Showing papers on "Contact area published in 1999"

Nanometre-scale rolling and sliding of carbon nanotubes

Abstract: is preferred over sliding, and it is expected to have an equally important role in the microscopic domain. Although progress has been made in our understanding of the dynamics of sliding at the atomic level 4 ,w e have no comparable insight into rolling owing to a lack of experimental data on microscopic length scales. Here we produce controlled rolling of carbon nanotubes on graphite surfaces using an atomic force microscope. We measure the accompanying energy loss and compare this with sliding. Moreover, by repro- ducibly rolling a nanotube to expose different faces to the substrate and to an external probe, we are able to study the object over its complete surface. The microscopic aspects of tribology have been explored by means of the surface force apparatus (SFA) 5 and atomic force microscopy (AFM) 6 , which have identified the intrinsic dependence of sliding friction on contact area 7,8 and crystallographic orientation 9,10 . Further, AFM has been used to perform friction mapping of surfaces with atomic resolution 4 and has identified stick-slip motion in the sliding of nanometre-scale objects 11 . Using AFM manipulation 12,13 of multiwall carbon nanotubes (CNTs), we have observed sliding and rolling. CNTs, with a range of available sizes down to the molecular scale, serve as interesting model systems for tribological studies. In addition, nanotubes are expected to play a part in future nanometre-scale electrical-mechanical devices, and rolling fullerenes have been proposed as ideal lubricants 14-16 . Our evidence for sliding and rolling comprises sequences of topogra- phical images of manipulated nanotubes and lateral force data acquired during the manipulation to measure energy loss. Samples were prepared by solvent evaporation on mica and graphite substrates from an ethanol solution of raw carbon soot produced by the carbon arc technique 17 . The multiwall CNTs were imaged and manipulated under ambient conditions. The nanomanipulator AFM system 18,19 , designed for manipulation, comprises an advanced visual interface, teleoperation capabilities for manual control of the AFM tip and haptic (touch) presentation of the AFM data (Topometrix Discoverer). Normal force, lateral force and AFM tip trajectory are recorded simultaneously for strict correlation. The AFM tip is used to apply lateral forces at locations along the tube to produce translations and rotations. These tubes are free of pinning material and move without deformation. The lateral force values have been calibrated from measured cantilever and tip geometry, literature values for the cantilever (Si) elastic moduli and the detector response from the z-translation of the cantilever. We estimate the absolute error in this calibration to be about 30%, with the largest contribution in the uncertainty coming

Modeling of Contact Mechanics and Friction Limit Surfaces for Soft Fingers in Robotics, with Experimental Results

Abstract: A new theory in contact mechanics for modeling of soft fingers is proposed to define the relationship between the normal force and the radius of contact for soft fingers by considering general soft-finger materials, including linearly and nonlinearly elastic materials. The results show that the radius of contact is proportional to the normal force raised to the power of, which ranges from 0 to 1/3. This new theory subsumes the Hertzian contact model for linear elastic materials, where D 1/3. Experiments are conducted to validate the theory using artificial soft fingers made of various materials such as rubber and silicone. Results for human fingers are also compared. This theory provides a basis for numerically constructing friction limit surfaces. The numerical friction limit surface can be approximated by an ellipse, with the major and minor axes as the maximum friction force and the maximum moment with respect to the normal axis of contact, respectively. Combining the results of the contactmechanics mo...

Integrated circuit memory cell having a small active area and method of forming same

Abstract: A method for manufacturing a memory device having a plurality of memory cells. Each memory cell has a non-volatile resistive memory element with a small active area. A plurality of memory cells are formed at selected locations of at least a portion of a semiconductor wafer. To form the memory cells, a lower electrode layer and a memory material layer are deposited over at least a portion of the wafer. Patterns are formed over desired contact locations of the memory material layer and etching is used to remove portions of the memory material layer. The etching step undercuts the patterns and forms memory elements having a protruding contact portion with an apex contact area. The pattern is removed, and an upper electrode is formed and electrically coupled to the contact area. Corresponding access devices and word/bit line conductor grids are provided and coupled to the memory cells.

