关键词： Eutectic alloy   Electromagnetic levitation   Solidification kinetics   Undercooling   Mechanical property  

摘要： The rapid solidification mechanism of Ni-Zr alloys was investigated by electromagnetic levitation (EML) technique assisted with high speed videography. A systematic analysis of the competitive growth mode between the primary phase and eutectic structure was conducted for three types of alloys (hypoeutectic Ni-5 at.%Zr, eutectic Ni-8.8 at.%Zr and hypereutectic Ni-13 at.%Zr alloys), whose maximum undercoolings reached up to 260, 192 and 270 K, respectively. With the increase of undercooling, the primary (Ni) phase of hypoeutectic Ni-5 at.% Zr alloy transferred from well-defined dendrites to dendrite fragments. Zr atom was difficult to diffuse into the Ni lattice because the radius of Zr atom was larger than Ni atom. For eutectic Ni-8.8 at.% Zr alloy, the competitive nucleation and growth between primary (Ni) phase and eutectics could happen when the undercooling exceeded 18 K. The refined dendrites of primary (Ni) phase and various eutectic morphologies were observed at the undercooling of 192 K. For the hypereutectic Ni-13 at.% Zr alloy, the growth morphology transition of the primary Ni5Zr intermetallic compound from faceted to non-faceted crystals occurred if the liquid alloy achieved a high undercooling. Furthermore, the atomic scale structure of the primary phase was explored by the transmission electron microscopy (TEM), which revealed that the solid solubility and the lattice constant were consistent at various undercoolings. Moreover, the compressive performance of these three type alloys were determined for EML solidified samples. Due to the effects of undercooling and composition on microstructures, Ni-5 at.%Zr and Ni-8.8 at.%Zr alloys exhibited plastic deformation at the large strain condition, while Ni-13 at.% Zr alloy showed brittle fracture feature. The compressive strength and yield strength displayed a non-linear relationship with the rise of undercooling.

关键词： Substituents   Amides   Reaction products   Reactivity   Aldehydes  

摘要： The rate constants for the aqueous reaction, between pH 0 and 14, have been determined for a series of amide substituted N-(hydroxybenzyl)benzamide derivatives, in H2O, at 25 degrees C, I = 1.0 M (KCl). The N-(hydroxybenzyl)-benzamide derivatives were found to react via three distinct mechanisms with the kinetically dominant mechanism being dependent on the pH of the reaction solution. It has been shown that the carbinolamides react via a specific-base catalyzed mechanism (E1cB-like) under basic and pH neutral conditions. At lower pH values, an acid-catalyzed mechanism was kinetically dominant and, last, a water reaction was postulated at pH values where neither the hydroxide dependent nor the general-acid-catalyzed mechanism was dominant. Contrary to earlier studies with N-(hydroxymethyl)benzamide compounds, no evidence for mechanistic variation based upon the nature of the amidic substituent was observed for any of the N-(hydroxybenzyl)benzamide derivatives studied between pH values of 0-14. The rate for the acid-catalyzed reaction (k(H), rho = 1.17), the apparent second-order hydroxide rate constant (k(1)', rho = 0.87), the hydroxide-independent rate (k(1), rho = 0.65), and the pK(a)'s of the hydroxyl group of the carbinolamide (rho = 0.23) are reported.

关键词： Silicon   Nucleation   Polysilicon   Magnetic nucleation   Condensation and nucleation rates   Silane pyrolysis kinetics   Silicon nanoparticles synthesis  

摘要： Chemical nucleation for the synthesis of solid nanoparticles from gas phase precursor is a fundamental process of technological relevance, yet, not well understood. In this work, a silane pyrolysis system was built to obtain insights in particle formation and growth. Primary particle analysis was used to determine the relative condensation and nucleation rates in the aerosol process of silane pyrolysis. TEM, XRD and SMPS allow the comparison between primary particle and agglomerate dynamics. The analysis reveals that nucleation is correlated to silane concentration to the power of 2/3. This relationship and other observations are used to assess the impact of magnetic interactions in classical Brownian agglomeration in the initial stages of primary particle dynamics in silane pyrolysis. Furthermore, this mechanism is consistent with key observations reported in the literature regarding nanoparticles of various materials, suggesting the proposed mechanism could be valid for more materials and deserves further attention. (C) 2019 Elsevier Ltd. All rights reserved.

关键词： alpha-Glucosidase   Inhibition   Cyanidin   Cinnamomum camphora   Molecular dynamics  

摘要： alpha-Glucosidase inhibitors are widely used to suppress postprandial glycemia in the treatment of type 2 diabetes mellitus. The present study evaluated the in vitro alpha-glucosidase inhibitory activity of three major pigment constituents of Cinnamomum camphora fruit, namely cyanidin, cyanidin 3-rutinoside, and cyanidin-3-O-glucoside. We found that cyanidin exerted strong inhibitory activity on alpha-glucosidase, with IC50 of 5.293 x 10(-3) mM, whereas cyanidin 3-rutinoside and cyanidin-3-O-glucoside did not show inhibitory activity on alpha-glucosidase. The inhibitory activity of cyanidin was stronger than that of acarbose (10(50) 1.644 mM), the current most commonly used drug for postprandial glycemia. Kinetic analysis indicated that cyanidin inhibited alpha-glucosidase through competition, with a K-1 value of 0.0183 mM. Fluorescence spectrum titration showed only one binding site between cyanidin and alpha-glucosidase, and the binding constant was calculated. Further, molecular docking was conducted to simulate the binding interactions between cyanidin and alpha-glucosidase. Cyanidin was found to interact with several residues close to the catalytic site of alpha-glucosidase through pi-pi stack interaction and hydrogen bonds. The calculated binding energy of the cyanidin and enzyme complex was - 105.031 kJ/mol. Molecular simulation and calculation showed that the van der Waals force played an essential role in the binding of alpha-glucosidase and cyanidin. (C) 2019 Elsevier B.V. All rights reserved.

