Xiao-Xiong Wang

A self-powered flexible hybrid piezoelectric–pyroelectric nanogenerator based on non-woven nanofiber membranes

Self-powered systems based on nanogenerators (NGs) that are capable of harvesting mechanical and thermal energies for electricity and are obtained using low-cost materials and simple preparation methods have received great attention. Herein, we report a lightweight and flexible self-powered hybrid NG based on the piezoelectric and pyroelectric effects of an electrospun non-woven poly(vinylidene fluoride) (PVDF) nanofiber membrane (NFM), which can be directly used as an active layer without any post-poling treatment. The flexibility of the NG was enhanced by using an electrospun thermoplastic polyurethane (TPU) NFM as a substrate and a conductive PEDOT:PSS-PVP NFM and a carbon nanotube layer as the electrodes. The capabilities of the NFM to harvest mechanical and thermal energies were demonstrated. Mechanical impacting and bending can effectively stimulate the piezoelectric output of the NG, which can directly light a LED. Moreover, a hybrid piezoelectric–pyroelectric current of the NG was detected upon simultaneous application of strain and a thermal gradient. Due to its flexible non-woven structure, the NG can further harvest energy from body motion and cold/hot airflows. Furthermore, its mechanical durability, robustness and practicality were investigated by charging a capacitor. The novel design of the NFM-constructed hybrid NG demonstrated here can be applied not only to self-powered wearable electronic textiles but also to power generation on a large scale.

A highly stretchable humidity sensor based on spandex covered yarns and nanostructured polyaniline

Stretchable sensors, as the important components of flexible electronic devices, have achieved progress in a variety of applications for monitoring physical or environmental conditions, such as sound, temperature, vibration, and pressure. However, it still remains a challenge to fabricate high performance stretchable humidity sensors. Herein, we present a novel stretchable humidity sensor, which was fabricated based on an ultrastretchable polyaniline composite fiber. Because of the composite fiber with a “twining spring” configuration (cotton fibers twining spirally around a polyurethane fiber) it maintains a stable electrical conductivity up to a strain of 200%. In addition, the conductivity of the composite fiber remains perfectly stable after 5000 cyclic stretching events of 200% strain. Incorporating the humidity sensitive properties of nanostructured polyaniline, the stretchable humidity sensor based on the composite fiber effectively maintains its humidity sensitivity at different elongations.

Bionic Single-Electrode Electronic Skin Unit Based on Piezoelectric Nanogenerator

Moravecs paradox shows that low-level sensorimotor skills are more difficult than high-level reasoning in artificial intelligence and robotics. So simplifying every sensing unit on electronic skin is critical for endowing intelligent robots with tactile and temperature sense. The human nervous system is characterized by efficient single-electrode signal transmission, ensuring the efficiency and reliability of information transmission under big data conditions. In this work, we report a sensor based on a single-electrode piezoelectric nanogenerator (SPENG) by electrospun polyvinylidene fluoride (PVDF) nanofibers that can realize steady-state sensing of pressure integrating cold/heat sensing on a single unit. Piezoelectric signals appear as square wave signals, and the thermal-sensing signals appear as pulse signals. Therefore, the two signals can be acquired by a single unit simultaneously. The SPENG overcomes the shortcoming of electronic skins based on a single-electrode triboelectric nanogenerator (STENG), which can sense only dynamic movement and cannot sense temperature variations. The new sensor configuration uses a capacitor instead of the STENGs ground wire as a potential reference, allowing it to be used for truly autonomous robots. At the same time, the traditional advantages of polymer piezoelectric materials, such as flexibility, transparency, and self-powered advantages, have also been preserved.

In Situ Assembly of Well-Dispersed Ag Nanoparticles throughout Electrospun Alginate Nanofibers for Monitoring Human Breath—Smart Fabrics

Alginate nanofibers assembled with silver nanoparticles throughout the whole nanofiber were fabricated by three steps including electrospinning of Na-alginate nanofibers, ion exchange between the sodium and silver ions, and in situ reduction of silver nanoparticles. The content, distribution, and size of the nanoparticles are controllable by tuning reaction conditions. Ag/alginate nanofibers exhibit good humidity sensitivity in a wide humidity range from 20% ambient relative humidity (RH) to 85% RH. Interestingly, these humidity sensors can be attached to a 3M-9001V mask for monitoring human breath during exercise and emotion changes, and this smart mask exhibits accurate and continuous human breath tracking, no matter how fast or slow as well as how deep or shallow is the human breathing. The obtained frequencies of respiration during normal, running, delight, and sadness conditions were 16, 13, 14, and 8 times per minute, respectively. Moreover, the signal waveform obtained under emotion changes is distinguishable, implying its potential applications in lie detection and interrogation. Thanks to this smart mask, it could accurately capture the rate and depth of respiration, providing an effective, low-cost, and convenient approach for tracking respiration, and it was utilized as smart fabrics in avoiding sleep apnea.

