Rapid low-temperature densification of Ag2Se bulk material: mass transfer through the dissociative adsorption reaction of Ag protrusions and Se saturated vapor.

In materials science, the impact of density on a material's capabilities is profound. Conventional sintering requires high temperatures and is energy-demanding, propelling the pursuit of less intensive, low-temperature densification methods. Electric field-assisted sintering has recently gained attention for its simplicity and effectiveness, offering a new frontier in low-temperature densification. In this study, dense bulk materials were produced by subjecting monophasic Ag2Se powders to electric field-assisted sintering, where a direct current with an average value of 4 A was applied, achieving a peak temperature of 344 K. The novel low-temperature densification mechanism unfolds thus: nanoscale silver protrusions, stimulated by electrical current, engage in a dissociative adsorption reaction with the ambient saturated selenium vapor. This process swiftly engenders the formation of fresh silver selenide (Ag2Se) compounds, initiating nucleation and subsequent growth. Consecutively, these compounds seamlessly occupy and expand, perpetually bridging the interstices amidst the powders. In a scant 8 s, the density swiftly surpassed 99%, yielding a bulk material that exhibited aZTvalue of 1.07 at 390 K. This investigation not only attains an unparalleled density at low temperatures but also charts a pioneering course for material densification in such conditions.

Dual-Cation CuAgSe-Based Material: Rapid Mass Transfer in Synthesis and High Thermoelectric Performance Realization.

Understanding mass transfer mechanisms is vital for developing new material synthesis and densification technologies. Ion transport, serving both mass and charge transfer, is essential for the rapid preparation of high-performance fast ionic conductor thermoelectric materials like Zn4Sb3 and Cu2Q (Q = S, Se). In the case of dual-cation fast ion conductor materials like CuAgSe, exploring the relationship between cation transport becomes pertinent. In this study, copper (Cu) and selenium (Se) undergo a reaction in the presence of an electric field (∼15 A), resulting in the formation of the CuSe compound. Subsequent to this initial reaction, a subsequent thermal environment facilitates the interaction among Cu, CuSe, and Ag2Se, culminating in the rapid formation and densification of CuAgSe (with a relative density exceeding 99%) in just 30 s. Evidently, the diffusion of copper ions substantiates a pivotal role in facilitating mass transfer. As a result, CuAg1+xSe samples with different silver contents (x = 0.01, 0.02, 0.03, 0.04 and 0.05) can effectively inhibit cation vacancy, and introduce highly ordered Ag nanotwins to enhance the electrical transport performance. For CuAg1.04Se, a peak ZT value of 1.0 can be achieved at 673 K, which is comparable to the literatures. This work will guide the future electric field-assisted rapid mass transfer of materials.

The Failure Mechanism of Micro Thermoelectric Devices under the Action of the Temperature Field.

The micro thermoelectric device (m-TED) boasts features such as adjustable volume, straightforward structure, and precise, rapid temperature control, positioning it as the only current solution for managing the temperature of microelectronic systems. It is extensively utilized in 5G optical modules, laser lidars, and infrared detection. Nevertheless, as the size of the m-TED diminishes, the growing proportion of interface damages the device's operational reliability, constraining the advancement of the m-TED. In this study, we used commercially available bismuth telluride materials to construct the m-TED. The device's reliability was tested under various temperatures: -40, 85, 125, and 150 °C. By deconstructing and analyzing the devices that failed during the tests, we discovered that the primary cause of device failure was the degradation of the solder layer. Moreover, we demonstrated that encapsulating the device with polydimethylsiloxane (PDMS) could effectively delay the deterioration of its performance. This study sparks new insights into the service reliability of m-TEDs and paves the way for further optimizing device interface design and enhancing the device manufacturing process.

The simulated cooling performance of a thin-film thermoelectric cooler with coupled-thermoelements connected in parallel.

