Showing papers on "Nucleation published in 1992"
TL;DR: The response of carbon soot particles and tubular graphitic structures to intense electron-beam irradiation in a high-resolution electron microscope is reported, suggesting that planar graphite may not be the most stable allotrope of carbon in systems of limited size.
Abstract: THE discovery1 of buckminsterfullerene (C60) and its production in macroscopic quantities2 has stimulated a great deal of research. More recently, attention has turned towards other curved graphitic networks, such as the giant fullerenes (Cn, n > 100)3,4 and carbon nanotubes5–8. A general mechanism has been proposed9 in which the graphitic sheets bend in an attempt to eliminate the highly energetic dangling bonds present at the edge of the growing structure. Here, I report the response of carbon soot particles and tubular graphitic structures to intense electron-beam irradiation in a high-resolution electron microscope; such conditions resemble a high-temperature regime, permitting a degree of structural fluidity. With increased irradiation, there is a gradual reorganization of the initial material into quasi-spherical particles composed of concentric graphitic shells. This lends weight to the nucleation scheme proposed9 for fullerenes, and moreover, suggests that planar graphite may not be the most stable allotrope of carbon in systems of limited size.
1,733 citations
TL;DR: In this paper, a periodic relation between shear stress and atomic shear displacement is assumed to hold along the most highly stressed slip plane emanating from a crack tip, which allows some small slip displacement to occur near the tip in response to small applied loading and, with increase in loading, the incipient dislocation configuration becomes unstable and leads to a fully formed dislocation which is driven away from the crack.
Abstract: Dislocation nucleation from a stressed crack tip is analyzed based on the Peierls concept. A periodic relation between shear stress and atomic shear displacement is assumed to hold along the most highly stressed slip plane emanating from a crack tip. This allows some small slip displacement to occur near the tip in response to small applied loading and, with increase in loading, the incipient dislocation configuration becomes unstable and leads to a fully formed dislocation which is driven away from the crack. An exact solution for the loading at that nucleation instability is developed via the J -integral for the case when the crack and slip planes coincide, and an approximate solution is given when they do not. Solutions are also given for emission of dissociated dislocations, especially partial dislocation pairs in fcc crystals. The level of applied stress intensity factors required for dislocation nucleation is shown to be proportional to √γ us , where γ us , the unstable stacking energy, is a new solid state parameter identified by the analysis. It is the maximum energy encountered in the block-like sliding along a slip plane, in the Burgers vector direction, of one half of a crystal relative to the other. Approximate estimates of γ us are summarized and the results are used to evaluate brittle vs ductile response in fcc and bcc metals in terms of the competition between dislocation nucleation and Griffith cleavage at a crack tip. The predictions seem compatible with known behavior and also show that in many cases solids which are predicted to first cleave under pure mode I loading should instead first emit dislocations when that loading includes very small amounts of mode II and III shear. The analysis in this paper also reveals a feature of the near-tip slip distribution corresponding to the saddle point energy configuration for cracks that are loaded below the nucleation threshold, as is of interest for thermal activation.
1,320 citations
Book•
01 Jan 1992
TL;DR: The coordination chemistry of the Hydrous Oxide--Water Interface is discussed in this paper. But the authors do not consider the effect of weathering on the coordination of the hydrous oxide-water interface.
Abstract: The Coordination Chemistry of the Hydrous Oxide--Water Interface. Surface Charge and the Electric Double Layer. Adsorption. The Kinetics of Surface Controlled Dissolution of Oxide Minerals: An Introduction to Weathering. Precipitation and Nucleation. Particle--Particle Interaction. Carbonates and Their Reactivities. Redox Processes Mediated by Surfaces. Heterogeneous Photochemistry. Regulation of Trace Elements by the Solid--Water Interface in Surface Waters. References. Index.
949 citations
TL;DR: In this paper, a model combining the effects of deposition and condensation-freezing nucleation is formulated based on data obtained from continuous-flow diffusion chambers, which indicate an exponential variation of ice-nuclei concentrations with ice supersaturation reasonably independent of temperatures between −7° and −20°C.
