Plasticity in single-crystalline Mg3Bi2 thermoelectric material.

Most of the state-of-the-art thermoelectric materials are inorganic semiconductors. Owing to the directional covalent bonding, they usually show limited plasticity at room temperature1,2, for example, with a tensile strain of less than five per cent. Here we discover that single-crystalline Mg3Bi2 shows a room-temperature tensile strain of up to 100 per cent when the tension is applied along the (0001) plane (that is, the ab plane). Such a value is at least one order of magnitude higher than that of traditional thermoelectric materials and outperforms many metals that crystallize in a similar structure. Experimentally, slip bands and dislocations are identified in the deformed Mg3Bi2, indicating the gliding of dislocations as the microscopic mechanism of plastic deformation. Analysis of chemical bonding reveals multiple planes with low slipping barrier energy, suggesting the existence of several slip systems in Mg3Bi2. In addition, continuous dynamic bonding during the slipping process prevents the cleavage of the atomic plane, thus sustaining a large plastic deformation. Importantly, the tellurium-doped single-crystalline Mg3Bi2 shows a power factor of about 55 microwatts per centimetre per kelvin squared and a figure of merit of about 0.65 at room temperature along the ab plane, which outperforms the existing ductile thermoelectric materials3,4.

Enhanced Thermoelectric Performance in Flexible Sulfur-Alloyed Ag2Se Thin Films.

Flexible thermoelectric generators can directly convert thermal energy harvested from the human body into electricity. The Ag2Se flexible film, a promising material for wearable thermoelectric generators, normally demonstrates an inferior electrical transport property due to its weakened in-plane mobility. In this study, the in-plane electrical transport properties of flexible Ag2Se films were optimized by alloying with additional sulfur. This optimization is achieved by leveraging the differences in elemental electronegativity and the preferred orientation of the Ag2Se films. The sulfur-alloyed Ag2Se thin film, with a nominal ratio of 3 atom %, can reach a maximum mobility of 1150 cm-2 V-1 s-1 at 300 K. So, the optimized room-temperature power factor increases to 1935 µW m-1 K-2. Furthermore, the Ag2Se film alloyed with 3 atom % sulfur exhibits excellent flexibility even after 1000 bending cycles with a radius of 5 mm, characterized by a relative resistance increment of less than 3%. In addition, the corresponding π-type flexible thermoelectric generator possesses a maximum power density of 51 W m-2 at a temperature difference of 50 K.

Revealing the Chemical Instability of Mg3Sb2-xBix-Based Thermoelectric Materials.

n-Type Mg3Sb2-xBix alloys have been regarded as promising thermoelectric materials due to their excellent performance and low cost. For practical applications, the thermoelectric performance is not the only factor that should be taken into consideration. In addition, the chemical and thermal stabilities of the thermoelectric material are of equal importance for the module design. Previous studies reported that the Mg3Sb2-xBix alloys were unstable in an ambient environment. In this work, we found that Mg3Sb2-xBix alloys reacted with H2O and O2 at room temperature and formed amorphous Mg(OH)2/MgO and crystalline Bi/Sb. The substantial loss of Mg resulted in a significant deterioration in thermoelectric properties, accompanied by the transition from n-type to p-type. With the increase in Bi content, the chemical stability decreased due to the higher formation energy of Mg3Bi2. A chemically stable Mg3Bi2 sample was achieved by coating it with polydimethylsiloxane to isolate H2O and O2 in the air.

High-Selectivity Planar Thermal Emitter with a Stable Temperature over 1400 K for a Thermophotovoltaic System.

A selective thermal emitter with superior thermal stability and perfect selective thermal emission in specific bands can facilitate the lifting of the thermophotovoltaic (TPV) energy conversion efficiency in TPV systems. Scalable planar selective thermal emitters with superior spectral selectivity and robust high-temperature stability are desired to meet the requirements of large-scale deployments of TPV systems. However, stably reradiating the available thermal photons at above 1273 K remains a significant challenge for selective thermal emitters. In this work, we demonstrated a high-selectivity planar thermal emitter based on the composite ceramic of ZrC and Al2O3. The prepared selective thermal emitter provides an emissivity of around 90% above the photon energy (0.6 eV) at 1423 K, a strong emission suppression effect below 0.6 eV, and superior thermal stability up to at least 1423 K. Therefore, the overall spectral efficiency can reach around 53%. Coupled with an InGaAs PV cell, the TPV system based on the selective thermal emitter demonstrates a predicted heat-to-electricity power conversion efficiency of 29.78% at 1423 K due to the matched spectral response of the emitter with the PV cell. Our work opens a new way forward for TPV systems based on planar selective thermal emitters.

