Table of Content

30 August 2020, Volume 26 Issue 4

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Review on photocatalytic removal of heavy metals from water

XU Zhenmin, SHI Liyi

2020, 26(4):  491-505.  doi:10.12066/j.issn.1007-2861.2245

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Treatment of heavy metal pollution is challenging, and hence, is the focus of tackling environmental problems. Photocatalysis-based technologies provide a green and sustainable route to heavy metal removal. TiO$_{2}$ is regarded as the most suitable photocatalyst because of its unique photo- and electrochemical performances. However, its application under visible light and natural sunlight is limited due to its large band gap and high carrier recombination rate. This review summarises a series of photo catalysts that have been developed in the recent years for heavy metal removal. Strategies for improving the photocatalytic activity of TiO$_{2}$, which involve surface modification, metal/non-metal doping, immobilisation, and coupling with other semiconductors, were summarised; the corresponding reaction mechanisms were also discussed. Moreover, the status and drawbacks of the current research on the photocatalytic removal of heavy metals were critically discussed.

Two-dimensional DOA estimation for wideband LFM signals based on Nested arrays

LI Xiaolin, ZHANG Wenjun

2020, 26(4):  506-517.  doi:10.12066/j.issn.1007-2861.2183

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Nested arrays have garnered significant interest owing to their large array apertures and more degrees of freedom. However, most studies have been focused on direction-of-arrival (DOA) estimation of narrow-band and stationary signals. Few studies have been conducted on wide-band signals, particularly nonstationary wide-band signals. In this paper, based on fractional Fourier transform and DOA matrix method, a two-dimensional angle measurement algorithm of nested array is proposed to accurately estimate the spatial DOA of wide-band linear-frequency-modulation signals without two-dimensional nonlinear search and parameter pairing. Simulation results confirm the superiority of the new algorithm.

Double filtration test point selection strategy for LBIST

DONG Chengliang, ZHANG Jinyi, QING Pei

2020, 26(4):  518-526.  doi:10.12066/j.issn.1007-2861.2067

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To solve the problem of low fault coverage rate in logic built-in self-test (LBIST) caused by test pattern using pseudorandom vector generator, an easily controlled and easily adjustable insertion technology of testing point has been applied. However, in the test points selection process of test point insertion (TPI), the usual criterion is ''fault coverage rate precedence'', which results in too high area overhead of some test points. To address the issue, a double filtration testing point selection strategy that can be used for LBIST has been proposed through analysis of existing mainstream selection strategies. This method first manages to obtain a single test point set of high fault coverage rate/low area overhead through pre-filtration to safeguard the whole quality of TPI. Next a single test point with high coincidence of fault coverage is filtered through filtration of global test points to complete TPI conforming to the boundary condition. Experimental results indicate that the method proposed in the paper, compared with presently novel compact unit perceived test point selection strategy, has the advantage of enhancing the fault coverage rate by 4.15%, and reducing the area overhead by 5.72%.

Recursive scanning algorithm for high-gray micro OLED display on silicon

SHEN Wenxin, RAN Feng, GUO Aiying, SHEN Huaming

2020, 26(4):  527-537.  doi:10.12066/j.issn.1007-2861.2079

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While the traditional scanning algorithm has the problem of time redundancy, the new development in silicon-based micro display shows a tendency for high gray and high frame rate. Through the research of digital pulse width modulation (PWM) scanning algorithm, weight scan algorithm and subfield scanning algorithm, a recursive scanning model is established and the scanning structure of the recursive scanning algorithm in any bit is deduced. The scanning process is abstracted into matrix form by mathematical model, the principle is to divide the whole display space into many sub spaces, and make full use of the waiting time in the data transmission process, so that the transmission efficiency can reach 100%. It is verified by FPGA hardware platform that the recursive scanning algorithm can make the conversion efficiency reach 100% and the frame rate can reach 85 Hz at the clock frequency 50 MHz and 256 gray scale. It can mainly meet the requirement of high frame rate, high gray scale and low clock frequency in high gray level micro display. The scanning algorithm can not only be applied to the image data transmission of the micro display, but also can be utilized in the flat panel, large screen display and other fields.