Atomic Scale Sliding and Rolling of Carbon Nanotubes

Abstract: Using molecular statics and dynamics methods we investigate the motion of nanotubes on a graphite surface. Each nanotube has unique equilibrium orientations with sharp potential energy minima which lead to atomic scale locking of the nanotube. The effective contact area and the total interaction energy scale with the square root of the radius. Sliding and rolling nanotubes have different characters. The potential energy barriers for sliding nanotubes are higher than that for perfect rolling. When the nanotube is pushed, we observe a combination of atomic scale spinning and sliding motion.

Effect of Ag particle size on electrical conductivity of isotropically conductive adhesives

Abstract: The present work is to introduce nanoparticles in micro-sized metal particles to study particle distribution in polymer matrix. Previous examinations of the silver-filled particles reveal that the micro-sized particle fillers appear as full density silver flakes, while nanoparticle fillers appear as highly porous agglomerates, similar to open-cell foams. Actually little work has been carried out to study the cross-sectional area of a particle-particle-contact in isotropically conductive adhesives (ICA). In this study, transmission electron microscope is chosen as a main measure to analyze the distribution of different-sized particles. The percentage of the nanoparticles varies from 20 wt% and 50 wt% to full percentage within micro-sized particles, and the total metal content in epoxy resin is 70 wt%. So the change of contact area and contact behavior with various volume ratio of nano-sized and micro-sized particles was investigated. At the same time, the electrical resistivity was measured, which is compared with the different level of the filler loading.

A mixed elastohydrodynamic lubrication model with asperity contact

Abstract: Most machine elements, such as gears and bearings, are operated in the mixed lubrication region. To evaluate lubrication performance for these tribological components, a contact model in mixed elastohydrodynamic lubrication is presented. This model deals with the EHL problem in the very thin film region where the film is not thick enough to separate the asperity contact of rough surface. The macro contact area is then divided into the lubricated area and the micro asperity contact areas by the contacted rough surfaces. In the case when asperity to asperity contact is present, Reynolds equation is only valid in the lubricated areas. Asperity contact pressure is determined by the interaction of two mating surfaces. The applied load is carried out by the lubricant film and the contacted asperities. FFT techniques are utilized to calculate the surface displacement (forward problem) by convolution and the asperity contact pressure (inverse problem) by deconvolution for non-periodic surfaces. With the successful implementation of FFT and multigrid methods, the lubricated contact problem can be solved within hours on a PC for the grids as large as one million nodes. This capability enables us to simulate random rough surfaces in a dense mesh. The load ratio, contact area ratio and average gap are introduced to characterize the performance of mixed lubrication with asperity contacts. Discussions are given regarding the asperity orientation as well as the effect of rolling-sliding condition. Numerical results of real rough topography are illustrated with effects of velocity parameter on load ratio, contact ratio, and average gap.

Mechanics of leukocyte deformation and adhesion to endothelium in shear flow

Abstract: The mechanics of leukocyte [white blood cell (WBC)] deformation and adhesion to endothelial cells (EC) in shear flow has been investigated. Experimental data on transient WBC–EC adhesion were obtained from in vivo measurements. Microscopic images of WBC–EC contact during incipient WBC rolling revealed that for a given wall shear stress, the contact area increases with time as new bonds are formed at the leading edge, and then decreases with time as the trailing edge of the WBC membrane peels away from the EC. A two-dimensional model (2D) was developed consisting of an elastic ring adhered to a surface under fluid stresses. This ring represents an actin-rich WBC cortical layer and contains an incompressible fluid as the cell interior. All molecular bonds are modeled as elastic springs distributed in the WBC–EC contact region. Variations of the proportionality between wall shear stress (τ w ) in the vicinity of the WBC and the resulting drag force (F s ), i.e., Fs/τw, reveal its decrease with WBC deformation and increasing vessel channel height (2D). The computations also find that the peeling zone between adherent WBC and EC may account for less than 5% of the total contact interface. Computational studies describe the WBC–EC adhesion and the extent of WBC deformation during the adhesive process. © 1999 Biomedical Engineering Society. PAC99: 8717-d, 8719Tt, 8717Aa

