Three-stage Mechanism Research Articles

Влияние температуры апконверсионных частиц NaYF4:Er,Yb на формирование люминесценции

The intensity of upconversion luminescence depends nonlinearly on the excitation intensity. The aim of this work is to study the effect of the temperature of NaYF4:Er,Yb upconversion particles on the dependence of the luminescence intensity on the excitation intensity. The synthesized particles were observed to have the shape of a hexagonal prism with a width of about 440 nm and a height of 445 nm. The upconversion luminescence spectra were obtained in the temperature range of 22–55° C with the excitation intensity in the range of 1.5–9.4 W/cm2. The obtained results show the green-band luminescence photons can be generated by means of two-step and three-step mechanisms: the contribution of these mechanisms depends on the temperature of the particles. As the temperature increases, the contribution of the three-stage mechanism of green luminescence excitation is enhanced.

Industrial structure transformation and provincial heterogeneity characteristics evolution of air pollution: Evidence of a threshold effect from China

As an important link between human economic activities and air quality, industrial structure is the key factor in resolving the contradiction between economic development and the environment. By constructing a panel threshold model between China's air pollution and the industrial structure, this study analyzes and explores how industrial transformation influences air pollution as well as its provincial heterogeneity characteristics evolution. The results show that industrial structure has a three-stage influential mechanism on NOx pollution and PM2.5 pollution, and a two-stage influential mechanism on SO2 pollution. A decrease in proportion of the secondary industry output in GDP can significantly reduce NOx pollution and SO2 pollution. Industrial structure can change the effects of economic development on air pollution. China’ s industrial structure has been continuously upgraded in the past decade. In the process of industrial structure transformation, energy consumption has always been a major factor in aggravating air pollution, whereas technological innovation has been an important way to reduce air pollution. Urbanization is another major factor exacerbating air pollution in the provinces where the service industry has always dominated the local economy. Based on the above research, the government should formulate corresponding industrial structure adjustment policies to reduce air pollution according to the provincial heterogeneity of industrial structure affecting different pollutants.

Hierarchical microfibrillar gels from evaporation-induced anisotropic self-assembly of in situ-generated nanocrystals

Whilst nanocrystal gels may be formed via destabilization of pre-functionalized nanocrystal dispersions, gelation via assembly of unfunctionalized nanocrystals into fibrillar networks remains a significant challenge. Here, we show that gels with hierarchical microfibrillar networks are formed from anisotropic self-assembly of in situ-generated mesolamellar nanocrystals upon evaporation of ZnO nanofluids. The obtained gels display the thermo-reversible behavior characteristic of a non-covalent physical gel. We elucidate a three-stage gelation mechanism. In the pre-nucleation stage, the cloudy ZnO nanofluid transforms into a transparent stable suspension, comprising multi-branched networks of aggregates self-assembled from in situ-generated layered zinc hydroxide (LZH) nanocrystals upon solvent evaporation. In the subsequent nucleation and anisotropic 1D fibre growth stage, further evaporation triggers nucleation and growth of 1D nanofibers through reorganization of the nanocrystal aggregates, before rapid nanofibre bundling leading to microfibrillar networks in the ultimate gelation stage. Our results provide mechanistic insights for hierarchical self-assembly of nanocrystals into fibrillar gels and open up facile fabrication routes using reactive transition metal-oxide nanofluids for new functional fibres and gels.

Screening of Hearing Impairment in High-Risk Neonates: A Study at Dr. R.N. Cooper Hospital and H.B.T. Medical College

Abstract Introduction The aim of this study was to find out the prevalence of hearing impairment in high-risk neonates born in Dr. R.N. Cooper Hospital and H.B.T Medical College and establish the fact that high-risk neonates have higher prevalence of hearing impairment compared with normal population. Materials and Methods A prospective observational study was conducted over a period of 1 year starting from March 2016 and involved three-stage screening of 410 neonates admitted to neonatal intensive care unit of Dr. R.N. Cooper Municipal General Hospital and H.B.T Medical College, Vile Parle (west), Mumbai. All enrolled neonateswere screened by a three-stage screening mechanism. First otoacoustic emission (OAE-1) screening was done within 24 to 72 hours of birth. Parents of neonates referred by OAE-1 were instructed to come back within 28 days for repeat OAE test (OAE-2). Those referred by OAE-2 were asked to come back after further 2 months for brainstem-evoked response audiometry. Data collected in the study were processed using Microsoft Excel. Results and Conclusion In the current study, out of 410 neonates who were screened by a three-stage screening mechanism, five including three girls and two boys were found to be suffering from profound sensorineural hearing loss. Observed prevalence of 12.20 (or 12 on rounding off to nearest digit) per 1000 in high-risk neonates is much higher compared with the prevalence of 1 to 6 per 1000 live births in overall population as reported by the American Speech-Language-Hearing Association.

