非相稱性輝鈷礦Ca3Co4O9+δ於室溫時擁有良好的熱電特性、包括高熱電力、低電阻率及低熱導率等，為良好的熱電材料。相較於 Bi2Te3、Zn4Sb3、PbTe及Si1-xGex，其在高溫的空氣中具有較高的化學穩定度。多晶相的Ca3Co4O9+δ 其熱電特性雖較單晶相的樣品低，但由於製作成本較低廉，因此更具有實用的價值。為了提高多晶材料的熱電特性，不同金屬元素掺雜取代Ca 或Co以增加載子濃度及其遷移率或熱傳導性質皆為可行之方法。本研究工作利用同步輻射光源進行X- 光吸收光譜量測，對 Mn 及 Fe 掺雜的 Ca3Co4-xMx(M = Fe, Mn)O9+δ 及鑭系元素掺雜的 Ca2.9Ln0.1(Ln = Ca, Dy, Ho, Er and Lu)Co4O9+δ 熱電材料作了深入而有系統的探討，以期更進一步瞭解這類材料的電子結構與其熱電性質之關聯性。
由研究結果得知，降低Co4+與Co3+比例有助於提高熱電力。增加Co4+ 3d與O 2p混層軌域有助於電阻率的降低。而藉由摻雜造成其晶格扭曲有助於降低其熱導率。同時我們也發展了可在真空環境裡進行實驗的電化學裝置，使的實驗並不只限於固體樣品的量測。

The misfit-layered cobaltites Ca3Co4O9+δ have shown large thermoelectric power and low electrical resistivity and thermal conductivity at room temperature. These cobaltites exhibit good chemical stability in air at high temperatures in comparing with intermetallic compounds, e.g., Bi2Te3, Zn4Sb3, PbTe and Si1-xGex alloys. The polycrystalline cobaltites are more reliable for practical applications even with a relatively lower performance compared to single crystalline cobaltites. However, single crystalline samples are too expensive for the practical fabrication of TE devices. Partial substitutions of various metal elements for Ca and/or Co to optimize the carrier concentration and to improve other electronic transport properties can result in better thermoelectric properties of these materials. We have performed systematic x-ray absorption spectroscopy measurements on two series of doped misfit-layered cobaltites, Ca3Co4-xMx(M = Fe, Mn)O9+δ and Ca2.9Ln0.1(Ln = Ca, Dy, Ho, Er and Lu)Co4O9+δ, in order to gain a deeper understanding of the correlations between the electronic structure and the thermoelectric properties in these materials. We found that reducing the ratio of Co4+/Co3+ will increase thermoelectric power. Increasing the hybridization of Co4+ (3d5) – O (2p) will decrease the electric resistivity. Introducing lattice distortion will decrease the thermal conductivity. We also developed the in-situ electrochemistry flow cell which can be performed with liquid samples in high vacuum environment.

Table of Contents

Acknowledgement	i
Abstract	iii
List of Figures	xi
List of Tables	xviii

1.  Introduction	1
1-1. Development History of Thermoelectric Materials	1
1-2. The Characteristics of Misfit Layered Cobaltites	4
1-3. Need for Spectroscopic Studies	9

2.  Experimental Techniques	11
2-1. Sample Preparation of Ca3Co4-xMx(M = Fe, Mn)O9+δ (x = 0, 0.05, 0.1, 0.15)	11
2-2. Samples Preparation of Ca2.9Ln0.1Co4O9+δ (Ln = Ca, Dy, Ho, Er, Lu)	11
2-3. Resistivity Measurements	12
2-4. Thermoelectric (TE) Power Measurements	14
2-5. Synchrotron radiation	15
2-6. Beamlines of Utilized	19
2-7. X-ray Absorption Spectroscopy	21

3.  Results on Ca3Co4O9+δ Series	27
3-1. Ca3Co4-xFexO9+δ (x = 0, 0.05, 0.1, and 0.15) series	27
3-2. Ca3Co4-xMnxO9+δ (x = 0, 0.05, 0.1, and 0.15) series	42
3-3. Ca2.9Ln0.1Co4O9+δ (Ln = Ca, Dy, Ho, Er and Lu) series	56

4.  In situ Electrochemistry Flow Cell 68

5.  Summary	74
5-1. Ca3Co4-xMx(M = Fe and Mn)O9+δ (x = 0, 0.05, 0.1, and 0.15) series	74
5-2. Ca2.9Ln0.1Co4O9+δ (Ln = Ca, Dy, Ho, Er and Lu) series	74
5-3. In situ Electrochemistry Flow Cell  75
Bibliography  78

