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Targeting for flexible, broadband ultrasound sensors, a new breed of nanographene platelet (NGP)/polyimide (PI) film is inkjet printed with morphologically optimized NGP/poly (amic acid) hybrid-based nanocomposite ink. The ink is produced with high-shear liquid phase exfoliation from inexpensive bulk graphite, manifesting good printability and high graphene concentration as high as 13.1 mg mL−1. Featuring an ultra-thin thickness (∼1 μm only), the inkjet-printed NGP/PI film sensor is demonstrated to possess excellent thermal stability and high adhesive strength reaching the American Society for Testing and Materials 5B level. The highly uniform and consolidated NGP/PI nanostructure in the sensor enables the formation of π-π interaction between NGPs and PI polymer matrix, and the quantum tunneling effect is triggered among NGPs when ultrasound traverses the sensor. This sensing mechanism endows the NGP/PI sensor with good sensitivity, fidelity and accuracy, showing comparable performance as prevailing commercial ultrasound sensors such as piezoelectric sensors. The film sensor has a proven gauge factor as high as 739, when sensing ultrasound at 175 kHz, and an ultrabroad responsive spectrum up to 1.6 MHz.

Certain traditional sensors like lead zirconate titanate (PZT) wafers and ultrasonic probes can respond to extremely weak disturbances such as ultrasonic guided waves (UGWs). However, their further development for applications to meet increasing engineering demands is limited on account of their hardness, brittleness, and complex manufacturing processes. Now, emerging nanotechnology ushers in a brand-new world for nanocomposite-based strain sensors, endowing them with higher flexibility, better surface compatibility and easier fabrication. Yet there are few reports of composites which can be used to perceive high frequency UGWs with an ultralow magnitude. Here, we present a novel graphene-based nanocomposite possessing strong sensitivity for sensing ultrasonic waves by virtue of a neoteric sensing mechanism − the tunneling effect. By designing and optimizing the microstructure of the conductive network in the nanocomposite sensor, we successfully capture ultrasonic waves with high signal-to-noise ratio in a broad frequency range up to 1 MHz. With the feature of high sensitivity and rapid response times, the graphene-based nanocomposite becomes a promising candidate for structural health monitoring in developing prospective applications.

We present a nano-engineered thin-film-type piezoresistive sensor, coatable or sprayable on a medium surface for in-situ acquisition of broadband ultrasound up to 1.4 MHz – a trait of nanocomposite-based piezoresistive sensing devices that has until now not been discovered and explored. With polyvinylidene fluoride as the matrix, fabrication of the spray-on sensor is attempted in a comparative manner, with multiscale nanofillers ranging from zero-dimensional carbon black, through one-dimensional multiwalled carbon nanotubes, to two-dimensional graphene nanoparticles. With a morphologically optimal nano-architecture, the quantum tunneling effect can be triggered in the percolating nanofiller network when ultrasound signals traverse the sensor, inducing dynamic alteration in the piezoresistivity manifested by the sensor. In-situ morphological analysis and experiment reveal high fidelity, ultrafast responses, and high sensitivity of the sensor to dynamic disturbance, from static strain to ultrasound in a regime of megahertz yet with an ultralow magnitude (of the order of microstrain or nanostrain). These findings are remarkable as no other investigation has probed the response of nanocomposite piezoresistive sensors over such a broad frequency spectrum.

