纳米沟槽表面黏着接触过程的分子动力学模拟研究

采用大规模分子动力学方法研究了刚性球型探头与具有不同纳米沟槽基体表面的黏着接触过程,探讨了表面沟槽结构对载荷-位移曲线、接触引力和拉离力以及材料转移的影响规律.研究结果表明:在相同的压入深度下,与原子级光滑表面的黏着接触过程相比,刚性探头与具有纳米沟槽结构基体表面的接触压力较小,接触加载过程中的引力作用范围较大,并伴随载荷的多次跳跃,且接触引力和拉离力均有减小;当沟槽深度相同时,随着沟槽宽度的增大,接触引力和拉离力逐渐减小,当沟槽宽度逐渐接近探头与光滑表面的接触直径时,接触引力和拉离力又逐渐增大,趋于接近探头与光滑表面的接触过程;当沟槽宽度相同时,随着沟槽深度的增大,接触引力相对减小,拉离力变化不大.     

微平面接触分离中弯月面力的计算

微平面间黏着力对微机电系统(MEMS)非常重要,常是决定其能量损耗乃至寿命长短的最主要因素.MEMS中的黏着力主要来源之一就是介于两互相接触平面间的弯月面力.弯月面力主要取决于相互接触的两平面间形成的弯月面形状.本文通过分析两微小平面在分离过程中弯月面形状的变化,得到在不同表面亲水/疏水性能、初始液面高度、分离距离等条件下的弯月面形状,计算得出在不同初始条件下断裂高度和弯月面力的数值以及随之变化的规律,为MEMS的性能分析和寿命计算提供依据。     

考虑接触热阻的纳米颗粒堆积热导率的分析

本文首先使用Callaway热导率模型对SiO₂纳米颗粒的热导率进行了近似计算,然后耦合堆积纳米孔隙内的导热和辐射、颗粒接触热阻,基于颗粒堆积单元结构模型的一维传热分析,最终推导得到了颗粒堆积有效热导率关于颗粒直径和温度、堆积孔隙率、颗粒热导率、气相热导率、辐射传热和接触热阻的关系式,并用该式进行了相关讨论。研究结果表明,对于纳米颗粒堆积,界面接触热阻不容忽略;在低孔隙率和颗粒不参与辐射的条件下,由于受到接触热阻的影响,存在最佳孔隙率（或密度）使得堆积热导率存在最大值。     

微机械纳米接触连续方法的建模和计算

为解决微机械中"微碰撞"、"纳米接触"问题,建立了纳米接触的刚性球-面模型,根据Hamaker3个假设和Lennard Jones势,利用连续方法推导出球同平面第1和第N层原子之间的作用力表达式,仿真出粘着力同原子层之间的关系,得出影响纳米接触的主要原子为接触区域少数几层原子的结论,从而为纳米接触、纳米摩擦的进-步研究提供理论基础.     

纳米晶氧化锡薄膜的接触特性

在 Ar和 O2 气体中 ,基片温度在 15 0～ 40 0℃的条件下 ,用直流磁控溅射的方法可以制备纳米晶透明导电薄膜。实验利用 TL M模型测试了纳米晶 Sn O2 透明导电薄膜的方块电阻、单位面积薄膜的接触电阻和电极与薄膜的结合力随热处理温度的变化情况     

热退火诱导纳米银膜形貌变化对表面等离激元共振特性的影响

采用磁控溅射技术制备并通过不同温度的快速热退火得到了不同表面形貌的纳米银膜。利用XRD,SEM和紫外-可见-近红外透射光谱等技术研究了纳米银膜的结构、表面形貌与光学性质。实验结果表明,随着退火温度的升高,银膜开口面积分数、银岛(纳米粒子)间距增大,长宽比减小,银岛由各向异性的蠕虫状变成各向同性的纳米球;表面等离激元共振带发生连续的蓝移,半高宽变窄。分析表明,纳米银膜的表面等离激元共振特性可以通过热退火诱导的表面形貌变化实现调整。     

AuBe/p-GaP接触体系界面特性随温度变化的研究

微合金条件对M-S接触的表面与界面特性产生重要的影响.借助光电子能谱分析了AuBe/p-GaP接触体系界面组分变化,用扫描电子显微镜观察其表面形貌；从冶金学观点解析了M-S接触界面反应的特征,并对其温度依赖关系进行了讨论.     

