Search

In this paper, the effects of temperature, organic solvents (benzene and toluene) and halogen salts on the electric conductivity of the four kinds of quaternary ammonium salt-type AlCl3 ionic liquids, including phenyltrimethylammonium chloride, benzyltrimethylammonium chloride, benzyltriethylammonium chloride, benzyltributylammonium chloride, and tetramethylammonium chloride, were studied. The experimental results showed that the conductivity of each ionic liquid system tested rises with increasing temperature. Among all the tested systems, the TMPAC-AlCl3 system shows the highest conductivity and lowest value of apparent activation energy calculated according to the Arrhenius equation of 19.29 kJ/mol. The addition of benzene (or toluene or ethylbenzene) to ionic liquids induces a considerable increase of the electric conductivity in mixed systems with a rising volume fraction of benzene (or toluene or ethylbenzene), and then when each kind of mixed system has about 50 vol % of benzene (or toluene or ethylbenzene), its electric conductivity generally reaches the highest value of 14.5 mS/cm. Among all of the alkaline halides tested, the solubility of LiCl in TMPAC-AlCl3 has the highest value, which has a maximum value of approximately 50 g L‒1. At temperatures above 60°C, the electrolyte containing 2 g LiCl has the highest conductivity of 7.45 mS/cm, when the concentration of LiCl in the electrolyte is close to saturation. [ABSTRACT FROM AUTHOR]

Isothermal extrusion is an important method for ensuring the quality of extruded profiles. On the basis of the actual indirect extrusion process of 7075 aluminum alloy with high strength and toughness, this study used the combination of a finite element model and deep neural network to construct a prediction model for the temperature series of the indirect extrusion outlet. The extrusion velocity–stroke control curves of the isothermal extrusion of aluminum profiles were obtained on the premise of ensuring maximum production efficiency and the absence of defects. Results showed that the constructed model had high robustness and performed well on the validation set with an average error of 1.8% and that the temperature difference at the outlet of the profile obtained through isothermal extrusion would not exceed 5 °C. By using machine learning, finite element simulation or experiments can be omitted from process optimization, and the indirect isothermal extrusion process can be rapidly optimized. Then, consistency along the extrusion direction was proven by microstructure evolution simulation. [ABSTRACT FROM AUTHOR]

In the electronics industry environment, rapid recognition of objects to be grasped from digital images is essential for visual guidance of intelligent robots. However, electronic components have a small size, are difficult to distinguish, and are in motion on a conveyor belt, making target detection more difficult. For this reason, the YOLOv4-tiny method is used to detect electronic components and is improved. Then, different network structures are built for the adaptive integration of middle- and high-level features to address the phenomenon in which the original algorithm integrates all feature information indiscriminately. The method is deployed on an electronic component dataset for validation. Experimental results show that the accuracy of the original algorithm is improved from 93.74 to 98.6%. Compared with other current mainstream algorithms, such as Faster RCNN, SSD, RefineDet, EfficientDet, and YOLOv4, the method can maintain high detection accuracy at the fastest speed. The method can provide a technical reference for the development of manufacturing robots in the electronics industry. [ABSTRACT FROM AUTHOR]

The green bacterial biosynthesis of lead sulfide nanocrystallites by l-cysteine-desulfurizing bacterium Lysinibacillus sphaericus SH72 was demonstrated in this work. Nanocrystals formed by this bacterial method were characterized using the mineralogical and morphological approaches. The results revealed that the microbially synthesized PbS nanocrystals assume a cubic structure, and are often aggregated as spheroids of about 105 nm in size. These spheroids are composed of numerous nanoparticles with diameter 5–10 nm. Surface characterization of the bacterial nanoparticles with FTIR spectroscopy shows that the l-cysteine coats the surface of PbS nanoparticle as a stabilizing ligand. The optical features of the PbS nanocrystallites were assessed by UV–Vis spectroscopy and PL spectroscopy. The maximum absorption wavelength of the bacterial PbS particles occurs at 240 nm, and the photoluminescence emission band ranges from 375 to 550 nm. The band gap energy is calculated to be 4.36 eV, compared to 0.41 eV for the naturally occurring bulk PbS, with this clear blue shift attributable to the quantum size effect. [ABSTRACT FROM AUTHOR]

