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The poor conductivity and deficient active sites of transition metal oxides lead to low energy density of supercapacitors, which limits their wide application. In this work, double transition metal oxide heterojunctions with oxygen vacancy (Vo-ZnO/CoO) nanowires are prepared by effective hydrothermal and thermal treatments. The formation of the heterojunction results in the redistribution of interface charge between ZnO and CoO, generating an internal electric field to accelerate the electron transport. Meanwhile, oxygen vacancies can enhance the redox reaction activity to further improve the electrochemical kinetics of the electrode material. Therefore, the prepared Vo-ZnO/CoO can provide a superior specific capacity of 845 C g−1 (1 A g−1). An asymmetric supercapacitor with the Vo-ZnO/CoO as positive electrode shows a higher energy density of 51.6 Wh kg−1 when the power density reaches 799.9 W kg−1. This work proposes a synergistic combination of defect and heterojunction engineering to improve the electrochemical properties of materials, providing an important guidance for material design in energy-storage field.

In this paper, a computationally efficient multifidelity local search algorithm for aerodynamic design optimization is presented. In this paper’s approach, direct optimization of a computationally ...

Polypyrrole (PPy) is a good candidate material for piezoresistive pressure sensors owing to its excellent electrical conductivity and good biocompatibility. However, it remains challenging to fabricate PPy-based flexible piezoresistive pressure sensors with high sensitivity because of the intrinsic rigidity and brittleness of the film composed of dense PPy particles. Here, a rational structure, that is, 3D-conductive and elastic topological film composed of coaxial nanofiber networks, is reported to dramatically improve the sensitivity of flexible PPy-based sensors. The film is prepared through surface modification of electrospun polyvinylidene fluoride (PVDF) nanofibers by polydopamine (PDA), in order to homogeneously deposit PPy particles on the nanofiber networks with strong interfacial adhesion (PVDF/PDA/PPy, PPP). This unique structure has a high surface area and abundant contact sites, leading to superb sensitivity against a subtle pressure. The as-developed piezoresistive pressure sensor delivers a low limit of detection (0.9 Pa), high sensitivity (139.9 kPa

For structures with both random and fuzzy uncertainty, this paper presents a novel method for determining the membership function in fuzzy reliability with the Automatic Updating Extreme Response Surface (AUERS) method. In the proposed method, fuzzy variables are initially converted into a value domain under the given cut level and the extreme point in the domain where the reliability reaches its extreme value is considered. Second, the Particle Swarm Optimization (PSO) algorithm is used to determine the extreme point according to the extreme responses for different sets of random sample inputs. A kriging response surface is subsequently constructed between the random variables and the corresponding extreme points. An automatic updating strategy is then introduced based on the Relative Mean Square Predicted Error (RMSPE) before performing every iteration of reliability analysis. By adding new sample points, the approximate quality of the kriging response surface is improved. Finally, reliability analysis is used to determine the reliability bound under the given cut level. The proposed method assures the accuracy and computation efficiency of the mixed uncertainty reliability analysis results while it prevents the solution from becoming trapped in a local optimum, which occurs in classical optimization methods. Two example analyses are used to demonstrate the validity and advantages of the proposed method.

Flexible ultrasensitive strain sensors are highly desirable in view of their widespread applications in wearable electronics, health monitoring systems, and smart robots, where subtle strain detection is required. However, traditional fabrication of such sensors was done to prepare sensitive layers on bare or single-sided structural substrates, leading to limited sensitivity. Herein, a stretchable resistive-type strain sensor was demonstrated by self-assembling conductive networks onto a monolithic polydimethylsiloxane substrate with a two-sided topological design, for example, a sinusoid/auxetic binary architecture. The sensitivity of the obtained sensor was greatly improved by 22-fold as compared to the traditional counterpart with a bare substrate. The remarkably good agreement between the experimental results and finite element analysis predictions confirmed that the superior sensitivity is a synergistic effect of local strain enhancement derived from the topological structure on the foreside and an additional strain concentration and a reduced Poisson's ratio from the auxetic arrays on the backside. Furthermore, this sensor can withstand an extreme mechanical force (750 N) because of the shear stiffening characteristic of the auxetic structure. Benefiting from the characteristics of ultrahigh sensitivity (gauge factor ∼1744 at 5%), low detection limit (0.05%), and long-term durability (500 loading cycles), this as-prepared sensor shows promise in practical applications of high-performance wearable electronics.

Cost and fabrication technique, as two important factors affecting the development and practical applications of electronic devices, have attracted extensive attention of researchers. In order to o...

Purpose Model-assisted probability of detection (MAPOD) is an important approach used as part of assessing the reliability of nondestructive testing systems. The purpose of this paper is to apply the polynomial chaos-based Kriging (PCK) metamodeling method to MAPOD for the first time to enable efficient uncertainty propagation, which is currently a major bottleneck when using accurate physics-based models. Design/methodology/approach In this paper, the state-of-the-art Kriging, polynomial chaos expansions (PCE) and PCK are applied to “a^ vs a”-based MAPOD of ultrasonic testing (UT) benchmark problems. In particular, Kriging interpolation matches the observations well, while PCE is capable of capturing the global trend accurately. The proposed UP approach for MAPOD using PCK adopts the PCE bases as the trend function of the universal Kriging model, aiming at combining advantages of both metamodels. Findings To reach a pre-set accuracy threshold, the PCK method requires 50 per cent fewer training points than the PCE method, and around one order of magnitude fewer than Kriging for the test cases considered. The relative differences on the key MAPOD metrics compared with those from the physics-based models are controlled within 1 per cent. Originality/value The contributions of this work are the first application of PCK metamodel for MAPOD analysis, the first comparison between PCK with the current state-of-the-art metamodels for MAPOD and new MAPOD results for the UT benchmark cases.

