Phase separation in bismuth doped Mg 2 Si 0.5 Ge 0.5 thermoelectric alloy.

Phase separation by the spinodal decomposition or nucleation and growth mechanisms is an established method for the generation of thermodynamically stable sub-micron features, capable of both reducing the lattice thermal conductivity, κ _l , and stabilizing its value, while obtaining high and stable thermoelectric (TE) figure of merit ZT values during practical applications. In the Mg ₂ (Si,Sn,Ge) class of TE materials, a miscibility gap and a thermodynamic tendency of phase separation were reported in the Mg ₂ Si-Mg ₂ Sn quasi-binary section of the ternary phase diagram, capable of enhancing and stabilizing the TE performance, by κ _l minimization. Yet, no such tendency was ever reported for the Mg ₂ Si-Mg ₂ Ge quasi-binary system, prohibiting the fulfillment of its TE potential. It is currently shown that a similar (yet, less pronounced) tendency of phase separation is also apparent in the Mg ₂ Si-Mg ₂ Ge quasi-binary system, into Mg ₂ Si- and Mg ₂ Ge-rich Mg ₂ Si _{0.5± δ} Ge _{0.5± δ} phases. This phenomenon is enhanced upon bismuth doping. Upon 1.5, 2, and 2.5% bismuth doping of Mg ₂ Si _0.5 Ge _0.5 , following induction melting and hot-pressing, the solubility limit of Bi was found as 1.5-2%, while increasing the bismuth content resulted in significant Mg ₃ Bi ₂ segregation into grain boundaries. The combined phase separation and segregation effects on κ _l reduction with the electronic effect of Bi doping resulted in a reasonably high maximal ZT of 0.9, which was observed upon 2.5% Bi doping.