关键词： Molecular Dynamics   Nanoscratch   Shear   Nanofriction   GaAs  

摘要： Nanomechanical characteristics of gallium arsenide (GaAs) are investigated applying nanoscratch approach combined with molecular dynamics (MD) simulation. Moreover, mechanical responses for the shear deformation and frictional behavior are obtained as well. Based on the MD analysis, the GaAs deforms plastically even for a tiny scratch depth of 0.3 nm. This plastic deformation results from mechanical twinning or dislocation motion during the scratching and therefore this paper also addresses the effects of the scratch depth and speed on the characteristics of interest over the GaAs surface. The shear deformation provided by the atomistic analysis well agrees with that determined by the previous experiment of atomic force microscopy (AFM). It further substantiates the validity of the present results.

关键词： PHASE transformations (Physics)   MATERIALS -- Mechanical properties   MOLECULAR dynamics   SIMULATION methods & models   CRYSTALS   POLYMORPHISM (Crystallography)   TEMPERATURE effect   HIGH pressure chemistry  

摘要： The phase transitions and mechanical properties of the energetic material octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) in three pure polymorphs crystals, alpha-, beta-, and delta-phases, have been studied theoretically. isothermal isobaric molecular dynamics (MD) simulations were performed in the temperature range of (5 to 555) K at 0 GPa and the pressure range of (0 to 40) GPa at 298 K. The lattice parameters and volumes show nonlinear dependence on the temperature and pressure. In the high-temperature regime, there exist two phase transitions, that is, from beta- to alpha-HMX at 360 K and from alpha- to delta-HMX at 440 K. Under high pressure, the beta-HMX transition to delta-HMX is shown at 27 GPa and 298 K. Within the range of temperature and pressure studies, it can be deduced that the beta-HMX at lower temperatures and higher pressures, the alpha-HMX at higher pressures, and the delta-HMX at lower temperatures have better malleability.

关键词： Energy utilization  

摘要： Debate exists regarding the merits and limitations of continuous versus pulsatile flow mechanical circulatory support. To characterize the hemodynamic differences between each mode of support, we investigated the acute effects of continuous versus pulsatile unloading of the failing left ventricle in a bovine model. Heart failure was induced in male calves (n = 14). During an acute study, animals were instrumented through thoracotomy for hemodynamic measurement. A continuous flow (n = 8) and/or pulsatile flow (n = 8) left ventricular assist device (LVAD) was implanted and studied during maximum support (similar to 5 L/min) and moderate support (similar to 2-3 L/min) modes. Pulse pressure (PP), surplus hemodynamic energy (SHE), and (energy equivalent pressure [EEP]/mean aortic pressure (MAP) - 1) x 100% were derived to characterize hemodynamic energy profiles during the different support modes. Standard hemodynamic parameters of cardiac performance were also derived. Data were analyzed by repeated measures one-way analysis of variance within groups and unpaired Student's t-tests across groups. During maximum and moderate continuous unloading, PP, SHE, and (EEP/MAP - 1) x 100% were significantly decreased compared with baseline and compared with pulsatile unloading. As a result, continuous unloading significantly altered left ventricular peak systolic pressure, aortic systolic and diastolic pressure, +/- dP/dt, and rate x pressure product, whereas pulsatile unloading preserved a normal profile of physiologic values. As continuous unloading increased, the pressure-volume relationship collapsed, and the aortic valve remained closed. In contrast, as pulsatile unloading increased, a comparable decrease in left ventricular volumes was noted. However, a normal range of left ventricular pressures was preserved. Continuous unloading deranged the physiologic profile of myocardial and vascular hemodynamic energy utilization, whereas pulsatile unloading preserved more normal phys

关键词： aerodynamic   flight   pectoralis   power  

摘要： There have been few comparisons between the relationship between the mechanical power requirements of flight and flight speed obtained using different approaches. It is unclear whether differences in the power-speed relationships reported in the literature are due to the use of different techniques for determining flight power or due to inter-specific differences. Here we compare the power-speed relationships in cockatiels (Nymphicus hollandicus) determined using both an aerodynamic model and measurements of in vitro performance of bundles of pectoralis muscle fibres under simulated in vivo strain and activity patterns. Aerodynamic power was calculated using different ranges of values for the coefficients in the equations: induced power factor (k 1.0-1.4), the profile (C-D,C-pro 0.01-0.03) and parasite drag (C-D,C-par 0.05-0.195) coefficients. We found that the aerodynamic power-speed relationship was highly sensitive to the values assumed for these coefficients and best fit the power calculated from in vitro muscle performance when k=1.2, C-D,C-pro=0.02 and C-D,C-par=0.13.

