The influence of mechanical constraint upon the switching of a ferroelectric memory capacitor

The role of mechanical constraint upon the switching response of a ferroelectric thin film memory capacitor is explored. The memory capacitor is represented by a two dimensional ferroelectric island whose non-linear behaviour is modelled by a crystal plasticity constitutive law within the finite element method. The switching response of the device, in terms of remnant charge storage, is determined as a function of geometry and constraint. Various types of constraint on the ferroelectric capacitor are considered, including the presence of a silicon dioxide passivation layer, a silicon substrate and metallic electrodes. The effect of the relative resistance to 90 degree switching and 180 degree switching is also explored in a tetragonal ferroelectric device. Throughout the study, the finite element calculations are compared with the behaviour of a material element subjected to various degrees of mechanical constraint.     

Numerical modelling of film cooling from converging slot-hole

This paper presents a numerical prediction of a new 3D film cooling hole geometry, the converging slot-hole or console. The console geometry is designed in order to improve the heat transfer and aerodynamic loss performance of turbine vane and rotor blade cooling systems without loosing the mechanical strength of a row of discrete holes. The cross section of the console changes from a circular shape at the inlet to a slot at the exit. Previous successful application of a new anisotropic DNS based two-layer turbulence model to cylindrical and shaped hole injections is extended to predict film cooling for the new console geometry. The suitability of the proposed turbulence model for film cooling flow is validated by comparing the computed and the measured wall-temperature distributions of cylindrical hole injections. The result shows that the anisotropic eddy-viscosity/diffusivity model can correctly predict the spanwise spreading of the temperature field and reduce the strength of the secondary vortices. Comparative computations of the adiabatic film cooling effectiveness associated with the three geometries tested in the present study (cylindrical, shaped, and console) show that the new console film-cooling hole geometry is definitely superior to the other geometries as shown by the uniform lateral spreading of the effectiveness with a slight enhancement downstream of the intersection of the two consoles.     

单畴外延铁电薄膜的相结构与稳定性

本文采用动态金茨堡-朗道（DGL）方程研究了薄膜厚度与错配应变对 取向单畴外延PbTiO3（PTO）铁电薄膜相结构与稳定性的影响。结合平面内松弛应变（等效应变）、表面效应与退极化场等机电耦合边界条件,通过数值求解DGL方程获得外延单畴铁电薄膜错配应变-厚度相图和错配应变-温度相图。数值分析结果显示,由于生成的界面位错松弛了薄膜内错配应变,在理论高应变区相图与传统分析结果有较大差别,文中发现在更广的理论错配拉应变区出现稳定的四方相（c相）结构和单斜相（r相）结构。结果也显示,随着薄膜厚度的减小,表面效应与退极化效应会把顺电相扩展到更低温度区域,从而压缩稳定的铁电相存在的温度区域。     

Mechanics of relaxing SiGe islands on a viscous glass

A process has been developed recently to fabricate a structure comprising, from top to bottom, a SiGe thin film, a glass layer, and a Si wafer. The SiGe film is a perfect crystal, and is under biaxial compression. The SiGe film is patterned into islands. On annealing, the glass flows and the islands relax. The resulting strain-free islands are used as substrates, to grow epitaxial optoelectronic devices. This article describes a series of studies on the annealing process, combining experiment and theory. A small island relaxes by expansion, starting at the edges and diffusing to the center. A large island wrinkles before the expansion reaches the center. After some time, the wrinkles either disappear, or cause the island to fracture. We model the island as an elastic plate, and the glass layer as a viscous liquid. The strains in the islands are measured by X-ray diffraction and Raman spectroscopy, and the wrinkle amplitudes by atomic force microscope. The data are compared with the theoretical predictions. We determine the conditions under which the islands relax by expansion without significant wrinking, and demonstrate that a cap layer suppresses wrinkles, relaxing a large island crack-free. The project supported by NSF (CMS-9820713), DARPA (N66001-00-1-8957), ARO (DAA655-98-1-0270), and New Jersey Science and Technology Commission     

Problem of Low-frequency Localized Oscillations in a Thin Film with Growing Islands

Influence of localization waves to islands growing on a thin film is investigated. The film is modelled as a fluid layer covered by an inertial surface with the variable density of mass and surface tension. Mathematically, the problem is reduces to analysis of a system of non-linear equations describing the growth of island nuclei and wave propagation in the films. The existence of trapped modes for the corresponding frequency-domain problem is established. We show that for large time wave localization near islands gives some contribution in the increase of the velocity of island growth.     

