Hollow Center Research Articles

Formation of hollow porous TiO2 nanospheres via the encapsulation of CO2 nanobubbles for high-performance adsorption and photocatalysis.

Porous TiO2 structures have been of great interest in various photocatalytic applications over the past decade. However, the synthetic methods reported so far in the literature are complex and time consuming. This study presents a simple one-pot fabrication method of hollow porous TiO2 nanospheres using a sol-gel process involving CO2 generation in aqueous solution. The CO2 nanobubbles produced in the reaction act as a template for the crystal growth of TiO2 resulting in highly porous hollow nanospheres. The unique nanocrystal structure with a hollow nanosphere centre surrounded by an outer shell of prickle-like porous nanocrystals was observed. The prepared hollow porous TiO2 nanospheres exhibited excellent adsorption properties as demonstrated by the adsorption measurements and dynamic extraction of methylene blue in aqueous solution. Furthermore, the hollow porous TiO2 nanospheres hydrothermally treated at 180 °C exhibit 3.2 times higher photocatalytic performance in the decomposition of methylene blue than the commercial P25 TiO2. The idea of this work provides a new direction for facile green synthesis routes of various metal oxides with cavities, channels or crevices for various technological applications.

Hydrogen storage capacity of alkali metal atoms decorated porous graphene

Porous graphene (PG), a kind of graphene-related material with nanopores in the graphene plane, exhibits novel properties different from those of pristine graphene, leading to its potential applications in many fields. Owing to periodic nanopores existing naturally in the two-dimensional layer, PG can be used as an ideal candidate for hydrogen storage material. High hydrogen storage capacity of Li-decorated PG has been investigated theoretically, but the effect of temperature on the stability of the H<sub>2</sub> adsorbed on Li-PG has been not discussed yet. In this paper, by using the first-principles method, the hydrogen storage capacity on alkaline metal atoms (Li, Na, K) decorated porous graphene is investigated in depth with generalized gradient approximation, and the effect of the temperature on the stability of the hydrogen adsorption system is elucidated by the <i>ab initio</i> molecular-dynamics simulation. The results show that the most favorable adsorption sites of Li, Na and K are the hollow center sites of the C hexagon, and four alkaline metal atoms can be adsorbed stably on both sides of PG unit cell without clustering. Alkaline metal adatoms adsorbed on PG become positively charged by transferring charge to PG and adsorbed H<sub>2</sub> molecules, and three H<sub>2</sub> molecules can be adsorbed around each alkaline metal atom. By analyzing the Mulliken atomic populations, charge density differences and density of states of H<sub>2</sub> adsorbed on Li-PG system, we find that the H<sub>2</sub> molecules are adsorbed on alkaline metal atoms decorated graphene complex by attractive interaction between positively charged alkaline metal adatoms and negatively charged H and weak van der Waals interaction. Twelve H<sub>2</sub> molecules are adsorbed on both sides of PG decorated with alkaline metal atoms. The average adsorption energy of H<sub>2</sub> adsorbed on Li-PG, Na-PG and K-PG are –0.246, –0.129 and –0.056 eV/H<sub>2</sub>, respectively. It is obvious that the hydrogen adsorption capacity of Li-PG system is strongest, and the hydrogen adsorption capacity of K-PG is weakest, thus K-PG structure is not suitable for hydrogen storage. Furthermore, by the <i>ab initio</i> molecular-dynamic simulation, in which the NVT ensemble is selected but the external pressure is not adopted, the effect of temperature on the stability of H<sub>2</sub> molecules adsorbed on Li-PG system is elucidated. The result shows that the configuration of Li-PG is very stable, H<sub>2</sub> molecules are stably adsorbed around the Li atoms at low temperature, and some H<sub>2</sub> molecules start to be desorbed from the Li atoms with the increase of temperature. At 200 K, H<sub>2</sub> molecules begin to move away from Li atoms, and two H<sub>2</sub> molecules escape from the binding of the Li atoms at 250 K. At 300 K, nine H<sub>2</sub> molecules can be stably absorbed on both sides of Li-PG, and the gravimetric hydrogen storage capacity can reach up to 9.25 wt.%, which is much higher than the the US Department of Energy target value of 5.5 wt.% for the year 2017. With the increase of temperature, more adsorbed H<sub>2</sub> molecules are desorbed, seven H<sub>2</sub> molecules can be desorbed at 400 K, and all H<sub>2</sub> molecules are completely desorbed in a temperature range of 600–700 K.

