Bi2Te3基熱電模組之界面反應

Interfacial reactions in the Bi2Te3-based thermoelectric modules

劉姿彣(Liu, Zi Wen)

指導教授 ： 陳信文

國立清華大學/工學院/化學工程學系/碩士(2016年)

摘要

熱電模組能夠將廢熱直接轉換成電能，進而提昇整體能源的使用效率，是目前非常重要的研究課題。Bi2Te3基的P型-(BixSb1-x)2Te3與N型-Bi2(TeySe1-y)3合金，是目前商業上最常使用的熱電材料。熱電模組通常包含了陣列的P-N組對熱電元件，並藉由軟銲將所有的熱電元件連結金屬電極，因此在熱電模組中存在著許多之接點。目前主要使用無鉛銲料，如Sn-3.0wt%Ag-0.5wt%Cu合金。為了避免銲料與熱電材料直接接觸，在熱電材料與銲料間常使用Ni(P)做為阻障層(barrier layer)材料。接點的界面反應，影響接點品質。而接點之品質，直接與熱電模組之可靠度相關。尤其熱電發電機模組應用於較高溫之環境，接點之接合界面反應探討更加重要。
本研究的界面反應探討，包括了二類樣品。第一類是相關子系統的反應偶，第二類是組裝完成的熱電模組。第一類的樣品的探討包括了Ni/Bi、Ni/Se、Ni/Sb、Ni/ Bi2Te3、與Ni/ Bi2Se3。於200oC與150oC反應，Ni/ Bi於界面處生成NiBi3相和NiBi相兩相，特別是僅在反應偶的角落處會發現NiBi相的生成。於200oC反應，Ni/Se界面處生成Ni3Se2和Ni1-xSe相。於200oC反應，Ni/ Sb界面處生成兩層介金屬相，分別為NiSb相與NiSb2相。於150oC與200oC進行固固反應，在Ni/Bi2Te3界面處生成NiTe2相與(Bi2)m(Bi2Te3)n相。Te是此界面反應的主要擴散元素，擴散路徑則為Ni/ NiTe2/(Bi2)m(Bi2Te3)n/ Bi2Te3。於150oC進行固固反應，Ni/ Bi2Se3界面生成一層Ni1-xSe相或Ni-Bi-Se三元相；然而當溫度提升為200oC時，界面處則生成二相混合結構的交錯型反應層。
第二類的樣品包含二種熱電模組，第一組為工研院所製備的Bi2Te3基熱電模組，第二組為俄羅斯某公司所製備的Bi2Te3基模組。工研院的熱電模組使用Sn-3.0wt%Ag-0.5wt%Cu合金為銲料，Au/Ni(P)為濕潤層(wetting layer)與阻障層，P-型合金為(Bi0.2Sb0.8)2(Te0.99Se0.01)3，N-型合金為Bi2(Te0.9Se0.1)3。俄羅斯的熱電模組，使用Sn-0.14wt%Cu-0.26wt%Bi合金為銲料，Ni(P)為阻障層，P-型合金為(Bi0.25Sb0.75)2Te3，N-型合金為Bi2(Te0.9Se0.1)3。反應溫度為150oC與100oC，反應時間長達八個月。對經過反應的模組中的界面生成相進行分析，並與文獻中的相關反應偶生成相進行比對。發現界面之介金屬相厚度與時間平方根均呈現線性正比的關係，顯示介金屬的成長是由擴散來控制。

關鍵字

Bi2Te3 ； 熱電模組 ； 界面反應

並列摘要

Thermoelectric modules are important research topic because they can enhance energy usage efficiency by directly converting waste heat into electricity. Bi2Te3-based p-type (Bi,Sb)2Te3 and n-type Bi2(Te,Se)3 alloys are the most commonly used thermoelectric materials in commercialthermoelectric modules. Typically, there are arrays of P-N thermoelectric devices connected to metallic electrodes in thermoelectric modules. Therefore, there are many joints inthermoelectric modules. Nowadays, Sn-based Pb-free solders, such as Sn-3.0wt.%Ag-0.5wt.%Cu, are widely used as solders. In order to prevent direct contact and significant interfacial reactions between solders and thermoelectric substrates, Ni(P) is frequently used as a diffusion barrier layer. Interfacil reactions affect joints’ properties, and the modules’reliablity is directly related with the properties of these joints. Especially, the thermoelectric generators are usually used at relatively high temperatures, understanding of theirinterfacial reactions are even more critical.
There are two kinds of interfacial reaction samples. The samples of the first kind are reaction couples made of the related material sub-systems. Those of the second kind are completed thermoelectric modules. The first kind samples include Ni/Bi, Ni/Se, Ni/Sb, Ni/Bi2Te3 and Ni/Bi2Se3 reaction couples. NiBi3 and NiBi phases are formed in the Ni/Bi couples reacted at 150 oC and 200 oC. NiBi phase is only observed at the corner of the couples. Ni3Se2 and Ni1-xSe phases are formed in the Ni/Se couples reacted at 200oC. In the Ni/Sb couples reacted at 200oC, NiSb and NiSb2 phases are observed. NiTe2 and (Bi2)m(Bi2Te3)n phases are formed in the Ni/Bi2Te3 couples both reacted at 150oC and 200oC. Te is the dominant diffusion species, and the reaction path is Ni/ NiTe2/ (Bi2)m(Bi2Te3)n/Bi2Te3. One reaction phase is formed in the Ni/Bi2Se3 couple reacted at 150oC. This phase could be the Ni1-xSe phase or a Ni-Bi-Se ternary phase. A two-phase mixture reaction zone is observed in the Ni/Bi2Se3 couple reacted at 200oC.
The samples of the second kind have two different groups of termoelectric modules. Thermoelctric modules of the first group are made by ITRI (Industrial Technology Research Instittue), and those of the second group are made by a Russian company. The ITRI-made modules use Sn-3.0wt%Ag-0.5wt%Cu solders and Au/Ni(P) wetting and barrier layers. The P-type thermoelectric substrate is (Bi0.2Sb0.8)2(Te0.99Se0.01)3 and the N-type thermoelectric substrate is Bi2(Te0.9Se0.1)3. The Russian-made modules use Sn-0.14wt%Cu-0.26wt%Bi solder and Ni(P) barrier layer. The P-type thermoelectric substrate is (Bi0.25Sb0.75)2Te3 and the N-type thermoelectric substrate is Bi2(Te0.9Se0.1)3. The reaction temperatures are at 150oC and 100oC. The reactions at each interface in the modules are examined. The results are compared with literature results. The reaction time is as long as 8 months, and the thickness of the reaction layers are proportional to the square root of reaction time which indicate the interfacial reactions are diffusion controlled.

並列關鍵字

Bi2Te3 ； thermoelectric modules ； Interfacial reactions

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