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。     

Thin film/substrate systems featuring arbitrary film thickness and misfit strain distributions. Part II: Experimental validation of the non-local stress/curvature relations

The classical Stoney formula relating local equibiaxial film stress to local equibiaxial substrate curvature is not well equipped to handle realistic cases where the film misfit strain, the plate system curvature, and the film thickness and resulting film stress vary with in-plane position. In Part I of this work we have extended the Stoney formula to cover arbitrarily non-uniform film thickness for a thin film/substrate system subject to non-uniform, isotropic misfit strains. The film stresses are found to depend non-locally on system curvatures. In Part II we have designed a demanding experiment whose purpose is to validate the new analysis for the case of radially symmetric deformations. To achieve this, a circular film island with sharp edges and a radially variable, but known, thickness is deposited on the wafer center. The plate system’s curvatures and the film stress distribution are independently measured by using white beam and monochromatic X-ray microdiffraction (μXRD) measurements, respectively. The measured stress field (from monochromatic μXRD) is compared to the predictions of various stress/curvature analyses, all of which have the white beam μXRD measurements as input. The results reveal the shortcomings of the “local” Stoney approach and validate the accuracy of the new “non-local” relation, most notably near the film island edges where stress concentrations dominate.     

Effect of surface non-flatness on the lubrication characteristics in the valve part of a swash-plate type axial piston pump

The objective of this application study was to investigate the effect of surface non-flatness on the lubrication characteristic of the bearing/sealing part between cylinder barrel and valve plate in a hydrostatic axial piston pump. A developed numerical algorithm facilitated the simultaneous calculation of time-varying cylinder pressure, rotating body motion, and fluid film pressure to observe fluid film geometry and power loss. It was shown that an ideally flat surface might not form full fluid lubrication film properly, and that small-scale machining error, surface waviness, may increase the film thickness to some degree. The shape model of surface waviness considered waviness unit shape as well as its surface lay. However the results demonstrated that surface non-flatness of such small scale did not form the desirable fluid film geometry which minimized the power loss yet. Providing some surface design tips, two particular curved surfaces whose pressure-generating mechanisms differ were selected and analyzed in variation with their shapes and operating conditions. This study asserted that a circumferentially wavy surface would make better performance of motion stability and power efficiency than a radially wedged land surface, and finally that the non-flatness design strategy should be applied with re-considering the clamping ability.     

Highly reliable bipolar resistive switching in sol-gel derived lanthanum-doped PbTiO3 thin film： Coupling with ferroelectricity？

Nanoscale PbxLa1-,Ti1-x/4O3 （PLT） thin film has been fabricated on Pt／Ti／SiO2／Si substrates by chemical solution deposition （CSD） method. Ferroelectricity of the fresh-made PLT thin film has been clearly detected through piezoelectric force microscopy （PFM） by writing reversible ferroelectric domains. However, PLT thin film also shows off-standard ferroelectric hysteresis loops highly dependent on frequency, indicating large amount of mobile space charges in the film. Subsequent current-voltage （C-V） studies show that sandwich-like Pt／PLT／Pt structure exhibits notable bipolar resistive switching （BRS） characteristics with high stability （〉 103 switching cycles）. It is found that the C-V curves of both high- and low-resistance states have the feature of space-charge-limited current （SCLC） conduction, indicating important roles of defects in the conduction. X-ray photoelectron spectroscopy measurement further verifies that oxygen vacancies based conductive filament mechanism is likely responsible for the observed RS effect. Our demonstration of stable RS effect in the PLT thin film and its possible coupling with ferroelectricity is promising in device development and applications, such as development of ferroelectric-tunable RS memories.     

