关键词： First-principles   Molecular dynamics   Chemical mechanical polishing   Ceria   Silica   Tribology  

摘要： We report a form of chemical mechanical polishing (CMP) in which ceria abrasive particles polish a silicon wafer surface in a water environment. The Car-Parrinello molecular dynamics (CPMD) method, which enables chemical reaction dynamics in non-equilibrium systems to be treated non-empirically, was used. The first application of CPMD was to a model including a silica surface rubbed by a ceria cluster to ascertain the fundamental tribochemical phenomena in the CMP process. The model consisted of a hydrogen-terminated silica surface (a representative model of an oxidized wafer surface) and H4Ce6O12 cluster placed in the vicinity of an asperity in the silica surface. H2O molecules were explicitly considered in order to reflect the presence of slurry water. An external load and shear force were applied to the cluster. Under conditions of finite temperature and friction, a Ce-O-Si bridging bond formed at the interface between the silica surface and the ceria cluster. Subsequently, another Ce-O-Si bridging bond formed. These newly formed bridging bonds changed the silicon atom into a five-coordinated one. Because this state was probably unstable, a pyramid structure was rapidly restored by breaking the original Si-O bond in the asperity, meaning that the wafer surface became flattened. By referring to static density functional theory (DFT) calculations, we concluded that this bond dissociation event was due to multiple Ce-O-Si bonds forming. To discuss the effect of slurry water, a similar CPMD simulation, but without H2O molecules (i.e., dry friction), was performed. Here, the Si-O bond dissociation reaction did not take place even if multiple Ce-O-Si bonds formed. This means slurry water is needed for CMP to occur and it probably helps to weaken the Si-O bond. The second application of CPMD was to a model consisting of a thin water layer sandwiched between silica and ceria plates for studying the effect of the surface orientation of the ceria. Two low-index CeO2 surfac

关键词： multifunctional device   MAPbI(3)-PVDF composite   energy harvester   photodetector   pressure sensor  

摘要： A single-structured multifunctional device capable of energy harvesting and sensing multiple physical signals has significant potential for a wide range of applications in the Internet of Things (IoT). In this study, the fabricated single-structured device based on methylammonium lead iodide-polyvinylidene fluoride (MAPbI(3)-PVDF) composite can harvest mechanical energy and simultaneously operate as a self-powered light and pressure sensor because of the combined photoelectric and piezoelectric/ triboelectric properties of the MAPbI(3)-PVDF composite. Light-dependent dielectric and piezoelectric properties of composite films are thoroughly investigated. Light and contact electrification effect on device performance in both piezoelectric and triboelectric modes is also systematically investigated. When the device is operated as a harvester in both piezoelectric and triboelectric modes, remarkable light-driven outputs were observed under illumination;the outputs decreased in the piezoelectric mode, while they increased in the triboelectric mode. Such light-controlled properties enabled the device to operate as a self-powered photodetector with outstanding responsivity (similar to 129.2 V/mW), rapid response time (similar to 50 ms), and satisfactory detectivity (similar to 1.4 x 10(10) Jones) in the piezoelectric mode. The same device could also operate as a pressure sensor that exhibited excellent pressure sensitivity values of 0.107 and 0.194 V/kPa in the piezoelectric and triboelectric modes, respectively. In addition, the device exhibits a fast response time with long-term on-off switching properties, excellent mechanical durability, and long-term stability.

关键词： Molecular dynamics   PDMS-Ni conductive Composites   Nanofillers   Modeling  

摘要： Molecular dynamics was applied to develop a novel PDMS and PDMS-Ni nanocomposite model. First, the PDMS model was validated in terms of the elastic and compressive moduli using available experimental data. The model was then used as a benchmark to add Ni nanoparticles and create a PDMS-Ni conductive nanocomposite model. This model was studied under uniaxial tensile and compressive loading conditions for different Ni filler ratios and mean particle diameters. The electrical resistivity of the model was then obtained as a function of pressure and the compressive stresses. The PDMS-Ni MD model was validated via the literature and shown to successfully predict electrical resistivity variations versus compressive stresses. The results show that as the Ni filler ratio increased from 3:1 to 6:1, the drop in initial electrical resistivity of the composites occurred much faster and at lower stresses. Furthermore, as the mean Ni particle diameter size was increased from 1 nm to 5 nm, the electrical conductivity of the PDMS-Ni improved at different filler ratios. The developed PDMS-Ni MD model can be used to study the electrical and mechanical behaviors of such materials in biosensor applications. It can further be used to study the effects of adding other fillers to PDMS matrix.

关键词： molecular dynamics   graphene   carbon nanotube   vdW heterostructure   mechanical properties  

摘要： We find stiffness properties of van der Waals heterostructures made of graphene and carbon nanotubes (CNTs) using molecular dynamics (MD) simulations. The variation of the stiffness tensor is studied at different temperatures for structures with changing CNT density, and orientations. Orthotropic structures with unidirectional CNTs have high stiffness similar to 300-500 GPa, in one direction. Structures with transverse isotropy show stiffness of similar to 150-350 GPa along two mutually perpendicular directions. The orthotropic structures also show greater stiffness with increasing CNT density, while the others are insensitive to it. In general,the stiffness constant values are decrease as temperatures increase. The results can aid in determining useful heterostructure configuration for a particular application and in finding overall stiffness of devices having additional components.

