Search

In order to achieve fast and accurate passive location of the target radiation source, this paper puts forward a Virtual Time Reversal Method Based on Uniform Linear Array (ULA-VTR) passive location algorithm. This framework employs the passive time reversal focusing theory, which does not to require the physical transmission of time reversal signals and does not require pre-estimation of the channel to realize passive direction finding. The above principle is utilized to perform the target radiation source signal through a uniform linear sensor array for detection. The article provides the signal model of linear time reversal array and the virtual retransmission process in the first place. Then utilizes ULA-VTR to establish a dual-station passive location framework and derives the coordinate transformation equation to find its observation angle, in addition, establishes the observation equation system for obtaining the site of the target radiation source. Ultimately, the paper analyses the computational complexity of the ULA-VTR algorithm and adopts Geometric Dilution of Precision(GDOP) to discuss the positioning performance. The experimental results indicate that the technique presented in this paper has higher positioning accuracy under the low SNR compared with other techniques.

Cation substitution in spinel cobaltites (e.g., ACo2O4, in which A = Mn, Fe, Co, Ni, Cu, or Zn) is a promising strategy to precisely modulate their electronic structure/properties and thus improve the corresponding electrochemical performance for water splitting. However, the fundamental principles and mechanisms are not fully understood. This research aims to systematically investigate the effects of cation substitution in spinel cobaltites derived from mixed-metal-organic frameworks on the oxygen evolution reaction (OER). Among the obtained ACo2O4 catalysts, FeCo2O4 showed excellent OER performance with a current density of 10 mA·cm−2 at an overpotential of 164 mV in alkaline media. Both theoretical calculations and experimental results demonstrate that the Fe substitution in the crystal lattice of ACo2O4 can significantly accelerate charge transfer, thereby achieving enhanced electrochemical properties. The crystal field of spinel ACo2O4, which determines the valence states of cations A, is identified as the key factor to dictate the OER performance of these spinel cobaltites.

Flexible and stretchable strain sensors are vital for emerging fields of wearable and personal electronics, but it is a huge challenge for them to possess both wide-range measurement capability and good sensitivity. In this study, a highly stretchable strain sensor with a wide strain range and a good sensitivity is fabricated based on smart composites of carbon black (CB)/wrinkled Ecoflex. The sensor exhibits a maximum recoverable strain of up to 500% and a high gauge factor of 67.7. It has a low hysteresis, a fast signal response (as short as 120 ms), and a high reproducibility (up to 5000 cycles with a strain of 150%). The sensor is capable of detecting and capturing wide-range human activities, from speech recognition and pulse monitoring to vigorous motions. It is also applicable for real-time monitoring of robot movements and vehicle security crash in an anthropomorphic field. More importantly, the sensor is successfully used to send signals of a volunteer’s breathing data to a local hospital in real time through a big data cloud platform. This research provides the feasibility of using a strain sensor for wearable Internet of things and demonstrates its exciting prospect for healthcare applications.

Flexible electronic devices are normally based on organic polymer substrate. In this work, an ultrathin glass-based flexible, transparent, and ultrasensitive ZnO/glass surface acoustic wave (SAW) humidity sensor is developed using a composite sensing layer of ZnO nanowires (NWs) and graphene quantum dots (GQDs). It shows much larger effective electromechanical coupling coefficients and signal amplitudes, compared to those of flexible polymer-based SAW devices reported in the literature. Attributed to large specific surface areas of ZnO NWs, large numbers of hydrophilic functional groups of GQDs, as well as the formation of p-n heterojunctions between GQDs and ZnO NWs, the developed ZnO/glass flexible SAW sensor shows an ultrahigh humidity sensitivity of 40.16 kHz/% RH, along with its excellent stability and repeatability. This flexible and transparent SAW sensor has demonstrated insignificant deterioration of humidity sensing performance, when it is bent on a curved surface with a bending angle of 30°, revealing its potential applications for sensing on curved and complex surfaces. The humidity sensing and human breathing detection have further been demonstrated for wearable electronic applications using ultrathin glass-based devices with completely inorganic materials.

Droplet impact on arbitrary inclined surfaces is of great interest for applications such as antifreezing, self-cleaning, and anti-infection. Research has been focused on texturing the surfaces to alter the contact time and rebouncing angle upon droplet impact. In this paper, using propagating surface acoustic waves (SAWs) along the inclined surfaces, we present a novel technique to modify and control key droplet impact parameters, such as impact regime, contact time, and rebouncing direction. A high-fidelity finite volume method was developed to explore the mechanisms of droplet impact on the inclined surfaces assisted by SAWs. Numerical results revealed that applying SAWs modifies the energy budget inside the liquid medium, leading to different impact behaviors. We then systematically investigated the effects of inclination angle, droplet impact velocity, SAW propagation direction, and applied SAW power on the impact dynamics and showed that by using SAWs, droplet impact on the nontextured hydrophobic and inclined surface is effectively changed from deposition to complete rebound. Moreover, the maximum contact time reduction up to ∼50% can be achieved, along with an alteration of droplet spreading and movement along the inclined surfaces. Finally, we showed that the rebouncing angle along the inclined surface could be adjusted within a wide range.

In this paper, CuCr–Zr alloys prepared by vacuum melting with adding La and Ni elementswere heat-treated and aged, followed by plastic deformation using low-energy cyclic impact tests, to simultaneously improve their mechanical and electrical properties. Results showed that the grain size of the casted Cu–Cr–Zr alloys was significantly reduced after the solid-solution aging and plastic deformation process. There were a lot of dispersed Cr and Cu5Zr precipitates formed in the alloys, and the numbers of dislocations were significantly increased. Accordingly, the hardness was increased from 78 to 232 HV, and the tensile strength was increased from 225 to 691 MPa. Electrical conductivity has not been significantly affected after these processes. The enhancement of overall performance is mainly attributed to the combined effects of solid-solution hardening, fine grain hardening, and precipitation/dislocation strengthening.

