关键词： 玉米生产   机械化   劳动力   替代弹性  

摘要： 本文在当前农村剩余劳动力减少、农业机械化水平提高的背景下,采用玉米生产的省级面板数据,研究劳动力工资变化与玉米机械化生产的相关关系。并利用随机前沿生产函数估计不同要素投入对玉米产出的贡献,进一步分析机械使用和劳动力投入之间的替代关系,以及农场规模对机械化水平的影响。研究结果表明,机械和劳动力的相对价格变动是影响机械化水平的主要因素之一,生产中劳动力与机械投入呈现显著的相互替代关系。

关键词： Grape dehydration kinetics   Alkaline pretreatment   Berry skin mechanical properties   Response surface methodology   Muscat of Alexandria grapes  

摘要： The dipping of berries in a dilute solution of sodium hydroxide during a short time was evaluated as pretreatment undertaken prior to convective dehydration of wine grapes. The impact of the sodium hydroxide content and dipping time on weight loss (WL) at different dehydration times was thoroughly assessed using central composite design (CCD) and response surface methodology (RSM). Furthermore, the effects of these two variables were also investigated on the skin mechanical properties of dehydrated grapes. The effect of these two pretreatment factors on the dehydration kinetics and skin hardness was satisfactorily fitted to regression models. The berry pretreatment with low sodium hydroxide contents (from 10 to 20 g/L) facilitated the dehydration process during the first 5 days when dipping times longer than 300 s were used. From the seventh day of dehydration, at which time the average WL% was close to 50, the highest values of WL% were obtained using intermediate sodium hydroxide contents and dipping times (around 45 g/L and 185 s, respectively). Because skin hardness affects the dehydration kinetics during postharvest withering, the strongest decrease in skin hardness corresponded to these last berry pretreatment conditions, whereas the greatest increase required the highest sodium hydroxide contents and longest dipping times. The quality of berries dehydrated may be influenced by the pretreatment conditions used, and the present study contributes to increase the knowledge on this effect to a better management of the alkaline pretreatment and dehydration process.

关键词： Strengthening and mechanisms   Mechanical properties   Bio-inspired nanocomposites   Molecular dynamics   Design optimisation  

摘要： Superior mechanical properties of biocomposites such as nacre and bone are attributed to their basic building blocks. These basic building blocks have nanoscale features and play a major role in achieving combined stiffening, strengthening and toughening mechanisms. Bioinspired nanocomposites based on these basic building blocks, regularly and stairwise staggered arrangements of hard platelets in soft matrix, have huge potential for developing advanced materials. The study of applicability of mechanical principles of biological materials to engineered materials will guide designing advanced materials. To probe the generic mechanical characteristics of these bioinspired nanocomposites, the model material concept in molecular dynamics (MD) is used. In this paper, the effect of platelets aspect ratio (AR) on the mechanical behaviour of bioinspired nanocomposites is investigated. The obtained Young's moduli of both the models and the strengths of the regularly staggered models agree with the available theories. However, the strengths of the stairwise staggered models show significant difference. For the stairwise staggered model, we demonstrate the existence of two critical ARs, a smaller critical AR above which platelet fracture occurs and a higher critical AR above which composite strength remains constant. Our MD study also shows the existence of mechanisms of platelet pull-out and breakage for lower and higher ARs. Pullout mechanism acts as a major source of plasticity. Further, we find that the regularly staggered model can achieve an optimal combination of high Young's modulus, flow strength and toughness, and the stairwise staggered model is efficient in obtaining high Young's modulus and tensile strength. (C) 2015 Elsevier Ltd. All rights reserved.

关键词： Embedding   Electrostatics   QM/MM   Electron density   Computational chemistry  

摘要： A new approach for performing Particle Mesh Ewald in ab initio quantum mechanical/molecular mechanical (QM/MM) simulations with extended atomic orbital basis sets is presented. The new approach, the Ambient-Potential Composite Ewald (CEw) method, does not perform the QM/MM interaction with Mulliken charges nor electrostatically fit charges. Instead the nuclei and electron density interact directly with the MM environment, but in a manner that avoids the use of dense Fourier transform grids. By performing the electrostatics with the underlying QM density, the CEw method avoids self-consistent field instabilities that have been encountered with simple charge mapping procedures. Potential of mean force (PMF) profiles of the p-nitrophenyl phosphate dissociation reaction in explicit solvent are computed from PBE0/6-31G* QM/MM molecular dynamics simulations with various electrostatic protocols. The CEw profiles are shown to be stable with respect to real-space Ewald cutoff, whereas the PMFs computed from truncated and switched electrostatics produce artifacts. PBE0/6-311G**, AM1/d-PhoT, and DFTB2 QM/MM simulations are performed to generate two-dimensional PMF profiles of the phosphoryl transesterification reactions with ethoxide and phenoxide leaving groups. The semiempirical models incorrectly produce a concerted ethoxide mechanism, whereas PBE0 correctly produces a stepwise mechanism. The ab initio reaction barriers agree more closely to experiment than the semiempirical models. The failure of Mulliken-charge QM/MM-Ewald is analyzed.