Spreading properties of a multi-Gaussian Schell-modelvortex beam in slanted atmospheric turbulence

The cross-spectral density function of a multi-Gaussian Schell-model vortex (MGSMV) beam propagating through slanted atmospheric turbulence was derived, and the influences of the MGSMV beam parameter and slanted atmospheric turbulence on the spreading properties of a MGSMV beam are studied. One can find that a MGSMV beam propagating in slanted atmospheric turbulence can evolve into the flat-topped beam, and a MGSMV beam with larger index N and topological charge M propagating in slanted atmospheric turbulence will lose the dark hollow center and evolve into the Gaussian beam more slowly than the MGSMV beam with smaller index N and topological charge M. It is also found that a MGSMV beam propagating in slanted atmospheric turbulence with larger strucutre parameter C will evolve into Gaussian beam faster, but the influences of zenith angle α on the spreading properties of MGSMV beam in the far field can be ignored.

Four-petal Lorentz-Gauss vortex beam and its propagation in free space

Considering the formations of vortex beam and four-petal Lorentz-Gauss beam produced by Liu (Optik, 179, 492, 2019), a new vortex beam called four-petal Lorentz-Gauss vortex beam is firstly introduced. The propagation equation of four-petal Lorentz-Gauss vortex beam in free space is derived, and the evolutions of intensity and phase are analyzed by using derived equations. With increasing the propagation distance, the intensity distribution with four-petal beams will be destroyed and evolve from array beam into a beam with dark hollow center, and the phase will become irregular. With increasing the topological M, the four-petal Lorentz-Gauss vortex beam will translate into a beam with larger dark hollow center, while the influences of N on the distribution can be neglected at the longer propagation distance.

A DFT study of the enhanced hydrogen storage performance of the Li-decorated graphene nanoribbons

The hydrogen storage performances of the pure graphene nanoribbon and the Li-decorated one were studied through the first-principle density functional theory. The results showed that the decorated Li atom could be chemisorbed on the hollow center of the C hexagon with the highness and the adsorption energy of 1.730 Å and −0.807 eV, respectively. The hydrogen molecule was found to be weakly physisorbed on the pure graphene nanoribbon but be strongly chemisorbed on the Li-decorated one with the high adsorption energy of −0.263 eV. The further calculation revealed that there were at most four hydrogen molecules stably chemisorbed and stored on the Li atom in the decorated graphene nanoribbon with the average adsorption energy of −0.235 eV. The number of the hydrogen molecules stored on the graphene nanoribbon could be successfully improved to eight through decorating it with two Li atoms on its two sides. Our results indicated that the graphene nanoribbon decorated with Li atoms could be a promising and excellent material to store hydrogen molecules, further implying that it is an effective strategy to enhance the hydrogen storage performance of graphene nanoribbon-based materials through modifying them with metal atoms.

Ultrastructural observations of anthers, staminodes, and pollen grains of mango (Mangifera indica L. var. Beneshan; Anacardiaceae)

The ultrastructure of anthers, staminodes, and pollen of Mangifera indica L. was studied using scanning electron microscopy (SEM), and pollen viability assessed using light (LM) and fluorescence microscopy (FM). Ultrastructural observation revealed anther surfaces with polygonal cells and hollow centres arranged in a reticulate manner, with swollen cells on the edges of the anther surfaces. Anther dehiscence is longitudinal, with pollen released through a long slit in both thecae. The average length and width of staminodes of male and hermaphroditic flowers measured 0.7 mm × 0.35 mm and 0.65 mm × 0.3 mm, respectively. Distinct ridge and hook-like outgrowths on the adaxial surface of staminodes are described, as are staminode surfaces comprised of long, irregular cells with stomata exuding nectar. Staminodes produced no pollen. Anthers of male flowers produced more pollen grains (536–537) than did anthers of hermaphrodite flowers (510–511). Pollen grains are tricolporate, have reticulate perforate exine ornamentation, and are bi-cellular at dispersal. Anther and staminode size and pollen production was greater but not significantly different in male versus hermaphrodite flowers. In contrast, the fluorochromatic reaction (FCR) test and FM observations found significantly higher pollen viabilities in hermaphroditic (50.1%) versus male (40.4%) flowers. This research provides new ultrastructural characters potentially useful in future taxonomic studies of mango and other Anacardiaceae. Results presented here may also be useful in pollination studies, and in the improvement of mango breeding programmes and commercial fruit production.