Strain-rate effects on Hardness of glassy polymers in the nanoscale range. Comparison between quasi-static and continuous stiffness measurements

Abstract: Strain rate effects on Hardness and Young's modulus of two glassy polymers, poly(diethylene glycol bis allyl carbonate) (CR39) and bisphenol-A polycarbonate (PC), were studied in the nanoscale range. Before analyzing material behaviors, we focused on a particular phenomenon prevailing at the first stage of the contact between the surface of these polymers and the Berkovitch diamond tip used in the experiments, leading to an apparent increase of the tip defect (i.e., the missing tip of the diamond from having a shape equivalent to a perfect cone). The common methods based on calibration functions of the tip appear to be inaccurate to calculate correctly the contact area at the nanoscale range for these polymers. A new method based on Loubet et al.'s model to calculate the contact area by taking account of the apparent tip defect is proposed. The hardness values obtained this way were compared to the compressive yield stress using Tabor's relationship. A hardness-yield stress ratio close to 2.0, as...

Peak Friction Behavior of Smooth Geomembrane-Particle Interfaces

Abstract: An investigation of shear mechanisms at interfaces between particles and relatively smooth materials using contact mechanics and basic friction theory reveals that a combination of sliding and plowing governs dense Ottawa 20/30 sand/smooth high density polyethylene geomembrane peak interface shear behavior. Contact area and the corresponding shear resistance during sliding increase at a slower rate than the applied normal stress, resulting in a decreasing friction coefficient and flattening of the peak strength envelope. Plowing of soil grains results in an increasing peak friction coefficient with increasing normal stress and can produce an upward curvature of the strength envelope above a critical stress level. Plowing is primarily controlled by the relative hardness of the interface materials and by grain shape with angular particles exhibiting plowing in all normal stress ranges, whereas nearly perfect spheres exhibit only sliding. High surface hardness is shown to constrain shear behavior to a sliding mode with little contribution from plowing. These findings are consistent with results reported in the tribology literature.

The contact resistance and reliability of anisotropically conductive film (ACF)

Abstract: The effect of bonding pressure on the electrical and mechanical properties of anisotropic conductive film (ACF) joint using nickel particles and metal-coated polymer ball-filled ACFs was investigated. The contact resistance decreases as the bonding pressure increases. Contact resistance of ACF is determined by the contact area change between particles and contact substrates. Electrical conduction through the pressure engaged contact area between conductive particles and conductor substrates is the main conduction mechanism in ACF interconnection. In addition, environmental effects on contact resistance and adhesion strength such as thermal aging, high temperature/humidity aging and temperature cycling were also investigated. Interestingly, the contact resistances of the excessively bonded samples deteriorated more than those of optimally bonded ones. Increasing contact resistance and decreasing adhesion strength after harsh environmental tests were mainly due to the loss of contact by thermal stress effect and moisture absorption, and also partially due to the formation of metal oxide on the conductive particles.

Effect of ionic strength on initial interactions of Escherichia coli with surfaces, studied on-line by a novel quartz crystal microbalance technique.

Abstract: A novel quartz crystal microbalance (QCM) technique was used to study the adhesion of nonfimbriated and fimbriated Escherichia coli mutant strains to hydrophilic and hydrophobic surfaces at different ionic strengths. This technique enabled us to measure both frequency shifts (Df), i.e., the increase in mass on the surface, and dissipation shifts (DD), i.e., the viscoelastic energy losses on the surface. Changes in the parameters measured by the extended QCM technique reflect the dynamic character of the adhesion process. We were able to show clear differences in the viscoelastic behavior of fimbriated and nonfimbriated cells attached to surfaces. The interactions between bacterial cells and quartz crystal surfaces at various ionic strengths followed different trends, depending on the cell surface structures in direct contact with the surface. While Df and DD per attached cell increased for nonfimbriated cells with increasing ionic strengths (particularly on hydrophobic surfaces), the adhesion of the fimbriated strain caused only low-level frequency and dissipation shifts on both kinds of surfaces at all ionic strengths tested. We propose that nonfimbriated cells may get better contact with increasing ionic strengths due to an increased area of contact between the cell and the surface, whereas fimbriated cells seem to have a flexible contact with the surface at all ionic strengths tested. The area of contact between fimbriated cells and the surface does not increase with increasing ionic strengths, but on hydrophobic surfaces each contact point seems to contribute relatively more to the total energy loss. Independent of ionic strength, attached cells undergo time-dependent interactions with the surface leading to increased contact area and viscoelastic losses per cell, which may be due to the establishment of a more intimate contact between the cell and the surface. Hence, the extended QCM technique provides new qualitative information about the direct contact of bacterial cells to surfaces and the adhesion mechanisms involved.

