孔洞材料具有著質量輕、密度低及高孔隙率等優點，目前常廣泛應用於各種不同領域。自然界的多孔材料、如羽毛、鳥喙及骨骼等，皆具有多階層孔洞結構之輕量化設計。其結構由不同尺度之孔洞有序排列而構成，搭配蛋白質等具高韌性之天然有機物，進而達到高孔隙率、輕量化、高強度、高韌性等特性。
本研究主要目標為觀察骨頭、鳥羽等天然多階層孔洞結構，以其做為啟發與設計靈感，以具有奈米孔洞之矽藻土(Fossil Shell Flour)為原料，利用冷凍鑄造法(Freeze Casting)合成多孔材料，再加入高分子合成強化 (聚乙烯醇(PVA))，製造出多功能、質量輕且具有多階層結構的仿生孔洞複合材料。掃描式電子顯微鏡(SEM)觀察證實利用上述冷凍鑄造技術與矽藻土陶瓷原料，可確實且有再現性地製造出具有多階層孔洞結構之輕量化陶瓷材料。而在機械性質方面，利用壓縮應力測試法測量並與同濃度之二氧化矽實心粉末鑄造之成品比較，發現兩者之最大壓縮應力與彈性係數相近，矽藻土成品則具有較佳的破裂韌性。此外，添加高分子強化，能增強陶瓷機多孔材料之機械性質，有效地提升強度與韌性。藉由調控不同參數，如降溫速率、漿料濃度、燒結溫度等，也可控制並改良其結構與機械性質。本研究亦量測仿生多孔複合材料之吸水、保水特性及熱阻抗，期望未來能以此多階層孔洞材料做為基礎，搭配各種合成技術，應用於各領域。

Porous composites are widely applied in various fields due to their lightweight, high surface/volume ratio and energy absorbing ability. Natural porous composites, such as bird feathers, beaks and bones, possess lightweight and superior mechanical properties due to their anisotropic, hierarchically porous structure. In this study, we synthesized ultra-lightweight, multifunctional scaffolds mimicking natural porous composites by the freeze casting technique utilizing diatomaceous earth (DE) as raw materials. By controlling the cooling rate, sintering condition and the formation of ice dendritic structures, scaffolds with aligned micro-scaled channels and nano-scaled pores were synthesized. Structural characterization was carried out by SEM and the compositional analysis was identified by XRD. Mechanical properties of DE scaffolds were measured by compressive tests and compared with silica scaffolds. Results showed that two scaffolds had similar ultimate compressive stress and elastic modulus yet DE scaffolds have better toughness and deformability. DE scaffolds were further infiltrated by polymer (PVA) to improve their mechanical properties. The water adsorption, water retention and thermal conductivity of DE scaffolds were also evaluated. The bio-inspired, hierarchically porous scaffolds can be further modified and applied in different fields.

List of Tables--v

List of Figures--vi

Chapter 1. Introduction--1

1.1 Background--1
1.2 Motivations and Goals--4

Chapter 2. Literature Review--7

2.1 Avian Feather--7
2.1.1 Morphology and Structure--7
2.1.2 Hierarchical Porous Structure--9

2.2 Bone--10
2.2.1 Hierarchical Structure--10
2.2.2 Mechanical Properties--12
2.2.3 Toughening Mechanisms--14

2.3 Diatomaceous Earth--15
2.3.1 Introduction--15
2.3.2 Classification and Morphology--16
2.3.3 Characteristics and Potential Applications--18

2.4 Freeze Casting--19
2.4.1 Theories of Freeze Casting--20
2.4.2 Experimental Procedures--23
2.4.3 Recent Developments of Freeze Casting--25

2.5 Fabrication of Porous Materials--27
2.5.1 Replica Technique--27
2.5.2 Sacrificial Template Method--28
2.5.3 Direct Foaming Methods--29

Chapter 3. Experimental Methods--45

3.1 Synthesis of Scaffolds--45
3.1.1 D.E. Scaffold Preparation--45
3.1.2 Polymer Infiltration--47
3.1.3 Purified D.E. & SiO2 Scaffold Preparation--48

3.2 Characterizations and Measurements--49
3.2.1 Structural Characterization and Elemental Analysis--49
3.2.2 Compressive Mechanical Testing--50
3.2.3 Water Absorption--50
3.2.4 Thermal Conductivity Measurement--51

Chapter 4. Results and Discussion--54

4.1 Freeze-Casted D.E. Scaffolds--54
4.1.1 Optimization--54
4.1.2 Effect of Solid Loading and Sintering Temperature--58

4.2 Comparisons in Mechanical Properties--63
4.2.1 Comparison with Silica Scaffolds--63
4.2.2 Comparison with Purified D.E. Scaffolds--64

4.3 Polymer Infiltration--66
4.3. Mechanical Properties--66

4.4 Functionality Measurement--67
4.4.1 Water Absorption--67
4.4.2 Water Retention--68
4.4.3 Thermal Conductivity Analysis--69

Chapter 5.--Conclusions--104

REFERENCES--106

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