Introductory Chapter: Thermoelectricity – Recent Advances, New Perspectives, and Applications

1. Introduction

Seeking for next-generation energy sources is crucial when facing the challenges of an energy crisis. The energy is expected to be economical, sustainable (renewable), and clean (environment-friendly). There exist lots of energy that can be utilized in industry, such as nuclear energy, radiation solar energy, hydrogen energy, etc. Generally, electricity is the most direct type of energy available to human society. And in most cases, all the other energies are firstly converted into electricity and then can be utilized. During the conversion of energy, heat plays a key role as shown in Figure 1. For instance, the power stations get heat from coal or fuel burning or nuclear radiating, which are used to heat water to produce steam. Then, the electricity generators are driven. Thus, in addition to the electricity that can be directly utilized, heat is the most common type for energy conversion and utilization. However, the traditional utilization of heat is usually to drive the electricity generator by heating water, which will cause a large waste, and the process is complex with lots of mechanical moving parts and noise.

Figure 1.

The key role of heat in the energy conversion and the key role of thermoelectrics in the utilization of heat.

2. Thermoelectrics

With the advantage of first-hand solid-state conversion to electrical power from thermal energy, especially from the reuse of waste heat, thermoelectricity shows its valued applications in reusing resources for the low cost of operation [1]. Compared to the traditional manners of converting heat to electricity, thermoelectric devices have lots of advantages, such as all-solid-state feature, no mechanical moving part, long lifetime, etc. Consequently, thermoelectricity has lots of potential applications. For instance, based on the nuclear radiation with a long half-life, thermoelectricity can supply electricity for space stations and spaceships in deep space far from stellar [2]. Besides, thermoelectricity can also supply electricity for flexible wearable devices, devices in the body, IoT devices, etc [3, 4]. Thus, with the numerous applications in lots of fields, thermoelectricity has received extensive attention and is expected to be helpful for the crisis of the environment and the saving of energy.

However, the wide applications in commercial and industrial fields have long been limited by thermoelectric conversion efficiency. The thermoelectric efficiency is characterized by the dimensionless figure of merit ZT = S²σT/κ [5, 6], where S is the Seebeck coefficient, σ is the electrical conductivity, T is the temperature, and κis the thermal conductivity contributed from both electron and phonon transport. The continuous improvement of thermoelectric performance and the strive to increase the power output under the same heat source are the key focus in thermoelectric technology. A lot of efforts have been dedicated to improving thermoelectric efficiency. However, it is a very challenging task because the parameters in the ZT formula are strongly correlated with each other. For instance, a large electrical conductivity is a benefit to the improvement of ZT, but a large thermal conductivity contributed from electron transport is simultaneously achieved, which plays a negative role in the improvement of ZT. Thus, the wide applications of thermoelectric devices are mainly limited by the low conversion efficiency from heat to electricity.

3. Improve thermoelectric performance

The efforts for boosting thermoelectric efficiency are mainly focused on two aspects. The first approach is to improve the electric performance while not affecting thermal transport. Another approach is to suppress the thermal transport while not affecting the electric properties. Moreover, the concept of “electron crystal & phonon glass” [7] has been proposed to improve the electric performance and to suppress the thermal transport simultaneously. All the manners for boosting thermoelectric efficiency include (1) designing or searching novel materials with excellent properties, especially for low-dimensional materials [1], and (2) regulating the performance using various manners such as strain engineering, doping, nanostructuring, etc. In addition, a new mechanism is also proposed for understanding the fundamentals of thermoelectrics. Significant progress has been made in the past few years, and thermoelectric efficiency has been largely improved. For instance, spin-related characteristics have been used to regulate the electron and phonon transport in magnetism materials and to achieve remarkable thermoelectric performance [8, 9]. Future studies are expected to further improve the thermoelectric efficiency following a similar procedure or finding a different way.

This book aims to comprehensively collect the progress made in thermoelectrics over the past few years with a focus on charge and heat carrier transport from both theoretical and experimental views. New perspectives are also presented for the challenges on the topics and for future studies, especially for the strategies to improve the thermoelectric figure of merit. Beyond that, new discussions on device physics and applications are also included to offer a guide to the research community.