Developing a facile, cost-saving, and environment-friendly method for fabricating a multifunctional humidity sensor is of great significance to expand its practical applications. However, most humidity sensors involve a complex fabrication process, resulting in their high cost and narrow application fields. Herein, a multifunctional paper-based humidity sensor with many advantages is proposed. This humidity sensor is fabricated using conventional printing paper and flexible conductive adhesive tape by a facile pasting method, in which the paper is used as both the humidity-sensing material and the substrate of the sensor. Owing to the moderate hydrophilicity of the paper and the rational structure design of the paper-based humidity sensor, the sensor exhibits an excellent humidity-sensing response of more than 103 as well as good linearity ( R2 = 0.9549) within the humidity range from 41.1 to 91.5% relative humidity. Furthermore, the paper-based humidity sensor has good flexibility and compatibility, endowing it with multifunctional applications for breath rate, baby diaper wetting, noncontact switch, skin humidity, and spatial localization monitoring. Although the resistance of the paper-based humidity sensor is relatively large, the humidity-sensing response signals of the sensor can be conveniently processed by the designed signal processing system. The readily available starting materials and facile fabrication technique provide useful strategies for the development of multifunctional humidity sensors.

Respiration is an important physiological process of human body that can not only act as vital signs for physical health management or “biomarkers” of some diseases, but also as a kind of energy that can be utilized to drive triboelectric nanogenerator (TENG). It is particularly desirable to develop a facile TENG that can be driven by human respiration itself for multifunctional human respiratory monitoring. In this work, a respiration-driven TENG based on Ce-doped ZnO-PANI nanocomposite film that can convert respiratory airflow energy into electric signals was developed for the detection of multiple respiratory parameters including trace-level NH3 concentration, human respiratory flow and respiratory frequency. It was found that the as-designed TENG exhibited excellent ability for respiratory flow monitoring (2–6 L/min), identification of human breathing frequency and recognition of different breathing patterns. Furthermore, output voltage of the self-powered NH3 sensor monotonously decreases with the increase of NH3 concentrations from 0.1 to 25 ppm. Especially, a good NH3-sensing performance was exhibited when exposed to a trace-level NH3 atmosphere ranging from 0.1 to 1 ppm, implying a promising potential in self-powered disease screening related to the detection of exhaled NH3 in human breathing. This research offers a facile method to establish a multifunctional respiratory monitoring system driven by respiration itself, which is of significant importance in self-powered human health monitoring through non-implantable respiratory diagnosis.

Biofouling has been a long-last problem in a variety of marine systems which causes energy waste and device damage. In this study, we present a self-activated anti-biofouling system enabled by electrical double layer disturbance, which could effectively suppress the initial formation of conditioning layers and subsequent microbe attachment. The small and low-frequency alternating electrical fields were produced by a triboelectric nanogenerator under water wave impacts. Systematic analyses confirmed that the anti-biofouling efficacy was directly related to the strength of the electric field and was effective in both fresh lake and sea water environments. An on-site demonstration was implemented at a calm lake shore for three weeks. The water wave-driven anti-biofouling exhibited excellent surface protection, which was significantly superior to several commercial anti-biofouling coatings. This development brings a novel, effective and eco-friendly solution for protecting a broad range of surfaces against biofouling.

Volatile organic sulfur compounds (VOSCs) pose serious threats to environment and human health, while detection techniques of VOSCs are expensive and complex. Here, this paper first-time reported a hydrogen-bond acidic (HBA) polymer-based surface acoustic wave (SAW) chemical sensor for the detection of four VOSCs (methyl mercaptan, dimethyl sulfide, dimethyl disulfide, and carbon disulfide). The results show that this sensor exhibited both excellent sensitivity (limit of detections ranging from 0.035 to 0.11 mg/m3) with fast response and recovery and high selectivity (2.23–49.6 times higher response than interference vapors) toward VOSCs at room temperature. The good performance is attributed to the hydrogen bonding between the sulfur atom of VOSCs and hydrogen atom of hexafluoroisopropanol (HFIP) groups in polymer chain. Fourier-transform infrared spectroscopy (FTIR) spectrum and first principle calculations were performed to further support this proposed mechanism.

An ultrasensitive self-powered ammonia (NH3) sensing system based on a vertical contact-separate mode triboelectric nanogenerator (TENG) has been proposed for room temperature detection of NH3 concentrations both in the ambient environment and in human exhaled gases. Owing to the special output characteristics of the TENG adjusted by the load resistance of the NH3 sensor, output voltage of the fabricated gas sensor has a proportional relationship with NH3 concentration, which is the fundamental working mechanism of the self-powered gas-sensing system. The triboelectric ammonia sensor (TEAS) based on PANI-MWCNTs composite thin film possesses a NH3-sensing response of 10% at 0.01 ppm NH3 and exhibits a great response of 255% at 100 ppm NH3. Meanwhile, the TEAS also holds fast response/recovery time (89–120 s/103–127 s for 0.6–100 ppm NH3), excellent long-term stability, satisfactory selectivity and good anti-bending ability. Furthermore, a preliminary test of human exhaled NH3 concentration has been carried out to further expand the practical application of the developed TEAS. The results not only provide an effective technique in ultrasensitive NH3 sensing but also greatly broaden the applicability of TENG in the detection of human kidney health through comparing to average NH3 concentrations within exhaled gases of healthy people.

Fully transparent electronics are of increasing interest for biological applications, where the combination of sensing and imaging can potentially improve patient healthcare diagnostics. Herein, we report high-performance, fully-transparent amorphous In-Ga-Zn-O field-effect transistor (a-IGZO FET) based biosensors fabricated directly on highly-curved glass substrates. The a-IGZO channel and indium tin oxide electrodes were patterned directly on glass tubes (1.0 mm radius) by microcontact printing self-assembled monolayers and etching the metal oxide films. This approach led to a-IGZO FETs with excellent electronic performance, with high on/off drain current ∼1.3 × 106, high average electron mobility > 7.4 cm2/Vs, low on/off hysteresis ∼0.6, V and low gate leakage current ∼10−10 A. The back-channel of the a-IGZO FETs were functionalized with glucose oxidase to make fully transparent biosensors with very high sensitivity to glucose. We have determined that the glucose limit of detection is 170 μM. These results provide insight into new methods for fabricating a-IGZO FETs and a-IGZO FET biosensors on non-planar substrates, and may open a range of new applications, including transparent sensing catheters, flexible active transparent electrode sensing arrays, and integration of FET based biosensors on optical fibers.