石墨烯纳米结构的原子级精准构造

相比单层石墨烯,特定构型的石墨烯纳米结构具有更加新奇的物理性质,被认为是构筑基于石墨烯功能纳米器件的基本单元。由于纳米结构的性质对其局域原子构型非常敏感,可控制备高质量的、原子级精确的石墨烯纳米结构并研究其奇特的电子学和自旋电子学性质是实现其器件应用的必要前提。目前,实验构筑锯齿形石墨烯纳米带及其面内异质结、纳米孔洞以及量子点已取得很多重要进展,但是具有复杂结构的功能化石墨烯纳米结构的构筑与物性研究仍存在挑战。文章介绍作者在原子级精确构造石墨烯纳米结构方面取得的系列科研进展。运用可控制备技术与高精度的极低温强磁场扫描隧道显微镜,探索在不同衬底上制备高质量的、具有特定构型的石墨烯纳米结构,并尝试对制备的石墨烯纳米结构进行原子级精准的操纵,实现构造“用户定制”的功能化石墨烯纳米结构。此系列工作发展了石墨烯纳米结构精确可控制备方法,为研究复杂纳米结构与物性提供了新的思路。     

欧姆接触中接触电阻率的计算

从欧姆接触电阻率的定义出发,先从理论上介绍了热离子发射、热离子场发射和场发射三种不同情况下欧姆接触电阻率的计算公式,然后详细地从实验上综合各种测试方法,并讨论了其利弊．     

纳米接触行为的三维多尺度数值模拟

应用分子动力学与有限元耦合的桥域多尺度算法,模拟三维刚性球形压头与光滑基体表面的纳米尺度接触行为,并与全原子分子模拟结果比较.考察在一定载荷下的系统弛豫行为、两种模型桥接区位移和应力的连续性、法向力和接触面积随压头位移变化等,结果表明:一定外载荷下,桥域多尺度算法能较快达到平衡状态,且压头的振荡幅度更小,系统初始温度为0 K时该算法的相对误差最小.在准静态加载过程中,该算法能够将原子区的位移、应力等连续的过渡到连续介质区,具有较好的耦合效果;法向力-压头位移和接触半径-压头位移曲线几乎与分子模拟结果重合,表明算法具有较高的计算精度.     

Relationship between the real contact area and contact force in pre-sliding regime

The pre-sliding regime is typically neglected in the dynamic modelling of mechanical systems. However, the change in contact state caused by static friction may decrease positional accuracy and control precision. To investigate the relationship between contact status and contact force in pre-sliding friction, an optical experimental method is presented in this paper.With this method, the real contact state at the interface of a transparent material can be observed based on the total reflection principle of light by using an image processing technique. A novel setup, which includes a pair of rectangular trapezoidal blocks, is proposed to solve the challenging issue of accurately applying different tangential and normal forces to the contact interface. The improved Otsu's method is used for measurement. Through an experimental study performed on polymethyl methacrylate(PMMA), the quantity of contact asperities is proven to be the dominant factor that affects the real contact area. The relationship between the real contact area and the contact force in the pre-sliding regime is studied, and the distribution of static friction at the contact interface is qualitatively discussed. New phenomena in which the real contact area expands along with increasing static friction are identified. The aforementioned relationship is approximately linear at the contact interface under a constant normal pressure, and the distribution of friction stress decreases from the leading edge to the trailing edge.     

A numerical study of contact force in competitive evacuation

Crowd force by the pushing or crushing of people has resulted in a number of accidents in recent decades. The aftermath investigations have shown that the physical interaction of a highly competitive crowd could produce dangerous pressure up to 4500 N/m, which leads to compressive asphyxia or even death. In this paper, a numerical model based on discrete element method(DEM) as referenced from granular flow was proposed to model the evacuation process of a group of highly competitive people, in which the movement of people follows Newton's second law and the body deformation due to compression follows Hertz contact model. The study shows that the clogs occur periodically and flow rate fluctuates greatly if all people strive to pass through a narrow exit at high enough desired velocity. Two types of contact forces acting on people are studied. The first one, i.e., vector contact force, accounts for the movement of the people following Newton's second law. The second one, i.e., scale contact force, accounts for the physical deformation of the human body following the contact law. Simulation shows that the forces chain in crowd flow is turbulent and fragile. A few narrow zones with intense forces are observed in the force field, which is similar to the strain localization observed in granular flow. The force acting on a person could be as high as 4500 N due to force localization, which may be the root cause of compressive asphyxia of people in many crowd incidents.     

