Phase separation in bismuth doped Mg2Si0.5Ge0.5 thermoelectric alloy

Abstract

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.