Showing papers on "Nucleation published in 1987"

A Continuum Model for Void Nucleation by Inclusion Debonding

Abstract: A cohesive zone model, taking full account of finite geometry changes, is used to provide a unified framework for describing the process of void nucleation from in­itial debonding through complete decohesion. A boundary value problem simulating a periodic array of rigid spherical inclusions in an isotropically hardening elastic-viscoplastic matrix is analyzed. Dimensional considerations introduce a characteristic length into the formulation and, depending on the ratio of this characteristic length to the inclusion radius, decohesion occurs either in a "ductile" or "brittle" manner. The effect of the triaxiality of the imposed stress state on nucleation is studied and the numerical results are related to the description of void nucleation within a phenomenological constitutive framework for progressively cavitating solids. 1 Introduction The nucleation of voids from inclusions and second phase particles plays a key role in limiting the ductility and toughness of plastically deforming solids, including structural metals and composites. The voids initiate either by inclusion cracking or by decohesion of the interface, but here attention is confined to consideration of void nucleation by interfacial decohesion. Theoretical descriptions of void nucleation from second phase particles have been developed based on both continuum and dislocation concepts, e.g., Brown and Stobbs (1971), Argon et al. (1975), Chang and Asaro (1978), Goods and Brown (1979), and Fisher and Gurland (1981). These models have focussed on critical conditions for separation and have not explicitly treated propagation of the debonded zone along the interface. Interface debonding problems have been treated within the context of continuum linear elasticity theory; for example, the problem of separation of a circular cylindrical in­clusion from a matrix has been solved for an interface that supports neither shearing nor tensile normal tractions (Keer et al., 1973). The growth of a void at a rigid inclusion has been analyzed by Taya and Patterson (1982), for a nonlinear viscous solid subject to overall uniaxial straining and with the strength of the interface neglected. The model introduced in this investigation is aimed at describing the evolution from initial debonding through com­plete separation and subsequent void growth within a unified framework. The formulation is a purely continuum one using a cohesive zone (Barenblatt, 1962; Dugdale, 1960) type model for the interface but with full account taken of finite geometry

A continuum model for void nucleation by inclusion debonding

Abstract: A cohesive zone model, taking full account of finite geometry changes, is used to provide a unified framework for describing the process of void nucleation from in­itial debonding through complete decohesion. A boundary value problem simulating a periodic array of rigid spherical inclusions in an isotropically hardening elastic-viscoplastic matrix is analyzed. Dimensional considerations introduce a characteristic length into the formulation and, depending on the ratio of this characteristic length to the inclusion radius, decohesion occurs either in a "ductile" or "brittle" manner. The effect of the triaxiality of the imposed stress state on nucleation is studied and the numerical results are related to the description of void nucleation within a phenomenological constitutive framework for progressively cavitating solids. 1 Introduction The nucleation of voids from inclusions and second phase particles plays a key role in limiting the ductility and toughness of plastically deforming solids, including structural metals and composites. The voids initiate either by inclusion cracking or by decohesion of the interface, but here attention is confined to consideration of void nucleation by interfacial decohesion. Theoretical descriptions of void nucleation from second phase particles have been developed based on both continuum and dislocation concepts, e.g., Brown and Stobbs (1971), Argon et al. (1975), Chang and Asaro (1978), Goods and Brown (1979), and Fisher and Gurland (1981). These models have focussed on critical conditions for separation and have not explicitly treated propagation of the debonded zone along the interface. Interface debonding problems have been treated within the context of continuum linear elasticity theory; for example, the problem of separation of a circular cylindrical in­clusion from a matrix has been solved for an interface that supports neither shearing nor tensile normal tractions (Keer et al., 1973). The growth of a void at a rigid inclusion has been analyzed by Taya and Patterson (1982), for a nonlinear viscous solid subject to overall uniaxial straining and with the strength of the interface neglected. The model introduced in this investigation is aimed at describing the evolution from initial debonding through com­plete separation and subsequent void growth within a unified framework. The formulation is a purely continuum one using a cohesive zone (Barenblatt, 1962; Dugdale, 1960) type model for the interface but with full account taken of finite geometry

Classification of dolomite rock textures

Abstract: Dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape. The classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics, and crystal boundary shape is controlled by growth kinetics. Size distributions are classified as unimodal or polymodal. Crystal boundary shapes are classified as planar or nonplanar. If the evidence permits, a complete classification includes a description of recognizable allochems, matrix, and void filling. Allochems and preexisting cements may be unreplaced, partially replaced, replaced mimically, or replaced nonmimically. Allochems may be dissolved, leaving molds. Matrix can be unreplace, partially replaced, or replaced by a unimodal or polymodal size dolomite. Unimodal size distributions generally indicated a single nucleation event on a unimodal substrate. Polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate. Planar crystal boundaries develop when crystals undergo faceted growth, and nonplanar boundaries develop when crystals undergo nonfaceted growth. Nonplanar boundaries are characteristic of growth at elevated temperature (> 50°C) and/or high supersaturation. Both planar and nonplanar dolomite can form as a cement, replacement of CaCO3, or neomorphism of a precursor dolomite.