关键词： Protein morphology   Protein film   Glycerol content   Polymerization   SAXS   WAXS  

摘要： Gliadin and glutenin proteins with 10, 20, 30 and 40% of glycerol were compression molded into films (130 degrees C) and evaluated for protein polymerization, beta-sheet structure and nano-structural morphology. Here, for the first time we show how different amounts of glycerol impact the nano-structure and functional properties of the gliadin and glutenin films. Most polymerized protein was found in the gliadin films with 20 and 30% glycerol, and in all the glutenin films (except 10%), by RP-HPLC. A beta-sheet-rich protein structure was found to be high in the 10 and 20% glycerol gliadin films, and in the 20 and 30% glycerol glutenin films by FT-IR. Glycerol content of 20, 30 and 40% impacted the nano-structural morphology of the gliadin glycerol films observed by SAXS, and to a limited extent for 10 and 20% glycerol gliadin films revealed by WAXS. No ordered nano-structure was found for the glutenin glycerol films. The 20%, 30% and 40% glycerol films were the most tunable for specific mechanical properties. For the highest stiffness and strength, the 10% glycerol protein films were the best choice.

关键词： 工程机械   液压系统   动力匹配  

摘要： 工程机械的液压动力系统的优化匹配控制技术是一门集合了现代多种理论和技术的高级系统技术。随着机电一体化技术的迅速发展和广泛应用。各种对于液压系统的阀门技术的发展都对工程机械液压系统动力匹配和控制技术提供了更为高效的控制技能,但同时也使工程机械液压系统动力匹配及控制技术的设计要求越来越复杂。详细讨论了工程机械液压系统动力匹配与控制技术、机电一体化技术,以及其技术发展的瓶颈,旨在为今后的工作提供一定的理论依据。

关键词： SILVER ions   AMMONIA   QUANTUM mechanics/molecular mechanics   HARTREE-Fock approximation   SOLVENTS  

摘要： Structures and dynamics of Ag+ in 18.6% aqueous ammonia have been studied using Quantum Mechanical Charge Field Molecular Dynamics (QMCF-MD) simulation at the Hartree-Fock (HF) level theory employing LANL2DZ ECP basis set for Ag+ and Dunning DZP for solvent molecules. Structural properties are in excellent agreement with previous QM/MM and experiments studies. [Ag(NH3)(2)(H2O)(3)](+) was found as dominant species during simulation time. For 20 ps of simulation time, a labile first solvation shell was observed with both fast ammonia and water ligands exchanges. QMCF-MD framework describes first solvation shell more labile than conventional QM/MM MD simulation. (C) 2016 Elsevier B.V. All rights reserved.

关键词： miniaturized mechanical resonator   anodic aluminum oxide   nanopatterning   nanomechanics  

摘要： We have fabricated porous miniaturized SiN resonators with various dimensions and studied their mechanical dynamics at their resonant modes. The surface modification of the resonators has been achieved by etching through a thin porous anodic aluminum oxide (AAO) mask, prepared by two-step anodization. Even though these porous resonators show well-defined Lorentzian line-shapes at their resonant modes, the corresponding fundamental flexural resonance frequencies are lower than those from typical non-porous resonators. The change in the resonance frequency is due to the presence of the pores on the surface, which reduces the effective tensile stress across the beam structure, as shown from both experimental measurements and the computational model. In addition, the observed quality factor reveals the level of dissipation originating from the surface modification. The principal dissipation mechanism is found to be gas damping in the free molecular flow regime. Based on the dissipation measurement, one can see an increase in the surface-to-mass ratio, which is responsible for the increased dissipation in the porous beam structure. The work presented here demonstrates simple integration of mechanical elements with a nanopatterning technique based on an AAO as well as the tuning of mechanics via surface modification at a small scale. Such a scheme could provide an additional degree of freedom in developing a mechanical sensing element with enhanced effective surface area.

关键词： 机械功   热能动力   应用   优化  

摘要： 在当前经济发展的背景之下,热能与动力工程也得到了一定程度的进步,对于推进社会经济发展有着十分重要的意义,基于此,论述了机械功在热能与动力工程之中的应用以及相关优化措施分析。

关键词： tight-binding quantum chemical molecular dynamics simulation   chemical mechanical polishing   copper   hydrogen peroxide   silica   density functional theory  

摘要： We applied our original chemical mechanical polishing (CMP) simulator based on the tight-binding quantum chemical molecular dynamics (TB-QCMD) method to clarify the atomistic mechanism of CMP processes on a Cu(111) surface polished with a SiO2 abrasive grain in aqueous H2O2. We reveal that the oxidation of the Cu(111) surface mechanically induced at the friction interface is a key process in CMP. In aqueous H2O2, in the first step, OH groups and O atoms adsorbed on a nascent Cu surface are generated by the chemical reactions of H2O2 molecules. In the second step, at the friction interface between the Cu surface and the abrasive grain, the surface-adsorbed O atom intrudes into the Cu bulk and dissociates the Cu Cu bonds. The dissociation of the Cu Cu back bonds raises a Cu atom from the surface that is mechanically sheared by the abrasive grain. In the third step, the raised Cu atom bound to the surface-adsorbed OH groups is removed from the surface by the generation and desorption of a Cu(OH)(2) molecule. In contrast, in pure water, there are no geometrical changes in the Cu surface because the H2O molecules do not react with the Cu surface, and the abrasive grain slides smoothly on the planar Cu surface. The comparison between the CMP simulations in aqueous H2O2 and pure water indicates that the intrusion of a surface-adsorbed O atom into the Cu bulk is the most important process for the efficient polishing of the Cu surface because it induces the dissociation of the Cu Cu bonds and generates raised Cu atoms that are sheared off by the abrasive grain. Furthermore, density functional theory calculations show that the intrusion of the surface-adsorbed O atoms into the Cu bulk has a high activation energy of 28.2 kcal/mol, which is difficult to overcome at 300 K. Thus, we suggest that the intrusion of surface-adsorbed O atoms into the Cu bulk induced by abrasive grains at the friction interface is a rate-determining step in the Cu CMP process.