关键词： Mechanical activation   DTA-TG   Sintering   Kinetics   Cordierite  

摘要： Since cordierite, 2MgO center dot 2Al(2)O(3)center dot 5SiO(2) (MAS), is a very useful high-temperature ceramic material, it is important to decrease its sintering temperature. In order to accelerate the process of sintering, 5.00 mass% MoO3 was added to the starting mixtures. The mechanical activation of the starting mixtures was performed in a high-energy ball mill in time intervals from 0 to 160 min. After the activation, starting mixtures were sintered at 1300 A degrees C for 2 h. In order to determine the impact of mechanical activation on particle size distribution and powders morphology, the mechanically treated powders were characterized by a laser light-scattering particle size analyzer and scanning electron microscopy. The phase composition of the starting mixtures and sintered samples was analyzed by the X-ray diffraction method. In order to determine temperature intervals of chemical reactions and phase transitions, differential thermal analyses (DTA) and thermo-gravimetric analysis were used. Kissinger's equation was employed to calculate apparent activation energies of various processes that occur within the system during heating. Based on the obtained DTA results, it was established that mechanical activation along with MoO3 additive has influence on sintering temperature which was decreased for more than 100 A degrees C, comparing to the literature data.

关键词： Hydrodynamic lubrication   Hydrodynamic forces   Short bearings   Slider-crank mechanisms  

摘要： The characterization of lubricated revolute joints configures a topic of great interest in mechanisms and machines since there is a significant demand for its proper design in many applications. Therefore, this work proposes a solution for the evaluation of the hydrodynamic force components of the journal bearings obtained from the integration of the Reynolds equation for infinitely short bearings. The main difficulty in obtaining these hydrodynamic forces lies on adequately defining the boundary conditions of the Reynolds equation. Differently from the boundary conditions proposed in classical literature, this work presents an approach to determine the integration boundaries, i.e., the angular position in the bearing reference system where the pressure is null. Therefore, the hydrodynamic forces are calculated taking into account only the positive pressure. These angles are time dependent, and its evaluation is very important for a more accurate estimation of the hydrodynamic force components. The computer simulations are accomplished for different length-diameter ratios (L/D). From the global results, it can be concluded that the proposed approach is promising to efficiently represent the hydrodynamic conditions in beatings with L/D ratios up to 0.5. (c) 2016 Elsevier Ltd. All rights resented.

关键词： Chemical analysis  

摘要： The purpose of the paper is to assess an impact of short- and long-term operational load on continuity and composition of TiN/Au layers deposited on the polymer Bionate II that the pulsatile ventricular assist device Religa Heart Ext is made of. The additional objective is to determine mechanical properties of the TiN/Au layers deposited on the polymer. The physical model of circulatory system was built using the Windkessel model. The tests with the use of energy dispersive spectroscopy and scanning electron microscopy were made on surfaces of samples made of the Religa Heart Ext material examined in the fatigue tests. Mechanical properties of the TiN/Au coatings deposited on the polymer were measured in the nanoindentation test by comparison of the coated and the uncoated samples.

关键词： Chemisorption   Physisorption   Enthalpy   Entropy   Pesticide  

摘要： This study reports the thermodynamic application and non-linear kinetic models in order to postulate the mechanisms and compare the carbofuran adsorption behavior onto rice husk and tea waste derived biochars. Locally available rice husk and infused tea waste biochars were produced at 700 degrees C. Biochars were characterized by using proximate, ultimate and surface characterization methods. Batch experiments were conducted at 25, 35, and 45 degrees C for a series of carbofuran solutions ranging from 5 to 100 mg L-1 with a biochar dose of 1 g L-1 at pH 5.0 with acetate buffer. Molar O/C ratios indicated that rice husk biochar (RHBC700) is more hydrophilic than tea waste biochar (TWBC700). Negative Delta G (Gibbs free energy change) values indicated the feasibility of carbofuran adsorption on biochar. Increasing Delta G values with the rise in temperature indicated high favorability at higher temperatures for both RHBC and TWBC. Enthalpy values suggested the involvement of physisorption type interactions. Kinetic data modeling exhibited contribution of both physisorption, via pore diffusion, pi*-pi electron donor-acceptor interaction, H-bonding, and van der Waals dispersion forces and chemisorption via chemical bonding with phenolic, and amine groups. Equilibrium adsorption capacities of RHBC and TWBC determined by pseudo second order kinetic model were 25.2 and 10.2 mg g(-1), respectively. (C) 2015 Elsevier Ltd. All rights reserved.

