Showing papers by "Dong Liu published in 2017"

Low Power Consumption Gas Sensor Created from Silicon Nanowires/TiO2 Core–Shell Heterojunctions

Abstract: Silicon nanowires/TiO2 (SiNWs/TiO2) array with core–shell nanostructure was created by sol–gel and drop-casting methods. The hybrid material displayed excellent sensing performance for CH4 detection at room temperature. The chemiresistor sensor has a linear response toward CH4 gas in the 30–120 ppm range with a detection limit of 20 ppm, which is well below most CH4 sensors reported before. The enhanced gas sensing performance at room temperature was attributed to the creation of heterojunctions that form a depletion layer at the interface of SiNWs and TiO2 layer. Adsorption of oxygen and corresponding gas analyte on TiO2 layer could induce the change of depletion layer thickness and consequently the width of the SiNWs conductive channel, leading to a sensitive conductive response toward gas analyte. Compared to conventional metal oxide gas sensors, the room temperature gas sensors constructed from SiNWs/TiO2 do not need an additional heating device and work at power at the μW level. The low power consump...

Distinguishing thermal lens effect from electronic third-order nonlinear self-phase modulation in liquid suspensions of 2D nanomaterials.

Abstract: The interaction of light with atomically thin nanomaterials has attracted enormous research interest in order to understand two-dimensional (2D) electron systems and develop novel opto-electronic devices. The observations of spatial self-phase modulation and the associated multiple diffraction ring patterns in liquid suspensions of 2D nanomaterials are believed to be excellent examples of strong laser interaction with 2D nanomaterials and this phenomenon has been attributed to their large electronic third-order susceptibilities. By performing a series of control experiments with liquid suspensions of graphene and graphene oxide flakes in different solvents at various temperatures under an increasing modulation frequency of laser illumination, we first show that the diffraction ring pattern has little dependence on the type of nanomaterial but strongly depends on the duration of laser illumination. A laser induced local refractive index change is then monitored by a weaker probe beam, resulting in the divergent diffraction of the probe beam that indicates a lower self-induced refractive index in the center of the pump laser beam than at its periphery: a clear signature of the thermal lens effect. Finally, we use computational fluid dynamics to simulate laser induced temperature and index changes of the suspensions. The evolution of diffraction rings is well correlated to the transient temperature distribution. Our understanding of complex laser interactions with nanomaterial suspensions and the associated thermal lens effect paves the way for further basic studies and fluid opto-electronic applications of 2D nanomaterials.

Laser streaming: Turning a laser beam into a flow of liquid

Abstract: Transforming a laser beam into a mass flow has been a challenge both scientifically and technologically. We report the discovery of a new optofluidic principle and demonstrate the generation of a steady-state water flow by a pulsed laser beam through a glass window. To generate a flow or stream in the same path as the refracted laser beam in pure water from an arbitrary spot on the window, we first fill a glass cuvette with an aqueous solution of Au nanoparticles. A flow will emerge from the focused laser spot on the window after the laser is turned on for a few to tens of minutes; the flow remains after the colloidal solution is completely replaced by pure water. Microscopically, this transformation is made possible by an underlying plasmonic nanoparticle-decorated cavity, which is self-fabricated on the glass by nanoparticle-assisted laser etching and exhibits size and shape uniquely tailored to the incident beam profile. Hydrophone signals indicate that the flow is driven via acoustic streaming by a long-lasting ultrasound wave that is resonantly generated by the laser and the cavity through the photoacoustic effect. The principle of this light-driven flow via ultrasound, that is, photoacoustic streaming by coupling photoacoustics to acoustic streaming, is general and can be applied to any liquid, opening up new research and applications in optofluidics as well as traditional photoacoustics and acoustic streaming.

