Comparative study of thermoelectric properties of Mg₂Si_0.3Sn_0.7 doped by Ag or Li

In recent decades, Mg₂(Si, Sn) solid solutions have long been considered as one of the most important classes of eco-friendly thermoelectric materials. The thermoelectric performance of Mg₂(Si, Sn) solid solutions with outstanding characteristics of low-price, non-toxicity, earth-abundant and low-density has been widely studied. The n-type Mg₂(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 Mg₂(Si, Sn) solid solutions are mainly improved by optimizing the carrier concentration. In this work, the thermoelectric properties for p-type Mg₂Si_0.3Sn_0.7 are investigated and compared with those for different p-type dopant Ag or Li. The homogeneous Mg₂Si_0.3Sn_0.7 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_{2(1–<i>x</i>)}Ag_2<i>x</i>Si_0.3Sn_0.7 (<i>x</i> = 0, 0.01, 0.02, 0.03, 0.04, 0.05) and Mg_{2(1–<i>y</i>)}Li_2<i>y</i>Si_0.3Sn_0.7 (<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>₀ of p-type Mg_{2(1–<i>x</i>)}Ag_2<i>x</i>Si_0.3Sn_0.7 and Mg_{2(1–<i>y</i>)}Li_2<i>y</i>Si_0.3Sn_0.7 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 4.64×10¹⁹ cm^–3 for <i>x</i> = 0.01 and 15.1×10¹⁹ cm^–3 for <i>y</i> = 0.08 at room temperature, respectively; the Li element has higher solid solubility in Mg₂(Si, Sn), which leads to higher carrier concentration and power factor <i>PF</i> ~1.62×10^–3 <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_1.92Li_0.08Si_0.3Sn_0.7 is 58% higher than that of Mg_1.9Ag_0.1Si_0.3Sn_0.7 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.