| Summary: | 碩士 === 國立清華大學 === 化學工程學系 === 104 === 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 125m. 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 5m at 600oC for 40 days. The Co/CoSb3 diffusion couples are prepared with electroplating, and the thickness of Co layer is about 100m. Two IMCs(CoSb, CoSb2) are observed in the reacted couples at 450oC for 44 days. The thickness of CoSb is 18m and the CoSb2 is 4m. In the reacted couples at 600oC for 42 days, the thicknesses of these two layers are 125m and 82m, respectively. Thethickness of Co layer reduces from 100m to 30m 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 175m 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 6m. Three IMCs(TiSb, TiSb2, TiCoSb) are observed at the reacted couples at 450oC for 21 days, the total thickness of IMCs are 3m. 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.
|
|---|