关键词： Temperature   Molecular dynamics   Mechanical property   Microscopic deformation mechanism   Single crystal tungsten nanowire  

摘要： High-purity single crystal tungsten nanowire was prepared by the metal-catalyzed vapor-phase reaction method, which was firstly proposed by our research group. Its tensile stress-strain curves and microscopic deformed structures at different temperatures were simulated by molecular dynamics method, in order to study the effects of temperature on its tensile mechanical properties and failure mechanisms. Research results show that the stress-strain curves can be all divided into five stages: elastic, damage, phase transition (dominant), hardening and failure stages. Elastic modulus, tensile strength and strain of phase transition are decreased with increase of temperature, while hardening peak stress is first increased and then decreased. There exists a critical temperature T-c for its phase transition mechanism. When T <= T-c, the phase transition occurs in one direction and leads to less damage and random failure in its damage zone. When T > T-c, the phase transition occurs in two directions and leads to more damage (twin plane) and failure in its twin plane. That is quite different from FCC metal, where there is only one direction of phase transition. Crown Copyright (C) 2016 Published by Elsevier B.V. All rights reserved.

关键词： force field   carotenoids   electronic excitation   light-harvesting  

摘要： Carotenoids are important actors both in light-harvesting (LH) and in photoprotection functions of photosynthetic pigmentprotein complexes. A deep theoretical investigation of this multiple role is still missing owing to the difficulty of describing the delicate interplay between electronic and nuclear degrees of freedom. A possible strategy is to combine accurate quantum mechanical (QM) methods with classical molecular dynamics. To do this, however, accurate force-fields (FF) are necessary. This article presents a new FF for the different carotenoids present in LH complexes of plants. The results show that all the important structural properties described by the new FF are in very good agreement with QM reference values. This increased accuracy in the simulation of the structural fluctuations is also reflected in the description of excited states. Both the energy order and the different nature of the lowest singlet states are preserved during the dynamics when the new FF is used, whereas an unphysical mixing is found when a standard FF is used. (C) 2016 Wiley Periodicals, Inc.

关键词： Bamberger rearrangement   Trifluoroacetic acid   4-aminophenol   Kinetics and mechanism  

摘要： The paper deals with the kinetics of Bamberger rearrangement of N-phenylhydroxylamine to 4-aminophenol in acetonitrile as a solvent catalyzed by TFA. The influence of the water is also investigated being the rearranging moiety in the mechanism. It is evident that the presence of water depresses the rate of the rearrangement suggesting the rate-determining step does not involve water as the key reagent. In acetonitrile, at lower temperature than that of reaction, we evidence the formation of a PHA-TFA complex, and its equilibrium has been measured between 288 K and 298 K. We observe also the addition of water destroys this complex but the ternary equilibrium PHA, water and TFA cannot easily measured because of the complex solvent effect on the Uv-vis and NMR signals. Starting from this evidence, we have proposed a kinetic model, which takes into account all the equilibria, and the results of the fitting gives reliable thermodynamic and kinetic parameters of the entire process. Finally, we have suggested that the PHA-TFA complex is the key intermediate, which determines the regioselectivity of the rearrangement. In fact, preliminary quantum chemistry calculation have shown that the TFA interaction with the NHOH group causes the hindering of the ortho position, thus favoring the attack of water to the para one. (C) 2016 Elsevier B.V. All rights reserved.