Scalable-produced 3D elastic thermoelectric network for body heat harvesting.

Flexible thermoelectric generators can power wearable electronics by harvesting body heat. However, existing thermoelectric materials rarely realize high flexibility and output properties simultaneously. Here we present a facile, cost-effective, and scalable two-step impregnation method for fabricating a three-dimensional thermoelectric network with excellent elasticity and superior thermoelectric performance. The reticular construction endows this material with ultra-light weight (0.28 g cm-3), ultra-low thermal conductivity (0.04 W m-1 K-1), moderate softness (0.03 MPa), and high elongation (>100%). The obtained network-based flexible thermoelectric generator achieves a pretty high output power of 4 µW cm-2, even comparable to state-of-the-art bulk-based flexible thermoelectric generators.

Revealing the Defect-Dominated Electron Scattering in Mg3Sb2-Based Thermoelectric Materials.

The thermoelectric parameters are essentially governed by electron and phonon transport. Since the carrier scattering mechanism plays a decisive role in electron transport, it is of great significance for the electrical properties of thermoelectric materials. As a typical example, the defect-dominated carrier scattering mechanism can significantly impact the room-temperature electron mobility of n-type Mg3Sb2-based materials. However, the origin of such a defect scattering mechanism is still controversial. Herein, the existence of the Mg vacancies and Mg interstitials has been identified by synchrotron powder X-ray diffraction. The relationship among the point defects, chemical compositions, and synthesis conditions in Mg3Sb2-based materials has been revealed. By further introducing the point defects without affecting the grain size via neutron irradiation, the thermally activated electrical conductivity can be reproduced. Our results demonstrate that the point defects scattering of electrons is important in the n-type Mg3Sb2-based materials.

Passive Radiative Cooling Enables Improved Performance in Wearable Thermoelectric Generators.

Wearable thermoelectric generators have great potential to be utilized as the power supply for wearable electronics. However, the limited temperature difference across the thermoelectric generators significantly degrades the output performance, which is anticipated to be improved by enhancing the thermal radiation at the cold side without extra energy consumption. In this paper, the impact of thermal radiation on the performance of thermoelectric generators in different environments is simulated and the enhanced performance in a wearable thermoelectric generator combined with a radiative cooling coating is experimentally verified. Compared with the pristine device, the wearable thermoelectric generator with radiative cooling coating can not only achieve an ≈128% improvement of output power in exposed environments, but also exhibit an ≈96% improvement of output power in non-exposed environments. The indoor output performance of the wearable thermoelectric generator with a radiative cooling coating due to its stable voltage output is extensively investigated, which shows an output power density of ≈5.5 µW cm-2 at the indoor temperature of 295 K, doubled that without a radiative cooling coating. This work paves a new way for further enhancing the performance of thermoelectric generators via passive radiative cooling.

High-Performance Spectrally Selective Absorber Using the ZrB2-Based All-Ceramic Coatings.

Enhancing the spectral selectivity and thermal stability of the absorber used in the concentrated solar power system would boom the conversion efficiency of solar energy to electricity. The ceramic coatings possess excellent thermal stability in optical films. Here, we design the ZrB2-based all-ceramic spectrally selective absorber with a quasioptical microcavity (QOM) structure, which shows an excellent performance with a solar absorptance of 0.965 and superior thermal stability. The pretty high absorptance is due to the design of QOM inducing the multiabsorption mechanisms composed of the intrinsic cermet absorption, the surface plasmon polaritons, and localized surface plasmon resonance proved by the electromagnetic power loss. The structure also demonstrates well-matched impedance with free space in the solar spectrum range, ensuring a high solar absorptance. The proposed absorber can survive at 800 °C in vacuum or 500 °C in air for 200 h, ascribed to the introduction of QOM and ultrahigh-temperature ceramic ZrB2. The total conversion efficiency of an ideal system with this absorber and an ideal thermal engineer can reach around 67% under the conditions of 800 °C and 1000 suns.

