Exciton binding energy has been regarded as a crucial parameter for mediating charge separation in polymeric photocatalysts. Minimizing the exciton binding energy of the polymers can increase the ...yield of charge‐carrier generation and thus improve the photocatalytic activities, but the realization of this approach remains a great challenge. Herein, a series of linear donor–acceptor conjugated polymers has been developed to minimize the exciton binding energy by modulating the charge‐transfer pathway. The results reveal that the reduced energy loss of the charge‐transfer state can facilitate the electron transfer from donor to acceptor, and thus, more electrons are ready for subsequent reduction reactions. The optimized polymer, FSO‐FS, exhibits a remarkable photochemical performance under visible light irradiation.
Modulating the charge‐transfer pathway in a series of linear donor–acceptor conjugated polymers controls their ability to minimize the exciton binding energy. A low exciton binding energy promotes the photoreaction for artificial photosynthesis
We experimentally, numerically and theoretically investigate the nonlinear interaction between a cavitation bubble and the interface of two immiscible fluids (oil and water) on multiple time scales. ...The underwater electric discharge method is utilized to generate a cavitation bubble near or at the interface. Both the bubble dynamics on a short time scale and the interface evolution on a much longer time scale are recorded via high-speed photography. Two mechanisms are found to contribute to the fluid mixing in our system. First, when a bubble is initiated in the oil phase or at the interface, an inertia-dominated high-speed liquid jet generated from the collapsing bubble penetrates the water–oil interface, and consequently transports fine oil droplets into the water. The critical standoff parameter for jet penetration is found to be highly dependent on the density ratio of the two fluids. Furthermore, the pinch-off of an interface jet produced long after the bubble dynamics stage is reckoned as the second mechanism, carrying water droplets into the oil bulk. The dependence of the bubble jetting behaviours and interface jet dynamics on the governing parameters is systematically studied via experiments and boundary integral simulations. Particularly, we quantitatively demonstrate the respective roles of surface tension and viscosity in interface jet dynamics. As for a bubble initiated at the interface, an extended Rayleigh–Plesset model is proposed that well predicts the asymmetric dynamics of the bubble, which accounts for a faster contraction of the bubble top and a downward liquid jet.
Large-scale photochemical synthesis of high value chemicals under mild conditions is an ideal method of green chemical production. However, a scalable photocatalytic process has been barely reported ...due to the costly preparation, low stability of photosensitizers and critical reaction conditions required for classical photocatalysts. Here, we report the merging of flow chemistry with heterogeneous photoredox catalysis for the facile production of high value compounds in a continuous flow reactor with visible light at room temperature in air. In the flow reactor system, polymeric carbon nitrides, which are cheap, sustainable and stable heterogeneous photocatalysts, are immobilized onto glass beads and fibers, demonstrating a highly flexible construction possibility for devices of the photocatalytic materials. As an example of the production of high value chemicals, important chemical structures such as cyclobutanes, which are basic building blocks for many pharmaceutical compounds, like magnosalin, are synthesized in flow with high catalytic efficiency and stability.
A peridynamic Reissner‐Mindlin shell theory Zhang, Qi; Li, Shaofan; Zhang, A‐Man ...
International journal for numerical methods in engineering,
15 January 2021, Letnik:
122, Številka:
1
Journal Article
Recenzirano
Summary
In this work, we have developed a state‐based peridynamics theory for nonlinear Reissener‐Mindlin shells to model and predict large deformation of shell structures with thick wall. The ...nonlocal peridynamic theory of solids offers an integral formulation that is an alternative to traditional local continuum mechanics models based on partial differential equations. This formulation is applicable for solving the material failure problems involved in discontinuous displacement fields. The governing equations of the state‐based peridynamic shell theory are derived based on the nonlocal balance laws by adopting the kinematic assumption of the Reissner and Mindlin plate and shell theories. In the numerical calculations, the stress points are employed to ensure the numerical stability. Several numerical examples are conducted to validate the nonlocal structure mechanics model and to verify the accuracy as well as the convergence of the proposed shell theory.
