극미량 La 도핑 SrTiO3 세라믹스의 결함 보상 전이와 유전 및 임피던스 특성 연구

다국어 초록 (Multilingual Abstract)

SrTiO3 is a representative ABO3 perovskite oxide whose dielectric and electrical properties are strongly affected by point defects and charge compensation behavior. Although La-doped SrTiO3 has been widely investigated as a donor-doped oxide, most previous studies have focused on relatively high doping levels. In contrast, the role of background impurities becomes increasingly important in the trace-level doping regime, where the intentional dopant concentration can be comparable to the impurity concentration in commercial starting powders. In this study, trace-level La-doped SrTiO3 ceramics were prepared using a liquid-phase ultrasonic doping process to achieve homogeneous La distribution and minimize contamination associated with conventional ball milling. La was added to commercial SrTiO3 powder in the range of 0–1000 ppm, and the resulting ceramics were characterized by X-ray diffraction, scanning electron microscopy, dielectric measurements, and impedance spectroscopy. XRD and Rietveld analyses confirmed that all compositions maintained a cubic SrTiO3 perovskite phase without detectable secondary phases. No systematic change in lattice parameters was observed with La addition, indicating that trace-level La doping did not significantly alter the long-range crystal structure. SEM observations also showed no remarkable compositional dependence of grain morphology or microstructure. Therefore, the changes in dielectric and electrical properties were mainly attributed to defect compensation behavior rather than structural or microstructural variations. Dielectric measurements revealed a non-monotonic compositional dependence of the relaxation response. The dielectric relaxation peak intensity increased up to 50 ppm and then decreased in the 50–1000 ppm range. Similarly, impedance analysis showed that the grain conductivity reached a maximum at 50 ppm and subsequently decreased with further La addition, whereas the grain boundary conductivity remained within a relatively narrow range. These results suggest that the electrical response in trace-level La-doped SrTiO3 is primarily governed by defect changes in the grain interior. The observed behavior was interpreted in terms of a defect compensation crossover associated with background acceptor impurities and intrinsic defect states. Below the crossover region, the increase in dielectric relaxation and grain conductivity suggests that electronic compensation of La donors may be dominant, leading to an enhanced oxygen vacancy/Ti3+-related electrical response. Above this region, ionic compensation involving acceptor-type cation vacancies such as strontium vacancy becomes more dominant, leading to a decrease in oxygen vacancy-related relaxation and grain conductivity. In the 1000 ppm composition, a pronounced decrease in high-temperature dielectric loss were observed, suggesting a further change in the compensation mechanism, possibly involving titanium vacancy. Based on these results, the A-site solubility limit of SrTiO3 is proposed to exist between 500 and 1000 ppm. This study demonstrates that even trace-level La doping can significantly affect the dielectric and electrical properties of SrTiO3 ceramics. The results indicate that background impurity concentration, initial intrinsic defect state, and solubility limit must be considered together when interpreting the properties of trace-level La doped SrTiO3 ceramics. Key words : trace-level doping SrTiO3, defect compensation, background impurity, intrinsic defect

목차 (Table of Contents)

• 제 1 장 서론 1
• 제 2 장 이론적 배경 4
• 2.1. 페로브스카이트 구조와 SrTiO3의 기본 특성 4
• 2.2. SrTiO3의 유전 특성과 유전 완화 7
• 2.3. SrTiO3의 intrinsic 결함 화학 10
• 2.3.1. Krӧger vink notation 11
• 2.3.2. Pure SrTiO3에서의 Schottky defect 12
• 2.3.3. 산소 공공과 Ti3+ 관련 전기적 반응 16
• 2.4. La-doped SrTiO3의 전하 보상 메커니즘 18
• 2.4.1. · 도너 결함 19
• 2.4.2. 전자 보상(Electronic compensation) 20
• 2.4.3. 이온 보상(Ionic compensation) 21
• 2.5. 극미량 도핑에서 배경 불순물(Background impurity)의 영향 22
• 2.6. 임피던스 분광법(Impedance spectroscopy)을 이용한 grain/grain boundary 전도 분석 25
• 2.7. 액상 초음파 혼합법과 극미량 도펀트 균일 분산 27
• 제 3 장 실험방법 및 특성평가 30
• 3.1. 실험방법 30
• 3.1.1. 분말혼합 30
• 3.1.2. 분말소결 31
• 3.2. 특성평가 32
• 3.2.1. X-선 회절분석 32
• 3.2.2. 미세구조 분석 32
• 3.2.3. 유전 특성 분석 33
• 3.2.4. 임피던스 특성 분석 33
• 제 4 장 실험결과 및 고찰 34
• 4.1. 극미량 La-doped SrTiO3 세라믹의 구조적 특성 34
• 4.1.1. X-선 회절 분석(XRD) & Rietveld 정량분석 34
• 4.1.2. 미세구조 분석 37
• 4.2. 극미량 La-doped SrTiO3 세라믹의 유전 특성 40
• 4.2.1. 온도 의존성 유전 상수 40
• 4.2.2. 유전 손실 및 Modulus 42
• 4.2.3. 고온 polaron 48
• 4.3. 극미량 La-doped SrTiO3 세라믹의 임피던스 특성 52
• 제 5 장 결론 59
• 참고문헌 61
• 국문 요약 70