Effect of Peel Ply on Resin Flow during Vacuum Infusion.

Although various simulations have been conducted for the vacuum infusion process, most of the studies have considered only fabrics and flow medium and ignored the effect of peel ply. However, peel ply can affect resin flow because it is placed between the fabrics and flow medium. To verify this, permeability of two types of peel plies was measured, and it was found that the permeability between the peel plies differed significantly. Moreover, the permeability of the peel plies was lower than that of the carbon fabric; thus, peel ply can cause a bottleneck in the flow in the out-of-plane direction. Some 3D flow simulations were conducted in cases of no peel ply and for two types of the peel plies to confirm the effect of peel ply, and experiments were also conducted for two types of the peel plies. It was observed that filling time and flow pattern were highly dependent on the peel plies. The smaller permeability of peel ply has, the greater effect of peel ply is. These results indicate that the permeability of peel ply is one of the dominant factors and should be considered in process design in vacuum infusion. Additionally, by adding one layer of peel ply and applying permeability, the accuracy of flow simulations can be improved for filling time and pattern.

Multifunctional Additives for High-Energy-Density Lithium-Ion Batteries: Improved Conductive Additive/Binder Networks and Enhanced Electrochemical Properties.

Cylindrical-type cells have been widely adopted by major battery and electric vehicle manufacturers owing to their price competitiveness, safety, and easy expandability. However, placement of electrodes at the core of cylindrical cells is currently restricted because of high electrode curvature and the lack of specialized electrodes and electrode materials. Here, we report the realization of highly flexible high-energy-density electrodes (active material loading of >98.4%) that can be used at the cores of cylindrical cells. The improved properties result from the introduction of a multifunctional poly(melamine-co-formaldehyde) (MF copolymer) additive, which yields a relatively more fluidic and well-dispersed slurry using only 0.08 wt %. MF copolymer-mediated formation of completely wrapped CNT/PVDF networks on LiCoO2 (LCO) and accompanying contact enhancement between LCO and carbon nanotubes (CNTs) resulted in an increase of electrical and mechanical properties of the two types (composites with or without collectors) of electrodes compared with those of additive-free electrodes. Flexibility tests were carried out by rolling electrodes onto cylinder substrates (diameters of ca. 1 and 10 mm); this process resulted in relatively lower resistance changes of the MF copolymer-containing electrodes than for the reference electrodes. In addition, capacity retention after 100 cycles for cells with and without MF copolymers was approximately 10-20% better for the samples with the MF copolymer than for those without. CNT/PVDF networks with MF copolymers were proven to induce a relatively thin and stable cathode electrolyte interface layer accompanying the chemical bond formation during cycling.

Sensitivity Improvement of Stretchable Strain Sensors by the Internal and External Structural Designs for Strain Redistribution.

Fiber strain sensors that are directly woven into smart textiles play an important role in wearable systems. These sensors require a high sensitivity to detect the subtle strain in practical applications. However, traditional fiber strain sensors with constant diameters undergo homogeneous strain distribution in the axial direction, thereby limiting the sensitivity improvement. Herein, a novel strategy of internal or external structural design is proposed to significantly improve the sensitivity of fiber strain sensors. The fibers are produced with directional increases in diameter (internal design) or polydimethylsiloxane (PDMS) microbeads attached to surfaces (external design) by combining hollow glass tubes used as templates with PDMS drops. The structural modification of the fiber significantly impacts the sensing performance. After optimizing structural parameters, the highest gauge factor reaches 123.1 in the internal-external structure design at 25% strain. A comprehensive analysis reveals that the desirable scheme is the internal structural design, which features a high sensitivity of 110 with a 100% improvement at ∼5-20% strain. Because of the sufficiently robust interface, even at the 800th cycle, fiber sensors still possessed an excellent stable performance. The morphology evolution mechanism indicates that the resistance increase is closely related with the increased peak width and distance, and the appearance of gaps. Based on the finite element modeling simulation, the quantified effective contributions of different strategies positively correlate with the improved sensitivity. The proposed fiber strain sensors, which are woven into the two-dimensional network structure, exhibit an excellent capability for displacement monitoring and facilitate the traffic control of crossroads.

Three-dimensional, millimeter-scale semiconducting SWCNT aerogels for highly sensitive ozone detection.

