冷凍鑄造法已被用於製造陶瓷多孔材料許久，其所合成的多孔材料具備互相連通的開放性孔洞結構以及擁有高度的異向性，這樣的結構已被證實可大幅降低材料的熱傳導性並用於隔熱領域。然而，過高的孔隙率會導致材料結構不穩定和增加裂縫形成的可能性，使得機械強度降低、材料功能性受限，因此透過傳統冷凍鑄造法製造的多孔材料存在孔隙率的上限。本研究中結合了溶膠-凝膠法和冷凍鑄造法，以不同的實驗參數合成高孔隙之氧化鋁多孔材料。利用異丙醇鋁作為溶膠凝膠法反應的前驅物，在水解後以不同的固含量配製成冷凍鑄造過程中使用的漿料。漿料在不同的冷卻速率下固化後，透過冷凍乾燥以去除冰晶，接著熱處理以催化凝膠化過程和增強機械性能，最終成功製造的氧化鋁多孔材料具有低密度（124.0-242.9 kg/m3）、低相對密度（0.0314-0.0615）和超高孔隙率（91.6-96.2%）。掃描式電子顯微鏡被用來觀察異向性多孔的板狀結構，著重於不同冷卻速率和固含量造成的結構變化。此外，在板狀結構表面上觀察到了大小介於87.7 nm到201.7 nm的奈米級孔洞，使此氧化鋁材料具備雙階層孔洞結構。相比傳統冷凍鑄造法，結合溶膠凝膠法合成出的超輕量氧化鋁多孔材料具有更高的比表面積，並表現出與緻密氧化鋁塊材相當的比壓縮強度(約560 kPa∙m³∙kg-1）。其獨特的各向異性結構、高孔隙率使得氧化鋁多孔材料具有低至0.2 W∙m-1∙K-1的熱傳導係數，並在熱傳導性上展現高度異向性，搭配穩定的機械強度使其有成為良好隔熱材料的潛力。此外，透過結合溶膠凝膠法和冷凍鑄造法，除了氧化鋁材料，也可合成具有不同功能性的陶瓷、玻璃多孔材料。

Ceramic-based scaffolds developed by the freeze-casting method exhibit anisotropic lamellar and interconnected porous structure and have been applied to thermal insulation in many research. However, the upper limit of porosity exists in the scaffolds fabricated by typical freeze-casting process since extremely high porosity can lead to structural instability and cracks formation, reducing mechanical strength and restricting functionalities of scaffolds. In this study, the sol-gel/freeze-casting hybrid method was developed to fabricate the highly porous alumina scaffolds with different processing parameters. The slurry used in freeze-casting process was prepared by the hydrolysis of the precursor, aluminum isopropoxide with different solid content. After the slurry was solidified, it underwent freeze drying and heat treatment to remove ice crystals, catalyze gelation process, and enhance mechanical properties. The successfully-synthesized alumina scaffolds were equipped with low bulk density (124.0-242.9 kg/m3), low relative density (0.0314-0.0615), and ultra-high porosity (91.6-96.2 %). The anisotropic porous lamellar structures were evaluated by SEM and the structural change with different cooling rates and solid content could be discerned. Besides, the nano-scale pores ranging from 87.7 nm to 201.7 nm were observed on the lamellae surface, forming the dual-scale porous structure inside the scaffolds. These ultra-lightweight porous alumina scaffolds fabricated by this hybrid method showed higher specific surface area compared to those produced by typical freeze-casting process and demonstrated remarkable specific compressive strength (about 560 kPa∙m3∙kg-1) in comparison with dense alumina bulk. The unique anisotropic structure, high porosity and stable mechanical properties enable the alumina scaffolds to deliver a low thermal conductivity of 0.2 W∙m-1∙K-1 and show high anisotropy in thermal properties, possessing great potential for thermal insulative materials in the future. Also, this hybrid sol-gel/freeze-casting approach can be extended to ceramic/glass scaffolds with varying functionalities.

中文摘要 IV
Abstract V
致謝 VII
Contents IX
List of Tables XII
List of Figures XIII

Chapter 1. Introduction 1
1.1 Backgrounds 1
1.2 Motivations 3

Chapter 2. Literature Review 5
2.1 Lightweight materials 5
2.1.1 Lightweight materials and structures in nature 5
2.1.2 Artificial lightweight materials and structural design 7
2.2 Introduction of Porous Ceramics 15
2.2.1 Synthesis strategies 15
2.2.2 Microporous, mesoporous and macroporous ceramics 19
2.2.3 Porous ceramics applied to thermal insulation 20
2.3 Freeze Casting 25
2.3.1 Principles of freeze casting process 25
2.3.2 Conventional Processing Procedures 28
2.3.3 Intrinsic structural control 31
2.3.4 Extrinsic structural control 33
2.4 Sol-gel method 38
2.4.1 Principles and Chemical Reaction 38
2.4.2 Effect of parameters 40
2.4.3 Synthesis of aluminum oxide by sol-gel method 42

Chapter 3. Experimental Methods 48
3.1 Fabrication of Alumina Scaffolds 48
3.1.1 Sol-gel Synthesis Procedure 48
3.1.2 Slurry Preparation 49
3.1.3 Freezing Casting 49
3.1.4 Sublimation of Ice-template and Heat Treatment 50
3.2 Intermediate Product Identification and Phase Analysis 54
3.2.1 Intermediate product identification 54
3.2.2 Phase analysis 54
3.3 Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) 55
3.4 Dimension Measurement and Porosity Analysis 57
3.4.1 Dimensional change of scaffolds 57
3.4.2 Calculated porosity and density 57
3.4.3 Experimental porosity and density 58
3.5 Structural Characterization 60
3.6 Pore Size Distribution 61
3.7 Compressive Mechanical Testing 61
3.8 Thermal Properties 63
3.8.1 Thermal Conductivity Measurements 63
3.8.2 Infrared (IR) Thermography 63

Chapter 4. Results and Discussion 67
4.1 Synthesis of alumina scaffold 67
4.1.1 Sol-gel process 67
4.1.2 Slurry preparation and freeze casting process 68
4.1.3 Effect of heat treatment and phase analysis 70
4.2 Physical properties of alumina scaffolds 77
4.3 Morphology and microstructural characterization 84
4.3.1 Two-dimensional structural characterization and pore size distribution 84
4.3.2 Three-dimensional structural characterization 88
4.4 Compression Test Results of Alumina Scaffolds 102
4.5 Thermal Insulation 108
4.5.1 Measurement of thermal conductivities 108
4.5.2 Effect of measurement direction on the infrared imaging result 112
4.5.3 Effect of solid content on the infrared imaging result 113
4.5.4 Effect of cooling rate on the infrared imaging result 114

Chapter 5. Conclusions 123

Chapter 6. Future Work 130
6.1 Extension of material selections 130
6.2 The infiltration of polymer to fabricate composite scaffolds 132

References 133

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