Covalent triazine frameworks (CTFs) have attracted a great deal of attention as an attractive new class of visible light-active, metal-free, and polymer-based heterogeneous photocatalysts. CTFs have ...demonstrated promising characteristics such as synthetic diversity, stability, nontoxicity, pure organic nature, and enhanced ordered structure. In this review, we aim to summarize the recent developments in CTFs ranging from novel preparation methods to critical factors that directly impact their photocatalytic efficiency. Various physical and chemical design strategies for morphology, band structure, charge separation, and transfer optimization described in the literature are discussed. Emphasis is placed on the enhancement and maximization of photocatalytic efficiencies of specific applications such as photoredox organosynthesis, water splitting, CO2 photoreduction, H2O2 generation, etc.
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•Four types of new refractory HEAs are designed.•These HEAs mainly consist of a simple BCC solid solution.•The formation mechanism of β-Ti is proposed.•Oxidation resistance is ...improved by adding Ti and Si, but reduced by adding V.
High-entropy alloys (HEAs) are defined as the alloys composed of at least five principal elements in equimolar or near equimolar ratios, which can facilitate the formation of simple solid solutions during solidification. Recent studies suggested that the refractory HEAs exhibited great promise for high temperature structural materials. However, their oxidation behavior had received little attention. In the present study, Cr, Al and Si elements were added to improve the oxidation resistance, four types of new refractory HEAs were designed and synthesized, including NbCrMoTiAl0.5 (H-Ti), NbCrMoVAl0.5 (H-V), NbCrMoTiVAl0.5 (H-TiV) and NbCrMoTiVAl0.5Si0.3 (H-TiVSi0.3). Their microstructures and oxidation behavior were studied. As expected, these refractory HEAs mainly consist of a simple body-centred cubic (BCC) refractory metal solid solution (RMss) due to the high mixing entropy effect. Solidification process and thermodynamic analysis were investigated to explain the formation mechanism of their microstructures. For all the refractory HEAs, the oxidation kinetics at 1300°C follows a linear behavior. The oxidation resistance of the HEAs is significantly improved with Ti and Si addition, but reduced with V addition.
This paper is concerned with the dynamics of large bubbles subject to various strengths of buoyancy effects, which are associated with applications for underwater explosion. The bubble is produced by ...electric discharge in a low-pressure tank to enhance the buoyancy effects. Experiments are carried out for a bubble in an infinite field, below a free surface and above a rigid boundary. The effects of buoyancy are reflected by the dimensionless parameter
${\it\delta}=\sqrt{{\it\rho}gR_{m}/(p_{amb}-p_{v})}$
, where
$R_{m}$
,
$p_{amb}$
,
$p_{v}$
,
${\it\rho}$
and
$g$
are the maximum bubble radius, ambient pressure, saturated vapour pressure, density of water and the acceleration of gravity respectively. A systematic study of buoyancy effects is carried out for a wide range of
${\it\delta}$
from 0.034 to 0.95. A series of new phenomena and new features is observed. The bubbles recorded are transparent, and thus we are able to display and study the jet formation, development and impact on the opposite bubble surface as well as the subsequent collapsing and rebounding of the ring bubble. Qualitative analyses are carried out for the bubble migration, jet velocity and jet initiation time, etc. for different values of
${\it\delta}$
. When a bubble oscillates below a free surface or above a rigid boundary, the Bjerknes force due to the free surface (or rigid boundary) and the buoyancy are in opposite directions. Three situations are studied for each of the two configurations: (i) the Bjerknes force being dominant, (ii) the buoyancy force being dominant and (iii) the two forces being approximately balanced. For case (iii), we further consider two subcases, where both the balanced Bjerknes and buoyancy forces are weak or strong. When the Bjerknes and buoyancy forces are approximately balanced over the pulsation, some representative bubble behaviours are observed: the bubble near free surface is found to split into two parts jetting away from each other for small
${\it\delta}$
, or involutes from both top and bottom for large
${\it\delta}$
. A bubble above a rigid wall is found to be subject to contraction from the lateral part leading to bubble splitting. New criteria are established based on experimental results for neutral collapses where there is no dominant jetting along one direction, which correlate well with the criteria of Blake et al. (J. Fluid Mech., vol. 170, 1986, pp. 479–497; J. Fluid Mech., vol. 181, 1987, pp. 197–212) but agree better with the experimental and computational results.