For conventional titanium matrix composites (TiMCs), there is always a trade-off issue between enhanced strength and ductility of these materials. In this study, we explore a new design methodology by reinforcing titanium alloy matrix with carbonaceous nanomaterials and investigate the mechanisms for achieving a good balance of their strength and ductility. The TiMCs were synthesized through a low-cost powder metallurgy route using pre-mixed Ti-6Al-4V (TC4) powders and various carbon based nanofillers, including graphite powders (GPs), graphene oxide nanosheets (GONs) and graphene nanoplates (GNPs), and were further rolled at a temperature of 1173 K with a deformation of 66.7%. Among these three types of carbon reinforcing sources, the GNPs are more easily reacted with TC4 matrix and form more contents of TiC phases after sintering owing to their larger amounts of defects than those of the GPs and GONs. TiC products are identified to play a bridging role for not only connecting the TC4 matrix but also forming coherent interfaces with the TC4 matrix, thus facilitating a strong interfacial bonding of the composites. The as-rolled GNPs/TC4 composites exhibit a 0.2% yield strength of 1146.36 MPa (with an elongation of ∼8.1%), which is 24.6%, 9.22% and 5.62% higher than those of pure TC4, GPs/TC4 and GONs/TC4 composites. The GNPs/TC4 nanocomposites show a better balance of strength and ductility than those of the other two types of nanocomposites. The synergetic strengthening mechanisms are identified to be Orowan strengthening effect, effective load transfer capability of GNPs, and in-situ formation of interfacial TiC structures, which provide optimum interfacial microstructures to achieve good mechanical properties of the TiMCs.

Surface acoustic wave (SAW) devices are widely used for physical, chemical, and biological sensing applications, and their sensing mechanisms are generally based on frequency changes due to mass-loading effects at the acoustic wave propagation area between two interdigitated transducers (IDTs). In this paper, a new sensing mechanism has been proposed based on a significantly enhanced mass-loading effect generated directly on Au IDT electrodes, which enables significantly enhanced sensitivity, compared with that of conventional SAW devices. The fabricated ultrahigh-frequency SAW devices show a significant mass-loading effect on the electrodes. When the Au-electrode thickness increased from 12 to 25 nm, the Rayleigh mode resonant frequency decreased from 7.77 to 5.93 GHz, while that of the higher longitudinal leaky SAW decreased from 11.87 to 9.83 GHz. The corresponding mass sensitivity of 7309 MHz·mm2·μg–1 (Rayleigh mode) is ∼8.9 × 1011 times larger than that of a conventional quartz crystal balance (with a frequency of 5 MHz) and ∼1000 times higher than that of conventional SAW devices (with a frequency of 978 MHz). Trinitrotoluene concentration as low as 4.4 × 10–9 M (mol·L–1) can be detected using the fabricated SAW sensor, proving its giant mass-loading effect and ultrahigh sensitivity.

Nickel dichalcogenides have received extensive attention as promising noble-metal-free nanocatalysts for a hydrogen evolution reaction. Nonetheless, their catalytic performance is restricted by the sluggish reaction kinetics, limited exposed active sites, and poor conductivity. In this work, we report on an effective strategy to solve those problems by using an as-designed new porous-C/Ni2SeS nanocatalyst with the Ni2SeS nanostubs anchored on with porous-carbon skeletons process. On the basis of three advantages, as the enhancement of the intrinsic activity using the ternary sulfoselenide, increased number of exposed active sites due to the 3D hollow substrate, and increased conductivity caused by porous-carbon skeletons, the resulting porous-C/Ni2SeS requires an overpotential of only 121 mV at a current density of 10 mA cm–2 with a Tafel slope of 78 mV dec–1 for hydrogen evolution in acidic media and a good long-term stability. Density functional theory calculations also show that the Gibbs free energy of hydrogen adsorption of the Ni2SeS was −0.23 eV, which not only is close to the ideal value (0 eV) and Pt reference (−0.09 eV) but also is lower than those of NiS2 and NiSe2; large electrical states exist in the vicinity of the Fermi level, which further improves its electrocatalytic performance. This work provides new insights into the rational design of ternary dichalcogenides and hollow structure materials for practical applications in HER catalysis and energy fields.

A microfiber interferometer based on a single-mode tapered-no-core single-mode (STNCS) fiber structure coated with a thin layer of graphene oxide (GO) was proposed and demonstrated as a relative humidity (RH) sensor. The STNCS fiber structure has a strong evanescent field, which can sensitively response to changes in the surrounding refractive index. The moisture-sensitive material GO was used to form the film on the no-core fiber by a dip impregnation method. The effects of GO concentration and the tapered waist diameter on RH sensitivity were studied experimentally. The experimental results showed that a thinner waist diameter provides higher RH sensitivity. When the concentration of GO solution is 0.01 mg/mL, the STNCS fiber structure with a waist diameter of 2.9 μm performs better in RH sensing; a high sensitivity of 0.461 nm/%RH in the range of 40-98% was achieved. The sensor has good stability with wavelength fluctuations of ±0.009 nm and ±0.008 nm over 60 mins at RHs of 50% and 80%, respectively. The proposed fiber optic humidity sensor has a high sensitivity over a wide measurement range and offers good stability showing promising potential application in the field of RH sensing.

Gas sensors based on metal oxide semiconductors (MOSCs) and reduced graphene oxide (rGO) for sensing of organic volatile compounds often suffer from high operation temperature, low responses, poor selectivity, or narrow detection range. Herein, we design and fabricate Bi-doped rGO/Co3O4 (BGCO) nanohybrids with a flower morphology, which have been applied as a sensing layer for an ethanol sensor. This BGCO sensor exhibits a maximum p-type response of 178.1 towards 500 ppm ethanol at an optimum working temperature of 120 °C. The sensor’s detection range for the ethanol concentration is from 500 ppb to 500 ppm, and the sensor has an excellent selectivity to ethanol compared to other types of organic volatile gases and oxidizing gas such as NO2. The enhanced ethanol sensing mechanism is attributed to the increased conductivity of Bi doped rGO/Co3O4 material. Additionally, incorporation of Bi dopant can promote the redox reaction, and the rGO/Co3O4 act as the catalyst.

Extensive applications of carbon dioxide (CO2) in various fields, such as food industry, agricultural production, medical and pharmacological industries, have caused a great demand for high-performance CO2 sensors. However, most existing CO2 sensors suffer from poor performance in a wet environment and often cannot work accurately in a high humidity condition. In this study, a quartz crystal resonator (QCR) coated with a uniform layer of reduced graphene oxide (RGO) is proposed to detect both the concentrations of CO2 and water molecules simultaneously, which can be used to significantly minimize the humidity interference. Unlike the other common gas sensors, the RGO-based CO2 QCR sensor can be operated in different humidity levels and the concentration of CO2 can be quantified precisely and effectively. Moreover, it has a fast response (∼0.4 s), which is also suitable for respiration monitoring. Our results showed that before and after a volunteer did a low-intensity exercise, the sensor could detect the differences of concentrations of CO2 in the exhaled breath (i.e., 4.50 % and 5.15 %, respectively).

