Investigation of interfacial reactions between metallic substrates and n-type bulk bismuth telluride thermoelectric material

In practical applications of bismuth telluride thermoelectric materials, the materials need to be connected with a metallic electrode before they can be used; however, the methodology for the direct joining of bismuth telluride to metallic materials remains undeveloped. Formation of liquid bismuth telluride should occur at the solidus temperature during the heating process, even if the chemical composition of a compound is only slightly different from the stoichiometric composition. It is possible that the liquid formed during heating can be used as a joining flux. To collect fundamental knowledge of the joining process at elevated temperatures, the present study experimentally confirmed the formation of liquid in the temperature region above its solidus and investigated the reactivity between the n-type bismuth telluride (n-BT) and metallic electrode materials at a temperature between the solidus and liquidus. We heated n-BT on three types of metallic substrates (Cu, Ni, and Fe) at 773 K in Ar. Among these metals, Cu is strongly reactive to n-BT, whereas Ni and Fe have modest reactivity to the n-BT sample. Our findings indicate that Ni or Fe could be candidate materials for the direct joining of n-BT with further optimization of the mechanical and electrical properties of the joined materials.