Aluminum-steel dissimilar robotic arc spot welding with auxiliary insert

The authors have developed the novel process for joining dissimilar metals, which uses gas metal arc welding and an auxiliary insert or element. In this process, the element or the flanged steel rivet that has a hollow center is inserted into the pre-hole prepared in an aluminum alloy sheet for the first step. Second, the hollow center is filled with steel weld metal by arc spot welding to join the element and a steel sheet at the fixing point, and thereby the element and the steel sheet are welded strongly enough to fasten the aluminum alloy sheet between the flange of the element and the steel sheet. This new joining process is called element arc spot welding (EASW). EASW has such advantages that it can easily be applied to a dissimilar metal joint with ultra-high-strength steel; it does not need to pinch the joining metals from the upper and lower sides of the joint with a clamping tool; and the resultant shear and peel strengths of the joint are equal to or greater than those by other dissimilar metal joining processes. The prototype robot system for automating the EASW process has also been developed. In this robot system, the position of the pre-hole in the aluminum alloy sheet is detected by an image sensor, and then the tool at the tip of the robot arm moves to the detected pre-hole position to insert the element correctly into the hole; in the next step, low-spatter arc welding is carried out to fasten the aluminum alloy sheet between the element and the steel sheet.

Model of a rectangular multi-Gaussian Schell-model vortex beam and its properties

Considering the theory of coherence, the rectangular multi-Gaussian Schell-model vortex beam (RMGSMV) beam generated by multi-Gaussian Schell-model source has been introduced, and the propagation properties of RMGSMV beam propagating in free space has been analyzed by using the derived equations. The effects of beam parameters on the properties of RMGSMV beam propagating in free space have been discussed. The results show that the RMGSMV beam propagating in free space can remain the dark hollow center at the short propagation distance, and the beam will gradually evolve into a beam with similar rectangular flat-topped profile as the propagation distance increases.

Properties of multi-Gaussian correlated partially coherent anomalous hollow beam propagating in underwater oceanic turbulence

The model of partially coherent anomalous hollow beam (AHB) generated by multi-Gaussian Schell-model source has first been introduced, and then the evolution properties of multi-Gaussian correlated partially coherent AHB propagating in underwater oceanic turbulence have been illustrated and analyzed using the numerical examples. The results show that the multi-Gaussian correlated partially coherent AHB propagating in stronger underwater oceanic turbulence will lose the initial dark hollow center with central solid core profile and evolve into Gaussian beam faster as the propagation distance increases. The results also show that the spectral degree of coherence for multi-Gaussian correlated partially coherent AHB with larger the number of Gaussian functions M will decreases more slowly.

Production of propagation-invariant vector beams with square array by use of 2D grating and prism

In this paper, we realized to produce the propagation-invariant vector beams with square array by use of one 2D grating and right angle prism. The vector beams have the square shape with the square hollow center. The simulation and experimental results demonstrate the feasibility of this method.

Propagation properties of a partially coherent anomalous hollow vortex beam in underwater oceanic turbulence

Based on the extended Huygens–Fresnel principle, the cross-spectral density function of a partially coherent anomalous hollow vortex beam (AHVB) propagating in underwater oceanic turbulence has been derived, and the influences of the beam parameters and underwater oceanic turbulence on the average intensity and coherence properties are investigated by using numerical examples. Our results show that the partially coherent AHVB with smaller coherence length and smaller M will evolve into the flat-topped faster as the propagation distance increases, and the partially coherent AHVB propagating in stronger underwater oceanic turbulence (the larger χT and ς or the smaller ε) will lose the initial dark hollow center and evolve into the Gaussian beam more rapidly. In the influences of oceanic turbulence on the spreading of partially coherent AHVB, it is found that the parameters χT and ς will cause the larger spreading than the parameter ε at the same propagation distance. This research is useful to the practical application of the partially coherent AHVB beam in underwater oceanic turbulence.

