Xizhong An

DEM study of crystallization of monosized spheres under mechanical vibrations

Abstract The crystallization (disorder-order transition) of monosized spheres under three-dimensional (3D) mechanical vibrations is studied using discrete element method (DEM). The crystallization dynamics and final structure are analyzed for two selected conditions: i.e. the packing of rough spheres (glass beads) with interval vibration and batch-wise feeding (Case I) and the packing of smooth spheres with continuous vibration and total feeding (Case II). The final packing densities are 0.728 and 0.712 for Cases I and II, respectively, higher than that of random close packings. Partial crystallization characterized by the {111}-oriented face centered cubic (FCC) structure can be observed in both packings, which is further confirmed from the analyses of coordination number, radial and angular distribution functions, and Q 6 bond order. Through the tracing of the particles (e.g. the evolutions of velocity and force fields), two crystallization mechanisms are identified: engulfed growth of two adjacent small crystals and epitaxial growth from existing ordered structures (nuclei).

Synthesis of V2O5@TiO2 core–shell hybrid composites for sunlight degradation of methylene blue

This study has demonstrated a facile but efficient synthesis method to prepare vanadium oxide@titanium oxide (V2O5@TiO2) core–shell nanostructures using a water bath at mild temperatures (≤100 °C). This method shows a few unique features, including a short reaction time for fabricating core–shell nanostructures, no requirement of high temperature calcination (>500 °C) for TiO2 crystallization, easily tunable TiO2 shell thickness, high yield, and good reproducibility. With characterization using several advanced techniques (TEM, BET, XRD, XPS and UV-vis spectroscopy), the as-prepared V2O5@TiO2 nanocomposites were found to exhibit a large surface area, and a good stability. The experimental results show that the V2O5@TiO2 core–shell composites show a superior sunlight photocatalytic activity compared to the pure TiO2 nanoparticles for the degradation of organic dyes (e.g., methylene blue), probably because of the matched energy bands between V2O5 and TiO2. These findings may bring new insights into the designing of TiO2-based core–shell and other nanocomposites with enhanced photocatalytic efficiencies for environmental remediation.

Synthesis of hierarchical nanosheet-assembled V2O5 microflowers with high sensing properties towards amines

Hierarchical three-dimensional nanosheet-assembled vanadium pentoxide (V2O5) microflowers are successfully synthesized by a hydrothermal method, followed by a high-temperature sintering treatment. Several advanced techniques are used to characterize the morphology and composition of the resulting nanostructures, such as TEM, HRTEM, SEM, XRD, and BET. The HRTEM image shows that the microflowers are assembled from the nanosheets with highly exposed {010} facets, as confirmed by selected area electron diffraction (SAED). According to N2 sorption isothermal studies, the as-prepared V2O5 microflowers show high specific surface area of 61.5 m2 g−1. The formation of the microflowers with assistance of NaHCO3, which may play a critical role in the self-assembly process, may be attributed to a “reproduction mechanism”. The gas sensing performances of both the V2O5 microflowers and the V2O5 nanosheets were evaluated towards several volatile organic compounds (VOCs), such as 1-butylamine, ethanol, acetone, and formaldehyde. The results show that the flower-like structure exhibits a superior sensing response and selectivity towards amines compared to that of the sheet-like structure at an optimum working temperature of ∼300 °C. The high selectivity towards 1-butylamine can be ascribed to the selective oxidation mechanism. This work will help explore vanadium oxides as gas sensors toward volatile organic compounds with high performance.

DEM simulation of binary sphere packing densification under vertical vibration

ABSTRACT The densification of random binary sphere packings subjected to vertical vibration was modeled by using the discrete element method (DEM). The influences of operating parameters such as the vibration conditions, sphere size ratio (diameter ratio of larger versus small spheres), and composition (volume fraction of large spheres) of the binary mixture on the fractional packing density φ (defined as the volume of spheres divided by the volume of container) were studied. Two packing states, i.e., random loose packing (RLP) and random close packing (RCP), were reproduced and their micro properties such as the coordination number (CN), radial distribution function (RDF), and force structure were characterized and compared. The results indicate that properly controlling vibration conditions can realize the transition of binary packing structure from the RLP to RCP state when the sphere size ratio and composition are fixed, and the fractional packing density for RCP after vibration can reach φRCP ≈ 0.86. Different packing characteristics from RLP and RCP identify that RCP shows much denser and more uniform structure than RLP. The current modeling results are in good agreement with those obtained from both the physical experiments and the proposed empirical models in the literature.