Passivation composition analysis and passivation process optimization of CdZnTe

ZHANG Ying, MIN Jiahua, LIANG Xiaoyan, LIU Zhaoxin, LI Ming, ZHANG Jijun, ZHANG Jiaxuan, ZHANG Delong, SHEN Yue, WANG Linjun

2020, 26(4):  538-543.  doi:10.12066/j.issn.1007-2861.2057

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The influence of different passivation time and annealing on the surface morphology, electrical properties and composition of CdZnTe is studied via scanning electron microscopy (SEM), current-voltage (I-V) and X-ray photoelectron spectroscopy (XPS). The result shows that the optimum passivation time for two-step passivation method is 30 minutes, and the leakage current is close to the minimum. After the wafers are annealed for 60 minutes at 100 ℃, metallography and SEM show that the surface morphology after heat treatment is to a more considerable degree in uniformity and density. XPS depth analysis shows that chemical reaction intermediate decomposition is relatively complete and that TeO₂ content is increased. I-V test shows that the leakage current is reduced, which effectively improves the detector's performance.

Synthesis and photoelectrical properties of hierarchical-structured anatase TiO₂ nanoflowers

YANG Huang, LI Yi, WANG Rui, YANG Weiguang

2020, 26(4):  544-551.  doi:10.12066/j.issn.1007-2861.2087

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Oxide semiconductor nanomaterials are used as aonde materials for dye- sensitized solar cells (DSSCs). One-dimensional nanostructures have enhanced electron collection of carriers and superior charge transport compared to spherical nanoparticles. However, they suffer from low dye-loading caused by small surface areas and unfavorable arrangement for a photo-generated electron extraction. Anatase TiO₂  nanorods-based nanoflowers have been successfully synthesized using a simple one-step hydrothermal method. Such hierarchical nanostructures can be controlled by reaction time. The dye absorption capacities of the as-obtained hierarchical nanostructures increase with the reaction time. The highest cell efficiency (7.29%) of the DSSC is achieved through the TiO₂  nanoflowers prepared for 70 h.

Ru nanoparticles supported on Mg-Al mixed metal oxide catalysts for selective oxidation of HMF methylfurfural into FDCA

WANG Lingchen, CHEN Chunlin, ZHANG Min

2020, 26(4):  552-561.  doi:10.12066/j.issn.1007-2861.2064

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Ru nanoparticles supported on Mg-Al mixed metal oxide catalysts have been prepared via the hydrothermal-reduction method. The structure, morphology and composition of the catalysts have been systematically analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), N₂ physisorption, CO₂-temperature programmed desorption（CO₂-TPD) and inductively coupled plasma-optical emission spectroscopy (ICP-OES). The characterization results show that the highly dispersed Ru nanoparticles benefit from the strongest alkalinity (142.81 mmol·g^-1 and the competitive specific surface area (97.0 m²·g^-1 of mixed metal oxides with n(Mg):n(Al)=5:1. The RO-5 catalyst with Ru loading of 2.0% can efficiently catalyze aerobic oxidation of 5-hydroxymethylfurfural (HMF), obtaining 100.0% HMF conversion and 99.0% 2,5-furandicarboxylic acid (FDCA) yield under the optimal reaction condition (120 ℃ , 2 MPa O₂, 5 h). According to the optimizing results, the reaction pathway for the conversion of HMF to FDCA is further confirmed for the formation of 2,5-diformylfuran,(DFF) and 5-formyl-2-furancarboxylic acid (FFCA) intermediates.

