「熱門文章」熱電元件與電極間的界面研究

由南方科技大學劉瑋書教授和中國科學院上海矽酸鹽所柏勝強研究員發表了這篇關於熱電元件和電極之間界面研究的綜述：通過過去幾年觀察，熱電材料在應用比例上有了明顯的提高。由於高ZT值的熱電材料在餘熱回收中的廣泛應用，使得熱電器件前景十分廣闊。然而，簡化的熱電材料的性能規模也將熱電能量轉換技術的重點從與器件相關的開發轉移到了材料層面的工作。但是，與熱電器件相關的工作沒有得到足夠的重視。直到最近幾年，大型設備級熱電的應用一直鮮有報導。除了開發優異性能的熱電材料外，熱電元器件中實際能量轉換效率和使用壽命在組裝過程和接觸界面的問題上也需要重點考慮。基於此方面，筆者們試圖在器件級應用上的聚焦，關注熱電元件和電極之間的熱電界面材料(TEIM)，也稱為金屬化層或焊料阻擋層。通過探討TEIM的範圍和相關技術的總結，包括結合強度、界面電阻和穩定性等。討論一些典型的例子，總結了其中的工作原理，為尋找合適的熱電轉換材料TEIM提供了建議。相關研究成果發表在Journal of Materiomics第5卷第3期，題目為Thermoelectric interface materials: A perspective to the challenge of thermoelectric power generation module.

Weishu Liu, Shengqiang Bai, Thermoelectric interface materials: A perspective to the challenge of thermoelectric power generation module. Journal of Materiomics 2019; 5 (3): 321-336.

請注意！又是一篇充滿了內涵的綜述~！贊~！

內容概述

熱電發電因其在以放射性同位素材料為熱源的深空探索飛行任務中,在雙航行太空飛行器上效力長達40年之久，其可靠的電力供應而聞名於世。在石油危機之後，人們對可持續發展的關注再次引發了對這種基於熱電效應材料的重新審視，即直接的「熱電」技術。熱電能量轉換技術以其簡單的系統結構、無振動操作；且具有從運輸車輛、工業操作和垃圾焚燒中廣泛獲取餘熱的優勢，再一次喚醒人們的渴求。在具有溫度無關輸運參數的熱力學理論的基礎上，將能量轉換效率（η）簡單地與卡諾效率和無量綱化圖(ZT)聯繫起來，ZT的熱電性能尺度是由A.F.Ioffe首先提出的，通過ZT=TS2σ/κ的關係，其中T、S、σ分別是溫度、Seebeck係數、電導率和熱導率。可以注意到，由於與溫度有關的傳輸特性，在實際情況下只有在較窄的溫度範圍內才有效。提高ZT值的「工程理念「是將一個準確的性能規模在一個廣泛的溫度範圍裡。在過去的三十年裡，人們看到了許多新的物理模型或概念，超晶格、納米複合材料、納米晶、納米包裹體、附加載流子口袋、扭曲的DOS、帶收斂、響尾狀的填料、複雜的晶體結構、液相亞晶格、自旋熵(增強S)和高熵合金化（還原κ）等。

目前，根據Web-Science資料庫，每年有超過2000多篇熱電材料相關的論文被發表，可以看出這是納米材料、能源材料領域的一個熱門話題。然而，事事都有兩面性。簡化的性能尺度將熱電能量轉換技術的重點從與器件相關的工作轉移到材料級別的工作。但是，與熱電器件相關的工作沒有得到足夠的重視。直到近幾年，熱電應用設備級的挑戰一直還是未知，好比冰山一角。從結構和組裝工藝的角度來看，熱電模塊是簡單的，它包括n/p型半導體單元模塊，由電極條串聯連接，並進一步封裝在兩個平行的陶瓷基板中。熱電腿和電極條可以設計成「π形狀」和「線型狀」。熱電模塊可以很容易地組裝在一個小車間和一個實驗艙中。然而，對於一個真正的熱電發電(TEG)系統，需要考慮它作為一個熱發動機，利用電子作為能量載體。一般TEG系統包括至少三個部分：熱電發電機，熱源和散熱器。

Fig 1(b)給出了一個用於汽車餘熱回收的TEG系統的示例，該系統包括400個模塊，每個模塊具有28個基於半赫斯勒材料的熱電對。通過對汽車柴油機排氣餘熱的轉化，達到了1kW的功率密度。在系統層面上，系統效率不僅由TEG子系統決定，而且還取決於TEG零件和熱源的連接方式，以及散熱器（風冷或水冷)散熱方式。此外，在實際熱電能量產生中，DC-DC升壓或調節器也是很有必要的。熱電發電確實是一項系統工程，它涉及凝結物理、材料科學、製造、熱管理和電子工程等知識。由於我們認知的限制，這篇綜述只淺談一下TEG模塊的挑戰，其中特別關注了熱電元件與電極之間的界面相關研究。

Fig. 1. Thermoelectric power generation. (a) The thermodynamic working principle, (b) a real TEG system for automotive waste heat recovery. Figure(b) was cited from Ref..

Fig. 2. Key materials of a thermoelectric power generation device. (a) thermoelectric conversion materials (TEcM), (b) thermoelectric interface materials [TEiM], (c) thermoelectric package materials [TEiM], (d) thermoelectric housing materials [TEiM].

