关键词： mechanical stability   non-B DNA   proteins   single-molecule laser tweezers   transition dynamics  

摘要： With the firm demonstration of the in vivo presence and biological functions of many non-B DNA structures, it is of great significance to understand their physiological roles from the perspective of structural conformation, stability, and transition kinetics. Although relatively simple in primary sequences compared to proteins, non-B DNA species show rather versatile conformations and dynamic transitions. As the most-studied non-B DNA species, the G-quadruplex displays a myriad of conformations that can interconvert between each other in different solutions. These features impose challenges for ensemble-average techniques, such as X-ray crystallography, NMR spectroscopy, and circular dichroism (CD), but leave room for single-molecular approaches to illustrate the structure, stability, and transition kinetics of individual non-B DNA species in a solution mixture. Deconvolution of the mixture can be further facilitated by statistical data treatment, such as iPoDNano (integrated population deconvolution with nanometer resolution), which resolves populations with subnanometer size differences. This Personal Account summarizes current mechanical unfolding and refolding methods to interrogate single non-B DNA species, with an emphasis on DNA G-quadruplexes and i-motifs. These single-molecule studies start to demonstrate that structures and transitions in non-B DNA species can approach the complexity of those in RNA or proteins, which provides solid justification for the biological functions carried out by non-B DNA species. DOI 10.1002/tcr.201200021

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.

关键词： 混合动力汽车   定子永磁型双机械端口电机   有限元分析   场路耦合   温度场   工作模式  

摘要： 目前，汽车工业的发展方向正从传统内燃机汽车逐渐发展到新能源汽车，混合动力汽车由于兼具传统汽车较高的动力性能和电动汽车节能减排的双重优势，己成为一种不可或缺的过渡车型，成为当前国、内外汽车行业的研究热点之一。本文研究了一种新型定子永磁型双机械端口电机，它继承了双转子电机和定子永磁型电机的优点，将该双机械端口电机应用在中/强度混合动力合成系统中。通过一定的控制方式灵活地切换电机工作模式，能实现定子永磁型双机械端口电机在内燃机和驱动轮之间的能量分配与调节，提高内燃机工作效率。\n 本文首先介绍了混合动力汽车应用背景，以及双凸极永磁电机和双机械端口电机的研究现状和发展趋势。在此基础上研究了一种用于混合动力合成系统的新型定子永磁型双机械端口电机及其关键技术，分析其结构和工作原理，还对该双机械端口电机常见工作模式的功率流情况以及对应的汽车行驶工况做了理论分析和仿真研究。\n 论文中建立了定子永磁型双机械端口电机的一般设计方法，并介绍了相关参数尺寸的计算与选择方法，设计了一个2kW的样机用来验证原理。根据计算所得参数，结合有限元分析软件，对电机进行参数化建模，详细分析了电机空载和负载磁场分布以及磁链、电感、反电势和定位力等静态特性。基于场路耦合法建立定子永磁型双机械端口电机的Maxwell-Simplorer仿真模型，对比了不同控制方法下的电机输出转矩特性，并总结了降低转矩脉动的方法。\n 最后，从电磁场和温度场耦合的角度对稳态运行时定子永磁型双机械端口电机内部温度场进行了分析，运用ANSYSWorkbench软件建立三维仿真模型。通过有限元软件Ansoft计算出电机不同情况下的损耗分布，导入ANSYS进行多物理场耦合仿真，分析了室温下仅依靠自身风冷散热条件下的定子永磁型双机械端口电机三维温度场温升分布。在此基础上模拟汽车运行环境，并对添加冷却设施后的电机进行仿真，验证了冷却系统的散热效果，进一步完善了电机的本体设计。

