Phase separation in bismuth doped MgSiGe 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, , and stabilizing its value, while obtaining high and stable thermoelectric (TE) figure of merit values during practical applications. In the Mg(Si,Sn,Ge) class of TE materials, a miscibility gap and a thermodynamic tendency of phase separation were reported in the MgSi-MgSn quasi-binary section of the ternary phase diagram, capable of enhancing and stabilizing the TE performance, by minimization. Yet, no such tendency was ever reported for the MgSi-MgGe 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 MgSi-MgGe quasi-binary system, into MgSi- and MgGe-rich MgSiGe phases. This phenomenon is enhanced upon bismuth doping. Upon 1.5, 2, and 2.5% bismuth doping of MgSiGe, 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 MgBi segregation into grain boundaries. The combined phase separation and segregation effects on reduction with the electronic effect of Bi doping resulted in a reasonably high maximal of 0.9, which was observed upon 2.5% Bi doping.