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Although soft electronic materials are of significant importance for flexible electronic devices, most of them are derived from commercial polymer elastomers, such as polydimethylsiloxane, polyurethane and Ecoflex. In this work, citric acid-based degradable polyester elastomers are prepared by a melt polycondensation process, utilizing citric acid, 1,8-octanediol and poly(ethylene glycol) (PEG) as monomers. Furthermore, poly(1,8-octanediol citrate acid) (POC)-PEG/silver nanowire (AgNW) conductive polyester elastomers (CPEs) are prepared by introducing a AgNW layer on the surface of the POC-PEG films. Scanning electron microscopy images reveal that the thickness of the AgNW layer is on the scale of several micrometers and the AgNWs form a continuous conductive network. Upon mechanical stimuli, POC-PEG exhibits recoverable deformation and induces variation in the AgNW conductive network, resulting in a conversion of strain to detectable resistance. When tensile strain is applied, the POC-PEG/AgNW CPEs achieve a gauge factor of 231.6, a response range of 0%-50%, a low response time of 35 ms and high stability. Moreover, the POC-10PEG/AgNW CPE also responds to bending deformation with a gauge factor of 3667.5, a response range of 0%-8.4%, a low response time of 62 ms and high stability. On the basis of strain sensitivity, wireless sensors are further assembled by integrating the POC-PEG/AgNW CPEs into a Bluetooth signal transmission system. Various human motions and physiological activities are successfully monitored using the wireless sensors. The results demonstrate that degradable citric acid-based polyester elastomers/AgNW CPEs are promising materials for next-generation sustainable and flexible electronic devices. LESS      Full article

Materials selection and microstructural design of the sensing part of flexible pressure sensors are of great significance in improving their performance. However, achieving synergy between the sensing material and the microstructure of the flexible sensors remains a challenge. Herein, compressible and stretchable sensors based on a carbon nanofiber/poly(styrene-butadiene-styrene) (CNF/SBS) compound are demonstrated with a template-stripped method for detecting various human motions, including pulses, finger bending and pressure distributions. Benefiting from the adjustable fingerprint microstructure and mass fraction of CNFs, the as-designed flexible pressure sensor dramatically achieves a high sensitivity of 769.2 kPa^-1, a low detection limit of 5 Pa and high reliability of over 1000 cycles. Moreover, the flexible sensor based on CNF/SBS can be stretched due to the outstanding tensile properties of SBS. The enhanced stretchable sensor remarkably possesses a high gauge factor of 105.6 with a stretch range of 0%-300% and up to 600% elongation. Importantly, the proposed pressure and tension strain sensors are investigated to monitor vigorous human motion, revealing their tremendous potential for applications in flexible compressible and stretchable wearable electronics. LESS      Full article

Sticky thermoelectric (TE) materials have been inversely designed to enable the mass production of flexible TE sheets through lamination or roll-to-roll processes without using electrically conductive adhesives. They have also been demonstrated as inorganic/organic hybrid materials consisting of TE inorganic particles and low-volatilizable organic solvents to exhibit Seebeck coefficients based on the TE particles and low thermal conductivities based on the organic matrix. To achieve energy harvesting of 250 µW for driving various electric devices using voltage boosters, herein, we employ p- and n-type Bi₂Te₃ particles due to their high Seebeck coefficients, and cover the Bi₂Te₃ bodies with Au skins because the interfacial electrical resistance depends on the electrical resistance of opposing substances at the interface. After controlling the plating amount to cover the Bi₂Te₃ particles with Au skins, we achieve a TE power generation two orders of magnitude greater than the previous study, i.e., 255 µW on a hot plate of 110 °C with a 6 × 6 module. Overall, with input from other organic devices, like organic light-emitting diodes and dye-sensitized solar cells, this study presents a hierarchical design for TE hybrid materials that suppresses the thermal conduction by hybridizing TE particles with the organic matrix at the microscale. This reduces the electrical resistance by modifying the interfaces of the TE particles at the nanoscale and optimizes the Seebeck coefficient of TE particles at the atomic scale. To compete with solid-state TE modules with regards to power generation capacity, the hierarchical design towards a possible further two orders of magnitude improvement is also discussed. LESS      Full article

Proper contacts between thermoelectric (TE) materials and electrodes are critical for TE power generation or refrigeration. The Bi-rich n-type Zintl material Mg_3+δBi_2-xSb_x exhibits very good TE performance near room temperature, which makes Mg_3+δBi_2-xSb_x-based compounds highly promising candidates to replace the Bi₂Te_3-ySe_y alloys, but ideal contacts that can match their TE performance have not yet been well studied. Here we investigate different metal (Ni and Fe) and metal alloy (NiFe, NiCr, NiCrFe, and stainless steel) contacts on n-type Mg_3+δBi_1.5Sb_0.5. It is first shown that the low Schottky barrier and narrow depletion region resulting from the band degeneracy and high carrier concentration of a heavily doped TE material are beneficial for the formation of a low-resistivity ohmic contact with a metal or a metal alloy. Most fully optimized TE materials can take advantage of this. Second, it is found that the NiFe/Mg_3+δBi_1.5Sb_0.5 contact exhibits excellent thermal stability and the lowest ohmic contact resistivity among those studied after aging for over 2100 h, which is attributed to the formation of metallic NiMgBi between the NiFe and Mg_3+δBi_1.5Sb_0.5 layers. As a buffer phase, NiMgBi can effectively prevent elemental diffusion without negatively affecting the electron transport. Benefiting from such low contact resistance, a Mg_3+δBi_1.5Sb_0.5/Bi_0.4Sb_1.6Te₃ unicouple exhibits competitive conversion efficiency, 6% with a 150 K temperature difference and a hot-side temperature of 448 K. LESS      Full article

Second-generation high-temperature superconducting (2G-HTS) tapes based on REBa₂Cu₃O_7-x (REBCO, RE: rare earth) materials enable the energy-efficient and high-power-density delivery of electricity, thereby promoting the development of clean energy generation, conversion, transmission, and storage. To overcome the weak grain-boundary connection and poor mechanical properties of these superconductors, a thin-film technology for epitaxy and biaxial textures based on flexible substrates has been developed. In recent years, high-quality 2G-HTS tapes have been produced at the kilometer scale and used in superconducting demonstration projects. This review first summarizes the development of HTS materials and briefly expounds the properties of REBCO superconducting materials. Subsequently, the structural characteristics, preparation methods, and current research progress of 2G-HTS tapes are given. In addition, the applications of REBCO tapes in constructing high-field magnets are also briefly reviewed. LESS      Full article

Fused deposition modeling (FDM) is an additive manufacturing technique with significant advantages, including cost effectiveness, applicability for a wide range of materials, user-friendliness and small equipment features. However, its poor resolution represents a hindrance for functional parts for commercial production. In this review, the key process parameters are presented with their factors and effects on the characteristics of FDM-printed polymeric products. Hence, better insights into the relationship between key parameters and three main printing characteristics, namely, surface roughness, mechanical strength and dimensional accuracy, in existing FDM research are provided. A conclusion that addresses the challenges and future research directions in this area is also presented. LESS      Full article