The nucleation of microcellular thermoplastic foam with additives: Part I: Theoretical considerations

Abstract: Microcellular foam is a polymeric foam with bubble sizes of 10 microns or less that is produced by saturating a polymer with gas and then utilizing the thermodynamic instabilities that result when the polymer is heated and the pressure is reduced to nucleate the cells. A model for the nucleation of microcellular foam in amorphous polymers with additives has been developed. The nucleation process depends on the solubility, concentration, and interfacial energy of any additives present. At very low levels, additives in solution act to increase the free volume of the polymer, resulting in homogeneous nucleation within the free volume Well above the solubility limit, heterogeneous nucleation dominates, as it lowers the activation energy for nucleation to levels below that for homogeneous nucleation. In the vicinity of the solubility limit of the additive, these two nucleation mechanisms compete. The polystyrene-zinc stearate system has been chosen for experimental evaluation.

A Chemical model for the cooperation of sulfates and carboxylates in calcite crystal nucleation : relevance to biomineralization

Abstract: Acidic matrix macromolecules involved in regulation of biological crystal growth often contain aspartic acid-rich domains and covalently bound sulfated polysaccharides. We propose that sulfates and β-sheet structured carboxylates cooperate in oriented calcite crystal nucleation. The sulfates concentrate calcium, creating the supersaturation necessary for nucleation on the structured carboxylate domains. An artificial model, composed of sulfonated polystyrene surfaces and adsorbed β-sheet poly(aspartate), demonstrates that the two components indeed act cooperatively with respect to two independent assays, both by induction of calcite nucleation off the (001) plane and by calcium association. Evidence is presented that a purified organic matrix acidic glycoprotein from mollusk shells may behave in vitro in a similar way.

Generation of misfit dislocations in semiconductors

Abstract: The acting slip mechanism for the generation of misfit dislocations in diamond‐type–semiconductor heterostructures is investigated with transmission electron microscopy. It is shown that dissociation of the 60°‐mixed dislocations can lead to a difference in strain accommodation for tensile and compressive strain. A strain/thickness relation is obtained from the energy expression for nucleation of half‐loops. This relation is compared with other theoretical relations and with experimental strain data for Si/GaP(001) and In0.07Ga0.93As/GaAs(001) , measured with transmission electron microscopy and ion blocking.

Theory of Nucleation Fields in Inhomogeneous Ferromagnets

Abstract: The effect of the microstructure on the coercive field of sintermagnets is investigated on the basis of the theory of micromagnetism. From the linearized micromagnetic equations nucleation fields are determinded for magnetic inhomogeneities with quasi-harmonic spatial variations of the magnetocrystalline anisotropy energy. For one- and two-dimensional nucleation modes the eigenvalues of the nucleation fields are derived as a function of the width and the strength of planar magnetic inhomogeneities.

An analysis of ductile rupture modes at a crack tip

Abstract: An elastic-Viscoplastic model of a ductile, porous solid is used to study the influence of the nucleation and growth of micro-voids in the material near the tip of a crack. Conditions of small scale yielding are assumed, and the numerical analyses of the stress and strain fields are based on finite strain theory, so that crack tip blunting is fully accounted for. An array of large inclusions or inclusion colonies, with a relatively low strength, results in large voids near the crack tip at a rather early stage, whereas small second phase particles in the matrix material between the inclusions require large strains before cavities nucleate. Various distributions of the large inclusions, and various critical strains for nucleation of the small scale voids between the inclusions, are considered. Localization of plastic flow plays an important role in determining the failure path between the crack tip and the nearest larger void, and the path is strongly sensitive to the distribution of the large inclusions. Values of the J-integral and the crack opening displacement at fracture initiation are estimated, together with values of the tearing modulus during crack growth, and these values are related to experimental results.

