Misfit dislocations and stress in In<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs heterostructures

We have estimated the elastic properties of In_{1 − x} Ga _x As/GaAs heterostructures and the characteristics of misfit dislocations in such heterostructures: misfit dislocation spacing, Burgers vector length in various interfaces, surface density of dangling bonds, film/substrate interface energy, critical film thickness below which pseudomorphic growth is possible without misfit dislocations, elastic strain energy of the film-substrate system, average elastic strain of a thin-film island as a function of its radius, thermal stresses induced by the thermal-expansion and lattice mismatches between the layers in contact, and crack length in the film.     

Reduced thermal stress in composites via negative thermal expansion particulate fillers

The use of negative thermal expansivity (NTE) particles as composite fillers is relatively new and the particle–matrix interface is not well studied. This lack of understanding of the particle–matrix interface is further complicated as in many engineering applications, such as microchip packaging, the composite is constrained by its surroundings and is not free to expand upon heating; an important consideration that is often not taken into account. This paper presents a systematic theoretical study of the behaviour at the particle–matrix interface under varying particle coefficient of thermal expansivity (CTE), Poisson’s ratios (including negative Poisson’s ratios), Young’s moduli, boundary conditions and particle separation distances via finite element modelling, and describes how to optimise composite formulation for problems of thermal mismatch through tailoring of particle–matrix interaction. The effects of reduced CTE are explored via models of electronic chip package assembly.     

Critical shear stress produced by interaction of edge dislocation with nanoscale inhomogeneity

According to the Mott and Nabarro’s model, the contribution to the critical shear stress of the material caused by the interaction between edge dislocations and nanoscale cylindrical inhomogeneities with interface stresses is obtained. The influence of the radius and the volume fraction of the inhomogeneity as well as the interface stresses on the critical shear stress is investigated. The important result is that, if the interface stress is considered, a maximum of the contribution to the critical shear stress produced by this interaction may be obtained when the radius of the inhomogeneity reaches a critical value.     

Modeling and simulation of material removal with particulate flows

In this work a multibody collision model, ame-nable to large-scale computation, is developed to simulate material removal with particulate flows. This model is developed by computing momentum exchange to account for different force interactions: (1) particle–particle interaction, (2) particle–fluid interaction, and (3) particle–surface interaction. For the particle-fluid interaction, a velocity field for the fluid is assumed to be known, and the drag force on the particles is computed from this field. In the particle–surface interaction, the Boussinesq solution for a point load on an elastic half-space is used along with the von-Mises yield criterion to determine the amount of material removed. Employing this model, inverse problems are then constructed where combinations of the abrasive particle size, the particle size distribution, the flow velocity, etc., are sought to maximize the efficiency of the process. A genetic algorithm is used to treat this inverse problem, and numerical examples are given to illustrate the overall approach.     

Interplay Between Wetting and Phase Behavior in Binary Polymer Films and Wedges: Monte Carlo Simulations and Mean Field Calculations

Confining a binary mixture, one can profoundly alter its miscibility behavior. The qualitative features of miscibility in confined geometry are ratheruniversal and shared by polymer mixtures as well as small molecules, but the unmixing transition in the bulk and the wetting transition are typically well separated in polymer blends. The interplay between wetting and miscibility of a symmetric polymer mixture via large-scale Monte Carlo simulations in the framework of the bond fluctuation model and via numerical self–consistent field calculations is studied. The film surfaces interact with the monomers via short ranged potentials, and the wetting transition of the semi–infinite system is of first order. It can be accurately located in the simulations by measuring the surface and interface tensions and using Young’s equation. If both surfaces in a film attract the same component, capillary condensation occurs and the critical point is close to the critical point of the bulk. If surfaces attract different components, an interface localization/delocalization occurs which gives rise to phase diagrams with two critical points in the vicinity of the pre-wetting critical point of the semi–infinite system. The crossover between these two types of phase diagrams as a function of the surface field asymmetry is studied. The dependence of the phase diagram on the film thickness Δ for antisymmetric surface fields is investigated. Upon decreasing the film thickness, the two critical points approach the symmetry axis of the phase diagram, and below a certain thickness Δ_tri, there remains only a single critical point at the symmetric composition. This corresponds to a second-order interface localization/delocalization transition even though the wetting transition is of first order. At a specific film thickness, Δ_tri, tricritical behavior is found. The behavior of antisymmetric films is compared with the phase behavior in an antisymmetric double wedge. While the former is the analog of the wetting transition of a planar surface, the latter is related to the filling behavior of a single wedge. Evidence for a second-order interface localization/delocalization transition in an antisymmetric double wedge is presented, and its unconventional critical behavior is related to the predictions of Parry et al. (Phys. Rev. Lett. 83:5535 (1999)) for wedge filling. The critical behavior differs from the Ising universality class and is characterized by strong anisotropic fluctuations.Invited paper presented at the Fifteenth Symposium on Thermophysical Properties, June 022–27, 2003, Boulder, Colorado, U.S.A.     