关键词： 空间机械臂   动力学建模   柔性分析   仿真验证  

摘要： 人类对外太空的不断探索使得空间机械臂技术颇受关注。美国、加拿大、日本等航天大国已经掌握了部分关键技术，根据需求研发了几种不同类型的空间站机械臂并已投入使用。我国也正着手加强这方面的研究，以便顺利实现我国空间三步走计划。空间机械臂动力学建模是空间机械臂研究的重要基础，在外太空复杂的环境下，建立能精确描述机械臂实际运行状态的模型，掌握机械臂系统动力学特性，对机械臂的精确控制尤为重要。因此，对空间机械臂动力学建模技术的研究是非常有必要和关键的。本文以大型空间柔性机械臂为研究对象，对机械臂动力学建模理论、仿真验证和动力学特性进行了深入的分析与研究。\n 首先，本文对目前空间机械臂的应用研究现状和动力学建模研究现状展开了详细调研，确定以大型空间双连杆机械臂为研究对象，综合考虑臂杆柔性和关节柔性对系统产生的影响。用假设模态法分析臂杆柔性，通过模态实验分析将柔性臂杆视为悬臂梁处理，截取前两阶振动模态。根据Spong关节模型简化原理，将柔性关节的变形近似等效为刚度系数为k的线性扭转弹簧。\n 其次，对空间柔性机械臂建模方法进行了研究。本文采用相对坐标法，对柔性臂上任意点的位置进行描述。为便于后续对机械臂从动力学分析向控制系统设计的转化，本文采用Lagrange方程结合虚功原理推导了模型动力学方程的一般形式。\n 再次，对动力学方程求解方法进行了研究。本文的动力学方程是一组非常复杂的、强耦合的、非线性的、时变的微分方程，本文先用 MAPLE软件辅助做复杂符号运算，求解出质量矩阵及其导数、刚度矩阵、广义力和总动能等的符号表达式，将这些表达式移植到 Matlab中，基于 Newmark-β原理开发了 Matlab数值仿真程序，求解了方程的数值解。\n 然后，对所推导的动力学模型进行仿真验证。用成熟的商业软件ADAMS和ANSYS联合设计并建立了柔性机械臂仿真模型，设定了多种工况，比较了两种模型的输出结果，证明了推导的动力学模型的准确性和建模方法的有效性。\n 最后，对柔性机械臂进行了瞬态响应和频率响应实验，还结合关节柔性、臂杆柔性、末端质量开展了仿真研究，分析了柔性机械臂的刚柔耦合动力学特性。

关键词： 产学研结合   控制工程   机械电子   浙江大学   科技发展   研究所   国家创新体系   科技创新能力  

摘要： 胡锦涛同志在党的十八大报告里指出,"要实施创新驱动发展战略","深化科技体制改革,推动科技和经济紧密结合,加快建设国家创新体系,着力构建以企业为主体、市场为导向、产学研相结合的技术创新体系"。这表明国家对提高科技创新能力有了更深层次的要求,进一步提高产学研结合的紧密程度,形成体系,才能给科技发展提供不竭动力。本刊记者带着一系列的问题采访了浙江大学机械电子控制工程研究所徐兵教授,请他给我们介绍浙江大学机械电子控制工程研究所产学研科技创新之路的工作情况。

关键词： thermal stability   imide   degradation kinetics   mechanical properties   thermoplastic elastomers   Poly(ester-ether)  

摘要： A novel thermally stable thermoplastic poly(ester-ether) (PEE) elastomer containing imide units was prepared from poly(tetramethylene glycol) (PTMG1000), 1,4-butanediol (BD) and a new imide dicarboxylic acid based on pyromellitic dianhydride (PMDA) and glycine through a traditional chemical two-step method. The structures of the synthesized imide dicarboxylic acid and novel PEE were confirmed by FT-IR spectroscopy. The mechanical properties of the novel PEE were investigated. Thermal stability and thermal degradation kinetics of the novel PEE were investigated by thermogravimetric analysis (TGA) under different heating rates. The kinetic parameters of the degradation process were determined by using Kissinger, Flynn-Wall-Ozawa and Friedman methods. The Coats-Redfern method was also used to discuss the probable degradation mechanism of this PEE. The results showed that introduction of the imide units into the poly(ester-ether) endowed the PEE with excellent thermal stability and good mechanical properties. The activation energy obtained by using the Kissinger method was in agreement with that using the Flynn-Wall-Ozawa method. The reaction order (n) and pre-exponential factor (A) were obtained by using the Friedman method. Analysis of the experimental results suggests that the decomposition reaction mechanism of the PEE was a F-3 type (random nucleation with three nuclei on the individual particle).