The thermoelements of the traditional thin-film thermoelectric cooler (TEC) are connected electrically in series, thus the performance of traditional thin-film TEC reduces sharply when there is something wrong with any thermoelement. On account of this deficiency, we proposed a novel thin-film TEC with a couple of thermoelements electrically connected in parallel and then electrically connected in series to the next couple of thermoelements. The performance and reliability of the novel thin-film TEC is compared with the traditional thin-film TEC. The maximum cooling capacity, the maximum cooling temperature, and the coefficient of performance of the novel and the traditional thin-film TEC are systematically studied and compared when 0, 2, and 4 thermoelements are disabled, respectively. The results show that the performance and reliability of the novel thin-film TEC are superior to that of the traditional thin-film TEC, while the optimal electric current of the novel thin-film TEC current is 2.14 times of that for the traditional thin-film TEC. This work is of great significance to improving the performance and reliability of thin-film thermoelectric devices consisting of dozens of small thermoelements.

Objective evaluation of wearable thermoelectric generator: From platform building to performance verification.

Wearable thermoelectric generators can harvest heat from the human body to power an intelligent electronic device, which plays an important role in wearable electronics. However, due to the complexity of human skin, there is still no unified standard for performance testing of wearable thermoelectric generators under wearable conditions. Herein, a test platform suitable for a wearable thermoelectric generator was designed and built by simulating the structure of the arm. Based on the biological body temperature regulation function, water flow and water temperature substitute blood flow and blood temperature, the silicone gel with some thickness simulates the skin layer of the human arm, thus achieving the goal of adjusting the thermal resistance of human skin. Meanwhile, the weight is used as the contact pressure to further ensure the reliability and accuracy of the test data. In addition, the environment regulatory system is set up to simulate the outdoor day. Actually, the maximum deviation of the performance of the thermoelectric generator worn on the test platform and human arm is ∼5.2%, indicating the accuracy of objective evaluation.

Enhancing the Thermoelectric and Mechanical Properties of Bi0.5Sb1.5Te3 Modulated by the Texture and Dense Dislocation Networks.

Bi2Te3-based materials are dominating thermoelectrics for almost all of the room-temperature applications. To meet the future demands, both their thermoelectric (TE) and mechanical properties need to be further improved, which are the requisite for efficient TE modules applied in areas such as reliable micro-cooling. The conventional zone melting (ZM) and powder metallurgy (PM) methods fall short in preparing Bi2Te3-based alloys, which have both a highly textured structure for high TE properties and a fine-grained microstructure for high mechanical properties. Herein, a mechanical exfoliation combined with spark plasma sintering (ME-SPS) method is developed to prepare Bi0.5Sb1.5Te3 with highly improved mechanical properties (correlated mainly to the dislocation networks), as well as significantly improved thermoelectric properties (correlated mainly to the texture structure). In the method, both the dislocation density and the orientation factor (F) can be tuned by the sintering pressure. At a sintering pressure of 20 MPa, an exceptional F of up to 0.8 is retained, leading to an excellent power factor of 4.8 mW m-1 K-2 that is much higher than that of the PM polycrystalline. Meanwhile, the method can readily induce high-density dislocations (up to ∼1010 cm-2), improving the mechanical properties and reducing the lattice thermal conductivity as compared to the ZM ingot. In the exfoliated and then sintered (20 MPa) sample, the figure-of-merit ZT = 1.2 (at 350 K), which has increased by about ∼20%, and the compressive strength has also increased by ∼20%, compared to those of the ZM ingot, respectively. These results demonstrate that the ME-SPS method is highly effective in preparing high-performance Bi2Te3-based alloys, which are critical for TE modules in commercial applications at near-room temperature.

Native Atomic Defects Manipulation for Enhancing the Electronic Transport Properties of Epitaxial SnTe Films.

P-type SnTe-based compounds have attracted extensive attention because of their high thermoelectric performance. Previous studies have made tremendous efforts to investigate native atomic defects in SnTe-based compounds, but there has been no direct experimental evidence so far. On the basis of MBE, STM, ARPES, DFT calculations, and transport measurements, this work directly visualizes the dominant native atomic defects and clarifies an alternative optimization mechanism of electronic transport properties via defect engineering in epitaxially grown SnTe (111) films. Our findings prove that positively charged Sn vacancies (VSn) and negatively charged Sn interstitials (Sni) are the leading native atomic defects that dominate electronic transport in SnTe, in contrast to previous studies that only considered VSn. Increasing the substrate temperature (Tsub) and decreasing the Te/Sn flux ratio during film growth reduces the density of VSn while increasing the density of Sni. A high Tsub results in a low hole density and high carrier mobility in SnTe films. The SnTe film grown at Tsub = 593 K and Te/Sn = 2/1 achieves its highest power factor of 1.73 mW m-1 K-2 at 673 K, which is attributed to the optimized hole density of 2.27 × 1020 cm-3 and the increased carrier mobility of 85.6 cm2 V-1 s-1. Our experimental studies on the manipulation of native atomic defects can contribute to an increased understanding of the electronic transport properties of SnTe-based compounds.

