Effect of Cobalt Doping on Thermoelectric Performance in Zr2FeNiSb2-based Double Half-Heusler Materials for n- and p-type Applications = Zr2FeNiSb2 기반 double half-Heusler 소재의 n-형 및 p-형 열전 특성에 대한 코발트 도핑 효과

다국어 초록 (Multilingual Abstract)

This dissertation presents multifaceted strategies for optimizing the performance of Zr2FeNiSb2-based double half-Heusler alloys for thermoelectric applications, with a particular focus on bridging the gap between theoretical modeling of lattice defects and experimentally observed thermal transport properties. To this end, a comprehensive review of the Debye–Callaway model was undertaken, consolidating its capacity to relate up to nine distinct phonon scattering mechanisms to material and defect features. Building upon this foundation, a novel Debye–Callaway simulator was developed as a user-friendly standalone tool. This program provides a practical platform for accelerating microstructural engineering by integrating theoretical predictions with experimental measurements of lattice thermal conductivity, thereby streamlining the optimization of thermoelectric materials.
The optimization of thermoelectric and mechanical properties in the Zr2FeNiSb2 system was systematically investigated through intrinsic Fe–Ni proportion variation and extrinsic cobalt lone doping, revealing distinct and asymmetric responses between p-type and n-type compositions. P-type optimization was limited by intrinsic material constraints, including poor dopant solubility, deleterious bulk inclusion formation, and lattice stiffening, resulting in negligible enhancement of the figure of merit (zT). In contrast, n-type compositions exhibited substantial improvements in zT, driven by synergistic mechanisms such as carrier donor effects, favorable band structure modifications, dopant-induced multiscale phonon scattering, and cobalt-induced lattice softening and disorder. These combined effects significantly enhanced both the power factor and thermal transport properties, yielding zT values of 0.51 in Zr2Fe0.4Co0.6NiSb2 and 0.43 in Zr2Fe0.7Ni1.3Sb2, representing a marked improvement over the baseline value of 0.001 in pristine Zr2FeNiSb2. Beyond thermoelectric performance, these strategies also strengthened mechanical robustness, as evidenced by Vickers hardness increases of 9% in Zr2Fe0.7Ni1.3Sb2 and 24% in p-type Zr2Fe0.4Co0.6NiSb2 relative to the pristine alloy. The enhanced viability for practical application was further confirmed through the fabrication and testing of a unileg thermoelectric generator, which demonstrated a 273% increase in peak output power for Zr2Fe0.4Co0.6NiSb2 compared to the pristine sample.
Overall, this work demonstrates that while p-type performance in Zr2FeNiSb2 remains constrained, the n-type branch provides a viable and tunable pathway toward high-efficiency thermoelectrics. Beyond advancing the understanding of carrier-type-dependent optimization in half-Heusler alloys, this dissertation contributes methodological and computational tools that will inform future design strategies for next-generation thermoelectric materials.

국문 초록 (Abstract)

