High-Performance p-Type Bi2Te3-Based Thermoelectric Materials Enabled via Regulating Bi-Te Ratio.

Bi2Te3-based alloys, as the sole commercial thermoelectric (TE) material, play an irreplaceable role in the thermoelectric field. However, the low TE efficiency, poor mechanical properties, and high cost have limited its large-scale applications. Here, high-performance p-type Bi2Te3-based materials were successfully prepared by ball milling and hot pressing. The optimized p-type Bi0.55Sb1.45Te3 + 2.5 wt % Bi shows a peak zT value of 1.45 at 360 K, and the average zT value of up to 1.24 at 300-480 K, which is completely comparable with previously reported Bi2Te3-based alloys with excellent performance. Such performance mainly results from the enhanced electrical conductivity and decreased lattice thermal conductivity via regulating carrier and phonon transport. Furthermore, this material shows good mechanical properties, in which the Vickers hardness and compressive strength are up to 0.95 GPa and 94.6 MPa, respectively. Overall, both the thermoelectric and mechanical performance of the materials fabricated by our processing technology are quite competitive. This may enlighten researchers concentrating on Bi2Te3-based alloys, thus further promoting their industrial applications.

High-Power-Density and Excellent-Flexibility Thermoelectric Generator Based on All-SWCNTs/PVP Composites.

Flexible polymer/single-wall carbon nanotube (SWCNT) composites are a vital component for wearable/portable electronics, but the development of their n-type counterpart is laggard. Furthermore, little attention is paid to the interaction between SWCNT and polymers, especially the unconjugated polymers, as well as the conversion mechanism of conduction characteristics. Here, the n-type flexible SWCNTs/Polyvinyl Pyrrolidone (PVP) films are successfully fabricated, where the oxygen atoms in PVP interacted with SWCNT via hydrogen bonds, which can lower the energy barrier of electron tunneling, providing the pathway for the electron transfer. Furthermore, with the increasing synthesis temperature, the hydrogen bonds strengthened and the thermal activation energy further improved, both of which enhanced the electron-donating ability of PVP, resulting in a high-power-factor value of 260 µW m-1  K-2 . Based on the optimized SWCNTs/PVP films, a thermoelectric module is assembled, which achieved a power density of 400 µW cm-2 at a temperature difference of 56 K, coupled with excellent flexibility, showing a less than 1% variation of resistance after 5000 bending cycles. It shows the highest output-performance and the best flexibility among the reported SWCNT-based thermoelectric modules. This work provides significant insights into the interaction mechanism and performance optimization of hybrid thermoelectric composites, based on SWCNTs/unconjugated polymers.

n-Type PVP/SWCNT Composite Films with Improved Stability for Thermoelectric Power Generation.

Single-walled carbon nanotubes (SWCNTs) are one of the promising thermoelectric materials in applications of powering wearable electronics. However, the electrical performance of n-type SWCNTs quickly decreases in air, showing a low stability, and low-cost and effective solutions to improving its stability are also lacking, all of which limit practical applications. In this study, we studied the stability of PVP/SWCNT composite films, where oxygen and moisture from air should be responsible for the decreased stability due to oxidation and hydration. In this case, we found that coating with a 0.20 g mL-1 PVP/0.002 g mL-1 PVDF layer on the surface of PVP/SWCNTs can prevent the penetration of oxygen and moisture from air, improving film stability, where there is almost no reduction in thermoelectric performance after they are exposed to air for 60 days. Based on the stable n-type PVP/SWCNT films, a thermoelectric generator was fabricated, where poly(dimethylsiloxane) (PDMS) was used to coat the surface of the thermoelectric leg to further improve its stability. This generator showed high output performance, which achieved an open-circuit voltage of 10.6 mV and a power density of 312.2 µW cm-2 at a temperature difference of 50 K. Particularly, it exhibited high stability, where the output performance kept almost unchanged after exposure to high-humidity air for 30 days. This coating technology is also applicable to other air-sensitive materials and promotes the development and application of thermoelectric materials and devices.

Emerging Flexible Thermally Conductive Films: Mechanism, Fabrication, Application.

