关键词： Cross linking   Elastic constants   Glass transition temperature   Molecular dynamics   Self-healing materials   Urea-formaldehyde  

摘要： Urea-formaldehyde polymers, which are utilized in the adhesives industry, have recently been shown to be suitable materials for synthesizing micro/nanocapsules for use in self-healing (nano)composites. In this study, molecular dynamics was employed to simulate the process in which urea and formaldehyde are cross-linked via methylene and ether cross linkers, and to study the structure and mechanical/thermal properties of simulated poly(urea-formaldehyde)s (PUFs). The elastic stiffness constants of the simulated materials were calculated using the constant-strain (static) method. A temperature cycle was applied to the cross-linked PUFs, and the glass transition behavior of each material was investigated through the mean squared displacement (MSD) and temperature evolution of the energy and the specific volume of the polymer. The simulation results confirmed that there was considerable improvement in the properties of the poly(UF) materials upon cross linking. The radial distribution function was also used to study the local structures of the polymers, and this revealed that increasing the temperature and cross linking density results in a significant drop in hydrogen bonding intensity in the cross-linked PUF systems.

关键词： Particulate nanocomposites   Size effects   Mechanical behaviour   Micromechanical models   Molecular Dynamics  

摘要： This paper aims at developing a method to account for a particle size effect on the mechanical behaviour of particulate nanocomposites. An introduction of specific information at the atomic scale, through Molecular Dynamics (MD) simulations, into homogenisation models of the overall mechanical behaviour of heterogeneous materials (micromechanical approaches) is proposed here. By studying virtual nanocomposites, which consist of silica inclusions embedded in a polymer matrix, MD simulations have shown the existence of an interphase of disturbed matrix surrounding the inclusions, whose thickness depends neither on the inclusion size nor on the volume fraction of inclusions. By considering this interphase of fixed thickness, a particle size effect is then deduced from a dilute micromechanical model which derives from the classical Eshelby's inhomogeneity problem. Effective elastic moduli of the composite strongly vary with the particle size for a fixed volume fraction of particles. Nevertheless, opposite trends are observed relative to the interphase behaviour. Whereas effective moduli increase with the particle size for an interphase softer than the matrix, they decrease in the reverse case. The confrontation between MD and micromechanical approaches and the characterisation of the interphase elastic moduli by an inverse method allow the stiffening effects observed by MD to be explained in the case of an interphase softer than the pure matrix. (C) 2013 Elsevier B. V. All rights reserved.

关键词： NdFeB alloy   Reaction milling in hydrogen   Disproportionation   Reaction kinetics  

摘要： The process of solid gas reaction milling in hydrogen was applied to achieve full disproportionation and microstructure nanocrystallization of NdFeB alloy. The NdFeB alloy was disproportionated by room-temperature reactive ball milling in hydrogen, with the mechanical energy serving as the driving force. The reaction progress during milling was examined by hydrogen absorption measurement, and the corresponding microstructure change was characterized by X-ray diffraction (XRD). The mechanism of mechanically activated hydrogenation disproportionation reaction of NdFeB alloy was investigated. The disproportionation kinetics of an as-cast Nd16Fe76B8 (atomic ratio) alloy during reaction milling was experimentally studied by in-situ monitoring of the hydrogen absorbed by the alloy in the vial. On the assumption that the impact energy to activate the reaction plays a role analogous to that of the thermal energy for the conventional thermally activated process, a theoretical model for the kinetics of the mechanically assisted disproportionation was proposed. By fitting the experimental data with the theoretical model, the kinetic equation and the energy utilization efficiency under different milling conditions were obtained. The effect of milling parameters and hydrogen pressure on the disproportionation kinetics was evaluated and the underlying mechanisms were discussed by referring to the mechanical energy input intensity and the Gibbs free energy change for the reaction. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

标准号: LY/T 2231-2013

摘要： 本标准规定了以汽油机为动力的手扶步进式割灌机(以下简称割灌机)的术语、定义、技术要求、试验方法、检验规则、标志、使用说明书、包装、运输和贮存。本标准适用于以汽油机为动力的手扶步进式割灌机,以其他内燃机为动力的割灌机亦可参照使用。

标准号: LY/T 1609-2013

摘要： 本标准规定了以汽油机为动力的随进式起草皮机的术语和定义、型号编制方法、基本参数、技术要求、试验方法、检验规则、标志、包装、使用说明书、运输和贮存。本标准适用于以汽油机为动力的随进式起草皮机(以下简称起草皮机)。

标准号: LY/T 2232-2013

摘要： 本标准规定了工作压力不低于3.45 MPa、流量不低于3 L/min的背负式高压细水雾灭火机(以下简称灭火机)的术语和定义、型号编制方法、技术要求、试验方法、检验规则、标志、使用说明书、包装、运输和贮存。本标准适用于以汽油机为动力的,液泵为活塞泵、柱塞泵的背负式高压细水雾灭火机。