Surface tension effects on the nonlinear behavior of long waves in a two layer flow

 The propagation of long waves of finite amplitude at the interface of two viscous fluids in the presence of interfacial tension is examined. The effect of capillarity on the shape of the waves at the interface of two superposed fluids is investigated for a wide range of density differences, viscosity ratios and imposed pressure gradients. It is found that in planar geometry surface tension stabilizes the interfacial disturbances. Attention is given to the case in which the upper fluid is more dense and comprises a thin film above the lower fluid. With the heavier fluid on the top the flow pattern is always unstable when surface tension effects are neglected. In this case the interfacial waves do not grow forever and reach a finite amplitude only when the interfacial tension is greater than a critical value.     

Solidification behaviour of ceramic particle reinforced Al-alloy matrices

Novel metal matrix composites have been produced by cast production route. TiC and WC ceramic reinforcing particles have been successfully introduced into Al 6060, Al 319, Al 356, Al–7Si–5Mg, Al–20Cu and Al 2007 alloys. Refined grain structure and various intermetallic phase formation have been observed. Particle–melt and particle–solidification front interactions, solidification sequence and particle–matrix interfacial characteristics have been examined by means of metallography, SEM examination and EDX analysis. Particle distribution, intermetallic phase formation and location and grain structure are discussed in terms of ceramic-melt wetting characteristics, alloying element interfacial segregation and particle–solidification front thermal behaviour.     

Behavior of magnesium and silicon in the formation of MgO/AlN composite

MgO/AlN composites have been fabricated by directed metal nitridation of Al–Si alloy in flowing N₂ at 1473 K. A mixture of magnesia particles and chemically pure magnesium powder was placed on the surface of Al–Si alloy block as reinforcement materials. Mg powder initiates the infiltration and nitridation of Al alloy melt by eliminating protective Al₂O₃ film at the reaction frontier. New Mg vapor from the interface reaction between Al and MgO particles, keeps as continuous deoxidization agent as the added Mg powder. The spinel layer thickness due to the reaction of Al melt with MgO particles is controlled by Mg content. Si not only reduces the surface tension and viscosity of Al alloy melt, but also leads to increase in N₂ content.     

Modeling and simulation of high speed sliding

We analyze the processes of thermal softening, melting and subsequent melt lubrication that occur during the high speed sliding of a metal piece on another metal block. The temperature rise in the slider arising from both high speed interface friction and from the energy dissipated during plastic deformations is computed using simple analysis and the finite element method. Subsequently, we propose a mathematical model for the transient lubrication problem that describes the behavior of the molten film at the slider–rail interface. This model successfully predicts the evolution process of the melt thickness and the melt front velocity of the liquid film; these predictions agree with the experimentally observed dynamics of molten film better than those from other existing models.     

Thermal Conductivity of AlN and SiC Thin Films

The thermal conductivity of AlN and SiC thin films sputtered on silicon substrates is measured employing the 3ω method. The thickness of the AlN sample is varied in the range from 200 to 2000 nm to analyze the size effect. The SiC thin films are prepared at two different temperatures, 20 and 500°C, and the effect of deposition temperature on thermal conductivity is examined. The results reveal that the thermal conductivity of the thin films is significantly smaller than that of the same material in bulk form. The thermal conductivity of the AlN thin film is strongly dependent on the film thickness. For the case of SiC thin films, however, increased deposition temperature results in negligible change in the thermal conductivity as the temperature is below the critical temperature for crystallization. To explain the thermal conduction in the thin films, the thermal conductivity and microstructure are compared using x-ray diffraction patterns.     

