关键词： Noise   Thermal properties   Amorphous materials   Oxides   Transition metals   Molecular dynamics   Two-level models  

摘要： The mechanical loss (Q−1) intrinsic to amorphous oxides is the limiting factor for sensitive, high-precision gravitational wave detectors and optical devices. Recent experimental work suggests that doping amorphous tantala with titania reduces Q−1, however, the physical processes underlying this reduction are unknown. Here, we calculate Q−1 for pure and titania-doped tantala using numerical methods combined with molecular dynamics simulations that have atomic levels of resolution. Our results match experimental trends that titania doping decreases the magnitude of the low-temperature loss peak characteristic of these materials, with 62% titanium cation doping minimizing Q−1 at low temperature. We provide a microscopic explanation for this reduced loss by examining how doping affects the potential energy landscape, strain coupling constant, relaxation time, and other properties of the amorphous materials within the framework of the double-well potential model. Analyzing configurational changes provides an atomic description of the transitions driving mechanical loss at various temperatures in these oxides. These results identify the important parameters contributing to Q−1 that are most affected by doping and provide guidance for how to screen for optimal doping combinations to minimize loss in other materials.

关键词： Bayesian inference   Species interactions   Model-guided experiments   Daphnia magna   Pseudokirchneriella subcapitata  

摘要： Our study describes the functioning of a 2-L laboratory microcosm of two species, daphnids (Daphnia magna) and microalgae (Pseudokirchneriella subcapitata), in two abiotic phases (water column and sediment). We modelled the dynamics of both species and their interactions using a mechanistic model based on coupled ordinary differential equations. The main processes occurring in this two-species microcosm were thus formalised, including growth and settling of algae and growth, survival and grazing of daphnids. We estimated model parameters by Bayesian inference, using simultaneously all data from multiple experiments specifically conducted for this study. Two types of model verifications were performed: (1) internal verification to validate model structure and parameter estimation method using all data simultaneously;and (2) external verification to validate the ability of the model to be applied under new sediment conditions. For all parameters, we obtained biologically realistic values and reasonable uncertainties. The first verification step allowed us to confirm the modelled processes and the benefits of our parameter estimation method. The second one confirmed the ability of the model to describe microcosm functioning under different abiotic conditions. This innovative combination of mechanistic modelling and model-guided experiments revealed successful to understand the algae-daphnid microcosm functioning. This approach appears promising and can be applied to various issues in the ecological and ecotoxicological fields. (C) 2015 Elsevier B.V. All rights reserved.

关键词： MECHANICAL behavior of materials   MOLECULAR simulation methods   FUSED silica   ALUMINUM oxide   WATER   CRYSTAL structure  

摘要： Structure and mechanical property of water-containing glasses were investigated with a molecular dynamics simulation. The glasses were prepared by substituting Na+ ion with H+ ion and inserting H2O molecule in 22Na(2)O-11Al(2)o(3)-67SiO(2) glass (N-0). The number of network rings in the ion-substituted hydrous glass (NH) was independent of total water content and almost the same as that in N-0, while that in the H2O molecule-inserted hydrous glass (NM) decreased with increasing total water content due to the reaction of H2O molecules with glass network;the decomposition of H2O molecule and the breakage of the network. However, NM had denser structure than NH so that Young's modulus of NM became larger than those of NH when total water content (OH + H2O) was the same. NM deformed more slowly than NH under stress, indicating that mechanical property of glass deteriorates more easily with OH group rather than H2O molecule. (C) 2015 Elsevier B.V. All rights reserved.

关键词： Porous graphene   Mechanical properties   Fracture pattern   Molecular dynamics simulations  

摘要： Synthesizing polyphenylene polymer, a two-dimensional hydrocarbon known as porous graphene, has led to the initiation of a new age in nanoscience. In this investigation, molecular dynamics (MD) simulations are carried out to study the mechanical properties of porous graphene such as Young's modulus, Poisson's ratio, bulk modulus and ultimate strength and strain. The fracture initiation and propagation pattern of porous graphene are also considered in this study. The results show that Young's and bulk moduli of porous graphene are lower than those of graphene, graphene and graphyne. Unlikely, it is also observed that its Poisson's ratio is considerably more than that of graphene, graphene and graphyne. Furthermore, it is found out that Young's and bulk moduli as well as fracture strain and ultimate stress are extremely size-dependent and also the porous graphene can be considered as an isotropic material. (C) 2015 Elsevier B.V. All rights reserved.

