Vicinity Of Contact Research Articles

Unraveling trap distribution in perovskite films: A qualitative analysis via diffusion to drift transition voltage of current–voltage characteristics

Traps cause accumulation of space charge near injecting contacts, due to which there is band bending in the vicinity of contacts. This leads to a lowering of voltage at which there is change from diffusion to drift dominated current under steady state, termed as transition voltage (Vα). We propose a technique to qualitatively understand the trap distribution by analyzing the Vα of a two terminal perovskite device. The validity of the proposed technique has been verified from the numerical simulations carried out using Sentaurus TCAD. Both density (NT) and energetic depth (ET) of traps were varied to study their effect on Vα. Devices with deeper and higher NT have been found to exhibit stronger band bending and thus lower Vα. In addition, Vα has been found to be sensitive to NT even when band bending due to accumulated ions or uncompensated dopants near interfaces is significant. This dependence has been further illustrated for a perovskite solar cell device. It has been concluded that Vα can be advantageous over VOC in optimizing perovskite film for trap-free solar cell. The availability of different methods to extract Vα from current–voltage (J–V) characteristics, and the extra sensitivity of Vα to NT paves a way to study traps effectively.

Theory-Aided Full Field Strain Measurement Method for the Accurate Characterization of Elastic Contact Deformation

AbstractAccurate characterization of contact deformation is of great difficulty especially for the area close to the interface. In this work, a theory-aided strain measurement method by combining modeling and experimental identification is developed for the characterization of contact deformation. The method consists of three procedures including displacement measurement, mechanical modeling, and strain identification. The displacement measurement introduces a modified digital image correlation to acquire the displacement field. In mechanical modeling, a contact model is developed so the relationship between the interfacial stress and the displacement is formulated. Finally, the strain identification procedure is used to optimize the unknown parameters by matching the modeled displacement to the experimental data, and then reconstruct the strain field using the model and the optimal parameters. By developing a discrete interfacial stress model and strain reconstruction algorithm, the proposed method can sensitively identify the strain concentration in the contact vicinity. Both simulation and experiment are carried out, and the effectiveness of the proposed method is discussed in detail.

Investigation on Elliptical Contact Starvation for High Slide-to-Roll Ratio: Application to Gears

Starvation can occur in lubricated systems due to a lack of lubricant. Though the starvation conditions of rolling element bearings are fairly well known, fewer studies are devoted to gears. In this study, experiments are conducted to analyze starvation for contact conditions similar to those of gears; that is, for high slide-to-roll ratio. A new oil-off test procedure on a twin-disc machine is proposed. The oil-off phase is analyzed through the friction coefficient and the bulk temperatures of discs. It is highlighted that low viscosities of residual oil located on the track have a positive effect on starved conditions. Otherwise, large ellipticities worsen starvation by reducing replenishment in the contact vicinity. Finally, a new inverse method is proposed to estimate the central oil film thickness during oil-off phases. The results obtained are consistent with existing models. This method proves to be suitable for severe starvation because it takes into account the lubrication regimes and the lubricant–material interactions.

Rate-dependent adhesion of cartilage and its relation to relaxation mechanisms

Cartilage adhesion has been found to play an important role in friction responses in the boundary lubrication regime, but its underlying mechanisms have only been partially understood. This study investigates the rate dependence of adhesion from pre-to post-relaxation timescales of cartilage and its possible relation to relaxation responses of the tissue. Adhesion tests on cartilage were performed to obtain rate-dependent cartilage adhesion from relaxed to unrelaxed states and corresponding relaxation responses. The rate dependence of cartilage adhesion was analyzed based on experimental relaxation responses. Cartilage adhesion increased about 20 times from relaxed to unrelaxed states. This rate-dependent enhancement correlated well with the load relaxation responses in a characteristic time domain. These experimental results indicated that the degree of recovery (or relaxation) in the vicinity of contact during unloading governed the rate dependence of cartilage adhesion. In addition, the experimentally measured enhancement of adhesion was interpreted with the aid of computationally and analytically predicted adhesion trends in viscoelastic, poroviscoelastic, and cohesive contact models. Agreement between the experimental and predicted trends implied that the enhancement of cartilage adhesion originated from complex combinations of interfacial peeling and negative fluid pressure generated within the contact area during unloading. These findings enhance the current understanding of rate-dependent adhesion mechanisms explored within short time scales and thus could provide new insight into friction responses and stick-induced damage in cartilage.

