Large Mobility Enables Higher Thermoelectric Cooling and Power Generation Performance in n-type AgPb18+xSbTe20 Crystals

Effective Diffusion Barrier Layer Achieves Thermal Stability for n-Type Bi2Te2.7Se0.3 Thermoelectric Generators

Bismuth telluride alloys are widely used in the commercial thermoelectric market due to their outstanding thermoelectric performance near room temperature. To improve the performance and stability of n-type Bi2Te3 devices aiming at power generation applications, this work focuses on the exploration of potential metal barrier materials between the thermoelectric material and the electrode, via the revelation of the diffusion behaviors for 11 metals in n-type Bi2Te2.7Se0.3. Ti was identified as a potential one due to its excellent bonding with Bi2Te2.7Se0.3 sintered at a relatively low temperature and low diffusion coefficient. As a demonstration, an n-type Bi2Te2.7Se0.3 single-leg device and module for power generation were fabricated using Ti as the barrier layer between the thermoelectric materials and weldable Ni electrodes. The nearly unchanged output power and conversion efficiency after several thermal cycles and long-term stability measurements at a hot-side temperature of 495 K for 160 h robustly demonstrate the excellent thermal stability for these generators using Ti as a barrier layer.

Favorable Contact with Low Interfacial Resistance for n-Type TiCoSb-Based Thermoelectric Devices

In the past decade, significant efforts have been made to develop efficient half-Heusler (HH) based thermoelectric (TE) materials. However, their practical applications remain limited due to various challenges occurring during the fabrication of TE devices, particularly the development of stable contacts with low interfacial resistance. In this study, we have made an effort to explore a stable contact material with low interfacial resistance for an n-type TiCoSb-based TE material, specifically Ti0.85Nb0.15CoSb0.96Bi0.04 as a proof of concept, using a straightforward facile synthesis route of spark plasma sintering. We tested many metals with compatible coefficients of thermal expansion to TiCoSb, like Fe and Co. Still, we failed to form proper atomic bonds with the TE material. In contrast, Ti metal bonded correctly but showed very high electrical contact resistance (∼300 mΩ at one side), reducing performance due to Ti diffusion and a high potential barrier at the interface. This issue was addressed by highly doped semiconductor (HDS) contact Ti0.7Nb0.3CoSb, which matched the TE material in terms of atomic bonding, crystal structure, and stability. The leg with the HDS contact demonstrated superior electronic transport performance and low interface resistance (∼15 mΩ at one side), achieving a maximum output power of 30.7 mW at ΔT = 451 K due to the sharp interface with a low barrier height. These findings suggest that using HDS material as a contact with the same HH TE material would be an effective way to develop a TE device with low interface resistance and high thermal stability.

Impact of Boron Nitride on the Thermoelectric Properties and Service Stability of Cu2-xSe

Improving the thermoelectric performance and service stability is essential for the effective use of cuprous selenide (Cu2-xSe). In this study, hexagonal boron nitride (h-BN) was incorporated into nano-Cu2-xSe, with the goal of enhancing thermoelectric performance and service stability. It was found that Cu2-xSe-0.005 wt % BN showed a higher thermoelectric figure of merit (zT) value (∼1.76 at 923 K), which was 11% greater than that of pure Cu2-xSe, mainly due to a significant reduction in lattice thermal conductivity (kL) to about 50% (0.13 W m-1 K-1). Additionally, the formation of an ion-blocking interface by hexagonal boron nitride effectively shortens the migration path of Cu+ ions, improving service stability while maintaining the contribution of Cu+ transitions to thermal conductivity. Finally, an increase in hardness of ∼11.1% was observed, reaching 0.7 GPa in Cu2-xSe-0.02 wt % BN. This research is a feasible approach to improving the service stability of Cu2-xSe.

Defect Engineering Advances Thermoelectric Materials

Defect engineering is an effective method for tuning the performance of thermoelectric materials and shows significant promise in advancing thermoelectric performance. Given the rapid progress in this research field, this Review summarizes recent advances in the application of defect engineering in thermoelectric materials, offering insights into how defect engineering can enhance thermoelectric performance. By manipulating the micro/nanostructure and chemical composition to introduce defects at various scales, the physical impacts of diverse types of defects on band structure, carrier and phonon transport behaviors, and the improvement of mechanical stability are comprehensively discussed. These findings provide more reliable and efficient solutions for practical applications of thermoelectric materials. Additionally, the development of relevant defect characterization techniques and theoretical models are explored to help identify the optimal types and densities of defects for a given thermoelectric material. Finally, the challenges faced in the conversion efficiency and stability of thermoelectric materials are highlighted and a look ahead to the prospects of defect engineering strategies in this field is presented.

Janus-like Structure and Resonance Level Actualized Ultralow Lattice Thermal Conductivity and Enhanced ZTave in Mg3(Sb, Bi)2-Based Zintls

Grain boundary (GB) engineering includes grain size and GB segregation. Grain size has been proven to affect the electrical properties of Mg3(Sb, Bi)2 at low temperatures. However, the formation mechanism of GB segregation and what kind of GB segregation is beneficial to the performance are still unclear. Here, the Ga/Bi cosegregation at GBs and Mg segregation within grains optimize the transport of electrons and phonons simultaneously. Ga/Bi cosegregation promotes the formation of Janus-like structures due to the diverse ordering tendencies of liquid Mg3Sb2 and Mg3Bi2 and the absence of a solid solution of Ga/Bi. The Janus-like structure significantly reduces the room-temperature lattice thermal conductivity by introducing diverse microdefects. Meanwhile, a coherent interface between the nano Mg segregation region and the matrix is formed, which reduces the thermal conductivity without affecting the carrier transport. Furthermore, the band structure calculations show that Ga doping introduces the resonance level, increasing the Seebeck coefficient. Finally, the lattice thermal conductivity reaches ∼0.4 W m-1 K-1, and a high average ZT of 1.21 between 323 and ∼773 K is achieved for Mg3.2Y0.02Ga0.03Sb1.5Bi0.5. This work provides guidance for improving the thermoelectric performance via designing cosegregation.

Thermoelectric Performance Improvement in the ZrNiSn-Based Composite via Modulating Si Addition

To ensure optimal performance, ZrNiSn is required to possess a low thermal conductivity and exhibit minimal bipolar effects under high-temperature conditions. This study demonstrates the integration of silicon (Si) at different doping levels into ZrNiSn. The composites consist of secondary phases of in situ ZrNiSi and Si. At a temperature of 873 K, the Seebeck coefficient experiences a 16% increase, despite the charge carrier concentration increasing three times as a result of the electron injection from ZrNiSi. The phenomenon can be elucidated by the introduction of Si, which causes energy filtering and inhibits the flow of minority charge carriers. When the doping levels in n- or p-type Si reach high levels (1019 to 1020 cm-3), the mixed interfaces ZrNiSn/ZrNiSi and ZrNiSn/Si reduce the thermal conductivity by 15%, resulting in a 50% increase in zT. These findings indicate that electron transfer in ZrNiSn can be regulated by precise doping in Si. They also demonstrate that incorporating an optimal p-type semiconductor can enhance the thermoelectric performance of n-type ZrNiSn. Additionally, a novel approach is proposed to separate electrical conductivity and the Seebeck coefficient by designing unique secondary phase interfaces.