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In this research, the synthesis and characterization of a shape memory–assisted self‐healing polyurethane (SHSMPU) coating based on strong quadrupole hydrogen bonding interaction have been investigated. The coating was fabricated with an acrylic polyol, an isocyanate functionalized ureidopyrimidinone (UPy‐NCO) as a hydrogen bonding provider, and methylene diphenyl diisocyanate (MDI) as chain extender. Fourier‐transform infrared spectroscopy (FT‐IR) confirmed that by incorporating the UPy‐NCO dimer in the polyurethane (PU) structure, strong hydrogen bonding appears, which can provide the healing function of the polymer. In addition, differential scanning calorimetry (DSC) showed that by increasing the UPy‐NCO amount, the overall Tg decreased and the material becomes more flexible. A rheometric investigation revealed that a sample containing 45% of UPy‐NCO as total diisocyanate shows good shape recovery and an healing efficiency of macroscale cracks of around 94.2%, which is much higher than the corresponding sample without UPy‐NCO (50.4%). Even better performances were obtained with microcracks, where tested samples were significantly healed at around 80°C in 10 min. Furthermore, the introduction of polyethylene glycol 400 (PEG400) into the polymer's backbone led to a reduced Tg and thus higher flexibility at room temperature. The combination of the observed properties makes these materials promising for application as durable and flexible polyurethane coatings. [ABSTRACT FROM AUTHOR]

The long‐term durability of triboelectric nanogenerators (TENGs) remains a main challenge for practical applications because of inevitable material abrasion and wear, especially for sliding TENGs. Herein, an inorganic triboelectric pair composed of diamond‐like carbon (DLC) and glass with excellent durability and triboelectric output for sliding‐mode TENGs is proposed. This triboelectric pair possesses a low coefficient of friction and little abrasion and accordingly excellent durability (>500 000 cycles). Moreover, compared with the traditional copper‐polytetrafluoroethylene (Cu‐PTFE) TENG with maximum transferred charges of 50 nC, those of the DLC‐glass TENG reaches 141 nC. Due to the low‐friction and high hardness of the triboelectric pair, the output quickly recovers after simply cleaning wear debris. The DLC‐glass TENG demonstrates an output power density of 530 mW m−2 and a fourfold faster capacitor charging speed than the Cu‐PTFE TENG. Compared to the reported durable TENGs via structure optimization and interface lubrication, the DLC‐glass TENG shows higher outputs and simpler structure. This DLC‐glass pair structure is also introduced into a spherical TENG for blue energy harvesting with excellent durability. The inorganic triboelectric pair with excellent mechanical durability and electrical performance proposed in this work shows huge prospects for practical applications of TENGs. [ABSTRACT FROM AUTHOR]

The development of lithium–sulfur (Li–S) batteries is hindered by the disadvantages of shuttling of polysulfides and the sluggish redox kinetics of the conversion of sulfur species during discharge and charge. Herein, the crystallinities of a titanium nitride (TiN) film on copper‐embedded carbon nanofibers (Cu‐CNFs) are regulated and the nanofibers are used as interlayers to resolve the aforementioned crucial issues. A low‐crystalline TiN‐coated Cu‐CNF (L‐TiN‐Cu‐CNF) interlayer is compared with its highly crystalline counterpart (H‐TiN‐Cu‐CNFs). It is demonstrated that the L‐TiN coating not only strengthens the chemical adsorption toward polysulfides but also greatly accelerates the electrochemical conversion of polysulfides. Due to robust carbon frameworks and enhanced kinetics, impressive high‐rate performance at 2 C (913 mAh g−1 based on sulfur) as well as remarkable cyclic stability up to 300 cycles (626 mAh g−1) with capacity retention of 46.5% is realized for L‐TiN‐Cu‐CNF interlayer‐configured Li–S batteries. Even under high loading (3.8 mg cm−2) of sulfur and relatively lean electrolyte (10 μL electrolyte per milligram sulfur) conditions, the Li–S battery equipped with L‐TiN‐Cu‐CNF interlayers delivers a high capacity of 1144 mAh g−1 with cathodic capacity of 4.25 mAh cm−2 at 0.1 C, providing a potential pathway toward the design of multifunctional interlayers for highly efficient Li–S batteries. [ABSTRACT FROM AUTHOR]