Fig. 3. (a) Challenges of the TEiM/TEcM contact interface: Bonding strength (Shear mode and Tensile Mode), Contact Resistance (Thermal and Electrical) and Stability (Diffusion control and Reaction control). (b) Connections between the interfacial requirements and designing principle.

Fig. 4. Mechanical failures of TEG devices after the thermal cycles test or accelerated aging process. (a–b) Comparison between the optical and infrared images of cracks at the TEiM/TEcM interface of Bi2Te3 device after thermal cycling test, (c–d) Comparison of SEM images of interface of Bi2Te3 device before and after thermal cycling test; (e–f) the TEiM/TEcM interface of a CoSb3 based Skutterudite device after aging test.

Fig. 5. Energy-band diagrams of metal-semiconductor contacts. Metal and semiconductor (a) in separated systems, and (b) connected into one system. As the gap δ (c) is reduced and (d) becomes zero.

Fig. 6. Interfacial resistance measurements by the scanning probe. (a) Ni/Ni3Te2/Ni2Te3/Bi2Te2.7Se0.3/Ni2Te3/Ni3Te2/Ni legs, made by hot pressing at 500 °C. The cross-section of the legs was 1.8*1.8 mm2, the current was 0.1 A. (b) Ni/BL-1/Bi2Te2.7Se0.3/BL-1/Ni and Ni/BL-2/Bi2Te2.7Se0.3/BL-2/Ni, made by hot pressing at 425 °C. BL-1: 90% Bi2Te2.7Se0.3 +10% NiSe2, BL-2: 1% SbI3 doped Bi2Te2.7Se0.3.

Fig. 7. SEM micrographs of CoSb3/Ti interface of a CoSb3/Ti/Mo-Cu joint aged at: (a) 525 °C for 16 days, (b) 525 °C for 30 days, (c) 575 °C for 4 days and (d) 575 °C C for 8 days. (e–f) schematic illustration of the interfacial reaction in the CoSb3/Ti/Mo-Cu joint at different stages during aging.

Fig. 8. (a) SEM micrograph of CoSb3/Ti/Mo-Cu joint aging at 600 °C for 4 days. (b) Shear strength of CoSb3/Ti/Mo-Cu joint with aging temperature and time.

Fig. 9. Microstructure of TEiM/TEcM interface after aging at 600 °C for 16 days: (a) Ti/Yb0.6Co4Sb12 (b) Ti88Al12/Yb0.6Co4Sb12; (c) diffusion layer thickness, and (d) contact resistivity as a function of the aging time for different Ti-Al/Yb-SKD joints aged at 600 °C.

Fig. 10. (a) Resistance of the as-sintered W-Si3N4/TiB2-Si3N4/Si80Ge20B0.6 TE joints with the cross-section area of 10 mm2 and (b) contact resistivity with the Si3N4 content (x) in (100-x) vol%TiB2+ x vol% Si3N4 intermediate layers of W-Si3N4/TiB2-Si3N4/Si80Ge20B0.6TE joints.

Fig. 11. Mapping results of typical interfaces between the Yb0.3Co4Sb12 substrate and particles of different barrier layer candidates (a) before and (b) after aging at 650 °C for 2 days in vacuum.

亮 點

• The thermoelectric device-level challenge was discussed with special focus on the contact interface.
• Thermoelectric interface materials (TEiM) was defined as one of key technique in the thermoelectric materials.
• The technique requirement for the design of TEiM was explained.
• Some efforts to search good TEiM for the classic TEcM, including Bi2Te3, CoSb3 and SiGe, will be reviewed.

作者介紹

Dr. Weishu Liu is an associate professor of materials science and engineering at the Southern University of Science and Technology (SUSTech). He obtained his Ph.D. degree from the University of Science and Technology of Beijing in2009. He was a post-doctor in UW and BC, an associate researcher in UH, and a principal scientist in Sheetak Inc. His research interests include novel thermal-to-electric technique, transport of electron and phonon, nanostructured materials and their application in the in waste heat harvesting, solar heat utilization, and wearable devices.

Dr. Shengqiang Bai, Received his bachelor degree from Zhejiang University in 2001, and his Ph.D. degree in Materials Physics & Chemistry from Shanghai Institute of Ceramics, Chinese Academy of Sciences in 2010. In 2001 he joined Shanghai Institute of Ceramics and was promoted to professor in 2017. His research activities primarily involve the mass production of high performance thermoelectric materials including skutterudites, half-Heusler, SiGe-based alloys and other new TE materials; design and fabrication of TE devices; evaluation and service behavior of TE devices; and also the applications of TE devices. He received the 2nd Prize of National Award for Technological Invention in 2014.

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The Journal of Materiomics is indexed by SCI (IF=5.797, rank in Q1 of Material Science, multidisciplinary) and Scopus (Citescore 8.6), aims to provide a continuous forum for the dissemination of research in the general field of materials science, particularly systematic studies of the relationships among composition, processing, structure, property, and performance of advanced inorganic or non-metallic materials. Supported by the Chinese Ceramic Society, the Journal of Materiomics is a peer-reviewed open-access journal, without publishing charges to authors.