Flexible nitrogen dioxide (NO2) gas sensors based on organic thin-film transistors (OTFTs) were developed in this work, in which conductive indium tin oxide (ITO) coated polyethylene naphthalene-2,6-dicarboxylate (PEN) was designed for the flexible substrates, and poly methyl methacrylate (PMMA) and poly(3-hexylthiophene) (P3HT) was spin-coated as the gate dielectric layer and spray-deposited as the active layer, respectively. The effects of PMMA concentrations and P3HT solution volume on performances of flexible OTFTs were systematically investigated. The results showed that the optimized flexible OTFT exhibited high field-effect mobility (9.51×10–3 cm2/(V s) at a gate bias of –50 V) and excellent response characteristics, including high sensitivity (up to 0.169 ppm–1), good repeatability and selectivity. The flexibility of the developed OTFT sensor was also investigated, and the results showed that the electrical and gas-sensing properties were affected by the bending cycles, thus further work should be done for improving the flexibility of the sensor. This work provides an effective approach in developing high performance flexible OTFT NO2 gas sensor.

In this paper, a two-step method is proposed to optimize the layout of the conventional serpentine film heater thus to improve the temperature uniformity. The optimization is made on transverse direction firstly with varied linewidths and pitches, and then on longitudinal direction with varied line shapes. Based on finite element simulation using a flow field approach, the heater geometry evolution dependent on temperature uniformity is investigated. The results show that the isothermal region extends greatly in transverse direction by the first step, while it extends subsequently in longitudinal direction by the second step and simultaneously shrinks in transverse direction. By a proper tradeoff between these two modification steps, the substrate area within 96.4% temperature uniformity increases by more than five times in comparison to the conventional serpentine heater. This area is even larger by 5% than that achieved by an optimized spiral heater, indicating that the serpentine heater is more powerful to adjust the temperature uniformity. Moreover, the optimized serpentine heater realizes a coverage ratio as high as 86.64% on a rectangular membrane, demonstrating that serpentine heaters are more flexible to substrate shapes than spiral heaters to achieve a uniform and large area temperature distribution.

The triboelectric nanogenerator (TENG) driven self-powered gas sensor has been developed for room temperature nitrogen dioxide (NO2) detection under UV illumination (365 nm). Powered by TENG, the concentration of NO2 can be spontaneously detected via chemoresistive gas sensor based on Zinc Oxide-Reduced graphene oxide (ZnO-RGO) hybrid films, where the voltage drop across the interdigital electrodes (IDEs) has a proportional relationship with NO2 concentration. The self-powered triboelectric gas sensor (TGS) composed by ZnO-RGO composite films exhibit superior response (~ 16.8) and sensitivity as well as selectivity than that for pure ZnO film. With respect to selectivity, the response of ZnO-RGO composite film based TGS to NO2 was at least 49-fold higher than those for other gases. Furthermore, through a signal processing circuit, a self-powered NO2 gas alert system was developed for real-time monitoring the ambient NO2 concentration. This work not only promotes the applicability of TENGs as self-powered sensors but also pushes forwards the active sensing network node for environmental monitoring by harvesting ambient energy.

A resistive-type flexible ammonia (NH3) sensor was proposed and developed in this work, which was prepared by depositing polyaniline-cerium dioxide (PANI-CeO2) nanocomposite thin film on flexible polyimide (PI) substrate through in-situ self-assembly method. The effect of CeO2 nanoparticles on the polymerization of aniline was studied by comparing the morphological, structural and chemical features of the pure PANI and PANI-CeO2 nanocomposite, and the dynamic polymerization processes were also recorded and investigated. In this process, an interesting phenomenon was found that the protonation and oxidation degrees of PANI in PANI-CeO2 nanocomposite were improved significantly according to the XPS spectra analysis, which should be ascribed to the synergetic oxidation of CeO2 nanoparticles and ammonium persulfate (APS). Meanwhile, the NH3-sensing performances of the pure PANI and PANI-CeO2 film sensors were evaluated at room temperature (∼25 °C), which showed that the PANI-CeO2 film sensor possessed enhanced response, reduced recovery time, perfect response-concentration linearity, good reproducibility, splendid selectivity, remarkable long-term stability, ultra-low detectable concentration (16 ppb) and theoretical detection limit (0.274 ppb), and outstanding flexibility without significant response decrease after 500 bending/extending cycles. It was speculated that the excellent sensing performances should probably benefit from the gas-sensing enhancement effect of p-n junction, the improved protonation degree and modified morphology of PANI by the addiction of CeO2 nanoparticles. And, the high flexibility might originate from the flexible structure of PANI chains, and the good adhesion and nano-mechanical performance of PANI-CeO2 film. Besides, the effect of relative humidity on the sensing properties of PANI-CeO2 film sensor was also discussed and analyzed. Therefore, the proposed high-performance flexible PANI-CeO2 thin film sensor holds great promise for application into hand-held or wearable electronic devices for trace-level NH3 detection at room temperature.

We report the fabrication and performance of a silicon-on-glass micro gas chromatography eight-capillary column based on microelectromechanical systems technology that is 50 cm long, 30 μm wide, and 300 μm deep. According to the theory of a gas chromatography column, an even gas flow among different capillaries play a vital role in the peak broadening. Thus, a flow splitter structure is designed by the finite element method through the comparison of the velocity distributions of the eight-capillary columns with and without splitter as well as an open tubular column. The simulation results reveal that eight-capillary column with flow splitters can receive more uniform flow velocity in different capillaries, hence decreases the peak broadening and in turn increases the separation efficiency. The separation experiment results show that the separation efficiency of about 22 000 plates/m is achieved with the chip column temperature programmed for analysis of odorous sulfur pollutants. This figure is nearly two times higher than that of the commercial capillary column coated the similar stationary phase. And the separation time of all the components in the microcolumn is less than 3.8 min, which is faster than the commercial capillary column.