LaFeO3 porous hollow micro-spindles for NO2 sensing

LaFeO3 micro-spindles with intriguing structural characteristics were synthesized using cyano-bridged coordination polymers as sacrificial templates. Within a longitudinal dimension of less than 5 µm, each micro-spindle features a hollow center contained in a mesoporous shell with hexagonal cross section. Conductometric gas sensors developed from these micro-spindles demonstrate a comprehensively improved NO2-sensing performance over other-reported LaFeO3 devices, including lower operating temperature (155 °C), higher sensitivity (response of 81.4% under 5 ppm) and expedited response and recovery (40 s/329 s). Such performance is enabled by a semiconducting mechanism governed by the formation and catalytic dissociation of nitrite species during NO2 adsorption on LaFeO3, facilitated by the special hierarchical structure of the micro-spindle.

Mesh Twisting Technique for Swirl Induced Laminar Flow Used to Determine a Desired Blade Shape

Swirling flow has been shown to increase heat transfer in heat exchangers. However, producing swirl while not presenting a severe pressure drop can be a challenge. In this paper, a desired shape of guidance blades for laminar swirl flow is determined by numerical simulation in OpenFOAM. Emphasis is on the mesh technique, where a predefined blade shape is formed by mesh twisting, or morphing. The validity of numerical simulations on a twisted mesh is shown by comparing it to the theoretical solution of laminar flow in a pipe without swirl and guidance blades. A sensitivity study shows that a cell size ratio of 0.025 of diameter is sufficient and affects the solution minimally. To determine the desired shape of guidance blades previously found optimal swirl decay and velocity profile for laminar swirling flow are utilized. Three blade shapes are explored: (I) with a twist angle that varies with axial location only; (II) having a deviation angle matching the theoretical deviation angle for laminar swirling flow; (III) same as II but with a hollow center. Simulations are performed for Re = 100 and swirl number S = 0.2 . Case II is able to sustain swirl longest while maintaining a low pressure drop and is therefore a desired swirler shape profile as predicted theoretically.

The effect of ball size on the hollow center cracked disc (HCCD) in Brazilian test

Hollow center cracked disc (HCCD) in Brazilian test was modelled numerically to study the crack propagation in the pre-cracked disc. The pre-existing edge cracks in the disc models were considered to investigate the crack propagation and coalescence paths within the modelled samples. The effect of particle size on the hollow center cracked disc (HCCD) in Brazilian test were considered too. The results shows that Failure pattern is constant by increasing the ball diameter. Tensile cracks are dominant mode of failure. These crack initiates from notch tip, propagate parallel to loading axis and coalescence with upper model boundary. Number of cracks increase by decreasing the ball diameter. Also, tensile fracture toughness was decreased with increasing the particle size. In this research, it is tried to improve the understanding of the crack propagation and crack coalescence phenomena in brittle materials which is of paramount importance in the stability analyses of rock and concrete structures, such as the underground openings, rock slopes and tunnel construction.

Temperature-Controllable Hydrogels in Double-Walled Microtube Structure Prepared by Using a Triple Channel Microfluidic System.

Hydrogels in the shape of double-walled microtubes possess great potential for development into artificial human blood vessels. In this work, we have prepared temperature-responsive tubular hydrogels with selectively controllable wall diameters, by using alginate templated photopolymerization in a triple channel microfluidic device. These tubular hydrogels mimic human blood vessels because of the separate thermally active inner and passive outer walls. The different behavior of each wall leads to the expansion of the hollow center volume with increasing temperature. This temperature-based control of the hollow center volume cannot be achieved in the case of conventional hydrogel microtubes. Furthermore, through this method, the hydrogels can be modified to achieve a controllable outer diameter while maintaining the hollow center dimensions simply by changing the position of the hydrogel walls. The ability to change the layer properties of the developed system indicates that the preparation of hydrogels with various monomers is possible.

