Enhanced thermoelectric properties of n-type Mg2.16(Si0.4Sn0.6)1−ySby due to nano-sized Sn-rich precipitates and an optimized electron concentration

Abstract

Due to the rich reserves of the raw materials, along with their low cost and nontoxic nature, Mg₂Si_1−xSn_x-based compounds have generated intense attention from the international thermoelectric community for their application in thermoelectric power generation within the intermediate temperature range. In this work, we have adopted a two-step solid state reaction followed by a spark plasma sintering process to prepare a series of Sb-doped Mg_2.16(Si_0.4Sn_0.6)_1−ySb_y (0 ≤ y ≤ 0.055) solid solutions. We discuss the influence of Sb doping and the microstructure on the thermoelectric properties. Our results confirm that Sb acts as an effective n-type dopant and we estimate the maximum amount of Sb the Mg₂Si_0.4Sn_0.6 structure can accommodate to be ∼2.3% by XRD, DSC and EPMA analyses. The electron transport properties and low-temperature electronic heat capacity measurements reveal that both the light conduction band and the heavy conduction band contribute to the transport in n-type Mg₂Si_0.4Sn_0.6 solid solutions. The highest density-of-states effective mass and power factor were observed for Mg_2.16(Si_0.4Sn_0.6)_0.985Sb_0.015 with an electron concentration of n ≈ 1.67 × 10²⁰ cm⁻³, which is likely to be due to the Fermi level positioned within ∼2k_BT of both the heavy and light conduction bands providing contributions from both bands. In addition, doping with Sb does not seem to affect the lattice thermal conductivity above room temperature. TEM analysis indicates the presence of Sn-rich precipitates with the size of several tens of nanometers dispersed in the Mg₂Si_0.4Sn_0.6 matrix. Such a nanophase may enhance the boundary scattering of phonons and contribute to a low lattice thermal conductivity. Owing to the above characteristics of the band structure and the microstructure, the Mg_2.16(Si_0.4Sn_0.6)_0.985Sb_0.015 solid solution with n = 1.67 × 10²⁰ cm⁻³ possessed excellent thermoelectric properties and achieved a high ZT value of 1.3 at 740 K. Further reductions in the lattice thermal conductivity could be achieved via optimization of the nanophase inclusions, leading to a further enhancement of the figure of merit for Mg₂Si_0.4Sn_0.6-based solid solutions.