Abstract: Two new primary ice-nucleation parameterizations are examined in the Regional Atmospheric Modeling System (RAMS) cloud model via sensitivity tests on a wintertime precipitation event in the Sierra Nevada region. A model combining the effects of deposition and condensation-freezing nucleation is formulated based on data obtained from continuous-flow diffusion chambers. The data indicate an exponential variation of ice-nuclei concentrations with ice supersaturation reasonably independent of temperatures between −7° and −20°C. Predicted ice concentrations from these measurements exceed values predicted by the widely used temperatures dependent Fletcher approximation by as much as one order of magnitude at temperatures warmer than −20°C. A contact-freezing nucleation model is also formulated based on laboratory data gathered by various authors using techniques that isolated this nucleation mode. Predicted contact nuclei concentrations based on the newer measurements are as much as three orders of mag...
842 citations
TL;DR: In this article, it has been shown for various types of glasses and glass-ceramics that the essential condition for them to bond to living bone is the formation of an apatite layer on their surfaces in the body.
Abstract: It has been shown for various types of glasses and glass-ceramics that the essential condition for them to bond to living bone is the formation of an apatite layer on their surfaces in the body. CaO,SiO 2 -based glasses formed the surface apatite layer in a simulated body fluid, whereas CaO,P 2 O 5 -based glasses did not form it. This means that the rate of the apatite nucleation on the surfaces of the former glasses is much higher than that of the latter glasses. The increase in the degree of the supersaturation of the surrounding fluid with respect to the apatite due to dissolution of the calcium ion from the CaO,SiO 2 -based glasses was almost equal to that due to dissolution of the phosphate ion from the CaO,P 2 O 5 -based glasses. The high rate of the apatite nucleation on the surface of the former glasses is therefore attributed to the lower interface energy between the apatite and the glass surfaces. The CaO,SiO 2 -based glasses form a silica hydrogel on their surfaces prior to formation of the apatite layer. This means that the hydrated silica provides specific favorable sites for the apatite nucleation.
827 citations
TL;DR: Mikumo et al. as discussed by the authors used a plane-strain model with spatially varying properties to demonstrate that accelerating slip precedes instability and becomes localized to a fault patch, and the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip.
695 citations
TL;DR: Recent theoretical and experimental advances in the study of homogeneous nucleation are reviewed, with emphasis placed on phase transitions involving single-component liquids (condensation, cavitation, and crystallization from the melt).
Abstract: Recent theoretical and experimental advances in the study of homogeneous nucleation are reviewed, with emphasis placed on phase transitions involving single-component liquids (condensation, cavitation, and crystallization from the melt). Extensions of classical nucleation theory are described and compared with new experiments that now directly measure nucleation rates. Novel methods of statistical mechanics, including density-functional theory and computer simulations, are presented. The recent rapid evolution of this field has opened up many new questions for further research.
603 citations
TL;DR: In this article, a large lattice constant on Si has been obtained by growing compositionally graded GexSi1−x on Si, and these buffer layers have been characterized with electron-beam-induced current, transmission electron microscopy and x-ray diffraction to determine the extent of relaxation, threading dislocation density, the surface morphology, and the optical properties.
Abstract: To obtain a large lattice constant on Si, we have grown compositionally graded GexSi1−x on Si. These buffer layers have been characterized with electron‐beam‐induced current, transmission electron microscopy, scanning electron microscopy, x‐ray diffraction, and photoluminescence to determine the extent of relaxation, the threading dislocation density, the surface morphology, and the optical properties. We have observed that it is possible to obtain completely relaxed GexSi1−x layers with 0.1
549 citations
TL;DR: In this article, it has been determined that hydrocarbon decomposition at elevated temperatures results in formation of a single layer of graphite on the Pt surface, which is observed to accumulate, forming a layer at the lower step edges and also forming large, regularly shaped islands on the terraces.
477 citations
TL;DR: The Kinetic Theory of Gases as discussed by the authors describes the physical structure of surfaces and their properties, including surface curvature, surface tension, and surface mobility, as well as surface chemistry.