CHARGE TRANSPORT FEATURES IN THE COMPOSITE ANODES OF SOLID OXIDE FUEL CELLS: MICROSTRUCTURAL AND IN-SITU RAMAN SPECTROSCOPY ANALYSES

In this work, we carry out the high-resolution electron microscopy of microstructure of grains boundaries of anion and electronic conductors in composite Ni/YSZ anodes before and after study of the current–voltage characteristics of model SOFCs. We propose a mechanism of 2-stage reaction of hydrogen oxidation occurring in the vicinity of triplephase boundary of Ni/YSZ SOFC anodes. On the first stage, metallic nickel is oxidized to nickel oxide by oxygen anion coming from the solid electrolyte membrane. On the second one, hydrogen reduces nickel oxide to metallic nickel, and water is formed. Decrease of the Ni grains size in the vicinity of contact with anion conductor grains is shown to be the result of NiO nano-grains appearance and their consequent reduction to metallic Ni during SOFC operation. High-resolution electron microscopy analysis demonstrates the significant changes in microstructure of grains boundaries of anion and electronic conductors in composite Ni/YSZ anodes after application of load current to SOFC. Nano-sized NiO grains appear in near-boundary regions of Ni grains after current tests. Orientation alignment between YSZ and nano-sized NiO lattices is unambiguous evidence of epitaxial growth of nickel oxide at YSZ surface as on a substrate that is possible only as a result of oxygen anion transport from anion conductor YSZ to the metal surface during current passage through the solid oxide fuel cell. We study the chemical transformations in the electrochemical reaction zone in SOFC composite electrodes depending on the current density passing the SOFC by new “in-situ” Raman spectroscopy technique. Increase of the current passing is shown to lead to growth in the intensity of Raman peak connected with symmetric oscillations of CeO 2 group. We connect this result with the change of the cerium cations charge state from Ce 3+ to Ce 4+ and consider this to be direct proof of the charge transfer in composite anode via oxygen anion transfer.

Multiaxial Fatigue Life Calculation Model for Engineering Components in Rolling-Sliding Contact

Number of important engineering components and elements such as gears, rollers, bearings operate in conditions of rolling-sliding contact loading. Determination of fatigue lives of such components and elements is very important for engineering practice but remains quite chalenging task due to complex states of stress and strain in the material in the vicinity of contact (multiaxiality, non-proportionality, rotation of principal axes, mean compressive stress) as well as complex contact conditions such as loading amplitude, complex geometry of bodies in contact, type of lubrication, value of coefficient of friction, etc. Proposed fatigue life calculation model for cases of rolling-sliding contact is based on critical plane approach in the form of Fatemi-Socie crack initiation criterion. Developed model was implemented in the case of gears teeth flanks in mesh and compared with results and fatigue lives of gears reported in literature. Good agreement was determined confirming validity of developed model. Further advantage of presented approach and developed model is obtained information on critical location(s) and critical plane(s) orientation which can subsequently be used for estimation of crack shapes in initial phases of their growth and later damage type into which they can be expected to develop.

Numerical modelling of the contact condition of a Brazilian disk test and its influence on the tensile strength of rock

The stress distribution and failure process in the Brazilian test disk are numerically studied using the continuum-based discrete element method. The stress distribution in the immediate contact vicinity between disk and jaws with two types of contact conditions are compared, and the position of failure initiation is determined based on the generalised Griffith criteria. Moreover, the whole process of the Brazilian test is modelled for disks with various material properties under two contact conditions and the modelled Brazilian tensile strengths of the disks are obtained. Numerical simulations show that the distribution of stress components in the vicinity of the contact have remarkable differences between the two contact conditions, and the difference increase with the increasing of loading level and disk's elastic modulus. However, the stress distributions exhibit almost no difference on the disk's centre area. The disk's elastic modulus and the ratio of compressive strength to tensile strength are the main factors that influence the distribution of stresses in the vicinity of the contact, thereby affecting the failure initiation and tensile strength, especially under no-cushion contact conditions. The thin plastic cushion can alleviate the stress concentration in the vicinity of the contact between the disk and jaws, and cracks initiating from the vicinity of the contact can be avoided for almost all of the rock-like materials. However, the enlarging contact ranges will result in a larger deviation between the test results and the real tensile strength. An improved Brazilian test method considering two contact conditions of with and without cushions for rock and rock-like material is proposed.

