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B2: Module design
Lead Telluride (PbTe) Materials owns unique thermoelectric characteristic, which has attracted considerable attention for mid-to-high temperature thermoelectric applications. The PbTe thermoelectric modules can serve as energy harvesting unit to convert heat into electricity under a high-temperature operating environment. However, the high-temperature bonding issue of PbTe module is still under investigation. This is because PbTe thermoelectric modules operated at elevated temperatures encounter the difficulty on soldering process. To address this issue, we propose a new SnAg-based solder with an additional titanium doping. Our study indicates that a well-controlled bonded interface between thermoelectric PbTe and copper (Cu) contact electrode can be reached by SnAgTi solder bonding under a process temperature of 400oC. The favorable blocking effect for ion diffusion near PbTe interface can be mainly ascribed to the use of Ni barrier and stable solder bonding property. The experimental procedure of PbTe thermoelectric module was described as follow. First, the PbTe elements through high-temperature sintering were processed by a sample preparation step including bulk cutting into 3×3×3 mm and surface polishing. After surface pre-treatment, a Ni barrier layer with a thickness of about 30 to 50 mm was electroplated on the PbTe element. Finally, the SnAgTi solder was used for interfacial bonding between Cu electrode and Ni barrier at a temperature of 400oC in vacuum with a pressure of 1×10-2 torr. To further investigate interface diffusion behaviors, these samples were inspected by scanning electron microscope (SEM) and electron probe X-ray microanalyzer (EPMA). From experimental results, we find that the titanium doping into SnAg solder not only can form thermally stable eutectic phase, but also enhance the interface bonding quality. The detailed material analysis and interface reaction mechanism will be provided in this work.