Activation mechanism of the effect of Ca on oxygen vacancy diffusion in grain boundary of alpha-Al₂O₃

MA Shuai, LI Yonghua, GAO Yubo

2020, 26(4):  562-569.  doi:10.12066/j.issn.1007-2861.2220

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The formation energy and diffusion barrier of oxygen vacancies (VO) were calculated using the first-principles density functional theory method, for the case of Ca segregated in the ∑3(1010) grain boundary of $\alpha alpha-Al₂O₃. The formation energy of Vo fell in the 3.05~4.04 eV range in the Ca doped grain boundary. This was roughly 2.5 eV lower than the undoped grain boundary. The activation energy of VO in the Ca-doped grain boundary changed to 2.30 eV, 1.8 eV less than the undoped grain boundary's maximum. Moreover, with the increase in the concentration of Ca, the crystal lattice near the grain boundary underwent significant expansion, further breaking the balance of charge. Therefore, the formation energy of VO was further reduced to -1.43 eV, and the diffusion activation energy was reduced to 1.27 eV. Thus, Ca doping proved advantageous in the formation and diffusion of VO in the grain boundary of $\alpha alpha-Al₂O₃.

Electron transfer of water-soluble perylene diimide compound and long-life radical anion generation

CHENG Zhengmin, ZHANG Jun, XING Feifei, ZHU Shourong

2020, 26(4):  570-577.  doi:10.12066/j.issn.1007-2861.2218

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A novel water-soluble perylene diimide compound, N,N'-di(polyethylene glycol)-1-(4-mercaptobenzene)-perylene-3,4,9,10-tetracarboxylic diimide (BFL), was synthesized by introducing PEG chains. Solvent-and temperature-dependent visible spectra were investigated and aggregation states were analysed. The results show that BFL in aqueous solution has only one absorption band at 530 nm, which is related to the ratio of monomer/aggregates. However, the change in the monomer/aggregates ratio with increasing concentration of BFL molecules is not obvious. Organic solvents and surfactants can disperse the compound, thereby significantly increasing the amount of the monomer. BFL can react with sodium sulfide in water;BFL is first reduced to anion radicals and then to dianions. The ability to produce a long-life free radical in the reduction reaction will make the BFL an excellent n-type material.

One-step ball-milled silicon/carbon composite as anode in lithium-ion batteries

CHEN Si, ZHANG Yong, ZHANG Xueqian, LÜ Liping

2020, 26(4):  578-585.  doi:10.12066/j.issn.1007-2861.2059

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In recent years, silicon-based materials have attracted a lot of research attention due to their high theoretical capacity, especially when applied as the anode materials of lithium ion batteries. In this paper, Si/C composite is prepared using a simple high-energy ball milling method, and the products with different milling time and carbon sources are compared. The Si/C composite material demonstrates excellent electrochemical performance when used as the anode of the lithium ion batteries. Moreover, the carbon in the product can effectively accommodate the volume expansion of the anode during the insertion and desertion of lithium ions. It is believed that the facile ball milling method, which synthesizes the silicon-based materials and which demonstrates good electrochemical performance, is beneficially deployed in the application of lithium ion batteries.

Preparation and electrochemical performance of hollow silicon sphere/graphene composite electrodes

GAO Yang, JIANG Yong, JIAO Zheng

2020, 26(4):  586-594.  doi:10.12066/j.issn.1007-2861.2056

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The silicon material of lithium-ion batteries with high theoretical capacity and low charge-discharge voltage platform, serving as the best anode material to replacing graphite, has become the most popular material for research. H-Si spheres and graphene oxide solutions form a three-dimensional porous graphene aerogel embedded silicon sphere nanocomposite (H-Si/GA) using hydrothermal reaction. H-Si spheres and positive graphene functionalizing with poly dimethyl diallyl ammonium (PDDA) electrostatically absorbed form a coating-like nanocomposite (H-Si/G) using solvothermal reaction. It was found that H-Si/G has better electrical properties because of compact coating through structural characterization and electrical performance tests. The hollow silicon spheres are embedded in the graphene nanosheets due to electrostatic adsorption. Graphene firmly fixes the silicon spheres, establishes stable conductive paths, buffers volume expansion and maintains electrode structure stability. And the void space inside the silicon spheres provides sufficient buffer space for volume expansion and shortens the electron and ion transmission channels.