Radial Flow of Non-Newtonian Power-Law Fluid in a Rough-Walled Fracture: Effect of Fluid Rheology

Fluid flow in a single rough-walled rock fracture has been extensively studied over the last three decades. All but few of these studies, however, have been done with Newtonian fluids and unidirectional flow in rectangular fractures. Notwithstanding the importance of such setups for theoretical understanding of fundamental issues in fracture flow, practical applications in drilling and petroleum engineering often involve radial flow of a non-Newtonian fluid. An example is a borehole intersecting a natural fracture during drilling in a fractured rock. In this study, steady-state incompressible radial flow from a circular well into a self-affine rough-walled fracture was simulated numerically using the lubrication theory approximation. The fluid rheology was power law. The flow behavior index was equal to 0.6, 0.8, 1.0 (Newtonian), 1.2, or 1.4. Asperities diverted the flow from an axisymmetric radial pattern that would be observed in a smooth-walled fracture. The extent of the deviation from radial flow was found to increase as the fluid became more shear-thickening. To reveal finer details of the flow, a tracer was introduced at the borehole wall and was transported by the flow. The front of the tracer propagating into the fracture was found to become slightly smoother with a more shear-thickening fluid. In the vicinity of contacts between fracture faces a more shear-thickening fluid could deliver the tracer closer to the contact spots.

Structure of geometrical nonlinearities in problems of liquid sloshing in tanks of non-cylindrical shape

Structure of geometrical nonlinearities in mathematical model of liquid sloshing in tanks of non-cylindrical shape is under consideration. In contrast to the case of cylindrical reservoir, some new types of nonlinearities occur in mathematical statement of the problem. They are connected with four main reasons. First, they are determined by new normal modes, which correspond to non-cylindrical shape of the tank and take into account some nonlinear properties of the problem (for example, they follow tank walls above level of a free surface). Second, determination of the potential energy of the liquid includes tanks geometry in close vicinity of cross-section of undisturbed free surface of the liquid and tank walls. Third type of manifestation of geometrical nonlinearities is connected with compensation of elevation of liquid level due to non-cylindrical type of tank shape for providing law of mass conservation. The fourth type of nonlinearities is connected with simultaneous manifestation of physical and geometrical nonlinearities. Investigation showed that mostly manifestation of nonlinear properties of liquid sloshing, connected with geometrical nature, is predetermined by inclination and curvature of tank walls in close vicinity of contact of undisturbed liquid with tank walls. We illustrated some general properties of geometrical nonlinearities by the example of three cases of tanks, namely, cylindrical, conic, and paraboloidal tank, which is selected such that its walls have the same inclination near free surface of the liquid as conic tank, but in this case curvature is manifested supplementary.

Methodology and verification of calculations for contact stresses in a wheel–rail system

The distribution of contact stresses in the wheel–rail system is a decisive factor for the wear of elements and the safety of rail transport. Analytical calculations of stresses based on the Hertz theory can only be applied to elastic deformation of materials. High dynamic loads leading to plastic deformation (not considered in the Hertz theory) are a predominant cause of problems in the contact vicinity. These problems can be successfully resolved by applying the finite-elements method. Two- and three-dimensional test models were generated to estimate an error in numerical calculations in the MSC.MARC program. We compared the results of numerical calculations with analytical calculations. Based on the obtained results we defined the effect of parameters describing the finite-elements mesh on the calculation error for contact stresses, and adjusted mesh parameters appropriately to achieve as low as possible error in numerical calculations. We also defined the effect of material characteristics on the value of contact stresses on the wheel–rail interface.

Fabrication of reinforcing nanostructured coatings by electron beam processing

In this work, electron beam welding by electron accelerator of tungsten and chromium carbides and subsequent pulse processing by low-energy electrons has been applied for fabrication of nano- and ultradisperse grain structures penetrated by the system of nanosized pores. It has been demonstrated that, in the region of secondary pulse exposure, the nanohardness and elasticity modulus achieve high values. The wear resistance of the modified structure depends on the value of the specific load in the vicinity of contact with indentor.

