Sommerfeld-Bethe analysis of ZT in inhomogeneous thermoelectrics

TL;DR

This paper analyzes the thermoelectric figure of merit ZT in inhomogeneous materials like GeTe and Mg3Sb2 using Sommerfeld-Bethe theory, revealing how microstructure influences thermal conductivity and ZT.

Contribution

It introduces a detailed analysis of electrical and thermal transport in inhomogeneous thermoelectrics, linking microstructure to ZT improvements.

Findings

Electrons significantly contribute to thermal conductivity.

Size and shape of grains affect phonon thermal conductivity.

Inhomogeneous structures influence temperature dependence of ZT.

Abstract

The development of good thermoelectric materials exhibiting high $ZT$ (=$\frac{PF}{\kappa} T$) requires maximizing power factor, $PF$, mainly governed by electrons, and minimizing thermal conductivity, $\kappa$, associated not only with electrons but also with phonons. In the present work, we focus on the GeTe and Mg$_3$Sb$_2$ as high $ZT$ materials with inhomogeneous structures and analyze both electrical conductivity, $L_{11}$, and Seebeck coefficient, $S$, with help of Sommerfeld-Bethe formula, resulting in understanding the temperature dependence of $PF$ and the identification of electrons contribution to thermal conductivity, $\kappa_{\rm el}$. Comparing the obtained $\kappa_{\rm el}$ and experimentally measured $\kappa$, the temperature dependence of phonons contribution to thermal conductivity, $\kappa_{\rm ph}=\kappa-\kappa_{\rm el}$, is inferred and analyzed based on the formula by Holland. Comparison of the GeTe and Mg$_3$Sb$_2$ with different types of crystal structures, i.e., GeTe being of a semiordered zigzag nanostructure like a disrupted herringbone structure while Mg$_3$Sb$_2$ of rather uniform amorphous structure, discloses that size effects on temperature dependence of $\kappa_{\rm ph}$ is large in the former, while very small in the latter. Hence, it is concluded that not only the size of the grain but also its shape has an important influence on $\kappa_{\rm ph}$ and then $ZT$.