关键词： Prediction   Bias   Precision   Species interaction   SDM   Protocol   Uncertainty  

摘要： Environmental change is expected to shift the geographic range of species and communities. To estimate the consequences of these shifts for the functioning and stability of ecosystems, reliable predictions of alterations in species distributions are needed. Projections with correlative species distribution models, which correlate species' distributions to the abiotic environment, have become a standard approach. Criticism of this approach centres around the omission of relevant biotic feedbacks and triggered the search for alternatives. A new generation of mechanistic process-based species distribution models aims at implementing formulations of relevant biotic processes to cover species' life histories, physiology, dispersal abilities, evolution, and both intra- and interspecific interactions. Although this step towards more structural realism is considered important, it remains unclear whether the resulting projections are more reliable. Structural realism has the advantage that geographic range shifting emerges from the interplay of relevant abiotic and biotic processes. Having implemented the relevant response mechanisms, structural realistic models should better tackle the challenge of generating projections of species responses to (non-analogous) environmental change. However, reliable projections of future species ranges demand ecological information that is currently only available for few species. In this opinion paper, we discuss how the discrepancy between demand for structural realism on the one hand and the related knowledge gaps on the other hand affects the reliability of mechanistic species distribution models. We argue that omission of relevant processes potentially impairs projection accuracy (proximity of the mean outcome to the true value), particularly if species range shifts emerge from species and community dynamics. Yet, insufficient knowledge that limits model specification and parameterization, as well as process complexity, increases proje

关键词： PALLADIUM   STOICHIOMETRY   CATALYSIS   NANOPARTICLES   ACETYLACETONE  

摘要： Palladium(0) nanoparticles continue to be important in the field of catalysis. However, and despite the many prior reports of Pd(0)(n) nanoparticles,, missing is a study that reports the kinetically controlled formation of Pd(0)(n) nanoparticles with the simple stabilizer [Bu4N](2)HPO4 in an established, balanced formation reaction where the kinetics and mechanism of the nanoparticle-formation reaction are also provided. It is just such studies that are the focus of the present work. Specifically, the present studies reveal that Pd(acac)(2), in the presence of 1 equiv of [Bu4N](2)HPO4 as stabilizer in propylene carbonate, serves as a preferred precatalyst for the kinetically controlled nucleation following reduction under 40 +/- 1 psig initial H-2 pressure at 22.0 +/- 0.1 degrees C to yield 7 +/- 2 nm palladium(0) nanoparticles. Studies of the balanced stoichiometry of the Pd(0)(n) nanoparticle-formation reaction shows that 1.0 Pd(acac)(2) consumes 1.0 equiv of H-2 and produces 1.0 equiv of Pd(0)(n) while also releasing 2.0 +/- 0.2 equiv of acetylacetone. The inexpensive, readily available HPO42- also proved, to be as effective a Pd(0)(n) nanoparticle stabilizer as the more anionic, sterically larger, "Gold Standard" stabilizer P2W15Nb3O629-. The kinetics and associated minimal mechanism of formation of the [Bu4N](2)HPO4-stabilized Pd(0)(n) nanoparticles are also provided, arguably the most novel part of the present studies, specifically the four-step mechanism of nucleation (A -> B, rate constant k(1)), autocatalytic surface growth (A + B -> 2B, rate constant k(2)), bimolecular agglomeration (B + B -> C, rate constant k(3)), and secondary autocatalytic surface growth (A + C -> 1.5C, rate constant k(4)), where A is Pd(acac)(2), B represents the growing, smaller Pd(0)(n) nanopartieles, and C represents the larger, most catalytically active Pd(0)(n) nanoparticles. Additional details on the mechanism and catalytic properties of the resultant Pd(0)(n)center dot HPO42- n

关键词： cure kinetics   curing mechanism   epoxy-anhydride   differential scanning calorimetry   quaternary phosphonium salts  

摘要： Mechanism and curing kinetics of bisphenol A epoxy resin-iso-methyltetrahydrophthalic anhydride compositions using quaternary phosphonium salts as accelerators were investigated by differential scanning calorimetry (DSC) and electrospray mass-spectrometry (ESI-MS). The DSC method was applied to investigate curing kinetics and apparent activation energy values for the overall curing process. The DSC results showed that some of the phosphonium salts lead to a lower activation energy, that means they are more effective accelerators for the curing of epoxy-anhydride systems. The mechanism of curing was studied by ESI-MS using the model reaction of epichlorohydrin (E) with phthalic anhydride (PA) in the presence of phosphonium salts or 2-methylimidazole. Products containing the alkyl moiety of the phosphonium salt in form of alkyl esters could be identified. This suggests that the phosphonium salts activate the anhydride by electrophilic attack. (C) 2015 Wiley Periodicals, Inc.