Functional and structural tailoring of three-dimensional (3D) conducting polymer nanoarchitectures is a promising route but remains challenging to develop high-performance electrodes for electrochemical energy storage. Herein, we design poly(3, 4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) 3D mesoporous aerogel electrodes through a simple and original one-pot mechanochemical processing route. The hierarchical structure of neat PEDOT:PSS aerogels displays a highly interpenetrated porous conductive network with a record high active surface area of 470 m 2 g −1 amongst the class of conducting polymer architectures. Robust structural and electrochemical performances are achieved with high gravimetric, areal and volumetric capacitance metrics of 120 F/g, 2.5 F cm −2 , and 124 F cm −3 , respectively, as the result of a mixed hybrid faradaic - capacitive charge storage mechanism. Moreover, these performances are attained in organic based electrolytes, reported so far to be incompatible or hinder the electrochemical activity of PEDOT:PSS. We undermine the fundamentals of the electrochemical operation in these electrodes and show that the electrolyte chemistry and the aerogel morphology particularly impact the charge storage performances demonstrating the superiority of the mesoporous architecture for charge transfer and ion exchange. Asymmetric cells are built having a specific capacitance of 40 F/g based on the total mass of electrodes and a good cyclic stability with 90% capacitance retention after 1000 charge and discharge cycles. This study offers a new route to enhance the electrochemical properties of conducting polymers and provides suggestive insights for developing high-performance polymer electrode materials for electrochemical energy storage.

In this work, a fundamental new approach for humidity detection was proposed. By coupling triboelectric nanogenerator (TENG) with resistive humidity sensor, the degree of ambient moisture can be actively detected at room temperature, where the voltage drops across the interdigital electrodes (IDEs) has a reversely proportional relationship with relative humidity. The prepared self-powered triboelectric humidity sensor (THS) holds high humidity response of ∼7, fast response/recovery time of 2.8 s and 3.5 s, stable repeatability and excellent long term stability. With respect to selectivity, the response of THS to gaseous water was at least 2-fold higher than those for other gases. Furthermore, through a signal processing circuit, a self-powered humidity alert system was developed for real-time monitoring the ambient humidity. Finally, the underlying working principle of the proposed sensor was discussed in detail. This research not only provides a simple, low-cost and efficient means in humidity sensing but also greatly broadens the applicability of TENGs as self-powered active sensors.

Increasing light absorption is of crucial importance for optimizing light-activated gas detection. However, the relevant research is still rather rare. Herein, high performance NO 2 gas sensing with plasmonic Au NPs decorated ZnO (AuZnO) nanorod array under visible light illumination at room temperature is demonstrated. It is achieved by the localized surface plasmon resonance (LSPR) and increased surface oxygen vacancies. The influence of light wavelength on the sensing behaviors was systematically investigated. It is found that the Au NPs decoration can largely promote the visible-activated gas sensing behaviors. In addition, the as-prepared sensors demonstrate robustness toward repeatability and selectivity in NO 2 sensing properties. Moreover, the sensing mechanism based on LSPR was discussed.

Model-assisted probability of detection (MAPOD) and sensitivity analysis (SA) are important for quantifying the inspection capability of nondestructive testing (NDT) systems. To improve the computational efficiency, this work proposes the use of polynomial chaos expansions (PCEs), integrated with least-angle regression (LARS), a basis-adaptive technique, and a hyperbolic truncation scheme, in lieu of the direct use of the physics-based measurement model in the MAPOD and SA calculations. The proposed method is demonstrated on three ultrasonic testing cases and compared with Monte Carlo sampling (MCS) of the physics model, MCS-based kriging, and the ordinary least-squares (OLS)-based PCE method. The results show that the probability of detection (POD) metrics of interests can be controlled within 1% accuracy relative to using the physics model directly. Comparison with metamodels shows that the LARS-based PCE method can provide up to an order of magnitude improvement in the computational efficiency.

Aerodynamic optimization based on computational fluid dynamics (CFD) is a powerful design approach because it significantly reduces the design time compared with the human manual design. However, CFD-based optimization can still take hours to converge because it requires repeatedly running computationally expensive flow simulations. To further shorten the design optimization time, we propose a fast, interactive design framework that allows us to complete an airfoil aerodynamic optimization within a few seconds. This framework is made efficient through a B-spline-based generative adversarial network model for shape parameterization, which filters out unrealistic airfoils for a reduced design space that contains all relevant airfoil shapes. Moreover, we use a combination of multilayer perceptron, recurrent neural networks, and mixture of experts for surrogate modeling to enable both scalar (drag and lift) and vector (pressure distribution) response predictions for a wide range of Mach numbers (0.3 to 0.7) and Reynolds numbers (104 to 1010). To verify our proposed framework, we compare the optimization results with the ones computed by direct CFD-based optimization for subsonic and transonic conditions. The results show that the optimal designs and the aerodynamic quantities (lift, drag, and pressure distribution) obtained by our proposed framework agree well with the ones computed by direct CFD-based optimizations and evaluations. The proposed framework is being integrated into a web-based interactive aerodynamic design framework that allows users to predict drag, lift, moment, pressure distribution, and optimal airfoil shapes for a wide range of flow conditions within seconds.

According to the theory of a gas chromatography column, the air flow distribution inside the channel plays a vital role in the peak broadening. Unlike the semi-packed columns which utilized embedded posts to obtain a more uniform lateral distribution compared to the open tubular columns, this paper reports the design and fabrication of a MEMS-based GC column folded in a wavy configuration to achieve an even flow velocity. The wavy column consists of upper arcs and lower arcs which are alternatively connected in series. The flow distributions of wavy columns with various arc angle and curvature radius are explored by the finite element method. The simulation reveals that the average flow velocity decreases with the increase of the arc angle, resulting in a successive improvement of the flow uniformity at the zones between two adjacent bends but also a more nonsymmetric distribution at the bends. These two effects counteract with each other and render an optimal arc angle of 136°. To verify the above design, the 136° wavy column and a serpentine column with the same dimensions are compared, where a better separation performance of the wavy column is demonstrated.

Fast and accurate airfoil design under uncertainty using non-intrusive polynomial chaos (NIPC) expansions and utility functions is proposed. The NIPC expansions provide a means to efficiently and accurately compute statistical information for a given set of input variables with associated probability distribution. Utility functions provide a way to rigorously formulate the design problem. In this work, these two methods are integrated for the design of airfoil shapes under uncertainty. The proposed approach is illustrated on a numerical example of lift-constrained airfoil drag minimization in transonic viscous flow using the Mach number as an uncertain variable. The results show that compared with the standard problem formulation the proposed approach yields more robust designs. In other words, the designs obtained by the proposed approach are less sensitive to variations in the uncertain variables than those obtained with the standard problem formulation.