본 학위논문에서는 Zr2FeNiSb2 기반 더블 하프-호이슬러(double half-Heusler) 합금의 열전 성능 최적화를 위한 다각적인 전략을 제시하였으며, 특히 격자 결함의 이론적 모델링과 실험적으로 관찰된 열 수송 특성 간의 격차를 해소하는 데 중점을 두었다. 이를 위해 Debye–Callaway 모델을 포괄적으로 고찰하였으며, 최대 아홉 가지의 개별적인 포논 산란 메커니즘을 재료 및 결함의 물리적 특성과 연계하여 모델의 적용 가능성을 체계적으로 정리하였다. 이러한 고찰을 토대로, 독립적으로 구동 가능한Debye–Callaway 시뮬레이터를 새롭게 개발하였다. 본 프로그램은 격자 열전도도의 이론적 예측과 실험 데이터를 통합하여 열전 재료의 미세구조 공학을 가속화하고, 그 결과로서 체계적인 최적화를 가능하게 하는 실용적인 도구로 활용될 수 있음을 입증하였다.
Zr2FeNiSb2 계의 열전 및 기계적 물성을 내재적 Fe/Ni 비율 조절과 외재적 Co 단일 도핑을 통해 체계적으로 고안하였으며, p-형과 n-형 조성 간에 뚜렷하고 비대칭적인 반응을 관찰하였다. p-형 조성의 경우, 도펀트 용해도 저하, 유해한 벌크 개재물 형성, 격자 경화 등과 같은 고유한 재료적 한계로 인해 zT의 향상이 미미하였다. 반면, n-형 조성에서는 전자 도너 효과, 밴드 구조의 유리한 변화, 도펀트 유도 다중 스케일 포논 산란, Co 도핑에 따른 격자 연화 및 무질서 증가 등의 복합적인 메커니즘이 작용하여 열전 성능이 크게 향상되었다. 이러한 복합적인 효과는 파워팩터와 열수송 특성을 동시에 크게 향상시켰고, 그 결과, Zr2Fe0.4Co0.6NiSb2와Zr2Fe0.7Ni1.3Sb2에서 각각 0.51, 0.43의 zT 값을 달성하였으며, 이는 기존 Zr2FeNiSb2(0.001) 대비 현저한 개선을 나타낸다. 열전 성능과 더불어, 이러한 전략은 기계적 강도 향상에도 기여하였으며, 비커스 경도는 기존 시료 대비Zr2Fe0.7Ni1.3Sb2와 p-형 Zr2Fe0.4Co0.6NiSb2에서 각각 9%, 24% 증가하였다. 실용화 가능성은 단일 다리형 열전 발전기(unileg thermoelectric generator)의 제작 및 출력 특성 평가를 통해 검증되었으며, Zr2Fe0.4Co0.6NiSb2의 경우 기존 시료 대비 최대 출력 전력이 273% 향상됨을 확인하였다.
종합적으로, 본 연구는 Zr2FeNiSb2 계에서 p-형 성능의 최적화는 여전히 제약을 받는 반면, n-형 조성은 고효율 열전 재료 개발을 위한실질적이며 조절 가능한 경로를 제시함을 보여주었다. 나아가, 하프-호이슬러 합금에서의 전하 운반자 유형 의존적 최적화에 대한 이해를 확장하는 데 그치지 않고, 차세대 열전 재료 설계전략 수립에 기여할 수 있는 방법론적 계산적 도구를 제공한다.

목차 (Table of Contents)

