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Currently, most biometric methods mainly use single features, making them easily forged and cracked. In this study, a novel triple-layers biometric recognition method, based on photoacoustic microscopy, is proposed to improve the security of biometric identity recognition. Using the photoacoustic (PA) dermoscope, three-dimensional absorption-structure information of the fingers was obtained. Then, by combining U-Net, Gabor filtering, wavelet analysis and morphological transform, a lightweight algorithm called photoacoustic depth feature recognition algorithm (PADFR) was developed to automatically realize stratification (the fingerprint, blood vessel fingerprint and venous vascular), extracting feature points and identity recognition. The experimental results show that PADFR can automatically recognize the PA hierarchical features with an average accuracy equal to 92.99%. The proposed method is expected to be widely used in biometric identification system due to its high security. This article is protected by copyright. All rights reserved.

Synchronized torrefaction and pelleting (STP) was developed as a laboratory scale process to produce torrefied pellets with a single biomass material loading. Two fuel upgrading actions (torrefaction and pelleting) happened simultaneously with the assistance of ultrasonic vibration. It was found that STP elevated biomass temperature to above 200 °C in less than one minute and initiated torrefaction while biomass was pelletized in a mold. A feasibility study showed that STP consistently produced torrefied pellets with improved physical, thermochemical, and hygroscopic properties as an upgraded fuel for biomass co-combustion. STP was effective at enhancing the density and durability of torrefied pellets. Elemental analysis of torrefied biomass material showed increased carbon content, indicating higher heating values of torrefied pellets over non-torrefied biomass. Thermogravimetric analysis and Fourier-transform infrared analysis revealed loss of hydrogen and oxygen-rich matters during STP. Finally, greater hydrophobicity of torrefied pellets was exhibited by less water and vapor absorption compared with non-torrefied biomass.

Helical structures exhibit novel optical and mechanical properties and are commonly used in different fields such as metamaterials and microfluidics. A few methods exist for fabricating helical microstructures, but none of them has the throughput or flexibility required for patterning a large surface area with tunable pitch. In this paper, we report a method for fabricating helical structures with adjustable forms over large areas based on multiphoton polymerization (MPP) using single-exposure, three dimensionally structured, self-accelerating, axially tunable light fields. The light fields are generated as a superposition of high-order Bessel modes and have a closed-form expression relating the design of the phase mask to the rotation rate of the beam. The method is used to fabricate helices with different pitches and handedness in the material SU-8. Compared to point-by-point scanning, the method reported here can be used to reduce fabrication time by two orders of magnitude, paving the way for adopting MPP in many industrial applications.

Biofuel production needs to be more efficient than its current status to increase its competitiveness. The multistep biofuel production is consisted of processes on biomass preprocessing and bioconversion stages. As a crucial parameter, biomass particle size has significant effects on both stages. It is essential to have an insightful understanding of the effects of particle size on sugar yield. Although numerous studies have been performed to meet this objective, there is no commonly accepted guideline on how to select particle size. One possible reason for this gap is the effects of particle size vary when different biomass pretreatment methods are employed. In this study, an assessment on the relationship between particle size and sugar yield was performed for four pretreatment methods. Three particle sizes (1, 4, and 8 mm) of corn stover and switchgrass biomass were used in supercritical CO2, dilute acid (H2SO4), dilute alkaline (Na2CO3), and metal oxide (MgO) pretreatments. Biomass compositional analyses were conducted before and after each pretreatment. Pretreatment solid recovery and sugar recovery rates were calculated. Enzymatic hydrolysis sugar yield and efficiency were used to evaluate the performance of hydrolysis and total sugar yield was used to interpret how much sugar a unit dry weight of biomass (before pretreatment) can yield through pretreatment and enzymatic hydrolysis combined. It was found that particle size was a weak indicator of enzymatic hydrolysis efficiency. There was little value in reducing particle size below 8 mm in order to overcome the resistance imposed by biomass structure on cellulose and xylan hydrolysis.

