Bismuth doping of induction furnace synthesized Mg₂Si, Mg₂Sn and Mg₂Ge thermoelectric compounds

Silicides and especially Mg₂X (X = Si, Sn, Ge) are considered as attractive thermoelectric materials, which are capable of an efficient heat to electricity energy conversion. They have low density, abundant and affordable raw constituents, and good mechanical properties. Yet, the synthesis of such Mg- based compounds is usually very challenging due to the reactivity, volatility and oxidation tendency of Mg. These challenges led many of the researchers exploring these materials, to low temperature synthesis routes, where, the most common is a solid-state mechanical alloying, using ball milling. In such synthesis approaches, Mg volatility was compensated upon the introduction of excess Mg into the source materials, beyond the stoichiometric composition. In the current research a novel synthesis route based on an induction melting in a closed Ta chamber is proposed. Using this technique undoped and Bi-doped Mg₂X (X = Si, Sn, Ge) compounds were synthesized and analyzed. Phase separation into Mg₂X- and Mg₃Bi₂-rich phases was observed for the Bi-doped compositions, with an associated significant reduction of the lattice thermal conductivity.