Abstract: SURFACE STRUCTURE, THERMODYNAMICS, AND MOBILITY. Atomic Structure of Surfaces. Electronic Structure of Surfaces. Surface Tension. Thermodynamics of One-Component Systems. Thermodynamics of Multicomponent Systems. Surface Mobility. GAS-SURFACE INTERACTIONS. The Kinetic Theory of Gases. Molecular Beam Formation. Gas Scattering. Adsorption-The Kinetic View. Physical Adsorption. Chemisorption. Surface Chemical Reactions. ENERGETIC PARTICLE-SURFACE INTERACTIONS. Electron-Surface Interactions. Ion-Surface Interactions. Photon-Surface Interactions. CRYSTAL GROWTH. Crystal Nucleation and Growth. Index.
464 citations
TL;DR: In this paper, the authors investigated the mechanisms behind the formation and growth of silica particles prepared from tetraalkoxysilanes in alcoholic solutions of water and ammonia, and concluded that the growth proceeds through a surface reaction-limited condensation of hydrolyzed monomers or small oligomers.
TL;DR: A model is proposed to help explain bias-enhanced nucleation on silicon, in hopes that this will improve the understanding of diamond nucleation, in general, and eventually result in the nucleation and growth of better-quality diamond films.
Abstract: An in-depth study has been performed of the nucleation of diamond on silicon by bias-enhanced microwave plasma chemical vapor deposition. Substrates were pretreated by negative biasing in a 2% methane-hydrogen plasma. The bias pretreatment enhanced the nucleation density on unscratched silicon wafers up to ${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ as compared with ${10}^{7}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ on scratched wafers. In vacuo surface analysis including x-ray photoelecton spectroscopy (XPS), Auger electron spectroscopy, and combined XPS and electron-energy-loss spectroscopy were used to study systematically both the initial-nucleation and growth processes. High-resolution cross-sectional transmission electron microscopy (TEM) was used to study the physical and structural characteristics of the diamond-silicon interface as well as to complement and enhance the in vacuo surface-analytical results. Raman spectroscopy confirmed that diamond was actually nucleating during the bias pretreatment. Scanning electron microscopy has shown that once the bias is turned off, and conventional growth is conducted, diamond grows on the existing nuclei and no continued nucleation occurs. If the bias is left on throughout the entire deposition, the resulting film will be of much poorer quality than if the bias had been turned off and conventional growth allowed to begin. Intermittent surface analysis showed that a complete silicon carbide layer developed before diamond could be detected. High-resolution cross-sectional TEM confirmed that the interfacial layer was amorphous and varied in thickness from 10 to 100 \AA{}. A small amount of amorphous carbon is detected on the surface of the silicon carbide and it is believed to play a major role in the nucleation sequence. A model is proposed to help explain bias-enhanced nucleation on silicon, in hopes that this will improve the understanding of diamond nucleation, in general, and eventually result in the nucleation and growth of better-quality diamond films.
Book•
01 Jan 1992
TL;DR: Physical and chemical characteristics of precipitation systems have been studied in this paper, including the kinetics of nucleation and crystal growth of precipitating experimental precipitation studies precipitator design practical aspects of precipitation.
Abstract: Physical and chemical characteristics of precipitation systems kinetics of nucleation and crystal growth kinetics of precipitating experimental precipitation studies precipitator design practical aspects of precipitation.
TL;DR: In this article, the free energy barrier for the formation of body-centered-cubic (bcc) crystallites from the melt was shown to be a surprisingly low free-energy barrier.
Abstract: We show how relatively standard Monte Carlo techniques can be used to probe the free-energy barrier that separates the crystalline phase from the supercooled liquid. As an illustration, we apply our approach to a system of soft, repulsive spheres [v(r)=(/r)12]. This system is known to have a stable face-centered-cubic (fcc) crystal structure up to the melting temperature. However, in our simulations, we find that there is a surprisingly low free-energy barrier for the formation of body-centered-cubic (bcc) crystallites from the melt. In contrast, there appears to be no `easy' path from the melt to the (stable) fcc phase. These observations shed new light on the results of previous simulations that studied the dynamics of crystal nucleation in the r–12 system. We argue that the techniques developed in this paper can be used to gain insight in the process of homogeneous nucleation under conditions where direct, dynamical simulations are inconclusive or prohibitively expensive.
