关键词： 粮食作物   农业机械化   系统动力学模型   仿真  

摘要： 分析了中国粮食作物机械化生产系统结构组成,构建了因果分析图和系统动力学流图。在此基础上,进行了系统动力学模型构建与仿真分析。结果表明:经济发展水平、土地经营规模、农业劳动力转移是影响粮食作物机械化生产系统的主要因素。只有通过提高粮食作物生产规模化程度和专业化程度并配合产业发展规律,才能从根本上提高机械化生产的效率和生产能力。

关键词： Rare-earth minerals   Mechanical activation   Tumbling mill   Kinetics   Leaching   Zinc ferrite  

摘要： Synthetic indium-bearing zinc ferrite (IBZF) was mechanically activated by a tumbling mill. Subsequently, the behavior and kinetics of leaching zinc and indium from the initial IBZF and activated IBZF in sulfuric acid solution were investigated. The results demonstrate that the tumbling ball milling enhances the leaching reactivity of IBZF and increases the zinc and indium extractions. The reaction temperature and H2SO4 concentration have an important effect on leaching zinc and indium from the initial IBZF and activated IBZF. The kinetic model -ln(1-x) = kt was used to describe the kinetics of leaching zinc and indium from the initial IBZF and activated IBZF and to calculate the kinetic parameters. The tumbling ball milling causes a decrease in activation energy and reaction order, which indicates that the dissolution of IBZF after mechanical activation by a tumbling mill becomes less sensitive to temperature and H2SO4 concentration. For the zinc leaching, the activation energy decreases from 73.9 to 70.3 kJ/mol and the reaction order decreases from 0.72 to 0.70. For indium leaching, the activation energy decreases from 78.1 to 73.5 kJ/mol and the reaction order decreases from 0.82 to 0.69. It is found that the dissolution rates of the initial IBZF and activated IBZF are controlled by surface chemical reactions. Minerals & Metallurgical Processing, 2013, Vol. 30, No. 1, pp. 45-52. An official publication of the Society for Mining, Metallurgy, and Exploration, Inc.

关键词： Epoxy nanocomposites   Dynamic cure kinetics   KAS method   SB (m, n) model   Thermal stability   Elastic moduli, microhardness  

摘要： Nanocomposites, based on tetrabromo-bisphenol-A epoxy and aniline formaldehyde condensates, containing 5 and 10 % organically modified montmorillonite (O-MMT), were prepared. The morphologies of these nanocomposites were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influences of O-MMT on the dynamic cure kinetics, thermal stability, and mechanical properties were investigated by differential scanning calorimetry, thermogravimetric analysis, and non-destructive ultrasonic testing techniques. The XRD and SEM results indicated a good dispersion of O-MMT within the epoxy matrix. The relation between the activation energy, E (a), and the degree of cure, alpha, for the examined systems was obtained by applying model-free isoconversional Kissinger-Akahira-Sunose method. As alpha increases, E (a) increases gradually, almost independent of the amount of O-MMT. The dynamic cure kinetics of the neat epoxy system as well as its nanocomposites were described by estA k-Berggren, [SB (m, n)], autocatalytic model. The O-MMT enhances the thermal stability of the examined epoxy system. The results of the mechanical properties indicated that the addition of O-MMT enhances the Young's and shear elastic modulus and microhardness. The values of these parameters increase with increasing O-MMT loading.

关键词： Poly(vinylidene fluoride)   Carbon nanotube   Molecular dynamics   Mechanical properties  

摘要： Molecular dynamics simulations and geometry optimizations based on the Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) force field were performed to understand the effect of Single Wall Carbon Nanotubes (SWCNTs) on the mechanical properties of Poly(vinylidene fluoride) (PVDF). In particular, the Young's modulus, bulk and shear modulus, pullout energy, pullout force, interfacial shear stress and interfacial bonding energy were calculated. The presence of the SWCNTs can increase the Young's modulus of the systems studied here by 1 GPa in the direction of the SWCNT axis, although this depends on the distance between neighboring SWCNTs. The calculated interfacial shear stress was between 100 and 129 MPa, which is in agreement with results obtained for other SWCNT-polymer systems. The results, and in particular those obtained for the bulk and shear modulus, show that SWCNTs do not have a significant effect on the bulk mechanical properties. Functionalizing the SWCNTs may yield stronger adhesion between the nanotube and the polymer, thereby achieving improved mechanical properties. (C) 2012 Elsevier B.V. All rights reserved.