This study reports a facile delignification and natural drying process for synthesizing cellulose films using the unique surface roughness, porosity, and lightness of the hollow stellate cellulose (HSC) united aerenchyma of the wetland weed Juncus effusus L. By controlling the grafted amino/fluorine‐bearing group content of various silane coupling agents, this work successfully manipulates the triboelectric polarities of HSC films after silanization. Subsequently, a layer of Ag nanowire electrodes is coated on one side of the silanized HSC friction layers, resulting in flexible, lightweight, semi‐transparent HSC‐based triboelectric nanogenerators (HSC‐TENGs) featuring both macro‐scale surface roughness and micro‐nano inner pores. These all‐in‐one HSC‐TENGs achieve the highest output voltage of 4.86 V, which is 28 times that of TENGs employing two pristine HSC films as triboelectric layers (PHSC‐TENG). Finally, the HSC‐TENG with the optimum output power is applied as a wearable self‐powered sensor for gait analysis, demonstrating stable and sustainable performances in distinguishing different body motions such as walking, running, jumping, and calf raising. This study not only proposes a new cellulose‐based TENG for future in‐depth body locomotion analysis but also paves the way for converting differently structured aerenchyma from abundant problematic aquatic or wetland weeds into promising structural templates in multifunctional cellulose‐based applications. [ABSTRACT FROM AUTHOR]

Lithium–sulfur (Li–S) batteries, although a promising candidate of next‐generation energy storage devices, are hindered by some bottlenecks in their roadmap toward commercialization. The key challenges include solving the issues such as low utilization of active materials, poor cyclic stability, poor rate performance, and unsatisfactory Coulombic efficiency due to the inherent poor electrical and ionic conductivity of sulfur and its discharged products (e.g., Li2S2 and Li2S), dissolution and migration of polysulfide ions in the electrolyte, unstable solid electrolyte interphase and dendritic growth on anodes, and volume change in both cathodes and anodes. Owing to the high specific surface area, pore volume, low density, good chemical stability, and particularly multimodal pore sizes, hierarchical porous carbon (HPC) materials have received considerable attention for circumventing the above problems in Li–S batteries. Herein, recent progress made in the synthetic methods and deployment of HPC materials for various components including sulfur cathodes, separators and interlayers, and lithium anodes in Li–S batteries is presented and summarized. More importantly, the correlation between the structures (pore volume, specific surface area, degree of pores, and heteroatom‐doping) of HPC and the electrochemical performances of Li–S batteries is elaborated. Finally, a discussion on the challenges and future perspectives associated with HPCs for Li–S batteries is provided. [ABSTRACT FROM AUTHOR]

Surface roughness of laser powder bed fusion (L-PBF) printed overhang regions is a major contributor to deteriorated shape accuracy/surface quality. This study investigates the mechanisms behind the evolution of surface roughness (Ra) in overhang regions. The evolution of surface morphology is the result of a combination of border track contour, powder adhesion, warp deformation, and dross formation, which is strongly related to the overhang angle (θ). When 0° ≤ θ ≤ 15°, the overhang angle does not affect Ra significantly since only a small area of the melt pool boundaries contacts the powder bed resulting in slight powder adhesion. When 15° < θ ≤ 50°, powder adhesion is enhanced by the melt pool sinking and the increased contact area between the melt pool boundary and powder bed. When θ > 50°, large waviness of the overhang contour, adhesion of powder clusters, severe warp deformation and dross formation increase Ra sharply. [ABSTRACT FROM AUTHOR]

The interface between materials and cells plays a critical role in many biomedical applications. Inspired by the hierarchical architecture of collagen, most abundant structure in the extracellular matrix (ECM), a multiscale hierarchical topography is designed to mimic the collagen nano/micro hierarchical topography. It is hypothesized that the ECM topography affects osteogenesis of human mesenchymal stem cells but until now, it cannot be studied without the biochemical and mechanical influences of the ECM. The multiscale hierarchical topography is achieved by innovatively using sequentially aligned topography preparation via a silicone stretch‐oxidation‐release method and imprinting lithography. The anisotropically hierarchical topography influences stem cell morphology, orientation, and osteogenic differentiation. Intriguingly, the design resembling that of assembled collagen, exhibits the highest degree of osteogenesis. The hierarchical topotaxis effects are further exemplified by the enhanced vinculin expression, cell contractility, and more pronounced nuclear translocation of Yes‐associated protein with the collagen‐mimicking topography, indicative for enhanced osteogenesis. The developed multiscale hierarchical system provides insights into the importance of specific biological ECM‐like topography by decoupling the biochemical influence. Various diseases, cancer, osteoarthritis, and fibrosis display impaired ECM structures, and therefore this system may have a great potential for tissue engineering approaches and developing in vitro disease models. [ABSTRACT FROM AUTHOR]

