Comparative study of thermoelectric properties of Mg<sub>2</sub>Si<sub>0.3</sub>Sn<sub>0.7</sub> doped by Ag or Li

Abstract

In recent decades, Mg<sub>2</sub>(Si, Sn) solid solutions have long been considered as one of the most important classes of eco-friendly thermoelectric materials. The thermoelectric performance of Mg<sub>2</sub>(Si, Sn) solid solutions with outstanding characteristics of low-price, non-toxicity, earth-abundant and low-density has been widely studied. The n-type Mg<sub>2</sub>(Si, Sn) solid solutions have achieved the dimensionless thermoelectric figure of merit <i>ZT</i> ~1.4 through Bi/Sb doping and convergence of conduction bands. However, the thermoelectric performances for p-type Mg<sub>2</sub>(Si, Sn) solid solutions are mainly improved by optimizing the carrier concentration. In this work, the thermoelectric properties for p-type Mg<sub>2</sub>Si<sub>0.3</sub>Sn<sub>0.7</sub> are investigated and compared with those for different p-type dopant Ag or Li. The homogeneous Mg<sub>2</sub>Si<sub>0.3</sub>Sn<sub>0.7</sub> with Ag or Li doping is synthesized by two-step solid-state reaction method at temperatures of 873 K and 973 K for 24 h, respectively. The transport parameters and the thermoelectric properties are measured at temperatures ranging from room temperature to 773 K for Mg<sub>2(1–<i>x</i>)</sub>Ag<sub>2<i>x</i></sub>Si<sub>0.3</sub>Sn<sub>0.7</sub> (<i>x</i> = 0, 0.01, 0.02, 0.03, 0.04, 0.05) and Mg<sub>2(1–<i>y</i>)</sub>Li<sub>2<i>y</i></sub>Si<sub>0.3</sub>Sn<sub>0.7</sub> (<i>y</i> = 0, 0.02, 0.04, 0.06, 0.08) samples. The influences of different dopants on solid solubility, microstructure, carrier concentration, electrical properties and thermal transport are also investigated. The X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images show that the solid solubility for Ag and for Li are <i>x</i> = 0.03 and <i>y</i> = 0.06, respectively. Based on the assumption of single parabolic band model, the value of effective mass ~1.2<i>m</i><sub>0</sub> of p-type Mg<sub>2(1–<i>x</i>)</sub>Ag<sub>2<i>x</i></sub>Si<sub>0.3</sub>Sn<sub>0.7</sub> and Mg<sub>2(1–<i>y</i>)</sub>Li<sub>2<i>y</i></sub>Si<sub>0.3</sub>Sn<sub>0.7</sub> are similar to that reported in the literature. The comparative results demonstrate that the maximum carrier concentration for Ag doping and for Li doping are 4.64×10<sup>19</sup> cm<sup>–3</sup> for <i>x</i> = 0.01 and 15.1×10<sup>19</sup> cm<sup>–3</sup> for <i>y</i> = 0.08 at room temperature, respectively; the Li element has higher solid solubility in Mg<sub>2</sub>(Si, Sn), which leads to higher carrier concentration and power factor <i>PF</i> ~1.62×10<sup>–3</sup> <inline-formula><tex-math id="Z-20190527102739-2">\begin{document}${\rm W}\cdot{\rm m^{–1}}\cdot{\rm K^{–2}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20190247_Z-20190527102739-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20190247_Z-20190527102739-2.png"/></alternatives></inline-formula> in Li doped samples; the higher carrier concentration of Li doped samples effectively suppresses the bipolar effect; the maximum of <i>ZT</i> ~0.54 for Mg<sub>1.92</sub>Li<sub>0.08</sub>Si<sub>0.3</sub>Sn<sub>0.7</sub> is 58% higher than that of Mg<sub>1.9</sub>Ag<sub>0.1</sub>Si<sub>0.3</sub>Sn<sub>0.7</sub> samples. The lattice thermal conductivity of Li or Ag doped sample decreases obviously due to the stronger mass and strain field fluctuations in phonon transport.