关键词： stacking fault energy   high manganese steel   TWIP   TRIP   shape memory alloys  

摘要： The thermodynamic properties of the Fe-Mn-C system were investigated by using an analytical model constructed by a CALPHAD approach. The stacking fault energy (SFE) of the fcc structure with respect to the hcp phase was always constant at T-0, independent of the composition and temperature when other related parameters were assumed to be constant. Experimental limits for the thermal hcp formation and the mechanical (deformation-induced) hcp formation were separated by the SFE at T-0. The driving force for the fcc to hcp transition, defined as a dimensionless value -dG(m)/(RT), was determined in the presence of Fe-rich and Mn-rich composition sets in each phase. Carbon tended to partition to the Mn-rich phase rather than to the Fe-rich phase for the compositions studied. The results obtained revealed a thermo-mechanical correlation with empirical yield strength, maximum true stress and maximum true strain. The proportionality between thermodynamics and mechanical properties is discussed.

关键词： Molecular Dynamics Simulation   Pyridines/chemistry   Solutions   Spectrum Analysis, Raman   Temperature  

摘要： The in situ observation of geometrical and electronic structural dynamics of a single molecule junction is critically important in order to further progress in molecular electronics. Observations of single molecular junctions are difficult, however, because of sensitivity limits. Here, we report surface-enhanced Raman scattering (SEAS) of a single 4,4'-bipyridine molecule under conditions of in situ current flow in a nanogap, by using nano-fabricated, mechanically controllable break junction (MCBJ) electrodes. When adsorbed at room temperature on metal nanoelectrodes in solution to form a single molecule junction, statistical analysis showed that nontotally symmetric b(1) and b(2) modes of 4,4'-bipyridine were strongly enhanced relative to observations of the same modes in solid or aqueous solutions. Significant changes in SERS intensity, energy (wavenumber), and selectivity of Raman vibrational bands that are coincident with current fluctuations provide information on distinct states of electronic and geometrical structure of the single molecule junction, even under large thermal fluctuations occurring at room temperature. We observed the dynamics of 4,4'-bipyridine motion between vertical and tilting configurations in the Au nanogap via b(1) and b(2) mode switching. A slight increase in the tilting angle of the molecule was also observed by noting the increase in the energies of Raman modes and the decrease in conductance of the molecular junction.

关键词： failure (mechanical)   fracture   molecular dynamics method   silicon compounds   stress-strain relations   thin films   Young's modulus  

摘要： The mechanical properties and failure mechanisms of the beta-Si3N4 thin layers in basal plane under uniaxial tension are investigated by using molecular dynamics simulations. It is found that the thin layers display a nonlinear stress-strain relationship first at epsilon<0.06, and then a linear response at 0.06

关键词： Graphyne   Mechanical properties   Vacancy defect   Molecular dynamics simulation  

摘要： This paper explores the mechanical properties of perfect and defective gamma-graphyne, a lattice of sp-sp(2)-hybridized carbon atoms, as a graphene allotrope using the classical molecular dynamics simulations. These simulations are carried out on the basis of the Tersoff-Brenner potential function with the Nose-Hoover thermostat algorithm in canonical ensemble. In this examination, the influences of vacancy defects on mechanical properties of gamma-graphyne such as Young's modulus, ultimate stress and strain and Poisson's ratio are studied. The results demonstrated lower strength and stiffness of this new graphene allotrope than those of graphene. Also, it is observed that in comparison with the mapped vacancy defects, the random vacancy ones are more responsible for the change in the mechanical properties of gamma-graphyne. Furthermore, the fracture pattern of defective gamma-graphyne is considered. (C) 2012 Elsevier B.V. All rights reserved.