Wrinkling of a charged elastic film on a viscous layer

A thin metallic film deposited on a compliant polymeric substrate begins to wrinkle under compression induced in curing process and afterwards cooling of the system. The wrinkle mode depends upon the thin film elasticity, thickness, compressive strain, as well as mechanical properties of the compliant substrate. This paper presents a simple model to study the modulation of the wrinkle mode of thin metallic films bonded on viscous layers in external electric field. During the procedure, linear perturbation analysis was performed for determining the characteristic relation that governs the evolution of the plane-strain wrinkle of the thin films under varying conditions, i.e., the maximally unstable wrinkle mode as a function of the film surface charge, film elasticity and thickness, misfit strain, as well as thickness and viscosity of the viscous layer. It shows that, in proper electric field, thin film may wrinkle subjected to either compression or tension. Therefore, external electric field can be employed to modulate the wrinkle mode of thin films. The present results can be used as the theoretical basis for wrinkling analysis and mode modulation in surface metallic coatings, drying adhesives and paints, and microelectromechanical systems (MEMS), etc.     

Periodic cracking of films supported on compliant substrates

When a tensile strain is applied to a film supported on a compliant substrate, a pattern of parallel cracks can channel through both the film and substrate. A linear-elastic fracture-mechanics model for the phenomenon is presented to extend earlier analyses in which cracking was limited to the film. It is shown how failure of the substrate reduces the critical strain required to initiate fracture of the film. This effect is more pronounced for relatively tough films. However, there is a critical ratio of the film to substrate toughness above which stable cracks do not form in response to an applied load. Instead, catastrophic failure of the substrate occurs simultaneously with the propagation of a single channel crack. This critical toughness ratio increases with the modulus mismatch between the film and the substrate, so that periodic crack patterns are more likely to be observed with relatively stiff films. With relatively low values of modulus mismatch, even a film that is more brittle than the substrate can cause catastrophic failure of the substrate. Below the critical toughness ratio, there is a regime in which stable crack arrays can be formed in the film and substrate. The depth of these arrays increases, while the spacing decreases, as the strain is increased. Eventually, the crack array can become deep enough to cause substrate failure.     

BUCKLING ANALYSIS OF STRETCHABLE FERROELECTRIC THIN FILM ON ELASTOMERIC SUBSTRATES

The thin stiff films on pre-stretched compliant substrates can form wrinkles, which can be controlled in micro and nanoscale systems to generate smart structures. Recently, buck- led piezoelectric/ferroelectrie nanoribbons have been reported to show an enhancement in the piezoelectric effect and stretchability, which can be applied in energy harvesting devices, sensors and memory devices instead of polymeric polyvinylidine fluoride （PVDF）. This paper studies the buckling and post-buckling process of ferroelectric thin films bonded to the pre-stretched soft layer, which in turn lies on a rigid support. Nonlinear electromechanical equations for the buckling of thin piezoelectric plates are deduced and employed to model the ferroelectric film poled in the thickness direction. Two buckling modes are analyzed and discussed： partially de-adhered buck- ling and fully adhered buckling. Transition from one buckling mode to the other is predicted and the effect of piezoelectricity on the critical buckling condition of piezoelectric film is examined.     

磁电复合材料中拓扑磁结构的力学调控

磁性斯格明子是在一些铁磁材料中存在的一种重要拓扑磁结构,由于其具有独特的磁-电-力-热多场耦合特性,在未来新型自旋电子器件中有着广泛的应用前景。然而,磁性斯格明子一般需要在外加磁场下才能稳定存在,极大地限制了其在自旋电子器件中的实际应用。本文基于实空间下磁电材料的相场模拟,发现铁电和铁磁复合薄膜中铁电斯格明子可以通过界面变形来稳定铁磁斯格明子。由于力电耦合效应,铁电层中铁电斯格明子的非均匀分布极化在界面产生周期性的非均匀界面变形。界面变形通过力磁耦合效应,使铁磁层中的磁性斯格明子在没有外加磁场的条件下能够稳定存在。本文的研究结果表明,基于磁电复合材料中的力-电-磁耦合效应,通过优化设计复合材料中不同组元的结构,可以实现拓扑磁结构的力学调控,从而为设计基于拓扑磁结构的新型自旋电子器件提供了新思路。     