Facile Fabrication of Multi-hierarchical Porous Polyaniline Composite as Pressure Sensor and Gas Sensor with Adjustable Sensitivity

A multi-hierarchical porous polyaniline (PANI) composite which could be used in good performance pressure sensor and adjustable sensitivity gas sensor has been fabricated by a facile in situ polymerization. Commercial grade sponge was utilized as a template scaffold to deposit PANI via in situ polymerization. With abundant interconnected pores throughout the whole structure, the sponge provided sufficient surface for the growth of PANI nanobranches. The flexible porous structure helped the composite to show high performance in pressure detection with fast response and favorable recoverability and gas detection with adjustable sensitivity. The sensing mechanism of the PANI/sponge-based flexible sensor has also been discussed. The results indicate that this work provides a feasible approach to fabricate efficient sensors with advantages of low cost, facile preparation, and easy signal collection.

Effects of Ce doping and humidity on UV sensing properties of electrospun ZnO nanofibers

Pure ZnO and Ce-doped ZnO nanofibers were synthesized via electrospinning-calcination technique. The morphology, composition, structure, humidity sensing and photoelectric properties were characterized. The field-effect curves showed that a single pure ZnO nanofiber is an n-type semiconductor and an individual Ce-ZnO nanofiber is a p-type semiconductor. The Ce doping and humidity have strong influence on the UV sensing properties of ZnO-based nanofibers. In the dark, the responses [(IVarious RH − I43% RH)/I43% RH] of pure ZnO increased gradually with the increase of humidity, while the responses of Ce-doped ZnO nanofibers decreased. When exposed to UV radiation, the response of pure ZnO nanofibers decreased with increasing humidity, while that of Ce-doped ZnO increased. And the highest responses are around 88.44 and 683.67 at 97% humidity for pure ZnO and Ce-ZnO nanofibers under UV irradiation. In addition, the UV response of Ce-ZnO with good stability and repeatability increases by two orders of magnitud...

Multicolor Tuning in Room‐Temperature Self‐Activated Ca2Nb2O7 Submicroplates by Lanthanide Doping

Self-activated phosphors are capable of generating optical emissions from the internal ion groups of host lattice before externally introducing luminescent ions. However, numerous self-activated phosphors only show luminescence at low temperature due to the thermally activated energy migration among ion groups at room temperature, severely confining their application conditions. In this letter, we propose a strategy to converting the low-temperature luminescence to a room-temperature one through changing the synthesis conditions to induce structural distortions and thus to limit energy migration. Room-temperature self-activated luminescence of Ca2 Nb2 O7 was accordingly achieved in submicroplates synthesized using the sol-gel method. By further coupling the blue broadband emission from Ca2 Nb2 O7 submicroplates with the characteristic luminescence of Ln3+ (Pr3+ , Sm3+ , and Dy3+ ) dopants, multicolor emissions were successively tuned through adjusting the concentration of Ln3+ . Our results are expected to expand the scope of designing room-temperature self-activated phosphors and tuning multicolor emission.

Preparation of Polypropylene Micro and Nanofibers by Electrostatic-Assisted Melt Blown and Their Application

In this paper, a novel electrostatic-assisted melt blown process was reported to produce polypropylene (PP) microfibers with a diameter as fine as 600 nm. The morphology, web structure, pore size distribution, filtration efficiency, and the stress and strain behavior of the PP nonwoven fabric thus prepared were characterized. By introducing an electrostatic field into the conventional melt-blown apparatus, the average diameter of the melt-blown fibers was reduced from 1.69 to 0.96 μm with the experimental setup, and the distribution of fiber diameters was narrower, which resulted in a filter medium with smaller average pore size and improved filtration efficiency. The polymer microfibers prepared by this electrostatic-assisted melt blown method may be adapted in a continuous melt blown process for the production of filtration media used in air filters, dust masks, and so on.

Solvent-free two-component electrospinning of ultrafine polymer fibers

Solvent recovery is a big challenge in conventional solution electrospinning for the large-scale production of ultrafine fibers because the precursor utilization ratio is usually less than 20 wt%. In this paper, we report an eco-friendly two-component electrospinning technique for the fabrication of acrylate composite fibers using a homemade device at room temperature. During this electrospinning process, the two-component spinning solutions (component A: t-butyl peroxy-2-ethyl hexanoate (BPOEH), isobornyl methacrylate (IBOMA), nitrile rubber (NBR), methacrylic acid (MAA); component B: N,N-dimethylaniline (DMA), IBOMA, NBR and MAA) are nearly all electrospun into ultrafine fibers, and the utilization ratio of the precursor can reach more than 90 wt%. The fiber solidification mechanism can be ascribed to the rapid polymerization of the IBOMA monomer in the presence of free radicals formed by a redox initiation system (BPOEH + DMA), which is different from the solvent evaporation in solution electrospinning or cooling solidification in melt electrospinning. Such a two-component electrospinning technique may stimulate the development of an eco-friendly approach to fabricate composite and functional ultrafine fibers.

Electrically Conductive TPU Nanofibrous Composite with High Stretchability for Flexible Strain Sensor

Highly stretchable and electrically conductive thermoplastic polyurethane (TPU) nanofibrous composite based on electrospinning for flexible strain sensor and stretchable conductor has been fabricated via in situ polymerization of polyaniline (PANI) on TPU nanofibrous membrane. The PANI/TPU membrane-based sensor could detect a strain from 0 to 160% with fast response and excellent stability. Meanwhile, the TPU composite has good stability and durability. Besides, the composite could be adapted to various non-flat working environments and could maintain opportune conductivity at different operating temperatures. This work provides an easy operating and low-cost method to fabricate highly stretchable and electrically conductive nanofibrous membrane, which could be applied to detect quick and tiny human actions.