In this paper, we for the first time report the catalytic activity and durability of nickel cobaltite telluride (NiCo2Te4) nanocluster bifunctional catalysts can be significantly boosted by surface modification with perylene-tetracarboxylic-dianhydride for overall water-splitting in neutral solution. We reveal that tuning energy distribution of nanoclusters via a simple surface ligand can drastically increase the catalytic activity towards efficient hydrogen and oxygen evolution reaction simultaneously. A two-electrode based water electrolysis cell using this newly developed nanocluster catalyst operates at a low bias voltage of 1.55 V to achieve a current density of 10 mA·cm−2 in near-neutral pH solution for overall water-splitting. This, to the best of our knowledge, represents the most efficient mixed-transition-metal-based electrode that has so far been reported for electrochemical water splitting.

Zinc cobalt sulfide (ZCS) is a promising and high performance electrode material for pseudocapacitors due to its good electrical conductivity, abundant active sites and rich valence states. In this work, zinc cobalt oxide (ZCO) nanoparticles are firstly synthesized via a hydrothermal method assisted by hexadecyltrimethyl ammonium bromide (CTAB), and then transformed into zinc cobalt sulfide nanoparticles (ZCS NPs) using a facile sulfuration process. The average diameter of ZCS NPs is estimated to be about 15 nm, which is beneficial for Faradaic redox reactions in energy storage process due to their numerous active surfaces. The ZCS NPs are then coated onto a nickel foam to form the working electrode of supercapacitors. Because the ZCS electrode has lower series and charge transfer resistance, and also higher ion diffusion rate than that of the ZCO electrode, it achieves a large specific capacitance of 1269.1 F g−1 at 0.5 A g−1 in 2 M KOH electrolyte, which is four times more than that of the ZCO electrode (e.g., 295.8 F g−1 at 0.5 A g−1). In addition, an asymmetric supercapacitor using the ZCS NPs as the positive electrode and activated carbon as the negative electrode is assembled, which delivers a high energy density of 45.4 Wh kg−1 at a power density of 805.0 W kg−1, with an excellent cycling stability (91.6% retention of the initial capacitance over 5000 cycles).

The influence of αs precipitates on electrochemical behavior and mechanical degradation of Ti-1300 alloy in artificial seawater have been studied. The results show that corrosion resistance and mechanical degradation have been significantly affected by the formation of acicular αs precipitates. The precipitated αs phase with an acicular shape around 40–60 nm in width are uniformly distributed inside β grain. Many αs precipitates are intersected each other and keep a well-defined Burgers orientation relationship with β matrix, which restricts the growth of other αs phases due to pinning effect. Within the electrolyte, the αs phases can form “microgalvanic cells” with their adjacent intergranular β phases, which dramatically deteriorate its corrosion resistance. The mechanical properties of the alloy are also degraded with the increase of immersion time due to the pitting reaction. The precipitated microstructure exhibits an inferior mechanical degradation behavior, and this is mainly because a lot of corrosion cavities are nucleate d and propagated at the interface between αs precipitates and prior β grains.

CuCo2S4 is regarded as a promising electrode material for supercapacitor, but has inferior conductivity and poor cyclic stability which restrict its wide-range applications. In this work, hierarchically hybrid composite of CuCo2S4/carbon nanotubes (CNTs) was synthesized using a facile hydrothermal and sulfuration process. The embedded CNTs in the CuCo2S4 matrix provided numerous effective paths for electron transfer and ion diffusion, and thus promoted the faradaic reactions of the CuCo2S4 electrode in the energy storage processes. The CuCo2S4/CNTs-3.2% electrode exhibited a significantly increased specific capacitance of 557.5 F g−1 compared with those of the pristine CuCo2S4 electrode (373.4 F g−1) and CuO/Co3O4/CNTs-3.2% electrode (356.5 F g−1) at a current density of 1 A g−1. An asymmetric supercapacitor (ASC) was assembled using the CuCo2S4/CNTs-3.2% as the positive electrode and the active carbon as the negative electrode, which exhibited an energy density of 23.2 Wh kg−1 at a power density of 402.7 W kg−1. Moreover, the residual specific capacitance of this ASC device retained 85.7% of its original value after tested for 10,000 cycles, indicating its excellent cycle stability.

The effects of loading rate on creep behavior and shear transformation zone (STZ) in magnetron sputtered La-Co-Al and Zr-Cu-Ni-Al amorphous alloy thin films were characterized through instrumented nanoindentation with a spherical indenter. It was revealed that with an increase in loading rate, both thin films became harder with a narrower distribution of mechanical response, and exhibited more pronounced creep displacement with higher creep strain rate. Based on cooperative shear model, both STZ volume and activation energy were calculated by measuring the strain rate sensitivity during creep, showing an increasing tendency with loading rate. However, the strain rate sensitivity was found to decrease with loading rate. Furthermore, structural heterogeneity density, estimated by analyzing the stressed volume at yielding, decreased with loading rate. Therefore, with increasing loading rate, the STZ event can only be activated at detectable structural heterogeneities with relatively larger volume, and the number of available fertile sites was decreased, resulting in postponed shear band formation and suppressed deformation mode transition.

Propagation of surface acoustic waves (SAWs) on bulk piezoelectric substrates such as LiNbO3 and quartz, exhibits an in-plane anisotropic effect due to their crystal cut orientations. Thin film SAW devices, such as those based on ZnO or AlN, offer potential advantages, including isotropic wave velocities in all in-plane directions, higher power handling capability, and potentially lower failure rates. This paper reports experimental and simulation results of nebulization behaviour for water droplets using ZnO/Si surface acoustic wave devices with focused interdigital transducers (IDTs). Post-deposition annealing of the films at various temperatures was applied to improve the quality of the sputtering-deposited ZnO films, and 500 °C was found to be the optimal annealing temperature. Thin film ZnO/Si focused SAW devices were fabricated using the IDT designs with arc angles ranging from 30° to 90°. Nebulization was significantly enhanced with increasing the arc angles of the IDTs, e.g., increased nebulization rate, reduced critical powers required to initialise nebulization, and concentration of the nebulised plume into a narrower size of spray. Effects of applied RF power and droplet size have been systematically studied, and increased RF power and reduced droplet size significantly enhanced the nebulization phenomena.