气流作用下同轴带电射流的不稳定性研究

通过对气体驱动同轴电流动聚焦的实验模型进行简化,开展了电场力和惯性力共同作用下同轴带电射流的不稳定性理论研究.在流动为无黏、不可压缩、无旋的假设下,建立了三层流体带电射流物理模型并得到了扰动在时间域内发展演化的解析形式色散关系,利用正则模方法求解色散方程发现了流动的不稳定模态,进而分析了主要控制参数对不稳定模态的影响.结果表明,在参考状态下轴对称模态的最不稳定增长率最大,因此轴对称扰动控制整个流场.外层气流速度越高,气体惯性力越大,射流的界面越容易失稳.内外层液-液同轴射流之间的速度差越大,射流越不稳定.表面张力对射流不稳定性起到促进作用.轴向电场对射流不稳定性具有双重影响:当加载电场强度较小时,射流不稳定性被抑制;当施加电压大于某一临界值时,轴向电场会促进射流失稳.临界电压的大小与界面上自由电荷密度和射流表面扰动发展关系密切.这些结果与已有的实验现象吻合,能够对实验的过程控制提供理论指导.     

塑封硅器件中介电薄膜的温湿效应分析

随着铜互连以及low-k电介质在超大规模集成电路中地广泛使用，low-k电介质的机械完整性及其对互连可靠性变得更加重要。影响介电膜的机械完整性和互连可靠性的因素包括介电膜的工艺制程，芯片与封装材料的相互影响，以及环境温度和湿度的影响。本文研究集中于了解环境温度和湿度对塑封硅器件中介电薄膜的可靠性影响。采用快速温度和湿度实验条件，对塑封硅器件中介电薄膜受水分和温度损伤的敏感性进行了分析。运用商业有限元（FEA）分析软件，对水分在塑封材料和硅器件中的扩散过程进行了建模及仔细分析。并对硅器件周边密封圈的防水分扩散效力进行了研究。通过这一系列实验与分析，对塑封硅器件中介电薄膜的温湿效应有了完整地了解，并提出和建立了相关的物理模型和经验公式。运用这物理模型和经验公式可对在各种使用环境温度和湿度条件下，塑封硅器件中介电薄膜的可靠性进行评估及分析。     

塑封硅器件中介电薄膜的温湿效应分析(英文)

随着铜互连以及low-k电介质在超大规模集成电路中地广泛使用,low-k电介质的机械完整性及其对互连可靠性变得更加重要。影响介电膜的机械完整性和互连可靠性的因素包括介电膜的工艺制程,芯片与封装材料的相互影响,以及环境温度和湿度的影响。本文研究集中于了解环境温度和湿度对塑封硅器件中介电薄膜的可靠性影响。采用快速温度和湿度实验条件,对塑封硅器件中介电薄膜受水分和温度损伤的敏感性进行了分析。运用商业有限元(FEA)分析软件,对水分在塑封材料和硅器件中的扩散过程进行了建模及仔细分析。并对硅器件周边密封圈的防水分扩散效力进行了研究。通过这一系列实验与分析,对塑封硅器件中介电薄膜的温湿效应有了完整地了解,并提出和建立了相关的物理模型和经验公式。运用这物理模型和经验公式可对在各种使用环境温度和湿度条件下,塑封硅器件中介电薄膜的可靠性进行评估及分析。     

Multiscale computations of thin films in multiphase flows

In multiphase flows thin films are often encountered when fluid masses collide. These films can become very thin and in direct numerical simulations (DNS) it is often impractical to resolve their thickness fully, even with adaptive grid refining. Here we examine the collision of a fluid drop with a wall and develop a multiscale approach to compute the flow in the film between the drop and the wall. By using a semi-analytical model for the flow in the film we capture the evolution of films thinner than the grid spacing reasonably well.     

磁敏弹性膜的研制、性能表征及应用研究进展

磁敏弹性膜是一种新型的智能材料,其力学、电学、磁学、声学等性质能够受外加磁场的控制,从而在多个领域展现在广泛的应用前景。本文首先从材料设计、制备工艺、结构设计等方面综述了磁敏弹性膜的研制方法,随后详细阐述了磁敏弹性膜的力学、电磁、声学等性能表征及内部机理,最后介绍了磁敏弹性膜在传感器、执行器、柔性机器人等领域的应用,在上述基础上,展望了磁敏弹性膜的发展,也提出了面临的问题及挑战。     

Spontaneous instability of soft thin films on curved substrates due to van der Waals interaction

The linear bifurcation theory is used to investigate the stability of soft thin films bonded to curved substrates. It is found that such a film can spontaneously lose its stability due to van der Waals or electrostatic interaction when its thickness reduces to the order of microns or nanometers. We first present the generic method for analyzing the surface stability of a thin film interacting with the substrate and then discuss several important geometric configurations with either a positive or negative mean curvature. The critical conditions for the onset of spontaneous instability in these representative examples are established analytically. Besides the surface energy and Poisson's ratio of the thin film, the curvature of the substrate is demonstrated to have a significant influence on the wrinkling behavior of the film. The results suggest that one may fabricate nanopatterns or enhance the surface stability of soft thin films on curved solid surfaces by modulating the mechanical properties of the films and/or such geometrical properties as film thickness and substrate curvature. This study can also help to understand various phenomena associated with surface instability.     

