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研究生： 黎氏懷
Thi, Hoai Le
論文名稱： 以溶膠凝膠法製備摻雜鉍和鎳之鈷酸鑭及其熱電性質研究
Sol-gel synthesis and thermoelectric properties of Bi and Ni doped LaCoO3
指導教授： 齊孝定
Qi, Xiao-Ding
學位類別： 碩士
Master
系所名稱： 工學院 - 尖端材料國際碩士學位學程
International Curriculum for Advanced Materials Program
論文出版年： 2018
畢業學年度： 106
語文別： 英文
論文頁數： 105
外文關鍵詞： lanthanum cobalt oxides, sol-gel process, thermoelectric, XPS
相關次數： 點閱：78  下載：1 
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    • Thermoelectric energy conversion is promising, but its potential for practical application depends on the discovery of high conversion-efficiency materials. Most of previous studies on thermoelectric materials have been focused on alloys and intermetallics. However, recent studies showed that oxide materials may also play an important role in this regard. Lanthanum cobalt oxide (LaCoO3), which has a perovskite structure, is one of the oxides displaying such a potential. However, its band gap is about 1.39 eV, which is too large to give a high enough carrier concentration for good electrical conductivity. On the other hand, LaCoO3 only keeps a large Seebeck coefficient at room temperature, which decreases to an unacceptable value at high temperature. It was reported that doping some cations either at La or Co sites improved the thermoelectric properties of LaCoO3. Therefore, we attempted to improve the power factor of LaCoO3 by doping Bi and Ni at La and Co sites, respectively. All polycrystalline samples of La1‑xBixCoO3 and LaCo1‑xNixO3 (x=0~0.05) were synthesized by the sol-gel process. The phase purity of the prepared samples was confirmed by X-ray diffraction (XRD). Structural analyses were then performed based on the XRD data by the Total Pattern Analysis Solution software. The results showed that the lattice constants of both a and c increased as the Bi concentration increased because of the larger ionic radii of Bi3+ (1.17 Å) than La3+ (1.032 Å). However, for the Co doped samples, the lattice constants increased as the doping level increased until x=0.04 and then dropped. Compositional analyses by energy dispersive X-ray spectroscopy indicated that the prepared samples had the right stoichiometry. Scanning electron microscopy showed that undoped LaCoO3 had a fairly uniform distribution of particle sizes around 1 m. The Bi doping did not change the particle size notably, while the particle size of Ni doped samples decreased greatly to about 0.2 m. X-ray photoelectron spectroscopy (XPS) was carried out to check the oxidation states in La1-xBixCoO3 and LaCo1-xNixO3. The results showed that some Co4+ coexisted with Co3+ in undoped LaCoO3. In Bi-doped LaCoO3, small amount of Bi4+ was found in addition to the majority of Bi3+ and yet the ratio of Co4+/Co3+ was increased by the Bi doping. In the Ni-doped LaCoO3, Ni2+ and Ni3+ coexisted and the samples contained more Co4+ than the Bi doped LaCoO3. Based on XPS results, the charge compensation mechanisms were proved for La1-xBixCoO3 and LaCo1-xNixO3, which may shed some light on the origins of charge carriers. Thermoelectric measurements showed that all the samples had positive Seebeck coefficients, indicating that the charge carriers were p-type. The magnitude of Seebeck coefficients was decreased by either the Bi or Ni doping. However, the decrement was only large around room temperatures and as the temperature increased, it became increasingly smaller. On the other hand, the electrical conductivities were increased by a factor of 5 or 100 in the Bi and Ni doped samples (x=0.05), respectively, owing to the increased charge carriers. Overall, the power factors were improved by both the Bi and Ni doing, which had the maximal values of 1.5 and 2.9 µV/cmK2 (measured at 550 K) for the Bi (x=0.04) and Ni (x=0.05) doped LaCoO3, respectively.
    Keywords: lanthanum cobalt oxides, sol-gel process, thermoelectric, XPS.

    TABLE OF CONTENTS ABSTRACT I ACKNOWLEDGMENTS III TABLE OF CONTENTS V LIST OF TABLES VIII LIST OF FIGURES IX CHAPTER 1. INTRODUCTION 1 1.1 Nanotechnology and nanomaterials. 1 1.2 Waste heat and challenges in thermoelectric research. 4 1.3 Thermoelectric materials. 6 1.4 Oxide as thermoelectric material. 8 1.5 Study motivation and purpose. 11 CHAPTER 2. LITERATURE REVIEW 13 2.1 Thermoelectric theory. 13 2.1.1 Seebeck effect. 13 2.1.2 Peltier Effect. 15 2.1.2.1 Thermoelectric devices. 16 2.1.3 Thomson Effect. 17 2.2 Physical properties of thermoelectric materials. 17 2.2.1 Thermoelectric performance. 17 2.2.1.1 Thermoelectric material efficiency. 19 2.2.1.1.1 Thermoelectric energy conversion efficiency. 19 2.2.1.1.2 Ways to improve the efficiency of thermoelectric generator. 21 2.2.2 Thermopower or Seebeck coefficient. 29 2.2.3 Electrical Conductivity 30 2.2.4 Thermal Conductivity. 31 2.3 Material selection. 32 2.4 Application of thermoelectric materials 33 2.5 Material introduction 34 2.5.1 Perovskite structure. 34 2.5.2 LaCoO3 as promising thermoelectric material. 37 CHAPTER 3. EXPERIMENTAL METHOD & PROCEDURES 43 3.1 Chemicals 43 3.2 Sample preparation. 43 3.2.1 The basic concepts 43 3.2.2 Sol-gel process. 44 3.2.3 Advantages and disadvantages of Sol-gel process. 46 3.2.4 Sol-gel applications. 47 3.3 Experimental steps 47 3.4 Structural characterization techniques 49 3.4.1 Thermogravimetry analysis/ Differential scanning calorimetry (TGA/DSC) …………………………………………………………………………...............49 3.4.2 X-Ray Diffraction (XRD) 49 3.4.3 Total pattern analysis solution, TOPAS. 51 3.4.4 Scanning Electron Microscopy (SEM) 56 3.4.4 Energy dispersive spectrum (EDS) 57 3.4.5 Archimedes density measurement. 58 3.4.6 Hall Effect Measurement System. 58 3.4.7 Seebeck coefficient measurement systems. 61 3.4.8 Electrical resistivity measurement system. 63 CHAPTER 4. RESULTS AND DISCUSSION 65 4.1 La1-xBixCoO3 (x=0~0.05) 65 4.1.1 Thermogravimetry analysis/ Differential scanning calorimetry (TGA/DSC) …………………………………………………………………………………...65 4.1.2 X-ray diffraction (XRD) 66 4.1.3 Surface appearance and elemental composition analysis. 70 4.1.4 Bulk density measurement results. 73 4.1.5 Hall measurement results. 74 4.1.6 Seebeck coefficient 76 4.1.7 Electrical conductivity. 78 4.1.8 Power factor (P). 79 4.2 LaCo1-xNixO3 (x=0~ 0.05) 81 4.2.1 Thermogravimetry analysis/ Differential scanning calorimetry (TGA/DSC). 81 4.2.2 X-ray diffraction (XRD) 82 4.2.3 Surface appearance and elemental composition analysis. 86 4.2.4 Bulk density measurement results. 88 4.2.5 Hall measurement results. 89 4.2.6 Seebeck coefficient 92 4.2.7 Electrical conductivity 94 4.2.8 Power factor of LaCo1-xNixO3 96 CHAPTER 4. CONCLUSION 98 REFERENCES 99

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