关键词： 机械动力学   有限元法   试验模态分析   综合性实验  

摘要： 针对目前机械动力学课程中悬臂梁结构模态分析实验存在的问题,引导学生将学习过的CAD/CAE软件操作知识运用于该实验,弥补了实验仪器和实验装置的不足,也提高了学生做实验的主动性和创新性,大大提升了实验效果。

关键词： Molecular dynamics simulation   Mechanical properties   Bismuth telluride   Thermoelectric material   Nanofilm  

摘要： A molecular dynamics (MD) simulation has been carried out to evaluate the mechanical properties of Bi2Te3 nanofilm, and all the simulation results are compared with that of Bi2Te3 bulk. After the stable free-relaxation state has been obtained, the radial distribution function of Bi2Te3 nanofilm is computed to validate its crystal structure. The distributions of potential energy and stress along the thickness direction are obtained. It is found that the surface atoms possess high potential energy;the surface of the nanofilm undergoes tensile stress, while the inside undergoes compressive stress. Tension simulations have been conducted to evaluate the mechanical properties (including Young's modulus, ultimate strength and failure strain) of the nanofilm. The effects of surface, size, and temperature on the calculating results of Bi2Te3 nanofilm are discussed in detail. (C) 2010 Elsevier B.V. All rights reserved.

关键词： 拖拉机   使用   指南   车辆修理  

ISBN: (纸本)9787544141475

摘要： 构建和谐新农村就是要坚持以科学发展观为指导，通过不懈的努力，实现农民“人与人，人与自然环境之间的相互依存、相互促进”的协调融洽关系，实现农村“生产发展、生活富裕、乡风文明、村容整洁、管理民主”的人居环境。《中共中央关于推进农村改革发展若干重大问题的决定》指出：“把建设社会主义新农村作为战略任务，把走中国特色农业现代化道路作为基本方向，把加快形成城乡经济社会发展一体化新格局作为根本要求”。农民、科技、政策是完成这一目标任务重中之重的三大要求。 一、 构建和谐新农村的主体作用 农民是构建和谐新农村的主体，首先必须充分发挥新型农民的主体作用。农村生产力的发展是构建和谐新农村的基础。有文化、懂技术、会经营的新型农民是农村生产力最具活力并起决定性作用的要素，是发展农业生产、繁荣农村经济的基本力量，加强对农民的培训教育，提高农民的科技文化素质和经营能力是构建和谐新农村工作的前提。 改革开放30年来，我国农村发生了翻天覆地的变化，农业发展取得了举世瞩目的巨大成就。两组基本数据可以说明这一点，即粮食生产总量和农民人均收入的增长数据。1978年我国的粮食产量是6095亿斤，2009年突破10616亿斤，产量增加4521亿斤，总增长率达到74%；1978年农民年人均收入是134元，2009年是5153元，扣除物价指数，平均每年增长超过7%，近4年来更是超过8%以上。如此高的年均增幅，从全球视野看都是了不起的。但横向比较，差距就显现出来了，以农民收入为例，1978年城乡收入差距是2.57﹕1，2009年是3.31﹕1,城乡差距不但未被“消灭”，反而明显在扩大。其中缘由，有自然条件，经济调控等多种主客观因素的影响，但是，毋庸置疑，劳动者素质，劳动效率等问题，影响更为严重，我国农业的劳动生产率且不说与世界发达国家相比，就是与发展中的农业先进国家都不能相提并论。中国农村的突出问题是人口多而人均占有的可利用土地少，这一特征注定了解决“三农”问题必然是一项“多管齐下”的综合工程，任何发达国家的经验都只能是“借鉴”而不能是“照搬”。城镇化是个发展方向，但决不能作为一条“捷径”来走。目前我国的城镇化水平是45.7%，这还是把大量进城农民工统计在内。美国、日本等发达国家城镇化率达到90%以上，农民的比例仅占5%左右。我国农村整体劳动力超过5亿人，城镇化除了住房、社保等问题，最重要的是要有就业机会。提供就业首先要有足够的生产能力，即使解决了生产能力，还要面对产品的市场出路。没有国内外统一且相对旺盛的市场需求，就无法保证城镇就业的稳定。频繁的农民城乡流动，算不上真正的城镇化。农村优质人力资源向城镇的单项流动更会严重影响城镇化的进程和水平。 到2030年我国人口将达到15亿，经过20年的艰苦努力，即使真正实现了70%的城镇化率，还有30%的人口在农村。面对4.5亿的巨大农村人口数字，构建和谐新农村的任务依然会很艰苦，任何外力只能起到引导与推动作用。培育一代掌握先进、适用农业技术，掌握现代经营管理知识的新型农民，充分激发调动农民的积极性、主动性、创造性，才是新农村建设的希望所在。 二、 构建和谐新农村的科技潜力 必须高度重视农业科学技术在构建和谐新农村的特殊作用，有以下几方面原因：其一，由于区域间、城乡间发展不平衡，耕地面积不断减少，农村优质人力资源在不断流失；其二，农村、农业的基础设施仍然相当滞后；其三，部分劳动资料投入过度导致的环境污染；其四，传统农业资源持续投入导致的边际效益下降；其五，市场对农业新产品以及农村对新技术的巨大需求。 我国农业科技的现实情况一方面是存在技术进步的多重需求刺激，另一方面 又表现出农业技术的严重滞后。目前我国农业科技的贡献率仅为48%，而发达国家可以达到80%~90%。如我们的化肥、农药的施用量年年增加，不仅造成资源投入浪费，还造成很大的环境污染；基础设施落后并不是科技进步的直接动因，但由于基础设施投入的严重不足，急需利用科技要素来弥补。即有科技进步的强烈需求，又存在科技应用的巨大空间，农业科学技术成为构建和谐新农村的最大潜力。 宏观上要加快推进农业产业技术体系建设和农业科技体制机制创新，利用农业部门得天独厚的，自上而下的技术推广系统推进农业技术转移和农业高新技术的推广普及，引导和促进农业科技创新要素向现实生产力转化，向农业生产实际需要集中。综合多部门多行业技术集成，配套能力，按照“高产、优质、高效、生态、安全”的要求，在品种培育等方面取得突破性进展。 在技术研究开发层面，不仅要重视无性繁育、无土栽培、生物无害、基因优选等种植、养殖领先技术的研发、推广，还要遵循和谐新农村的规划要求，创新、完善、应用、普及沼气、太阳能、沙石道路、绿色建材等适应不同农村地域特点的实用技术和适用技术。 三、构建和谐新农村的政策保障 纵观世界各发达国家工业化的发展