Honeycomb-like lignin-derived carbon (LC) foams doped with reduced graphene oxide (RGO) are fabricated via unidirectional ice-templating followed by freeze-drying and carbonization. The dimensions, constituent contents, and density of the foams could be easily adjusted. By manipulating the density, high electrical conductivity and good mechanical properties could be achieved at relatively low RGO contents. In addition to the high conductivity and interfaces between LC and RGO, aligned pores also boost electromagnetic interference (EMI) shielding effectiveness (SE) of the foams owing to induced multiple reflections. X-band EMI SE of the LC-based foams with 2-mm thickness could reach 28.5 to 70.5 dB at very low densities. Furthermore, the normalized surface specific SE (SE divided by density and thickness) could be as high as 28750 dB·cm2/g, which is much higher than those of other types of carbon foams and most shielding materials ever reported. With the outstanding EMI shielding performance, good mechanical properties and more sustainable raw material, the biomass-based carbon foams offer promising prospects for lightweight, robust and high-performance EMI shielding materials. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

In this paper, we report graphene nanoscrolls bridged by crumpled graphene nanosheets as an effective conductive framework for confining SnS2 nanosheets for lithium-ion battery (LIB) applications. The nanoscroll/nanosheet hybrid aerogels (GNAs) with confined SnS2 nanosheets are facilely prepared via fast quenching, freeze-drying and thermal annealing. During quenching, wrapping SnS2 nanosheets in the nanoscrolls and bridging the one-dimensional nanoscrolls by the two-dimensional nanosheets occur simultaneously, and the ratio of nanoscrolls/nanosheets can be controlled by simply adjusting quenching conditions. The optimized SnS2/GNA is highly porous with a large specific surface area of 127.1 m2 g−1 and multi-scale pore structure, which can effectively prevent SnS2 aggregation to provide abundant lithiation/delithiation sites, and buffer volumetric change and pulverization of SnS2 nanosheets. Moreover, the three-dimensional conductive network formed in the hybrid aerogels can remarkably improve its electrical conductivity while providing sufficient channels for the transportation of lithium ions and charges. As a result, the optimized SnS2/GNA nanocomposite exhibits enhanced electrochemical performance with a high initial reversible capacity (1514.8 mAh g−1 at 0.1 A g−1), excellent rate capacity (665.4 mAh g−1 at 5 A g−1) and good cyclic stability (1050 mA h g-1 at the 50th cycle). This may provide an efficient generic approach for encapsulation of transition metal dichalcogenide materials in conductive nano/micro confined spaces for fabrication of high-performance LIB anodes. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

Oyster reefs, which can maintain coastal ecosystems by absorbing storm surge energy, are composed of aggregated oyster shells bonded by bioadhesive secretions containing inorganic minerals. Bioadhesive secretions are formed by cross-linking extracellular liquid, which is rich in organic and inorganic ions, with sand, bacteria, or diatoms. Inspired by such bioprocesses, a mineral hydrogel with excellent 3D printability, rapid self-healing ability (85% recovery within 1 min), high stretchability (>1500% tensile elongation) and ionic conductivity was synthesized by the chelation of polyacrylic acid (PAA) with calcium ions (Ca2+) and subsequent physical cross-linking of PAA with modified amorphous calcium phosphate (ACP) nanoparticles. With such mineral hydrogels, ionic skin was successfully prepared, which could recognize the bending degrees of the index finger and sensitively sense the tensile strain (approximately 33 ms). Moreover, the ionic skin can distinguish external temperature stimuli and work stably at a temperature of 75 °C. These performances exhibit the great mechano-sensing and thermosensing abilities of ionic skin, showing promising applications in various fields, such as wearable sensors, for withstanding high ambient temperatures and the next generation of soft intelligent robots.

Nanocomposites fabricated based on a compound of carbon nanofiller and polymer matrix exhibit extraordinary piezoresistive performance, and the capacity of nanocomposite sensors in measuring structural dynamic strains has been widely demonstrated. However, the sensitivity of existing nanocomposite strain sensors is limited, particularly in aspects of sensing precision and response frequency. This study focuses on sensitivity investigations of flexible nanocomposite sensors fabricated using a compound of carbon black (CB) fillers and polyvinylidene fluoride (PVDF) matrix, which has shown promising potential in perceiving extremely weak strain signals within a considerably broad range of response frequencies. As a highly important indication of sensitivity, sensing precision was characterized in a quantitative manner. Broadband spectrum analysis was conducted as an effective way to examine sensor performance in perceiving minimal disturbances in dynamic strains. Disturbances were generated in two experiments, by introducing local material and geometric variations into the inspected structures, in terms of mass attachment and structural damage. The effectiveness of the nanocomposites in structural damage detection was demonstrated. Moreover, the experimental results indicate the promising potential of nanocomposite-based strain sensors for applications such as advanced bioelectronics, ultrasonic inspection, and in - situ structural health monitoring.

