Phase separation in bismuth doped Mg₂Si_0.5Ge_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 Mg₂(Si,Sn,Ge) class of TE materials, a miscibility gap and a thermodynamic tendency of phase separation were reported in the Mg₂Si-Mg₂Sn 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 Mg₂Si-Mg₂Ge 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 Mg₂Si-Mg₂Ge quasi-binary system, into Mg₂Si- and Mg₂Ge-rich Mg₂Si_0.5±δGe_0.5±δ phases. This phenomenon is enhanced upon bismuth doping. Upon 1.5, 2, and 2.5% bismuth doping of Mg₂Si_0.5Ge_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 Mg₃Bi₂ 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.