A molecular mechanics model for covalent and ionic double-network polymer composites was developed in this study to investigate mechanisms of mechanochemically induced unzipping and scission of chemical bonds. Morse potential function was firstly applied to investigate mechanical unzipping of the covalent bonds, and then stress-dependent mechanical energy for the interatomic covalent bonds was discussed. A new mechanochemical model was formulated for describing the mechanically induced ionic bond scissions based on the Morse potential model and equations for electrostatic forces. Based on this newly proposed model, mechanochemical behaviors of several common interatomic interaction types (e.g., A+B−, A2+B2−/A2+2B−/2A+B2− and A3+B3−/A3+3B−/3A+B3−) of the ionic bonds have been quantitatively described and analyzed. Finally, mechanochemical unzipping of the covalent bonds and dissociation of the ionic bonds have been characterized and analyzed based on the molecular mechanics model, which has well predicted the chemical and mechanochemical activations in the covalent and ionic double-network polymer composites.

Surface acoustic wave (SAW) sensors have great advantages in real-time and in-situ gas detection due to their wireless and passive characteristics. Using nanostructured sensing materials to enhance the SAW sensor’s responses has become a research focus in recent years. In this paper, solution-processed PbS colloidal quantum dots (CQDs) were integrated into quartz SAW devices for enhancing the performance of NO2 detection operated at room temperature. The PbS CQDs were directly spin-coated onto ST-cut quartz SAW delay lines, followed by a ligand exchange treatment using Pb(NO3)2. Upon exposure to 10 ppm of NO2 gas, the sensor coated with untreated PbS CQDs showed response and recovery times of 487 s and 302 s, and a negative frequency shift of −2.2 kHz, mainly due to the mass loading effect caused by the absorption of NO2 gas on the surface of the dense CQD film. Whereas the Pb(NO3)2-treated sensor showed fast response and recovery times of 45 s and 58 s, and a large positive frequency shift of 9.8 kHz, which might be attributed to the trapping of NO2 molecules in the porous structure and thus making the film stiffer. Moreover, the Pb(NO3)2-treated sensor showed good stability and selectivity at room temperature.

In this work, effects of 10 keV argon ion implantation on laser-induced damage threshold (LIDT) of fused silica were systematically investigated with ion fluences ranged from 1 × 1016 ions/cm2 to 1 × 1018 ions/cm2. Results show that only when the ion fluence increases above 1 × 1017 ions/cm2, the surface roughness apparently increases due to the formation of argon bubbles in the surface of fused silica. The concentration of defects decreases with the increased fluences up to 1 × 1017 ions/cm2 but then increases further, especially for the oxygen deficient center (ODC) defect. Based on the nanoindentation test results, Ar ion implantation generates large compressive stress and strengthens the surface of fused silica by surface densification. With the increase of the Ar ion fluences, the LIDTs of the samples increase due to the increases in both surface compressive stress and defects annihilation. However, at higher ion fluences, the increase of the densities of defects and argon bubbles are identified as the key reasons for the decrease of the LIDTs. Therefore, Ar ion implantation can improve the LIDTs of fused silica at moderate fluences.

This paper presents a passive direction finding of a virtual time reversal method based on cross antenna array (CROSS-VTR), which aim is to solve the problem that the number of antenna elements of planar arrays is too large and the layout of circular arrays is complex which requires high hardware design. In order to verify the feasibility and accuracy of the algorithm, the mathematical model of the cross antenna array is first established. The search area of yaw angle and a pitch angle of the CROSS-VTR algorithm is determined according to the basic principle of passive time reversal focusing, and then, the time-reversed signal of each antenna element completes the time delay compensation at each scanning point in the search area. The compensated signals of each antenna element are added up to obtain the signal energy of each scanning point. The value of the yaw angle and pitch angle is obtained by searching the maximum energy among scanning points along the search route of yaw angle and pitch angle, respectively. Finally, the validity of the method is verified and compared with the music algorithm (multiple signal classification) based on the cross antenna array. The results show that the method has good direction finding accuracy at low signal-to-noise ratio.

Soft elastomers with their ability to integrate strain-adaptive stiffening and coloration have recently received significant research attention for application in artificial muscle and active camouflage. However, there is a lack of theoretical understanding of their complex molecular dynamics and mechanochromic coupling/decoupling. In this study, a topological dynamics model is proposed to understand the anchoring-mediated topology signature of self-assembled elastomers. Based on the constrained molecular junction model, a free-energy function is firstly formulated to describe the working principles of strain-adaptive stiffening and coloration in the self-assembled elastomer. A coupled ternary "rock-paper-scissors" model is proposed to describe the topological dynamics of self-assembly, mechanochromic coupling and mechanoresponsive stiffening of the self-assembled elastomers, in which there are three fractal geometry components in the topology network. Finally, the proposed models are verified using the experimental results reported in the literature. This study provides a fundamental approach to understand the working mechanism and topological dynamics in the self-assembled elastomers, with molecularly encoded stiffening and coloration.

Understanding working principles and thermodynamics behind phase separations, which have significant influences on condensed molecular structures and their performances, can inspire to design and fabricate anomalously and desirably mechanoresponsive hydrogels. However, a combination of techniques from physicochemistry and mechanics has yet been established for the phase separation in hydrogels. In this study, a thermodynamic model is firstly formulated to describe solvent-aided phase and microphase separations in the hydrogels, which present significantly improved mechanoresponsive strengths. Flory–Huggins theory and interfacial energy equation have further been applied to model the thermodynamics of concentration-dependent and temperature-dependent phase separations. An intricately detailed phase map has finally been formulated to explore the working principle. The thermodynamic methodology of phase separations, combined with the constitutive stress–strain relationships, has a great potential to explore the working mechanisms in mechanoresponsive hydrogels.