Low-energy and volumetric density of biomass has been a major challenge, hindering its large-scale utilization as a bioenergy resource. Torrefaction is a thermochemical pretreatment process that can significantly enhance the properties of biomass as a fuel by increasing the heating value and thermal stability of biomass materials. Densification of biomass by pelleting can greatly increase the volumetric density of biomass to improve its handling efficiency. Currently, torrefaction and pelleting are processed separately. So far, there has been little success in dovetailing torrefaction and pelleting, which only requires a single material loading to produce torrefied pellets. Synchronized ultrasonic torrefaction and pelleting has been developed to address this challenge. Synchronized ultrasonic torrefaction and pelleting can produce pellets of high energy and volumetric density in a single step, which tremendously reduces the time and energy consumption compared to that required by the prevailing multistep method. This novel fuel upgrading process can increase the biomass temperature to 473–573 K within tens of seconds to create torrefaction. Studying the temperature distribution is crucial to understand the fuel upgrading mechanism since pellet energy density, thermal stability, volumetric density, and durability are all highly related to temperature. A rheological model was established to instantiate biomass behaviors when undergoing various ultrasonic vibration conditions. Process parameters including ultrasonic amplitude, ultrasonic frequency, and pelleting time were studied to show their effects on temperature at different locations in a pellet. Results indicated that the volumetric heat generation rate was greatly affected by both ultrasonic amplitude and frequency. This model can help to understand the fuel upgrading mechanism in synchronized ultrasonic torrefaction and pelleting and also to give guidelines for process optimization to produce high-quality fuel pellets.

In this paper, we investigate thermal rectification in hybrid pristine and silicon-functionalized graphene nanoribbons (GNRs) using nonequilibrium molecular dynamics simulations. The heat flux is larger from the functionalized section to the pristine section than that in the opposite direction. It is found that thermal rectification can be tuned through the Si/C ratio and silicon distribution. A moderate Si/C ratio and patterned arrangement are preferable to obtain a higher rectification factor. The rectification factor is also sensitive to the temperature difference and the mean temperature. As the length of the system increases, the rectification factor decreases gradually due to the emerging of diffusive effect in nanoscale GNRs. Furthermore, we calculate the vibrational density of states to explain the underlying mechanism of thermal rectification.

Microfabrication based on photopolymerization is typically achieved by scanning a focal spot within the material point by point, which significantly limits the fabrication speed. In our previous study, the authors explored a method for rapid fabrication of high-aspect-ratio micro- and nanostructures by scanning the Bessel beam in the plane transverse to the direction of beam propagation. However, the structure fabricated by this method suffers from the surface texture. In this work, the origin of these effects is investigated by the in situ measurement of the photopolymerization process. By scanning the laser beam at a speed faster than the polymerization that takes place at any given position, we show that it is possible to eliminate the surface texture and obtain smooth surface finish.

The thermal boundary resistance (TBR) between SiO2-GaN film-sapphire is reconstructed by 3 omega method in the temperature range of 260-480 K. The TBR at SiO2/GaN film interface is obtained on different samples with different thickness SiO2 films at high frequency of 1 KHz-20 KHz and the TBR at GaN film/sapphire interface is isolated by extending the frequency to a wide range. The measured TBR with strong temperature dependence at GaN film/sapphire interface increases with increasing temperature, and is found to be 1.02 x 10(-7) m(2) K W-1 at room temperature. The measured TBR with weak temperature dependence at SiO2/GaN film interface decreases with increasing temperature. The temperature dependence of the TBR differs appreciably from the predicted trend by phonon inelastic scattering model and may be affected partially by the lack of some sources of inelastic boundary scattering and the increased diffusive boundary scattering. The predicted results corrected by phonon attenuation process agree well with the experimental values especially at high temperature. The temperature dependence and the larger value of the TBR contain contributions from the near-interface effect including the real boundary between the GaN film and its adjacent layers and the phonon inelastic scattering near the interface region. (C) 2014 Elsevier Masson SAS. All rights reserved.