关键词： Model comparison   Maximum caliber   Organic matter dynamics   Litter decomposition   Mineral nitrogen   Microbial biomass   C-13 labelling   Priming effect  

摘要： First order kinetics characterize most models of soil organic matter dynamics. Although first order kinetics often provide a good description of litter decomposition, their general applicability has recently been challenged by numerous observations of priming effects. A priming effect can be defined as a change in native soil organic matter decomposition rate following the addition of some labelled exogenous substrate. Recently two new formalisms were developed which predict a priori the existence of priming effects, whether positive or negative. The Extended Mass Action (EMA) formalism is a generalization of enzyme kinetics at the microbial scale. The Maximum Caliber (MAXCAL) formalism describes the most probable dynamics of a system that arises when the multiple ways feasible macroscopic dynamics can be realized at the microscopic particle scale are accounted for. Here those two formalisms were applied to a common soil compartimentation scheme and their predictions confronted with an appropriate set of priming observations. We show that the two formalisms generate distinct, testable predictions and that the MAXCAL formalism performed better than the EMA formalism. We discuss the determinants of priming effects as predicted by the Maximum Caliber formalism. (c) 2010 Elsevier Ltd. All rights reserved.

关键词： Nanostructured intermetallics   Phase transformation   Twinning   Work hardening   Simulations   atomistic  

摘要： The asymmetric stress strain behavior under tension/compression in an initial < 100 > B-2-NiAl nanowire is investigated considering two different surface configurations i.e., < 100 >/(0 1 0) (0 0 1) and < 100 >/(0 1 1) (0 - 1 1). This behavior is attributed to two different deformation mechanisms namely a slip dominated deformation under compression and a known twinning dominated deformation under tension. It is also shown that B2 -> BCT (body-centered-tetragonal) phase transformation under tensile loading is independent of the surface configurations for an initial < 100 > oriented NiAl nanowire. Under tensile loading, the nanowire undergoes a stress-induced martensitic phase transformation from an initial B2 phase to BCT phase via twinning along {110} plane with failure strain of similar to 0.30. On the other hand, a compressive loading causes failure of these nanowires via brittle fracture after compressive yielding, with a maximum failure strain of similar to-0.12. Such brittle fracture under compressive loading occurs via slip along {110} plane without any phase transformations. Softening/hardening behavior is also reported for the first time in these nanowires under tensile/compressive loadings, which cause asymmetry in their yield strength behavior in the stress strain space. Result shows that a sharp increase in energy with increasing strain under compressive loading causes hardening of the nanowire, and hence, gives improved yield strength as compared to tensile loading. (C) 2010 Elsevier Ltd. All rights reserved.

