複合材料。結果顯示大約有15莫爾百分比的二氧化鋯溶入金紅石中，且微米級ZrTiO4顆粒可在金紅石晶粒內轉至 [010]a//[011]r ; (001)a // (011)r (即 [100]a // [100]r; (001)a // (011)r)之磊晶關係，且具(101)a/(211)r晶癖面。顆粒在基材晶粒中調整晶向關係的現象，可以用顆粒的布朗運動來解釋，由界面能量低谷驅動，並受Zr-Ti-O之相互擴散與固溶的影響，以致於界面斷鍵，並啟動顆粒旋轉。當在900oC退火50小時後，在基材ZrTiO4內形成暫穩定態的週整結構，可能為在900oC穩定的斯里蘭卡石之先驅物。

In part I, MgO and Co1-xO powders in 9:1 and 1:9 molar ratio (denoted as M9C1 and M1C9 respectively) were sintered and homogenized at 1600oC followed by annealing at 850 and 800oC, respectively to form defect clusters and precipitates. Analytical electron microscopic (AEM) observations indicated the protoxide remained as rock salt structure with complicated planar diffraction contrast for M9C1 sample, however with spinel paracrystal precipitated from the M1C9 sample due to the assembly of charge- and volume-compensating defects of the 4:1 type, i.e. four octahedral vacant sites surrounding one Co3+-filled tetrahedral interstitial site. The spacing of such defect clusters is 4.5 times the lattice spacing of the average spinel structure of Mg-doped Co3-dO4, indicating a higher defect cluster concentration than undoped Co3-dO4. The {111} faulting of Mg-doped Co3-dO4/Co1-xO in the annealed M1C9 sample implies the possible presence of zinc blend-type defect clusters with cation vacancies assembled along oxygen close packed (111) plane.

In part II, the Mg2TiO4/MgO composites prepared by reactive sintering MgO and TiO2 powders (9:1 molar ratio) at 1600oC and then air-cooled or further aged at 900oC were studied by X-ray diffraction and (AEM) in order to characterize the microstructures and formation mechanism of nanosized Mg2TiO4 spinel precipitated from Ti-doped MgO. Expulsion of Ti4+ during cooling caused the formation of (001)-specific G.P. zone under the influence of thermal/sintering stress and then the spinel precipitates, which were about 30 nm in size and nearly spherical with {111} and {100} facets to minimize coherency strain energy and surface energy. Secondary nano-size spinel was precipitated and became site saturated during aging at 900oC, leaving a precipitate free zone at the grain boundaries of Ti-doped MgO. The intergranular spinel became progressively Ti-richer upon aging 900oC and showed <110>-specific diffuse scatter intensity likely due to short range ordering and/or onset decomposition.

In part III, the Co1-xO/Co2TiO4 composite prepared by reactive sintering CoO and TiO2 powders (9:1 molar ratio) at 1450oC and then air-cooled were studied by X-ray diffraction and AEM in order to characterize the microstructures and formation mechanism of nanosized Co2TiO4 spinel precipitated from Ti-doped Co1-xO. Slight expulsion of Ti4+ during cooling caused the precipitation of nanosize Co2TiO4 spinel. Bulk site saturation also caused impingement of the Co2TiO4 precipitates upon growth. The Co3-dO4 spinel, as an oxidatin product of Co1-xO, was found to form at free surface and the Co1-xO/Co2TiO4 interface. The Co2TiO4 spinel particles formed by reactive sintering rather than precipitation were able to detach from the Co1-xO grain boundaries to reach parallel epitaxial orientation with respect to the host Co1-xO grains via Brownian-type rotation of the embedded particles.

In part IV, AEM was used to study the defect microstructures of Zr-dissolved TiO2 prepared via reactive sintering the ZrO2 and TiO2 powders (8:92 in molar ratio, designated as Z8T92) at 1600oC for 24 h and then aged at 900oC for 2-200 h in air. The Zr-dissolved TiO2 with rutile structure showed dislocation arrays, defect clusters, G.P. zone, superlattice, nanometer-size domains incommensurate and commensurate superstructure, may be the precursor of ZrTi2O6 precipitates at 900oC. The rutile showed diffuse diffractions along [001] direction as a result of Zr4+ substitution for Ti4+ with volume compensating defect clusters. Incommensurate and commensurate structures, as indicated by diffraction splitting and extra diffraction along <100> and <010> directions may be attributed to the ordering and clustering process of Zr and Ti atoms in these directions.

