Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe1-xSex through Isoelectronic Substitution

The Remarkable Role of Indium in Synergistically Optimizing Carrier Concentration and Phase Distribution of AgCuTe-Based Materials

Carrier regulation has proven to be an effective approach for optimizing the thermoelectric performance of materials. One common method to adjust the carrier concentration is through element doping. In the case of AgCuTe-based materials, it tends to form with cation vacancies, resulting in a high hole concentration and complex phase composition at low temperatures, which also hinders material stability. However, this also offers additional opportunities to manipulate the carrier concentration. In this study, the improved performance of AgCuTe through indium doping is reported, which leads to a reduction in hole concentration. In combination with a significant increase in the effective mass of the carriers, the enhanced Seebeck coefficient is also realized. Particularly, a notable improvement in power factor is observed in the hexagonal phase near room temperature. Furthermore, a lower electron thermal conductivity is achieved, contributing to an average figure of merit value of ≈1.21 (between 523 and 723 K). Additionally, the presence of indium inhibits the formation of the second phase and ensures a homogeneous phase distribution, which reduces the instability arising from phase transition. This work significantly enhances the potential of AgCuTe-based materials for low to medium-temperature applications.

Silver Copper Chalcogenide Thermoelectrics: Advance, Controversy, and Perspective

Thermoelectric technology, which enables a direct and pollution-free conversion of heat into electricity, provides a promising path to address the current global energy crisis. Among the broad range of thermoelectric materials, silver copper chalcogenides (AgCuQ, Q = S, Se, Te) have garnered significant attention in thermoelectric community in light of inherently ultralow lattice thermal conductivity, controllable electronic transport properties, excellent thermoelectric performance across various temperature ranges, and a degree of ductility. This review epitomizes the recent progress in AgCuQ-based thermoelectric materials, from the optimization of thermoelectric performance to the rational design of devices, encompassing the fundamental understanding of crystal structures, electronic band structures, mechanical properties, and quasi-liquid behaviors. The correlation between chemical composition, mechanical properties, and thermoelectric performance in this material system is also highlighted. Finally, several key issues and prospects are proposed for further optimizing AgCuQ-based thermoelectric materials.

Improvement of Mechanical and Thermoelectric Properties of AgCuTe by Pinning Effect and Enhanced Liquid-Like Behavior via SiC Alloying

With the development and application of thermoelectric (TE) devices, it requires not only high-performance of TE materials but also high mechanical properties. Here, we report a medium-temperature liquid material, AgCuTe, with high mechanical properties. The results demonstrate that AgCuTe possesses a multiphase structure characterized by abundant grain boundaries, resulting in reduced lattice thermal conductivity and inherently high mechanical strength. Furthermore, nano-SiC was alloyed into the AgCuTe material to further improve its mechanical and TE properties. Nano-SiC exhibited a button-like distribution within the grain boundaries, introducing a pinning effect that significantly elevated the Vickers hardness of the samples. Additionally, nano-SiC induced strong lattice distortion energy in the vicinity, which promotes Ag/Cu ions to escape from the lattice and enhances the liquid-like behavior of Ag/Cu ions. Finally, these enhancements led to a 21% improvement in the mechanical properties and a 40% improvement in the TE properties for AgCuTe. Notably, AgCuTe achieved its peak TE performance, with a latest peak ZT value of 1.32 at 723 K. This research expands the potential applications of AgCuTe.

Extremely Low Lattice Thermal Conductivity and Significantly Enhanced Near-Room-Temperature Thermoelectric Performance in α-Cu2Se through the Incorporation of Porous Carbon

In this work, a series of Cu2Se/x wt % porous carbon (PC) (x = 0, 0.2, 0.4, 0.6, 0.8, 1) composite materials were synthesized by ball milling and spark plasma sintering (SPS). The highly ordered porous carbon was synthesized by a hydrothermal method using mesoporous silica (SBA-15) as the template. X-ray diffraction results show that the incorporation of porous carbon induces a phase transition of Cu2Se from the β phase to the α phase. Meanwhile, the addition of porous carbon reduces the carrier concentration from 2.7 × 1021 to 2.45 × 1020 cm-3 by 1 order of magnitude. The decrease of the carrier concentration leads to the reduction of electrical conductivity and the increase of the Seebeck coefficient, which results in the enhancement of the power factor. On the other hand, the incorporation of porous carbon into Cu2Se increases the porosity of the composites and also introduces more interfaces between the two materials, which is evidenced by positron annihilation lifetime measurements. Both pores and interfaces greatly enhance phonon scattering, leading to extremely low lattice thermal conductivity. In addition, the decrease of electrical conductivity also causes a sufficient reduction in electronic thermal conductivity. Due to the above synergistic effects, the thermoelectric performance of the Cu2Se/PC composite is significantly enhanced with a maximum ZT value of 0.92 at 403 K in the Cu2Se/1 wt % PC composite, which is close to that of the Bi2Te3-based materials. Our work shows that α-Cu2Se has great potential for near-room-temperature thermoelectric materials.

Ag2 Q-Based (Q = S, Se, Te) Silver Chalcogenide Thermoelectric Materials

Thermoelectric technology provides a promising solution to sustainable energy utilization and scalable power supply. Recently, Ag₂ Q-based (Q = S, Se, Te) silver chalcogenides have come forth as potential thermoelectric materials that are endowed with complex crystal structures, high carrier mobility coupled with low lattice thermal conductivity, and even exceptional plasticity. This review presents the latest advances in this material family, from binary compounds to ternary and quaternary alloys, covering the understanding of multi-scale structures and peculiar properties, the optimization of thermoelectric performance, and the rational design of new materials. The "composition-phase structure-thermoelectric/mechanical properties" correlation is emphasized. Flexible and hetero-shaped thermoelectric prototypes based on Ag₂ Q materials are also demonstrated. Several key problems and challenges are put forward concerning further understanding and optimization of Ag₂ Q-based thermoelectric chalcogenides.