关键词： MD simulation   Nanoporous   Tensile properties   Single crystal  

摘要： In this research, the mechanical behavior of aluminum nanofoam was studied by the method of molecular dynamics simulation. The effective parameters in mechanical properties such as porosity, pore size, temperature, and strain-rate were investigated. The results showed that Young's modulus, yield strength, and ultimate tensile strength were increased by decreasing the porosity and pore size. To accurately calculate the Young's modulus (E-s), the yield strength (sigma(YS)) and the ultimate tensile strength, combining the stress-strain curves with the potential and kinetic energy-strain curves was suggested as a standard method. The investigations about the effects of temperature and strain-rate on the tensile behavior showed that increasing temperature cause decrease in E-s, sigma(YS), and sigma(UTS) of foams. Moreover, the sigma(UTS) increased significantly while the E-s and the sigma(YS) of the samples exhibited minor change with an increase in strain-rate. Nanofoam failure also occurred by stress localization, nucleation, and growth of cracks on the surface of pores.

关键词： 钻柱   水平井   非线性   接触   机械系统动力学自动分析  

摘要： 钻柱在高曲率、长水平段的水平井中与井壁接触状态十分复杂,接触状态又直接影响钻进过程中钻柱的摩阻扭矩,进而影响井的延伸极限。为确定试验装置上接触力的测量位置,准确描述钻柱与井壁的接触状态,进而保证摩阻扭矩测试试验结果准确可靠。基于机械系统动力学自动分析(automatic dynamic analysis of mechanical systems,ADAMS)软件,结合力平衡理论、Hertz接触理论和相似原理,建立了与真实测试试验装置具有一致钻井参数的水平井全井钻柱-井壁动态非线性接触模型,分析了全井钻柱接触力分布情况,研究了起钻速度、下钻速度、钻压和转速对接触的影响规律。结果表明:接触力峰值出现在弯曲段;起、下钻速度均对弯曲段钻柱接触状态影响明显,速度相同时起钻比下钻时接触力大;增加钻压可减小全井钻柱的平均接触力;钻柱旋转会在水平段中前部出现小幅密集接触。仿真结果为钻柱摩阻扭矩测试试验装置提供了合适的钻柱与井壁的接触力测量位置,便于利用该装置开展钻柱摩阻扭矩等相关研究。

关键词： Cytochrome P450   hepatocytes   mechanism-based inhibition   microsomes   time-dependent inhibition  

摘要： Time-dependent inhibition (TDI) may confound drug interaction predictions. Recently, models were generated for an array of TDI kinetic schemes using numerical analysis of microsomal assays. Additionally, a distinct terminal inactivation step was identified for certain mechanism based inhibitors (MBI) following reversible metabolite intermediate complex (MIC) formation. Longer hepatocyte incubations potentially allow analysis of slow TDI and terminal inactivation. In the experiments presented here, we compared the quality of TDI parameterization by numerical analysis between hepatocyte and microsomal data. Rat liver microsomes (RLM), suspended rat hepatocytes (SRH) and sandwich-cultured rat hepatocytes (SCRH) were incubated with the prototypical CYP3A MBI troleandomycin and the substrate midazolam. Data from RLM provided a better model fit as compared to SRH. Increased CYP3A expression after dexamethasone (DEX) induction improved the fit for RLM and SRH. A novel sequential kinetic scheme, defining inhibitor metabolite production prior to MIC formation, improved the fit compared to direct MIC formation. Furthermore, terminal inactivation rate constants were parameterized for RLM and SRH samples with DEX-induced CYP3A. The low expression of CYP3A and experimental error in SCRH resulted in poor data for model fitting. Overall, RLM generated data better suited for elucidation of TDI mechanisms by numerical analysis.

关键词： 航空工业   航空事业   空气动力学   试验基地   航天飞行器   设计阶段   自主设计   航空领域  

摘要： 20世纪中期,新中国第一架自主设计的飞机试飞成功,标志着我国航空事业走向全新纪元。改革开放后,随着大量外资及先进技术的引入,更是加快了我国航空工业迈向自主研发及设计阶段的脚步,发展至今,我国航空领域已形成基本完善的开发设计体系及生产试验基地。

关键词： 虚拟同步机   机电比拟   机械导纳模型   转矩–频率动力学   功率振荡  

摘要： 为满足高渗透新能源并网的惯性和阻尼需求,虚拟同步机(virtual synchronous generator,VSG)技术应运而生。然而,在指令功率或负载发生扰动时,易引发VSG间产生暂态功率环流和功频振荡,现有小信号分析模型难以全面描述上述问题。因此,该文运用机电比拟原理将同步发电机(synchronous generator,SG)的转子动力学模型类比为机械网络,进而提出一种基于机械导纳法的转矩–频率动力学建模方法。构建单台SG/VSG的机械导纳模型,进而拓展为多台SGs/VSGs并联的机械导纳模型。该模型可采用电路理论分析系统的功频特性,不仅可以给出详细的响应表达式,而且对采用不同控制策略的逆变器均具有一定的适用性。仿真结果验证了所建模型及理论分析的准确性和有效性。

关键词： Bathe积分算法   机械系统多体动力学方程   广义阻尼矩阵   Baumgarte违约稳定项  

摘要： 准确与高效的求解算法一直是多体系统动力学领域的关键问题.重点研究了将Bathe积分算法应用于机械系统多体动力学方程的求解,将多体系统动力学方程整理为显含广义阻尼矩阵的一般形式.利用Bathe积分策略,推导了基于此形式动力学方程的求解流程,并将广义阻尼矩阵用于迭代计算时雅克比矩阵初值的选择,减少迭代计算次数.为了减小违约的影响,动力学方程中添加了Baumgarte违约稳定项.数值算例表明:利用Bathe积分算法求解多体系统动力学方程具有高准确性、良好的稳定性和较低的数值耗散,显含广义阻尼矩阵的动力学方程形式也使求解更加高效.