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A2: Medium temperature materials

Nickel Barrier Layers on Thermoelectric Zinc Antimonite Materials for High Temperature Bonding Applications

H. H. Hsu1,*, C. H. Cheng2, Y. R. Chen1, B. Y. Sung1, C. M. Liu1, R. C. Juang1, Y. L. Lin1, and C. P. Cheng2
1Green Energy and Environment Research Lab., Industrial Technology Research Institute, Taiwan2Dept. of Mechatronic Tech. National Taiwan Normal University, Taiwan

Among thermoelectric materials, the bismuth telluride (PbTe) is regarded as mid-to-high temperature thermoelectric. However, the thermoelectric PbTe with low Gibbs energy of formation of PbTe (-100 kJ/mol) is still challenging study for high-temperature module bonding. Besides, the high vapor pressure (3×10-4 mmHg at 300oC) of tellurium is also sensitive to the deposition process, which can lead to an unstable control on compound stoichiometry. In this study, we adopt a zinc antimonite (ZnSb) thermoelectric material with eco-friendly lead-free advantage to perform the diffusion barrier study based on different film structures of Ni/PbTe and Ni/ZnSb. The experimental procedure was described as the following. The ZnSb elements through sintering were pre-treated by surface polishing to increase the electroplating adhesion. After that, a nickel (Ni) barrier layer was electroplated on PbTe and ZnSb thermoelectric element and followed by different post annealing temperatures. Finally, these samples were inspected by scanning electron microscope (SEM) and energy dispersive spectrometry (EDX) and also tested by electrical measurement. To investigate the complex phase mixture in Ni/PbTe and Ni/ZnSb systems, the further information regarding the crystallinity phases and grain size was obtained from x-ray diffraction (XRD). This study investigates the feasibility of introducing Ni barriers on n-type PbTe and p-type ZnSb thermoelectric elements. From our experimental results, the formation of Te-based intermetallic compounds would lead to a rapid inter-diffusion between Ni and PbTe interface and accelerate the degradation of thermoelectric property. The atomic inter-diffusion at Ni/PbTe interface and tellurium out-diffusion phenomenon after high-temperature annealing have been evaluated by high-resolution SEM and EDX. However, the different diffusion behaviors occurred between Ni barrier and thermoelectric ZnSb are also observed at various annealing temperatures. The related interfacial thermodynamic properties shall be provided after comparing the two thermoelectric material systems.