Vegetation plays an important role in the energy exchange, water cycle, carbon cycle, biogeochemical cycle, and maintenance of surface ecosystems. In recent years, regional vegetation cover has changed significantly. This study used statistical analyses, including the Mann-Kendall trend test, the Hurst exponent, and Pettitt test, to analyze the characteristics of temporal and spatial variation of vegetation coverage in the Xijiang River basin from 2000 to 2013. The results showed that vegetation coverage of 98.76% of the Xijiang River basin is weakly variable (Cv < 0.1). The area with significantly increased vegetation accounts for 43.45% of the total area (p < = 0.05). A total of 19.47% of vegetation coverage in the Xijiang River basin had significant change-points from 2004 to 2008 (p < = 0.05), and the area of concave change-points accounted for 25.99% of the total area of point increased the vegetation coverage. At an altitude of 500–2000 m, the altitude has an inhibitory effect on vegetation coverage. When the slope is less than 35 degrees, the slope has a promoting effect on vegetation coverage. Rich precipitation resources are the main source of soil water supply, and higher temperature provides better thermal energy resources, which may have a significant impact on vegetation growth in the future and cause time lag effects of climatic factors on vegetation coverage. The vegetation coverage and the area affected by the precipitation and temperature (time lag factors) accounted for 32.99% and 31.47% of the total watershed, respectively. The correlation between climatic factors, topographic factors, and vegetation coverage increased over time. The results from this study will help to further deepen the understanding of vegetation cover and its influencing factors, and provide a scientific basis for ecological restoration projects such as vegetation restoration in the Xijiang River basin of China. [ABSTRACT FROM AUTHOR]

We propose an efficient colocated multiple-input multiple-output radar waveform-design method based on two-step optimizations in the spatial and spectral domains. First, a minimum integrated side-lobe level strategy is adopted to obtain the desired beam pattern with spatial nulling. By recovering the hidden convexity of the resulting fractional quadratically constrained quadratic programming non-convex problem, the global optimal solution can be achieved in polynomial time through a semi-definite relaxation followed by spectral factorization. Second, with the transmit waveforms obtained via spatial optimization, a phase changing diagonal matrix is introduced and optimized via power method-like iterations. Without influencing the shape of the optimized beam pattern, the transmit waveforms are further optimized in the spectral domain, and the desired spectral nulling is formed to avoid radar interference on the overlaid licensed radiators. Finally, the superior performance of the proposed method is demonstrated via numerical results and comparisons with other approaches to waveform design. [ABSTRACT FROM AUTHOR]

Based on a distributed-parameter model, the forced vibration of a cantilever pair excited by a sinusoidal base movement is analyzed. Two cantilevers are coupled at their free ends by a linear spring. A nonlinear concentrated magnetic force acts on the tip of one cantilever, serving at the nonlinear boundary condition of the continuous model. The magnetic force is modeled as a fractional function, strongly dependent on the distance between two magnets. Via the method of multiple scales, the primary resonance is analyzed for all modes. A second-order approximate solution and its stability condition are analytically captured. It is revealed that the frequency-response curves are sensitive to the distance between the two magnets. The curve may exhibit the hardening-type, softening-type or linear behavior due to the existence of the quadratic nonlinearity. The outcomes are supported by the numerical simulations very well. [ABSTRACT FROM AUTHOR]

In this paper, equilibrium bifurcations of an axially moving Timoshenko beam are studied in the supercritical region. For the first time, Timoshenko beam theory is applied to investigate nonlinear dynamics of high-speed axially moving structures. The static equilibrium equation is deduced from the governing equation of transverse vibration of the axially moving Timoshenko beam. Two kinds of boundary conditions are considered. The non-trivial equilibrium solutions are analytically determined. Moreover, the equilibrium equations are discretized by using the finite difference method. Therefore, equilibrium configurations are numerically verified by proposing an iterative scheme. This investigation shows that non-trivial equilibrium solutions of Timoshenko beams bifurcate with axially moving speed. By comparing with Euler-Bernoulli beam theory, this study finds that the critical speed, determined by the Timoshenko beam, is remarkably smaller. Nevertheless, the equilibrium deformation of the moving Timoshenko beam is obviously larger. Furthermore, the present work derives the critical speed of the axially moving Timoshenko beam. At last, the effects of the system parameters on the equilibrium bifurcation and the critical speed are presented. [ABSTRACT FROM AUTHOR]