Enhancement of the Thermoelectric Performance of Cu2GeSe3 via Isoelectronic (Ag, S)-co-substitution

Recently, ternary Cu-based Cu-IV-Se (IV = Sb, Ge, and Sn) compounds have received extensive attention in the thermoelectric field. Compared with Cu-Sb-Se and Cu-Sn-Se, Cu-Ge-Se compounds have been less studied due to its poor Seebeck coefficient and high thermal conductivity. Here, the Cu₂GeSe₃ material with high electrical conductivity was first prepared, and then, its effective mass was increased by doping with S, which led to the Seebeck coefficient of the doped sample being 1.93 times higher than that of pristine Cu₂GeSe₃ at room temperature. Moreover, alloying Ag at the Cu site in the Cu₂GeSe_2.96S_0.04 sample could further cause a 5.16 times increase in the Seebeck coefficient at room temperature, and the lattice thermal conductivity was remarkably decreased because of the introduction of the dislocations in the Cu₂GeSe₃ sample. Finally, benefitted from the high Seebeck coefficient and low thermal conductivity, a record high ZT = 0.9 at 723 K was obtained for the Cu_1.85Ag_0.15GeSe_2.96S_0.04 sample, which increased 345% in comparison with the pristine Cu₂GeSe₃, and it is among the highest reported values for Cu₂GeSe₃-based thermoelectric.

Review on Wearable Thermoelectric Generators: From Devices to Applications

Wearable thermoelectric generators (WTEGs) can incessantly convert body heat into electricity to power electronics. However, the low efficiency of thermoelectric materials, tiny terminal temperature difference, rigidity, and neglecting optimization of lateral heat transfer preclude WTEGs from broad utilization. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of flexibility and high normalized power density in thermoelectric generators by establishing the links among materials, TE performance, and advanced design of WTEGs (structure, heatsinks, thermal regulation, thermal analysis, etc.) based on inorganic bulk TE materials. Each section starts with a concise summary of its fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of wearable thermoelectric devices and potential applications. Overall, this review will serve to help materials scientists, electronic engineers, particularly students and young researchers, in selecting suitable thermoelectric devices and potential applications.

Enhancing Thermoelectric Performance of AgSbTe2-Based Compounds via Microstructure Modulation Combining with Entropy Engineering

Modulation of the microstructure and configurational entropy tuning are the core stratagem for improving thermoelectric performance. However, the correlation of evolution among the preparation methods, chemical composition, structural defects, configurational entropy, and thermoelectric properties is still unclear. Herein, two series of AgSbTe₂-based compounds were synthesized by an equilibrium melting-slow-cooling method and a nonequilibrium melting-quenching-spark plasma sintering (SPS) method, respectively. The equilibrium method results in coarse grains with a size of >300 μm in the samples and a lower defect concentration, leading to higher carrier mobility of 10.66 cm² V^-1 s^-1 for (Ag₂Te)_0.41(Sb₂Te₃)_0.59 compared to the sample synthesized by nonequilibrium preparation of 1.83 cm² V^-1 s^-1. Moreover, tuning the chemical composition of nonstoichiometric AgSbTe₂ effectively improves the configurational entropy and creates a large number of cation vacancies, which evolve into dense dislocations in the samples. Owing to all of these in conjunction with the strong inharmonic vibration of lattice, an ultralow thermal conductivity of 0.51 W m^-1 K^-1 at room temperature is achieved for the (Ag₂Te)_0.42(Sb₂Te₃)_0.58 sample synthesized by the equilibrium preparation method. Due to the enhanced carrier mobility, optimized carrier concentration, and low thermal conductivity, the (Ag₂Te)_0.42(Sb₂Te₃)_0.58 sample synthesized by the equilibrium preparation method possesses the highest ZT of 1.04 at 500 K, more than 60% higher than 0.64 at 500 K of the same composition synthesized by nonequilibrium preparation.

Phase Transition Behaviors and Thermoelectric Properties of CuAgTe1-xSex near 400 K

Phase transition is an effective strategy to engineer thermal conductivity and electrical transports. Recently, p-type CuAgTe_1-xSe_x materials were reported to show excellent thermoelectric performance at 300-450 K, but the data are controversial due to the cooccurrence of phase transition in this temperature range. Accurately measuring and analyzing the electrical and thermal transport properties in the narrow phase transition temperature range is a quite challenging task. In this work, we systemically investigate the phase transition behavior, and electrical and thermal transport properties of p-type CuAgTe_1-xSe_x (x = 0.3, 0.4, and 0.5) near 400 K. CuAgTe_1-xSe_x (x = 0.3, 0.4, and 0.5) materials show similar phase transition temperatures but quite different phase transition speeds. The phase transition has a weak influence on the electrical transport properties of CuAgTe_0.7Se_0.3 and CuAgTe_0.6Se_0.4, but a strong influence on those of CuAgTe_0.5Se_0.5. Likewise, an obvious underestimation of thermal diffusivity, with a maximum deviation about 20% off the real value, is observed during the phase transition temperature range for CuAgTe_1-xSe_x. Finally, CuAgTe_0.7Se_0.3 shows a peak zT around 0.9 at 390 K. The present study proves that CuAgTe_1-xSe_x solid solutions are one kind of promising near-room-temperature thermoelectric material.