Fast ion transport for synthesis and stabilization of ß-Zn4Sb3.

Mobile ion-enabled phenomena make ß-Zn4Sb3 a promising material in terms of the re-entry phase instability behavior, mixed electronic ionic conduction, and thermoelectric performance. Here, we utilize the fast Zn2+ migration under a sawtooth waveform electric field and a dynamical growth of 3-dimensional ionic conduction network to achieve ultra-fast synthesis of ß-Zn4Sb3. Moreover, the interplay between the mobile ions, electric field, and temperature field gives rise to exquisite core-shell crystalline-amorphous microstructures that self-adaptively stabilize ß-Zn4Sb3. Doping Cd or Ge on the Zn site as steric hindrance further stabilizes ß-Zn4Sb3 by restricting long-range Zn2+ migration and extends the operation temperature range of high thermoelectric performance. These results provide insight into the development of mixed-conduction thermoelectric materials, batteries, and other functional materials.

Mechanical Properties and Thermal Stability of the High-Thermoelectric-Performance Cu2Se Compound.

The Cu2Se compound possesses extraordinary thermoelectric performance at high temperatures and shows great potential for the application of waste heat recycling. However, a thermoelectric device usually undergoes mechanical vibration, mechanical and/or thermal cycling, and thermal shock in service. Therefore, mechanical properties are of equal importance as thermoelectric performance. However, the mechanical performance and stability of the Cu2Se compound during long-term service at high temperatures have rarely been reported. In this study, we systematically investigated the mechanical properties of Cu2Se compounds synthesized by three varied methods (melting (M), self-propagating high-temperature synthesis (SHS), and a combination of SHS and ultrasonic treatment (UT)) and investigated the thermal stability of the SHS-UT compound under different annealing temperatures. The SHS-UT process effectively refines the grain size from 19 µm for the melting sample to 5 µm for the SHS-UT sample. The high density of grain boundaries in the SHS-UT sample effectively dissipates the energy of crack propagation; thus, the mechanical properties are greatly improved. The compressive strength, bending strength, and Vickers hardness of the SHS-UT sample are 147 MPa, 52.6 MPa, and 0.46 GPa, respectively, which are 21.5, 16.6, and 35.3% higher than those of the melting sample, respectively. Moreover, excellent thermal stability is achieved in the compound prepared by SHS and ultrasonication at a temperature below 873 K. After annealing at temperatures up to 873 K for 7 days, the excellent thermoelectric performance of the Cu2Se compound is well maintained with a ZT value exceeding 1.80 at 873 K. However, with further increasing the annealing temperature to 973 K, the volatilization of Se and the precipitation of Cu result in the instability and significantly deteriorated thermoelectric performance of the material. This work provides an avenue for boosting the mechanical properties and commercial application of Cu2Se.

Strong Anisotropy and Bipolar Conduction-Dominated Thermoelectric Transport Properties in the Polycrystalline Topological Phase of ZrTe5.

ZrTe5 has unique features of a temperature-dependent topological electronic structure and anisotropic crystal structure and has obtained intensive attention from the thermoelectric community. This work revealed that the sintered polycrystalline bulk ZrTe5 possesses both (020) and (041) preferred orientations. The transport properties of polycrystalline bulk p-type ZrTe5 exhibits an obvious anisotropic characteristic, that is, the room-temperature resistivity and thermal conductivity, possessing anisotropy ratios of 0.71 and 1.49 perpendicular and parallel to the pressing direction, respectively. The polycrystalline ZrTe5 obtained higher ZT values in the direction perpendicular to the pressing direction, as compared to that in the other direction. The highest ZT value of 0.11 is achieved at 350 K. Depending on the temperature-dependent topological electronic structure, the electronic transport of p-type ZrTe5 is dominated by high-mobility electrons from linear bands and low-mobility holes from the valence band, which, however, are merely influenced by valence band holes at around room temperature. Furthermore, external magnetic fields are detrimental to thermoelectric properties of our ZrTe5, mainly arising from the more prominent negative effects of electrons under fields. This research is instructive to understand the transport features of ZrTe5 and paves the way for further optimizing their ZTs.