Numerical analysis of constitutional supercooling during directional solidification of alloys

Some limitations of Tiller’s morphological stability criterion are discussed in the present study. This criterion assumes a purely diffusive regime in the melt as well as a planar solid–liquid interface and a constant solidification rate. But experimental works in agreement with previous numerical modeling have shown a significant decrease of the growth rate and a variable interface curvature during the concentrated semiconductor alloys solidification. The mathematical expression of the morphological stability criterion was derived by using Tiller’s equation, predicting the solute distribution in the liquid. The numerical computations performed in this study show a significant disagreement between the numerical results and Tiller’s formula. Numerical modeling conducted in conditions when the supercooling should occur, show that the Tiller’s stability criterion cannot predict the moment of interface destabilization. The interface destabilization is numerically observed when some fluctuations appear in the liquid solutal profiles and cause the appearance of a supercooled zone inside the liquid at small distance from the interface. The present numerical results are not in contradiction with the basic elements of the classical constitutional supercooling theory, providing only that the stability criterion cannot predict the moment of the interface destabilization.     

Modeling inclusion approach to the steel/slag interface

The removal of non-metallic inclusions from a steel melt to the upper slag phase involves the movement of buoyant particles from a lower, less viscous phase (steel) to an upper, more viscous phase (slag). The film that forms ahead of the impinging inclusion must drain and rupture for particle capture by the slag. This deformation and drainage process has been modeled for a Newtonian fluid using no-slip boundary conditions at all surfaces and a pressure balance across the liquid–liquid interface for the given interfacial shape. Computer implementation of this model shows that particles of 5 μm in radius can be delayed up to two seconds by the resultant drag, with delays of a tenth of a second for 100 μm particles. Decreasing the interfacial tension between the lower and upper phases [corresponding to the presence of sulfur (of activity 0.7) at the steel–slag interface] can increase this time by 1 or 2%.     

Generation of porous particle structures using the void expansion method

The newly developed “void expansion method” allows for an efficient generation of porous packings of spherical particles over a wide range of volume fractions using the discrete element method. Particles are randomly placed under addition of much smaller “void-particles”. Then, the void-particle radius is increased repeatedly, thereby rearranging the structural particles until formation of a dense particle packing. The structural particles’ mean coordination number was used to characterize the evolving microstructures. At some void radius, a transition from an initially low to a higher mean coordination number is found, which was used to characterize the influence of the various simulation parameters. For structural and void-particle stiffnesses of the same order of magnitude, the transition is found at constant total volume fraction slightly below the random close packing limit. For decreasing void-particle stiffness the transition is shifted towards a smaller void-particle radius and becomes smoother.     

2D Flow Structure in a Thin Liquid Layer Under Thermal Wave Propagation

The paper is devoted to numerical modeling of 2-D steady-state flow structure in a thin non-isothermal liquid layer on horizontal substrate (or under the zero-gravity condition). The layer is locally heated by a moving heat source (flame for example). Simulation is carried out on the base of derived equation describing the thickness of the layer in the accompanying frame of reference. The deformation of the layer is caused by thermocapillarity, and non-uniformity of the temperature distribution at the free surface plays the main role in the phenomenon. The temperature distribution is assumed to be known from experiment. It takes into account local heating and subsequent cooling down of the liquid due to thermal radiation and heat conductivity. The impact of different physical mechanisms in the layer deformation is analyzed. It is shown that main stabilizing effect is provided by inertial force. The influence of surface tension isn’t significant for long-wave deformations of the free surface. The calculated stream function reveals the presence of a vortex in the layer if the value of temperature gradient isn’t too small.     

Crack propagation in a glass particle-filled epoxy resin

Crack propagation in an epoxy resin reinforced with spherical glass particles has been followed using a double-torsion test. In particular the effect of strain rate, volume fraction and particle size upon the stability of propagation, the Young's modulus, the critical stress intensity factor,K _Ic and the fracture energy,G _Ic has been studied. It has been shown that the crack propagation behaviour can be explained principally in terms of crack pinning, although it has been found that propagation is also affected by blunting the breakdown of the particle—matrix interface. It has been demonstrated that crack-front pinning is consistent with a critical crack opening displacement criterion.     

