Interfacial Reactions and Electrical Properties of Thermoelectric Modules Manufactured with Solid-Liquid Interdiffusion Bonding
Date Issued
2014
Date
2014
Author(s)
Yeh, Wei-Ting
Abstract
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.
Subjects
熱電材料
固液擴散接合
接觸電阻
擴散耦合
擴散阻障層
Type
thesis
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