銻化鉍系列化合物為目前室溫下熱電性質表現最佳的材料系統，且普遍應用於商用熱電致冷器模組中。而薄膜型熱電元件組裝中，熱電材料與金屬間所產生的額外接觸電阻與熱阻由於元件尺寸的縮小而不能忽略，這些阻抗將會造成熱電元件效能的低落。因此本研究的主題在於熱電材料與金屬接觸界面性質分析。在本研究中，熱電材料與金屬間接觸電阻率將以TLM(transmission-line-model)方法來量測，而接觸熱阻將以3ω法量測。本實驗成功建立量測系統並能得到可靠的量測數值。實驗結果得知Bi-Sb-Te/metal界面間的接觸電阻率約在10^-5ohm-cm²的數量級。而Bi-Sb-Te/metal界面間的接觸熱阻分則為在約10^-8 m²K/W的數量級。本研究將以不同的金屬界面來比較，藉由量測出的實驗數值來探討界面間電子與聲子的傳輸機制，以期對於熱電材料與金屬間的界面性質有更多的了解。最後由實驗值與理論值的分析，發現金屬的功函數與熱電材料的費米能量之間的差異會對接觸電阻率造成影響。
Bismuth telluride based compounds have been considered as promising candidates for thin-film thermoelectric (TE) devices due to their superior thermoelectric figure-of-merit at room temperature regime. Thermoelectrics/metal electrode junctions usually cause extra resistance to electron flow and heat flux in a typical TE module, and hence degrade the efficiency of TE devices. Such electrical/thermal contact resistance may become a performance killer, especially for thin-film TE devices. Thus, a methodology of evaluating the electrical/thermal contact resistance of TE/metal interfaces becomes essential. In this study the electrical and thermal contact resistances of Bi-Sb-Te thin film/metal were measured using the transmission-line-model (TLM) and the transient 3ω techniques, respectively. In this study, the results show that the electrical contact resistivities of Bi-Sb-Te/metal are in the order of 10^-5ohm-cm² and the thermal contact resistances of the Bi-Sb-Te/metal are in the order of 10^-8 m²K/W, respectively. The effects of Fermi energy of Bi-Sb-Te compounds and the work function of metals on the electrical and thermal contact resistances of Bi-Sb-Te film/metal interfaces are investigated.
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運用製程調整提高N型熱電材料的熱電優質(Figure of merit)，意為提高熱電材料的發電或致冷效率，主要方向為提升電導率，提升西貝克系數，降低熱導率。本實驗室使用粉末冶金的製程，運用行星式球磨將塊才打碎使晶粒細化，再置入模具中壓錠成塊材，細晶可使聲子散射以達到降低熱導的目的，然而球磨後的粉末結構內仍有許多多餘的缺陷使電子的傳輸受到阻礙，本實驗利用高溫退火的製程消除粉體或塊材內部多餘的缺陷，以降低電阻率，而達到材料最佳化的結果。由於近年來有很多N型熱電材料的氧化的研究，氫氣還原是很熱門的去氧化物的方式，再加熱的過程中通入氫氣，氫氣固然有機會還原氧化物，但是還原過程中的高溫亦大幅改變了粉末內部的結構，所以退火也是改善材料的一個重要途徑，本實驗提供兩種退火的製程，一種是壓錠前粉末的預退火，另一種是壓錠後的塊材退火，經過各種熱電性質量測，確實發現這兩種製程能提供不同缺陷消除效果，就粉末與粉末之間與塊材內部顆粒之間的擴散狀況，在退火之後會造成載子遷移率明顯的差異。粉末預退火抑制晶粒成長與嚴重空孔的形成，也可以預先去除球磨部分的缺陷，有潛力可以大幅提升材料的性質。
The manufacturing processes are modified to improve the figure of merit of N-type thermoelectric material, which characterize the electric power generating and cooling performances of the material. The main ways to improve the performance are increasing electrical conductivity, Seebeck coefficient, and decreasing thermal conductivity. We applied ball milling to crush the ingot and get fine grains, and then pressed into bulk specimen by powder metallurgy method. The fine grains scattered the transporting phonons, so the thermal conductivity is lowered. However, there are too many superfluous crystal defects in the material after ball milling, so the transportation of electrons is also retarded. We applied annealing to annihilate the superfluous crystal defects to improve electrical conductivity, and then optimize the material. There are many researches on oxidation effects of N-type bismuth telluride recently, and hydrogen reduction is a common process to reduce the oxide. During the reducing process, hydrogen may reduce the oxide, however, the major effect is about the high reducing temperature causing great change of defects structure of material. Here we notice that the high temperature annealing process is an effective way to improve the properties of the material. Then, we provide two kinds of annealing treatment, one is heating powders prior pressing, the other is heating the pressed bulk specimens. After measuring all kinds of thermoelectric transport properties, we find that the sequence of annealing treatment: prior and posterior indeed affected differently on the annihilation of defects. The diffusion conditions of powders and pressed bulk cause significant differences on carrier mobility after annealing. The annealing on powders restrains grain growth and severe void formation, and removes defects from ball milling process, so it shows great potential in improving thermoelectric transport properties.
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更新日期： 2012-08-08