CoSb3熱電元件接點之界面反應與相關材料系統相圖

Interfacial reactions at the joints in the CoSb3-based thermoelectric devices and phase diagrams of the related material systems

曲華德(Chu, Alan Hwader)

指導教授 ： 陳信文 ； 汪上曉

國立清華大學/工學院/化學工程學系/碩士(2016年)

摘要

熱電材料具有熱電轉換特性，主要應用於廢熱回收，在能源短缺及環保意識抬頭的現今備受關注。熱電元件之結構通常包含陣列熱電半導體，連接Cu導電基板以組成元件，因此熱電元件中存在許多接點。CoSb3是具代表性之中溫型熱電材料，於450-600oC有優異之熱電表現，Ag-Cu銲料合金是個適合的選擇，為了避免銲料擴散至熱電材料，需要在銲料與熱電材料之間加上一層阻障層。因此，熱電元件中接點為「熱電材料/障層/銲料合金」的型式。接點之界面反應與元件之可靠度與穩定性有極大的關係。本研究進行CoSb3於450oC及600oC之界面反應共有六組：Ag-Cu/CoSb3、Ag-Cu/Co、Co/CoSb3、Ag-Cu/Co/CoSb3、Ag-Cu/Ni/CoSb3以及Ti/CoSb3；相圖部分為Ag-Cu-Co之600oC等溫橫截面圖。Ag-Cu/CoSb3之反應劇烈，當兩者液固接合時，已有125m之反應層生成，將反應偶於600oC進行3天之熱處理，CoSb3熱電材料已完全被消耗而生成CoSb相，藉此結果可知，阻障層之引入是必須的。Ag-Cu/Co之界面反應進行於450oC以及600oC，可觀察到富Cu相之堆積，當於600oC之反應40天時，其反應層厚度約為5m。Co/CoSb3之反應偶以電鍍之方式製備，電鍍上之Co層厚度約為100m，當於450oC反應44天時，可在界面上觀察到CoSb及CoSb2相，厚度分別為18m及4m。將反應偶於600oC進行42天之界面反應，可觀察到CoSb及CoSb2相，其厚度分別為125及82m。Ag-Cu/Co/CoSb3之界面反應於600oC進行42天，Co層之厚度由原本之100m反應剩下30m，當反應持續進行，Co層終究被消耗殆盡。而Ag-Cu/Ni/CoSb3之界面反應於600oC進行12天，介金屬相之厚度約為175m，與Ag-Cu/Co/CoSb3之界面反應結果進行比較，Ni與CoSb3之反應性較強，不適合為CoSb3熱電元件之障層選擇。Ti/CoSb3之反應偶以濺鍍之方式製備，Ti層之厚度約為6m，當於450oC反應21天時，可以觀察到三層介金屬相之生成，分別是TiSb、TiSb2以及TiCoSb，而總厚度為3m。將Co/CoSb3、Ni/CoSb3以及Ti/CoSb3之反應性進行比較，可以發現Ti與CoSb3之反應性最低，對於CoSb3之熱電元件，Ti將是一個較理想的擴散阻障層。Ag-Cu-Co 600oC 等溫橫截面圖已經利用實驗及CALPHAD進行建構，此相圖中存在一個三相區:Ag+Cu+Co，並沒有發現三元相之存在。本研究成果對中溫熱電元件開發具重要參考價值，亦提供了相關材料系統之重要基礎瞭解。

關鍵字

熱電材料 ； 熱電元件 ； CoSb3 ； Ag-Cu銲料 ； 界面反應

並列摘要

Thermoelectric materials can directly convert heat and electricity, and are promising in the usage of waste heat recovery. Due to the energy crisis concerns and environmental issues, thermoelectric materials have been intensively investigated recently. Thermoelectric modules usually consist of areas of thermoelectric semiconductors connected together to Cu plate. Thus, there are many solder/braze joints in the thermoelectric modules. The CoSb3 compound is a skutterudite compound with good thermoelectric properties especially in the mid-temperature range, i.e. 450-600oC. The suitable Pb-free solder for application in mid-temperature range is Ag-Cu eutectic alloy. In order to prevent the solder/braze diffusion into the thermoelectric materials, diffusion barrier layers, Co, Ni and Ti, are introduced between solder/braze and substrates. Therefore, the solder joints in the thermoelectric modules are “thermoelectric material/barrier layer/solder alloy”. The interfacial reactions between various materials at the joints are crucial to the properties and the reliabilities of thermoelectric modules.This study plans to investigate the interfacial reactions at the joints in the mid-temperature thermoelectric modules and the phase diagram of their related materials system. The interfacial reactions studies include the following six systems: Ag-Cu/CoSb3, Ag-Cu/Co, Co/CoSb3, Ag-Cu/Co/CoSb3, Ag-Cu/Ni/CoSb3, and Ti/CoSb3. Phase diagrams studies focus on Ag-Cu-Co 600oC isothermal section.The thickness of reaction layer in quenched Ag-Cu/CoSb3 diffusion couple is about 125m. After the heat treatment at 600oC for 3 days, the CoSb3 substrates completely transform into CoSb phase. It is thus very clear that a barrier layer is needed to be introduced between CoSb3 substrates and Ag-Cu alloys. Cu-rich phase is present in the reaction layer of Ag-Cu/Co diffusion couples heat-treated at 450oC and 600oC, and the maximum thickness of the reaction layer is about 5m at 600oC for 40 days. The Co/CoSb3 diffusion couples are prepared with electroplating, and the thickness of Co layer is about 100m. Two IMCs(CoSb, CoSb2) are observed in the reacted couples at 450oC for 44 days. The thickness of CoSb is 18m and the CoSb2 is 4m. In the reacted couples at 600oC for 42 days, the thicknesses of these two layers are 125m and 82m, respectively. Thethickness of Co layer reduces from 100m to 30m in the Ag-Cu/Co/CoSb3 reacted couples at 600oC for 42 days. As the aging time increases, the CoSb3 keep reacting with Co. Eventually; Co will be consumed completely, exposing Ag-Cu/CoSb3. Co layer is not a suitable candidate for barrier layer. The thickness of IMCs is about 175m in the reacted Ag-Cu/Ni/CoSb3 couples at 600oC for 12 days. Compare the results of Ag-Cu/Co/CoSb3, Ni is not a suitrbla barrier layer candidate. Ti/CoSb3 diffusion couples are prepared with sputter, and the thickness of Ti layer is about 6m. Three IMCs(TiSb, TiSb2, TiCoSb) are observed at the reacted couples at 450oC for 21 days, the total thickness of IMCs are 3m. Compared to Co/CoSb3 and Ni/CoSb3, the consumption of diffusion barrier layer by CoSb3 in Ti/CoSb3 is the least. Ti can be a suitable layer for CoSb3-based thermoelectric modules. The 600oC isothermal section of Ag-Cu-Co ternary system has been determined by experiment and calculation. There is one tie-triangle, Ag+Cu+Co, in the Ag-Cu-Co system at 600oC, and no ternary compound is found.The results of this study are valuable for the development and reliability evaluations of thermoelectric modules. Furthermore, the phase diagrams and interfacial reaction results provide fundamentally important knowledge of the related materials systems.

並列關鍵字

thermoelectric materials ； thermoelectric modules ； CoSb3 ； Ag-Cu solder alloys ； interfacial reactions

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