Semiconducting frameworks possessing porous structure are promising platforms for the detection of hazardous gas molecules. In this study, we propose a facile route to fabricate millimeter-scale, three-dimensional semiconducting SWCNT (s-SWCNT) aerogels and demonstrate deactivation of the co-existing metallic SWCNT (m-SWCNT) network via electrical breakdown process. In particular, the on-off ratio of the modulated semiconducting aerogel after the electrical breakdown process was 205, which is an increase of 18.9 times over that before the process. The modulated semiconducting SWCNT aerogels with a large specific surface area (∼1270 m2 g-1) demonstrated their applicability for highly sensitive ppb-level ozone detection. The modulated semiconducting networks led to a 1310 % increase in the magnitude of response to 30-ppb ozone gas injection compared with that of pristine SWCNT aerogels. Furthermore, the prepared aerogels could detect 3 ppb of ozone within 40 s and retain stable reversible ozone detection for 200 cyclic operations over 100 h. Thus, the proposed semiconducting SWCNT aerogels are a promising candidate for highly sensitive environmental gas sensors.

An Organic/Inorganic Nanocomposite of Cellulose Nanofibers and ZnO Nanorods for Highly Sensitive, Reliable, Wireless, and Wearable Multifunctional Sensor Applications.

Organic and inorganic one-dimensional nanomaterials were synthesized and combined into a nanocomposite film for a wearable sensor. Reproducible ZnO nanorod (NR) synthesis was achieved by the addition of an appropriate amount of water. Cellulose nanofibers (CNFs) were used due to their porous matrix formation. The interconnected channels of brittle ZnO NRs were well-fabricated in the flexible network of CNFs. The surface morphology, thermal, and mechanical properties of the CNF/ZnO NR nanocomposite film were characterized. The interfacial interactions between these two nanomaterials were also studied. The nanocomposite film is sufficiently flexible so that it shows no electrical resistance changes even after repeated bending tests with a minimum bending radius of 1.5 mm. In addition, ZnO NRs with different lengths were synthesized. The composite of longer ZnO NRs and CNF showed 2.8 × 103 times higher photocurrent and responsivity performance. The humidity sensing performance of the composite was also suggested. The CNF/ZnO NR film shows reasonable electrical signal changes enabling the evaluation of a calibration curve. Finally, a smart band including a CNF/ZnO NR film sensor was fabricated and connected to a smartphone by Bluetooth. These results open an avenue for developing wearable sensors by overcoming the brittleness of inorganic materials.

Highly stretchable multi-walled carbon nanotube/thermoplastic polyurethane composite fibers for ultrasensitive, wearable strain sensors.

Here, we report a novel highly sensitive wearable strain sensor based on a highly stretchable multi-walled carbon nanotube (MWCNT)/Thermoplastic Polyurethane (TPU) fiber obtained via a wet spinning process. The MWCNT/TPU fiber showed the highest tensile strength and ultra-high sensitivity with a gauge factor (GF) of approximately 2800 in the strain range of 5-100%. Due to its high strain sensitivity of conductivity, this CNT-reinforced composite fiber was able to be used to monitor the weight and shape of an object based on the 2D mapping of resistance changes. Moreover, the composite fiber was able to be stitched onto a highly stretchable elastic bandage using a sewing machine to produce a wearable strain sensor for the detection of diverse human motions. We also demonstrated the detection of finger motion by fabricating a smart glove at the joints. Due to its scalable production process, high stretchability and ultrasensitivity, the MWCNT/TPU fiber may open a new avenue for the fabrication of next-generation stretchable textile-based strain sensors.

Experimental and Numerical Studies on Fiber Deformation and Formability in Thermoforming Process Using a Fast-Cure Carbon Prepreg: Effect of Stacking Sequence and Mold Geometry.

A fast-cure carbon fiber/epoxy prepreg was thermoformed against a replicated automotive roof panel mold (square-cup) to investigate the effect of the stacking sequence of prepreg layers with unidirectional and plane woven fabrics and mold geometry with different drawing angles and depths on the fiber deformation and formability of the prepreg. The optimum forming condition was determined via analysis of the material properties of epoxy resin. The non-linear mechanical properties of prepreg at the deformation modes of inter- and intra-ply shear, tensile and bending were measured to be used as input data for the commercial virtual forming simulation software. The prepreg with a stacking sequence containing the plain-woven carbon prepreg on the outer layer of the laminate was successfully thermoformed against a mold with a depth of 20 mm and a tilting angle of 110°. Experimental results for the shear deformations at each corner of the thermoformed square-cup product were compared with the simulation and a similarity in the overall tendency of the shear angle in the path at each corner was observed. The results are expected to contribute to the optimization of parameters on materials, mold design and processing in the thermoforming mass-production process for manufacturing high quality automotive parts with a square-cup geometry.