In recent years, graphene has attracted enormous attention and has been utilized in the investigation of flexible strain sensors due to its prominent mechanical and electrical properties. In this paper, the naturally viscous material polyethylenimine (PEI) and reduced graphene oxide (rGO) were used to fabricate flexible strain sensors by facile chemical layer-by-layer self-assembly (CLS) and thermal reduction methods. The morphology, spectroscopy and thermal properties of the as-prepared sensing films were measured by SEM, AFM, FTIR and TGA. The influences of GO solution concentration and PEI–GO bilayer number on the sensing performance were studied. The optimal sensor obtained a remarkable performance with a high gauge factor (754 under 5% stretch deformation) and an ultralow limit of detection (0.1% strain). A linear relationship between the normalized response of the sensors and the stretch deformation was observed in the low strain range. The proposed sensor achieved durable properties after 500 stretching–relaxing cycles and could work and withstand a strain up to 50%. Moreover, the proposed sensor was able to detect the subtle motion of a knuckle. This research proposed a facile strategy for the large-scale fabrication of a flexible strain sensor with high sensitivity and excellent repeatability.

This work presents the investigation of reduced graphene oxide (RGO) as a new type of stationary phase for gas chromatographic separation on a microcolumn which is fabricated on a silicon wafer by the application of microelectromechanical system (MEMS) techniques. A ZnO film synthesized by a sol–gel method is utilized as a supporting material between RGO and the column wall for inwall roughening to form a stable RGO stationary phase and increase gas-stationary phase interaction. The RGO/ZnO grafted on the column wall was characterized by SEM and FT-IR spectra. The experimental results demonstrate that good separations for mixtures of a wide range of organic compounds including alkanes and esters are achieved by the microcolumn coated with bilayer film. And the maximum column efficiency of 11 363 theoretical plates per meter for n-dodecane is obtained, which is higher than any previously reported values for chromatography columns coated with graphene and their analogs to the best of our knowledge. However, the RGO stationary phase shows obvious peak tailings for polar alcohols, which mainly originate from its specific π–π stacking interaction as well as the hydrophobic effect between analytes and the π-electron on the high surface area of reduced graphene oxide. This work greatly favors RGO coated chip columns for gas chromatographic separation in achieving micro-total analytical systems.

In this work, a novel graphene oxide (GO)/Poly(ethyleneimine) (PEI) layered film was designed and prepared on quartz crystal microbalance (QCM) for humidity detection through a feasible spray process at room temperature. The morphological and chemical properties of GO/PEI layered film were examined by means of field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Fourier Transform infrared spectroscopy (FTIR), and Raman spectroscopy. The effect of GO solution concentration on humidity-sensing characteristics was investigated and an optimized QCM humidity sensor was obtained. An intriguing phenomenon was observed that a three-dimension structure of GO sheets was formed due to the redissolution effect at the interface between PEI and GO layers. Results showed that a layered film QCM sensor exhibited superior humidity sensing properties than a pure GO film or a PEI one including high response, short response/recovery time, small humidity hysteresis (less than 1%RH), excellent repeatability, selectivity and long-term stability. Moreover, the humidity features of sensors were also researched via the method of impedance analysis. Lastly, a convincing sensing mechanism model was established to interpret the enhanced humidity-sensing performance of layered film sensors. The results demonstrated the potential application of GO/PEI layered films to humidity sensors.

In this paper, a protonated polyethylenimine (P-PEI)-graphene oxide (GO) nanocomposite thin film, referred as P-PEI-GO, was deposited on the quartz crystal microbalance (QCM) by a facile dipping process for the development of high performance humidity sensors. The morphological and structural characteristics of P-PEI-GO thin film were analyzed by scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscope. The humidity-sensing properties, such as response and recovery, sensitivity, repeatability, selectivity, humidity hysteresis and long-term stability, were investigated in a wide range of working humidity (11–97% RH). The resultant P-PEI-GO nanocomposite thin film possesses much larger specific surface area and richer hydrophilic groups than pure P-PEI film, leading to higher sensitivities and shorter response/recovery times of the sensor. Especially, the sensor preserves excellent durability, small hysteresis, good selectivity and stability compared with individual P-PEI or GO film sensor, which could be ascribed to the high mechanical modulus of GO and weak interaction between P-PEI and GO. Finally, the humidity sensing mechanism of proposed sensor was discussed in details. This research provides a feasible and effective approach to construct high performance humidity sensor operated at room temperature based on GO nanocomposites.

A single-electrode-based segmented triboelectric nanogenerator (S-TENG) was developed. By utilizing the wind-induced vibration of a fluorinated ethylene propylene (FEP) film between two copper electrodes, the S-TENG delivers an open-circuit voltage up to 36 V and a short-circuit current of 11.8 μA, which can simultaneously light up 20 LEDs and charge capacitors. Moreover, the S-TENG holds linearity between output current and flow rate, revealing its feasibility as a self-powered wind speed sensor. This work demonstrates potential applications of S-TENG in wind energy harvester, self-powered gas sensor, high altitude air navigation.

Cobalt/copper-codoped ZnO nanoparticles, synthesized with different Co concentrations by a sol–gel method using ethanol as solvent, were studied via XPS. Hexagonal wurtzite structure was found in all samples, with no evidence of any secondary phase. The average crystallite size of the samples was around 20–30 nm, altered significantly with increasing Co concentration. Copper ions and Cobalt ions are indeed substituted into the ZnO lattice at the Zn2+ site, as shown by XRD and XPS. Further studies showed dramatic changes of Cu valence from +2 to +1 as the Co concentration level exceeds 1%, accompanied by a blue-shift of the optical bandgap from 3.01 to 3.13 eV. Ferromagnetism of the Co-doped Zn0.95Cu0.05O thin films was observed and found to be tunable – a phenomenon associated with the valence state of the Cu ions and the existence of some defects like oxygen vacancies in the films.