Equilibrium shapes of strained islands with non-zero contact angles

The equilibrium morphology of a strained island on an elastic substrate is determined. The island is assumed to partially wet the substrate (Volmer-Weber growth) and thus makes a non-zero contact angle with the surface. Both isotropic and anisotropic misfit strain are allowed. Two- and three-dimensional equilibrium island shapes are determined by using expressions for the elastic strain energy in the small-slope approximation. In this limit, the problem can be reduced to a singular integral-differential equation for the island thickness. We find that when there is a non-zero contact angle, all island shapes, for a given ratio of the elastic stress to surface energy, attain a form that is independent of the specific contact angle under an appropriate scaling. We show that for islands with non-zero contact angles, as the island volume increases, the shape approaches the geometry of a completely wetting island. But when the volume decreases, these islands approach a point while islands with a zero contact angle, approach a finite length line segment of zero volume. Multiple-hump equilibrium shapes are found. Single-humped islands are shown to have a lower chemical potential than multiple-humped islands, implying that they are the most stable. This conclusion is shown to be consistent with a stability analysis of the two-dimensional case. The effects of a tetragonal misfit strain on the three-dimensional island shape is investigated.     

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.     

激光诱导应力波测量薄膜界面强度研究进展

薄膜界面强度是影响多层薄膜装置性能的重要参数。激光诱导应力波技术是在可控制和非接触条件下定量测量薄膜界面强度最有效的技术之一。在采用高强度应力波短脉冲加载实现界面层裂的同时，通过光学测量薄膜自由面瞬态位移，并利用应力波理论计算得到临界界面强度。通过精确控制试样几何形状及尺寸，包括拉伸、剪切和复合型在内的各种界面加载模式都可以实现。本文对激光诱导应力波测量薄膜界面强度研究进展进行了综述，并特别强调了不同加载模式的实现方法，高强度超薄薄膜的界面测量技术，以及如何通过辨别薄膜自由面瞬态位移光学干涉信号中的某些特殊性征来实时判断和测量界面的层裂。     

MINIMUM SIZE OF 180 DEGREE DOMAINS IN FERROELECTRIC THIN FILMS COVERED BY ELECTRODES

Ferroelectric domain switching under low voltage or short pulses is of interest for the development of high-density random access memory (FRAM) devices. Being necessarily very small in size, instability and back switching often occur when the external voltage is removed, which creates serious problems. In this investigation, a general approach to determine the minimum size of ferroelectric domain to avoid back switching was developed, and as an example, a 180°domain in a ferroelectric thin film covered by the upper and lower electrodes was considered in detail. We note that our approach is generally applicable to many other fields, including phase transformation, nucleation and expansion of dislocation loops in thin films, etc.     

Controlled buckling of thin film on elastomeric substrate in large deformation

Electronic systems with large stretchability have many applications. A precisely controlled buckling strategy to increase the stretchability has been demonstrated by combining lithographically patterned surface bonding chemistry and a buckling process. The buckled geometry was assumed to have a sinusoidal form, which may result in errors to determine the strains in the film. A theoretical model is presented in this letter to study the mechanics of this type of thin film/substrate system by discarding the assumption of sinusoidal buckling geometry. It is shown that the previous model overestimates the deflection and curvature in the thin film. The results from the model agree well with finite element simulations and therefore provide design guidelines in many applications ranging from stretchable electronics to micro/nano scale surface patterning and precision metrology.     