TL;DR: This work provides a detailed scaling theory for the complete distribution of island sizes and separations, both with the ratio of diffusion to deposition rate and with time, and quantifies the depletion in the concentration of pairs of islands at small separations.
Abstract: We examine the competition between diffusion-mediated irreversible nucleation and growth of islands during submonolayer deposition on perfect substrates. We provide a detailed scaling theory for the complete distribution of island sizes and separations, both with the ratio of diffusion to deposition rate and with time. Scaling functions and exponents are obtained by simulation. The leading scaling behavior is independent of details of the island structure. These results are supplemented by an analysis of rate equations for the island-size distribution whose unconventional form appropriately describes island nucleation and growth mechanisms. The exponents agree with the simulations and the island-size distribution shows qualitative agreement. We further provide simulation results for the scaling of the island-separation distribution, quantifying, in particular, the depletion in the concentration of pairs of islands at small separations.
TL;DR: The diamond was characterized by Raman spectroscopy and scanning electron microscopy as discussed by the authors, showing that approximately 50% of the initial diamond nuclei appear to be aligned with the C(001) planes parallel to the SiC(001), and C[110] directions parallel to SiC within 3°.
Abstract: Textured diamond films have been deposited on β‐SiC via microwave plasma chemical vapor deposition preceded by an in situ bias pretreatment that enhances nucleation. Approximately 50% of the initial diamond nuclei appear to be aligned with the C(001) planes parallel to the SiC(001), and C[110] directions parallel to the SiC[110] within 3°. The diamond was characterized by Raman spectroscopy and scanning electron microscopy.
TL;DR: Results in this study showed that loss of hydraulic conductivity occurred in stem segments pressurized in a pressure chamber while the xylem water was under positive pressure, consistent with the air-seeding hypothesis of how embolism are nucleated, but not with the nucleation of embolisms at hydrophobic cracks.
Abstract: Loss of hydraulic conductivity occurs in stems when the water in xylem conduits is subjected to sufficiently negative pressure. According to the air-seeding hypothesis, this loss of conductivity occurs when air bubbles are sucked into water-filled conduits through micropores adjacent to air spaces in the stem. Results in this study showed that loss of hydraulic conductivity occurred in stem segments pressurized in a pressure chamber while the xylem water was under positive pressure. Vulnerability curves can be defined as a plot of percentage loss of hydraulic conductivity versus the pressure difference between xylem water and the outside air inducing the loss of conductivity. Vulnerability curves were similar whether loss of conductivity was induced by lowering the xylem water pressure or by raising the external air pressure. These results are consistent with the air-seeding hypothesis of how embolisms are nucleated, but not with the nucleation of embolisms at hydrophobic cracks because the latter requires negative xylem water pressure. The results also call into question some basic underlying assumptions used in the determination of components of tissue water potential using "pressure-volume" analysis.
01 Jan 1992
TL;DR: In this paper, a pH-stat method was used to detect calcite seeds in solutions of known calcite saturation state at 25 C for 1--2 days before transferring to an SFM fluid cell for continued reaction in the same solutions.