Control Of Surface Fouling Of Membrane Modules In Membrane Bioreactors

The article considers the approaches in the field of membrane fouling control in membrane bioreactors used for wastewater treatment. It is highlighted that a significant number of research world-wide is devoted to the study of membrane fouling, which makes it possible to point out and identify the main factors affecting membrane fouling. Generally, such factors are the parameters of operation of the biological treatment reactor with a submerged membrane module, the properties of the biomass of activated sludge, as well as the characteristics of the membranes themselves, while the process of fouling is a three-stage mechanism. This article considers two approaches to control the process of the surface fouling of membrane modules: by improving the functioning of the aeration system and using floating biomass carriers. For aeration systems the concept of the minimum allowable aeration intensity is defined as a value which positively affects the reduction of fouling. The use of biomass carriers in addition to the function of biomass accumulation makes it possible to create additional shear forces on the surface of the membrane module, which contribute to more intensive removal of contaminants from the surface. It is established that the combination of several ways to control fouling is particularly effective, which has a positive effect on the operation of the treatment systems.

Understanding the origin of the enantioselectivity and the mechanism of the asymmetric reduction of ketimine generated from acetophenone with oxazaborolidine catalyst

The enantioselectivity of the oxazaborolidine-catalysed asymmetric reduction reaction of ketimine generated from acetophenone has been investigated by transition state theory at the hydride transfer step using DFT methods at the B3LYP/6-31G(d,p) level of theory. The obtained results indicate that the hydride attack at the Si face is more favourable than the Re one. Analyses of non-covalent interactions and molecular electrostatic potential indicate that the several favourable interactions at the Si hydrogen transfer mode are the origin of the enantioselectivity observed experimentally. Electron Localisation Function topological analysis indicates that the studied hydride transfer step takes place via a non-concerted three-stage mechanism.

Insights into oil recovery, soil rehabilitation and low temperature behaviors of microwave-assisted petroleum-contaminated soil remediation.

As an environmental contaminant, petroleum-contaminated soil will pollute water, stunt agricultural growth, and cause serious harms to human health if it wasn't safely disposed and remediated. Here, low-temperature microwave-assisted treatment of petroleum-contaminated soil was developed for simultaneous soil rehabilitation, oil recovery and revegetation. The results indicated the concentration of petroleum contaminant was decreased to regulatory standard after 20 min treatment at 250-300 °C. 91.6% of the oil was recovered, and it mainly consisted of C11-C30 hydrocarbons. Using the soil remediated at 250 °C for plant growth test, no adverse effect or fertility loss was observed and an optimal clover germination rate was reached. The results of microwave thermogravimetric analysis, dielectric property and EDX mapping revealed that the efficient remediation was attributed to the presence of hot spots, and the efficient heat /mass transfer during microwave heating. A three-stage petroleum removal mechanism was proposed, where the hydrocarbons were gradually removed via steam stripping, thermal desorption, and pyrolysis/carbonization as the temperature increased.

Thermal studies on the starch-g-copolymers prepared from two terpene acrylate monomers under oxidative conditions

The simultaneous thermal studies (TG/DTG/DSC) coupled with the FTIR analysis of the gaseous decomposition products created under oxidative heating of starch-g-poly(neryl acrylate) and starch-g-poly(geranyl acrylate) copolymers have been presented. To these studies, the copolymers with the following grafting percents (G) were selected: starch-g-poly(neryl acrylate) copolymers: 36.6 ± 0.3%, 40.3 ± 0.4%, 42.8 ± 0.4% and starch-g-poly(geranyl acrylate) copolymers: 28.9 ± 0.2%, 32.4 ± 0.6%, 35.6 ± 0.4%. The performed tests proved that the thermal resistance of the copolymers was strongly dependent on their G values, despite a small difference in the G values between the samples. The slight increase (ca. 6.2–6.7%) in the G value caused the significant drop of the thermal stability of all the studied materials. The TG/DTG/DSC studies confirmed at least three-stage decomposition mechanism of the copolymers where simultaneous pyrolysis, oxidation, dehydration and decarboxylation processes took place. The TG/FTIR analyses showed the emission of various structure fragments; among them, one can mention the creation of some organic fragments such as aldehyde, acid, alkene, alkane, furan fragments, CH4 and inorganic species (CO2, CO, H2O) as a result of the oxidative decomposition processes of the studied copolymers. In addition, the conducted studies demonstrated similar decomposition course and mechanism for both types of the copolymers, regardless of the monomer type used to the graft process.