关键词： 农用设备   农业机械   控制装置   液压联轴器   液压传动   液压系统   液压装置   识别   处理   惰性气体铠装电弧焊   作标记   数字代码   操作元件   遥控   拖拉机   牵引车  

标准号: BS ISO 11471-1995

关键词： HYDROGEN fluoride   QUANTUM theory   BORN-Oppenheimer approximation   MOLECULAR dynamics   ELECTRONIC excitation   DIPOLE moments  

摘要： The electronic properties of liquid hydrogen fluoride (HF) were investigated by carrying out sequential quantum mechanics/Born-Oppenheimer molecular dynamics. The structure of the liquid is in good agreement with recent experimental information. Emphasis was placed on the analysis of polarisation effects, dynamic polarisability and electronic excitations in liquid HF. Our results indicate an increase in liquid phase of the dipole moment (similar to 0.5 D) and isotropic polarisability (5%) relative to their gas-phase values. Our best estimate for the first vertical excitation energy in liquid HF indicates a blue-shift of 0.4 +/- 0.2 eV relative to that of the gas-phase monomer (10.4 eV). (C) 2010 Elsevier B.V. All rights reserved.

摘要： Microtubules (MTs) are the largest type of cellular filament, essential in processes ranging from mitosis and meiosis to flagellar motility. Many of the processes depend critically on the mechanical properties of the MT, but the elastic moduli, notably the Young's modulus, are not directly revealed in experiment, which instead measures either flexural rigidity or response to radial deformation. Molecular dynamics (MD) is a method that allows the mechanical properties of single biomolecules to be investigated through computation. Typically, MD requires an atomic resolution structure of the molecule, which is unavailable for many systems, including MTs. By combining structural information from cryo-electron microscopy and electron crystallography, we have constructed an all-atom model of a complete MT and used MD to determine its mechanical properties. The simulations revealed nonlinear axial stress-strain behavior featuring a pronounced softening under extension, a possible plastic deformation transition under radial compression, and a distinct asymmetry in response to the two senses of twist. This work demonstrates the possibility of combining different levels of structural information to produce all-atom models suitable for quantitative MD simulations, which extends the range of systems amenable to the MD method and should enable exciting advances in our microscopic knowledge of biology.

关键词： PHBV   P3/4HB   miscibility   crystallization kinetics   morphology  

摘要： Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV)/poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3/4HB) blend films were prepared by solvent-cast method. The nonisothermal crystallization results showed that PHBV and P3/4HB are miscible due to a single glass transition temperature (T-g), which is dependent on blend composition. The isothermal crystallization results demonstrate that the crystallization rate of PHBV becomes slower after adding amorphous P3/4HB with 19.2 mol%, 4HB, which could be proved through depression of equilibrium melt point (T-m(o)) from 183.7 degrees C to 177.6 degrees C. For pure PHBV and PHBV/P3/4HB (80/20) blend, the maximum crystalli-zation rate appeared at 88 degrees C and 84 degrees C, respectively. FTIR analysis showed that PHBV/P3/4HB blend films would maintain the helical structure, similar to pure PHBV. Meanwhile, with increasing P3/4HB content, the inter- and intra-interactions of PHBV and P3/4HB decrease gradually. Besides, a lower elastic modulus and a higher elongation at break were obtained, which show that the addition of P3/4HB would make the brittle PHBV to ductile materials. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 838-848, 2010

关键词： Animals   Biosensing Techniques   Calibration   Cattle   Cell Line   Cell Movement/physiology   Fluorescent Dyes   Focal Adhesions/metabolism   Humans   Mice   Microscopy, Confocal   Movement   Optical Tweezers   Spectrometry, Fluorescence   Stress, Mechanical   Vinculin/chemistry   Vinculin/deficiency   Vinculin/genetics   Vinculin/metabolism  

摘要： Mechanical forces are central to developmental, physiological and pathological processes(1). However, limited understanding of force transmission within sub-cellular structures is a major obstacle to unravelling molecular mechanisms. Here we describe the development of a calibrated biosensor that measures forces across specific proteins in cells with piconewton (pN) sensitivity, as demonstrated by single molecule fluorescence force spectroscopy(2). The method is applied to vinculin, a protein that connects integrins to actin filaments and whose recruitment to focal adhesions (FAs) is force-dependent(3). We show that tension across vinculin in stable FAs is similar to 2.5 pN and that vinculin recruitment to FAs and force transmission across vinculin are regulated separately. Highest tension across vinculin is associated with adhesion assembly and enlargement. Conversely, vinculin is under low force in disassembling or sliding FAs at the trailing edge of migrating cells. Furthermore, vinculin is required for stabilizing adhesions under force. Together, these data reveal that FA stabilization under force requires both vinculin recruitment and force transmission, and that, surprisingly, these processes can be controlled independently.