Electrically rotating suspended films of polar liquids

Controlled rotation of a suspended soap water film, simply generated by applying an electric field, has been reported recently. The film rotates when the applied electric field exceeds a certain threshold. In this study, we investigate the phenomenon in films made of a number of other liquids with various physical and chemical properties. Our measurements show that the intrinsic electrical dipole moments of the liquid molecules seems to be vital for the corresponding film rotation. All the investigated rotating liquids have a molecular electric dipole moment of above 1 Debye, while weakly polar liquids do not rotate. However, the liquids investigated here cover a wide range of physical parameters (e.g. viscosity, density, conductivity, etc.). So far, no significant correlation has been observed between the electric field thresholds and macroscopic properties of the liquids.     

大气环境下YBa_2Cu_3O_7薄膜的摩擦行为和表面形貌

利用球 -盘摩擦试验装置考察了大气环境下 YBa2 Cu3O7薄膜和多晶硅薄膜同蓝宝石球及钢球对摩时的摩擦行为 ,并用原子力显微镜对其表面形貌进行了分析 .结果表明 :表面相对较粗糙的 YBa2 Cu3O7薄膜与蓝宝石球对摩时的摩擦系数低于多晶硅薄膜 /蓝宝石球的摩擦系数 ;而 2种薄膜与钢球对摩时的摩擦系数相当 ;YBa2 Cu3O7薄膜与蓝宝石球对摩时的启动摩擦非常稳定 ,而多晶硅薄膜的启动摩擦很不稳定 ;与钢球对摩时两者的启动摩擦则都比较稳定 .对 YBa2 Cu3O7薄膜的磨损表面形貌分析研究表明 ,其表面突出物结合疏松 ,容易从基体表面磨损脱落 .     

激光冲击对氧化石墨烯薄膜的结构及力电性能影响

采用激光烧蚀氧化石墨烯薄膜,可实现其微尺度图案化加工,以应用于微纳米电子器件。但激光冲击下氧化石墨烯薄膜的结构及力、电性能变化直接影响了器件稳定性和可靠性。为研究超高应变率加载对氧化石墨烯薄膜的结构及性能的影响,采用不同功率激光冲击氧化石墨烯薄膜,通过对其表面形貌、化学成分表征揭示薄膜结构的改变机理,通过对薄膜冲击前后的硬度、弹性模量、导电率测试探索合理的激光加工参数。结果表明:在1.14 W功率的二氧化碳激光冲击下,可实现加工区氧化石墨烯薄膜的还原且不造成薄膜烧蚀断裂,其电导率可达到1.727×103 S/m,弹性模量为49.97 GPa,硬度为5.71 GPa。     

Tensile response of passivated films with climb-assisted dislocation glide

The tensile response of single crystal films passivated on two sides is analysed using climb enabled discrete dislocation plasticity. Plastic deformation is modelled through the motion of edge dislocations in an elastic solid with a lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation incorporated through a set of constitutive rules. The dislocation motion in the films is by glide-only or by climb-assisted glide whereas in the surface passivation layers dislocation motion occurs by glide-only and penalized by a friction stress. For realistic values of the friction stress, the size dependence of the flow strength of the oxidised films was mainly a geometrical effect resulting from the fact that the ratio of the oxide layer thickness to film thickness increases with decreasing film thickness. However, if the passivation layer was modelled as impenetrable, i.e. an infinite friction stress, the plastic hardening rate of the films increases with decreasing film thickness even for geometrically self-similar specimens. This size dependence is an intrinsic material size effect that occurs because the dislocation pile-up lengths become on the order of the film thickness. Counter-intuitively, the films have a higher flow strength when dislocation motion is driven by climb-assisted glide compared to the case when dislocation motion is glide-only. This occurs because dislocation climb breaks up the dislocation pile-ups that aid dislocations to penetrate the passivation layers. The results also show that the Bauschinger effect in passivated thin films is stronger when dislocation motion is climb-assisted compared to films wherein dislocation motion is by glide-only.     

