The flexible tactile sensor has attracted widespread attention because of its great flexibility, high sensitivity, and large workable range. It can be integrated into clothing, electronic skin, or mounted on to human skin. Various nanostructured materials and nanocomposites with high flexibility and electrical performance have been widely utilized as functional materials in flexible tactile sensors. Polymer nanomaterials, representing the most promising materials, especially polyvinylidene fluoride (PVDF), PVDF co-polymer and their nanocomposites with ultra-sensitivity, high deformability, outstanding chemical resistance, high thermal stability and low permittivity, can meet the flexibility requirements for dynamic tactile sensing in wearable electronics. Electrospinning has been recognized as an excellent straightforward and versatile technique for preparing nanofiber materials. This review will present a brief overview of the recent advances in PVDF nanofibers by electrospinning for flexible tactile sensor applications. PVDF, PVDF co-polymers and their nanocomposites have been successfully formed as ultrafine nanofibers, even as randomly oriented PVDF nanofibers by electrospinning. These nanofibers used as the functional layers in flexible tactile sensors have been reviewed briefly in this paper. The β-phase content, which is the strongest polar moment contributing to piezoelectric properties among all the crystalline phases of PVDF, can be improved by adjusting the technical parameters in electrospun PVDF process. The piezoelectric properties and the sensibility for the pressure sensor are improved greatly when the PVDF fibers become more oriented. The tactile performance of PVDF composite nanofibers can be further promoted by doping with nanofillers and nanoclay. Electrospun P(VDF-TrFE) nanofiber mats used for the 3D pressure sensor achieved excellent sensitivity, even at 0.1 Pa. The most significant enhancement is that the aligned electrospun core-shell P(VDF-TrFE) nanofibers exhibited almost 40 times higher sensitivity than that of pressure sensor based on thin-film PVDF.

Sodium (Na) metal anodes stand out with their remarkable capacity and natural abundance. However, the dendritic Na growth, infinite dimensional changes, and low Coulombic efficiency (CE) present key bottlenecks plaguing practical applications. Here, heteroatom‐doped (nitrogen, sulfur) hollow carbon fibers (D‐HCF) are rationally synthesized as a nucleation‐assisting host to enable a highly reversible Na metal. The “sodiophilic” functional groups introduced by the heteroatom‐doping and large surface area (≈1052 m2 g−1) synchronously contribute to a homogenous plating morphology with dissipated local current density. High “sodiophilicity” of the D‐HCF is confirmed by first‐principle calculations and experimental results, where strong adsorption energy of −3.52 eV with low Na+ nucleation overpotential of 3.2 mV at 0.2 mA cm−2 is realized. As such, highly reversible plating/stripping is achieved at 1.0 mA cm−2 with average CE approximating 99.52% over 600 cycles. The as‐assembled Na@D‐HCF symmetric cells exhibit a prolonged lifetime for 1000 h. A full‐cell paired with Na3V2(PO4)3 cathode further demonstrates stable electrochemical behavior for 200 cycles at 1 C along with excellent rate performance (102 mAh g−1 at 5 C). The results clearly show the effectiveness of the D‐HCF in manipulating Na+ deposition and thus the significance of nucleation control in realizing dendrite‐free metal anodes.

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Low-cost growth of patterned zinc oxide (ZnO) nanorod arrays (NAs) has attracted much attention with the rapid development of electronics and nanotechnology. Mechanoelectrospinning-assisted continuous hydrothermal synthesis method (MES-CHSM) is proposed to direct-write the precursor patterns for growth of ZnO-NAs, in a digital, low-cost, and mask-free manner. The morphology and distribution of hierarchical ZnO nanorods, having a tremendous impact on gas response, are determined by the process parameters of MES-CHSM. It is highly desirable that the diameter, interval, orientation and distribution of ZnO nanorods can be tuned proactively by changing growth time, solution concentration, the nature of precursor layer, and the pattern by MES. ZnO-NAs exert excellent Ohmic contact with interdigital electrodes when exposure to dry air, NO2 gas and dry air again. The gas response of ZnO sample is surface-reaction-determining. Gas sensing results show highly sensitive and repeatable response-recovery cycles with NO2 gas exposure and the air purge, respectively. The dynamic response of gas sensor shows temperature-dependent response to NO2, even at low concentrations (1-50 ppm). The best gas response is located between 200 ºC and 225 ºC. Gas sensors, prepared by different process parameters, show two laws between the corresponding responses and NO2 concentrations: approximately linear and saturation regions. The optimal process parameters is presented to postpone the occurrence of saturation region, to enlarge measuring range. Please do not adjust marginsPlease do not adjust margins where the MES is an improved electrohydrodynamic direct-writing process with high controllability on the morphology of fibers 32, 33 . This method can achieve highly aligned ZnO nanorods on sub-10micrometer printed patterns, to form hierarchical structures of the highly sensitive sensors. Further, the morphology and distribution of ZnO-NAs are tunable with different process parameters, such as growth time, ZnAc precursor concentration, Zn(NO3)2 concentration in growth solution and the printed micropattern of MES. Their relationships with gas sensing performance are investigated in detail to discover the optimal parameters of process and working. Experimental section PreparationZinc acetate (ZnAc, Zn(CH3COO)22H2O, AR), zinc nitrate (Zn(NO3)2, AR) hydrate and hexamethylenetetramine (HMTA, (CH2)6N4) were purchased from Sinopharm Chemical Reagent Co., Ltd.. Polyethylene oxide (PEO) used in this study was purchased from Aldrich. The relative molecular mass (Mr) of PEO was 600,000. An aqueous solution containing PEO (6wt%) and ZnAc was fabricated by magnetic stirring for 10 hours (h). Aqueous solutions of Zn(NO3)2 (0.2 M) and HMTA (0.2 M) were prepared by magnetic stirring for >0.5 h respectively, then were mixed with equal volume and stirred evenly to fabricated mixed solution of Zn(NO3)2 and HMTA.Interdigitated electrodes of Ag were screen printed on alumina substrate (1 cm*0.8 cm). The distance between adjacent electrodes was 800 µm. Hierarchical...

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The security of water resources is the core content and ultimate goal of urban water resource management agencies. The management of water resources is directly related to the needs of urban residents’ lives and the area’s socio-economic development. How to determine the effective evaluation indicators and methods is an important prerequisite to solving the water resource security problem. This study took Luoyang City as the research area and constructed a water resource security evaluation index system based on pressure-state-response framework. An analytic hierarchy process and entropy weight method were used to determine the index weight. A set pair analysis model was then introduced to evaluate the security of water resources in Luoyang from 2006 to 2016. The results of this study show that the standard of water resource security generally improved in Luoyang in the latter years of the study period. From 2006 to 2008, Luoyang was graded at the Insecurity Level. This compares to a slightly improved grading of Critical Security Level from 2009 to 2016 (except for 2013). However, the overall grade is still low. The pressure on the Luoyang water resource system mainly comes from the development of the urban socio-economy, which in turn has caused problems for both the quantity and quality of water resources. Therefore, a series of countermeasures have been introduced as a means of improving the water resource security of Luoyang, and these measures have achieved certain results. However, further improvements to the efficiency of water resource utilization and strengthening the management and protection of water resources remain necessary.