In this work, we show the simultaneous enhancement of electrical transport and reduction of phonon propagation in p-type PbTe codoped with Tl and Na. The effective use of advanced electronic structure engineering improves the thermoelectric power factor S2σ over the temperature range from 300 to 825 K. A rise in the Seebeck coefficient S was obtained due to the enhanced effective mass m*, coming from the Tl resonance state in PbTe. Due to the presence of additional carriers brought by Na codoping, electrical conductivity became significantly improved. Furthermore, Tl and Na impurities induced crystal lattice softening, remarkably reducing lattice thermal conductivity, which was confirmed by a measured low speed of sound vm and high internal strain CeXRD. Eventually, the combination of both the attuned electronic structure and the lattice softening effects led to a very high ZT value of up to ∼2.1 for the Pb1-x-yTlxNayTe samples. The estimated energy conversion efficiency shows the extraordinary value of 15.4% (Tc = 300 K, Th = 825 K), due to the significantly improved average thermoelectric figure of merit ZTave = 1.05. This work demonstrates that the combination of impurity resonance scattering and crystal lattice softening can be a breakthrough concept for advancing thermoelectrics.

High Thermoelectric Performance of p-Type PbTe Enabled by the Synergy of Resonance Scattering and Lattice Softening.

Development of the high performance thermoelectric unicouple based on Bi2Te3 compounds

Phase boundary mapping and Suppressing Pb vacancies for Enhanced Thermoelectric Properties in n-type Sb doped PbTe Compounds

Bi2Te3-based alloys are the main materials for the construction of low- and medium-temperature thermoelectric modules. In this work, the microstructure and thermoelectric properties of Cl-doped Bi2Te3–xSex alloys were systematically investigated considering the high anisotropy inherent in these materials. The prepared samples have a highly oriented microstructure morphology, which results in very different thermal transport properties in two pressing directions. To accurately separate the lattice, electronic, and bipolar components of the thermal conductivity over the entire temperature range, we employed a two-band Kane model to the Cl-doped Bi2Te3–xSex alloys. It was established that Cl atoms act as electron donors, which tune the carrier concentration and effectively suppress the minority carrier transport in Bi2Te3–xSex alloys. The estimated value of the lattice thermal conductivity was found to be as low as 0.15 Wm–1 K–1 for Bi2Te3–x–ySexCly with x = 0.6 and y = 0.015 at 673 K in parallel to the pressing direction, which is among the lowest values reported for crystalline materials. The large reduction of the lattice thermal conductivity in both pressing directions for the investigated Bi2Te3–xSex alloys is connected with the different polarities of the Bi-(Te/Se)1 and Bi-(Te/Se)2 bonds, while the lone-pair (Te/Se) interactions are mainly responsible for the extremely low lattice thermal conductivity in the parallel direction. As a result of the enhanced power factor, suppressed bipolar conduction, and ultralow lattice thermal conductivity, a maximum ZT of 1.0 at 473 K has been received in the Bi2Te2.385Se0.6Cl0.015 sample.

Ultralow Lattice Thermal Conductivity and Improved Thermoelectric Performance in Cl-Doped Bi2Te3–xSex Alloys

Nanostructure engineering is a key strategy for tailoring properties in the fields of batteries, solar cells, thermoelectrics, and so on. Limited by grain coarsening, however, the nanostructure effect gradually degrades during the materials’ manufacturing and in‐service period. Herein, a strategy of cleavage‐fracture for grain shrinking is developed in a Pb0.98Sb0.02Te sample during sintering, and the grain size remains stable after repeated tests. Moreover, the initial grain boundary is filled by fractured slender grains and enriched by dislocations, evolving into a hierarchical grain‐boundary structure. The lattice thermal conductivity (klat) is greatly reduced to approach the amorphous limit. As a result, a record‐high ZT value of ≈1.9 is obtained at 815 K in the n‐type Pb0.98Sb0.02Te sample and a decent efficiency of 6.7% in thermoelectric device. This strategy for grain shrinking will shed light on the application of nanostructure engineering under high temperature and extreme conditions in other material systems.

Dramatic Enhancement of Thermoelectric Performance in PbTe by Unconventional Grain Shrinking in the Sintering Process

Searching for novel low-cost and eco-friendly materials for energy conversion is a good way to provide widespread utilization of thermoelectric technologies. Herein, we report the thermal behavior, phase equilibria data, and thermoelectric properties for the promising argyrodite-based Cu7P(SxSe1-x)6 thermoelectrics. Alloying of Cu7PSe6 with Cu7PS6 provides a continuous solid solution over the whole compositional range, as shown in the proposed phase diagram for the Cu7PS6-Cu7PSe6 system. As a member of liquid-like materials, the investigated Cu7P(SxSe1-x)6 solid solutions possess a dramatically low lattice thermal conductivity, as low as ∼0.2-0.3 W m-1 K-1, over the entire temperature range. Engineering the configurational entropy of the material by introducing more elements stabilizes the thermoelectrically beneficial high-symmetry γ-phase and promotes the multivalley electronic structure of the valence band. As a result, a remarkable improvement of the Seebeck coefficient and a reduction of electrical resistivity were observed for the investigated alloys. The combined effect of the extremely low lattice thermal conductivity and enhanced power factor leads to the significant enhancement of the thermoelectric figure of merit ZT up to ∼0.75 at 673 K for the Cu7P(SxSe1-x)6 (x = 0.5) sample with the highest configurational entropy, which is around twice higher compared with the pure selenide and almost four times higher than sulfide. This work not only demonstrates the large potential of Cu7P(SxSe1-x)6 materials for energy conversion but also promotes sulfide argyrodites as earth-abundant and environmentally friendly materials for energy conversion.

Entropy-Induced Multivalley Band Structures Improve Thermoelectric Performance in p-Cu7P(SxSe1-x)6 Argyrodites.

Thermoelectric (TE) technologies realize the generation of electrical energy from the waste heat. The one bottleneck, which significantly restricts the wide use of these technologies, relates to the low energy conversion efficiency of the commercial devices. In this work, the double-tuned functionally graded thermoelectric material (DT-FGTM) approach was proposed to achieve the high-performance TE leg through the increase in the average TE figure of merit (ZT)ave. The essence of this idea is connected with the precise control of the bandgap Eg and chemical potential μc over the entire temperature range. Considering Pb0.75Sn0.25Te solid solution, as an example, and using the three band Kane model, we evaluated the best conditions for the highest thermoelectric performance in this material. Within the offered herein DT-FGTM approach, we fabricated the thermoelectric n-type Pb0.75Sn0.25Te1−xIx leg and measured its output energy characteristics. The efficiency of energy conversion for the prepared DT-FGTM leg reaches a very high value of ∼12.0% at temperature difference ΔT = 540 K. Furthermore, the thermal treatment of the fabricated leg should not injure the carrier concentration distribution through the leg, as the hot end of the leg is heavily doped, and the chemical diffusion between segments would be only beneficial. Our demonstration shows that the DT-FGTM approach has significant practical interest and can be utilized for the other TE materials.

Taras Parashchuk

Papers

High Thermoelectric Performance of p-Type PbTe Enabled by the Synergy of Resonance Scattering and Lattice Softening.

Entropy-Induced Multivalley Band Structures Improve Thermoelectric Performance in p-Cu7P(SxSe1-x)6 Argyrodites.

Evaluation of the double-tuned functionally graded thermoelectric material approach for the fabrication of n-type leg based on Pb0.75Sn0.25Te