電流效應對Bi-Se-Te奈米結構塊材熱電性質之研究

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研究生：	林幸□ Lin, Sin-Shien
論文名稱：	電流效應對Bi-Se-Te奈米結構塊材熱電性質之研究 Effect of electric current stressing on thermoelectric properties of Bi-Se-Te based bulk nanostructured materials
指導教授：	廖建能 Liao, Chien-Neng
口試委員:	甘炯耀朱旭山廖建能
學位類別：	碩士 Master
系所名稱：	工學院 - 材料科學工程學系 Materials Science and Engineering
論文出版年：	2011
畢業學年度：	99
語文別：	英文
論文頁數：	80
中文關鍵詞：	熱電效應、電流效應、晶體缺陷、傳輸機制
外文關鍵詞：	Thermoelectric, electrical stressing, crystal defects, transport properties
相關次數：	點閱：2 下載：0
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熱電材料為一種可將熱能與電能互相轉換的材料。由於近年來的能源危機，熱電材料被視為具有潛力的替代性能源。鉍銻系列化合物在室溫應用範圍，具有最佳熱電優值，因此在商業上也是最普遍被應用的熱電材料。本研究將針對擁有奈米結構的n型Bi-Se-Te化合物做有系統的分析與探討。奈米結構塊材之製備為利用行星式球磨製成奈米尺度粉末並加壓成形，再以電流輔助熱燒結製成具良好性質之熱電塊材。電流效應對於微結構及熱電傳導性質之影響為本研究之重點。
實驗結果顯示經由電流輔助熱退火可有效的提升材料之熱電性質：主要提升來自於電導率與Seebeck係數，整體可將zT值提升約兩倍。經電流輔助退火之材料具有較低的載子濃度與較高的遷移率，而這些性質的提升是無法單純藉由加熱退火所能達到的。因此，電流與材料晶體缺陷之交互作用對材料性質之影響扮演著極為重要的角色。研究發現，電流輔助熱退火會使晶粒界面處產生局部高溫，進而使晶體內部產生極大的溫度梯度。除此之外，電流能使缺陷遷移所需的能量降低，進而使材料缺陷容易移動。綜合以上兩個效應，晶體缺陷可有效且快速的被推往晶界而消除，使載子濃度下降，載子遷移率大幅提升。
本文中提出電流效應對於消除材料缺陷的機制，以解釋觀察到的性質提升。藉由本研究所提出的電流輔助熱退火的製程，可進一步提升熱電材料之性能。

Thermoelectric materials, which convert thermal energy into electrical energy and vice versa, have long been considered as a prospective solution for energy harvesting. Bismuth telluride based compounds are widely used in commercial thermoelectric devices due to their superior thermoelectric figure-of-merit at room temperature regime. In this research, a systematic study of the nanostructured n-type Bi-Se-Te compound prepared by powder metallurgy and subsequent electric-current assisted sintering process has been conducted. The effects of electric current stressing on the microstructure and thermoelectric transport properties of the sintered Bi-Se-Te materials are the main investigated subject.
The experimental results shows significant improvement of thermoelectric properties after electric current sintering: the electrical conductivity and the Seebeck coefficient of Bi-Se-Te compounds are greatly improved which could double the overall zT value. The electrical stressed Bi-Se-Te material appears to have lower carrier concentration but much higher mobility than the untreated ones. These enhancements due to electric current stressing are so significant that cannot be simply attained by heating the sample and sintering the sample at the similar temperature. Thus, the electric current should play an important role in modulating crystal defects in the material. Our study found that during electric current stressing, the highly local temperature gradient between the interfaces and grains and the electric current induced decrease in the mobility activation energy of defects enhance the motion of atoms and defects. Therefore the crystal defects are effectively eliminated by easy motion toward defect sinks as grain boundaries, leading to lower carrier concentration and much higher electron mobility than simply thermal annealed one. Based on the mechanism involving electric-current induced atomic migration, the material can be further optimized by the electric-current assisted sintering process, leading to higher thermoelectric efficiency.

誌謝……………………………………………………I
Abstract………………………………………………II
摘要……………………………………………………IV
Contents ……………………………………………V
List of Figures …………………………………VIII
List of Tables ………………………………………XII
Chapter 1 Introduction………………………………1
1 Thermoelectric material………………………1
1.1 Seebeck effect…………………………………2
1.2 Peltier effect…………………………………3
1.3 Thomson effect…………………………………3
2 Application and conversion efficiency……4
2.1 Thermoelectric generator……………………4
2.2 Thermoelectric cooler…………………………5
2.3 Efficiency of thermoelectric materials ……6
3 Transport theory of thermoelectric………………7
Chapter 2 Literature review……………………………12
1 Bismuth-telluride based compounds…………………12
1.1 Crystal structure of Bi2Te3 ………………………12
1.2 Transport properties of Bi2Te3……………………14
1.3 Defects in Bi2Te3……………………………………14
2 Recent improvement in thermoelectric field ………16
2.1 Reduction of thermal conductivity…………………17
2.2 Enhancement of Seebeck coefficient…………………20
2.3 Enhancement of electrical conductivity……………21
3 Motivation……………………………………………………22
Chapter 3 Experimental design…………………………………25
1 Bi-Se-Te nanostructured bulk preparation……………25
2 Thermal annealing and electrical stressing…………26
3 Characterization of electrical transport properties…27
3.1 Seebeck coefficient………………………………………27
3.2 Electrical resistivity…………………………………28
3.3 Carrier concentration and mobility…………………30
4 Thermal transport properties characterization………31
4.1 Thermal diffusivity……………………………………31
4.2 Specific heat……………………………………………32
4.3 Volume density…………………………………………33
4.4 Thermal conductivity…………………………………33
5 Microstructure and phase determination……………34
Chapter 4 Results and Discussion…………………………35
1 Bi-Se-Te ingot properties……………………………35
2 Effect of ball milling process………………………38
2.1 Microstructure of powder as function of milling time……39
2.2 Electrical transport properties as function of milling time……40
2.3 Thermal transport properties as function of milling time………45
3 Effect of electrical current stressing on cold pressed materials………46
3.1 Electrical transport properties after electrical stressing………47
3.2 Thermal transport properties after electrical stressing…………49
3.3 Microstructure properties after electrical stressing…………49
3.4 Mechanism of electrical current stressing………51
4 Effect of electrical current stressing on hot pressed materials………59
4.1 As hot-pressed material………………………………59
4.2 Electrical transport properties after electrical stressing………60
4.3 Thermal transport properties after electrical stressing………62
4.4 Microstructure properties after electrical stressing………62
4.5 Mechanism of electrical current stressing…………63
4.6 Temperature dependent Hall measurement………66
5 Effect of oxidation on thermoelectric properties………72
Chapter 5 Conclusion and Future prospect………75
1 Conclusion……………………………………………76
2 Future prospect………………………………………76
References……………………………………………………77

                                

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