Preface

As we face the challenges of an energy crisis, it is crucial to seek out next-generation energy sources that are economical, sustainable (renewable), and clean (environmentally friendly). There are many types of energy that can be utilized in industry, such as nuclear energy, radiation solar energy, hydrogen energy, and so on. In most cases, all energy is converted into electricity before it can be utilized. Heat plays a key role in energy conversion and utilization. Heat is traditionally used to create electricity, but the process is complex and noisy and creates large amounts of waste. With the advantage of first-hand solid-state conversion to electrical power from thermal energy, especially from the reuse of waste heat, thermoelectricity has valuable applications in reusing resources, making it costeffective. Compared to the traditional methods of converting heat to electricity, thermoelectric devices have many advantages, such as all-solid-state features, no mechanical moving parts, long lifetime, and more. Consequently, thermoelectricity has many potential applications. For example, based on nuclear radiation with a long half-life, thermoelectricity can supply electricity for space stations and spaceships in deep space. In addition, thermoelectricity can also supply electricity for flexible wearable devices. As such, thermoelectricity has received extensive attention and is expected to be helpful for combating environmental pollution and saving energy.

The wide applications of thermoelectrics in commercial and industrial fields have long been limited by thermoelectric conversion efficiency. The continuous improvement of thermoelectric performance and the strive to increase the power output under the same heat source are the key foci in thermoelectric technology. Many efforts have been dedicated to improving thermoelectric efficiency, which is a very challenging task because of the strongly correlated properties involved in thermoelectrics. Efforts to boost thermoelectric efficiency focus on two aspects. The first approach focuses on improving electric performance while not affecting thermal transport. The second approach focuses on suppressing thermal transport while not affecting electric properties. All methods for boosting thermoelectric efficiency include designing or searching for novel materials with excellent properties, especially low-dimensional materials, and regulating performance via techniques such as strain engineering, doping, nanostructuring, and so on. Significant progress has been made in the past few years and thermoelectric efficiency has been largely improved. Future studies are expected to improve thermoelectric efficiency even further.

This book presents a comprehensive overview of the progress made in thermoelectrics over the past few years with a focus on charge and heat carrier transport from both theoretical and experimental viewpoints. It also presents new strategies to improve thermoelectricity. In addition, the book discusses device physics and applications to guide the research community.

This book project has lasted more than two years and in that time many things have happened. I would like to acknowledge my wife and daughter for their support. Also, I would like to thank my sister for her helpful suggestions and contributions during the editing of this book.

## **Guangzhao Qin**

State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, College of Mechanical and Vehicle Engineering, Hunan University, Changsha, China

## **Chapter 1**