Observation of proportionality between friction and contact area at the nanometer scale

Abstract: The nanotribological properties of a hydrogen‐terminated diamond(111)/tungsten‐carbide interface have been studied using ultra‐high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten‐carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer‐scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin–Muller–Toporov continuum mechanics model.

Influence of cell deformation on leukocyte rolling adhesion in shear flow.

Abstract: Blood cell interaction with vascular endothelium is important in microcirculation, where rolling adhesion of circulating leukocytes along the surface of endothelial cells is a prerequisite for leukocyte emigration underflow conditions. HL-60 cell rolling adhesion to surface-immob ilized P-selectin in shear flow was investigated using a side-view flow chamber, which permitted measurements of cell deformation and cell-substrate contact length as well as cell rolling velocity. A two-dimensiona l model was developed based on the assumption that fluid energy input to a rolling cell was essentially distributed into two parts: cytoplasmic viscous dissipation, and energy needed to break adhesion bonds between the rolling cell and its substrate. The flow flelds of extracellular fluid and intracellular cytoplasm were solved using finite element methods with a deformable cell membrane represented by an elastic ring. The adhesion energy loss was calculated based on receptor-ligand kinetics equations. It was found that, as a result of shear-flow-induced cell deformation, cell-substrate contact area under high wall shear stresses (20 dyn/crrf) could be as much as twice of that under low stresses (0.5 dyn/cm'). An increase in contact area may cause more energy dissipation to both adhesion bonds and viscous cytoplasm, whereas the fluid energy input may decrease due to the flattened cell shape. Our model predicts that leukocyte rolling velocity will reach a plateau as shear stress increases, which agrees with both in vivo and in vitro experimental observations.

Transfer of Contaminants from Surface to Hands: Experimental Assessment of Linearity of the Exposure Process, Adherence to the Skin, and Area Exposed During Fixed Pressure and Repeated Contact with Surfaces Contaminated with a Powder

Abstract: Estimation of dermal exposure in the workplace resulting from contact with contaminated surfaces is important in risk assessment. Models have been developed to describe the process of exposure due to transfer, but for major input parameters - that is, contact area surface and adherence - defaults are used. This study examines the effect of one single-hand press contact and repeated contacts with a contaminated glass plate on both skin area exposed and loading of the skin for three volunteers. A fluorescent whitening agent was used to study the process of exposure and to determine the increase of the area exposed as well as the adherence of the compound to the skin surface after 1 to 12 consecutive contacts by a video imaging technique. In addition, loading of the skin after 12 contacts was compared to loading of a cotton glove monitor with similar hand pressures. The results show that after one single-hand contact only 4 to 16 percent of the total surface of the palm of the hand was exposed, whereas after 12 contacts this was increased to about 40 percent. The efficiency of transfer was ≤2 percent of the contamination of the surface. The adherence to the skin was 1.07 μg/cm2 after 12 contacts and tended to increase non-linearly with increase in contacts. Because defaults of adherence for use in exposure models are generally a factor 500 to 5,000 higher, and the area exposed is assumed to be the total surface of the hand, overestimation of dermal exposure due to a single hand-surface contact in workplaces may occur. Therefore, additional studies on multi- contact exposure scenarios are indicated to adjust defaults for hand-surface transfer processes.