Contaminant Migration in the Vicinity of a Grease Lubricated Bearing Seal Contact

Lubricating grease is commonly used for lubricating “sealed and greased for life” rolling element bearings. This grease also provides an additional sealing function to protect the bearing against ingress of contaminants. In this work the sealing function of lubricating grease in the vicinity of the seal lip contact has been studied experimentally by measuring the migration of spherical fluorescent contaminant particles in the vicinity of the contact, as a function of shaft speed and lubricant type. The experimental results reveal that in some greases contaminant particles migrate towards the sealing contact where the shear rate reaches its highest value. However, for other greases, Newtonian base oils, and elastic fluids, this is not necessarily the case and contaminant particles consistently migrate away from the sealing contact. Various physical phenomena have been investigated to explain the difference in migration behavior. It is concluded that migration towards the sealing contact is driven by the viscosity gradient and migration away from the sealing contact is related to the Weissenberg number. The sealing function of grease in the vicinity of the sealing contact is due to the migration of contaminant particles. The migration reduces the probability of particles to reach the sealing and bearing contacts.

Electron transport through the contact of quantum wire and 3D electrodes

The effect of the electron interaction on the electron transport through the contacts of 1D conductor with 2D and 3D metal electrodes is theoretically studied. The spin-polarized electrons that are described using the Tomonaga-Luttinger Hamiltonian are considered. The boundary conditions for the current and charge density at the boundary of the 1D conductor are derived as functions of the electric potential across electrodes, and the boundary conditions for the fluctuations are obtained. In the case of the nonadiabatic contact, the Friedel oscillations of the charge density that emerge in the vicinity of contact in the 1D system strongly suppress the conduction in the system similarly to the effect of impurities in the systems of 1D electrons with repulsion. When the applied voltage exceeds threshold level Vthr, the conductivity sharply increases and the flow of dc current {ie1218-1} is accompanied by the generation of the current oscillations with the frequency {ie1218-2}.

EBIC study of resistive photosensitive elements based on HgCdTe

Photosensitive resistive elements based on HgCdTe with decreased photosensitivity as a result of prolonged operation are studied by the method of electron-beam-induced current in a scanning electron microscope. It is shown that the degradation processes in these elements are related to the appearance of regions with a lowered sensitivity in the vicinity of contacts. Distribution of the induced current for these inhomogeneous elements was simulated. It is shown that a comparison of the measured and calculated distributions of the induced-current signal makes it possible to reveal the most probable causes of the inhomogeneous decrease in the photosensitivity. The comparison performed showed that the most likely cause of a decrease in the photosensitivity of the elements under study was an increase in the donor concentration in the near-contact regions.

Density functional for structures of colloids confined in a slit-like pore

A density functional theory is applied to calculating the local density profiles of colloids confined in a slit-like pore as well as the radial distribution functions of bulk colloids. The interaction between the colloidal particles is described using a hard-core Yukawa model. The excess Helmholtz energy functional is a combination of the modified fundamental measure theory of Yu and Wu (2002) for the hard-core contribution and a corrected mean-field theory for the attractive contribution. Comparison with the results from the Monte Carlo simulations shows that the corrected theory improves the density profiles of colloids in the vicinity of contact over the original mean-field theory. Both the present corrected theory and simulations suggest that there are depletion and desorption for the colloid with strong attraction between particles at low temperature.

Structures and adsorption of binary hard-core Yukawa mixtures in a slitlike pore: Grand canonical Monte Carlo simulation and density-functional study

The grand canonical ensemble Monte Carlo simulation and density-functional theory are applied to calculate the structures, local mole fractions, and adsorption isotherms of binary hard-core Yukawa mixtures in a slitlike pore as well as the radial distribution functions of bulk mixtures. The excess Helmholtz energy functional is a combination of the modified fundamental measure theory of Yu and Wu [J. Chem. Phys. 117, 10156 (2002)] for the hard-core contribution and a corrected mean-field theory for the attractive contribution. A comparison of the theoretical results with the results from the Monte Carlo simulations shows that the corrected theory improves the density profiles of binary hard-core Yukawa mixtures in the vicinity of contact over the original mean-field theory. Both the present corrected theory and the simulations suggest that depletion and desorption occur at low temperature, and the local segregation can be observed in most cases. For binary mixtures in the hard slitlike pore, the present corrected theory predicts more accurate surface excesses than the original one does, while in the case of the attractive pore, no improvement is found in the prediction of a surface excess of the smaller molecule.

