Attempts to improve the performance of thermoelectric materials called dimensionless figure of merit zT, have been made over the past several decades. Owing to the efforts, recently, materials having a high zT enough to achieve a conversion efficiency of 10% or more, which is a standard criterion for practical use of thermoelectric power generation, has been reported. This chapter reviews recent trends in thermoelectric materials and thermoelectric power generation applications. Regarding the former, we will describe conventional methods for improving zT, and their limitations, and then introduce examples of new materials design guidelines that have the potential to break through those limitations. Regarding the latter, here are some examples of thermoelectric power generation applications that have made remarkable progress in recent years.
Si is nontoxic and the second most abundant element in the Earth’s crust, which makes it quite attractive as an environmentally friendly thermoelectric (TE) material. The TE performance of Si is, however, quite poor mainly due to its high thermal conductivity. This demerit might be overcome by nanostructuring since enhanced phonon scattering at nanostructures results in reduced thermal conductivity. From a practical point of view, self-assembled nanostructure is desirable. This chapter aims at introducing recent studies on Si with automatically formed nanostructures. To this end, the TE properties of Si are described first. Since the most reliable data can be obtained from single crystals, TE properties of P- and B-doped single crystal Si are shown. The carrier concentrations of these single crystal Si are at most 1 × 1020 cm−3, which are not enough for the evaluation of the optimum carrier concentration. Thus, the TE properties of Si are discussed based on simulations using models built based on data taken from single crystal Si. The calculation suggests that the best carrier concentration that maximizes the power factor is 5 × 1020 cm−3 for both p- and n-type Si. The TE properties of polycrystalline Si with the best carrier concentration are provided as the TE properties of optimized bulk Si. Next, the best nanostructure size is proposed to be 10 nm based on the calculated mean free paths of phonons and electrons. Then, three studies on Si with self-assembled nanostructures are introduced. The first one is Si/silicide nanocomposite films. Si and silicide nanocrystals are formed by the phase separation from amorphous phase. Significantly low thermal conductivity was achieved in this film. The second study is on nanometer-sized eutectic structures of Si/ CrSi2. Characteristic maze-like structure is formed by melt spinning of eutectic Si-CrSi2 melt. The third study is about Si with nanoprecipitates and dislocations. P acts as an n-type dopant in Si, and when P is added to Si above its solution limit, the excess P atoms form nanoprecipitates in the Si matrix, improving the TE performance of Si. Dislocations are able to be formed in this excess P-doped Si through liquid phase sintering and reduce the thermal conductivity further by enhanced phonon scattering. Since the size of the nanostructures formed by these approaches is larger than the best size of 10 nm, there seems to be still space for further improvement of TE performance of Si.
Materials having structures with size scales of various orders of magnitude could cause crucial effects on their properties. In this chapter, first, fundamentals of thermodynamic theories governing phase diagrams will be given. Next, the ways to control the morphology and size scales of microstructure based on phase transformation theories will be provided.
Heusler materials have attracted substantial attention over decades because of their impressive magnetic and electronic properties. Half-Heusler compounds, attributed to their semiconducting properties, are promising candidates for thermoelectric devices in the medium to high temperature range. In this regard, the chapter provides a thorough summary on the properties of high efficiency half-Heusler thermoelectrics. In the first section, the crystal structure and molecular orbital diagrams of full-Heusler and half-Heusler materials are analyzed for a better understanding of their electronic properties. Following, phase diagram, electronic structure, and thermoelectric properties of n-type XNiSnbased and p-type XCoSb half-Heusler compounds (X = Ti, Zr, Hf) are discussed in detail. Subsequently, recent progress in high performance NbFeSb-based and ZrCoBi-based materials are addressed, and some suggestions are provided for further investigations. Lastly, the development of thermoelectric modules based on half-Heusler materials is summarized to demonstrate the advancements towards industrial applications.
Since the discovery of Mg2Si as a promising thermoelectric (TE) material, extensive studies have been devoted to improve the TE performance of Mg2Si and its derivatives. The chapter review recent progress in the understanding of lattice defects and secondary phases in these materials, which greatly influence the conduction type and electrical transport properties.
This chapter presents a comprehensive review of the high-performance p-type thermoelectric (TE) oxide materials. The benefits of using oxides for TE applications include high stability and durability at elevated temperature, abundancy, low-cost, and non-toxicity. The research on TE oxides has increased substantially in the last two decades. The most popular candidates for p-type TE oxides include NaxCoO2, Ca3Co4O9, delafossite oxides, and BiCuSeO. They are the main subject of this chapter. There are reports on many aspects of these oxides, for example, crystal structure, defect chemistry, microstructure, electronic structure, fabrication process, or other properties. Hence, in this chapter, we only focus on the TE properties at elevated temperature of high-performance p-type oxides, with large dimensionless figure-ofmerit (ZT). The details of the results are summarized for the two main objectives: why and how such oxides exhibit high ZT. Polycrystalline NaxCoO2 shows the relatively large ZT values of 0.4-0.8, which can be enhanced to >0.9 with doping. The ZT of the pristine Ca3Co4O9 is relatively lower (ZT < 0.4), but with proper doping, the recorded ZT value of 0.74 at 800 K for this oxide is achieved. Delafossite oxides are another class of materials that have been extensively studied. However, the ZT of the delafossite oxides is still much lower than other p-type oxides. The thermoelectric properties of BiCuSeO were firstly reported in 2010, and since then, the compound has been intensively studied. Recently, the ZT value of BiCuSeO has climbed up exponentially to 1.5, which is comparable to or even better than the current state-of-the-art thermoelectric materials, This makes BiCuSeO a very promising p-type thermoelectric oxide for hightemperature applications.
Carrier energy filtering effect boosting the thermoelectric performance by blocking “cold” carriers is here reviewed and summarized from theoretical and experimental perspectives. Among them, theoretical models including phenomenological interpretation, cut-off model and relax-time model are summarized to discuss the potential of carrier energy filtering effect enhancing the thermoelectric performance. Furthermore, experimental observations are also presented in some common thermoelectric materials systems, such as PbTe, Bi2Te3, Skutterudite, Half-Heusler systems. According to theory model, it still need the further optimization of the carrier concentration, barrier height, and barrier separated distance by experimental operation to be close to theoretical predication of the enhancement of power factor.
Thermoelectric modules have applications in power generation, refrigeration, active cooling, and thermal switching. In all these applications, the key is to have a thermoelectric material with large power factor (PF). Simultaneously, large electrical conductivity and Seebeck coefficient values are needed to achieve large thermoelectric PF. Heavily doped semiconductors are by far the most studied class of thermoelectric materials. At low temperatures, the carrier mobility of heavily doped semiconductors is limited by the ionized impurity scattering. This chapter is an overview of the strategies that are proposed to enhance the charge carrier mobility of heavily doped semiconductors in their nanostructured form, including: selective doping, modulation doping, surface doping, and percolation. These strategies are demonstrated in multiphase samples in which the charge transfer may occur at the vicinity of the heterointerfaces. Therefore, interfaces are unavoidable and their impact on the thermoelectric transport is crucial in determining the performance of the thermoelectric materials.
Thermal conductivity is one of the most important parameters determining the performance of a thermoelectric material. Recently, various strategies for effective phonon scattering have been developed which have boosted the thermoelectric figure of merit, ZT of many materials. All-scale phonon scattering is one such strategy where a combination of several scattering mechanisms come into play to impede the phonon transport. In this Chapter we will review pathways of achieving all-scale phonon scattering in traditional and new thermoelectric materials.