電化學法製備熱電致冷器之Bi2Te3薄膜及奈米線之研究

Preparation of Bi2Te3 Thin Film and Nanowires for Thermoelectric Refrigeration by Electrochemical Method

王文琳(Wen-Lin Wang)

指導教授 ： 王詠雲 ； 萬其超

國立清華大學/工學院/化學工程學系/博士(2008年)

https://doi.org/10.6843/NTHU.2008.00015

摘要

Bi2Te3有極高的熱電優值，並常被用作熱電致冷器的材料。在這篇報告裡將研究以電鍍的方式來製備Bi2Te3合金。當電鍍反應發生時，電解液裡的電化學活性物質之輸送現象將以石英振盪微平衡器(QCM)與旋轉電極 (RDE)加以研究。我們發現，在相同的電位極化條件下，沉積物的成份可以藉由擴散─控制的方式加以改變。QCM和RDE的實驗結果皆顯示Bi3+ 和 HTeO2+的供應對反應物成份和反應速率有很大的影響。首先，薄膜的成份主要是由電極表面反應物的濃度所決定的，並非單靠整體的濃度來改變，因此，擴散─控制的方式就提供了一個絕佳的方法來控制反應物的成份卻不會改變電極極化的情況。再者，當沉積電壓相對於飽和甘汞電位低於-0.14伏特時，其沉積的反應速率式可用下列式子表示
r (Bi-Te) = 8.569x10^-6 x [Bi3+]^0.315 x [HTeO2+]^0.245
另外，由於奈米線的熱電性質預期會較塊材改善許多，我們也採用陽極氧化鋁薄膜來製備Bi2Te3奈米線陣列。我們採用旋轉電極製備Bi2Te3 奈米線陣列，發現在高深寬比的氧化鋁模板之中，物質的輸送是決定反應物成份與反應速率的一個重要因素，因為在這些奈米極的孔洞內，物質的輸送完全只靠擴散。除了很慢的沉積速率之外，單靠擴散的效應也會使沉積的奈米線產生一個濃度梯度延著奈米線的底部和頂端生成。藉由電極的旋轉，可使電極表面達到一個穩定的動態平衡，物質濃度損耗的區域會從溶液的深處縮短到奈米孔的開口處，因此奈米線的成份濃度梯度會減少，奈米線的成長速率也會加快。最後，在薄模板的限制下，奈米線的成分已證實可由改變轉速來加以控制。
最後，將量測不同組成之Bi2Te3薄膜的性質，薄膜的成分由我們提出的擴散-控制的方式來改變：在一個固定厚度擴散層內，藉由控制Bi3+/HTeO2+在此擴散層內的比例，可達到成分控制的目的。此方法所沉積的薄膜具有均勻、緻密和精準的成份以及固定的優選結構。X-ray繞射分析與掃瞄式電子顯微鏡皆顯示不同的Te含量會改變沉積物的晶格常數與晶粒大小。當沉積物的Te含量大於60%時 (Te-rich)，Seebeck常數會隨Te的增加而增加。Te-rich的薄膜有較高的電荷載子濃度，但載子的移動力卻較Bi-rich的薄膜差。我們觀察到在所沉積的薄膜中，電荷載子濃度與載子的移動力有反比的關係，同樣的反比關係也存在於Seebeck常數與電導度。因此，成分最接近Bi2Te3比例的薄膜具有最佳的功率因子。
若將電鍍沉積的Bi2Te3薄膜作熱處理，會改善薄膜的結晶度， Seebeck常數與電導度會呈現正比的關係，明顯地提升薄膜的功率因子。

關鍵字

熱電 ； 電鍍 ； 薄膜 ； 奈米線

並列摘要

Bismuth telluride has the highest figure of merit, and is thus widely employed in thermoelectric refrigeration. In this study, the method of fabrication of Bi2Te3 alloy by electrodeposition was investigated. When electrodepositing Bi2Te3 thin film, the importance of mass transport of the electroactive species has been evaluated by quartz crystal microgravity (QCM) and rotating disc electrode (RDE). Under a constant polarized condition, the composition of the deposit was confirmed to be adjustable by the diffusion-controlled method. Both QCM and RDE studies reveal the importance of supplying Bi3+ and HTeO2+ for altering the film composition and the reaction rate. Firstly, the film composition depends mainly on the surface concentrations of the active species rather than the bulk concentrations. Thus the diffusion-controlled method contributes an effective route to determine the film composition without changing the polarized condition. Secondly, when the potential is below -0.14 V vs. SCE, the deposition rate can be expressed by the relation,
r (Bi-Te) = 8.569x10^-6 x [Bi3+]^0.315 x [HTeO2+]^0.245
Also, we have fabricated Bi2Te3 nanowire arrays via anodic alumina membrane, which have potential applications in thermoelectric devices due to the predicted enhanced thermoelectric properties of nanowires relative to the bulk. A rotating electrode was employed to investigate the electrodeposition of Bi2Te3 nanowires. We found that mass transport of electrolytes into alumina templates of high aspect ratio plays a significant role in determining the properties of the obtained wires. Diffusion is the rate-determining mechanism of mass transport within these nano-channels. In addition to slow growing rate, the effect of mass transport causes a slight composition variation from the bottom to the top of the wires. With a rotating electrode, the composition variation along the wires can be reduced by shortening the concentration depleted zone from the bulk electrolyte to the opening of pores. The wire growing rate can consequently be increased. Moreover, the wire compositions were confirmed to be adjustable by varying the rotation speed when a thin template was used as substrate.
The characterization of thermoelectric properties of electrodeposited Bi2Te3 thin films was discussed in chapter 4. The film composition was varied by our proposed diffusion-controlled method, which is related to the change of Bi3+/HTeO2+ ratios in a controlled diffusion layer. A homogeneous and dense film with precise chemical composition could thus be obtained under constant electrode polarization. Meanwhile, the sole dependence of film properties on composition change of both Te-rich and Bi-rich films were investigated. Firstly, the studies of XRD and FE-SEM show that different Te contents in deposit would lead to different dimensions of unit cell and grain sizes. The Seebeck coefficient increased apparently when the Te content was over 60 at.%Te. Te-rich films had higher carrier concentration but lower mobility than Bi-rich films. Inverse relations were observed between carrier concentration and carrier mobility and between Seebeck coefficient and conductivity. Therefore, an optimal power factor of 7×10^-4 W/m K2 was realized near the stoichiometric Bi2Te3.
If heat treatment was employed on the deposited films, the Seebeck coefficient is possible to increase with the electrical conductivity due to improved crystallinity. The power factor could significantly be increased to a value of 15×10^-4 W/m K2 by heat treatment.

並列關鍵字

thermoelectric ； electrodeposition ； Bi2Te3 ； nanowire

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