关键词： Coarse-grained   Uniaxial tension   Multi-layer graphene   Mechanical properties  

摘要： In this work, we investigate the effect of temperature, defect, and strain rate on the mechanical properties of multi-layer graphene using coarse-grained molecular dynamics (CGMD) simulations. The simulation results reveal that the mechanical properties of multi-layer graphene tend to be less sensitive to temperature as the layer increases, but they are sensitive to the distribution and coverage of Stone Wales (SW) defects. For the same number of defect, there is less decline in the fracture stress and Young's modulus of graphene when the defects have a regular distribution, in contrast to random distribution. In addition, Young's modulus is less influenced by temperature and defect, compared to fracture stress. Both the fracture stress and Young's modulus have little dependence on strain rate. (C) 2016 Elsevier B.V. All rights reserved.

关键词： mechanically renewed silver electrode   kinetics of electroreduction of anions   impedance method   voltammetry   modeling  

摘要： The kinetics of electroreduction of peroxodisulfate anions on a mechanically renewed silver electrode is studied by voltammetric and impedance methods. Impedance diagrams obtained in the region of negatively charged metal surface are successfully modeled by the equivalent circuit formed by the solution resistance and also the reaction resistance connected in parallel with the constant-phase element substituted for the double layer capacitance. The analysis of experimental data carried out in terms of this circuit and their comparison with the literature data makes it possible to assume that the reduction kinetics of S2O (8) (2-) anions on this electrode can be adequately described in the framework of the phenomenological theory of slow discharge. The reasons for deviation of the literature data on the kinetics of this reaction on polycrystalline Ag electrode from the relationships following from the slow discharge theory are put forward and substantiated.

关键词： GRAPHENE   POLYMERIC composites   MOLECULAR simulation methods   TRIBOLOGY   YOUNG'S modulus   ABRASION (Engineering)  

摘要： Molecular models for graphene reinforced polymer composites are developed to study the enhanced mechanical and tribological properties of the composites by introduction of graphene as reinforcements. Strain constant method is applied to evaluate the Young's modulus and shear modulus of the polymer composites. A layer model containing iron atoms as the top nanorod and the graphene reinforced polymer composites as the core are constructed to study the tribological properties of polymer composites, i.e. friction coefficient and abrasion rate, by sliding the top iron nanorod on the surface of the polymer matrix. The molecular dynamics simulation results show that increases of about 150% in Young's modulus, 27.6% in shear modulus and 35% in hardness of the polymer composites are obtained respectively by incorporation of graphene reinforcements. In addition, decreases of around 35% and 48% in the average friction coefficient and abrasion rate of the polymer composites are achieved. The research findings are discussed and interpreted by monitoring the interaction energy between graphene and the polymer matrix, the angle, bond and kinetic energy of the polymer chains of the polymer composites. (C) 2016 Elsevier Ltd. All rights reserved.

关键词： Wetland modeling   Nitrogen   Carbon   Process based  

摘要： In this article, extension and application to variably saturated wetland conditions of a process-based wetland model is demonstrated. The new model described in this article is an improved version of an earlier model, which was only capable of capturing nutrient dynamics in continuously ponded wetlands. The upgraded model is capable of simulating nutrient cycling and biogeochemical reactions in both ponded and unsaturated wetland zones. To accomplish this goal, a comprehensive module for tracking water content in wetland soil was implemented in the model, and biogeochemical relationships were added to explain cycling of nitrogen (N) and carbon (C) in variably saturated zones of wetlands. The developed model was applied to a small, restored wetland receiving agricultural runoff, located on Kent Island, Maryland. On average, during the two-year study period, the ponded compartment of the study wetland covered 65% of the total 1.2 ha area. Through mass balance analysis, it was revealed that the mass of nitrogen lost to denitrification at the variably saturated compartment of the study wetland was about three times higher than that of the ponded compartment (32.7 +/- 29.3 kg versus 9.5 +/- 5.5 kg), whereas ammonia volatilization at the variably saturated compartment was a fraction of that of ponded compartment (1.2 +/- 1.9 kg versus 11.3 +/- 11.8 kg). Sensitivity analysis showed that cycling of carbon-related constituents in variably saturated compartment had high sensitivity to temperature and available soil moisture. (C) 2016 American Society of Civil Engineers.