Experimental studies on heat and mass transfer performance of a coiled tube absorber for R134a-DMAC based absorption cooling system

Absorber is an important component in vapor absorption refrigeration system and its performance has greater influence in overall efficiency of absorption machines. Falling film heat and mass transfer in an absorber is greatly influenced by fluid properties, geometry of heat exchanger and its operating parameters. This paper presents on the results of experimental studies on the heat and mass transfer characteristics of a coiled tube falling film absorber, using 1,1,1,2-Tetrafluroethane(R-134a) and N-N Dimethyl Acetamide (DMAC) as working fluids. The effects of film Reynolds number, inlet solution temperature and cooling water temperature on absorber heat load, over all heat transfer coefficient and mass of refrigerant absorbed are presented and discussed. Normalized solution and coolant temperature profiles and refrigerant mass absorbed along the height of absorber are also observed from the experimental results. The optimum over all heat transfer coefficient for R-134a–DMAC solution found to be 726 W/m²K for a film Reynolds number of 350. The R-134a vapour absorption rate is maximum in the normalized coil height of 0.6 to 1.     

Numerical and experimental study on effect of wall geometry on wall impingement process of hollow-cone fuel spray under various ambient conditions

The spray–wall impingement process in gasoline direct injection (GDI) engines, which is caused by the interaction among spray, wall and air to move the air–fuel mixture near the spark plug, directly influences the engine performance and emissions. Therefore, a detailed understanding of this process is very important in designing an injection system and controlling a strategy of GDI engines. The purpose of this study is to understand the spray–wall impingement characteristics for more efficient designing of the injection system in GDI engines and to supply the fundamental data under engine operation conditions. The wall impingement processes of hollow-cone fuel spray according to ambient gas conditions and wall geometry are calculated by validated spray models. The calculated results were compared with the experimental results obtained by the laser-induced exciplex fluorescence (LIEF) technique. It was found that the spray and vortex cloud at the high ambient pressure were distributed at inner area of cavity and the more fuel film mass observed at this condition. The fuel film mass decreased with the increase of ambient temperature, while the fuel film mass increased at high cavity angles.     

Numerical analyses of rectangular micro-textures in hydrodynamic lubrication regime for sliding contacts

Hydrodynamic lubrication of rectangular micro-textures on sliding contact surfaces is investigated using numerical calculation methods. The theoretical models for the slider surface are developed and the film pressure is used to evaluate the hydrodynamic lubrication based on the Reynolds equation. Meanwhile, the geometry and distribution of the rectangular dimples are optimized for maximizing the average film pressure. Results show that the film pressure is dependent on the geometry and distribution of the rectangular micro-dimples. The optimal geometry of the single rectangular dimple is obtained, and the spacing has an important influence on the film pressure. The distribution types of rectangular dimples affect the hydrodynamic lubrication significantly and the interlaced array of the rectangular micro-dimples is beneficial to enhancing the hydrodynamic lubrication. Meanwhile, the rectangular dimples with 72° interlaced angle exhibits the best effectivity.

     

Atomistic simulation of stress evolution during island growth

We report the results of a series of hybrid molecular dynamics simulations of the growth of islands on a substrate for several different island/substrate interface energies. When the interface energy is small, the islands tend to be thin and broad and the magnitude of the compressive stress-thickness product is relatively large. As the interface energy increases, the islands become taller and thinner and the magnitude of the compressive stress-thickness product decreases. This trend is consistent with experimental observations. The island aspect ratio dependence on interface energy follows from consideration of the equilibrium wetting angle. The effect of interface energy on the stress-thickness product shows that the island shape, surface/interface stresses and island stresses are self-equilibrated. A simple theory is developed that shows that the stress-thickness product is simply proportional to the substrate coverage and the substrate surface stress. The present simulations yield a simple, accurate, validated theory for stress development during the pre-coalescence stage of film growth.     

Effects of interface dislocations on properties of ferroelectric thin films

Effects of interfacial dislocations on properties of thin-film ferroelectric materials, such as the self-polarization distribution, Curie temperature, dielectric constant and the switching behaviors, are investigated via the system dynamics based on the Landau-Devonshire functional. Dislocation generation in the film is found to reduce the overall self-polarization and the Curie temperature. The spatial variations are both very strong, particularly in the immediate neighborhood of the dislocation cores. In agreement with previous results based on a stationary model, a dead layer exists near the film/substrate interface, in which the average self-polarization is much reduced. Moreover, it is evident from our results that interface dislocations play an important role in suppressing the remnant polarization and the coercive field of the polarization.