Comparative study on photocatalytic and bactericidal activity between Ag@TiO 2 core-shell nanoparticles and Ag@TiO 2 surface doped nanostructures

Silver@titanium dioxide (Ag@TiO2) core-shell nanostructures and Ag surface doped TiO2 particles (TiO2@Ag) have been designed and synthesized by sol-gel and hydrothermal methods under mild conditions. These two types of Ag-TiO2 nanocomposites were characterized in terms of their properties. Specifically, the photocatalytic performance and antibacterial behavior of such nanocomposites have been investigated and compared. It was found that the Ag@TiO2 core-shell nanostructures exhibit superior photocatalytic property to the Ag surface doped TiO2 particles under the reported conditions, while the same situation happened in the bactericidal test. This is probably because the Ag cores tend to facilitate charge separation for TiO2, producing greater hydroxyl radicals on the surface of the TiO2 particles. These findings would be useful for the design and synthesis of Ag-TiO2 nanocomposites with desirable photocatalytic and antimicrobial activity for environmental applications.

Topological and metrical property characterization of radical subunits for ternary hard sphere crystals

Quantitative characterization on the topological and metrical properties of radical subunits (polyhedra) for two new ternary hard sphere crystals was studied. These two ideal crystalline structures are numerically constructed by filling small and medium spheres into interstices (corresponding to regular tetrahedral and octahedral pores) of perfect face centered cubic (FCC) and hexagonal close packed (HCP) crystals formed by the packing of large spheres. Topological properties such as face number, edge number, vertex number of each radical polyhedron (RP), edge number of each RP face and metrical properties such as volume, surface area, total perimeter and pore volume of each RP, area and perimeter of each RP face were analyzed and compared. The results show that even though the overall packing densities for FCC and HCP ternary crystals are the same, different characteristics of radical polyhedra for corresponding spheres in these two crystals can be identified. That is, in the former structure RPs are mor...

Experimental study on the packing densification of mixtures of spherical and cylindrical particles subjected to 3D vibrations

ABSTRACT To identify the dense packing of cylinder–sphere binary mixtures (spheres as filling objects), the densification process of such binary mixtures subjected to three-dimensional (3D) mechanical vibrations was experimentally studied. Various influential factors including vibration parameters (such as vibration time t, vibration amplitude A, frequency ω, vibration acceleration Γ) as well as particle size ratio r (small sphere vs. large cylinder), composition of the binary mixtures XL (volume fraction of cylinders), and container size D (container diameter) on the packing density ρ were systematically investigated. The results show that the optimal vibration parameters for different binary cylinder–sphere mixtures are different. The smaller the size ratio, the less vibration acceleration is needed to form a stable dense packing. For each binary mixture, high packing density can be obtained when the volume fraction of large cylindrical particles is dominant. Meanwhile, increasing the container size can decrease the container wall effect and get higher packing density. The proposed analytical model has been proved to be valid in predicting the packing densification of current cylinder–sphere binary mixtures.

Preparation of plasmonic porous Au@AgVO3 belt-like nanocomposites with enhanced visible light photocatalytic activity

This study reports a visible light-driven plasmonic photocatalyst of Au deposited AgVO3 nanocomposites prepared by a hydrothermal method, and further in situ modification of Au nanoparticles by a reducing agent of NaHSO3 in an aqueous solution at room temperature. Various characterization techniques, such as SEM, TEM, XRD, EDS, XPS, and Brunauer-Emmett-Teller, were used to reveal the morphology, composition, and related properties. The results show that belt-like AgVO3 nanoparticles with a width of ∼100 nm were successfully synthesized, and Au nanoparticles with controlled sizes (5-20 nm) were well distributed on the surface of the nanobelts. The UV-vis absorption spectra indicate that the decoration of Au nanoparticles can modulate the optical properties of the nanocomposites, namely, red shift occurs with the increase of Au content. The photocatalytic activities were measured by monitoring the degradation of Rhodamine B (RhB) with the presence of photocatalysts under visible light irradiation. The photodegradation results show that AgVO3 nanobelts exhibit good visible light photocatalytic activities with a degradation efficiency of 98% in 50 min and a reaction rate constant of 0.025 min-1 towards 30 ppm RhB. With the modification of Au nanoparticles, photocatalytic activity basically increases with the molar ratio of Au to V. Among the Au@AgVO3 nanocomposites, the 3% (molar ratio) Au decorated AgVO3 nanobelts showed the highest photocatalytic activity, and the k (0.064 min-1) was almost two times higher than that of the pure AgVO3 nanobelts. This can be attributed to several factors including specific surface areas, optical properties, and the energy band structure of the composites under visible light illumination. These findings may be useful for the practical use of visible light-driven photocatalysts with enhanced photocatalytic efficiencies for environmental remediation.