关键词： Transformer   Sulfur corrosion   Cellulose   Hydrogen sulfide   Molecular dynamics simulation  

摘要： Insulation faults caused by corrosive sulfur contained in oil-paper insulation systems in transformers are a key factor affecting the safety and stable of operation of this critical equipment. To date, sulfur corrosion phenomena have been studied from an experimental point of view;however, little attention has been paid to the microscale mechanism of sulfur corrosion. On the basis of molecular dynamics simulations, we analyze the effects of hydrogen sulfide content on the mechanical and thermal properties of cellulose insulating paper in terms of hydrogen bonding, diffusion coefficients, mean square displacement, and mechanical parameters. The results of our kinetic simulations show that the mechanical properties of cellulose decrease as the content of hydrogen sulfide increases. At a hydrogen sulfide content of 8%, the elastic modulus of cellulose decreases by 33.1%, the volume modulus decreases by 46.4%, the shear modulus decreases by 37%, and the glass transition temperature decreases by 83 K. An increase of hydrogen sulfide content weakens interactions between cellulose chains, leading to increased chain motion, which thereby reduces the thermal stability of cellulose. The effects of hydrogen sulfide molecules on the mechanical and thermal properties of cellulose are mainly attributed to the formation of many hydrogen bonds between hydrogen sulfide molecules and cellulose, which destroy the original hydrogen bond network of the cellulose. At a high hydrogen sulfide content, van der Waals forces between cellulose molecules are weakened, lowering intermolecular interactions and reducing the thermal stability of the cellulose.

关键词： Chromophores  

摘要： This paper presents the nonadiabatic molecular dynamics simulation in the solution phase using the spin-flip time-dependent density functional theory (SF-TDDFT). Despite the single-reference level of theory, the SF-TDDFT method can generate the correct topology of S-0/S-1 crossing points, thus providing a natural S-1 -> S-0 nonadiabatic transition. We extend the gas-phase trajectory surface hopping simulation with the SF-TDDFT [N. Minezawa and T. Nakajima, J. Chem. Phys. 150, 204120 (2019)] to the hybrid quantum mechanical/molecular mechanics (QM/MM) scheme. To this end, we modify the code to evaluate the electrostatic interaction between the QM and MM atoms and to extract the classical MM energy and forces from the MM program package. We apply the proposed method to the photoisomerization reaction of aqueous E-azomethane and anionic green fluorescent protein chromophore in water and compare the results with those of the previous simulation studies based on the multireference methods. Published under license by AIP Publishing.

关键词： Lead compounds  

摘要： We investigated the atomistic and dynamical mechanism of polaron formation in methylammonium lead iodide perovskite (MAPbI(3)), which is a representative perovskite solar cell absorber, through the quantum mechanical molecular dynamics simulations. The simulations were conducted on the spatial scale of several nanometres, which can describe charge localization and the associated structural deformation, using the divide-and-conquer-type density-functional tight-binding method, which enables a quantum chemical treatment of systems comprising thousands of atoms. We found that both the structural parts of MAPbI(3), namely, the inorganic framework (PbI3-) and the MA cations, involve the structural deformation associated with polaron formation. We elucidated that in the process of polaron formation, charge localization is invoked by thermal structural fluctuation, and a further structural deformation is caused by the relaxation of the charge carrier. Finally, importance of the two structural parts, PbI3- and MA, was examined from the energetical viewpoint.

关键词： Catalysis  

摘要： Metal-ligand cooperation catalysis is currently the prevailing strategy in the field of homogeneous catalyst research, and is widely used in direct catalytic hydrogenation and transfer hydrogenation reactions. Herein, a density functional theory (DFT) study is conducted to clarify the origin of the different activities of Mn(i) bifunctional catalysts bearing similar Lewis base functional ligands, and amine and hydroxyl groups. The results indicate that a Mn(i) catalyst with an OH group as a bifunctional group requires a higher activation free energy barrier relative to the catalyst with amine as an active ligand, which is in line with the experimental observations. By comparing the electronic structures of the key intermediates in the two catalytic systems, it is found that the Mn-O complex catalyst is thermodynamically unstable and may lead to irreversible decomposition, which accounts for its lower catalytic activity. Moreover, the inductive effect between the OH group and the metal hydride increases unfavorable orbital interactions in the Mn-O system. Consequently, the generation of a metal hydride intermediate becomes a thermodynamically uphill process, further leading to a lack in driving force for the dehydrogenation of (PrOH)-Pr-i. Further investigation suggests that the driving force of the catalyst can be tuned by changing the different oxidation states of the metal centers, revealing a crucial role for the metal center in M-L bond cooperation mode in MLC catalysis. This study highlights that although the hydroxyl and amine groups are both Lewis base functional ligands, subtle differences in the electronic effects of ligand have a significant impact on the activities of the metal-ligand cooperation (MLC) catalysts.

关键词： Well posed problems   design sensitivity analysis   manifold theoretic ODE  

摘要： Manifold theoretic ordinary differential equations of motion for holonomic mechanical systems that depend on problem data, or design variables, are shown to be well posed;i.e., they have a unique solution that depends continuously on problem data. It is proved that these differential equations are equivalent to the d?Alembert variational formulation and the index 3 Lagrange multiplier formulation of differential-algebraic equations of motion, which are also shown to be well posed. These results provide a foundation for dynamic system design sensitivity analysis, which requires differentiability of solutions of the equations of motion with respect to design variables.