Abstract: Experimental conditions of Busenberg and Plummer for 1atm p[sub CO[sub 2]] and variable saturation states were reproduced and growth mechanisms were examined in situ at these conditions. Using a pH-stat method, calcite seeds were reacted in solutions of known calcite saturation state at 25 C for 1--2 days before transferring to an SFM fluid cell for continued reaction in the same solutions. The authors observed that when solution saturations with respect to calcite were >1--2, precipitation began with the formation of surface nuclei. These nuclei spread, coalesced, and continued growing. Only after more than 1 hour was there a transition to a mechanism resembling spiral growth. At these long reaction times, migrating steps assumed individual heights of 1--3 monolayers. The influence of phosphate was examined and observations suggested two inhibition mechanisms, depending on surface history. Phosphate (6 [mu]mol PO[sub 4]) introduced during the nucleation stage results in the formation of nuclei with amorphous shapes. Phosphate introduced during layer growth disrupts the relatively straight steps produced during PO[sub 4]-free growth to form jagged steps. A comparison of their observations with kinetic interpretations reported by solution chemistry studies of calcite precipitation suggests that some rate experiments may include the early influence ofmore » a rapid nonlinear growth phase caused by surface nucleation. If true, the analysis of data and interpretations of growth mechanisms may be affected and suggests that rates of stable calcite growth in natural systems may be slower than experimentally determined values would predict. Both of the phosphate-calcite surface interactions are consistent with mechanisms proposed in previous studies.« less
TL;DR: In this paper, the effects of dilute sulphate on metastable and stable pitting of 304 stainless steel in chloride solution have been studied, showing that the presence of sulphate causes the distribution of available pit sites to be shifted to a higher potential, implying that pit nucleation is inhibited.
AeA1
TL;DR: In this article, the authors used two high purity model alloys, one of which was doped with sulphur, and laser surface melting of the alloys to detect and follow the evolution of anodic current transients (unstable micropits).
TL;DR: In this article, the dissolution rate of calcite far from equilibrium was measured to be 3.1 × 10 −10 −10 mol · cm −2 · s −1 in free-drift rotating disk experiments at 1160 rpm, 25°C and pH 8.6.
01 Jan 1992
TL;DR: In this article, the approaches of kinetic nucleation theory and of "thick" film growth modeling are outlined and the structures developed as a function of substrate temperature and particle energy are presented.
Abstract: Nucleation and film growth determine the coating structure and the coating properties. In certain film/substrate systems an experimental study of the processes in the very initial stage of film formation is possible. They reflect the results of modeling using the kinetic nucleation theory or the thermodynamic approach. Tor practical systems, i.e. non-UHV conditions, alloy and compound deposition, the relation are much more complex, but the structures developed are very similar to those obtained for pure metal deposits. In the present paper the approaches of kinetic nucleation theory and of “thick” film growth modeling are outlined and the structures developed as a function of substrate temperature and particle energy are presented. In principal the theoretical investigations are very helpful even if they don’t allow to draw quantitative conclusions for complex coating/substrate systems.
TL;DR: In this article, the formation mechanisms and properties of TiSi2 on Si are investigated and the particular emphasis is in relating the nucleation, morphology, and phase stability of the films.
Abstract: The formation mechanisms and properties of TiSi2 on Si are investigated. The particular emphasis is in relating the nucleation, morphology, and phase stability of the films. TiSi2 films were prepared by deposition of Ti on atomically clean silicon substrates in ultrahigh vacuum. The silicide formation was initiated either by in situ annealing or deposition onto heated substrates. The island formation of TiSi2 and surface and interface morphologies of TiSi2 were examined by scanning electron microscopy and transmission electron microscopy. The TiSi2 formation process was monitored with in situ Auger electron spectroscopy and low‐energy electron diffraction to analyze the surface concentration and the surface structures, respectively. Raman spectroscopy was used for phase identification of the TiSi2. Titanium film thicknesses from 50 to 400 A were examined. For all thicknesses studied, the C49 TiSi2 phase is observed to nucleate. Immediately after low‐temperature deposition, the interface morphology was smo...
TL;DR: The properties of glass-ceramics depend upon both composition and microstructure as mentioned in this paper, and the bulk chemical composition controls the ability to form a glass and its degree of workability.