关键词： 动力机械设备   故障   诊断  

摘要： 在动力机械设备运行的过程中,一个小小的故障都会影响到设备的安全性及可靠性,不过,通过故障诊断技术,动力机械设备的安全性及可靠性得到了有效的保证.故障诊断技术将会发展的更加完善,从而促使动力机械设备更好的发挥功能.鉴于此,本文主要分析动力机械设备的故障诊断技术.

关键词： 空间机械臂   地面试验模型   相似设计   动力学特性  

摘要： 随着空间站建设和外太空探索的深入,空间机械臂在航天领域的应用越来越广泛。由于很难实现在轨修复等操作,所以空间机械臂发射到指定位置工作之前,必须经过严格的地面试验。针对空间机械臂的两大特点展开相似动力学特性的研究:一是机械臂的大型结构,给地面试验带来诸多不便,且制造成本高;二是微重力或者低重力的空间环境。国内外学者多采用水浮法、气浮法等模拟空间机械臂的工作环境,导致模型试验存在操作不方便、风险大、成本高等问题。将两者考虑在内,提出克服重力加速度条件下的地面试验模型相似设计方法,完成模型对原型动力学特性的预测分析,在模型设计过程中通过改变材料、尺寸等,可以节省成本,简化操作。并且通过模型试验调节控制参数,可提高原型机械臂的控制精度,因此空间机械臂地面试验模型的相似动力学特性研究具有重要意义。本文首先针对空间机械臂简化的刚性结构,在动力学建模的基础上,分别依据积分模拟法和量纲分析法,展开相似关系的推导,并通过算例数值仿真验证相似关系的正确性;其次,针对空间柔性机械臂的广泛应用,展开考虑连杆柔性在内的相似试验模型设计方法研究,采用混合坐标法和假设模态法动力学建模,基于量纲分析法推导相似关系,通过算例仿真进行简单柔性特征分析,基于系统固有参数的识别,展开振动抑制方法的研究;再者,针对关节柔性引起的运动稳定性问题,开展考虑关节柔性的空间机械臂相似试验模型设计,通过关节柔性简化和力矩传递进行动力学建模,基于相似关系的推导进行算例仿真,研究重要关节参数对系统动力学特性的影响。然后,同时考虑连杆和关节柔性进行多关节连杆机械臂地面试验模型的相似关系推广,并确定相似设计流程。最后,分别通过软件仿真和试验,对理论研究进行验证。

关键词： 选择性转移   农业机械化   替代弹性  

摘要： 传统理论认为农业劳动力成本的上升将通过替代效应引致农业机械化的普及，然而这种效应在中国并不明显。本文首先从收入效应、教育效应与结构效应三个方面构建了农村劳动力选择性转移对于农业机械化的影响机制，并采用2004-2012年中国25个省份种植业的投入产出数据，改进了超越对数成本函数模型，分别计算了种植业各投入要素之间的CPE替代弹性与MES替代弹性。结果表明，考虑了农村劳动力选择性转移之后，劳动力与机械投入之间的CPE替代弹性与MES替代弹性均分别为0.469及0.529，小于仅考虑劳动力价格变化的结果。劳动力成本的提高并不能显著的提高农业机械投入水平。劳动力选择性转移因素解释了部分机械投入的增长。