Heteroskedasticity and autocorrelation consistent (HAC) covariance matrix estimation, or HAC estimation in short, is one of the most important techniques in time series analysis and forecasting. It serves as a powerful analytical tool for hypothesis testing and model verification. However, HAC estimation for long and high-dimensional time series is computationally expensive. This paper describes a pipeline-friendly HAC estimation algorithm derived from a mathematical specification, by applying transformations to eliminate conditionals, to parallelise arithmetic, and to promote data reuse in computation. We discuss an initial hardware architecture for the proposed algorithm, and propose two optimised architectures to improve the worst-case performance. Experimental systems based on proposed architectures demonstrate high performance especially for long time series. One experimental system achieves up to 12 times speedup over an optimised software system on 12 CPU cores. [ABSTRACT FROM AUTHOR]

A new kind of bio-inspired, lightweight structure was designed and built from carbon fibre prepreg based on the cross-sectional microstructure of a beetle's elytra. The compression strength and failure process of the resulting structure was analysed using the finite element method; while at the same time, a quasi-static compression experiment was performed using an electronic universal testing machine to verify the effectiveness and accuracy of this finite element method. This bio-inspired structure was compared against a conventional honeycomb structure using FEM, revealing that for a given porosity and load parallel to the axis of the core tubes the respective compressive and specific compressive strengths of the bio-inspired structure are much higher at 84.3 MPa and 194.7 MPa/(g cm); thus demonstrating that this bio-inspired structure has superior compressive capability. [ABSTRACT FROM AUTHOR]

Steady-state periodic responses of nonlinear coupled planar motions are investigated for transporting beams in the supercritical transport speed ranges. The straight equilibrium configuration bifurcates into multiple equilibrium positions in the supercritical regime. The finite-difference schemes are developed to calculate the non-trivial static equilibrium and the steady-state response under simply supported or clamped boundary conditions. The forced vibration is assumed to be spatially uniform and temporally simple harmonic. Based on the long time series, the steady-state transversal amplitudes of nonlinear planar motions are recorded with changing load frequencies. A resonance exists if the external load frequency approaches the fundamental frequency. The effects of material parameters and vibration amplitude on the resonance responses are investigated. The coupled planar model can be reduced to two nonlinear models on transversal vibrations, an integro-partial-differential equation and a partial-differential one. Numerical examples are displayed for the pros and cons between the two transversal models. It is also revealed that the increased axial speed converts the hardening-type behavior into the softening-type one. [ABSTRACT FROM AUTHOR]

On the basis of the microstructure of the cross-section of a beetle's elytra, three bio-inspired lightweight structures were designed and built from acrylonitrile butadiene styrene plastic with a three-dimensional printer. The mechanical properties of three lightweight structures were analyzed and compared employing the finite element method, and quasi-static compression experiments and a three-point bending test on the structure samples were carried out using an electronic universal testing machine to verify the effectiveness of the finite element method. The results show that all three bio-structures were lightweight and had excellent mechanical properties. In particular, the bio-structure with spherical holes and hollow columns perpendicular to the top and bottom surfaces best imitated the microstructure of the cross-section of the Cybister elytra and had the greatest specific strength/stiffness in compression and bending. Finally, a preliminary optimization design was obtained for this bio-structure to further improve its specific strength and specific stiffness to 31.82 kN m/kg and 108.73 kN m/kg respectively. [ABSTRACT FROM AUTHOR]

The study of the adhesion of millions of setae on the toes of geckos has been advanced in recent years with the emergence of new technology and measurement methods. The theory of the mechanism of adhesion by van der Waals forces is now accepted and broadly understood. However, this paper presents limitations of this theory and gives a new hypothesis of the biomechanism of gecko adhesion. The findings are obtained through measurements of the magnitude of the adhesion of setae under three different conditions, to show the close relationship between adhesion and status of the setae. They are reinforced by demonstrating two setal structures, follicle cells and hair, the former making the setae capable of producing bioelectrical charges, which play an important role in attachment and detachment processes. It is shown that the abundant muscular tissues at the base of the setae cells, which are controlled by peripheral nerves, are instrumental in producing the foot movement involved in attachment and detachment. Our study will further uncover the adhesion mechanism of geckos, and provide new ideas for designing and fabricating synthetic setae. [ABSTRACT FROM AUTHOR]

