다공성 기능소재의 구조적 안정성 향상을 위한 성형 및 소결 공정 기술 개발 = Development of forming and sintering process technologies for enhancing the structural stability of porous ceramics

다국어 초록 (Multilingual Abstract)

Oxide-based bulk ceramics are fabricated through shaping and sintering, and the structure formed in shaping stage together with the microstructural evolution during sintering determines the final properties. Improving the performance of such materials relies on understanding the key phenomena at each stage and controlling the relevant processing parameters accordingly. This strategy has been widely adopted in ceramic research and industry, and the increasing focus on carbon neutrality and energy-efficient processing has intensified the development of materials and processes. In this study, the key microstructural formation behaviors in the shaping and sintering stages are examined independently. The shaping stage is treated in terms of structural formation and mechanical stability, whereas the sintering stage is considered with respect to densification and anisotropic shrinkage in freeze-cast porous alumina. Chapter 1 analyzes how changes in the binder composition affect the mechanical stability and pore structure of activated carbon pellets, and Chapter 2 shows the origin of anisotropic shrinkage during the sintering of freeze-cast alumina and presents processing strategies to control it. The abstracts of each chapter are provided below.
Activated carbon (AC) is widely used for wastewater treatment, and practical applications require bulk forms such as pellets with a well-developed porous structure and sufficient mechanical stability. Binder addition is essential for improving the stability of AC pellets, making both binder composition and binder content critical factors in bulk production. This study optimized a mixed-binder system using polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP). Binder compositions were evaluated at total binder contents of 10, 20, 30 wt.%, and the 10 wt.% condition showed the best performance. A PVA:PVP ratio of 3:7 at this content provided a 146% increase in compressive strength and superior resistance to agitation compared with commercial AC pellets. Additionally, the mixed binder generated a favorable hierarchical porosity, improving fluid permeability and indicating strong potential for filtration applications.
The origin of anisotropic shrinkage during the sintering of porous Al2O3 fabricated via freeze casting was investigated, and the influence of densification mechanisms on this anisotropy was experimentally analyzed. In-situ directional shrinkage behavior was monitored using a laser dilatometer during Conventional Sintering (CS). Starting at approximately 1396°C, shrinkage in the vertical direction began to exceed that in the horizontal direction by ~0.4%, eventually reaching the maximum anisotropy of ~1.45%. To reduce this anisotropy, Two-Step Sintering (TSS) was introduced, resulting in an 8-fold reduction compared to CS. To clarify the influence of densification mechanisms on anisotropic shrinkage, shrinkage equations were applied to identify orientation-dependent diffusion mechanisms at the initial stage, and temperature-dependent dominant mechanisms at the intermediate and final stages were compared between CS and TSS. These findings provide valuable insights into optimizing sintering processes to minimize anisotropic shrinkage in freeze-cast porous ceramics.

목차 (Table of Contents)

• Chapter 1 10
• I. 서론 10
• II. 문헌조사 14
• 1. 활성탄 14
• III. 실험방법 18
• 1. 분말준비 및 성형 18
• 2. 소결 21
• IV. 분석 22
• 1. 상대 밀도 (Relative Density) 22
• 2. 상 및 미세구조 (Microstructure) 22
• 3. 기계적 안정성 (Mechanical Stability) 23
• 4. 비표면적 및 기공 특성 분석 (Brunauer-Emmett-Teller Analysis) 25
• V. 결과 및 고찰 26
• 1. 바인더 조성 최적화 26
• 2. 열처리 33
• 3. 바인더 함량 최적화 37
• 4. 기계적 안정성 41
• 5. 기공 분포 및 형상 46
• VI. 결론 53
• VII. 참고문헌 56
• Chapter 2 62
• I. 서론 62
• II. 문헌조사 65
• 1. 다공성 세라믹 (Porous Ceramics) 65
• 2. 동결 주조 (Freeze-casting) 67
• 3. 수축 이방성 (Anisotropic Shrinkage) 69
• 4. TSS (Two-Step Sintering) 71
• III. 실험방법 73
• 1. 분말준비 및 성형 73
• 2. 소결 (Sintering) 75
• 1) Conventional Sintering 75
• 2) Two-Step Sintering 75
• IV. 분석 77
• 1. 미세구조 (Microstructure) 77
• 2. 수축률 77
• 3. 소결 초기 수축률 분석 78
• V. 결과 및 고찰 80
• 1. 미세구조 80
• 2. 수축 이방성 83
• 3. In-Situ Shrinkage 86
• 4. CS와 TSS의 열 이력 및 수축 이방성 93
• 5. 수축 이방성 메커니즘 규명 96
• VI. 결론 100
• VII. 참고문헌 102
• Abstract 109
• 감사의 글 111