A two-channel gas chromatography column and a single-channel column were made by deep reactive-ion etching technology. The two short columns were coated with different stationary phases, and then linked without a modulator. This is to aim at increasing the sample capacity and achieving a higher separation efficiency in complex environments. The results show that the capacity of the connected column is approximately 4 and 1.5 times larger than that of the single- and two-channel columns, respectively. The linked column was utilized to separate a six-component mixture, composed of three simulants of nerve and blister agents and three interfering vapors. The results demonstrate that the combined column has a remarkably higher separation efficiency than the individual columns, and an acceptable resolution is achieved although the total length of the linked column is only 1.5 m.

A promising approach was proposed to enhance the responsivity of a ZnO nanowires array based photodetector with metal-semiconductor-metal structure through piezo-phototronic effect. The ZnO nanowires arrays were synthesized via hydrothermal method to function as both piezoelectric and light-trapping layer. The photoresponses of the ZnO nanowires based photodetector were tuned by the modulation of local Schottky barrier heights (SBH) at the metal-ZnO interfaces as a result of strain-induced polarization charges. Subjected to a −0.62% compressive strain, the responsivity of the prepared photodetector was increased by as much as 176%. Furthermore, the devices hydrothermally synthesized at 0.10 M have a larger photoresponse and higher stress sensitivity compared to the ones prepared at 0.05 M and 0.15 M. This work not only strengthens the fundamental understanding of piezo-phototronic effect on photodetectors but also provides an efficient means for optimization/improvement of piezoelectric semiconductor nanowires based optoelectronic devices.

For the fabrication of pyroelectric polyvinylidene fluoride (PVDF) film with high performance, a novel method is presented which combines the usual two steps of solution casting and stretching into one. It demonstrates that a stretch ratio as large as 15 as well as low quantity of defects and high uniformity are achieved by this new technique. The characterization results reveal that baking temperature (Tb) after stretching has significant and similar influences on relative β-phase fraction (Fβ), remnant polarization (Pr), dielectric constant and loss (e′ and e″) and pyroelectric coefficient of the samples; the highest pyroelectric coefficient, occurring at baking temperature of 80 °C without thermal poling, is 12.4 μC m−2 K−1, which is larger than that of the usually-stretched sample undergoing thermal poling. In addition, by using the prepared film as sensitive element, a novel chopperless pyroelectric infrared detector is constructed by electrostatically modulating the incident thermal irradiation; the measurement results demonstrate that the detector has excellent pyroelectric voltage response to the thermal radiation of the laser source and hot (warm) water with a sensitivity of 2.65 × 104 V/W, showing its good potential for fully flexible infrared detector and temperature sensor.

Approximately 22% of the 86 million people in the United States living with prediabetes are unaware of their condition. Detection of acetone in human respiration offers an effective and painless approach for the diagnosis of prediabetes. In this work, a wearable active acetone biosensor employing chitosan and reduced graphene oxide (RGO) as sensitive materials was developed to non-invasively diagnose prediabetes. When operated under 97.3% relative humidity at room temperature, the prepared chitosan and RGO composite film-based sensor exhibited a good sensing response of 27.89% under 10 ppm acetone in respiratory gases, which is about 5 times higher than the sensing response of pure chitosan film-based devices. In addition, finite element analysis and phase-field simulation were conducted to provide theoretical support for the active sensing mechanism. This work not only presents a wirelessly powered wearable active acetone biosensor, but also paves the way for a new method of non-invasive prediabetes diagnosis.

An reduced graphene oxide (RGO) resistive gas sensor was prepared to detect ammonia at room temperature, the result indicated that the desorption of gas (NH 3 ) molecules from a graphene-based sensor was difficult, which lead to a baseline drift. The responses of different concentrations were compared and studied. It was found that both the response rate and its acceleration were affected by the gas concentration. An Intermolecular Forces Based Model was established to explain the adsorption and desorption dynamic response curves. A new method was proposed based on this model. The first and second derivative extrema (FSDE) of the response curve can be attained quickly to calibrate the gas concentrations. The experiment results demonstrated that this new method could eliminate the baseline drift and was capable of increasing the efficiency of gas calibration significantly.

Polyvinylidene fluoride (PVDF) films possess multifunctional ability for piezo/pyro/ferroelectronic applications. One critical challenge of the traditional techniques is the complicated fabrication process for obtaining the poled films. In this work, the PVDF film is facilely prepared by the solution cast on hydrophilically treated substrates. The obtained PVDF films exhibit fairly good pyroelectricity comparable to those fabricated by thermal poling, indicating the film is self-polarized. This result is attributed to the hydrogen-bonding-induced orderly arrangement of the first sub-nanolayer at the bottom, which serves as a “seed layer” and triggered alignment of the rest of the film in a layer-by-layer approach. Additionally, to suppress the piezoelectric noise, a pyroelectric sensor with a novel bilayer structure is developed using the as-prepared PVDF film. Compared with the conventional monolayer sensor, the signal-to-noise ratio of the bilayer one is drastically improved to 38 dB from 18 dB. The above results provide great possibilities for achieving a high-performance wearable pyroelectric sensor with reduced cost and simple procedures. Electronic supplementary material The online version of this article (10.1186/s11671-019-2906-1) contains supplementary material, which is available to authorized users.

Recent advances in glucose sensing have focused on the integration of sensors into contact lenses to allow noninvasive continuous glucose monitoring. Current technologies focus primarily on enzyme-based electrochemical sensing which requires multiple nontransparent electrodes to be integrated. Herein, we leverage amorphous indium gallium zinc oxide (IGZO) field-effect transistors (FETs), which have found use in a wide range of display applications and can be made fully transparent. Bottom-gated IGZO-FETs can have significant changes in electrical characteristics when the back-channel is exposed to different environments. We have functionalized the back-channel of IGZO-FETs with aminosilane groups that are cross-linked to glucose oxidase and have demonstrated that these devices have high sensitivity to changes in glucose concentrations. Glucose sensing occurs through the decrease in pH during glucose oxidation, which modulates the positive charge of the aminosilane groups attached to the IGZO surface. The change in charge affects the number of acceptor-like surface states which can deplete electron density in the n-type IGZO semiconductor. Increasing glucose concentrations leads to an increase in acceptor states and a decrease in drain-source conductance due to a positive shift in the turn-on voltage. The functionalized IGZO-FET devices are effective in minimizing detection of interfering compounds including acetaminophen and ascorbic acid. These studies suggest that IGZO FETs can be effective for monitoring glucose concentrations in a variety of environments, including those where fully transparent sensing elements may be of interest.