This work demonstrates the application of electrospun single and bundled carbon nanofibers (CNFs) as piezoresistive sensing elements in flexible and ultralightweight sensors. Material, electrical, and nanomechanical characterizations were conducted on the CNFs to understand the effect of the critical synthesis parameter—the pyrolyzation temperature on the morphological, structural, and electrical properties. The mechanism of conductive path change under the influence of external stress was hypothesized to explain the piezoresistive behavior observed in the CNF bundles. Quasi-static tensile strain characterization of the CNF bundle-based flexible strain sensor showed a linear response with an average gauge factor of 11.14 (for tensile strains up to 50%). Furthermore, conductive graphitic domain discontinuity model was invoked to explain the piezoresistivity originating in a single isolated electrospun CNF. Finally, a single piezoresistive CNF was utilized as a sensing element in an NEMS flow sensor to demonstrate air flow sensing in the range of 5–35 m/s. [ABSTRACT FROM AUTHOR]

The effect of the initial structure on the microstructure and texture of the friction stir welded Monel 400 alloy joints was investigated. For this aim, Monel 400 alloy of two initially treated conditions, annealed and rolled, was studied. Orientation imaging microscopy with a step size of 70 nm was used to characterise the microstructures of the joints. The mechanical properties were evaluated using Vickers hardness and nanoindentation tests. In the joint of the initial annealed alloy, continuous dynamic recrystallisation governed the grain structure formation. However, in the case of the initial rolled base material, discontinuous dynamic recrystallisation also occurred to cause more random texture, smaller grains, much more high-angle grain boundaries and higher dislocation densities. In the stir zone, the hardness and yielding strength increased respectively from 177 HV to 192 HV and 215.9 to 238.8 MPa by changing the base metal from the initial annealed to rolled condition. In addition, the microstructural features of the different zones containing the texture components and grain boundaries were scrutinised thoroughly. [ABSTRACT FROM AUTHOR]

A non-equilibrium heat treatment was used to produce secondary phase (β) in the single phase brass (α), which caused the modification of the grain structure and mechanical properties after friction stir processing. For this aim, the single phase brass plate containing 37 wt pct Zn was heated at 810 °C for 1 hour, and then quenched in water, which caused the formation of non-equilibrium secondary β phase. After the non-equilibrium heat treatment, bead on plate friction stir processing was employed. The origin of the modification by β phase was studied using high resolution electron backscattered diffraction and transmission electron microscopy. The results showed that the average grain size, yield strength, and strain-hardening exponent were changed, respectively, from 4.8 µm, 166 MPa and 0.28 to 2.1 µm, 213 MPa and 0.25 in the presence of β phase. The β phase promoted the discontinuous dynamic recrystallization by particle-stimulated nucleation mechanism, and then transformed to α resulting in a finer microstructure with more random texture. In addition, the nanometer-sized β particles were retained at the grain boundaries, which reduced their mobility and hence the grain growth was inhibited. [ABSTRACT FROM AUTHOR]

This letter reports that a pre-notched WS2/a-C tribocoating surface can be fully healed through sliding. The healed notches act as superior lubricant micro-reservoirs affording frictionless responses leading to an ultralow coefficient of friction (0.02). HR-TEM observations reveal WS2 (002) platelets in the healed notch parallel to the outer coating surface and to the V-notch/coating interface. Direct evidence of the rearrangement of WS2 at the curved interface elucidates that basal plane reorientation does not occur necessarily parallel to the counterpart sliding direction as was taken for granted in literature. The patchy tribofilm provides a new avenue for self-healing efficiency in tribology. This research reveals a new avenue for self-healing and repairing of damage in tribocoatings whereby one may lift the requirement of producing defect-free coatings for triboapplications. [ABSTRACT FROM AUTHOR]

In the present study, the effect of resistance spot welding scheme (i.e. single and double pulse welding) on the mechanical behaviour of resistance spot-welded DP1000-GI steel is investigated. It is shown that double pulse welding at low welding current decreases the maximum cross-tension strength and mechanical energy absorption capability of the welds. The factors that lead to the lower mechanical performance of double pulse welds are scrutinised. Local residual stress mapping reveals that the compressive residual stress perpendicular to the plane of the pre-crack either decreases or is fully released at the weld edge of double pulse welds. Orientation imaging microscopy analyses show that the martensite formed in front of the pre-crack of double pulse weld has a lower fraction of high-angle grain boundaries and a coarser structure of Bain groups as opposed to the corresponding area of single pulse weld. Lower mechanical performance of double pulse welds produced at lower welding current is ascribed to the lower compressive residual stress normal to the plane of crack and the formation of martensitic structure in front of the pre-crack with a lower fraction of high-angle grain boundaries and coarser Bain groups. [ABSTRACT FROM AUTHOR]