Lateral RF MOS device having a combined source structure

Abstract: A lateral RF MOS device having a combined source connection structure is disclosed. The combined source connection structure utilizes a diffusion area and a conductive plug region. In one embodiment, the diffusion source area forms a contact region connecting the top surface of the semiconductor material to a highly conductive substrate of the lateral RF MOS transistor structure. In another embodiment, the diffusion source area is located completely within the epitaxial layer of the lateral RF MOS transistor structure. The conductive plug region makes a direct physical contact between a backside of the semiconductor material and the diffusion contact area.

IC chip package with directly connected leads

Abstract: A semiconductor device (60) is disclosed, comprising: a semiconductor die (62) including top and bottom surfaces, the semiconductor die (62) further comprising a source contact area (64), a gate contact area (66), and a drain contact area, wherein the contact areas (64, 66) are arranged on a selected one of the top and bottom surfaces; a drain lead (72) comprising a top surface, a bottom surface (72c), and a coupling portion (72a) coupled to the drain contact area of the semiconductor die (62); and an encapsulant (74) enclosing a portion of the semiconductor die (62) and a portion of the drain lead (72) such that the bottom surface (72c) of the drain lead (72) remains exposed.

A theoretical model of micro-pool lubrication in metal forming

Abstract: A model of a secondary hydrodynamic lubrication mechanism, which is called micro-pool or micro-plasto hydrodynamic lubrication, has been developed. It shows that, with sufficiently high viscosity and sliding speed, the lubricant trapped in the micro-pools between the tool and workpiece can be drawn into the interface. The friction force is either increased or decreased, depending on the viscosity and sliding speed. Without bulk stretching, the product of the lubricant viscosity and sliding velocity can be used as an index to indicate whether or not micro-pool lubrication will occur. Stretching the workpiece may make a strong influence not only on the thickness of the permeating film but also on the asperity contact area.

Image fixing apparatus

Abstract: The image fixing apparatus includes a heater having a heat generating resistor on a substrate, a heater holder for holding said heater, and a backup roller for forming a nip portion in cooperation with said heater, wherein said heater holder has, in a direction along a short side of said heater, a contact area facing said resistor forming area and coming into contact with said heater, and non-contact areas provided on both sides of the contact area and not coming into contact with said heater, and the contact area has a width equal to or larger than the resistor forming area. With this configuration, it is possible to provide the image fixing apparatus capable of restraining a stress applied to the heater and an image heating apparatus capable of restraining a rise in temperature of a sheet non-feeding portion.

Stick–slip oscillations: Dynamics of friction and surface roughness

Abstract: properties of interacting surfaces and on the dynamics of the system containing them. At a microscopic level, the true contact area changes as the surfaces move relative to each other. Thus at a macroscopic level, total friction and normal forces are time-dependent phenomena. This paper introduces a more detailed friction model, one that explicitly considers deformation of and adhesion between surface asperities. Using probabilistic surface models for two nominally flat surfaces, the stick‐slip model sums adhesive and deformative forces over all asperities. Two features distinguish this approach from more traditional analyses: ~i! Roughness distributions of the two interacting surfaces are considered to be independent, ~ii! Intersurface contacts occur at both asperity peaks, as in previous models, and on their slopes. Slope contacts, in particular, are important because these oblique interactions produce motion normal to the plane of sliding. Building the model begins by analyzing local friction forces as composites of resistance to elastic deformation and shear resistance arising from adhesion between asperity surfaces. By extending the expressions obtained for normal and tangential friction forces over the macroscopic surfaces, the model then describes the stick‐slip behavior frequently observed in dynamic systems and permits simulating a rigid body on a moving platform. Numerical results for several surface and system parameters illustrate both time-dependent and time-averaged frictional forces. These analyses also show that, although total averaged friction remains constant with respect to sliding velocity for the cases considered, the relatively small deformation component exhibits resonancelike behavior at certain speeds. Stick‐slip occurs only within a narrow range around these critical speeds of a system. External damping can prevent stick‐slip motion, and both deformative and adhesive frictional forces must be present for it to occur at all. © 1999 Acoustical Society of America.@S0001-4966~99!03701-7#