Cellular automaton model for U-turn vehicles at T-crossing

YUAN Dingqiang, GUO Mingmin, ZHANG Peng, LIN Zhiyang, FANG Rui

2020, 26(4):  595-605.  doi:10.12066/j.issn.1007-2861.2107

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Based on the left part of the T-crossing, this paper establishes a two-way, six-lane road system and studies the two cases whereby the U-turn lane is located in the left lane and the middle lane. The straight road and the curved road are divided into cells according to the actual vehicle trajectories, and the traffic lights are introduced at the T-crossing. The simulation is based on the NaSch model. The traffic flow of each lane is counted and analysed. The results show that when the U-turn lane is located in the left lane, it is beneficial to the U-turn vehicles. When the U-turn lane is in the middle lane, it is advantageous for vehicles intending to turn from right to left, but it has a great impact on the traffic flow of the three lanes on the right.

Numerical study on gasous chemical reaction mechanism and detonation wave in the detonation chamber of engine

RAO Feixiong, DING Jue, WENG Peifen, LI Xiaowei

2020, 26(4):  606-616.  doi:10.12066/j.issn.1007-2861.2216

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The strong nonlinear coupling of gas dynamics and chemical reactions exists in the structure of detonation wave. Based on sensitivity analysis, a 20-step element reaction model and 9-step reaction model are utilized with Euler equations. Roe scheme is used to solve the convective term and second order upwind format is used for interpolation. A numerical study on the detonation of  H₂/O₂ in a pulse detonation engine is conducted. Calculated detonation parameters agree with CJ theory and experimental data, which show that the physical models with chemical reactions used in this study are correct. In addition, the detonation cellular structure is numerically analyzed. The 20-step reaction model can describe the collision and evolution of the triple point more accurately than the 9-step reaction model. It can also better explain the decoupling between reaction zone and incident shock wave as well as strong coupling with Mach stem. Therefore, the effects of inert-gas (Ar) dilution and the initial pressure of the gas phase on detonation in a detonation chamber are discussed based on the 20-step reaction model.

Multi-Monte Carlo simulation for bicomponent coalescence in finite system

ZUO Hao, SHEN Jie, LU Zhiming

2020, 26(4):  617-627.  doi:10.12066/j.issn.1007-2861.2055

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The mixing state of a bicomponent population of granules is characterized by the total variance of component A, which measures the deviation of the composition of each granule from the overall mean. By means of the multi-weighted Monte Carlo method, first the fundamental influence of the homogeneity index lambda of the kernel is verified, and the scaling rate of segregation index, which is a function of homogeneity index lambda, is obtained. Second, two stages in the evolution of mixing are identified. Before the critical time, both interaction between components and the kernel take important effects on the coalescence. And the repulsive interaction is much stronger than the attractive interaction. After the critical time, only the kernel itself affects the coalescence and the effects of interaction between components are ignored. Finally, a power-law function between the critical time and alpha is obtained, whereas an exponential function relationship between the critical time and $\lambda$ is identified by a curve fitting technique. The results are of great importance in the pharmaceutical engineering.

Mechanical properties of bentonite-calcium carbonate mixture

QIN Aifang, FU Xianlei, RUAN Kunlin, JIA Xu

2020, 26(4):  628-639.  doi:10.12066/j.issn.1007-2861.2066

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Different amounts of CaCO₃ are mixed in bentonite for the purpose of simulating the characteristic changes of bentonite mixed with CaCO₃ that precipitates from groundwater around high-level radioactive waste (HLW) repositories. In the process of the project, four groups of bentonite-CaCO₃ mixtures are prepared with different contents of CaCO₃. Then a series of experiments including swell-under-load, compression and direct shear tests are carried out. The permeability coefficient is calculated through Terzaghi consolidation theory. The microstructure changes are photographed by the scanning electron microscopy (SEM). Test results indicate that: (1) The final swelling strains are related to vertical stress and CaCO₃ content. While vertical stress and CaCO₃ content increase, the final swelling strains decrease. (2) The compression index deceases slightly as the content of CaCO₃ crystals increases. (3) The permeability turns worse with increasing CaCO₃ content. (4) With the content of CaCO₃ crystals changing, the shear strength of unsaturated samples will come to its peak value. Contrary to saturated samples, the cohesive of unsaturated samples increases firstly and then increases with the increasing CaCO₃ content. (5) The photographs show that the existence of CaCO₃ fills the pores which makes the structure denser.