Recrystallization of amorphized polycrystalline silicon films on SiO2: Temperature dependence of the crystallization parameters

Abstract: This paper presents a theoretical and experimental study of the recrystallization behavior of polycrystalline silicon films amorphized by self‐implantation. The crystallization behavior was found to be similar to the crystallization behavior of films deposited in the amorphous state, as reported in the literature; however, a transient time was observed, during which negligible crystallization occurs. The films were prepared by low‐pressure chemical vapor deposition onto thermally oxidized silicon wafers and amorphized by implantation of silicon ions. The transient time, nucleation rate, and characteristic crystallization time were determined from the crystalline fraction and density of grains in partially recrystallized samples for anneal temperatures from 580 to 640 °C. The growth velocity was calculated from the nucleation rate and crystallization time and is lower than values in the literature for films deposited in the amorphous state. The final grain size, as calculated from the crystallization param...

Nucleation of microcellular foam: Theory and practice

Abstract: Microcellular polymer foams exhibit greatly improved mechanical properties as compared to standard foams due to the formers' small bubble size. Microcellular foams have bubbles with diameters on the order of 10 microns, volume reductions of 30 to 40 percent, and six or seven times the impact strength of solid parts. They are produced through the use of thermodynamic instabilities without the use of foaming agents. This method leads to a very uniform cell size throughout a part's cross section. A theoretical model for the nucleation of microcellular foams in thermoplastic polymers has been developed and experimentally confirmed. This model explains the effect of various additives and processing conditions on the number of bubbles nucleated. At levels of secondary constituents below their solubility limits, an increase in the concentration of the additive or the concentration of gas in solution with the polymer increases the number of bubbles nucleated. Nucleation in this region is homogeneous. Above the solubility limit of additives, nucleation is heterogeneous and takes place at the interface between second phase inclusions and the polymer. The number of bubbles nucleated is dependent on the concentration of heterogeneous nucleation sites and their relative effect on the activation energy barrier to nucleation. In the vicinity of the solubility limit, the two mechanisms compete.

The nucleation of microcellular thermoplastic foam with additives: Part II: Experimental results and discussion

Abstract: Experiments were performed to validate the model for the nucleation of microcellular foams in amorphous thermoplastic polymers. The polystyrene-zinc stearate system was chosen as the model system. Other additives such as stearic acid and carbon black were also investigated. Molecular weight and orientation effects were studied. Nitrogen and carbon dioxide were used to produce the microcellular bubbles. Results show that amounts of soluble additives at levels just below their solubility limit and high gas saturation pressures yield the most acceptable foams—ones with a large number of uniform small bubbles. In this region, the bubble number is sensitive to both the gas saturation pressure and the concentration of solutes. Increasing the concentration of soluble additives above the solubility limit has little effect on bubble number and almost eliminates the dependence on saturation pressure. Molecular weight and orientation had no effect on the number of bubbles produced. Similarly, carbon black, which is insoluble in and which bonds well to polystyrene, produced no effect on bubble numbers. The agreement between theoretical predictions and experimental results is reasonably good.

The density of supercooled water. II. Bulk samples cooled to the homogeneous nucleation limit

Abstract: We have measured the density of supercooled water (H2O) in the range −33.41≤T≤−5.23 °C. Our samples were held in glass capillary tubing with an approximate inside diameter of 0.3 mm=300 μ. These samples were prepared by Mossop’s method and could be cooled to their homogeneous nucleation limit before freezing. We compare our density data to other measurements using capillaries and demonstrate what appears to be an excess density in smaller capillaries which is inversely proportional to the capillary inside diameter. The origins of this excess density are unknown, but we show its effect is insignificant on our measurement. The thermal expansivities derived from our data are fit to a power law in temperature relative to a singular temperature. These results are inconclusive due to a poor knowledge of the background expansivity.

The growth of diamond in microwave plasma under low pressure

Abstract: The synthesis of diamond particles in a low pressure plasma has been studied, with emphasis on the investigation of the substrate effect and the plasma conditions. It was found that a special pre-treatment of silicon substrate made it possible to form dense films, and a thickness of about 15μm could be reached by 20 h discharge. Unfortunately, however, the prepared films had poor adhesion. Observations by scanning electron microscope (SEM) showed that the poor adhesion was due to the fact that the film consisted of large particles with a diameter of about 10μm, and each particle had contacted to the substrate only by a point, not by a face. In addition, the plasma diagnostics of optical and ultraviolet emission spectroscopy (OES, 200–750 nm) revealed that CH and H radicals have come to be criteria for the formation of diamonds, and the ratio of radicals drastically affected the characteristics of the deposits. Nucleation and growth mechanism are also discussed.