This article concentrates on the jerkytype phenomenon of surfaces in relative motion (i.e., a breakdown of the Coulomb friction law) in nanocomposite materials. Physical arguments are provided to understand the dependence of friction on sliding velocity in the sense of self-lubrication. Also described is the influence of water vapor adsorption from the testing environment. From a structural design viewpoint, nanocomposite coatings exhibit an optimized combination of functionalities when the nanoparticle becomes of approximately the same size as the separation. [ABSTRACT FROM AUTHOR]

By employing carbon dioxide as one source of reaction gases, carbon-doped Ti–O films were fabricated via reactive magnetron sputtering deposition. The chemical bonding configurations and microstructure of the films were analyzed by Raman spectrum and SEM, respectively. The effect of pH on the photocatalytic activities of the films was determined via evaluation of the decolorization rate of methyl orange under alkali, acid and neutrality conditions using UV light irradiation. Electrochemical impedance spectroscopy and potentiodynamic polarization tests were employed to determine the anti-corrosion properties. Compared with the C-free Ti–O film, the C-doped Ti–O films exhibit superior corrosion resistance. Furthermore, the results of the photodegradation experiment suggest that the C-doped Ti–O films have excellent photocatalytic activities and, for methyl orange, those with higher carbon content exhibit hyper-photodegradative effect under the alkali condition. [ABSTRACT FROM AUTHOR]

This study investigates the self‐healing capacity of a WS2/amorphous carbon (a‐C) tribocoating. It is found that prenotches up to 45 µm wide in the WS2/a‐C coating surface can be completely healed under the stimulus of sliding operations. The in situ tribotest of 100, 500, 2000, and 6000 laps confirms a dynamic filling of tribofilms that patch the voids and prenotched damages. The stabilized coefficient of friction (CoF) remains at an ultralow value down to 0.02, independent of the prenotched damage at the top of coating. The sites of notched damage in fact act as lubricant reservoirs to accumulate the otherwise "wasted" debris, which are restored as a superlubricant by the sliding operation. High resolution transmission electron microscopy reveals that WS2 (002) nanoplatelets in the healed notch are parallel to the top coating surface but conformal to the coating/notch interface. The patchy tribofilm holds excellent promise for the self‐repair of damages in the field of tribology. [ABSTRACT FROM AUTHOR]

Sensor designs found in nature are optimal due to their evolution over millions of years, making them well-suited for sensing applications. However, replicating these complex, three-dimensional (3D), biomimetic designs in artificial and flexible sensors using conventional techniques such as lithography is challenging. In this paper, we introduce a new processing paradigm for the simplified fabrication of flexible sensors featuring complex and bioinspired structures. The proposed fabrication workflow entailed 3D-printing a metallic mold with complex and intricate 3D features such as a micropillar and a microchannel, casting polydimethylsiloxane (PDMS) inside the mold to obtain the desired structure, and drop-casting piezoresistive graphene nanoplatelets into the predesigned microchannel to form a flexible strain gauge. The graphene-on-PDMS strain gauge showed a high gauge factor of 37 as measured via cyclical tension-compression tests. The processing workflow was used to fabricate a flow sensor inspired by hair-like 'cilia' sensors found in nature, which comprised a cilia-inspired pillar and a cantilever with a microchannel that housed the graphene strain gauge. The sensor showed good sensitivity against both tactile and water flow stimuli, with detection thresholds as low as 12 µm in the former and 58 mm/s in the latter, demonstrating the feasibility of our method in developing flexible flow sensors. [ABSTRACT FROM AUTHOR]

Polydimethylsiloxane (PDMS) is a silicone elastomer-based material that is used in various applications, including coatings, tubing, microfluidics, and medical implants. PDMS has been modified with hydrogel coatings to prevent fouling, which can be done through UV-mediated free radical polymerization using benzophenone. However, to the best of our knowledge, the properties of hydrogel coatings and their influence on the bulk properties of PDMS under various preparation conditions, such as the type and concentration of monomers, and UV treatment time, have never been investigated. Acrylate-based monomers were used to perform free radical polymerization on PDMS surfaces under various reaction conditions. This approach provides insights into the relationship between the hydrogel coating and bulk properties of PDMS. Altering the UV polymerization time and the monomer concentration resulted in different morphologies with different roughness and thickness of the hydrogel coating, as well as differences in the bulk material stiffness. The surface morphology of the coated PDMS was characterized by AFM. The cross section and thickness of the coatings were examined using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. The dependence of coating development on the monomer type and concentration used was evaluated by surface hydrophilicity, as measured by water contact angle. Elongation-until-break analysis revealed that specific reaction conditions affected the bulk properties and made the coated PDMS brittle. Therefore, boundary conditions have been identified to enable high quality hydrogel coating formation without affecting the bulk properties of the material. [ABSTRACT FROM AUTHOR]