On the tuning of electrical and thermal transport in thermoelectrics: an integrated theory–experiment perspective

Abstract

During the last two decades, we have witnessed great progress in research on thermoelectrics. There are two primary focuses. One is the fundamental understanding of electrical and thermal transport, enabled by the interplay of theory and experiment; the other is the substantial enhancement of the performance of various thermoelectric materials, through synergistic optimisation of those intercorrelated transport parameters. Here we review some of the successful strategies for tuning electrical and thermal transport. For electrical transport, we start from the classical but still very active strategy of tuning band degeneracy (or band convergence), then discuss the engineering of carrier scattering, and finally address the concept of conduction channels and conductive networks that emerge in complex thermoelectric materials. For thermal transport, we summarise the approaches for studying thermal transport based on phonon-phonon interactions valid for conventional solids, as well as some quantitative efforts for nanostructures. We also discuss the thermal transport in complex materials with chemical-bond hierarchy, in which a portion of the atoms (or subunits) are weakly bonded to the rest of the structure, leading to an intrinsic manifestation of part-crystalline part-liquid state at elevated temperatures. In this review, we provide a summary of achievements made in recent studies of thermoelectric transport properties, and demonstrate how they have led to improvements in thermoelectric performance by the integration of modern theory and experiment, and point out some challenges and possible directions. INTRODUCTION Thermoelectric (TE) materials are materials that can generate useful electric potentials when subjected to a temperature gradient (known as the Seebeck effect). Conversely, they also transfer heat against the temperature gradient when a current is driven against this potential (known as the Peltier effect). They are promising energy materials with many applications, such as waste heat harvesting, radioisotope TE power generation, and solid state Peltier refrigeration, all of which are driving growing research interest. A key challenge is to improve the TE properties in order to obtain more efficient energy conversion and in turn enable new practical applications. Good TE materials must have excellent electrical transport properties, measured by the TE power factor ( = S 2 σ, where S is the Seebeck coefficient and σ is the electrical conductivity), and also a very low thermal conductivity κ (composed of the electronic contribution κ e , the lattice contribution κ L , and the bipolar contribution κ bi ). Combining the two aspects gives us the dimensionless figure of merit ZT,

Valleytronics in thermoelectric materials

“…12 Lattice thermal expansion is always regarded as the first reason leading to the increase of electronic band gap with increasing temperature (the lowering L valleys in energy position, seen in Fig. 3b).…”

“…One is the reduction of κ L and the other is the increase of TE power factor (PF = α 2 σ). 12,13 The current realization of large zT enhancement (zT > 2) is mainly due to the significantly reduced κ L , e.g., with allscale hierarchical phonon scattering, 14 using materials with strong anharmonicity, 15 and discovering liquid-like phonon behavior. 16 Some excellent reviews focusing on the thermal transport aspects of TE study 17,18 as well as several good comprehensive TE reviews introducing the synergistic design strategies of TE materials have been published in recent years.…”

Unique Role of Refractory Ta Alloying in Enhancing the Figure of Merit of NbFeSb Thermoelectric Materials

“…Strategies for increasing zT have focused on the reduction of the lattice thermal conductivity by hierarchical microstructure, [3,4] nanostructuring [5,6] point defects, [7,8] and the enhancement of the power factor (PF = α 2 σ) by optimal doping and band engineering [9][10][11] as well as the employment of complex crystal structures that possess intrinsically low lattice thermal conductivity. [12][13][14][15][16] Half-Heusler (HH) alloys, with a valence electron count of 8 or 18, have been extensively studied as potential hightemperature TE materials due to their excellent electrical properties, mechanical properties, and high temperature stability.…”

Intrinsically low thermal conductivity from a quasi-one-dimensional crystal structure and enhanced electrical conductivity network via Pb doping in SbCrSe3

“…15 Moreover, the asymmetrical local atomic coordination with a large distortion structure, which possess both strong and weak bond types, is beneficial for increasing the bond anharmonicity, which is another important aspect that affects the lattice thermal conductivity. 16 The bond anharmonicity serves as a measure of the deviation in lattice vibrations from a perfect harmonic motion and is characterized by the effective Grüneisen parameter. Generally, structures with a chemical bond hierarchy [17][18][19][20][21] and anisotropic multicenter bonding features 9 or those that possess a lone pair of electrons [22][23][24][25][26] exhibit a stronger anharmonicity.…”