The correlation between electronic structure and thermoelectric properties of misfit-layered cobaltites studied by x-ray absorption spectroscop

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Title:	The correlation between electronic structure and thermoelectric properties of misfit-layered cobaltites studied by x-ray absorption spectroscop
Other Titles:	以x光吸收光譜研究非相稱性輝鈷礦之電子結構及其與熱電性質之關聯性
Authors:	陳政龍;Chen, Jeng-Lung
Contributors:	淡江大學物理學系博士班張經霖
Keywords:	X光吸收能譜;熱電材料;非相稱性輝鈷礦;XAS;Thermoelectric Materials;Misfit-layered Cobaltites
Date:	2012
Issue Date:	2013-04-13 11:02:40 (UTC+8)
Abstract:	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.
Appears in Collections:	[Graduate Institute & Department of Physics] Thesis

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