Structure of Inhomogeneous Attractive and Repulsive Hard-Core Yukawa Fluid: Grand Canonical Monte Carlo Simulation and Density Functional Theory Study

A density functional theory is proposed for an inhomogeneous hard-core Yukawa (HCY) fluid based on Rosenfeld's perturbative method. The excess Helmholtz energy functional is derived from a modified fundamental measure theory for the hard-core repulsion and a quadratic functional Taylor expansion for the long-ranged attractive or repulsive interactions. To test the established theory, grand canonical ensemble Monte Carlo simulations are carried out to simulate the density profiles of attractive and repulsive HCY fluid near a wall. Comparison with the results from the Monte Carlo simulations shows that the present density functional theory gives accurate density profiles for both attractive and repulsive HCY fluid near a wall. Both the present theory and simulations suggest that there is depletion for attractive HCY fluid at low temperature, but no depletion is found for repulsive HCY fluid. The calculated results indicate that the present density functional theory is better than those of the modified version of the Lovett-Mou-Buff-Wertheim and other density functional theories. The present theory is simple in form and computationally efficient. It predicts accurate radial distribution functions of both attractive and repulsive HCY fluid except for the repulsive case at high density, where the theory overestimates the radial distribution function in the vicinity of contact.

Experimental and theoretical study of acceptor activation and transport properties in p-type AlxGa1−xN/GaN superlattices

Experimental and theoretical results of Mg-doped superlattices consisting of uniformly doped AlxGa1−xN, and GaN layers are presented. Acceptor activation energies of 70 and 58 meV are obtained for superlattice structures with an Al mole fraction of x=0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped bulk GaN. At room temperature, the doped superlattices have free-hole concentrations of 2×1018 cm−3 and 4×1018 cm−3 for x=0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures is 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x=0.10 and 0.20, respectively. X-ray rocking curve data indicate excellent structural properties of the superlattices. We discuss the origin of the enhanced doping, including the role of the superlattice and piezoelectric effects. The transport properties of the superlattice normal and parallel to the superlattice planes are analyzed. In particular, the transition from a nonuniform to a uniform current distribution (current crowding) occurring in the vicinity of contacts is presented. This analysis provides a transition length of a few microns required to obtain a uniform current distribution within the superlattice structure.

Simulation of dynamics of interacting rigid bodies including friction I: General problem and contact model

This is the first of two papers that deal with the problem of modeling contact (impact, sliding, rolling) between unconstrained rigid bodies for computer simulation of their motion. Modeling impacts and sustained contact is difficult because of the lack of a concise theory for three-dimensional (3-D) impact with friction, the complex non-holonomic constraints involved, general complexity of friction, etc. The traditional rigid body ‘hard contact’ approach leads to a quadratic program formulation for determining the contact forces, which branches into cases and does not model multiple simultaneous contacts adequately. We have formulated a computational theory based on the ‘soft contact’ approach that models contact through localized non-permanent deformation in the vicinity of contact. The model of mechanical contact between polyhedral objects that we propose, strikes a balance between realism and computability. It is simple enough to permit numerical integration through periods of contact in reasonable time, yet rich enough to represent well a variety of contact behaviors. The main cost associated with our model is the need for small time steps during contact, which slows down the simulation. Starting with the motivation for our work, this paper describes the governing equations and constitutive laws for the interacting bodies. Issues pertaining to the solution approaches are then discussed. The final sections explain the mathematical details of our contact model. A companion paper [1] describes details of our computational theory and its integration into a software system.

An accurate integral equation for molecular fluids

The Percus-Yevick (PY), HNC, nonspherical bridge function (NSB), and VM integral equation theories are used to study the structure and thermodynamics of the hard linear homonuclear triatomic fluid. Results for the spherical harmonic coefficients of the pair distribution function and for the compressibility factor are compared with computer simulation data for a range of triatomic densities and elongations up to 1·6. The VM theory gives excellent results for the harmonic coefficients in the site-site and centre-site molecular coordinate frames, the PY theory results are fair, and the NSB and HNC theory results are poor at low distances. None of the theories satisfactorily describes the g 000 harmonic coefficient in the centre-centre frame in the vicinity of contact at high densities. The VM compressibility factors agree quantitatively with those of the simulations. The NSB equation of state results are also excellent. The PY values are too low and the HNC values are too high. The thermodynamic consistency of the VM theory is excellent, that of the NSB theory very good, and that of the PY and HNC theories poor, especially at high densities.