Cu2Se as Textured Adjuvant for Pb-Doped BiCuSeO Materials Leading to High Thermoelectric Performance.

Exploring the origin of intrinsic low thermal conductivity in BiCuSeO is of great significance for searching new oxide thermoelectric (TE) materials. In addition, from the perspective of material preparation, it is of great value to further develop the TE performance optimization strategy of BiCuSeO-based materials. In this work, the low-temperature TE transport properties of Pb-doped BiCuSeO-based materials are investigated. It is found that Pb doping can greatly optimize the carrier concentration, soften the lattice, and reduce the lattice thermal conductivity. The addition of Cu2Se significantly enhanced the grain texture and then increased the interface concentration parallel to the pressure direction in the sintering process, which further reduced the lattice thermal conductivity of the material. Finally, the ZT value of Bi0.96Pb0.04CuSeO-6 mol % Cu2Se bulk material is as high as 0.85 at 840 K. This provides important guidance to improve the properties of TE materials via interface engineering.

Blocking Ion Migration Stabilizes the High Thermoelectric Performance in Cu2 Se Composites.

The applications of mixed ionic-electronic conductors are limited due to phase instability under a high direct current and large temperature difference. Here, it is shown that Cu2 Se is stabilized through regulating the behaviors of Cu+ ions and electrons in a Schottky heterojunction between the Cu2 Se host matrix and in-situ-formed BiCuSeO nanoparticles. The accumulation of Cu+ ions via an ionic capacitive effect at the Schottky junction under the direct current modifies the space-charge distribution in the electric double layer, which blocks the long-range migration of Cu+ and produces a drastic reduction of Cu+ ion migration by nearly two orders of magnitude. Moreover, this heterojunction impedes electrons transferring from BiCuSeO to Cu2 Se, obstructing the reduction reaction of Cu+ into Cu metal at the interface and hence stabilizes the ß-Cu2 Se phase. Furthermore, incorporation of BiCuSeO in Cu2 Se optimizes the carrier concentration and intensifies phonon scattering, contributing to the peak figure of merit ZT value of ≈2.7 at 973 K and high average ZT value of ≈1.5 between 400 and 973 K for the Cu2 Se/BiCuSeO composites. This discovery provides a new avenue for stabilizing mixed ionic-electronic conduction thermoelectrics, and gives fresh insights into controlling ion migration in these ionic-transport-dominated materials.

Realizing High Thermoelectric Performance in Sb-Doped Ag2Te Compounds with a Low-Temperature Monoclinic Structure.

To recover the low-grade waste heat (300-500 K), it is of urgent importance to develop and improve the thermoelectric performance at a low-temperature region. Herein, we have realized a record high ZT value of 1.4 at 410 K and a record high average ZT value of 0.6 in the temperature interval from 300 to 400 K for Sb-doped Ag2SbxTe1-x (x = 0-0.03) compounds, which show an improvement of 180 and 120% compared to pristine Ag2Te, respectively. Sb doping increases the carrier concentration and electrical conductivity, leading to a remarkable improvement of electrical transport properties. The Ag2Sb0.015Te0.985 sample obtains the maximal power factor of 1.07 × 10-3 W m-1 K-2 at 410 K, which is increased by 80% in comparison to that of pristine Ag2Te. Moreover, as a result of the intensified alloying phonon scattering by Sb doping, Ag2Sb0.01Te0.99 possesses the minimum lattice thermal conductivity of 0.35 W m-1 K-1 at 300 K, which demonstrates a decline of 57% compared to that of pristine Ag2Te. All of these produce a great enhancement on the thermoelectric performance of Ag2Te materials, which shows great potential in the application of recycling the low-grade waste heat at a low-temperature region.

Enhanced Thermoelectric Performance of Bi0.46Sb1.54Te3 Nanostructured with CdTe.