A type of flexible, broadband electromagnetic microwave absorber is designed, simulated, manufactured and characterized. The multilayer-structured absorber is composed of a screen-printed carbon-based resistive film sandwiched between two layers of silicon rubber/multi-walled carbon nanotube composites and in turn backed by aluminum foil. The theoretical analysis based on the equivalent circuit model and the electromagnetic simulations is instructive for the design of the absorbers with the desired operation frequency by simply tuning the thickness of the dielectric substrates and the utilization of the one carbon film. The absorber indicates a broadband absorption, with a 90% absorption bandwidth of 13.5 GHz, and the thickness of the dielectric substrate of 5 mm. The fabricated absorber also shows a flexibility that it can be bent, rolled and twisted without breakdown of the structure and degradation of the absorption performance.

A novel nanocomposite-inspired in-situ broadband ultrasonic sensor previously developed, with carbon black as the nanofiller and polyvinylidene fluoride as the matrix, was networked for acousto-ultrasonic wave-based passive and active structural health monitoring (SHM). Being lightweight and small, this kind of sensor was proven to be capable of perceiving strain perturbation in virtue of the tunneling effect in the formed nanofiller conductive network when acousto-ultrasonic waves traverse the sensor. Proof-of-concept validation was implemented, to examine the sensor performance in responding to acousto-ultrasonic waves in a broad frequency regime: from acoustic emission (AE) of lower frequencies to guided ultrasonic waves (GUWs) of higher frequencies. Results have demonstrated the high fidelity, ultrafast response and high sensitivity of the sensor to acousto-ultrasonic waves up to 400kHz yet with an ultra-low magnitude (of the order of micro-strain). The sensor is proven to possess sensitivity and accuracy comparable with commercial piezoelectric ultrasonic transducers, whereas with greater flexibility in accommodating curved structural surfaces. Application paradigms of using the sensor for damage evaluation have spotlighted the capability of the sensor in compromising "sensing cost" with "sensing effectiveness" for passive AE- or active GUW-based SHM.

A series of composite radar absorbing structures (RAS) with resistive frequency selective surface (FSS) have been designed and optimized in high efficiency using the transfer matrix method together with the adaptive genetic algorithm. The composite structures show broadband absorption property verified by both numerical simulation and experimental measurement. Especially, the optimal 5 mm-thick composite RAS inserted with a single layer of resistive FSS can produce 90% absorption bandwidth of around 11.8 GHz, which is much wider compared with the multilayered metallic metamaterial absorber with the same thickness. The composite RAS can be easily manufactured by dielectric substrates in combination with screen-printed resistive FSSs.

