中高溫熱電模組之擴散阻障層研究

Abstract

熱電材料是近年來興起的一種乾淨綠色能源，能將熱能與電能互相轉換，然而單一熱電材料之熱電轉換效率有限，因此工業上以P型和N型交互串聯做成的熱電模組應用最具優勢，而傳統熱電材料與電極間的接合方式常以軟銲或硬焊為主，由於熱電材料需在高溫環境下操作，因此軟銲及硬焊皆有其不適用之地方，為了克服傳統接合的缺失，本研究選用中高溫熱電材料Zn4Sb3、CoSb3為母材、Cu為電極，先以Ni作為擴散阻隔層，Ag、Sn分別作為高熔點及低熔點金屬進行固液擴散接合，且在不同時間及溫度的接合參數下，進行界面微結構分析、接合強度測試以及破斷面分析。 實驗結果顯示，CoSb3預鍍錫後強度皆能提升到10 MPa以上，Zn4Sb3則是不需預鍍錫就能有25 MPa左右的強度，但於長時間的反應下，熱電材料Zn4Sb3可大量地提供Zn使得γNi5Zn21不斷增厚直到Ni被完全消耗殆盡，CoSb3亦是如此，顯示兩種熱電材料皆會有Ni擴散阻障層被消耗進而無法阻擋各層金屬原子間的互相擴散，最終導致整體元件的破裂與失效。因此嘗試利用濺鍍金屬Ti薄膜或金屬玻璃的方式來取代，發現以金屬玻璃Zr53Ni30Cu9Al8不論接合時間多長，與Zn4Sb3形成的IMC layer就只維持約10 um；而濺鍍上Ti作為buffer layer後，再鍍上WTi膜，實驗發現將有助於改善先前因熱膨差異、附著性不佳及連續性不足等問題，且亦能成功擋住Zn的擴散。

In the recent years, thermoelectric(TE) materials are a very competitive renewable energy. They can generate an electric potential while in a temperature gradient. In order to enhance the conversion efficiency, thermoelectric devices are usually composed of arrays of modules made by P-type and N-type thermoelectric materials in industrial. And, there are numerous solder or braze joints connecting these arrays of modules in thermoelectric devices conventionally. However, traditional soldering bonding is not suitable for operating in high temperature. We choose Zn4Sb3 and CoSb3 for mid-high temperatured TE materials, copper for electrode, silver for high melting point metal and tin for low melting point metal. After the SLID bonding procedure, we will process a series of analysis on interfacial morphology, shear strength test and fracture surface observation in various bonding temperature and time. The results show that the shear strength can be up to 10 MPa when the CoSb3 was pre-coated with tin layer and heated before SLID bonding. However, the shear strength of Zn4Sb3 is approximately 25 MPa without pre-coated with tin layer. To prevent direct contact and interfacial reactions between solders or brazes and thermoelectric materials, nickel is often used as the barrier layer. However, in long-term reaction, Zn4Sb3 can sufficiently offer Zn to form γNi5Zn21 until nickel is completely consumed. It might cause the device failure in someday. So, nickle is not suitable for being the diffusion barrier anymore. Therefore, we tried to use sputtering Ti or metallic glass thin film to replace the original nickle diffusion barrier. The results show that the thickness of IMC is only 10 um no matter how long the metallic glass Zr53Ni30Cu9Al8 bonding with Zn4Sb3. On the other hand, the problem of thermal residual stress and poor adherence can be solved when we sputter Ti as buffer layer before WTi layer. And, this double barrier layer also successfully blocks the diffusion of zinc into high melting metal.