Cd-containing polycrystalline Bi0.46Sb1.54Te3 samples with precisely controlled phase composition were synthesized by conventional melting-quenching-annealing technique and a melt-spinning method. The pseudo ternary phase diagram for Cd-Bi/Sb-Te in the region near Bi0.46Sb1.54Te3 was systematically studied. Cd serves as an acceptor dopant contributing holes, whereas for samples doped with CdTe, the combined effects of the substitution of Sb/Bi with Cd and the formation of Sb/BiTe antisite defects leads to the increase in hole concentration. Moreover, upon doping with Cd, the lattice thermal conductivity decreases significantly owing to the intensified point defect phonon scattering. The sample with Cd content of 0.01 attains the maximum ZT of 1.15 at 425 K. The utilization of melt-spinning method brings about the in situ nanostructured CdTe and grain size refinement, which further reduce the lattice thermal conductivity while preserving excellent electrical performance. As a result, a higher ZT of 1.30 at 425 K is realized with CdTe content x = 0.005.

Vacancy-Based Defect Regulation for High Thermoelectric Performance in Ge9Sb2Te12-x Compounds.

Defect engineering is the core strategy for improving thermoelectric properties. Herein, cation doping along with modulation of cation vacancy has been developed in GeTe-based materials as an effective method to induce vacancy-based defects to boost their thermoelectric performance. A series of ternary compounds of Ge9Sb2Te12-x (x = 0, 0.03, 0.06, 0.09, 0.12, 0.15) was prepared by vacuum-melting and annealing combined with the spark plasma sintering (SPS) process. The role of Sb doping and cation vacancy on thermoelectric properties was systematically investigated. It is found that alloying Sb2Te3 into GeTe increases the concentration of cation vacancies, which is corroborated by both positron annihilation measurements and theoretical calculations. The vacancies, stacking faults, and planar defect interactions determine the thermoelectric transport properties. Adjusting the deficiency of Te effectively tunes the concentration of cation vacancies and dopant defects in the structure. In turn, this tunes the carrier concentration close to its optimum. A high power factor of 32.6 µW cm-1 K-2 is realized for Ge9Sb2Te11.91 at 725 K. Moreover, large strains induced by the defect structures, including Sb dopant, vacancy, staking faults, as well as planar defects intensify phonon scattering, leading to a significant decrease in the thermal conductivity from 7.6 W m-1 K-1 for pristine GeTe to 1.18 W m-1 K-1 for Ge9Sb2Te11.85 at room temperature. All of the above contribute to a high ZT value of 2.1 achieved for the Ge9Sb2Te11.91 sample at 775 K.

Large Thermal Conductivity Drops in the Diamondoid Lattice of CuFeS2 by Discordant Atom Doping.

Doping in a lattice refers to the introduction of very small quantities of foreign atoms and has a generally small effect on decreasing the lattice thermal conductivity, unlike alloying which involves large fractions of other elements and strongly enhances point defect phonon scattering. Here, we report that, by alloying only 3% of In on the Cu sites of the diamond-like lattice of CuFeS2 chalcopyrite compound (Cu1-xInxFeS2, x = 0.03) has a disproportionally large effect in reducing the lattice thermal conductivity of the compound from 2.32 to 1.36 Wm-1K-1 at 630 K. We find that In is not fully ionized to +3 when on the Cu sublattice and exists mainly in the +1 oxidation state. The 5s2 lone pair of electrons of In+ makes this atom incompatible (referred to as discordant) with the tetrahedral geometry of the crystallographic site. This causes strong local bond distortions thereby softening the In-S and Cu-S chemical bonds and introducing localized low frequency vibrations. The latter couple with the base phonon frequencies of the CuFeS2 matrix enhancing the anharmonicity and decreasing the phonon velocity, and consequently the lattice thermal conductivity. The control material in which the In doping is on the Fe3+ site of the structure at the same doping level (and found in the site-compatible In3+ state), has a far smaller effect on the phonon scattering.

Enhanced Mechanical Properties of Na0.02Pb0.98Te/MoTe2 Thermoelectric Composites Through in-Situ-Formed MoTe2.