Influence of tire-pavement contact stress distribution on development of distress mechanisms in pavements

Abstract: It is demonstrated that accurate characterization of tire-pavement contact stress distribution is important for the correct prediction of distress evolution in flexible pavements. First, an analysis of tire imprints and measured tire-pavement contact stress distribution leads to the conclusion that the shape of the contact area depends on the load and tire pressure and that the contact stress distribution is nonuniform. Second, software for a linear-layered elastic medium contrasts the stress distribution in a pavement due to two loads: a uniformly distributed pressure on a circular area and a distribution reported earlier by de Beer and Fisher. The analysis herein demonstrates that nonuniform contact stress distribution leads to considerably larger stresses in the pavement relative to the uniform stress distribution case. Consequently, both rut and crack evolution prediction would be different if they were based on true distribution rather than on the uniform distribution assumption. It is also shown tha...

Electrical response during indentation of piezoelectric materials: A new method for material characterization

Abstract: The electrical responses of piezoelectric materials subjected to spherical microindentation are evaluated. Theoretical analysis based on normal indentation of a transversely isotropic, linear piezoelectric solid by a conducting steel sphere with zero potential bias is compared to experimental results. The materials considered are PZT-4 and (Ba0.917Ca0.083)TiO3. Effects of poling, poling direction, indentation velocity, and polarization loss due to annealing were investigated. All the basic trends predicted by the theory are confirmed by the experiments. The current induced into the indentor due to the polarized layer on the contact surface of the piezoelectric specimen increases with time as the contact area increases. Experimentally, observed power dependence of current to indentation velocity is close to the theoretical value of 1.5. The relation between induced current and indentation time is specific to the properties of the material. It is demonstrated that in addition to some of the material constan...

Electrical Contact Resistance: Fundamental Principles

Abstract: All solid surfaces are rough on the microscale. Surface microroughness consists of peaks and valleys whose shape, height variations, average separation and other geometrical characteristics depend on fine details of the surface generation process (1). Contact between two engineering bodies thus occurs at discrete spots produced by the mechanical contact of asperities on the two surfaces, as illustrated in Fig. 1.1. For all solid materials, the area of true contact is thus a small fraction of the nominal contact area, for a wide range of contact loads (1,2). The mode of deformation of contacting asperities is either elastic, plastic or mixed elastic-plastic depending on local mechanical contact stresses and on materials properties such as elastic modulus and hardness. In a bulk electrical interface where the mating components are metals, the contacting surfaces are often covered with oxide or other electrically insulative layers. Generally, the interface becomes electrically conductive only when metal-to-metal contact spots are produced, i.e. where electrically insulative films are ruptured or displaced at contacting surface asperities. In a typical bulk electrical junction, the area of electrical contact is thus appreciably smaller than the area of true mechanical contact.

Corrosion resistant imager

Abstract: A radiation imager is disclosed that is resistant to degradation due to moisture by either contact pad corrosion, guard ring corrosion or by photodiode leakage. A contact pad of a large area imager is disclosed that is formed into three distinct and electrically connected regions. The resulting structure of the contact pad regions forms reliable contact that is resistant to corrosion damage. Also disclosed is a data line of an imager, or a display, the resistance of which is reduced by patterning an aluminum (Al) line on top of a transistor island structure, with the formed data line preferably being encapsulated. In addition, a guard ring having first and second regions and photosensitive element are disclosed. The second region comprises an electrical contact between ITO and underlying metal and a second tier which acts as a moisture barrier and is preferably disposed at the corner of the guard ring and separated from the contact pads of the imager in such a manner as to minimize corrosion. The photosensitive element has a multitier passivation layer disposed between the top contact layer and an amorphous silicon photosensor island except for a selected contact area on the top surface of the photosensor island, where the top contact layer is in electrical contact with the amorphous silicon material of the photosensor island. The passivation layer includes a first tier inorganic barrier layer which is disposed at least over the sidewalls of the photosensor island.