Crosswalk navigation for people with visual impairments on a wearable device

Abstract: Detecting and reminding of crosswalks at urban intersections is one of the most important demands for people with visual impairments. A real-time crosswalk detection algorithm, adaptive extraction and consistency analysis (AECA), is proposed. Compared with existing algorithms, which detect crosswalks in ideal scenarios, the AECA algorithm performs better in challenging scenarios, such as crosswalks at far distances, low-contrast crosswalks, pedestrian occlusion, various illuminances, and the limited resources of portable PCs. Bright stripes of crosswalks are extracted by adaptive thresholding, and are gathered to form crosswalks by consistency analysis. On the testing dataset, the proposed algorithm achieves a precision of 84.6% and a recall of 60.1%, which are higher than the bipolarity-based algorithm. The position and orientation of crosswalks are conveyed to users by voice prompts so as to align themselves with crosswalks and walk along crosswalks. The field tests carried out in various practical scenarios prove the effectiveness and reliability of the proposed navigation approach.

Combustion Characteristics of Single Particles from Bituminous Coal and Pine Sawdust in O2/N2, O2/CO2, and O2/H2O Atmospheres

Abstract: Burning fuels in an O2/H2O atmosphere is regarded as the next generation of oxy-fuel combustion for CO2 capture and storage (CCS). By combining oxy-fuel combustion and biomass utilization technology, CO2 emissions could be further reduced. Therefore, this work focuses on investigating the combustion characteristics of single particles from bituminous coal (BC) and pine sawdust (PS) in O2/N2, O2/CO2 and O2/H2O atmospheres at different O2 mole fractions (21%, 30%, and 40%). The experiments were carried out in a drop tube furnace (DTF), and a high-speed camera was used to record the combustion processes of fuel particles. The combustion temperatures were measured by a two-color method. The results reveal that the particles from BC and PS all ignite homogeneously. Replacing N2 by CO2 results in a longer ignition delay time and lower combustion temperatures. After substituting H2O for N2, the ignition delay time is shortened, which is mainly caused by the steam gasification reaction (C + H2O → CO + H2) and steam shift reaction (CO + H2O → CO2 + H2). In addition, the combustion temperatures are first decreased at low O2 mole fractions, and then increased at high O2 mole fractions because the oxidation effect of H2O performs a more important role than its volumetric heat capacity and thermal radiation capacity. At the same condition, particles from PS ignite earlier because of their higher reactivity, but the combustion temperatures are lower than those of BC, which is owing to their lower calorific values.

Nanostructure and Oxidation Reactivity of Nascent Soot Particles in Ethylene/Pentanol Flames

Abstract: As byproducts of the combustion process of hydrocarbon fuels, soot particles are difficult to remove, and they can greatly harm human health and pollute the environment. Therefore, the formation and growth processes of the soot particles has become a study focus of researchers. In this paper, the nanostructure and oxidation reactivity of carbonaceous particles collected from ethylene inverse diffusion flames with or without the additions of three pentanol isomers (1-pentanol, 3-methyl-1-butanol, and 2-methyl-1-butanol) were investigated in detail. The nanostructure and oxidation characteristics of nascent soot particles were characterized using high resolution transmission electron microscopy (HRTEM), X-ray diffractometry (XRD) and thermogravimetric analysis (TGA). It was found that the nascent soot cluster of pure ethylene flame had a loose structure, while the additions of pentanol isomers made the soot agglomerates more compact and delayed the growth of graphitic structures. The pentanol isomer additions also contributed to a higher disorder of the crystallite arrangement in the soot nanostructure. According to the TGA experiments, the results showed that the addition of pentanol isomers enhanced the oxidation reactivity of soot particles, which could help to reduce soot particle emissions.

Simple Rectangular Gratings as a Near-Field "Anti-Reflection" Pattern for GaSb TPV Cells.

Abstract: We show theoretically that 2D rectangular gratings on the surface of GaSb can serve as an “anti-reflection” pattern for nano-gap thermophotovoltaic (TPV) devices, which significantly enhances near-field radiative flux from the emitter to a GaSb cell, thus improving output power and conversion efficiency. The system in this study is a 200-nm gap TPV power generation system with a planar infrared plasmonic emitter and GaSb cell. Rigorous coupled-wave analysis is used to calculate the spectral near-field radiative flux involving periodic structures. The simulation shows that when coupled with a near-infrared plasmonic bulk emitter, adding gratings on the GaSb cell surface results in strong spectral enhancement above the cell’s bandgap and suppression for low-energy photon transmission, an effect that cannot be fully predicted by the effective medium theory. The resultant peak spectral heat flux is 2.8 times as high as the case without surface structures and the radiative transfer efficiency increased to 24.8% from the original 14.5% with the emitter temperature at 1800 K. The influence of the grating’s geometry parameters on the enhancement and peak frequency is further discussed with rigorous calculation of the spatial distribution of thermal radiative transfer that provided insight into the physical mechanism.