List of Figures
Chapter 1
1-1 Diagram of a single couple made of a p-type and an n-type TE material. I
represents the current flow. (a) When a temperature difference is added on the device,
useful power can be extracted. (b) Electrons pass from p-type to n-type material, and
the temperature will decrease when heat is absorbed. Then heat is dissipated into
surrounding environment ............................................................................................... 2
1-2 Schematic illustration of the NaxCoO2 structure. .................................................... 5
1-3 Schematic illustration of the Ca3Co4O9 structure .................................................... 6
Chapter 2
2-1 Schematic apparatus of resistivity measurement ................................................... 13
2-2 Schematic apparatus of thermoelectric power measurement. ............................... 14
2-3 Schematic diagram of synchrotron radiation extending from the infrared to the
x-ray regions.. .............................................................................................................. 16
2-4 National synchrotron radiation center (NSRRC) in Taiwan. ................................. 17
2-5 Schematic diagram of a synchrotron radiation source. .......................................... 18
2-6 Schematic illustration of x-ray absorption spectroscopy process. ......................... 21
2-7 Energy levels, absorption edges, and characteristics line emissions of a
muti-electron atom . ..................................................................................................... 22
2-8 X-ray absorption spectrum of Ca3Co4O9+δ at Co K-edge, as an example which
corresponds to excitation of a Co 1s electron into empty p state. The spectrum is
divided into XANES and EXAFS. ............................................................................... 23
2-9 (a) The illustration of photoelectron scattering process in the single scattering
regime, and (b) in the multiple scattering regime. ....................................................... 24
2-10 The schematic view of x-ray absorption detecting modes................................... 25
2-11 The decay mechanism for the total electron yield and the total fluorescence
modes. .......................................................................................................................... 26
Chapter 3
3-1 Co K-edge XANES spectra of Ca3Co4−xFexO9+δ (x= 0, 0.05, 0.10 and 0.15) and
the reference oxides. .................................................................................................... 29
3-2 Co K-edge derivative spectra (symbolic lines) of Ca3Co4O9+δ and the reference
oxides..........................................................................................................................30
3-3 Normalized Fe K-edge XANES spectra of Ca3Co4−xFexO9+δ (x= 0, 0.05, 0.10 and
0.15) and the reference oxides. .................................................................................... 31
3-4 Fe K-edge derivative spectra (symbolic lines) of Ca3Co3.85Fe0.15O9+δ and the
reference oxides............................................................................................................32
3-5 (a) O K-edge XANES spectra of Ca3Co4−xFexO9+δ (x= 0, 0.05, 0.10 and 0.15) and
the reference oxides. (b) The inset shows the magnified pre-edge peak after
background subtraction using a best fitted Gaussian curve indicated by the dotted line
(Fig. 3-5a)…………………………………………………………………………….34
3-6 Co L2,3-edge XANES spectra of Ca3Co4−xFexO9+δ (x= 0, 0.05, 0.10 and 0.15) and
the reference oxides. The inset shows the magnified L3-edge after background
subtraction using an arctangent function indicated by the dotted line……………….35
3-7 Plot of magnetic susceptibility verse temperature for Ca3Co3.95Fe0.05O9+δ in an
applied field of 5000 Oe. The solid line is a fit to the Curie-Weiss law. ...................... 36
3-8 Integrated intensities of the Co L3 - and pre-edge features of O K-edges, Fe K- and
Co K-edges for Ca3Co4−xFexO9+δ (x=0, 0.05, 0.10, and 0.15). The inset is the variation
of the room-temperature resistivity with x…………………………………………...40
3-9 Integrated intensity of Co L2,3 -edge (Co) and thermoelectric power (S) for
Ca3Co4-xFexO9+δ (x = 0, 0.05, 0.1 and 0.15)……………………………………..…...41
3-10 Normalized Co K-edge XANES spectra of Ca3Co4-xMnxO9+δ (x = 0, 0.05, 0.1
and 0.15) and standard oxides.……..….......................................................................44
3-11 (a) Magnified Co K-edge XANES spectra of Ca3Co4-xMnxO9+δ (x = 0, 0.05, 0.1
and 0.15). (b) The variation of A1 intensity with x…………………………………..45
3-12 Normalized Mn K-edge XANES spectra of Ca3Co4-xMnxO9+δ (x = 0, 0.05, 0.1
and 0.15) and standard oxides………………………………………………………..46
3-13 Mn K-edge derivative spectra (symbolic lines) of Ca3Co3.85Mn0.15O9+δ and the
reference oxides………………………………………………………………………47