Abstract: Glass-ceramics are microcrystalline solids produced by the controlled devitrification of glass. Glasses are melted, fabricated to shape, and then converted by heat treatment to a predominantly crystalline ceramic. The basis of controlled crystallization lies in efficient internal nucleation ( 1), which allows development of fine, randomly oriented grains without voids, microcracks, or other porosity. A unique manufacturing advantage of glass-ceramics over conventional ceramics is the ability to use high-speed plastic forming processes developed in the glass industry (e.g. pressing, blowing, rolling, etc.) to create complex shapes essentially free of internal inhomogeneities. Because glass-ceramic compositions are designed to crystallize, however, they can not be held for long periods at temperatures below the liquidus during the forming process. Therefore, the viscosity at the liquidus temperature is critical both in the choice of a forming process and in the choice of a glass composition. The properties of glass-ceramics depend upon both composition and microstructure. The bulk chemical composition controls the ability to form a glass and its degree of workability. In order to achieve internal nucleation, suitable nucleating agents are melted into the glass. Bulk com position also directly determines the potential crystalline phase assem blage, and this in turn governs the general physical and chemical charac teristics, e.g. hardness, density, acid resistance, etc. Secondly, but equally important, is the importance of microstructure. Microstructure is the key
TL;DR: Ohnaka et al. as discussed by the authors proposed a physical model for short-term precursors of an earthquake, which is based on a quasistatic to quasidynamic rupture nucleation process.
TL;DR: In this paper, a method of computing nonequilibrium, steady flows of wet steam in two-and quasi-three-dimensional turbine cascades is described, where mixture conservation equations are solved in an Eulerian reference frame using an inviscid time-marching method that includes the effects of the centrifugal and Coriolis acceleration terms in rotating blade rows.
Abstract: The paper describes a method of computing nonequilibrium, steady flows of wet steam in two- and quasi-three-dimensional turbine cascades. The mixture conservation equations are solved in an Eulerian reference frame using an inviscid time-marching method that includes the effects of the centrifugal and Coriolis acceleration terms in rotating blade rows. Nucleation and growth of water droplets are computed by integrating the relevant equations along true streamlines in a Lagrangian reference frame. Steam properties are computed using equations that display commercial steam table accuracy for pressures below 10 bar. Special procedures for grouping the range of droplet sizes present are described that allow an accurate representation of the droplet size distribution to be retained without requiring a large increase in CPU time. All types of wet-steam flow, including those involving secondary nucleation, can be computed. Examples are presented that display the sensitivity of the calculation procedure in computing nucleation affected by the shock- and expansion-wave structure in the region of a turbine blade trailing edge. Typical CPU time requirements for nonequilibrium solutions involving primary or secondary nucleations are about three times those for perfect gas calculations.
TL;DR: In this article, the thermal evolution of amorphous TiO2 powders, consisting of spherical particles and prepared by hydrolysis of a titanium ethoxide aerosol, was studied by using Raman spectroscopy.
Abstract: The thermal evolution of amorphous TiO2 powders, consisting of spherical particles and prepared by hydrolysis of a titanium ethoxide aerosol, was studied by using Raman spectroscopy. On calcination at 350°C, the solid crystallized, giving anatase as a major phase. A small amount of rutile was also detected and attributed to small seeds localized at the particle outlayer. The nucleation of rutile at so low a temperature was ascribed to the presence of organic impurities in the powders. The transformation of anatase into rutile was clearly observed after heating at 660°C.
TL;DR: In this paper, the growth of the holes resulted in roughly circular agglomerated areas and the area fraction transformed was measured as a function of time for several temperatures in oxygen and argon.
TL;DR: In this article, the lateral surface free energy parameter σ in the nucleation constant K g in the relation G exp[-K g /T(ΔT)f] that describes the growth rate of polymer crystals from the melt at low-tomoderate undercoolings ΔT.
Abstract: A theory is presented for the lateral surface free energy parameter σ in the nucleation constant K g in the relation G exp[-K g /T(ΔT)f] that describes the growth rate of polymer crystals from the melt at low-to-moderate undercoolings ΔT. The theory forms a connection between nucleation theory and the statistics of polymer chain dimensions and provides a new approach to the determination of the characteristic ratio C ∞. It is predicted that σ varies as constxC ∞ -1 , where the constant involves known quantities. The effect of chain structure resides principally in C ∞
TL;DR: In this paper, a new finding is reported concerning some structural change in the amorphous state of the polymer which occurs during the so-called induction period before the start of crystallization.