Amorphous indium gallium zinc oxide (IGZO) field effect transistors (FETs) are a promising technology for a wide range of electronic applications. Herein, we fabricated and characterized FETs with a nanostructured IGZO network as a sensing transducer. The IGZO was patterned using colloidal lithography and electrohydrodynamic printing, where an 8 μm wide nanostructured close-packed hexagonal IGZO network was obtained. Electrical characterization of the nanostructured IGZO network FET demonstrated a drain–source current on–off ratio of 6.1 × 103 and effective electron mobilities of 3.6 cm2 V−1 s−1. The nanostructured IGZO network was functionalized by aminosilane groups with cross-linked glucose oxidase. The devices demonstrated a decrease in drain–source conductance and a more positive VON with increasing glucose concentration. These changes are ascribed to the acceptor-like surface states associated with positively charged aminosilane groups attached to the nanostructured IGZO surface. Continuous monitoring of the drain–source current indicates a stepwise and fully reversible response to glucose concentrations with a short response time. The specific catalytic reaction between the GOx enzyme and glucose eliminates interference from acetaminophen/ascorbic acid. We demonstrate that nanostructured IGZO FETs have improved sensitivity compared to non-nanostructured IGZO for sensing glucose and can be potentially extended to other biosensor technologies.

Superhydrophobic surfaces have been shown to produce significant drag reduction in both laminar and turbulent flows by introducing an apparent slip velocity along an air-water interface trapped within the surface roughness. In the experiments presented within this study, we demonstrate the existence of a surface tension gradient associated with the resultant Marangoni flow along an air-water interface that causes the slip velocity and slip length to be significantly reduced. In this study, the slip velocity along a millimeter-sized air-water interface was investigated experimentally. This large-scale air-water interface facilitated a detailed investigation of the interfacial velocity profiles as the flow rate, interfacial curvature and interface geometry were varied. For the air-water interfaces supported above continuous grooves (concentric rings within a torsional shear flow) where no surface tension gradient exists, a slip velocity as high as 30% of the bulk velocity was observed. However, for the air-water interfaces supported above discontinuous grooves (rectangular channels in a Poiseuille flow), the presence of a surface tension gradient reduced the slip velocity and in some cases resulted in an interfacial velocity that was opposite to the main flow direction. The curvature of the air-water interface in the spanwise direction was found to dictate the details of the interfacial flow profile with reverse flow in the center of the interface for concave surfaces and along the outside of the interface for convex surfaces. The deflection of the air-water interface was also found to greatly affect the magnitude of the slip. Numerical simulations imposed with a relatively small surface tension gradient along the air-water interface were able to predict both the reduced slip velocity and back flow along the air-water interface.

The paper presents a computational framework for assessing quantitatively the detection capability of structural health monitoring (SHM) systems for flat plates. The detection capability is quantified using the probability of detection (POD) metric, developed within the area of nondestructive testing, which accounts for the variability of the uncertain system parameters and describes the detection accuracy using confidence bounds. SHM provides the capability of continuously monitoring the structural integrity using multiple sensors placed sensibly on the structure. It is important that the SHM can reliably and accurately detect damage when it occurs. The proposed computational framework models the structural behavior of flat plate using a spring-mass system with a lumped mass at each sensor location. The quantity of interest is the degree of damage of the plate, which is defined in this work as the difference in the strain field of a damaged plate with respect to the strain field of the healthy plate. The computational framework determines the POD based on the degree of damage of the plate for a given loading condition. The proposed approach is demonstrated on a numerical example of a flat plate with two sides fixed and a load acting normal to the surface. The POD is estimated for two uncertain parameters, the plate thickness and the modulus of elasticity of the material, and a damage located in one spot of the plate. The results show that the POD is close to zero for small loads, but increases quickly with increasing loads.

Probability of detection (POD) is used for reliability analysis in nondestructive testing (NDT) area. Traditionally, it is determined by experimental tests, while it can be enhanced by physics-based simulation models, which is called model-assisted probability of detection (MAPOD). However, accurate physics-based models are usually expensive in time. In this paper, we implement a type of stochastic polynomial chaos expansions (PCE), as alternative of actual physics-based model for the MAPOD calculation. State-of-the-art least-angle regression method and hyperbolic sparse technique are integrated within PCE construction. The proposed method is tested on a spherically-void-defect benchmark problem, developed by the World Federal Nondestructive Evaluation Center. The benchmark problem is added with two uncertainty parameters, where the PCE model usually requires about 100 sample points for the convergence on statistical moments, while direct Monte Carlo method needs more than 10000 samples, and Kriging based Monte Carlo method is oscillating. With about 100 sample points, PCE model can reduce root mean square error to be within 1% standard deviation of test points, while Kriging model cannot reach that level of accuracy even with 200 sample points.

In this work, RGO/SnO 2 nanocomposites films were airbrushed on interdigitated electrodes to prepare gas sensors. The films were characterized by SEM, XRD and Raman. The sensing performance to NO 2 gas was investigated at room-temperature. The experimental results showed that moderate SnO 2 nanoparticles enhanced the response of the sensors. The optimum amount of SnO 2 is 40mg, whose response is 39.1% toward 50ppm NO 2 .

In this work, a fundamental new approach for humidity detection was proposed. By coupling triboelectric nanogenerator (TENG) with resistive humidity sensor, the degree of ambient moisture can be actively detected at room temperature, where the voltage drop across the interdigital electrodes (IDEs) has a reversely proportional relationship with relative humidity. The prepared self-powered triboelectric humidity sensor (THS) holds high humidity response of ∼7, fast response/recovery time of 2.8 s and 3.5 s, stable repeatability and excellent long term stability. With respect to selectivity, the response of THS to gaseous water was at least 2-fold higher than those for other gases. Furthermore, through a signal processing circuit, a self-powered humidity alert system was developed for real-time monitoring the ambient humidity. Finally, the underlying working principle of the proposed sensor was discussed in detail. This research not only provides a simple, low-cost and efficient means in humidity sensing but also greatly broadens the applicability of TENGs as self-powered active sensors.

