Thermal Transport Driven by Extraneous Nanoparticles and Phase Segregation in Nanostructured Mg₂(Si,Sn) and Estimation of Optimum Thermoelectric Performance

Solid solutions of magnesium silicide and magnesium stannide were recently reported to have high thermoelectric figure-of-merits (<i>ZT</i>) due to remarkably low thermal conductivity, which was conjectured to come from phonon scattering by segregated Mg₂Si and Mg₂Sn phases without detailed study. However, it is essential to identify the main cause for further improving <i>ZT</i> as well as estimating its upper bound. Here we synthesized Mg₂(Si,Sn) with nanoparticles and segregated phases, and theoretically analyzed and estimated the thermal conductivity upon segregated fraction and extraneous nanoparticle addition by fitting experimentally obtained thermal conductivity, electrical conductivity, and thermopower. In opposition to the previous speculation that segregated phases intensify phonon scattering, we found that lattice thermal conductivity was increased by the phase segregation, which is difficult to avoid due to the miscibility gap. We selected extraneous TiO₂ nanoparticles dissimilar to the host materials as additives to reduce lattice thermal conductivity. Our experimental results showed the maximum <i>ZT</i> was improved from ∼0.9 without the nanoparticles to ∼1.1 with 2 and 5 vol % TiO₂ nanoparticles at 550 °C. According to our theoretical analysis, this <i>ZT</i> increase by the nanoparticle addition mainly comes from suppressed lattice thermal conductivity in addition to lower bipolar thermal conductivity at high temperatures. The upper bound of <i>ZT</i> was predicted to be ∼1.8 for the ideal case of no phase segregation and addition of 5 vol % TiO₂ nanoparticles. We believe this study offers a new direction toward improved thermoelectric performance of Mg₂(Si,Sn)