Thermoelectrics: From Thermoelectric Figure of Merit to Device Design

Abstract

Thermoelectrics is defined as direct energy conversion between the heat and electricity associated with thermoelectric power generation and refrigeration. Significant progress has been made in the theoretical understanding, materials and device preparation technology, and performance improvement of thermoelectrics since the Seebeck effect was found 200 years ago. Herein, the article starts with the basic physics concepts underlying the thermoelectric phenomenon, and the key descriptor for evaluating thermoelectric performance, the thermoelectric figure of merit ( ZT ), is introduced. The corresponding issues and strategies for pursuing higher ZT are discussed in detail, and a brief introduction of organic thermoelectric materials is given. The leading engineering challenges from the material to the device are further elaborated based on the state‐of‐the‐art device design. This article aims to provide the reader with a comprehensive overview of the scope of activities related to this area of scientific endeavor.

Device‐Level Optimization of n‐Type Mg3(Sb, Bi)2‐Based Thermoelectric Modules toward Applications: A Perspective

“…For example, Zn 4 Sb 3 experiences a phase transition at 425 K, which makes it unsuitable for high-temperature power generation in terms of structural reliability. [54,89] Half-Heusler (NbFeSb) [90] and skutterudite (CoSb 3 ) compounds [91] have much lower coefficients of thermal expansion than n-type Mg 3 (Sb, Bi) 2 , [65,83] which may generate significant thermal stress in the associated device during high-temperature operation due to substrate constraints. The widespread use of GeTe, PbTe, Bi 2 Te 3 , and TAGS (Ag 6.52 Sb 6.52 Ge 36.96 Te 50 ) is limited by either the toxicity or the high cost of their constituent elements.…”

“…The following equations are generally employed to establish a direct correlation between the zT avg and the COP or 𝜂 for thermoelectric devices: [54]…”

“…The following equations are generally employed to establish a direct correlation between the zT avg and the COP or η for thermoelectric devices: [ 54 ] $$$$\begin{eqnarray}{\rm{COP}} = \frac{{{T}_{\rm{c}}}}{{{T}_{\rm{h}} - {T}_{\rm{c}}}}{\rm{\ \ }}\frac{{\sqrt {1 + z{T}_{{\rm{avg}}}} - \frac{{{T}_{\rm{h}}}}{{{T}_{\rm{c}}}}}}{{\sqrt {1 + z{T}_{{\rm{avg}}}\ } + \ 1}}\end{eqnarray}$$$$ $$$$\begin{eqnarray}\eta = \frac{{{T}_{\rm{h}} - {T}_{\rm{c}}}}{{{T}_{\rm{h}}}}{\rm{\ \ }}\frac{{\sqrt {1 + z{T}_{{\rm{avg}}}} - 1}}{{\sqrt {1 + z{T}_{{\rm{avg}}}\ } + \ \frac{{{T}_{\rm{c}}}}{{{T}_{\rm{h}}}}}}\end{eqnarray}$$$$…”

“…However, its actual performance may differ from the theoretical prediction based on these equations due to various factors that are often overlooked, leading to an overestimation of the device's performance. As shown in Figure 3, thermoelectric device fabrication involves more complex scientific and engineering issues than material-level optimization, such as those regarding the contact layer between the thermoelectric leg and the substrate, matching between the n-and p-type thermoelectric legs, solder and welding techniques, material thermal stability, packaging protection processes, etc.. [54][55][56][57][58] Currently, increasing numbers of researchers are focusing on n-type Mg 3 (Sb, Bi) 2 -based devices, and many thermoelectric modules have been prepared for laboratory research, in which the performance of n-type Mg 3 (Sb, Bi) 2 has been further verified by COP or 𝜂 at the device level for cooling or power generation, respectively. Here we summarize the main challenges and current research achievements for ntype Mg 3 (Sb, Bi) 2 -based thermoelectric devices from the aspects of contact design, structural optimization, and material matching, and we discuss in detail how these variables may impact device functionality.…”

Near‐Room‐Temperature Thermoelectric Performance Enhancement Via Phonon Spectra Mismatch in Mg3(Sb, Bi)2‐Based Material by Incorporating Multi‐Walled Carbon Nanotubes

“…[ 1–5 ] The performance of a thermoelectric material is evaluated using its figure‐of‐merit, $$zT\ = \frac{{{S}^2\sigma }}{\kappa }\ T$$, which combines the material's Seebeck coefficient ( S ), electrical conductivity ( σ ), and thermal conductivity ( κ ), and the absolute temperature ( T ). [ 6,7 ] To achieve high energy conversion efficiency in thermoelectric devices, significant effort has been directed toward the development of high‐ zT materials, such as GeTe, [ 8,9 ] PbTe, [ 10 ] and silicides, [ 11 ] that show promise for sustainable power generation applications.…”