In this paper, we explore the acoustofluidic performance of zinc oxide (ZnO) thin-film surface acoustic wave (SAW) devices fabricated on flexible and bendable thin aluminum (Al) foils/sheets with thicknesses from 50 to 1500 μm. Directional transport of fluids along these flexible/bendable surfaces offers potential applications for the next generation of microfluidic systems, wearable biosensors and soft robotic control. Theoretical calculations indicate that bending under strain levels up to 3000 με causes a small frequency shift and amplitude change (

Flexible acoustic wave devices (FAWDs) have been explored for various applications where bending is inevitable. However, theoretical investigations of bending behavior of FAWDs hitherto are mostly done in the linear deformation regime. Herein, we develop a multi-sublayer model based on a stiffness matrix method for analysis of frequency shifts of surface acoustic wave (SAW) and Lamb waves under elasto-plastic deformations. Using this model, we calculate the frequency shifts for the cases of both an elastic bending and an elasto-plastic bending. Experimental frequency shifts of ZnO/Al flexible devices show good agreements with the theoretical results in the elastic bending tests (with a relative error of strain sensitivity < 3%), and also show relatively good agreements with the qualitative theoretical predictions in the nonlinearly elasto plastic bending tests. For three successive bending and recovery processes, the experimentally obtained frequency shifts show good repeatability in the elastic and elasto-plastic bending stages, demonstrating maximum relative errors of strain sensitivities less than 6.1% and 18.2%.

Surface acoustic wave (SAW) based technologies have recently been explored for various sensing and microfluidic applications, and numerous experimental studies and numerical modelling of SAW streaming and liquid-solid interactions have been performed. However, the large deformation of droplet interface actuated by SAWs has not been widely explored, mainly due to the complex physics of SAW-droplet interactions and interfacial phenomena. In this paper, a computational interface tracking method is developed based on the couple level set the volume of fluid (CLSVOF) approach to simulate the interactions between liquid and acoustic waves and deformation of the liquid-air surface. A dynamic contact angle boundary condition is developed and validated by experimental results to simulate the three-phase contact line dynamics. The modified CLSVOF method is then used to study the droplet jetting and internal streaming behaviours by analyzing the energy terms within the liquid medium. Furthermore, by applying the numerical model, effects of configurations and positions of two interdigital transducers (IDTs) on droplet actuation have been investigated to achieve efficient mixing, separation, and jetting. Results show that two perfectly aligned IDTs are optimal for mixing applications. In contrast, two offset IDTs are optimal for concentration and separation applications. The maximum jetting velocity and minimum jetting time are achieved by using a pair of aligned IDTs, whereas by using the two offset IDTs, effective liquid mixing and jetting are observed which can be used in bioprinting applications.

Temperature control within a microreactor is critical for biochemical and biomedical applications. Recently acoustothermal heating using surface acoustic wave (SAW) devices made of bulk LiNbO3 substrates have been demonstrated. However, these are generally fragile and difficult to be integrated into a single lab-on-a-chip. In this paper, we propose a rapid and controllable acoustothermal microheater using AlN/Si thin film SAWs. The device’s acoustothermal heating characteristics have been investigated and are superior to other types of thin film SAW devices (e.g., ZnO/Al and ZnO/Si). The dynamic heating processes of the AlN/Si SAW device for both the sessile droplet and liquid within a polydimethylsiloxane (PDMS) microchamber were characterized. Results show that for the sessile droplet heating, the temperature at a high RF power is unstable due to significant droplet deformation and vibration, whereas for the liquid within the microchamber, the temperature can be precisely controlled by the input power with good stability and repeatability. In addition, an improved temperature uniformity using the standing SAW heating was demonstrated as compared to that of the travelling SAWs. Our work shows that the AlN/Si thin film SAWs have a great potential for applications in microfluidic heating such as accelerating biochemical reactions and DNA amplification.

This paper reports a high-efficiency raindrops energy harvester using the interdigital electrode. With the new electrode deployment and connection scheme, the proposed raindrops energy harvester shows excellent energy conversion effeciency up to 2.5%, over 250 times higher than the conventional interdigital electrode-based triboelectric nanogenerator with efficiency of only 0.01%. The performance under the different finger electrodes was investigated. Note that the first electrode of the interdigital electrode is designed for the energy harvesting and the rest are utilized to detect the raindrops speed. We also find out that the open-circuit voltage varies with the tilt angle of the structure, of which the inclination angle of 45 degrees shows the best output performance. And under the condition of 0 degree, the open-circuit voltage first decreases and then gradually remains stable, rather than showing no voltage output. The proposed raindrops generator pushes forward a significant step of realizing raindrops energy harvesting in real-world application.

Origami, an ancient art of paper folding with a history spanning over 1000 years, can devise ingenious patterns with unique advantages in terms of great flexibility, lightweight, extreme simplicity, and excellent shape adaptability. The poor stability of electret / triboelectric devices in a humid environment is a key challenge faced by researchers worldwide. Inspired from Japanese Muira-origami structures, we propose a "W-tube" shaped triboelectric nanogenerator (WTN) with the two pieces of electret films encapsulated inside to isolate the humid environment. Besides, it's capable of capturing energy from various moving scenes, including horizontal or vertical pressing, and lateral bending. We have tested multiple parameters of WTN in detail, including the influence of the number of generating units, the original height of equipment, the acceleration and the excitation direction on the output performance. This work combines the ancient origami art with energy conversion technology to create a multifunctional waterproof energy harvesting device.

Different acoustic wave modes are required for effective implementation of biosensing and liquid actuation functions in an acoustic wave-based lab-on-a-chip. For efficient sensing in liquids, shear waves (either a thickness-shear bulk wave or a shear-horizontal surface acoustic wave) can achieve a high sensitivity, without significant loss of acoustic wave energy. On the other hand, longitudinal bulk waves or out-of-plane displacement waves (such as Rayleigh waves) enable efficient sampling functions and liquid manipulation. However, there are significant challenges in developing a lab-on-a-chip to efficiently generate multiple wave modes and perform both these functions on a single piezoelectric substrate, especially when a single crystalline orientation is available. This paper highlights the latest progress in the theories and techniques to deliver both sensing and microfluidic manipulation functions using engineered inclined-angled piezoelectric films, allowing for the simultaneous generation of longitudinal (or Rayleigh) and thickness-shear bulk (or shear-horizontal surface acoustic) waves. Challenges and theoretical constraints for generating various wave modes in the inclined films and techniques to efficiently produce inclined columnar and inclined crystalline piezoelectric films using sputtering deposition methods are presented. Applications of different wave modes in the inclined film-based lab-on-chips with multiple sensing and acoustofluidic functions are also discussed.