Effective thermal management is quite urgent for electronics owing to their ever-growing integration degree, operation frequency and power density, and the main strategy of thermal management is to remove excess energy from electronics to outside by thermal conductive materials. Compared to the conventional thermal management materials, flexible thermally conductive films with high in-plane thermal conductivity, as emerging candidates, have aroused greater interest in the last decade, which show great potential in thermal management applications of next-generation devices. However, a comprehensive review of flexible thermally conductive films is rarely reported. Thus, we review recent advances of both intrinsic polymer films and polymer-based composite films with ultrahigh in-plane thermal conductivity, with deep understandings of heat transfer mechanism, processing methods to enhance thermal conductivity, optimization strategies to reduce interface thermal resistance and their potential applications. Lastly, challenges and opportunities for the future development of flexible thermally conductive films are also discussed.

One Structure, Two Elements-LuGe2 Superconductor vs Ordinary Metallic Conductor LuSn2. A Case Study on How Site-Selective Germanium for Tin Atom Substitution Leads to Modulating of the Charge Distribution.

The substitution of chemically similar elements in a given crystal structure is an effective way to enhance physical properties, but the understanding on such improvements is usually impeded because the substitutions are random, and the roles of the different atoms cannot be distinguished by crystallographic symmetry. Herein, we provide a detailed crystallographic analysis and property measurements for the continuous solid solutions LuGexSn2-x (0 < x < 2). The results show that there is no apparent change of the global symmetry, with the end-members LuGe2 and LuSn2, as well as the intermediate LuGexSn2-x compositions adopting the ZrSi2 type structure (space group Cmcm, Pearson index oC12). Yet, the refinements of the crystal structures from single-crystal X-ray diffraction data show that Ge-Sn atom substitutions are not random, but occur preferentially at the zigzag chain. The patterned distribution of two group 14 elements leads to a significant variation in chemical bonding and charge ordering within the other structural fragment, the 2D square nets, thereby resulting in tuned electron transport. The enhancement is greater than that for the typical Bloch-Gruneisen model and more akin to that for the parallel-resistor model. Magnetization measurements on single crystals show bulk superconductivity in all LuGexSn2-x samples with shielding fractions as high as 90%. Specific heat data confirm the effect to originate from residual metallic tin in the material, indicating that Sn atom substitutions in the 2D square nets cause disruptions of the hypervalent bonding and local anisotropy, which ultimately leads to vanishing of the superconducting state in the end-member LuGe2. This work sheds light on how the complexity in chemical interactions by two different carbon congeners leads to changes in the physical properties and how they can be correlated with the induced charge distribution. These studies also provide a general approach to modulation of charge density and. thus, of emerging physical properties in other classes of intermetallic systems based on the main-group elements of groups 13 to 15.

High-Performance Ag-Modified Bi0.5Sb1.5Te3 Films for the Flexible Thermoelectric Generator.

Bi-Sb-Te-based semiconductors possess the best room-temperature thermoelectric performance, but are restricted for application in the wearable field because of their inherent brittleness, rigidity, and nonscalable manufacturing techniques. Therefore, how to obtain thermoelectric materials with excellent thermoelectric properties and flexibility through the batch production process is a serious challenge. Here, we report the fabrication of flexible p-type thermoelectric Ag-modified Bi0.5Sb1.5Te3 films on flexible substrates using a facile approach. Their optimized power factors are ∼12.4 and ∼14.0 µW cm-1 K-2 at 300 and 420 K, respectively. These high-power factors mainly originate from the optimized carrier transport of the composite system, through which a high level of electrical conductivity is achieved, whereas a remarkably improved Seebeck coefficient is simultaneously obtained. Bending tests demonstrate the excellent flexibility and mechanical durability of the composite films, and their power factors decrease by only about 10% after bending for 650 cycles with a bending radius of 5 mm. A flexible thermoelectric module is designed and constructed using the optimized composite films and displays a power density of ∼1.4 mW cm-2 at a relatively small ΔT of 60 K. This work demonstrates the potential of inorganic thermoelectric materials to be made on flexible/wearable substrates for energy harvesting and management devices.

N-Type Mg3Sb2-x Bi x Alloys as Promising Thermoelectric Materials.

