Thermoelectric Materials

Abstract

The field of thermoelectric materials has flourished over the last two decades. Significant successes achieved in the development of materials notwithstanding, novel and highly efficient materials are in steep demand due to a wide range of potential and current applications, ranging from wearable electronics to space missions. The main challenge in the improvement of the thermoelectric figure of merit ( zT ) is to optimize the intertwined intrinsic charge and thermal transport properties. High electrical conductivity and thermopower, along with low thermal conductivity, need to be achieved simultaneously to excel the thermoelectric efficiency. In this review, the thermoelectric concepts, basics of transport properties, and strategies for zT optimization are discussed in the first section. In the following sections, the predominate structure types associated with high‐performance bulk thermoelectric materials are discussed in detail: Bi 2 Te 3 , PbTe, TAGS/LAST systems, SnSe, Cu 2− x Se, chalcopyrites, tetrahedrites, clathrates, skutterudites, zinc antimonides, Yb 14 MnSb 11 , Heusler and half‐Heusler intermetallics. The basic crystal structures, the possibilities for doping and substitutions to adjust charge carrier concentration and thermal conductivity, baseline thermoelectric properties, and strategies for efficiency improvement are considered for each structure type. Examples of recent work highlighting these strategies are briefly presented.

Clathrate XI K58Zn122Sb207: A New Branch on the Clathrate Family Tree

“…The electronic contribution to the thermal conductivity was estimated using the Weidemann‐Franz law and found to be negligible, 0.0005 % of the total thermal conductivity at room temperature. For crystalline solids thermal conductivity typically has a maximum at 50–150 K range and decreases at higher temperatures due to Umklapp phonon‐phonon scattering [47] . For glass materials such maximum is absent due to absence of the grain boundary scattering at low temperatures.…”

“…Thee lectronic contribution to the thermal conductivity was estimated using the Weidemann-Franz law and found to be negligible, 0.0005 %o ft he total thermal conductivity at room temperature.F or crystalline solids thermal conductivity typically has am aximum at 50-150 Kr ange and decreases at higher temperatures due to Umklapp phonon-phonon scattering. [47] Forglass materials such maximum is absent due to absence of the grain boundary scattering at low temperatures.S ome Sn and Ge clathrates exhibit glass-like thermal conductivity which was associated with displacement of the guest cations from the ideal position in the center of polyhedral cages. [48][49][50] Similar glass-like behavior is observed for the title clathrate which also exhibit complex displacement of Ka toms inside oversized Zn-Sb cages.…”

The Smaller the Better: Hosting Trivalent Rare-Earth Guests in Cu–P Clathrate Cages

“…Nonetheless, the total and lattice thermal conductivities for all the clathrates studied are very low and comparable with those for state-of-the-art thermoelectric materials. 12,13,23,[35][36][37][38][39] Rare-earth incorporation leads to enhancement of the Seebeck coefficient and suppression of the total thermal conductivity, resulting in a significant improvement of the thermoelectric figure of merit, zT = S 2 T/rk, where S is the thermopower, T is the absolute temperature, r is the electrical resistivity, and k is the total thermal Table S2 and Figures S10-S13.…”

“…Ba 6.4 La 1.6 Cu 16 P 30 shows the highest efficiency, 0.11 at 300 K to 0.63 at 900 K, compared with the poor efficiency of Ba 8 Cu 16 P 30 , 0.01 at 300 K to 0.09 at 900 K; 700% enhancement of thermoelectric efficiency due to rare-earth doping is a substantial achievement compared with incremental 20%-40% increases that are typical in the thermoelectric field. 35 Moreover, the flexible Cu-P clathrate framework provides room for further improvement of the thermoelectric efficiency.…”

Phonon glass behavior beyond traditional cage structures: synthesis, crystal and electronic structure, and properties of KMg4Sb3

“…Further temperature increase caused the thermal conductivity to decrease down to 0.78 Wm -1 K -1 at 400 K due to increased Umklapp contribution of phonon-phonon scattering. [64] Several samples of KMg4Sb3 with geometrical densities of 87 and 95% were studied and all exhibit similar values of the total thermal conductivity ( Figure S3). The thermal conductivity can be deconvoluted to a charge carrier contribution and a lattice contribution, κtotal = κe + κL.…”