A Simple Solid-State Direct Bonding Process for Fabrication of Ohmic Contacts on n-type Mg3Sb2-xBix-based Thermoelectric Materials

N-type Mg3Sb2-xBix materials are expected to replace n-type bismuth telluride because they are more abundant in the Earth's crust and lower in cost. However, the metallization process of this material, which is essential to develop modules, has been relatively under-researched. According to the literature, one-step sintering process based on powder metallurgy is mostly used to form an ohmic bonding layer on n-type Mg3Sb2-xBix materials. However, this method is not reproducible nor practical, and is unsuitable for industrial mass production. This paper presents a new, simple method for metallization, the solid-state direct bonding of Mg and Cu foils on sintered n-type Mg3Sb2-xBix. It employs interdiffusion between the thermoelectric material and the metal layer at elevated temperatures to form a tight and robust bond. Mgwas chosen as the contact metal so that the interdiffusion of Mg would not cause a Mg deficiency in the Mg3Sb2-xBix near the interface. Then, a Cu layer was selected as the second metal to wrap around the Mg layer so that the subsequent soldering process could be the same as that of bismuth telluride. Solid-state bonding with the Mg/Cu double layer formed a structurally perfect joint at 723 K. When the temperature of the solid-state bonding exceeded 750 K, the eutectic point of Mg and Cu, the Mg layer was lost due to liquid phase formation. Solid-phase bonding at 723 K produced no noticeable change in the Seebeck coefficient near the interface, which would be caused by the outdiffusion of Mg from the n-Mg3Sb2-xBix. The specific contact resistance was about 53.8 μΩ cm2. This is superior to the values reported for n-type Bi2Te3-based materials. In this study, we also fabricated a Mg3Sb2-xBix (n)-Bi2Te3 (p) hybrid thermoelectric module and evaluated the output characteristics, which confirmed the high applicability of our solid-state bonding process.