A 2D-SnSe film with ferroelectricity and its bio-realistic synapse application.

Catering to the general trend of artificial intelligence development, simulating humans' learning and thinking behavior has become the research focus. Second-order memristors, which are more analogous to biological synapses, are the most promising devices currently used in neuromorphic/brain-like computing. However, few second-order memristors based on two-dimensional (2D) materials have been reported, and the inherent bionic physics needs to be explored. In this work, a second-order memristor based on 2D SnSe films was fabricated by the pulsed laser deposition technique. The continuously adjustable conductance of Au/SnSe/NSTO structures was achieved by gradually switching the polarization of a ferroelectric SnSe layer. The experimental results show that the bio-synaptic functions, including spike-timing-dependent plasticity, short-term plasticity and long-term plasticity, can be simulated using this two-terminal devices. Moreover, stimulus pulses with nanosecond pulse duration were applied to the device to emulate rapid learning and long-term memory in the human brain. The observed memristive behavior is mainly attributed to the modulation of the width of the depletion layer and barrier height is affected, at the SnSe/NSTO interface, by the reversal of ferroelectric polarization of SnSe materials. The device energy consumption is as low as 66 fJ, being expected to be applied to miniaturized, high-density, low-power neuromorphic computing.

Enhanced Thermoelectric Performance of c-Axis-Oriented Epitaxial Ba-Doped BiCuSeO Thin Films.

We reported the epitaxial growth of c-axis-oriented Bi1-xBaxCuSeO (0 ≤ x ≤ 10%) thin films and investigated the effect of Ba doping on the structure, valence state of elements, and thermoelectric properties of the films. X-ray photoelectron spectroscopy analysis reveal that Bi3+ is partially reduced to the lower valence state after Ba doping, while Cu and Se ions still exist as + 1 and - 2 valence state, respectively. As the Ba doping content increases, both resistivity and Seebeck coefficient decrease because of the increased hole carrier concentration. A large power factor, as high as 1.24 mWm-1 K-2 at 673 K, has been achieved in the 7.5% Ba-doped BiCuSeO thin film, which is 1.5 times higher than those reported for the corresponding bulk samples. Considering that the nanoscale-thick Ba-doped films should have a very low thermal conductivity, high ZT can be expected in the films.

Microstructure and thermoelectric transport properties of BiCuSeO thin films on amorphous glass substrates.

c-Axis-textured BiCuSeO thin films were grown directly on amorphous glass substrates by pulsed laser deposition. The resistivity (∼27.1 mΩ cm) of the films at room temperature was found to be much lower than those reported for polycrystalline bulk samples with the same nominal composition, and a three dimensional variable-range-hopping conduction process was suggested to govern the electrical transport properties of the films below room temperature. Moreover, detailed microstructural analysis revealed the existence of amorphous grain boundaries throughout the films, which would lead to a significant decrease of thermal conductivity. Thus, the thermoelectric performance of the present BiCuSeO thin films is expected to be greatly enhanced as compared to that of bulk samples with the same nominal composition, demonstrating the potential application in the thermoelectric thin film devices.

Temperature-dependent LITT effect in c axis inclined BiCuSeO thin films.

Light-induced transverse thermoelectric (LITT) effect in c axis inclined BiCuSeO thin films was investigated in the temperature range of 80-300 K by using three different lasers with the wavelength changing from visible to mid-IR. Obvious open-circuit voltage signals were all detected when the film surface was illuminated by these lasers, and the amplitude Vp of the induced voltage signals was found to increase with the measured temperature. The improvement of Vp at a higher temperature can be explained by the enhanced anisotropy of the Seebeck coefficient as well as the reduced thermal conductivity of the film. This work demonstrates the potential application of BiCuSeO thin film as a broadband light detector working at wide temperature ranges, including a very low temperature environment.