Background: Until now, the conventional treatment of stromal tumors has been primarily open surgery or laparoscopic excision. The use of combined laparoscopic/endoscopic surgeries has been investigated, but endoscopic therapy alone has been limited to en bloc resection or nucleus removal of intracavitary tumors with diameters <2 cm. Nonintracavitary and intramural gastric stromal tumors preclude the use of endoscopic resection due to the risk of gastric perforation. This study was designed to show the safety and effectiveness of full-thickness endoscopic resection of nonintracavitary stromal tumors based on our direct experience. Methods: A total of 109 consecutive patients with nonintracavitary gastric stromal tumors <4 cm in diameter underwent surgical treatment; 66 patients received endoscopic surgery and 43 patients received laparoscopic surgery. Results: No significant differences existed between the two groups in terms of demographics and clinical characteristics, and no tumor exceeded 3.5 cm in size. Median operation times (endoscopic group, 53.6 min; laparoscopic group, 139 min) and hospitalization fees of the endoscopic group were significantly lower than those of the laparoscopic group with significant median hospital stays (8 days for endoscopic group; 6 days for laparoscopic group). No intraoperative complications occurred in the laparoscopic group and complete removal of tumors was achieved in the endoscopic group. Postoperative complications occurred in 6 patients of 43 who underwent laparoscopic surgery and 17 patients of 66 who underwent endoscopic surgery, representing a significant difference; the size of the lesion correlated positively with the occurrence of complications. Conclusions: Endoscopic resection is safe and effective for treating nonintracavitary stromal tumors. The endoscopic natural-cavity technique produced less surgical injury to the patients and preserved the anatomy of intra-abdominal structures. In addition, the endoscopic technique reduced operative times, postoperative bleeding, and costs. [ABSTRACT FROM AUTHOR]

The mechanical properties and the topological structure of cross-sections found in Cybister elytra are presented. SEM images show the similar geometric structure in transverse and longitudinal sections. Elytra consist of several layers: a dense, black epicuticle, an exocuticle that is braided by several parallel chitin fiber layers in a helix structure, and many bridge piers formed by the fiber bundles that connect the exocuticle to the endodermis, forming a composite, light-weight biomaterial. The mechanical properties along the edge of elytra are measured using a nano-indenter. The hardness and modulus of fresh cybister elytra is 0.31 GPa and 6.13 GPa, respectively. The ultimate stresses of fresh elytra, measured by using a tensile test machine, are 169.2 MPa and 194.5 MPa in the transverse and longitudinal directions, respectively, which is twice higher than that of the dried one. The difference in the two directions indicates that this bio-material has been topologically optimized during evolution. The results may provide a biological template to inspire lightweight structure design for aerospace engineering. [ABSTRACT FROM AUTHOR]

To study the modulation mechanism of the middle brain of the Gekko to the locomotion, we introduced a stereotaxic method first in literature and developed an apparatus suitable for Gekko gecko, the biggest wall and ceiling climber in nature. We defined the bregma and nasal points as reference points, selected the bilateral infraorbital margin and top point of the maxillary tooth for locating and fixing, and set up the line passing through the bregma and paralleling to a line connecting the bilateral infraorbital margin as x axis. Then, we defined a horizontal plane in the stereotaxic instrument, passing through x axis and the certain point which is 4.8 mm exactly above the nasal point, as the XOY plane; the sagittal plane, i.e. the YOZ plane, is the plane which is perpendicular to x axis and passes through the bregma; the plane, i.e. the XOZ plane, which passes through x axis and is perpendicular to the horizontal plane is the coronal plane. We designed a set of head holder which includes three parts: bilateral infraorbital margin clamps, a gecko adaptor holding the rostral side of the upper jaw. The allocation and operation of the head holder is accurate and simple, and the device is low in cost and compatible with standard stereotaxic instrument. [ABSTRACT FROM AUTHOR]

Scanning electron microscopy (SEM) and histological techniques were used to observe and study the setae structures of two gecko species ( G. gecko and G. swinhonis) and the relationships between these structures and the adhesive forces. The SEM results showed that the setae of these two species were densely distributed in an orderly fashion, and branched with curved tips. The setae of G. gecko had cluster structures, each cluster containing 4–6 setae whose terminal branches curved towards the center of the toes at ∼ 10°, the tips of the branches like spatulae and densely arrayed at an interval of less than 0.2–0.3 μm. On the contrary, the branch tips in the setae of G. swinhonis were curled, and the terminal parts of setae curved towards the center of the toes at various angles. Usually the setae of these gecko species branch twice at the top at intervals greater than that of G. gecko. The histological observation found that inside the setae of these two species there were plenty of unevenly distributed contents, such as epithelia, fat cells, pigmental cells and muscle tissue, but no gland cells existed. The results of functional experiments suggested that modifying the structure of gecko’s setae could reduce its adhesive ability dramatically, demonstrating the positive correlation between the structure of the gecko’s setae and its adhesive ability. The above results provide important information in designing bio-mimic setae and bio-gecko robots. [ABSTRACT FROM AUTHOR]