Perturbation of formate pathway and NADH pathway acting on the biohydrogen production

Abstract: The formate pathway and NADH pathway as two common hydrogen-producing metabolic pathways have been well characterized to understand and improve biohydrogen production. These two pathways have been thought to be separate and have been independently investigated. However, in this study, perturbation of genes (hycA, fdhF, fhlA, ldhA, nuoB, hybO, fdh1, narP, and ppk) in Enterobacter aerogenes related to the formate pathway or NADH pathway revealed that these two pathways affected each other. Further metabolic analysis suggested that a linear relationship existed between the relative change of hydrogen yield in the formate pathway or NADH pathway and the relative change of NADH yield or ATP yield. Thus, this finding provides new insight into the role of cellular reducing power and energy level in the hydrogen metabolism. It also establishes a rationale for improving hydrogen production from a global perspective.

The pyrolysis and gasification performances of waste textile under carbon dioxide atmosphere

Abstract: In order to obtain the waste textiles’ pyrolysis and the corresponding char’ gasification performance and mechanism under CO2 atmosphere, the pyrolysis and gasification tests of different kinds of waste textiles (polyester fiber (PF), cotton and woolen) were conducted in thermogravimetric-Fourier transform infrared spectrometer at atmospheric pressure. Scanning electron microscopy and Raman spectrometry were used to characterize the char structure. The pyrolysis and gasification processes were quantitatively evaluated by kinetic analysis using the shrinking core model. Results indicated that the pyrolysis and gasification characteristic of the three textiles under CO2 atmosphere were different because the kinetic and thermal dynamic of the three textiles in pyrolysis and gasification processes were different. In addition, the apparent pyrolysis activation energies for PF, cotton and woolen were 162.38, 103.05 and 110.02 kJ mol−1, respectively, while the gasification activation energies were 178.15, 144.49 and 79.37 kJ mol−1, respectively, which were in good agreement of the Raman tests.

Design of iodine absorption cell for high-spectral-resolution lidar.

Abstract: Iodine absorption cells are extensively employed by high-spectral-resolution Lidars (HSRLs) for aerosol optical properties and atmosphere state parameters profiling. To the best of our knowledge, the optimal design of the parameters of iodine cells has not been talked about systematically. In this paper, a heuristic method based on multi-objective concept is proposed for the design of iodine cells employed in HSRLs for aerosol profiling, and the method can be also applied to different types of HSRLs. The bi-objective model is established based on the retrieval error analysis of HSRL and then the Pareto optimal solutions are obtained through the Non-dominated Sorting Genetic Algorithm II (NSGA-II). The performance of different absorption lines are compared according to the Pareto solution sets, and the stability of transmittance characteristics of different absorption lines are discussed through sensitivity analysis. The results are expected to provide guidance for the design of HSRLs based on iodine absorption filters.

IR stereo RealSense: Decreasing minimum range of navigational assistance for visually impaired individuals

Abstract: Introduction of RGB-D sensors is a revolutionary force that offers a portable, versatile and cost-effective solution of navigational assistance for the visually impaired. RGB-D sensors on the market such as Microsoft Kinect, Asus Xtion and Intel RealSense are mature products, but all have a minimum detecting distance of about 800mm. This results in the loss of depth information and the omission of short-range obstacles, posing a significant risk on navigation. This paper puts forward a simple and effective approach to reduce the minimum range that enhances the reliability and safety of navigational assistance. Over-dense regions of IR speckles in two IR images are exploited as a stereo pair to generate short-range depth, as well as fusion of original depth image and RGB image to eliminate misjudgment. Besides, a seeded growing algorithm of obstacle detection with extended depth information is presented. Finally, the minimum range of Intel RealSense R200 is decreased by approximately 75%, from 650mm to 165mm. Experiment results show capacity of detecting obstacles from 165mm to more than 5000mm and improved performance of navigational assistance with expansion of detection range. The presented approach proves to be of qualified accuracy and speed for guiding the visually impaired.