3-14 (a) O K-edge XANES spectra of Ca3Co4−xMnxO9+δ (x= 0, 0.05, 0.10 and 0.15).
(b) The inset shows the magnified pre-edge peak after background subtraction using a
best fitted Gaussian curve indicated by the dotted line (Fig. 3-14a). ........................... 48
3-15 Normalized Co L2,3-edge spectra of Ca3Co4-xMnxO9+δ (x = 0, 0.05, 0.1 and 0.15)
and the background arctangent function (the dotted line).. ......................................... 50
3-16 Plot of magnetic susceptibility as a function of temperature for
Ca3Co3.85Mn0.15O9+δ in an applied field of 5 T. The solid line is a fit to the Curie-Weiss
law. ............................................................................................................................... 51
3-17 Integrated intensity of Co L2,3 -edge (Co) and thermoelectric power (S) for
Ca3Co4-xMnxO9+δ (x = 0, 0.05, 0.1 and 0.15)………………………………………...54
3-18 Integrated intensity of the Co4+ (3d5) – O 2p hybridization and the resistivity for
Ca3Co4-xMnxO9+δ (x = 0, 0.05, 0.1 and 0.15)………………………………………...55
3-19 Co K-edge XANES spectra of a series of Ca2.9Ln0.1Co4O9+δ (Ln = Ca, Dy, Ho, Er
and Lu) and the two standard oxides. .......................................................................... 58
3-20 Co K-edge derivative spectra (symbolic lines) of Ca3Co4O9+δ and the reference
oxides. .......................................................................................................................... 59
3-21 Normalized Co L2,3-edge XANES spectra of a series of Ca2.9Ln0.1Co4O9+δ (Ln =
Ca, Dy, Ho, Er and Lu). ............................................................................................... 60
3-22 The plots of Co L2,3-edge (including A2 and B2) magnitude after background
subtraction and the room temperature thermoelectric power………………………...61
3-23 Normalized O K-edge XANES spectra of a series of Ca2.9Ln0.1Co4O9+δ (Ln = Ca,
Dy, Ho, Er and Lu). ...................................................................................................... 64
3-24 The intensity of Co4+ 3d-O 2p hybridized states and the room temperature
electrical resistivity……………………………………………………….…………..65
3-25 Normalized Ca L2,3-edge XANES spectra of a series of Ca2.9Ln0.1Co4O9+δ (Ln =
Ca, Dy, Ho, Er and Lu). ............................................................................................... 66
3-26 The intensity of Ca L2,3-edge after arctangent background subtraction. ............. 67
Chapter 4
4-1 Schematic drawing of the electrochemical cell assembly for in situ x-ray
absorption spectroscopy studies. a) Si3N4 window, b1) electrical connection to Si3N4
window (working electrode), b2) reference electrode, b3) counter electrode, c) PEEK
body, d) support tube assembly. The green arrow indicates the liquid flow. ............... 69
4-2 a) Cyclic voltammogram of a Cu thin film working electrode in 0.1 M NaHCO3
solution at a scan rate of 0.02 Vs−1. Pt and Ag wires were used as counter and
pseudo-reference electrodes, respectively. b) Total fluorescence yield XAS of Cu
L2,3-edge. (1) Reference metallic Cu foil. (2) Cu thin film (∼300 nm) evaporated on
the Si3N4 membrane (oxidized during transfer in air). Spectra (3) and (4) were
recorded after insitu oxidation at nominally 0.2 V and after reduction at nominally
-0.9 V in 2 mM NaHCO3 solution, respectively. ......................................................... 71
4-3 Time evolution of the Cu L2,3-edge XAS spectra showing the a) oxidation and b)
reduction kinetics in 2 mM NaHCO3 solution at nominal potential of -0.1 V and -1.0
V, respectively. The spectra were collected sequentially with a time difference of
approximately 15 min. ................................................................................................. 72
Chapter 5
5-1 The variation of Co4+ (3d5) 3d – O 2p hybridized area with room-temperature
electrical resistivity of Ca3Co4-xMx(M = Fe and Mn)O9+δ (x = 0, 0.05, 0.1, and 0.15)
series. ........................................................................................................................... 76
5-2 The variation of Co L2,3-edge intensity with room-temperature thermoelectric
power of Ca3Co4-xMx(M = Fe and Mn)O9+δ (x = 0, 0.05, 0.1, and 0.15) series……...77

List of Tables
Chapter 1
1-1 Various physical parameters for NaCo2O4 (in-plane) and Bi2Te3 at 300 K, where ρ,
S, and μ are, electrical resistivity, TE power and mobility, respectively. ...................... 4
1-2 The ratio g3/g4 in Heikes formula for possible cases. .............................................. 8
Chapter 3
3-1 Effective magnetic moment of Co, carrier concentration, average oxidation
number, and oxygen content of Ca3Co4−xFexO9+δ (x= 0 - 0.1). .................................... 37
3-2 Effective magnetic moment of Co, average oxidation number and oxygen content
of Ca3Co4−xMnxO9+δ (x = 0, 0.05, 0.10 and 0.15). ....................................................... 52

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