소결 변수 제어를 통한 산화물계 벌크 세라믹의 기계적 특성 향상 = Sintering Strategies for Improving the Mechanical Properties of Oxide Bulk Ceramics

다국어 초록 (Multilingual Abstract)

Oxide-based bulk ceramics are manufactured through ceramic powder processing, and improving their performance requires understanding the key phenomena at each stage. This approach has been widely adopted in both academic and industrial ceramic research, and its importance has grown with increasing demands for carbon neutrality and energy efficiency. This thesis focuses on high-temperature sintering, where densification, grain growth, and phase evolution determine the final microstructure and properties of bulk ceramics. Chapter 1 investigates the sintering behavior and mechanical properties of freeze-cast porous Al2O3, and Chapter 2 examines CAC, emphasizing phase formation and microstructural changes during the sintering process. Overall, these studies highlight essential processing-microstructure-phase relationships
and provide strategies for enhancing the mechanical performance of oxide-based bulk ceramics.
Porous Al2O3 requires sufficient mechanical strength to ensure structural integrity in practical applications. Strength enhancement can be achieved by controlling densification and grain growth, thereby improving the density-to grain size ratio. In this study, two-step sintering (TSS) was combined with MgO doping to reinforce freeze-cast porous Al2O3. The applicability of TSS was established through activation energy analysis, revealing a significant difference between densification (63.48 kJ/mol) and grain growth (806.11 kJ/mol). Under optimized conditions, a higher relative density (45.44 % of theoretical) with finer grain size (0.814 µm) was obtained, leading to improved compressive strength (9.37 MPa). The additional role of MgO doping was confirmed by comparison with undoped specimens. These findings highlight the synergistic effect of TSS and MgO doping, offering an effective processing strategy for enhanced mechanical performance of porous alumina.
The effect of SiO2 addition on the phase fraction and flexural strength (σf) of calcium aluminate cement (CAC) castable was examined. With increasing SiO2 content from 0.26 to 4.0 wt.%, the CA6 phase fraction increased from 9.8 to 28.6 wt.% through CA → CA2 → CA6 transformations. After sintering 1500 oC, the highest σf was obtained with SiO2 content of 2.0 wt. %. Higher SiO2 content produced more CA6 and hence degradation of flexural strength due to large voids among the CA6 plates. However, after three-cycle heat treatment at 1500 oC simulating practical environment, the voids were filled by liquid phases especially in sample with 4.0 wt.% SiO2. A superior σf (86.73 MPa) was obtained with the highest SiO2 content (4.0 wt.%). These results explain the contribution of SiO2 to long-term mechanical stability and thus highlight the importance of optimization of SiO2 concentration in practical applications.

목차 (Table of Contents)

• 목차
• Chapter 1 8
• I. 서론 8
• II. 문헌조사 13
• 1. 다공성 세라믹(Porous ceramics) 13
• 2. 동결주조(Freeze-casting) 15
• 3. MgO 도핑 효과 17
• 4. Two-step sintering (TSS) 18
• III. 실험방법 21
• 1. 분말준비 및 성형 21
• 2. 소결 24
• 1) Conventional sintering 24
• 2) Two-step sintering (TSS) 25
• IV. 분석 26
• 1. 상대 밀도 (Relative Density) 26
• 2. 미세구조 분석 (Microstructure) 27
• 3. 상 및 결정구조 (XRD, HSC Chemistry) 28
• 4. 활성화 에너지 (Activation energy) 29
• 5. 기계적 강도 (Mechanical strength) 29
• V. 결과 및 고찰 30
• 1. 다공성 구조의 미세구조 30
• 2. Lamellar wall 내부 입자 성장 양상 33
• 3. 입자 성장 활성화 에너지 41
• 4. 치밀화 활성화 에너지 52
• 5. TSS 공정 설계 53
• 6. 기계적 강도 58
• VI. 결론 61
• VII. 참고문헌 63
• Chapter 2 69
• I. 서론 69
• II. 문헌조사 72
• 1. Calcium Aluminate Cement (CAC) 72
• 2. SiO2의 영향 73
• III. 실험방법 76
• 1. 분말준비 76
• 2. 성형체 제작 76
• 3. 소결 77
• 1) Conventional sintering 77
• 2) Laser Dilatometery 78
• 3) Cycle test 78
• IV. 분석 79
• 1. 상 및 결정구조 (XRD, HSC Chemistry) 79
• 2. Laser Dilatometery 80
• 3. 미세구조 분석 (Microstructure) 80
• 4. 밀도 (Density) 81
• 5. 기계적 강도 (Mechanical strength) 81
• 6. 장기 안정성 평가 (Long-term stability) 82
• V. 결과 및 고찰 83
• 1. 상전이 (Phase transition) 83
• 2. 팽창/수축 거동 분석 (Laser Dilatometery) 89
• 3. 미세구조 92
• 4. 밀도 및 기계적 특성 96
• 5. 장기 안정성 및 파단 경로 98
• VI. 결론 102
• VII. 참고문헌 104
• Abstract 113
• 감사의 글 115