Highly Sensitive Wearable Textile-Based Humidity Sensor Made of High-Strength, Single-Walled Carbon Nanotube/Poly(vinyl alcohol) Filaments.

Textile-based humidity sensors can be an important component of smart wearable electronic-textiles and have potential applications in the management of wounds, bed-wetting, and skin pathologies or for microclimate control in clothing. Here, we report a wearable textile-based humidity sensor for the first time using high strength (∼750 MPa) and ultratough (energy-to-break, 4300 J g-1) SWCNT/PVA filaments via a wet-spinning process. The conductive SWCNT networks in the filaments can be modulated by adjusting the intertube distance by swelling the PVA molecular chains via the absorption of water molecules. The diameter of a SWCNT/PVA filament under wet conditions can be as much as 2 times that under dry conditions. The electrical resistance of a fiber sensor stitched onto a hydrophobic textile increases significantly (by more than 220 times) after water sprayed. Textile-based humidity sensors using a 1:5 weight ratio of SWCNT/PVA filaments showed high sensitivity in high relative humidity. The electrical resistance increases by more than 24 times in a short response time of 40 s. We also demonstrated that our sensor can be used to monitor water leakage on a high hydrophobic textile (contact angle of 115.5°). These smart textiles will pave a new way for the design of novel wearable sensors for monitoring blood leakage, sweat, and underwear wetting.

Flexible Textile-Based Organic Transistors Using Graphene/Ag Nanoparticle Electrode.

Highly flexible and electrically-conductive multifunctional textiles are desirable for use in wearable electronic applications. In this study, we fabricated multifunctional textile composites by vacuum filtration and wet-transfer of graphene oxide films on a flexible polyethylene terephthalate (PET) textile in association with embedding Ag nanoparticles (AgNPs) to improve the electrical conductivity. A flexible organic transistor can be developed by direct transfer of a dielectric/semiconducting double layer on the graphene/AgNP textile composite, where the textile composite was used as both flexible substrate and conductive gate electrode. The thermal treatment of a textile-based transistor enhanced the electrical performance (mobility = 7.2 cm²·V-1·s-1, on/off current ratio = 4 × 105, and threshold voltage = -1.1 V) due to the improvement of interfacial properties between the conductive textile electrode and the ion-gel dielectric layer. Furthermore, the textile transistors exhibited highly stable device performance under extended bending conditions (with a bending radius down to 3 mm and repeated tests over 1000 cycles). We believe that our simple methods for the fabrication of graphene/AgNP textile composite for use in textile-type transistors can potentially be applied to the development of flexible large-area electronic clothes.

Highly Conductive Graphene/Ag Hybrid Fibers for Flexible Fiber-Type Transistors.

Mechanically robust, flexible, and electrically conductive textiles are highly suitable for use in wearable electronic applications. In this study, highly conductive and flexible graphene/Ag hybrid fibers were prepared and used as electrodes for planar and fiber-type transistors. The graphene/Ag hybrid fibers were fabricated by the wet-spinning/drawing of giant graphene oxide and subsequent functionalization with Ag nanoparticles. The graphene/Ag hybrid fibers exhibited record-high electrical conductivity of up to 15,800 S cm(-1). As the graphene/Ag hybrid fibers can be easily cut and placed onto flexible substrates by simply gluing or stitching, ion gel-gated planar transistors were fabricated by using the hybrid fibers as source, drain, and gate electrodes. Finally, fiber-type transistors were constructed by embedding the graphene/Ag hybrid fiber electrodes onto conventional polyurethane monofilaments, which exhibited excellent flexibility (highly bendable and rollable properties), high electrical performance (µh = 15.6 cm(2) V(-1) s(-1), Ion/Ioff > 10(4)), and outstanding device performance stability (stable after 1,000 cycles of bending tests and being exposed for 30 days to ambient conditions). We believe that our simple methods for the fabrication of graphene/Ag hybrid fiber electrodes for use in fiber-type transistors can potentially be applied to the development all-organic wearable devices.

Mussel-inspired green synthesis of silver nanoparticles on graphene oxide nanosheets for enhanced catalytic applications.

We report a facile green approach to the synthesis of silver nanoparticles (Ag NPs) on the surface of graphene oxide nanosheets functionalized with mussel-inspired dopamine (GO-Dopa) without additional reductants or stabilizers at room temperature. The resulting hybrid Ag/GO-Dopa exhibits good dispersity and excellent catalytic activity in the reduction of nitroarenes.