Establishment of safety structure theory

In order to reveal the essence of “safety”, to realize the active identification of hazard and the active prevention and control of accidents, the safety structure theory (SST) is established in this paper. A new definition of “safety” is given from the view of the interaction between activities and environment. And the definition of the instability domain of safety events is given based on the definition of connotation in logic. According to the factor space theory (FST), factors are used to express activities and environment, then the environment factors space cane (EFSC) is developed to “bearing” the states of environment and safety events, and the atomic activity factor space (AAFS) is developed to “bearing” the states of activities. The mapping from EFSC × AAFS to EFSC is established to describe the interaction between the atomic activity factor (AAF) and atomic environment factor (AEF), then the safety structure consisting of AAF, AEF, and the interaction between the two is developed. The evolution mechanism and evolution process of safety issues are analyzed according to the changes of states of safety structures and the relationships between the states of simple environment factors (SEFs) and instability domain of safety events. Then the definition of hazard based on safety structures and the active identification method are given, and the guiding ideology for establishing dynamic risk measurement and prediction models of safety events is given. According to the risk assessment and prediction, a three-stage echelon active safety control mechanism for safety events consisting of advanced feedforward control, feedforward control and feedback control is established. SST is a new safety science theory, which can provide a new analysis method for revealing the essence of “safety” and the evolution mechanism of safety issues, and open up a new way for the development of active safety control.

An Investigation into the Stages of Alloy 625 Crevice Corrosion in an Ocean Water Environment: Initiation, Propagation and Repassivation in a Remote Crevice Assembly

In this paper we investigate the initiation, propagation and repassivation of alloy 625 crevice corrosion in ASTM artificial ocean water using a remote crevice assembly. This assembly was equipped to monitor crevice corrosion current at a constant potential and consisted of two alloy 625 washers fastened together at a set torque (gap) with an alloy 625 nut and bolt. Applied potential controlled initiation time with higher potentials having a shorter induction period. Independent of applied potential, crevice corrosion always initiated farthest from the mouth of the crevice, at the tip. This initial attack was shallow, consistent with transpassive dissolution even though the applied potential was in the passive region. As time progressed, an active corrosion front moved toward the mouth and the crevice current increased. At a critical distance from the mouth, forward movement stopped, and the damage changed to deep trenching. The crevice repassivation potential, as determined using the Tsujikawa-Hisamatsu method, was a function of the position of the active front with depths closer to the mouth having more positive repassivation potentials. These results are discussed within the context of a three-stage mechanism for crevice initiation and propagation in alloy 625.

Enhancing Upconversion Luminescence Efficiency via Chiral β-NaYF4:Er3+/Yb3+ Microcrystals Based on Mesoscale Regulation.

Chirality, universal characteristics of nature, introduces asymmetry in synthetic materials. Revealing the microscopic asymmetry of macroscopically symmetric materials is the key to control the growth of chiral materials and give full play to their application potential. Materials for photon upconversion (UC) are of great interest for many applications owing to their anti-Stoke luminescence process, especially for chiral UC materials. For the preparation of UC materials, a tiny change in reaction parameters will lead to variations in morphology, phase components, and fluorescence intensity, as well as its chirality. Because of the strict reaction conditions for the formation of chiral UC materials, there are no reports of the successful synthesis of chiral UC materials. Therefore, a facile method for the controllable synthesis of chiral UC materials is highly desired. Herein, chiral-assembled hexagonal prisms of β-NaYF4:Er3+/Yb3+ microcrystals were synthesized to realize the smart manipulation of their morphology as well as a great improvement of the fluorescence efficiency. We proposed a three-stage doped β-NaYF4 crystal growth mechanism on mesoscale regulation, where the fluorescence enhancement principle of chirality was revealed. The enhancement of fluorescence efficiency of chiral UC materials endows their promising application in luminescent displays.

Facile synthesis of three-dimensional ZnO hierarchical microspheres composed of well-ordered nanorods by hydrothermal method

Abstract Three-dimensional (3D) ZnO hierarchical microspheres composed of well-ordered nanorods were synthesized by a facile one-step hydrothermal method without any surfactants. The effects of reaction conditions including reaction temperature, reaction time on the microstructure of hierarchical microspheres were investigated. The results demonstrated that the reaction conditions played an important role in the morphology of 3D ZnO hierarchical microspheres. A three-stage reaction mechanism of 3D ZnO hierarchical microspheres was proposed. Firstly, Zn2+ reacts with OH− and NH4+ to produce nanoparticles. Then the nanoparticles are self-assembled into nucleis. Finally is the formation and growth of the hierarchical structure on the nuclear surface. When the as-synthesized ZnO were used as the anode of the dye sensitization solar cells (DSSCs), the 3D ZnO hierarchical microspheres composed of well-ordered nanorods exhibits the best photoelectric properties, which was attributed to the increased surface area and advantageous electronic transmission path.