关键词： Research Article  

摘要： Mechanistic modeling of DNA double strand break (DSB) rejoining is important for quantifying and medically exploiting radiation-induced cytotoxicity (e.g. in cancer radiotherapy). Most radiation-induced DSBs are quickly-rejoinable and are rejoined within the first 1-2 hours after irradiation. Others are slowly-rejoinable (persist for several hours), and yet others are essentially unrejoinable (persist for >24 hours). The dependences of DSB rejoining kinetics on radiation dose and dose rate remain incompletely understood. We hypothesize that the fraction of slowly-rejoinable and/or unrejoinable DSBs increases with increasing dose/dose rate. This radiation-dependent (RD) model was implemented using differential equations for three DSB classes: quickly-rejoinable, slowly-rejoinable and unrejoinable. Radiation converts quickly-rejoinable to slowly-rejoinable, and slowly-rejoinable to unrejoinable DSBs. We used large published data sets on DSB rejoining in yeast exposed to sparsely-ionizing (electrons and gamma-rays, single or split-doses, high or low dose rates) and densely-ionizing (alpha-particles) radiation to compare the performances of the proposed RD formalism and the established two-lesion kinetic (TLK) model. These yeast DSB rejoining data were measured within the radiation dose range relevant for clonogenic cell survival, whereas in mammalian cells DSB rejoining is usually measured only at supra-lethal doses for technical reasons. The RD model described both sparsely-ionizing and densely-ionizing radiation data much better than the TLK model: by 217 and 14 sample-size-adjusted Akaike information criterion units, respectively. This occurred because: the RD (but not the TLK) model reproduced the observed upwardly-curving dose responses for slowly-rejoinable/unrejoinable DSBs at long times after irradiation;the RD model adequately described DSB yields at both high and low dose rates using one parameter set, whereas the TLK model overestimated low dose rate data. T

关键词： Susanne G. Hering  

摘要： A novel instrument has been developed for generating highly monodisperse aerosol particles with a geometrical standard deviation of 1.05 or less. This aerosol generator applies a periodic mechanical excitation to a micro-liquid jet obtained by aerodynamic flow-focusing. The jet diameter and its fastest growth wavelength have been optimized as a function of the flow-focusing pressure drop and the liquid flow rate. The monodisperse aerosol generated by this instrument is also charge neutralized with bipolar ions produced by a non-radioactive, corona discharge device. Monodisperse droplet generation in the 15- to 72-mu m diameter range from a single 100-micron nozzle has been demonstrated. Both liquid and solid monodisperse particles can be generated from 0.7- to 15-mu m diameter by varying solution concentration, liquid flow rate, and excitation frequency. The calculated monodisperse particle diameter agrees well with independent measurements. The operation of this new monodisperse aerosol generator is stable and reliable without nozzle clogging, typical of other aerosol generators at the lower end of the operating particle size ranges.

关键词： 机械设计制造   学院   三峡大学   能源与动力工程   重点实验室   机械电子工程   材料成型   机械工程  

摘要： 2014年1月原机械与材料学院更名为机械与动力学院，学院现设有机械设计制造及其自动化、机械电子工程、材料成型及控制工程、工业工程、能源与动力工程、核工程与核技术六个本科专业，其中机械设计制造及其自动化为国家特色专业、第二批卓越工程师教育培养计划专业和省本科品牌专业，材料成型及控制工程为第三批卓越工程师教育培养计划专业。拥有机械工程一级硕士点、机械工程专业学位点、机械工程领域工程硕士点、工业工程专业学位点，机械制造及其自动化为湖北省重点学科。目前，在校全日制本科学生近2000人，在读研究生160余人，毕业生的一次就业率一直保持在90％以上。学院拥有水电机械设备设计与维护湖北省重点实验室，工程训练中心等，实验室建筑面积1．2万平米，设备仪器总值2000余万元。

关键词： ANSYS   FMS   机械臂   动力学   谐响应   模态  

摘要： FMS系统中的搬运机械臂底座是非标件且为关键部位,其在运行过程中因使用频率高,故经常出现弯曲变形、断裂和过载磨损等现象。利用ANSYS Workbench对搬运机械臂底座进行模态分析和谐响应分析,得出数据并进行优化分析,为底座的改进提供依据。

关键词： 动力机械及工程   专业英语   教学  

摘要： 专业外语教学目前在课程教学中存在诸多不足,使得教学质量不理想,最终影响了学习效果。为此作者在多年的课程教学经验的基础上,总结出一些有利于教学的教改措施,较好地改善了教学效果,为专业外语的教学提供了一定的价值参考。

关键词： MATLAB软件   多参数   动力系统   混沌  

摘要： 针对多参数非线性机械系统的分析问题,提出了在MATLAB环境下仿真系统的设计方法。该方法的核心是将在参数和初始值的变化过程中,各个参数和初始值及其对应的系统运动相图保存起来,根据相图对系统的运动状态进行初步判断,找出与研究主题最相关的相图及其对应的参数和初始点。利用该参数和初始点重新对系统进行仿真,再利用系统其他运动状态的判据,对系统的运动状态进一步分析,如功率谱和最大Lyapunov指数,从而重新确定出对应系统某种运动状态的参数和初始值。继续上述程序运行,便可完成多参数变化对系统运动状态影响的分析。以一个两自由度机械系统为例,说明了该方法的实现过程和有效性。