Part V, deals with the reactive sintering of ZrO2 and TiO2 powders (1:4 molar ratio) at 1400 to 1600oC in air to form orthorhombic ZrTiO4 (a-PbO2-type structure, denoted as a) and to study its epitaxial reorientation in the matrix of tetragonal TiO2 (rutile) grains with Zr4+ (15 mol %) dissolution. The epitaxial relationship of intragranular ZrTiO4 and Zr-dissolved rutile (denoted as r) was determined by electron diffraction as [010]a//[011]r; (001)a // (011)r (i.e. [100]a // [100]r; (001)a // (011)r). The reorientation of the intragranular particles in the composites can be reasonably explained by rotation of the nonepitaxial particles above a critical temperature (T/Tm > 0.8) and below a critical particle size for anchorage release at interface with respect to the host grain. Reactive sintering facilitated the reoreientation process for the particles about to detach from the grain boundaries. The Brownian rotation of the confined ZrTiO4 particles in rutile grains was activated by a beneficial lower interfacial energy for the epitaxial relationship, typically forming lath-like ZrTiO4 with (101)a/(211)r habit plane having fair match of oxygen atoms at the interface. Further aging at 900oC for 50 h in air caused modulated and periodic antiphase domains in ZrTiO4 matrix, as likely precursor of equilibrium ZrTi2O6.

Contents

Abstract i
Contents iv
List of Tables vii
List of Figures viii
List of Appendixes xix

Research Background 1
1. Paracrystals in transition metal monoxides 1
2. Precipitation of equilibrium phase and metastable G.P. zones 3
3. Brownian motion/rotation of the confined particles in the solid state 5

Part I
Defect clusters and precipitation/oxidation of MgO-Co1-xO solid solution 7
1. Introduction 7
2. Experimental 8
3. Results 9
4. Discussion 11
4.1 Defect chemistry 11
4.2. Heterogeneous nucleation of defect clusters and spinel 14
4.3. Implications of spacing between defect clusters 14
5. Conclusions 15

Part II
On the precipitation of coherent spinel nanoparticles in Ti-doped MgO 31
1. Introduction 31
2. Experimental 33
3. Results 33
4. Discussion 37
4.1. Defect chemistry 37
4.2. G.P. zone 39
4.3. Controlled nucleation and growth of nano-Mg2TiO4 precipitate 39
4.4. Defects of intergranular Mg2TiO4 spinel 40
5. Conclusions 41

Part III
Spinel formation in CoO-TiO2 system by reactive sintering, exsolution and oxidation 55
1. Introduction 55
2. Experimental Procedures 55
3. Results 57
4. Discussion 59
4.1 Ti4+-Doped Co1-xO 59
4.2 Intragranular Co2TiO4 spinel particles 62
5. Conclusions 64

Part IV
Defect and Microstructures of Zr-dissolved TiO2 System 89
1. Introduction 89
2. Experimental Procedures 90
3. Results 91
4. Discussion 93
4.1 Solid solubility and metastable exsolution of Zr-dissolved rutile 93
4.2 Thermal and surface effects on the Ti3+/Ti4+ ratio of the TiO2-x polycrystals 93
4.3 Zr4+ dissolved rutile induced defect clusters 95
5. Conclusions 96

Part V
On the reorientation of ZrTiO4 particles during reactive sintering of TiO2-ZrO2 116
1. Introduction 116
2. Experimental 118
3. Results 118
4. Discussion 121
4.1. Interdiffusion-induced nucleation of ZrTiO4 121
4.2. Microstructural development of ZrTiO4/rutile composite 121
4.3. Reorientation of embedded nonepitaxial ZrTiO4 particles in the composites 123
4.4. Energetics of epitaxial relationships for ZrTiO4 /rutile 125
4.5. PAPB’s and superlattice of ZrTiO4 126
5. Conclusions 127

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