Evaporation and flow dynamics of thin, shear-driven liquid films in microgap channels

Thin and ultra-thin shear-driven liquid films in a narrow channel are a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Such flows experience complex, and as yet poorly understood, two-phase flow phenomena requiring significant advances in fundamental research before they could be broadly applied. This paper focuses on the results obtained in experiments with locally heated shear-driven liquid films in a flat mini-channel. A detailed map of the flow sub-regimes in a shear-driven liquid film flow of water and FC-72 have been obtained for a 2 mm channel operating at room temperature. While the water film can be smooth under certain liquid/gas flow rates, the surface of an intensively evaporating film of FC-72 is always distorted by a pattern of waves and structures. It was found, that when heated the shear-driven liquid films are less likely to rupture than gravity-driven liquid films. For shear-driven water films the critical heat flux was found of up to 10 times higher than that for a falling film, which makes shear-driven films (annular or stratified two-phase flows) more suitable for cooling applications than falling liquid films.     

Effect of the electric field on the wave flow regimes of a thin film of a viscous dielectric fluid

Waves on the surface of a thin film of a viscous dielectric fluid flowing down the inner surface of one plate of a plane capacitor with alternating voltage applied is considered. It is shown that the volume forces acting from the inhomogeneous electric field are negligibly small in the case of long waves, and the influence of the electric field reduces to the influence of additional pressure onto the film surface. A model equation for determining the deviation of the film thickness from the undisturbed value is derived in the long-wave approximation. Some numerical solutions of this equation are given.     

薄膜润滑中双电层效应的理论分析与实验研究

建立了考虑双电层效应的有限宽组合滑块薄膜润滑数学模型,并利用组合滑块与圆盘的滑动摩擦试验对双电层效应进行研究,利用实验结果修正了润滑过程中双电层效应的计算,给出电粘度的计算公式并进行数值分析.结果表明:在薄膜厚度较薄的情况下,双电层效应使得流体的等效粘度随膜厚减小而迅速增加;随着膜厚增加,双电层的电粘度效应逐渐减弱;随着电场强度增加,双电层的电粘度效应增加,当电场强度达到一定程度时,双电层的电粘度效应开始减弱.     

Cracking mechanisms in lithiated silicon thin film electrodes

The massive cracking of silicon thin film electrodes in lithium ion batteries is associated with the colossal volume changes that occur during lithiation and delithiation cycles. However, the underlying cracking mechanism or even whether fracture initiates during lithiation or delithiation is still unknown. Here, we model the stress generation in amorphous silicon thin films during lithium insertion, fully accounting for the effects of finite strains, plastic flow, and pressure-gradients on the diffusion of lithium. Our finite element analyses demonstrate that the fracture of lithiated silicon films occurs by a sequential cracking mechanism which is distinct from fracture induced by residual tension in conventional thin films. During early-stage lithiation, the expansion of the lithium-silicon subsurface layer bends the film near the edges, and generates a high tensile stress zone at a critical distance away within the lithium-free silicon. Fracture initiates at this high tension zone and creates new film edges, which in turn bend and generate high tensile stresses a further critical distance away. Under repeated lithiation and delithiation cycles, this sequential cracking mechanism creates silicon islands of uniform diameter, which scales with the film thickness. The predicted island sizes, as well as the abrupt mitigation of fracture below a critical film thickness due to the diminishing tensile stress zone, is quantitatively in good agreement with experiments.     

FLOW-INDUCED VIBRATION OF FREE EDGES OF THIN FILMS

During manufacturing processes of thin materials such as paper, photographic film, and magnetic film, which are handled as continuous sheets and subjected to drying air-flows, the interaction of the air with the web can cause the free edges to vibrate violently. This phenomenon is related to the waving motion of a flag in the wind, except that the thin films under consideration are under tension in the direction of the air-flow or at right angles to it. A travelling-wave analysis was done based on incompressible potential-flow theory; the critical flow speed, wave speed, wavelength, and flutter frequency were predicted. A closed-form solution of the critical flow speed is suggested. Experiments were carried out with stationary thin films mounted in a wind tunnel where the direction of tension was perpendicular to the flow direction. It was shown that the analysis, which assumes that the film is infinitely long in the flow direction, could successfully predict the critical flow speed above which violent edge vibrations occur.     

润滑条件下钢表面氧化膜结构的SAM研究

作者在球-盘式摩擦磨损试验机上采用阶梯式加载和20~#机油滴注式润滑进行了GCr15钢球对45~#钢盘的摩擦磨损试验。通过摩擦系数的变化记录了在不同滑动速度下摩擦磨损状态发生转化时的临界载荷,从而获得了反映摩擦副摩擦学特性的P-V图,并用扫描俄歇微探针仪(SAM)研究了该摩擦副处于不同状态时表面氧化膜的结构,提出了钢表面氧化膜的结构层次模型,论证了钢的摩擦磨损特性与氧化膜结构之间的关系。