关键词： 机械动力   格局   变化  

摘要： 分析了21世纪以来黑龙江省农业机械动力变化,以地方和农垦总局进行对比,总结二者不同机械动力格局的构成。介绍了农垦总局大马力拖拉机状况。

关键词： Solid-liquid reactions   Kinetics   Leaching   Mass transfer   Morphology  

摘要： A systematic approach was developed to consider liquid-solid reactions with rough solid particles and shrinking particle model. The model is able to predict the reactivities of both non-porous and porous solid particles;the reaction order with respect to the solid material varies from zero (non-porous slab) to one (porous particle). As a model system, leaching of zinc sulphide (sphalerite) with ferric iron in an acidic environment was considered. The modelling was based on experimental data obtained in a batch reactor system, for which both conventional mixing and ultrasound was applied. Rival models based on plausible reaction mechanisms were derived and discriminated qualitatively and with regression analysis. The best model described the leaching reaction as a stepwise process, where ferricions react with solid zinc sulphide in consecutive surface reaction steps. Shrinking particle model a long with the surface roughness approach was used. The model predicts first order behaviour with respect to zinc sulphide, while the reaction order with respect to ferric iron varies from one to two as there action progresses. This is in accordance with experimental observations. The intrinsic kinetics, liquid-solid mass transfer and the effect of ultrasound were well described by the best kinetic model. (C) 2010 Elsevier Ltd. All rights reserved.

关键词： 分子机械   模拟   动力学   建模  

摘要： 从纳米尺度了解结构和材料的动力学性能有着重要的意义,对分子机械动力学的研究现状进行了概括和总结,介绍了目前常用的分析模型,展望了未来的研究方向和热点。

关键词： 并联机械手   动力性能   尺度综合  

摘要： 研究一种二平动自由度高速并联机械手的动力尺度综合方法.在建立逆刚体动力学模型基础上,提出一种基于单轴最大驱动力矩全域最大值最小的动力学性能评价指标,该指标可表示为系统尺度和惯性参数及位形的显函数形式,可直接用于观察奇异位形的出现条件.建立雅可比矩阵奇异值的显式,提出一组兼顾速度、精度和刚度的传递角约束方程,通过考察奇异值与运动学性能的关系,可将传递角约束方程转化为性能约束方程,同时配以工作空间/机构尺度等约束条件,利用工程实例研究性能约束对尺度参数和动力学性能评价指标的影响规律,由此综合出在满足上述约束条件下使得系统动力学性能最优的尺度参数.

关键词： 工程机械   发动机   供应商   中国   潍柴动力股份有限公司   市场地位   获奖产品   颁奖典礼  

摘要： 3月29日，2009年中国工程机械年度产品TOP50评选颁奖典礼在北京举行，本次活动评选出了2009年中国工程机械市场各方面表现优异的50个产品，无论在技术创新、市场表现或应用贡献方面，这些获奖产品均在各自领域有着极高的市场地位，受到专家、用户及媒体代表的高度认可。在发动机产品评选项目中，潍柴动力股份有限公司作为唯一获奖企业，被评为“2009年中国工程机械年度发动机供应商”。

关键词： 微构件   振动模型   固有频率   动力学分析  

摘要： 微电子机械系统中的微构件如定子、转子,结构尺寸微小,一般在数百微米量级。制造微构件的材料主要是硅,硅具有优良的机械性能,能够满足微结构对材料力学性能的要求。机械(力学)理论基础如弹性体基本方程、运动学方程、动力学方程、弹性梁基本方程、振动理论等对微构件依然适用。笔者对一种新型微电机的驱动机理进行了研究。基于振动学理论,建立了微机械构件的振动数学模型,计算了自由振动固有频率以及推导出激励下的振动方程,并在此基础上进行了运动学和动力学分析。