• 1. Introduction 1
• 2. Literature Review 8
• 2.1. Thermoelectric effect 8
• 2.1.1. Seebeck effect 8
• 2.1.2. Peltier effect 9
• 2.1.3. Thompson effect 10
• 2.2. Thermoelectric devices 13
• 2.2.1. Unileg thermoelectric device 13
• 2.2.2. π-leg thermoelectric device 13
• 2.2.3. Thermoelectric module 14
• 2.2.4. Commercial viability setbacks 17
• 2.3. Thermoelectrics' efficiency 19
• 2.3.1. Device efficiency 19
• 2.3.2. Material's efficiency 20
• 2.3.3. The Figure of merit 20
• 2.4. Charge transport 21
• 2.4.1. Carrier concentration tuning 22
• 2.5. Thermoelectric quality factor 23
• 2.6. Band structure engineering 26
• 2.6.1. Introduction to band structures 26
• 2.6.2. Band flattening 28
• 2.6.3. Band convergence 31
• 2.6.4. Band degeneracy 31
• 2.6.5. Bandgap tuning 32
• 2.6.6. Fermi-level energy tuning 34
• 2.6.7. Density functional theory 35
• 2.7. Phonon transport 38
• 2.7.1. Introduction to phonon transport 38
• 2.7.2. Lattice thermal conductivity 44
• 2.7.3. Heat capacity 44
• 2.7.4. Suppressing phonon group velocity 47
• 2.7.5. Suppressing phonon mean free path 49
• 2.7.6. Glass-limit and diffuson models for lower bounds of κlatt 50
• 2.8. Special performance enhancement strategies 52
• 2.8.1. Energy filtering effect 52
• 2.8.2. Modulation doping 53
• 2.8.3. Entropy engineering 56
• 2.9. Mechanical properties 59
• 2.9.1. Microhardness 59
• 2.9.2. Sound velocity-related mechanical properties 60
• 2.10. Measurement of thermoelectric properties 62
• 2.10.1. Density 62
• 2.10.2. Electrical conductivity and Seebeck coefficient 62
• 2.10.3. Hall measurement 63
• 2.10.4. Laser flash analysis 64
• 2.10.5. Vickers hardness test 65
• 2.11. Thermoelectric materials 65
• 2.11.1. Classes of thermoelectric materials 65
• 2.11.2. Thermoelectric materials' performance 66
• 2.11.3. Half-Heusler alloys 66
• 2.11.4. Double half-Heusler alloys 68
• 3. Developing the Debye Callaway Simulation software 71
• 3.1. Introduction 71
• 3.2. Debye-Callaway model review 75
• 3.2.1. Phonon normal processes scattering 76
• 3.2.2. Phonon Umklapp scattering 76
• 3.2.3. Electron-phonon scattering 77
• 3.2.4. Point defect scattering 77
• 3.2.5. Grain boundary scattering 80
• 3.2.6. Inclusions scattering 80
• 3.2.7. Vacancy-phonon scattering 82
• 3.2.8. Dislocation-phonon scattering 83
• 3.3. Simulation software development 87
• 3.3.1. User Interface and data input 87
• 3.3.2. Theoretical κlatt computation 87
• 3.3.3. Dynamic control of physical parameters 89
• 3.3.4. Input validation and user guidance 89
• 3.3.5. Chemical parameter handling and automated Γ/Γs calculation 89
• 3.3.6. Selective activation of scattering mechanisms 90
• 3.3.7. χ² calculation and automated differential-evolution fitting 90
• 3.3.8. Manual fine-tuning via logarithmic and linear sliders 91
• 3.3.9. Dynamic axis scaling and visualization enhancements 92
• 3.3.10. κmin enforcement using glass-limit and diffuson models 92
• 3.3.11. Data export and workflow integration 92
• 3.3.12. Advancements beyond prior implementations 94
• 3.4. Case study for Debye-Callaway simulator applications 94
• 3.4.1. Case study 1: GeTe materials 94
• 3.4.2. Case study 2: SnTe TE materials 99
• 3.4.3. Case study 3: NbFeSb half-Heusler TE alloys 102
• 3.5. Conclusion 105
• 4. Optimizing Thermoelectric Performance in Zr2FeNiSb2 Double half-Heusler via Intrinsic Fe–Ni Ratio Tuning 106
• 4.1. Introduction 106
• 4.2. Experimental methods 109
• 4.2.1. Materials and synthesis 109
• 4.2.2. Microstructure and structural characterization 111
• 4.2.3. Physical properties measurement 111
• 4.2.4. Electronic structures 111
• 4.3. Results and discussion 112
• 4.3.1. Structural characterization. 112
• 4.3.2. Phase and compositional characterization 115
• 4.3.3. Electronic properties 122
• 4.3.4. Density functional theorem 127
• 4.3.5. Thermal properties 130
• 4.3.6. Figure of merit and B-factor 137
• 4.3.7. Mechanical properties 139
• 4.4. Conclusion 141
• 5. Enhancing n- and p-type TE Performance in Zr2FeNiSb2 Double Half-Heusler via Extrinsic Cobalt Doping 142
• 5.1. Introduction 142
• 5.2. Experimental methods 146
• 5.2.1. Materials and synthesis 146
• 5.2.2. Microstructural and structural characterization 148
• 5.2.3. Physical property measurements 148
• 5.2.4. Theoretical simulation of band structures 148
• 5.2.5. Fabrication and performance evaluation of the unileg TEGs 149
• 5.3. Results and discussion 151
• 5.3.1. Phase and structural characterization 151
• 5.3.2. Electronic properties 157
• 5.3.3. Density functional theorem 161
• 5.3.4. Thermal conductivity 164
• 5.3.5. Phonon scattering centres' contribution to phonon transport suppression 169
• 5.3.6. Experimental isolation of nanoinclusion and grain boundary scattering 173
• 5.3.7. Lattice softening changes induced by Co doping 177
• 5.3.8. Deconvolution of lattice thermal conductivity reduction 181
• 5.3.9. Figure of merit 185
• 5.3.10. Power output of the fabricated unileg thermoelectric generators 188
• 5.3.11. Mechanical properties 190
• 5.4. Conclusion 192
• 6. Comprehensive Conclusion 193
• References 195
• (Abstract) 210
• (초 록) 212