In this work, utility theory is introduced to optimum aerodynamic shape design under uncertainty (also called robust aerodynamic design). Specifically, utility theory is used to formulate the objective function of the optimization problem. The advantage of the proposed approach over the commonly used weighted sum method is that it does not require the use of weighting factors or the addition of constraints on the statistical moments. The polynomial chaos expansion metamodel with the least-angle regression and the hyperbolic truncation scheme are used to accelerate the uncertainty propagation and the optimization process. The proposed approach is demonstrated on the optimum airfoil shape design under uncertainty at steady transonic conditions. Two design cases are considered. In both cases the drag coefficient is to be minimized subject to constraints on the lift coefficient and the airfoil thickness at two chordwise locations. The first case treats the Mach number as uncertainty parameter and the second case treats the Mach number and the lift coefficient at target as uncertain. The proposed method is compared with deterministic single-point optimization and multi-point optimization, and the standard robust design formulations. Results show that the proposed approach is capable of obtaining airfoil design of the lowest mean drag coefficient with the smallest standard deviation as compared to the other approaches. In the first case, the proposed approach yields a design with a mean drag coefficient comparable to the standard robust design (around 97.6 drag counts, cts), but with a significantly lower variance (0.6 cts versus 2.5 cts). In the second case, the proposed approach yielded a design with a higher mean drag coefficient than the standard approach (98.4 cts versus 97.6 cts), but with a lower standard deviation (4.1 cts versus 4.8 cts). In both cases, the deterministic approaches yield designs with comparable or lower drag coefficients but with significantly larger standard deviations.

This work introduces multifidelity metamodeling for reliability analysis of nondestructive testing (NDT) systems. Specifically, the Cokriging metamodel is utilized to accelerate the uncertainty propagation within model-assisted probability of detection (MAPOD) analysis of ultrasonic testing (UT) systems. The Cokriging multifidelity metamodel fuses a limited amount of data obtained from high-fidelity (HF) physics-based UT models, which are accurate but time-consuming to evaluate, with a conservative to large amount of data from low-fidelity (LF) physics-based UT models, which are less accurate but faster to evaluate. The resulting Cokriging metamodel is fast to evaluate and yields an accurate estimate of the output of the HF model. The proposed approach is demonstrated on three benchmark UT MAPOD cases involving spherically-void and pill-box shaped defects in flat aluminum plates using planar and focused transducers. The results show that a two-level Cokriging metamodel is capable of yielding estimations of the HF model that are globally accurate within 1% of the standard deviation of the testing points. Furthermore, the result show that the Cokriging metamodeling approach needs around one order of magnitude fewer training data points when compared to the current state-of-the-art approaches that rely on metamodels constructed with Kriging.

Human respiration is rich in physiological and pathological information for health assessment and illness prediction. In this work, an integrated triboelectric self-powered respiration sensor (TSRS) was developed for simultaneously monitoring human respiratory behaviors and NH3 concentration in exhaled gases. The TSRS based on Ce-doping ratio of 0.004 M exhibits a better selectivity and larger sensitivity of 20.13 ppm−1 compared with those synthesized in other doping ratios. Under the same moisture as the breathing gas (97.5%RH), the sensor holds greater sensitivity toward NH3 at low concentration (0.1–1 ppm) than that at high concentration (1–10 ppm), implying the capability in detecting trace level NH3 biomarker in human breathing gases. Furthermore, triggered by the expansion and contraction of chest during breathing, TSRS can spontaneously monitor human respiratory patterns and physiological process after physical exercise. Meanwhile, a theoretical modeling was established in terms of permittivity change under gas molecules adsorption and verified via finite element simulation to interpret the self-powered sensing behaviors. This research not only proposes a promising approach to develop an all-in-one wearable respiration monitoring system for personal health diagnosis but also sheds some light on the theoretical modeling of triboelectric self-powered gas-sensing in terms of permittivity change.

Nitrogen dioxide sensors with high sensitivity and low energy consumption are demanded for atmosphere sensing networks. Here, a self-powered room temperature NO2 sensor has been developed based on the conjugation between the triboelectric and photoelectric effect. By converting the mechanical motions into electricity, a triboelectric nanogenerator (TENG) serves as a power source to simultaneously drive chemoresistive gas sensing and UV illumination. Under a 5 Hz external impact, the output voltage drop across interdigital electrodes has a proportional relationship with the NO2 concentration. A self-powered optomechatronic gas sensor (OGS) with hydrothermal ZnO nanowires synthesized at a concentration of 0.035 mol/l exhibits a superior response (∼14.8) and sensitivity (0.302 ppm−1) than those synthesized at other concentrations. Furthermore, the influence of the external force frequency and rectification on the gas sensing properties was systematically investigated. It is found that the TENG induced built-in electric field can effectively modulate the internal quantum efficiency and thus the sensing performance of OGSs. This work not only paves the way for constructing self-powered optomechatronic devices and systems but also pushes forward the active multifunctional network node for environmental monitoring.

This paper reports the fabrication and performance of a semi-packed column with wavy channel configuration. The 2-m long, 80-μ m wide, and 300-μ m deep column is made by deep reactive-ion etching (DRIE) technology, and contains embedded circular posts of 30 μ m in diameter. The posts and curved groove increase the overall surface area of the column, which leading to higher separation efficiency. In order to achieve an even flow velocity in the channel, the arrangement position, shape, size of embedded posts are explored by the finite element method (FEM) in the semi-packed wavy gas chromatography column. The simulation results show that the gas samples experience less variation in the velocity profile of the semi-packed wavy column than the semi-packed rectangular MEMS column, and possess shorter mass transfer distance than the open one. The micro-fabricated column is used to separate normal alkanes from C5 to C12 to test its performance. The experimental separation results indicate that the theoretical plate number obtained by the proposed semi-packed wavy gas chromatography device is about 4 times higher than that obtained from the open rectangular column under the same injection conditions.