Reduced Thermal Conductivity of Mg2(Si, Sn) Solid Solutions by a Gradient Composition Layered Microstructure.

Solid solutioning of Mg2(Si, Sn) has been a promising approach in reducing thermal conductivity and leads to improvement of thermoelectric performance. In addition to the Mg2(Si, Sn) solid solutions, we have noticed a layered structure with a gradient composition, which is formed by nonequilibrium solidification and peritectic reaction process and can provide further reduction of thermal conductivity of the Mg2(Si, Sn) solid solutions. All layers of the layered structure have the same face-centered cubic-based structure but varying Sn/Si concentration ratios in each layer. The interfaces between the layers are semi-coherent, reticulating with different numbers of misfit dislocations. Such an interfacial structure brings large numbers of phonon-scattering sources, resulting in further reduction of thermal conductivity in the Mg2(Si, Sn) solid solutions. Consequently, the undoped Mg2Si0.75Sn0.25 containing a higher density of the layered structure has relatively lower thermal conductivity, 1.9 W m-1 K-1 at 523 K, than Mg2Si0.25Sn0.75 with a much lower density of the layered structure, 2.3 W m-1 K-1 at 523 K.