In situ nanostructure design leading to a high figure of merit in an eco-friendly stable Mg2Si0.30Sn0.70 solid solution

Abstract

The relationship between the temperature and the composition as well as the microstructure of a Sb-doped Mg₂Si_0.30Sn_0.70 solid solution was systematically studied according to the Mg₂Si–Mg₂Sn pseudo-binary phase diagram. This work shows that the composition, distribution pattern, and fraction of in situ nanostructures can be controlled by the heat treatment carried out at a specific peritectic reaction temperature. A large number of in situ nanoprecipitates were observed when the sample was quenched at 900 K or 1130 K. However, quenching at 900 K resulted in the formation of agglomerates while quenching at 1130 K yielded a more even dispersion of nanoprecipitates. Monochromatic X-ray studies, back-scattering images and HRTEM results showed that the composition of the nanoprecipitates when quenched at 900 K was the Mg₂Si-rich phase, while the nanostructure was the Mg₂Sn-rich phase when quenched at 1130 K. The lattice thermal conductivity decreased dramatically due to the well distributed Mg₂Sn-rich nanoprecipitates, and the maximum ZT of about 1.2 was achieved at around 750 K. Moreover, the average value of the figure of merit ZT_average reached about 0.9 in the range of 300–800 K, about a 15% higher value than in the sample composed of Mg₂Si-rich nano-agglomerates. This study demonstrates that carrying out heat treatments at the phase transformation temperatures is a simple and controllable method to design and form in situ nanostructures, which is of vital importance in the fabrication of nanocomposites and optimization of the thermoelectric performance.