Shear performance of new joint for prestressed I-shaped retaining piles

ZHAO Huiling, WANG Zhenjiang

2020, 26(4):  640-650.  doi:10.12066/j.issn.1007-2861.2217

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The lengths of precast piles are normalised, and a reliable pile joint can facilitate the application of prefabricated retaining piles. In this study, the joint used for connecting the ends of prestressed concrete I-piles was composed of an L-shaped flange steel plate, a steel connection plate, and high-strength bolts. The shear failure mode and capacity of the joint were evaluated through shear tests, and a finite element model was established to simulate the shear failure process of the joint. Based on the experimental verification, the effects of the strength of concrete, the strength of the connecting steel plate, and the number of PC steel bars on the shear bearing capacity of pile joints were analysed through finite element simulation. The results showed that the three-dimensional fine numerical simulation considering plastic damage could predict the joint response. With the increase in the shear load, the connecting steel plate and top PC steel bars near the joint yielded before other parts. The variations in concrete strength and the number of top longitudinal PC bars significantly influenced the shear capacity of the joint. However, the increase in the yield strength of the connecting steel plate had no significant effect on the bearing capacity.

Influence of intermittent fracture grouting electro-osmosis for dredged slurry strengthening

LIU Feiyu, ZHOU Jie, WANG Jun, LI lingyu

2020, 26(4):  651-661.  doi:10.12066/j.issn.1007-2861.2219

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During electro-osmosis, the electrical resistance of the soil increases and electro-osmotic efficiency decreases owing to the appearance of fractures in the soil. Fracture grouting is a method that can be applied to solve this problem. In this study, under the equipotential premise gradient, experimental laboratory tests through electro-osmosis were performed to investigate the effects of various fracture-grouting intermittence frequencies using 1.5% NaOH solution on each of five dredged slurry samples. The results were analysed based on the extracted water mass, drainage rate, surface settlement, electric current, energy consumption effect, water content, and undrained vane shear strength. The results showed that fracture grouting could improve the effect of electro-osmosis. In addition, the experimental findings indicated that an optimal grouting ratio existed, which effectively decreased the water content, increased the vane shear strength, and reduced the energy consumption effect.

Factorization of   X^2m-1 over Z_(2m-1)^k

YANG Jiansheng, SUN Yanan

2020, 26(4):  662-670.  doi:10.12066/j.issn.1007-2861.2053

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This paper mainly studies the factorization ofX^2m-1 over Z_(2m-1)^k, and it obtains the constrained relationship between coefficients of irreducible factors of X^2m-1

Application of adaptive level set method in shape optimization for Stokes problem

DUAN Xianbao, DANG Yan, QIN Ling

2020, 26(4):  671-680.  doi:10.12066/j.issn.1007-2861.2212

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We present a level-set-based adaptive mesh method for solving the drag minimization problem of incompressible flow governed by the Stokes equations. A shape sensitivity analysis of the cost functional is presented. Two levels of meshes are employed during the optimization. A uniform coarse mesh for evolving the level set function is defined over the entire computational domain. Additionally, the level set function serves as a refinement indicator. The coarse mesh comprising the interfaces is then further divided into a uniform fine mesh. The computation is performed mainly near the interfaces. Therefore, the computational cost is significantly reduced compared with the uniform refined mesh over the whole domain that achieves the same resolution. Furthermore, the shape derivative on the boundary can be obtained implicitly, which is a very challenging task in classical optimal shape design problems.