Bent conformation of a backbone pilin N-terminal domain supports a three-stage pilus assembly mechanism

Effective colonization of host cells by some Gram-positive bacteria often involves using lengthy, adhesive macromolecular structures called sortase-dependent pili. Among commensals, the gut-adapted Lactobacillus rhamnosus GG strain encodes the operons for two varieties of these pili (SpaCBA and SpaFED), with each structure consisting of backbone, tip, and basal pilin subunits. Although the tertiary structure was recently solved for the backbone subunit (SpaA) of the SpaCBA pilus, no structural information exists for its counterpart in the SpaFED pilus. Here, we report several crystal structures for the SpaD backbone pilin, two of which capture the N-terminal domain in either the closed (linear) or open (bent) conformation. To our knowledge, this is the first observation of the bent conformation in Gram-positive pilin structures. Based on this bent conformation, we suggest a three-stage model, which we call the expose-ligate-seal mechanism, for the docking and assembly of backbone pilins into the sortase-dependent pilus.

Three-stage Evolution Mechanism of Multiple Stability Problems and Control Strategies of AC/DC Power System with Large-scale Wind Turbine Generators

There exists interactive evolution process of multiple stability problems of AC/DC power system with large-scale HVDC and wind power, such as voltage or frequency fluctuation and power oscillation, which can be caused by HVDC commutation failure. Further research is required on the evolution mechanism and control strategies of this kind of power system. In this paper, firstly three-stage evolution mechanism is proposed, including large-scale wind turbine generators tripping off caused by transient overvoltage, transient angle instability caused by power flow transferring in weak transmission section, and out-of-step separation caused by cross-regional power oscillation. Secondly, evolution processes and typical models of different stability problems are studied deeply. The prevention and control strategies are proposed. Finally, simulation results and analysis based on real-sized power grid in China prove the rationality and effectiveness of the proposed three-stage evolution mechanism and control strategies.

Response of high-strength steel beam and single-storey frame in fire: Numerical simulation

The design principles for high strength steel (HSS) structures exposed to fire are under development. In this paper, the response of HSS structures in fire is studied through numerical simulation of a beam and a two-bay frame. Geometrical imperfections and residual stresses are introduced into the structural models. Deformation limit criteria are used to compare the critical temperatures of the structures made of HSS and mild steel. The comparisons show that HSS structures have higher strength reserve than mild steel structures. Using the mechanical properties at elevated temperatures from literature sources, it is observed that the deflection behaviour of the studied structures depends on the ratio of strength to elastic modulus. The deflection of the studied beam is sensitive to yield strength reduction factors as the beam fails by plastic hinge mechanism. Whereas, the deflection of the HSS frame is sensitive to the reduction factors of the elastic modulus as the frame fails by inelastic instability. The above-mentioned observations on the studied structures are made using a three-stage mechanism which is developed for interpreting the deformation response.

In situ monitoring and analysis of void evolution in unidirectional prepreg

In the layup of prepreg laminates, air is inevitably entrapped between adjacent prepreg plies, yet the removal of this inter-ply air under vacuum bag cure conditions is not well understood. In this study, an in situ visualization technique was developed to dynamically observe inter-ply air removal during the cure of an out-of-autoclave prepreg. The technique was used to investigate mechanisms of air removal and void evolution in unidirectional prepreg. Prepreg impregnation was also tracked by inspection of laminate cross-sections prepared at different times during the cure cycle. From these data, a three-stage air removal mechanism was documented based on the relationships between void content, resin properties, and tow impregnation as functions of time. Furthermore, a positive correlation was observed between the rate of evacuation and bubble elongation. Though discussed here in the specific context of voids in unidirectional laminates, the in situ observation technique developed for this work has broad potential to enhance understanding of processing phenomena associated with out-of-autoclave prepregs.

Theoretical Insight into the Mechansim and Origin of Ligand-Controlled Regioselectivity in Homogenous Gold-Catalyzed Intramolecular Hydroarylation of Alkynes.

