Strategies for engineering phonon transport in thermoelectrics

Abstract

In this review, we discuss some of the representative strategies of phonon engineering by categorizing them into the methods affecting each component of phonon thermal conductivity, i.e., specific heat, phonon group velocity, and mean free path. In terms of specific heat, a large unit cell is beneficial in that it can minimize the fraction of thermal energy that can be transported since most of the energy is stored in the optical branches. In an artificial structure such as the superlattice, phonon bandgaps can be created through constructive interference by Bragg reflection, which reduces phonon group velocity.We further categorize the mean free path, i.e., scattering processes, into grain boundary scattering, impurity scattering, and phonon-phonon scattering. Rough-surfaced grains, nano-sized grains, and coated grains are discussed for enhancement of the grain boundary scattering. Alloy atoms, vacancies, nanoparticles, and nano-sized holes are treated as impurities, which limit the phonon mean free path. Lone pair electrons and acoustical-to-optical scattering are suggested for manipulating phonon-phonon scattering. We also briefly mention the limitation and temperature range in which the Wiedemann-Franz law is valid in order to achieve a better estimation of electronic thermal conductivity. This paper provides an organized view of phonon engineering so that this concept can be implemented synergistically with power factor enhancement approaches for design of thermoelectric materials. Fig. 1 Strategies of phonon engineering categorized into methods affecting each component of phonon thermal conductivity, i.e., specific heat, phonon group velocity, and scattering processes. Some of the figures are from the literature. 71,80 Fig. 3 Thermal conductivity of the (SrTiO 3 ) m /(CaTiO 3 ) n superlattice versus period thickness. 23 A TEM figure shows the interface quality of the superlattice. Also, a phonon dispersion curve showing phonon bandgaps is presented.

Valleytronics in thermoelectric materials

“…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. 1,11,19,20 Nevertheless, there exists a threshold for zT improvement from reducing the thermal conductivity, which is limited to a theoretical minimum (amorphous limit).…”

Atomistic origin of the reduced lattice thermal conductivity of silicon nanotubes

“…[8][9][10] Thermal conductivity of Si nanostructures can be significantly influenced by surface roughness, crystalline orientation and diameter, which can modulate the thermal transport properties. [11][12][13][14][15][16] Therefore, the ultralow thermal conductivity of Si nanostructures as thermoelectric materials is highly desirable.…”

Computation-Driven Materials Search for Thermoelectric Applications

“…With more defects on grain boundaries, the thermal conductivity can be lower but it is mostly attributed to reduced phonon transmissivity. 33 With coated grain boundaries, 34 it is possible to further reduce the k L by lowering phonon transmissivity at the grain boundaries 35 and prevent grain growth at high temperatures. With further decreased a, both the electrical and thermal conductivities will be scaled down with a as classical size effects, but S will remain the same with unchanged carrier concentration, resulting in saturated Z T .…”