关键词： 音叉式微机械陀螺   动力学耦合   正交耦合   同相-反相耦合   振动灵敏度   模态优化  

摘要： 微机械陀螺仪是一种基于哥氏效应来测量物体角速度的惯性传感器。它具有体积小、重量轻、成本低、功耗低等优点,可广泛应用于军事和民用领域。随着微陀螺性能的不断提高,其应用对象越来越多,应用环境也越来越恶劣。由于微陀螺不可避免地受到耦合和外界振动的作用,导致微陀螺存在动力学耦合及振动灵敏度(外界振动引起的输出误差)问题。因此需要对微陀螺动力学耦合及振动灵敏度的抑制或消除进行深入的研究,以此来提高微陀螺的性能。基于以上背景,本文以自主设计的双质量块音叉式微机械陀螺为研究对象,通过理论研究、仿真分析、实验测试等手段,分别从结构和电学两个层面对音叉式微陀螺的动力学耦合特性及振动灵敏度进行了研究,为高性能微陀螺的优化设计提供重要的理论依据和参考价值。本文的主要研究工作和创新性研究成果简述如下:(1)考虑到微陀螺受到外界加速度引起的惯性力、耦合刚度力及耦合阻尼力的作用,分别建立了单质量块微陀螺的两自由度动力学一般方程、双质量块音叉式微陀螺的四自由度动力学一般方程和四质量块音叉式微陀螺的八自由度动力学一般方程,为微陀螺的动力学分析提供了理论前提。同时,着重分析了双质量块音叉式微陀螺存在的正交耦合、同相-反相耦合以及模态优化与振动灵敏度等问题,为后续问题的解决提供了理论基础。(2)为实现音叉式微机械陀螺正交耦合的解耦,提出了结构解耦和电学解耦两种解耦方法。首先,利用能量法首次推导出了梁长相等但梁宽不相等的单折叠梁刚度耦合系数,给出了降低弹性梁刚度耦合系数的关键参数,通过降低弹性梁的刚度耦合系数实现了正交耦合的结构解耦。同时,设计了用来消除正交耦合的电极,推导出了该电极产生的静电正交耦合刚度力,提出了通过控制一个质量块的正交耦合刚度来实现正交耦合的电学解耦,并通过静电-结构多场耦合仿真分析和实验测试验证了正交耦合电学解耦的有效性。(3)针对同相-反相耦合影响音叉式微陀螺的振动灵敏度与品质因数的问题,提出了结构解耦和电学解耦的方法来抑制或消除同相-反相耦合。首先,通过提高同相反相刚度差比和弹性梁的宽度实现了同相-反相耦合的结构解耦,并通过仿真分析和实验测试验证了同相-反相耦合的结构解耦可以有效降低音叉式微陀螺的振动灵敏度。其次,设计了刚度匹配电极,推导出了刚度匹配电极产生的静电负刚度力,提出了通过静电负刚度效应的刚度匹配来实现同相-反相耦合的电学解耦,并通过正弦扫描振动实验验证了同相-反相耦合的电学解耦降低振动灵敏度的有效性。另外,考虑到基底的能量损耗,又建立了理想和非理想音叉式微陀螺的三自由度振动模型,首次给出了考虑基底阻尼的非理想结构反相模态品质因数的表达式,并通过实验验证得到反相模态(工作模态)品质因数可以通过同相-反相耦合的电学解耦达到最大值。(4)为解决传统的直接耦合式音叉微陀螺的自身灵敏度与振动灵敏度之间的矛盾,设计了新式的锚点环形梁耦合式音叉微陀螺和锚点菱形梁耦合式音叉微陀螺,并首次构建了双质量块音叉式微陀螺的两自由度锚点耦合振动模型。首先,利用坐标变换法对模态优化和共模振动输出进行了理论计算,并通过有限元仿真分析验证了理论模型的正确性。同时,对比分析了锚点耦合式和直接耦合式音叉微陀螺的模态优化与振动灵敏度,得到锚点耦合式音叉微陀螺在不牺牲陀螺自身灵敏度的前提下,可以极大地优化模态顺序和降低振动灵敏度。(5)综合考虑了音叉式微机械陀螺的动力学耦合特性、振动灵敏度及模态优化等因素,设计了基于锚点杠杆梁耦合机制的音叉式微陀螺,并构建了双质量块音叉式微陀螺的四自由度耦合振动模型,理论分析了所设计微陀螺的动力学响应特性。采用SOG工艺加工了微陀螺,并利用LCC-44封装管壳对加工后的管芯进行气密性封装。最后设计了基于FPGA的音叉式微陀螺的数字化控制电路,采用基于CORDIC算法与LMS算法的闭环驱动和开环检测方式对音叉式微陀螺进行了性能测试。

关键词： 机械与能源动力类专业   创新训练中心   地方院校   创新训练项目  

摘要： 大学生创新训练中心是地方院校培养具有实践能力、创新意识和创业精神的应用型高素质人才的重要保障。机械与能源动力类专业大学生创新训练中心从搭建基于校企合作的"四位一体"创新训练平台、构建"五层次渐进式"创新训练体系、开展形式多样的综合创新实践项目、认真组织参与各类学科专业竞赛、举办主题鲜明的大学生创新创业大赛等方面进行了探索和实践,取得良好的成效。