固液擴散接合製作熱電模組之界面反應及其電性之研究

Abstract

單一熱電元件能夠傳輸或轉換的電能非常有限，必須利用金屬電極將多組熱電元件連結成為熱電模組，才能夠提供足夠的熱電輸送功率，因此熱電材料的應用必須先解決熱電元件與Cu或Ni電極的接合問題。文獻上對於如何製備熱電材料以提升其熱電優值(Figure of merit)ZT進而增進熱電轉換效率已有大量的探討，對於熱電元件模組可靠度等性質之提升則少有著墨。一般熱電材料模組是利用金屬電極將n type及p type熱電材料依序串聯，上下兩側為陶瓷基板所製，在兩側之陶瓷基板製造溫差即可產生熱電效應。本研究著重於多層結構已預鍍Sn之Bi0.5Sb1.5Te3/Sn/Ni/SAC305-Cu 及未預鍍Sn之Bi0.5Sb1.5Te3/Ni/SAC305-Cu並以軟銲方式接合之比較。以及針對Bi0.5Sb1.5Te3/Sn/Ni/SAC30-Cu做時效處理(aging)的討論。研究結果顯示當我們在熱電材料先預鍍一層Sn並加熱使反應，其接合強度將明顯提升至接近母材(Bi0.5Sb1.5Te3)本身之強度，證明該預鍍Sn層可有效增強低溫熱電材料Bi0.5Sb1.5Te3與Ni層之間之接合強度。 實驗的另一部分將選用中溫熱電材料PbSnTe與Cu電極之SLID接合實驗，而由於中溫熱電元件將在高溫環境下操作，以傳統之軟、硬焊接合方法面臨許多挑戰。本研究遂於PbSnTe熱電材料上電鍍Ni後，採用Ag/Sn(or In)薄膜作為固液擴散接合技術之材料，並和Cu/Ag電極接合，利用其低溫接合，高溫應用的特性，接點之介金屬將在相對低溫即可產生，同時在高溫仍然可以保有其穩定性。實驗探討PbSnTe/Ni以及Ag/Sn(or In)薄膜與Cu/Ag電極之界面反應觀察，計算介金屬生長動力學，以及針對不同接合參數下之接點強度做量測。 實驗結果顯示，於PbSnTe熱電材料與Ni層間電鍍一層Sn 並加熱使與熱電材料反應，將能再提升PbSnTe及Ni層之界面接合強度。於250°C以上之溫度接合5分鐘以上，PbSnTe/Sn/Ni/Ag與Sn(or In)/Ag/Cu接合之界面即可把中間4μm之Sn層完全消耗完畢並形成Ag3Sn、Cu6Sn5及Cu3Sn三種介金屬，其中隨著接合溫度與時間上升，Ag3Sn及Cu3Sn將會有消長之勢，且Cu6Sn5將快速被Cu3Sn 取代。 在確定預鍍Sn製程之PbSnTe/Sn/Ni/-Sn(or In)/Ag/Cu樣品確實能提升其接合強度後，接下來將進一步驗證該預鍍Sn層對熱電元件的整體電性是否會有不良影響，實驗結果發現預鍍Sn製程之各系統樣品之各接合區界面接觸電阻率佔整體總電阻率的3％以內，顯示此預鍍Sn之SLID製程並不會令熱電模組的整體電性下降太多。 研究的最後部分則為探討擴散阻障層的選擇，因為當我們選用中溫熱電材料Zn4Sb3來製作熱電模組時，傳統上常作為擴散阻障層之Ni層在此已不適用，理由是Zn的活性過大，所以擴散阻障Ni層將會在使用過程中消耗過速。而後續在嘗試各種不同的擴散阻障層時，發現Ti會是極適合的選擇，並且在未來的應用上可直接以Ti同時作為擴散阻障層與電極，並直接以擴散耦合(diffusion couple)的方式接合之。

The ability for single thermoelectric components to transfer or convert electric energy is very limited. In order to provide sufficient thermoelectric power, metallic electrodes are used to connect a series of thermoelectric components into a thermoelectric module. As a result, solving the joint issues between the Cu or Ni electrode with thermoelectric components is the first step to the application of thermoelectric materials. The process improvement for enhancing the figure of merit, ZT, in thermoelectric material production, which then increases the efficiency of thermoelectric conversion efficiency has been thoroughly discussed in the literature. However, the property enhancement of thermoelectric components and modules, such as reliability, is seldom discussed. In general, thermoelectric modules are connected in series through their metal electrodes in a sequence of n- and p-type thermoelectric materials. The top and bottom sides are made from ceramic substrates. The temperature difference from the ceramic substrate thereby can produce a thermoelectric effect. The focus in this research was to compare the bonding between two multilayer structures: Bi0.5Sb1.5Te3/Sn/Ni/SAC305-Cu and Bi0.5Sb1.5Te3/Ni/SAC305-Cu. In addition, the aging effect of Bi0.5Sb1.5Te3/Sn/Ni/SAC305-Cu was also discussed. Results also showed when the thermoelectric material was pre-coated with a Sn layer and then heated, the joint strength significantly increased to nearly the strength of the base material (Bi0.5Sb1.5Te3) itself. This demonstrated that the Sn layer can effectively increase the adhesion strength between the Bi0.5Sb1.5Te3 and Ni layer. Theother part of the experiment will choose medium temperature thermoelectric PbSnTe which is bonded with Cu electrode, because high temperature environment is needed for thermoelectric components to work properly, the traditional soldering or brazing bond between components and electrodes is facing new challenges. In this research, we studied the property of PbSnTe plated with Ni and with a thin film of Ag/Sn(or In) as the solid-liquid interdiffusion bonding material. Also, with Cu/Ag electrode joint and its low temperature bonding and being applicable to high temperature, IMC can be produced in relatively low temperature and still being stable in high temperature. Experiments are done to check the interfaces between PbSnTe/Ni, Ag/Sn(or In) thin film, and Ag/Cu electrode. We calculated the growth dynamics of intermetallic compound and measured the strength of the joints for different experiment parameters. The results show that electroplating Sn in between thermoelectric material PbSnTe and Ni layer, and then heating it to ensure that there are reactions between thermoelectric material and Sn, which will enhance the strength of the interface between PbSnTe and Ni layer. Heating with temperature over 2500C and over five minutes will ensure Sn layer with 4μm between PbSnTe/Sn/Ni/Ag and Sn(or In)/Ag/Cu is consumed completely. This will create the following three kinds of intermetallic compound, Ag3Sn, Cu6Sn5, and Cu3Sn. With rising temperature and more time, Ag3Sn and Cu3Sn will decline and grow mutually. Also, Cu6Sn5 will be quickly replaced by Cu3Sn. After confirming that the bonding strength of PbSnTe/Sn/Ni/-Sn(or In)/Ag/Cu module can be increased by pre-coating Sn, the next step is to verify the pre-coated Sn layer does not cause negative effect on the electricity of the thermoelectric component. The results indicate the resistance of the reaction region with pre-coated Sn layer is only less than 3% of the overall electrical resistance in the sample. Hence, the pre-coated Sn layer does not affect the overall electrical performance. The last part of the research is to discuss the choice of diffusion barrier layer in the Zn4Sb3 thermoelectric module. The traditional choice of Ni as diffusion barrier layer is not suitable for Zn4Sb3 because of the high Zn activity. Thus, the depletion rate of Ni as diffusion barrier layer is over-accelerated. After testing many different materials as diffusion barrier layer, Ti turns out to be a great option. Theoretically, Ti can be applied for both diffusion barrier and electrode via diffusion couple method.