This work aims at understanding the mechanism and regioselectivity in ligand-controlled gold-catalyzed divergent intramolecular hydroarylation of alkynes reported by Jiang et al. ( J. Am. Chem. Soc. 2016 , 138 , 5218 - 5221 ). Focusing on a representative alkyne, N-propargyl-N-tosylaniline, we conducted a detailed computational study on the ortho- and para-position hydroarylation of the alkyne catalyzed by gold(I) catalysts with different ligands. Both the ortho- and para-position hydroarylation reactions are found to follow a similar three-stage mechanism: electrophilic cyclization, proton loss, and protiodeauration. The initial electrophilic cyclization was identified as the rate- and regiochemistry-determining step. With the flexible electron-deficient phosphite ligand, the ortho-position cyclization is identified as the energetically more favorable pathway, while with the rigid electron-abundant phosphine (Xphos) ligand, the dominant pathway turns to the para-position cyclization. The theoretical results are in good agreement with the experimental observations. The π-π interaction between alkynyl phenyl and the directing acylamino group are found to be mainly responsible for the observed ortho-selectivity, while a combination of favorable noncovalent CH···π interaction and steric repulsion between Xphos ligand and alkynyl group contributes to the observed exclusive para-selectivity. The present calculations provide deeper insight into the mechanism and origin of regioselectivity of the title reaction.

Monitoring of the Course of the Silanolate-Initiated Polymerization of Cyclic Siloxanes. A Mechanism for the Copolymerization of Dimethyl and Diphenyl Monomers

Homopolymerization of octamethylcyclotetrasiloxane, D4, and two copolymerizations of this monomer with octaphenylcyclotetrasiloxane, D4Ph2, and hexaethylcyclotrisiloxane, D3Et2, were investigated by monitoring the courses of their occurrence using three different methods: size exclusion chromatography (SEC), thermal gravimetric analysis (TGA), and 29Si NMR spectroscopy. The results obtained showed that while homopolymerization of D4 and its copolymerization with D3Et2 occurred in a nearly identical manner, suggesting that reactivities of the two monomers toward the silanolate active species were practically identical, the copolymerization of D4 with D4Ph2 exhibited a distinct initial induction period resulting from two factors: (a) very low solubility of solid D4Ph2 in D4 and (b) significantly higher reactivity of D4Ph2 than D4 toward the dimethylsilanolate. On the basis of these results, we propose a new three-stage mechanism for the copolymerization of D4 with D4Ph2, which includes (a) initial formation of B–A–B triblocks of dimethylsiloxane (A) and diphenylsiloxane (B) segments, followed by (b) subsequent formation of A–B–A–B–A pentablock species, and (c) redistribution of the former multiblocks through a siloxane equilibration reaction into copolymers containing single diphenylsiloxy units separated by extended polydimethylsiloxane segments.

Mechanism of Urea Crystal Dissolution in Water from Molecular Dynamics Simulation.

Molecular dynamics simulations are used to determine the mechanism of urea crystal dissolution in water under sink conditions. Crystals of cubic and tablet shapes are considered, and results are reported for four commonly used water models. The dissolution rates for different water models can differ considerably, but the overall dissolution mechanism remains the same. Urea dissolution occurs in three stages: a relatively fast initial stage, a slower intermediate stage, and a final stage. We show that the long intermediate stage is well described by classical rate laws, which assume that the dissolution rate is proportional to the active surface area. By carrying out simulations at different temperatures, we show that urea dissolution is an activated process, with an activation energy of ∼32 kJ mol-1. Our simulations give no indication of a significant diffusion layer, and we conclude that the detachment of molecules from the crystal is the rate-determining step for dissolution. The results we report for urea are consistent with earlier observations for the dissolution of NaCl crystals. This suggests that the three-stage mechanism and classical rate laws might apply to the dissolution of other ionic and molecular crystals.

Design and control of a novel asymmetrical piezoelectric actuated microgripper for micromanipulation

Microgripper is an important tool in high precision micromanipulation task, which directly affects the quality and efficiency of micromanipulation. This paper presents the design and control of a novel asymmetrical microgripper driven by a piezoelectric (PZT) actuator. The developed microgripper is designed as an asymmetrical structure with one movable jaw, so it has the advantages of no dense mode and fixed locating datum compared with the symmetrical microgripper with two movable jaws. The main body of microgripper is a compact flexure-based mechanical structure with a three-stage amplification mechanism. Based on the three-stage amplification structure, large-range, robust and parallel grasping operation can be realized. The characteristics analyses of the developed microgripper are carried out by finite element analysis (FEA). A position-force switching control strategy is utilized to regulate the position and grasping force of movable jaw. Discrete-time sliding model (DSM) controller is designed to control the position and grasping force. Experimental studies are conducted and the experiment results show that the microgripper exhibits good performance and high precision grasping operations can be realized through the developed control strategy.