Lead telluride (PbTe) is one of the best thermoelectric materials in the intermediate temperature range, which shows great potential for waste heat recycling. However, its low strength and high brittleness limit its large-scale application because the thermoelectric device usually undergoes mechanical vibration, mechanical and/or thermal cycling, and thermal shock in service. In this study, the enhanced mechanical properties and thermoelectric properties of PbTe are realized simultaneously through introducing dispersive transition-metal dichalcogenide MoTe2 (molybdenum telluride). The in-situ-formed MoTe2 precipitations with a size in the range from 2 to 5 µm and the tight and smooth interface between the PbTe matrix and precipitates contribute to the obvious crack deflection, crack bridging, and pull-out of long grains, dissipating more energy during crack propagation and resulting in a tortuous propagation path. Because of the toughening and the dispersion strengthening effect, the compressive strength, bending strength, and fracture toughness of the sample with a composite amount of 1% are 109 MPa and 50 MPa and 0.65 MPa·m1/2, respectively, which are increased by about 37, 117, and 67% compared to the Na0.02Pb0.98Te matrix. Additionally, the in situ MoTe2 precipitates intensify the interface phonon scattering and thus decrease the lattice thermal conductivity. As a result, the Na0.02Pb0.98Te-1%MoTe2 sample achieves a maximum ZT value of 1.46 at 700 K, which is 11% higher than that of Na0.02Pb0.98Te without any MoTe2 nanoprecipitation.

Role of Cation Vacancies in Cu2SnSe3 Thermoelectrics.

In this study, a series of Cu2-xSnSe3 (x = 0.075-0.175) and Cu2Sn1-ySe3 (y = 0.06-0.1) compounds were synthesized by self-propagating high-temperature synthesis combined with plasma-activated sintering. The effects of different cation vacancies (Cu vacancies and Sn vacancies) on the thermoelectric properties are systematically studied. Both Cu vacancies and Sn vacancies enhance the carrier densities and move the Fermi level deeply into the valence band, promoting the multiband from Γ and S points involved in the electrical transport and increasing the effective mass, which is further corroborated by the theoretical calculation. Due to the stronger carrier scattering caused by Cu vacancies, the mobilities of samples with Cu deficiencies reduce leading to decreased power factors. The power factors of samples with Sn deficiencies increase owing to the increased carrier concentration and attaining a maximum power factor of 10.17 µW cm-1 K-2 at 800 K. Besides, the deficiencies of both Cu and Sn strengthen the phonon scattering, and samples with Cu deficiencies obtain lower thermal conductivity than samples with Sn deficiencies due to the lower electronic thermal conductivity. All the samples with cation deficiencies have improved thermoelectric properties. For Cu1.875SnSe3, ZT reaches 0.95 at 800 K, which is 83% higher than those of undoped samples, while for Cu2Sn0.93Se3, ZT reaches 0.87 at 800 K, which is a 67% improvement.

Synergistically Improved Electronic and Thermal Transport Properties in Nb-Doped NbyMo1-ySe2-2xTe2x Solid Solutions Due to Alloy Phonon Scattering and Increased Valley Degeneracy.

The layered 2H-MoSe2-based compounds have recently attracted considerable attention as novel thermoelectric (TE) materials for medium-to-high temperature power generation. In this research, for the first time, dissolving Te in binary MoSe2 and thus forming MoSe2-2xTe2x solid solutions is shown to be very effective for reducing lattice thermal conductivity (κL) due to strong alloy scattering of phonons. Along the ⊥P direction, MoSe1.2Te0.8 achieves the lowest κL of 2.87 W m-1 K-1 at room temperature among all solid solutions, an 85% decrease from that of pristine MoSe2 (18.69 W m-1 K-1). Band structure calculations and experiments have verified that Te alloying effectively increases the density-of-states effective mass of MoSe2-2xTe2x solid solutions via increased valley degeneracy, leading to an enhanced Seebeck coefficient. Moreover, Nb doping modulates the density of holes of NbyMo1-ySe1.2Te0.8 samples to its optimum level and gives rise to the maximum power factor of 0.85 mW m-1 K-2 at 823 K. On account of synergistic optimization of the electronic and thermal transport, Nb0.05Mo0.95Se1.2Te0.8 has achieved the highest ZT value of 0.34 at 823 K, representing a 70% enhancement as compared to the best result previously reported for MoSe2. This research documents that alloying combined with doping is an effective avenue to greatly enhance the TE performance of NbyMo1-ySe2-2xTe2x solid solutions.