The Roles of Grain Boundaries in Thermoelectric Transports

Abstract

Grain boundary scattering was long recognized and widely utilized to regulate the transports of charge carriers and phonons in thermoelectric materials; nevertheless, the understanding of grain boundaries in lots of literatures seems somewhat lopsided and way too simplified, without distinguishing the exact microstructural characters for each individual case. In this perspective, we first review the well-known functions of grain boundaries on the electrical and thermal transport properties based on some representative thermoelectric series. Next, we try to deepen the understanding of the roles of grain boundaries in the following two aspects: (1) whether the grain boundaries are large-angle or small-angle ones, and what the difference between them in affecting electrical/thermal performance is; (2) whether the grain boundaries are clean and sharp as lattice discontinuity defects, if not, how can the grain boundaries as an individual phase affect the thermoelectric transports. At last, we suggest that further investigations engaging more detailed microstructural information of grain boundaries are urgently necessary in order to better realize a positive trade-off between lattice thermal conductivity and electrical power factor for an overall enhanced thermoelectric performance.

Unraveling the Role of Entropy in Thermoelectrics: Entropy-Stabilized Quintuple Rock Salt PbGeSnCdxTe3+x

“…The performance of thermoelectric materials is evaluated by the dimensionless figure of merit ZT , which is determined as ZT = S 2 σΤ/(κ e + κ L ) . Therefore, to obtain a high thermoelectric performance, a large Seebeck coefficient S and high electrical conductivity σ are desired, while the thermal conductivity κ, comprising the electronic contribution κ e and the lattice contribution κ L , should be minimized. − Over the past decades, several advanced thermoelectric materials have been found in the group IV–VI semiconductors, such as PbQ, − SnQ (Q = S, Se, and Te), − and GeTe. , Various strategies have been developed and successfully applied to optimize their thermoelectric performances, such as band convergence, − resonant level, , nanostructuring, ,− discordant atoms, − and intrinsically large anharmonicity, , and tremendous progress has been achieved. However, because of their binary composition and simple crystal structure, most of the developed group IV–VI semiconductors still exhibit a relatively high intrinsic thermal conductivity.…”

Two Mixed-Anion Semiconductors in the Ba–Sn–Te–S System with Low Thermal Conductivity

“…Thermoelectric materials can convert heat energy directly into electrical energy, which can be used to improve energy utilization efficiency in various fields. − The dimensionless figure of merit ZT determines the energy conversion efficiency of thermoelectric materials. The ZT is defined as ZT = (σ S 2 T )/κ, where σ represents the electrical conductivity, S represents the Seebeck coefficient, and T represents the working temperature in Kelvin. ,− κ is the total thermal conductivity, which includes the lattice thermal conductivity (κ lat ) and the carrier thermal conductivity (κ ele ).…”

Contrasting the Roles of Cu Interstitials and Sb Substitutions in Regulating Ferroelectric Distortions and Thermoelectric Properties of α-GeTe

“…Thermoelectric materials are capable of converting heat into electricity without noise and pollution. − The conversion efficiency is quantified by the dimensionless thermoelectric figure of merit ZT = S 2 σ T /κ tot = S 2 σ T /(κ lat + κ ele ), where S is the Seebeck coefficient, σ is electrical conductivity, T stands for absolute temperature, and κ tot is the total thermal conductivity determined by the contributions from the lattice (κ lat ) and the carriers (κ ele ). − It is tough to individually optimize ZT values since these parameters ( S , σ, and κ tot ) are strongly coupled via the carrier concentration. , The high ZT values are generally accomplished through increasing the power factor (PF = S 2 σ) or dampening the κ lat , for example, band convergence, , nanostructuring, , and lattice anharmonicity. − …”

“…4−9 It is tough to individually optimize ZT values since these parameters (S, σ, and κ tot ) are strongly coupled via the carrier concentration. 10,11 The high ZT values are generally accomplished through increasing the power factor (PF = S 2 σ) or dampening the κ lat , 12 for example, band convergence, 13,14 nanostructuring, 15,16 and lattice anharmonicity. 17−20 As we know, the pristine α-GeTe possesses an intrinsically high carrier concentration of ∼10 21 cm −3 at ambient temperature.…”