In recent years, surface acoustic wave (SAW) devices have demonstrated great potentials and increasing applications in the manipulation of nano- and micro-particles including biological cells with the advantages of label-free, high sensitivity and accuracy. In this letter, we introduce a novel tilted-angle SAW devices to optimise the acoustic pressure inside a microchannel for cancer-cell manipulation. The SAW generation and acoustic radiation force are improved by seamlessly patterning electrodes in the space surrounding the microchannel. Comparisons between this novel SAW device and a conventional device show a 32% enhanced separation efficiency while the input power, manufacturing cost and fabrication effort remain the same. Effective separation of HeLa cancer cells from peripheral blood mononuclear cells is demonstrated. This novel SAW device has the advantages in minimizing device power consumption, lowering component footprint and increasing device density.

Surface acoustic wave (SAW)-based acoustofluidics shows broad applications in biomedicine and chemistry. Conventional manufacturing process for SAW devices uses photolithography and metal deposition, thus requires accessing cleanroom facilities. This study presents an efficient and versatile technique based on a flexible printed circuit board (FPCB) for developing SAW acoustofluidic devices. By mechanically clamping interdigital electrodes (IDEs) made on the FPCB onto a piezoelectric substrate, SAWs can be effectively generated with an additional matching network. The SAW amplitudes were measured by a laser vibrometer, which increases with the applied input voltage. The FPCB-SAW device has been applied to actuate 10- $\mu \text{m}$ microspheres to form strong streaming vortices inside a droplet, and to drive a sessile droplet for transportation on the substrate surface. The use of the FPCB rather than a rigid printed circuit board (PCB) can help cut down on the overall footprint of the device and save space. The low requirement in assembling the FPCB-SAW device can facilitate versatile acoustofluidic applications by providing fast prototyping devices.

Graphene has been extensively explored to enhance functional and mechanical properties of metal matrix nanocomposites for wide-range applications due to their superior mechanical, electrical and thermal properties. This article discusses recent advances of key mechanisms, synthesis, manufacture, modelling and applications of graphene metal matrix nanocomposites. The main strengthening mechanisms include load transfer, Orowan cycle, thermal mismatch, and refinement strengthening. Synthesis technologies are discussed including some conventional methods (such as liquid metallurgy, powder metallurgy, thermal spraying and deposition technology) and some advanced processing methods (such as molecular-level mixing and friction stir processing). Analytical modelling (including phenomenological models, semi-empirical models, homogenization models, and self-consistent model) and numerical simulations (including finite elements method, finite difference method, and boundary element method) have been discussed for understanding the interface bonding and performance characteristics between graphene and different metal matrices (Al, Cu, Mg, Ni). Key challenges in applying graphene as a reinforcing component for the metal matrix composites and the potential solutions as well as prospectives of future development and opportunities are highlighted.

An ultra-high sensitivity label-free optical fiber biosensor for inactivated Staphylococcus aureus (S. aureus) detection is proposed and investigated in this study, with additional advantages of robust and stability compared to traditional tapered fiber structure. The proposed fiber biosensor is based on a tapered singlemode- no core-singlemode fiber coupler (SNSFC) structure, where the no core fiber was tapered to small diameter (taper-waist diameter of about 10 μm) and functionalized with the pig immunoglobulin G (IgG) antibody for detection of S. aureus. The measured maximum wavelength shift of the sensor for an S. aureus concentration of 7 × 101 CFU/mL (colony forming unit per milliliter) is 2.04 nm, which is equivalent to a limit of detection (LOD) of 3.1 CFU/mL (a highest LOD reported so far for optical fiber biosensors), considering the maximum wavelength variation of the sensor in phosphate buffered saline (PBS) is ±0.03 nm over 40 min, where 3 times of maximum wavelength variation (3 × 0.03 = 0.09 nm) is defined as measurement limit. The response time of the developed fiber sensor is less than 30 min. The ultra-sensitive biosensor has potential to be widely applied to various areas such as disease, medical diagnostics and food safety inspection.

In this research work, optoelectronic properties of Indium bismuth selenide (InBi2Se4) thin films are studied for their potentials for photovoltaic applications. The InBi2Se4 films are prepared via a thermal co-evaporation technique on glass substrate using Bi2S3 powders and indium granules. The as-deposited films are then annealed at different temperatures to convert into InBi2Se4 thin films. Results show that the obtained InBi2Se4 films possess excellent optoelectronic properties as an optimum bandgap of 1.2 eV was obtained for the film annealed at 350 °C. Based on characterisation results of current and voltage realiationships, both as-deposited and annealed InBi2Se4 thin films show a linear relationship between current and annealing temperature. It was also noted that with increasing grain-size of the film, the current is also increased at a fixed applied voltage.

Versatile, in situ sensing and continuous monitoring capabilities are critically needed but challenging for components made of solid woven carbon fibers in aerospace, electronics and medical applications. In this work, we proposed a unique concept of integrated sensing technology on woven carbon fibers through integration of thin film surface acoustic wave (SAW) technology and electromagnetic metamaterials, with capabilities of non-invasive, in-situ and continuous monitoring of environmental parameters and biomolecules wirelessly. Firstly, we fabricated composite materials using a three-layer composite design, in which the woven carbon fiber cloth was firstly coated with a polyimide (PI) layer followed by a layer of ZnO film. Integrated SAW and metamaterials devices were then fabricated on this composite structure. Temperature of the functional area of the device can be controlled precisely using the SAW devices, which can provide a proper incubation environment for biosampling processes. As a demonstration for an ultraviolet light sensor, the SAW device could achieve a good sensitivity of 56.86 ppm/(mW∙cm-2). On the same integrated platform, the electromagnetic resonator based on the meta-materials has been demonstrated to work as a glucose concentration monitor with a sensitivity of 0.34 MHz/(mg/dL).

Acoustofluidic devices have been demonstrated effectively for liquid manipulation functionalities. Likewise, electromagnetic metamaterials have been employed as highly sensitive and wireless sensors. In this work, we introduced a new design combining the concepts of acoustofluidics and electromagnetic metamaterials on a single device realised on a flexible PVDF substrate. We characterise the operation of the device at acoustic and microwave frequencies. The device can be used in wearable biosensors with integrated liquid sampling and continuous wireless sensing capabilities.

