Engineering electrical transport in α-MgAgSb to realize high performances near room temperature.

α-MgAgSb shows promise as a potential new low-temperature thermoelectric (TE) material and has been widely researched recently. We explored the effects of sintering conditions on the properties of MgAgSb-based thermoelectric materials through manipulating a spark plasma sintering system (SPS), where Ag vacancies and Mg point defects play a dominant role. The transport properties of MgAgSb were optimized effectively and efficiently, especially for electrical transport. As a result, we obtained a steady power factor (PF) of ∼17 µW cm-1 K-2, owing to the optimal carrier concentration of 9.8 × 1019 cm-3. Additionally, α-MgAgSb exhibits an ultralow lattice thermal conductivity of around 0.45 Wm-1 K-1 at 375 K. More importantly, a high ZT value of 0.85 was achieved below 375 K, approaching room temperature.

Modeling critical thermoelectric transports driven by band broadening and phonon softening.

Critical phenomena are one of the most captivating areas of modern physics, whereas the relevant experimental and theoretical studies are still very challenging. Particularly, the underlying mechanism behind the anomalous thermoelectric properties during critical phase transitions remains elusive, i.e., the current theoretical models for critical electrical transports are either qualitative or solely focused on a specific transport parameter. Herein, we develop a quantitative theory to model the electrical transports during critical phase transitions by incorporating both the band broadening effect and carrier-soft TO phonon interactions. It is found that the band-broadening effect contributes an additional term to Seebeck coefficient, while the carrier-soft TO phonon interactions greatly affects both electrical resistivity and Seebeck coefficient. The universality and validity of our model are well confirmed by experimental data. Furthermore, the features of critical phase transitions are effectively tuned. For example, alloying S in Cu2Se can reduce the phase transition temperature but increase the phase transition parameter b. The maximum thermoelectric figure of merit zT is pushed to a high value of 1.3 at the critical point (377 K), which is at least twice as large as those of normal static phases. This work not only provides a clear picture of the critical electrical transports but also presents new guidelines for future studies in this exciting area.

Structural Modularization of Cu2 Te Leading to High Thermoelectric Performance near the Mott-Ioffe-Regel Limit.

To date, thermoelectric materials research stays focused on optimizing the material's band edge details and disfavors low mobility. Here, the paradigm is shifted from the band edge to the mobility edge, exploring high thermoelectricity near the border of band conduction and hopping. Through coalloying iodine and sulfur, the plain crystal structure is modularized of liquid-like thermoelectric material Cu2 Te with mosaic nanograins and the highly size mismatched S/Te sublattice that chemically quenches the Cu sublattice and drives the electronic states from itinerant to localized. A state-of-the-art figure of merit of 1.4 is obtained at 850 K for Cu2 (S0.4 I0.1 Te0.5 ); and remarkably, it is achieved near the Mott-Ioffe-Regel limit unlike mainstream thermoelectric materials that are band conductors. Broadly, pairing structural modularization with the high performance near the Mott-Ioffe-Regel limit paves an important new path towards the rational design of high-performance thermoelectric materials.

Core-shell nanostructures introduce multiple potential barriers to enhance energy filtering for the improvement of the thermoelectric properties of SnTe.

Using dispersed nanostructures to induce an energy filtering effect is an easy and effective mechanism to optimize the performance of bulk thermoelectric materials. Compared with other nanostructures, core-shell nanostructures possess more interfaces and multiple potential barriers, which would lead to a significant impact on the thermal and electrical properties of materials. In this paper, after BiCuSeO alloy doping into SnTe, SnO2 layers were formed at the interfaces and the BiCuSeO nanoparticles were wrapped in the SnO2 shell during the following high temperature solid state reaction. The formation of SnO2 layers could be observed and confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). BiCuSeO@SnO2 core-shell nanostructures can introduce multiple potential barriers to enhance the energy filtering effect. Once the BiCuSeO doping concentration was over 3%, the carrier concentration could decrease to about 10% while the mobility increases to 350% compared to the values of the undoped sample at room temperature. Meanwhile, the Seebeck coefficients were improved to 176.05 µV K-1 at 835 K. Additionally, due to the scattering of core-shell nanostructures for the phonons, a lower thermal conductivity is achieved with a value of 1.04 W m-1 K-1 at 835 K in Sn1.03Te-5% BiCuSeO. Combined with the improvement of thermal and electrical properties by the BiCuSeO@SnO2 core-shell, a high ZT value of ∼1.21 was achieved for Sn1.03Te-5% BiCuSeO at 835 K, which was enhanced by 190% compared to pristine SnTe.

Enhancement of Thermoelectric Properties in Pd-In Co-Doped SnTe and Its Phase Transition Behavior.

SnTe has attracted more and more attention due to the similar band and crystal structure with high performance thermoelectric materials PbTe. Here, we introduced Pd into SnTe and the valence band convergence was confirmed by first-principles calculation. In the experimental process, we found that Pd-doped SnTe exhibit a reduced thermal conductivity because of softening chemical bonds and grain refining effects. To further improve the thermoelectric performance, Pd-In codoped SnTe samples were prepared, and the abnormal change of thermal conductivity was observed. The results of synchrotron powder diffraction suggest that the local phase transition (local structural distortions) near 400 K results in the first turn on thermal conductivity. Similarly, the second local phase transition in near 600 K observed by neutron powder diffraction lead to a decrease thermal conductivity of the sample. Finally, a peak thermoelectric figure of merit (ZT) ≈ 1.51 has been obtained in Sn0.98Pd0.025In0.025Te at 800 K.

High Thermoelectric Performance of Bi0.46Sb1.54Te3-SnTe: Synergistic Modulation of Electrical and Thermal Transport by the Introduction of Thermoelectric Hetero Nano Region.

The thermoelectric hetero nano region, as a new strategy, can effectively modulate the electrical and thermal transport properties. In this study, the thermoelectric hetero nano region is explored to improve the thermoelectric performance for Bi0.46Sb1.54Te3 material at room temperature, and a high ZT of 1.45 at 325 K has been achieved. We introduce the thermoelectric hetero nano SnTe regions in a Bi0.46Sb1.54Te3 matrix by mechanical alloying and spark plasma sintering technique, which decouples the relation between electrical and thermal transport properties. The improved electrical conductivity can be attributed to the increase in carrier concentration due to the increased point defects and Bi/SbTe antisite defects. Thermoelectric hetero nano regions effectively scatter the acoustic phonon and thus induce the low lattice thermal conductivity of 0.33 W m-1 K-1. Due to the synergistic modulation of electrical and thermal transport by the introduction of the thermoelectric hetero nano region, a high ZT value of 1.45 is realized at 325 K.