Non-reflecting boundary conditions (NRBCs) play an important role in computational fluid dynamics (CFD). A novel NRBC based on the method of characteristics using timeline interpolations is proposed ...for fluid dynamics solved by smoothed particle hydrodynamics (SPH). It is performed by four layers of particles whose pressures and velocities are obtained through the Lagrange interpolation in the time domain which is derived from the propagation of characteristic waves between particles. The proposed NRBC can allow outward travelling pressure and velocity messages to pass through the boundary without obvious reflection. That is, with the implementation of the NRBC, the solution in a finite computational domain of interest is close to that in an infinite domain. Several numerical tests show that this NRBC is robust and applicable for a broad variety of hydrodynamics ranging from low to high speed.
Electron transfer is the rate‐limiting step in photocatalytic water splitting. Viologen and its derivatives are able to act as electron‐transfer mediators (ETMs) to facilitate the rapid electron ...transfer from photosensitizers to active sites. Nevertheless, the electron‐transfer ability often suffers from the formation of a stable dipole structure through the coupling between cationic‐radical‐containing viologen‐derived ETMs, by which the electron‐transfer process becomes restricted. Herein, cyclic diquats, a kind of viologen‐derived ETM, are integrated into a 2,2′‐bipyridine‐based covalent organic framework (COF) through a post‐quaternization reaction. The content and distribution of embedded diquat‐ETMs are elaborately controlled, leading to the favorable site‐isolated arrangement. The resulting materials integrate the photosensitizing units and ETMs into one system, exhibiting the enhanced hydrogen evolution rate (34600 μmol h−1 g−1) and sustained performances when compared to a single‐module COF and a COF/ETM mixture. The integration strategy applied in a 2D COF platform promotes the consecutive electron transfer in photochemical processes through the multi‐component cooperation.
Viologen‐derived electron‐transfer mediators (ETMs) are integrated into a 2,2′‐bipyridine‐containing covalent organic framework (COF) by a controllable post‐synthetic method. The site‐isolated ETM modules cooperate well with photosensitizer modules on the platform of COFs for enhanced electron‐transfer ability, thereby remarkably promoting the photocatalytic H2 evolution performance (34600 μmol h−1 g−1) from water splitting.
The construction of multiple heteroatom-doped porous carbon with unique nanoarchitectures and abundant heteroatom active sites is promising for reversible oxygen-involving electrocatalysis. However, ...most of the synthetic methods required the use of templates to construct precisely designed nanostructured carbon. Herein, we introduced an ultrasound-triggered route for the synthesis of a piperazine-containing covalent triazine framework (P-CTF). The ultrasonic energy triggered both the polycondensation of monomers and the assembly into a nanoflower-shaped morphology without utilizing any templates. Subsequent carbonization of P-CTF led to the formation of nitrogen, phosphorus, and fluorine tri-doped porous carbon (NPF@CNFs) with a well-maintained nanoflower morphology. The resultant NPF@CNFs showed high electrocatalytic activity and stability toward bifunctional electrolysis, which was better than the commercial Pt/C and IrO2 electrocatalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively. As a further demonstration, employing NPF@CNFs as air electrode materials resulted in an excellent performance of liquid-state and solid-state Zn-air batteries, showing great potentials of the obtained multiple heteroatom-doped porous carbon electrocatalysts for wearable electronics.
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