关键词： Electrochemistry  

摘要： Quantum mechanical theory of electrochemical kinetics based on Fermi's golden rule was formulated by introducing the concept of electron-transfer distance. The expressions for the exchange current density and standard rate constant in electrochemistry were derived in analytical form, as well as exponential current overpotential dependence. The theory corresponds well to the electrode kinetics based on the transition state theory that is familiar at present with many electrochemists. It was applied to various kinds of electrode reactions to analyze the standard rate constants and the exchange current densities reported in past literature. The evaluated magnitudes of the electron exchange energy were very small, being in the order of 10(-3)-10(-5) eV. This theory supports the ordinary electron-transfer mechanism due to the overlap of wave functions between the electrode and redox species, denying tunneling mechanism.

关键词： composites   crystallization   kinetics   mechanical properties  

摘要： The isothermal crystallization and mechanical behavior of polycaprolactone (PCL) with zinc oxide (ZnO) with oleic acid and glycerol monooleate (GMO) were studied. Theoretical melting points calculated by the Flory-Huggins and Thompson-Gibbs models were thoroughly compared with differential scanning calorimetry experimental observations. The isothermal crystallization kinetic parameters by Avrami analysis showed that crystallization was controlled by nucleation, crystal growth was spherical, and the nucleation type changed between thermal and athermal nucleation. X-ray diffraction showed that when the additives were used together both the crystal thickness and the degree of crystallinity increased. A multiple-response regression analysis was made with the ZnO, oleic acid, and GMO concentrations as variables and the crystallinity as output. Interaction parameters by the Pukanzky model were calculated from the tensile strength at the yield point and indicated that the addition of oleic acid or GMO improved the interface between the ZnO particles and PCL. (C) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1259-1275, 2013

关键词： RESONATORS   NANOELECTROMECHANICAL systems   ELECTROMECHANICAL devices   OPTOMECHANICS   ATOMIC force microscopes  

摘要： We quantify the Brownian driven, stochastic dynamics of an elastic nanobeam immersed in a viscous fluid that is partially wrapped around a microdisk optical resonator. This configuration has been proposed as an optomechanical and nanoscale analog of the atomic force microscope [Srinivasan et al., Nano Lett. 11, 791 ( 2011)]. A small gap between the nanobeam and microdisk is necessary for the optomechanical transduction of the mechanical motion of the nanobeam. We compute the stochastic dynamics of the nanobeam in fluid for the precise conditions of the laboratory using deterministic finite element simulations and the fluctuation dissipation theorem. We investigate the dynamics of a nanobeam in water and in air and quantify the significance of the fluid-solid interaction between the nanobeam and the optical resonator. Our results in air show that, despite the complex geometry of the nanobeam, it can still be represented approximately as a damped simple harmonic oscillator. On the other hand, when the nanobeam is immersed in water there are significant deviations from the dynamics of a simple harmonic oscillator. The small gap between the nanobeam and the microdisk is found to be a significant source of additional dissipation. In air, the quality factor of the mechanical oscillation of the nanobeam is reduced by an order of magnitude due to the presence of the microdisk, however, the dynamics remain underdamped even in the presence of the microdisk. On the other hand, when placed in water, the dynamics without the microdisk is underdamped and with the microdisk the dynamics become strongly over damped. (C) 2013 AIP Publishing LLC.

关键词： SILVER compounds   METALLIC oxides   DISSOLUTION (Chemistry)   IMPLANTABLE cardioverter-defibrillators   ELECTRIC batteries   SERVICE life (Engineering)  

摘要： Material design strategies for energy storage applications can be considered in two major categories: (1) control of structure and composition and (2) material dimensional control such as the implementation of nanomaterials. Characterization of electrochemical properties determines energy content and possible viability for potential application. Equally critical yet more challenging is quantifying the non-Faradaic parasitic reactions of the active materials and the relationship to battery life. Understanding the significant factors associated with battery lifetimes for the implantable cardioverter defibrillator (ICD) is critical for the development of new ICD batteries. In situ dissolution of the cathode material has been identified as a major factor in premature end of life for ICD batteries. This study contains the kinetic analyses of silver and vanadium dissolution from the benchmark silver vanadium oxide (SVO) material and two silver vanadium phosphorous oxide (SVPO-H and SVPO-R) materials with differing physical properties in a non-aqueous ICD battery electrolyte. A comparison of the kinetic and mechanistic results for SVO, SVPO-H and SVPO-R provides insight for future material design approaches. [ABSTRACT FROM AUTHOR]