倒圆锥面基底蒸发镀膜均匀性理论分析

通过计算得出了蒸发源位于倒圆锥面正下方外部镀膜时锥面上各点的膜厚方程,并对整个锥面上膜厚均匀性进行了理论分析。结果表明：当圆锥面形状固定时,蒸发源与圆锥底圆圆心距离增大使锥面上膜厚均匀性变好;当蒸发源固定时,增大底圆半径导致锥面上膜厚均匀性变差。在同样的配置下,蒸发源为点源或小平面源时锥面上膜厚均匀性的变化趋势一致,小平面源蒸镀比点源蒸镀时圆锥面上膜厚均匀性差。     

Mechanical stability of sol-gel films

The relationship between film cracking and film thickness has been studied experimentally for films of ceria gel deposited by spinning on to stainless steel substrates from an aqueous ceria sol. A critical film thickness below which films were crack-free was observed at about 0.6 m. For films thicker than the critical thickness the crack spacing was approximately ten times the film thickness. Existing models for the mechanical stability of the films were examined to explain the observations, encompassing different forms of relaxation of the stress in the vicinity of a crack through the film. The model in best accord with the experimental observations is one in which stable delamination cracks are formed at the film-substrate interface on both sides of a crack through the film. However, for the model to be applicable some rather restrictive conditions must be assumed to be satisfied.     

An explicit elastic solution for a brittle film with periodic cracks

A two-dimensional explicit elastic solution is derived for a brittle film bonded to a ductile substrate through either a frictional interface or a fully bonded interface, in which periodically distributed discontinuities are formed within the film due to the applied tensile stress in the substrate and consideration of a “weak form stress boundary condition” at the crack surface. This solution is applied to calculate the energy release rate of three-dimensional channeling cracks. Fracture toughness and nominal tensile strength of the film are obtained through the relation between crack spacing and tensile strain in the substrate. Comparisons of this solution with finite element simulations show that the proposed model provides an accurate solution for the film/substrate system with a frictional interface; whereas for a fully bonded interface it produces a good prediction only when the substrate is not overly compliant or when the crack spacing is large compared with the thickness of the film. If the section is idealized as infinitely long, this solution in terms of the energy release rate recovers Beuth’s exact solution for a fully cracked film bonded to a semi-infinite substrate. Interfacial shear stress and the edge effect on the energy release rate of an asymmetric crack are analyzed. Fracture toughness and crack spacing are calculated and are in good agreement with available experiments.     

Evaluation of the Residence Time of a Moving Fuel Bed on a Forward Acting Grate

The objective of this study is to identify numerical approaches and to employ them to evaluate the residence time of a moving bed on a forward acting grate. Obtaining residence times of fuel particles on a grate favours more reliable design of grate furnaces. The moving bed was represented by spherical particles, whereby a varying size accounts for the variety of particle geometries in a combustion chamber. In order to describe accurately the motion of a moving bed e.g. its particles, the discrete element method was applied. Thus, detailed data on all the particle’s paths and velocities are available. These data were used within two statistical approaches to estimate the residence time of a moving bed. One approach is based on a spatial averaging procedure, while the other relies on tracking the particle’s path. Both methods yielded satisfactory agreement with measurements, however, better predictions were obtained by tracking particles.Academic visitor to the Lithuanian Energy Institute     

The effect of mutual location of heaters on the falling film dynamics

Thin nonisothermal liquid film flowing down under action of gravity is considered. Investigation of the influence of the spanwise and streamwise arrangement of the rectangular heaters on 3-D structures, occurring at the film surface, is the main objective of the present work. Three-dimensional time-dependant mathematical model for calculation of gas-liquid interface deformations and evolution of temperature fields was developed. Our numerical investigations have shown that interaction, imposition and mutual influence of the 3D structures (bumps, lateral waves ...) takes place. In the case of streamwise arrangement of the heaters film rupture is most likely on the second heater. There is a critical backlash between the heaters, at which film deformations, including film thinning, are the largest. For the spanwise arrangement of the heaters distance between them practically do not effect on the minimum film thickness, but mutual imposition of the lateral waves and film thickening exists.