Rational Design of Flexible Thermoelectric Generators based on Nano-carbon Materials

Abstract

This research was performed in purpose of rationally designing flexible thermoelectric generator based on nano-carbon materials. Thermoelectric (TE) materials, harvesting electrical energy directly from temperature gradients, have attracted tremendous attention due to their potentials for realizing next-generation power generators and waste-heat-recovery systems. For several decades, many attempts have been made to improve the TE materials efficiency, now TE generators based on semiconductors are commercially used in various applications including an automobile and aerospace industries. Recently, following the trend of consumer electronics, there is a strong demand for TE devices to provide new function other than simply converting heat energy to electricity. In particular, use as a flexible power generator that can supply continuous power to wearable smart devices has been highlighted and many researches are ongoing to make this technology a reality using various TE materials. Inorganic semiconductors with narrow band-gap for a large thermopower, especially, the bismuth-tellurium-antimony-selenium (Bi-Te-Sb-Se) alloy family have been widely investigated. However, despite their high TE performances, the mechanical endurance of TE modules based on inorganic semiconductors cannot be guaranteed owing to their brittleness, and with their energy-intensive process, exquisite or large-area flexible TE device are inconceivable. As possible candidate for flexible TE materials, organic materials including conducting polymers and their hybrid composites have been recently investigated because of their unique advantages including facile processability, scalability, and flexibility as well as low cost and weight. However, most of their performance could not satisfy the needs, yet. Although the performance of flexible TE materials have significantly improved, they are generally sensitive to humidity in ambient conditions, resulting limited practical applications. Therefore, we should spontaneously consider many factors including performance, flexibility, mechanical and chemical stability, and processability to develop TE materials for flexible TE generator. In addition, both N and P type TE materials are necessary for efficient TE generator, and proper module design and fabrication process of TE generator should also be considered. Nano-carbon materials having great mechanical, chemical, electrical properties have been studied as promising TE materials to meet these requirements, but its use was limited to supplement electrical conductivity as fillers in composite system, systematic study to enhance the TE performance themselves have been lacked. The reason is that even if the electrical conductivity was improved by such as chemical doping, its thermopower was reduced because those two factors lie in a trade-off relation, leading to less effective improvement in TE performance. To solve these fundamental problems and dramatically enhance the TE performance of nano-carbon materials, effective way is to control the mobility of carriers, and not the carrier concentration. This was derived from the theoretical relationship between the carrier concentration and mobility. Based on these backgrounds, this research is systemically studied on what rational design of flexible TE materials is needed to enhance the fundamental TE performance of nano-carbon materials. Chapter 1 provides a general introduction of TE generators, especially in needs of flexible generator for wearable electronic device. Theoretical considerations based on thermal and electrical viewpoint are also summarized. The state-of-the art of researches on flexible TE generators and their limitations are discussed. On the basis of them, the aims, strategies, and scopes of this work are presented. Chapter 2 introduces apparatus for measuring the TE performances of materials and generators. It is consisted of two parts which are evaluation system of TE properties of materials and power measuring system for TE generator. The design of our hand-made equipment is explained and the advantages of it are presented. Chapters 3 discuss about the TE performance of manufactured carbon nanotube (CNT) films and yarn with different degree of orientation. Highly oriented CNT yarn with superior mobility shows greatly enhanced electrical conductivity, whilst maintaining relatively high thermopower. This results in drastic improvement of TE performance. It was demonstrated that the mobility engineering of nano-carbon materials is an effective strategy to enhance the TE performance of them. Chapter 4 and 5 focus on hybridization of nano-carbon and inorganic materials for the simultaneous enhancement of both electrical conductivity and thermopower. These works have started to overcome the performance limitation of carbon materials from trade-off relation between the electrical conductivity and thermopower. Binary and ternary nano-carbon / inorganic hybrid system by controlling the work function of nano-carbons are designed, and synergetic enhancement of TE performance by effective energy filtering at heterojunctions is demonstrated. By the energy filtering, the whole mobility of composite materials is slightly decreased, but TE performance of it could be improved by enhanced energy transport efficiency of carrier. It was demonstrated that these materials design is also effective way to achieve high TE performance in terms of effective mobility engineering at hybrid interfaces. Chapter 6 introduces novel design and high performance of flexible TE generators based on nano-carbon. While flexible TE materials show excellent TE properties, facile module fabrication and high power density of generator are also important for practical applications. For the effective design of TE generator, high current mobility of circuit leads to reduced thermal energy loss which maximize the power generation of TE module. Thus, to minimize the contact resistance, novel design of TE generators without metal electrodes was developed, and that electric power was actually generated from body heat was demonstrated. In conclusion, this study has significant meaning in engineering research, because not only high performance nano-carbon based TE materials and modules was rationally designed, but also commercialization potential was realized by successfully fabricating prototype module. Furthermore, carrier mobility engineering strategies employed in this research can be a good guideline for researchers aiming to achieve high performance flexible TE generator.