A novel microfiber sensor is proposed and demonstrated based on a singlemode-tapered hollow core-singlemode (STHS) fiber structure. Experimentally a STHS with taper waist diameter of $26.5~\mu \text{m}$ has been fabricated and RI sensitivity of 816, 1601.86, and 4775.5 nm/RIU has been achieved with RI ranges from 1.3335 to 1.3395, from 1.369 to 1.378, and from 1.409 to 1.4175 respectively, which agrees very well with simulated RI sensitivity of 885, 1517, and 4540 nm/RIU at RI ranges from 1.3335 to 1.337, from 1.37 to 1.374, and from 1.41 to 1.414. The taper waist diameter has impact on both temperature and strain sensitivity of the sensor structure: (1) the smaller the waist diameter, the higher the temperature sensitivity, and experimentally 26.82 pm/°C has been achieved with a taper waist diameter of $21.4~\mu \text{m}$ ; (2) as waist diameter decrease, strain sensitivity increase and 7.62 pm/ $\mu \varepsilon $ has been achieved with a taper waist diameter of $20.3~\mu \text{m}$ . The developed sensor was then functionalized for human chorionic gonadotropin (hCG) detection as an example for biosensing application. Experimentally for hCG concentration of 5 mIU/ml, the sensor has 0.5 nm wavelength shift, equivalent to limit of detection (LOD) of 0.6 mIU/ml by defining 3 times of the wavelength variation (0.06 nm) as measurement limit. The biosensor demonstrated relatively good reproducibility and specificity, which has potential for real medical diagnostics and other applications.

A Love-mode surface acoustic wave (SAW) biosensor based on ST-cut quartz was developed for highly selective and label-free detection of carcinoembryonic antigen (CEA). The delay line area of an interdigital transducer (IDT) based SAW device was coated with gold and then chemically modified through thioglycolic acid–EDC/NHS reaction mechanism. A self-assembled monolayer of anti-CEA was further immobilized on the bioreceptors through the coupling layer. The biosensing capability of the SAW device was evaluated using solutions of CEA with various concentrations and limit of detection was obtained at 0.31 ng/ml of CEA, which is better than the results reported by the literatures available for CEA detection using SAW device. The real-time detection capability of the biosensor was evaluated using clinical serum samples and selectivity was evaluated using mixed solutions of CEA with other common tumor marking proteins. Long-term stability of the biosensor was also evaluated over a period of 30 days and the immunoassay response has shown only 8% decrease in performance within the whole period. The binding of CEA onto the bioreceptor was evaluated through Langmuir and Freundlich sorption isotherm kinetic studies as well.

Self-powered photodetectors with great potential for implanted medical diagnosis and smart communications have been severely hindered by the difficulty of simultaneously achieving high sensitivity and fast response speed. Here, we report an ultrafast and highly sensitive self-powered photodetector based on two-dimensional (2D) InSe, which is achieved by applying a device architecture design and generating ideal Schottky or ohmic contacts on 2D layered semiconductors, which are difficult to realize in the conventional semiconductors owing to their surface Fermi-level pinning. The as-fabricated InSe photodiode features a maximal lateral self-limited depletion region and a vertical fully depleted channel. It exhibits a high detectivity of 1.26 × 1013 Jones and an ultrafast response speed of ∼200 ns, which breaks the response speed limit of reported self-powered photodetectors based on 2D semiconductors. The high sensitivity is achieved by an ultralow dark current noise generated from the robust van der Waals (vdW) Schottky junction and a high photoresponsivity due to the formation of a maximal lateral self-limited depletion region. The ultrafast response time is dominated by the fast carrier drift driven by a strong built-in electric field in the vertical fully depleted channel. This device architecture can help us to design high-performance photodetectors utilizing vdW layered semiconductors.

Gallium nitride (GaN) is a compound semiconductor which has advantages to generate new functionalities and applications due to its piezoelectric, pyroelectric, and piezo-resistive properties. Recently, surface acoustic wave (SAW) based acoustic tweezers were developed as an efficient and versatile tool to manipulate nano- and micro-particles aiming for patterning, separating and mixing biological and chemical components. Conventional piezoelectric materials to fabricate SAW devices such as lithium niobate suffers from its low thermal conductivity and incapability of fabricating multiphysical and integrated devices. This work piloted the development of a GaN-based Acoustic Tweezer (GaNAT) and its application in manipulating micro-particles and biological cells. For the first time, the GaN SAW device was integrated with a microfluidic channel to form an acoustofluidic chip for biological applications. The GaNAT demonstrated its ability to work on high power (up to 10W) with minimal cooling requirement while maintaining the device temperature below 32C. Acoustofluidic modelling was successfully applied to numerically study and predict acoustic pressure field and particle trajectories within the GaNAT, which agree well with the experimental results on patterning polystyrene microspheres and two types of biological cells including fibroblast and renal tumour cells. The GaNAT allowed both cell types to maintain high viabilities of 84.5% and 92.1%, respectively.

In this paper, we propose and experimentally demonstrate for the first time, the integration of a radio-over-fiber (RoF) communication system and a Brillouin optical time-domain reflectometry (BOTDR) distributed sensor system using a single optical fiber link. In this proof-of-concept integrated system, the communication system is composed of three modulation formats of quadrature phase-shift keying (QPSK), 16-quadrature amplitude modulation (16-QAM) and 64-QAM, which are modulated onto an orthogonal frequency division multiplexing (OFDM) signal. Whereas, the BOTDR sensor system is used for strain and/or temperature monitoring over the fiber distance with a spatial resolution of 5 m using a 25 km single-mode silica fiber. The error vector magnitude (EVM) is analyzed in three modulation formats in the presence of various BOTDR input pump powers. Using QPSK modulation, optimized 18 dBm sensing and 10 dBm data power, the measured EVM values with and without bandpass filter are 3.5% and 14.5%, respectively. The proposed system demonstrates a low temperature measurement error (&plusmn, 0.49 &deg, C at the end of 25 km) and acceptable EVM values, which were within the 3GPP requirements. The proposed integrated system can be effectively applied for practical applications, which significantly reduces the fiber infrastructure cost by effective usage of a single optical fiber link.

A Love mode surface acoustic wave (SAW) humidity sensor based on bacterial cellulose (BC) coated ST-cut quartz was developed in this study. The BC film is composed of ultrafine interwoven fibers to form a highly porous network, and its surface contains a large amount of hydroxyl groups, which significantly improve the adsorption capability of SAW sensing layer for water molecules. This results in significant mass loading effects and enhanced responsivity of the SAW sensor. The resonant frequency of the sensor changes linearly with RH at lower relative humidity (RH) values (e.g., RH30%), but when RH80%, an exponential increase in frequency shift as a function of RH is obtained due to the enhanced mass loading effect. A frequency shift of 89.8 kHz was measured using a sensor with a BC film with a thickness of 148 nm thick when the RH was increased from 30% to 93%. The frequency of the sensor can be fully shifted back to the original reading when the RH was returned back to 30%, with the response and recovery times of 12 s and 5 s, respectively. The SAW sensor also exhibits good short-term repeatability and long-term stability for humidity sensing.