N-type Mg3Sb2-x Bi x alloys have been extensively studied in recent years due to their significantly enhanced thermoelectric figure of merit (zT), thus promoting them as potential candidates for waste heat recovery and cooling applications. In this review, the effects resulting from alloying Mg3Bi2 with Mg3Sb2, including narrowed bandgap, decreased effective mass, and increased carrier mobility, are summarized. Subsequently, defect-controlled electrical properties in n-type Mg3Sb2-x Bi x are revealed. On one hand, manipulation of intrinsic and extrinsic defects can achieve optimal carrier concentration. On the other hand, Mg vacancies dominate carrier-scattering mechanisms (ionized impurity scattering and grain boundary scattering). Both aspects are discussed for Mg3Sb2-x Bi x thermoelectric materials. Finally, we review the present status of, and future outlook for, these materials in power generation and cooling applications.

Interactions between amphiphilic Janus nanosheets and a nonionic polymer in aqueous and biphasic systems.

It is of great significance to understand the interactions between nanoparticles and polymers since they guide the development of tremendous applications, for example, in optoelectronic devices, biomedicine, and enhanced oil extraction. However, few studies have probed into this fundamental science as the emerging amphiphilic Janus nanosheets have a more complicated structure than homogeneous nanoparticles, which makes their interactions more complex. In this work, we try to understand the interactions between amphiphilic Janus nanosheets and a model nonionic polymer, hydroxyethyl cellulose (HEC), under different electrolyte and temperature conditions in both aqueous and biphasic systems by employing molecular dynamics simulations as well as experiments. It is found that the attachment of HEC onto the nanosheet surfaces exhibits ion-concentration-dependent behavior in the aqueous phase, helping to colloidally stabilize the nanosheets even in an environment with an extremely high salt concentration for a long duration. In the oil and water biphasic system, only elevated temperature promotes both Janus nanosheets and HEC to individually remain at the interface.

Highly (00l)-oriented Bi2Te3/Te heterostructure thin films with enhanced power factor.

Introducing nanoscale heterostructure interfaces into material matrix is an effective strategy to optimize the thermoelectric performance by energy-dependent carrier filtering effect. In this study, highly (00l)-oriented Bi2Te3/Te heterostructure thin films have been fabricated on single-crystal MgO substrates using a facile magnetron co-sputtering method. Bi2Te3/Te heterostructure thin films with Te contents of 63.8 at% show an optimized thermoelectric performance, which possess a Seebeck coefficient of -157.7 µV K-1 and an electrical conductivity of 9.72 × 104 S m-1, leading to a high power factor approaching 25 µW cm-1 K-2. The partially decoupled behavior of the Seebeck coefficient and electrical conductivity is contributed to Bi2Te3/Te heterostructure interfaces, which causes interfacial barrier filtering and scattering effects; thus, a high level of the Seebeck coefficient is obtained. Meanwhile, carrier transport in a-b plane can benefit from the highly preferred orientation, which guarantees a remarkably high electrical conductivity. We anticipate that our strategy may guide the way for preparing high-performance thermoelectric materials by microstructure design and regulation.

Removal of CdTe in acidic media by magnetic ion-exchange resin: a potential recycling methodology for cadmium telluride photovoltaic waste.

Sulfonated magnetic microspheres (PSt-DVB-SNa MPs) have been successfully prepared as adsorbents via an aqueous suspension polymerization of styrene-divinylbenzene and a sulfonation reaction successively. The resulting adsorbents were confirmed by means of Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS) and vibrating sample magnetometer (VSM). The leaching process of CdTe was optimized, and the removal efficiency of Cd and Te from the leaching solution was investigated. The adsorbents could directly remove all cations of Cd and Te from a highly acidic leaching solution of CdTe. The adsorption process for Cd and Te reached equilibrium in a few minutes and this process highly depended on the dosage of adsorbents and the affinity of sulfonate groups with cations. Because of its good adsorption capacity in strong acidic media, high adsorbing rate, and efficient magnetic separation from the solution, PSt-DVB-SNa MPs is expected to be an ideal material for the recycling of CdTe photovoltaic waste.