On the Response of Nascent Soot Nanostructure and Oxidative Reactivity to Photoflash Exposure

Abstract: Soot particles are a kind of major pollutant from fuel combustion. To enrich the understanding of soot, this work focuses on investigating detailed influences of instantaneous external irradiation (conventional photoflash exposure) on nanostructure as well as oxidation reactivity of nascent soot particles. By detailed soot characterizations flash can reduce the mass of soot and soot nanostructure can be reconstructed substantially without burning. After flash, the degree of soot crystallization increases while the soot reactive rate decreases and the activation energy increases. In addition, nanostructure and oxidative reactivity of soot in air and Ar after flash are different due to their different thermal conductivities.

Retrieving the seawater volume scattering function at the 180° scattering angle with a high-spectral-resolution lidar.

Abstract: A high-spectral-resolution lidar (HSRL) is proposed to retrieve the seawater volume scattering function at the 180° scattering angle βπ without the assumption of the lidar extinction-to-backscatter ratio. A field-widened Michelson interferometer is employed as the ultra-narrow spectral discriminator to reject particulate scattering and molecular Rayleigh scattering but transmit molecular Mandelshtam-Brillouin scattering. The theoretical framework to retrieve βπ is presented in detail based on a dual-channel HSRL configuration. Simulation on the retrieval and error estimation shows that, the proposed oceanographic HSRL based on the ship or aircraft can perform well to extract the profile of βπ and has a real potential in the oceanographic remote sensing.

Use of Debye’s series to determine the optimal edge-effect terms for computing the extinction efficiencies of spheroids

Abstract: The extinction efficiencies of atmospheric particles are essential to determining radiation attenuation and thus are fundamentally related to atmospheric radiative transfer. The extinction efficiencies can also be used to retrieve particle sizes or refractive indices through particle characterization techniques. This study first uses the Debye series to improve the accuracy of high-frequency extinction formulae for spheroids in the context of Complex angular momentum theory by determining an optimal number of edge-effect terms. We show that the optimal edge-effect terms can be accurately obtained by comparing the results from the approximate formula with their counterparts computed from the invariant imbedding Debye series and T-matrix methods. An invariant imbedding T-matrix method is employed for particles with strong absorption, in which case the extinction efficiency is equivalent to two plus the edge-effect efficiency. For weakly absorptive or non-absorptive particles, the T-matrix results contain the interference between the diffraction and higher-order transmitted rays. Therefore, the Debye series was used to compute the edge-effect efficiency by separating the interference from the transmission on the extinction efficiency. We found that the optimal number strongly depends on the refractive index and is relatively insensitive to the particle geometry and size parameter. By building a table of optimal numbers of edge-effect terms, we developed an efficient and accurate extinction simulator that has been fully tested for randomly oriented spheroids with various aspect ratios and a wide range of refractive indices.

Combustion characteristics of nanofluid fuels in a half-opening slot tube

Abstract: Combustion characteristics of nanofluid fuels containing aluminum nanoparticles were investigated in half-opening slot tubes from the fundamental view. The effects of particle loading rates (0.25% and 2.5% by weight), type of base fuels (ethanol and butanol), and fuel flow rates (0.2, 0.6, and 1 mL/min) were studied in details. The combustion characteristics of the nanofluid fuels and pure based fuels were also examined to provide a comparison. Flame was unstable with reignition, stable state, nearly extinguishment repeatedly at low flow rate. At medium flow rate, flame height was increased and flame tended to be stable. At high flow rate, flame became unstable and was disturbed by the droplet forming and dripping significantly. Al atoms inside the oxide layer should be melted before the particles combustion, while Al oxide layer should be melted before the particles aggregates combustion. The effects of particles on the combustion characteristics, especially on the evaporation rate of base fuel, were discussed. The reasons for various combustion phenomena of nanofluid fuels were given, which can provide the useful guidance for the experimental research and practical applications of nanofluid fuels.