(AlAs)0.5(GaAs)0.5 fractional‐layer superlattices grown on (001) vicinal surfaces by metalorganic chemical vapor deposition

Abstract: (AlAs)0.5(GaAs)0.5 fractional‐layer superlattices with a new periodicity perpendicular to the growth direction was successfully grown by metalorganic chemical vapor deposition on (001) GaAs substrates slightly misoriented toward [110]. The atomic structures were analyzed by x‐ray superlattice satellite diffraction. Superlattice periods were exactly the same as the mean distance of each atomic step on the (001) vicinal surface. The results indicate that lateral growth from nucleation at the step edge is the dominant process compared with the two‐dimensional nucleation on atomically flat terraces.

Control of Grain Boundary Location By Selective Nucleation Over Amorphous Substrates

Abstract: A selective nucleation based crystal-growth-technique over amorphous substrates is originated. The method manipulates nucleation sites and periods and hence, controls the grain boundary location by modifing the substrate surface. In Si, small Si 3 N 4 nucleation sites are formed, 1–2 pm in diameter, 100 μm in period, over Sio 2 . One Si nucleus is formed exclusively in the small area of Si 3 N 4 by CVD. The highly faceted and periodically located nuclei grow over SiO 2 up to 100 μm in diameter before impingement. A MOS-FET fabricated inside the island operates comparably to the bulk Si control

Effect of yield surface curvature and void nucleation on plastic flow localization

Abstract: The effect of void nucleation is incorporated in a recently proposed material model that accounts for a combination of kinematic hardening and isotropic hardening of a porous ductile material. Since each of plastic dilatancy, void nucleation and yield surface curvature have a strong influence on predictions of plastic flow localization, the present material model can be used to study the interaction of these effects. Nucleation controlled by the plastic strain as well as nucleation controlled by the maximum normal stress on the particle-matrix interface are modelled. The predictions of the material model, for various combinations of parameters, are illustrated by analyses of shear band formation under plane strain or axisymmetric conditions, and by analyses of necking in biaxially stretched sheets.

Raman spectroscopic study of the synthesis of zeolite Y

Abstract: The formation of zeolite Y from colloidal silica and soluble silicate species was investigated by Raman spectroscopy. The role of aging of the reactant mixture was studied. During the nucleation period, the solid amorphous phase consists of predominantly six-membered aluminosilicate rings, which act as building blocks for the formation of zeolite Y. It is essential to have polymeric, highly condensed silicate units as a reactant if zeolite Y crystallization is to take place.

Kinetics of high-temperature thermal decomposition of SiO2 on Si(100)

Abstract: The decomposition of oxide films of 50 to 500 A on Si(100) during ultrahigh vacuum anneal has been studied in a scanning Auger microscope. The decomposition of the oxide occurs locally, in that voids form in the oxide and grow laterally with time and temperature, leaving the oxide areas in between unperturbed. Void growth kinetics data are derived from in situ absorbed current and secondary electron imaging during vacuum anneal. The growth of the void diameter is found to be linear in time with an activation energy of 2.0±0.25 eV. This implies that oxide decomposition rates after the void nucleation phase are dominated by chemical reactions and/or diffusion processes near the circumference, not by the nature of the defect which nucleated the void.

A kinetic model for solid-state silicide nucleation

Abstract: We present a model for solid‐state silicide nucleation at the interface of a thin metal film and a silicon substrate, starting with the assumption that compound nucleation in these systems is a kinetically controlled process. We consider the work being done on other systems, such as dendrite nucleation and growth, and snowflake generation, where the processes are kinetically self‐limited by the structures that form. We show that the requirements for kinetically self‐limited systems can be formally satisfied at the interfaces for silicide forming systems. In particular, in analogy with the selection of metastable shapes for snowflake structures, we would in general expect metastable chemical structures to form at metal‐semiconductor interfaces under conditions of large supercooling. We discuss the selection criteria (maximum energy degradation rate), the probable forms of these structures, and the conditions required for their growth. We then discuss an interpretation of phenomenological rules for selectio...

Hydrates in supersaturated binary sulfuric acid–water vapor: A reexamination

Abstract: The influence of hydrates formation on the energetics of nucleation has been reexamined. The distribution Nh of hydrates containing one acid molecule and h water molecules has been calculated as a function of the relative humidity and acid vapor concentration. A new formula giving the reversible work required to form a droplet has been derived. Hydrate formation can exert an appreciable effect on the energetics of nucleation in the H2SO4–H2O system and must be included in any theory of nucleation rate.