Average intensity of anomalous hollow beam with orbital angular momentum in atmospheric turbulence

The propagation equation of average intensity of anomalous hollow beam (AHB) with optical angular momentum (OAM) in atmospheric turbulence has been derived, and the influences of constant of refraction index structure of atmospheric turbulence and the parameters of AHB with OAM on the average intensity and spreading properties are analyzed. It is found that AHB with OAM propagating in atmospheric turbulence can almost the dark hollow center as the propagation distance increases, and the AHB with OAM propagating in free space can keep its dark hollow center, and the beam propagating in stronger atmospheric turbulence will lose its initial dark hollow beam profiles more rapidly. This obtained results are useful to the practical application in free space optical communication using the AHB with OAM.

Optimization of thermoelectric generator (TEG) integrated with three-way catalytic converter (TWC) for harvesting engine’s exhaust waste heat

The exhaust-based thermoelectric generator (TEG) is a promising way to recover the waste heat of internal combustion engines (ICEs). Integrating TEG with three-way catalytic converter (TWC) can improve the output power of TEG and reduce additional pressure drop in the exhaust system. In the present study, some optimization designs for the integration structure of TEG and TWC are carried out through a complete 3-D numerical model. Moreover, the overall performance improvement between the present TEG and a single TEG is illustrated. The results show that the uniform temperature distribution and high output power can be obtained with TWC placed in the rear of heat exchanger. Increasing the number of thermoelectric modules (TEMs) can’t always increase the maximum output power of TEG, but can reduce the conversion efficiency of TWC because more heat is absorbed from the exhaust gas. On the condition of maintaining high conversion efficiency of TWC, a moderate height of hollow center body contributes to higher maximum output power of TEG with reasonable pressure drop. Compared with a single TEG, the maximum output power of present TEG is increased by 16%, and the maximum net output power is increased by 37% with considering the power loss caused by the additional pressure drop and increased weight.

A novel hydrogen storage medium of Ca-coated B40: First principles study

Using first principles study, we have investigated the hydrogen storage capacity of Ca-coated B40. Our result shows that Ca prefers to adsorb on the top hollow center of heptagonal ring of B40 due to the large binding energy of −2.820 eV. Bader charges calculation indicates that charges transfer from Ca to B40 result in an induced electric field so that H2 molecules are polarized and adsorbed onto the surface of B40 without dissociation. The Ca6B40 complex can adsorb up to 30 H2 molecules with average adsorption energy of −0.177 eV/H2 and the hydrogen storage gravimetric density reaches up to 8.11 wt.%, higher than the goal from DOE by the year 2020. These findings will suggest a new and potential structure for hydrogen storage in the future.

Metalens optical 3D-trapping and manipulating of nanoparticles

A principal design of a single-beam optical trap is proposed based on the planar metalens assembled from an ordered array of dielectric microspheres arranged in a closely packed micro-assembly with a hollow center. By means of FDTD numerical simulation, we study in detail the spatial structure of the optical field within the trap active zone and present an example of metalens-trap operation demonstrating the capturing and propulsion of a glass nanoparticle by the optical field.

Preferential Pt Nanocluster Seeding at Grain Boundary Dislocations in Polycrystalline Monolayer MoS2.

We show that Pt nanoclusters preferentially nucleate along the grain boundaries (GBs) in polycrystalline MoS2 monolayer films, with dislocations acting as the seed site. Atomic resolution studies by aberration-corrected annular dark-field scanning transmission electron microscopy reveal periodic spacing of Pt nanoclusters with dependence on GB tilt angles and random spacings for the antiphase boundaries ( i.e., 60°). Individual Pt atoms are imaged within the dislocation core sections of the GB region, with various positions observed, including both the substitutional sites of Mo and the hollow center of the octahedral ring. The evolution from single atoms or small few atom clusters to nanosized particles of Pt is examined at the atomic level to gain a deep understanding of the pathways of Pt seed nucleation and growth at the GB. Density functional theory calculations confirm the energetic advantage of trapping Pt at dislocations on both the antiphase boundary and the small-angle GB rather than on the pristine lattice. The selective decoration of GBs by Pt nanoparticles also has a beneficial use to easily identify GB areas during microscopic-scale observations and track long-range nanoscale variances of GBs with spatial detail not easy to achieve using other methods. We show that GBs have nanoscale meandering across micron-scale distances with no strong preference for specific lattice directions across macroscopic ranges.