Particulate matter (PM)-induced air pollution has become a serious threat to public health in many regions. There is, therefore, a need for development of advanced respirator materials that can efficiently capture PM and also exhibit both low breathing resistance and good moisture wicking properties to ensure physiological comfort of the wearers. Herein, we report a new type of multilayer membranes that exhibit excellent filtration efficiencies for PM of various sizes, low air resistance and good directional moisture transport properties. We incorporated a type of metal-organic framework, ZIF-8, into polyacrylonitrile (PAN) to prepare microfibers with high surface roughness, and alternatingly stacked layers of rough microfibers with layers of PAN nanofibers via electrospinning, resulting in multi-level structured membranes. Moreover, subsequent acid and base treatments could not only tailor the hydrophilicity of fiber surfaces, but also induce multiscale surface roughness, facilitating both moisture wicking and PM adsorption. This allowed us to construct a unique multilayer membrane composed of a super-hydrophilic outer layer made of PAN fibers with multiscale surface roughness, a hydrophilic composite intermediate layer consisting of porous PAN-ZIF-8 microfibers and PAN nanofibers, and a hydrophobic polystyrene fibrous inner layer, which could offer excellent directional moisture transport performance and a high water vapor transmission rate of 10.56 kg m−2 d−1 due to strong capillary force and push-pull effect. More importantly, benefiting from the large specific surface area, rough fiber surface, and hierarchical pore structure, the membrane exhibits high removal efficiencies of 99.973% for 0.3-μm particles and ≧99.99% for particles of other sizes, including the more harmful ultrafine particles, at a low pressure drop of 80.1 Pa. Moreover, the multilayer composite filter still maintains a high removal efficiency of 99.951% after continuous air purification for 48 h in a high PM2.5 concentration (>300 μg m−3) environment. These make such membranes promising high-performance filtration media for respirator applications.

A type of lightweight and flexible multi-walled carbon nanotube (MWCNT)/waterborne polyurethane (WPU) composites is fabricated, which show superior shielding effectiveness (SE) of electromagnetic interference in the X-band even under the thin thickness of samples. The thickness values 0.05, 0.32 and 0.8 mm correspond to SE of 24, 49 and 80 dB, respectively. This attributes to the extremely high MWCNT loading up to 76 wt%. Moreover, the composites show much higher specific SE (up to 3408 dB cm 2 /g) than other carbon-based polymer composites with similar filler amount. Shielding mechanisms of the composites with wide ranges of MWCNT loadings are discussed based on the concentration, thickness and conductivity. High concentration of MWCNT/WPU composites at low thicknesses indicates higher capability of shielding by absorption compared to reflection, which is adverse to composites with relatively low MWCNT mass ratios. A comparison between experimental and theoretical SE results is made in detail based on observed microstructures by scanning electron microscopy. The MWCNT/WPU composite films fabricated on large-area polyimide and cloth substrates are also demonstrated.

Multi-walled carbon nanotubes are used to fabricate a type of environment-friendly electrothermal bimetallic actuators with the matrix of waterborne polyurethane or silicone rubber. Even under a relatively low DC driven voltage of 7 V, the actuator can achieve a bending displacement up to 28 mm, or a curvature up to 0.29 cm −1 , which are greater than most of other electrothermal actuators reported. The actuator has considerable controllability, large mechanical output and long life-time, and the working power can be reduced down to 25 mW/mm 3 in the atmospheric environment. The actuation mechanisms owe not only to the mismatch in the coefficients of thermal expansion, but also to the unique negative temperature coefficient effect of nanotube composites. Furthermore, the bimetallic actuators based on various polymer matrices are supposed to provide revelations for electrothermal bimorph actuators.

In this article, we design stable, highly porous PLLA/PDLA blend nanofibrous aerogels (NFAs) fabricated by cross-linking the electrospun nanofibers via spontaneous inter-fiber stereocomplexation, and the potential of the NFAs as an air filter for capturing ultrafine airborne particles. It is found that the extent of crosslinking can be easily controlled by adjusting the molecular weights of the polymers and their ratio. Compression tests reveal that the PLLA/PDLA blend NFAs exhibit significantly higher compressive resistance than their PLLA counterparts, demonstrating the effectiveness of the cross-linking. When combined with a layer of HEPA filter to form a composite filter, the NFAs show the potential to significantly improve the filtration efficiency for ultrafine particles with a slight increase in pressure drop. This is attributed predominantly to the high porosity of the NFAs, as well as the slip flow effect of nanofibers. The greater thickness of the highly porous NFAs leads to increased residence time of the ultrafine particles and hence enhances the probability of capturing particles through Brownian motion. This unique approach to spontaneously cross-linking electrospun nanofibers may also be applied to other stereocomplex systems or used for applications in other fields.