It is a huge challenge to develop a highly precision sensor with good selectivity for target gas. In this work, In2O3 nano-cubes, prepared using a cetyltrimethyl ammonium bromide assisted solvothermal process, were used to make gas sensors for H2S and NO2 detections. The In2O3 nano-cube based sensor exhibited a good temperature-dependent selectivity toward H2S and NO2. At room temperature of 25 °C, the sensor exhibited a good selectivity towards H2S with a high response (1461 for 60 ppm H2S), fast response/recovery times (82 s/102 s) and a superior detection limit (0.005 ppm). Whereas at an operation temperature of 100 °C, the sensor showed a poor sensitivity to H2S, but an excellent selectivity towards NO2 with high response (336 for 100 ppm NO2), fast response/recovery times (18 s/31 s) and a superior detection limit (0.001 ppm). The sensor also showed good reversibility, reproducibility and long-term stability at two optimized operation temperatures. The different sensing mechanisms for H2S and NO2 were discussed and the temperature dependent selectivity was explained.

Ternary copper sulphides, especially copper-bismuth-sulphide (Cu-Bi-S), are alternative solar absorber materials due to their earth-abundant and non-toxic constituent elements, compared to the conventional copper indium gallium sulphide and cadmium telluride films. In this study, Cu-Bi-S thin films were deposited onto soda lime glass substrates using a one stage co-evaporation process from Cu2S and Bi2S3 sources, with the deposition temperatures varied from room temperature to 400 °C. X-ray diffraction analysis confirmed that Cu3BiS3 was the dominant phase in the Cu-rich films, and the crystalline quality of the films was significantly improved with increasing the deposition temperature. An optical bandgap of 1.4 eV was achieved for the film deposited at 400 °C, which demonstrated a Hall mobility of 3.95 cm2/V-s and a carrier concentration of 7.48 × 1016 cm−3. Cu3BiS3 films deposited at 375 and 400 °C were implemented into superstrate solar cell structures (glass/ITO/n-CdS/p-Cu3BiS3/Al).

Multi-shape memory effect (multi-SME) in amorphous shape memory polymers (SMPs) linked with collective and cooperative rearrangements and accommodations of monomeric segments, thus leading to generation of complex thermodynamic modes. In this study, an extended domain size model is initially formulated to describe various temperature-dependent relaxation behaviors and domain transitions in amorphous SMPs. According to the Adam-Gibbs theory, a cooperative model is employed to identify the principle role of domain size in the collective dynamics of multi-SME in amorphous SMPs. The phase transition theory is then combined with multi-branch Kelvin model to describe the collective and cooperative relaxation behaviors of the SMPs with multiple transition domains. It is shown that the proposed model is able to characterize the thermomechanical transitions and multiple shape recovery processes. Finally, the model is applied to predict shape recovery behavior of SMPs with triple- and quadruple-SME, respectively, and the theoretical results are well validated by the experimental ones.

ZnO-Al2O3 nanocomposite was synthesized and developed as a high performance sensitive and selective layer for surface acoustic wave (SAW) sensor, aiming for in-situ detection of H2S gas in ppb level operated at room temperature. ZnO-Al2O3 nanocomposite, synthesized though a sol-gel method, was spin-coated onto a quartz based SAW resonator. This composite layer inherits the mesoporous structure of the Al2O3 layer and good affinity to H2S gas molecules of the ZnO layer, and thus can selectively adsorb and react with H2S gas molecules to form ZnS compounds on its surface. This reaction leads to significant decreases of both pore sizes and total pore volume of the layer, an increase of layer’s elastic modulus, thus causing a large positive shift of the frequency responses of the SAW sensor. The sensor operated at room temperature shows a frequency response of ∼500 Hz to 10 ppb H2S, with an excellent selectivity and good recovery property.

A nanostructural catalyst with long-term durability under harsh conditions is very important for an outstanding catalytic performance. Herein, a new ultrastable PtCo/Co3O4-SiO2 nanocatalyst was explored to improve the catalytic performance of carbon monoxide (CO) oxidation by virtue of the surface active lattice oxygen derived from strong metal-support interactions. Such a structure can overcome the issues of Co3O4-SiO2 inactivation by water vapor and the Pt inferior activity at low temperature. Further, Co3O4-SiO2 nanosheets endow superior structure stability under high temperatures of up to 800 °C, which gives long-term catalytic cyclability of PtCo/Co3O4-SiO2 nanocomposites for CO oxidation. Moreover, the large specific surface areas (294 m2 g-1) of the nanosheet structure can expose abundant surface active lattice oxygen, which significantly enhanced the catalytic activity of CO oxidation at 50 °C over 30 days without apparent aggregation of PtCo nanoparticles after 20 cycles from 50 to 400 °C. It can be expected to be a promising candidate as an ultrastable efficient catalyst.

Due to its good chemical stability and outstanding redox properties, CeO2 has been regarded as a promising electrode material for supercapacitors, but its specific capacity is quite low which restricts its wide-range applications. To enhance its specific capacity, in this study, specially designed mesoporous Zr-doped CeO2 nanostructures with large surface area, extraordinarily high porosity and abundant oxygen vacancies were fabricated using a hydrothermal method and an assisted calcination process. The synthesized mesoporous CeO2-Zr-1 nanostructures (with an atomic ratio of Ce:Zr = 10:1) were composed of nanocrystals with an average size of 6.7 nm, and had a large surface area of 81.0 m2 g−1, and abundant mesopores with a volume of 0.2108 cm3 g−1. In 2 M KOH electrolyte, the CeO2-Zr-1 electrode generated a much larger specific capacity (448.1 C g−1) than that of the pristine CeO2 (249.3 C g−1) at a current density of 1 A g−1. An asymmetric supercapacitor of CeO2-Zr-1//activated carbon produced a high energy storage density of 23.3 Wh kg−1 at 398.5 W kg−1, and an excellent long-term cycling stability with 96.4% capacity retention after 6000 cycles.