DEM construction of binary hard sphere crystals and radical tessellation

In this paper, four binary hard sphere crystals were numerically constructed by discrete element method (DEM) through different packing modes under three-dimensional (3D) mechanical vibration. For each crystal, a modified Voronoi tessellation method (called radical tessellation) was utilized to quantitatively investigate the topological and metrical properties of radical polyhedra (RPs). The topological properties such as the number of faces, edges, vertices per RP and the number of edges per RP face as well as the metrical properties such as perimeter, surface area, volume, and relative pore size per RP were systematically characterized and compared. Meanwhile, the mechanism of the binary hard sphere crystallization was also investigated. The results show that the packing sequence and pattern of the large spheres can determine the structure of the binary hard sphere crystal. The RP structures and their metrical and topological properties of the four binary hard sphere crystals (even the packing density of the two crystals is the same) are quite different. Each property can clearly reflect the specific characteristics of the corresponding binary hard sphere crystalline structure. The obtained quantitative results would be useful for the deep understanding of the structure and resultant properties of binary hard sphere crystals.In this paper, four binary hard sphere crystals were numerically constructed by discrete element method (DEM) through different packing modes under three-dimensional (3D) mechanical vibration. For each crystal, a modified Voronoi tessellation method (called radical tessellation) was utilized to quantitatively investigate the topological and metrical properties of radical polyhedra (RPs). The topological properties such as the number of faces, edges, vertices per RP and the number of edges per RP face as well as the metrical properties such as perimeter, surface area, volume, and relative pore size per RP were systematically characterized and compared. Meanwhile, the mechanism of the binary hard sphere crystallization was also investigated. The results show that the packing sequence and pattern of the large spheres can determine the structure of the binary hard sphere crystal. The RP structures and their metrical and topological properties of the four binary hard sphere crystals (even the packing density o...

Microscopic analyses of stress profile within confined granular assemblies

Bottom pressure of confined granular assemblies saturates at a certain value even this packing bed is being continuously charged. Corresponding formulation has been established to describe this interesting phenomenon. In this work, the influences of particle size and friction on the bottom stresses of granular matter were numerically investigated by discrete element method (DEM). It is found that the Janssen model can well predict the stress profile only when the size ratio of the container versus the particle is larger than 16. Moreover, a hydrostatic linear relation between apparent mass and filling mass can be obtained when the friction coefficient becomes insignificant (μ ≤ 0.01). To further interpret the Janssen effects, the granular assemblies are characterized and evaluated from the overall interactions with sidewalls, angular distribution function, void size distribution, coordination number, contact networks, contact orientation and distributions of contact forces within the packing structure. It is believed that these results will be helpful to comprehend the granular behaviors and may offer instructive reference to industrial processes in related fields.Bottom pressure of confined granular assemblies saturates at a certain value even this packing bed is being continuously charged. Corresponding formulation has been established to describe this interesting phenomenon. In this work, the influences of particle size and friction on the bottom stresses of granular matter were numerically investigated by discrete element method (DEM). It is found that the Janssen model can well predict the stress profile only when the size ratio of the container versus the particle is larger than 16. Moreover, a hydrostatic linear relation between apparent mass and filling mass can be obtained when the friction coefficient becomes insignificant (μ ≤ 0.01). To further interpret the Janssen effects, the granular assemblies are characterized and evaluated from the overall interactions with sidewalls, angular distribution function, void size distribution, coordination number, contact networks, contact orientation and distributions of contact forces within the packing structure. It...