Comparative study of thermoelectric properties of Mg₂Si_0.3Sn_0.7 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.

Influence of spherical inclusions on effective thermoelectric properties of thermoelectric composite materials

“…The effective figure of merit for the thermoelectric composite material can be defined as Z = ᾱ2 σ / k from the effective properties. It is noted that the derivation process of the effective electric conductivity, heat conductivity and Seebeck coefficient are independent of the geometrical configuration of the internal inclusions, therefore equations (20) and ( 22) are suitable for all the problem of thermoelectric composites containing internal inclusions when the local concentration factors H and Ḡ are obtained. However, the H and Ḡ are determined based on the knowledge of the geometrical configuration of the internal inclusions.…”

“…[15] As a result, most efforts dedicating to improving the conversion efficient have involved alloying, doping, quantum, and other size effects in thin thermoelectric films and mesoscopic systems. [16][17][18][19][20][21][22][23][24][25][26][27] Lots of papers have focused on improving the properties of thermoelectric materials by using size effect and interface effect, but very few researchers have examined the classical composite mixtures as a route to enhancing the thermoelectric characteristics. In such thermoelectric composites, the granularity and heterogeneity are on a large enough scale that the physical processes can be well described in each individual phase by the classical thermodynamic theory.…”