把“拉倒法测量薄膜附着力”引入大学物理实验

介绍了如何把"拉倒法测量薄膜附着力"的实验引入到大学物理实验教学中.用简单的直流溅射镀膜仪在玻璃衬底上沉积银薄膜,用设计制作的拉倒法测量薄膜附着力的实验装置测量了银薄膜在衬底上单位面积的附着力.本实验是对已经在北京科技大学国家工科物理基础课程教学基地开设的大学物理实验内容的新的扩充.     

Effect of micro-dimple patterns on capillary pull-off force and friction force of silicon surface

A microtribometer is used to measure and compare pull-off forces and friction forces exerted on (a) micro-dimpled silicon surfaces, (b) bare silicon surfaces, and (c) octadecyltrichlorosilane (OTS) treated silicon surfaces at different relative humidity (RH) levels separately. It is found that above a critical RH level, the capillary pull-off force increases abruptly and that the micro-dimple textured surface has a lower critical RH value as well as a higher pull-off force value than the other two surfaces. A micro topography parameter, namely sidewall area ratio, is found to play a major role in controlling the capillary pull-off force. Furthermore, micro-dimpled silicon surface is also proved to be not sensitive to variation in RH level, and can realize a stable and decreased friction coefficient compared with un-textured silicon surfaces. The reservoir-like function of micro dimples is considered to weaken or avoid the breakage effect of liquid bridges at different RH levels, thereby maintaining a stable frictional behaviour.     

Adhesion elastic contact and hysteresis effect

In this paper, we study the relationship between the pull-off force and the transition parameter (or Tabor number) as well as the variation of the pull-off radius with the transition parameter in the adhesion elastic contact. Hysteresis models are presented to describe the contact radius as a function of external loads in loading and unloading processes. Among these models, we verified the hysteresis model from Johnson－Kendall－Roberts theory, based on which the calculated results are in good agreement with experimental ones.     

Scratch direction and threshold force in nanoscale scratching using atomic force microscopes

The nanoscaled tip in an AFM (atomic force microscope) has become an effective scratching tool for material removing in nanofabrication. In this article, the characteristics of using a diamond-coated pyramidal tip to scratch Ni-Fe thin film surfaces was experimentally investigated with the focus on the evaluation of the influence of the scratch or scan direction on the final shape of the scratched geometry as well as the applied scratch force. Results indicated that both the scratched profile and the scratch force were greatly affected by the scratch direction. It has been found that, to minimize the formation of protuberances along the groove sides and to have a better control of the scratched geometry, the tip face should be perpendicular to the scratching direction, which is also known as orthogonal cutting condition. To demonstrate the present findings, three groove patterns have been scratched with the tip face perpendicular to the scratching direction and very little amount of protuberances was observed. The threshold scratch force was also predicted based on the Hertz contact theory. Without considering the surface friction and adhesive forces between the tip and substrate, the threshold force predicted was twice smaller than the measurement value. Finally, recommendations for technical improvement and research focuses are provided.     

表面黏附导致的接触行为转变

应用大规模分子动力学方法, 模拟了具有不同原子级粗糙形貌的两种刚性球形探头与弹性平面基体的黏附接触行为. 研究了载荷与真实接触面积、接触界面排斥力与真实接触面积, 以及黏附力与真实接触面积之间的关系. 分子模拟得到的载荷与真实接触面积的关系, 与连续力学接触理论预测很好地定性一致. 无论是原子级光滑探头还是粗糙探头, 黏附接触下的排斥力与真实接触面积的关系, 都与无黏附接触时的规律相一致, 即黏附力对接触行为的影响作用, 可以等效为附加在真实外载荷基础上的虚拟载荷, 将对黏附接触行为的分析转变为无黏附接触分析. 两种探头的黏附力随真实接触面积都呈幂函数形式的增长, 但是, 原子级光滑探头的幂指数大于1, 而原子级粗糙探头的幂指数小于1. 关键词： 接触行为 表面黏附 分子动力学模拟     

PZT铁电薄膜纳米尺度铁电畴的场致位移特性

利用扫描力显微术的压电响应模式，并基于逆压电效应原理，研究了梯度组成的PZT铁电薄膜纳米尺度铁电畴的场致位移特性.获得了源于纳米尺度铁电畴的压电效应和电致伸缩效应贡献的场致位移回滞线，以及源于线性压电效应和电畴反转效应综合贡献的纳米尺度压电位移 场强蝶形曲线，证实了Caspari Merz理论在纳米尺度上的有效性.发现了梯度铁电薄膜存在纳米尺度印刻现象，认为该现象的内因源于薄膜中的内偏场. 关键词： PZT铁电薄膜 场致位移 纳米尺度 扫描力显微术