Showing papers on "Nucleation published in 1996"

On the nucleation and growth of amyloid beta-protein fibrils: detection of nuclei and quantitation of rate constants

Abstract: We have studied the fibrillogenesis of synthetic amyloid beta-protein-(1-40) fragment (A beta) in 0.1 M HCl. At low pH, A beta formed fibrils at a rate amenable to detailed monitoring by quasi-elastic light-scattering spectroscopy. Examination of the fibrils with circular dichroism spectroscopy and electron microscopy showed them to be highly similar to those found in amyloid plaques. We determined the hydrodynamic radii of A beta aggregates during the entire process of fibril nucleation and growth. Above an A beta concentration of approximately 0.1 mM, the initial rate of elongation and the final size of fibrils were independent of A beta concentration. Below an A beta concentration of 0.1 mM, the initial elongation rate was proportional to the peptide concentration, and the resulting fibrils were significantly longer than those formed at higher concentration. We also found that the surfactant n-dodecylhexaoxyethylene glycol monoether (C12E6) slowed nucleation and elongation of fibrils in a concentration-dependent manner. Our observations are consistent with a model of A beta fibrillogenesis that includes the following key steps: (i) peptide micelles form above a certain critical A beta concentration, (ii) fibrils nucleate within these micelles or on heterogeneous nuclei (seeds), and (iii) fibrils grow by irreversible binding of monomers to fibril ends. Interpretation of our data enabled us to determine the sizes of fibril nuclei and A beta micelles and the rates of fibril nucleation (from micelles) and fibril elongation. Our approach provides a powerful means for the quantitative assay of A beta fibrillogenesis.

Numerical calculation of the rate of crystal nucleation in a Lennard‐Jones system at moderate undercooling

Abstract: We report a computer‐simulation study of the rate of homogeneous crystal nucleation and the structure of crystal nuclei in a Lennard‐Jones system at moderate undercooling. The height of the nucleation barrier has been determined using umbrella sampling, whereas the barrier crossing rate is calculated using molecular dynamics simulation. The simulations clearly show that the barrier crossing is a diffusive process. Nevertheless, the kinetic prefactor in the nucleation rate is found to be some two orders of magnitude larger than predicted by classical nucleation theory. The height of the barrier is in good agreement with the theoretical prediction. Although the Lennard‐Jones system has a stable face‐centered cubic (fcc) phase below the melting line, the precritical nuclei are found to be mainly body‐centered cubic (bcc) ordered. As they grow to their critical size, they become more fcc ordered in the core. However, the critical and postcritical nuclei retain a high degree of bcc ordering in the interface. Furthermore it is found that in the interface the density falls off faster than the structural order parameter, which is in agreement with the predictions of density functional calculations. [P. Harrowell and D. W. Oxtoby, J. Chem. Phys. 80, 1639 (1984)].

Defect structure of metal‐organic chemical vapor deposition‐grown epitaxial (0001) GaN/Al2O3

Abstract: Defect structures were investigated by transmission electron microscopy for GaN/Al2O3 (0001) epilayers grown by metal‐organic chemical vapor deposition using a two‐step process. The defect structures, including threading dislocations, partial dislocation bounding stacking faults, and inversion domains, were analyzed by diffraction contrast, high‐resolution imaging, and convergent beam diffraction. GaN film growth was initiated at 600 °C with a nominal 20 nm nucleation layer. This was followed by high‐temperature growth at 1080 °C. The near‐interfacial region of the films consists of a mixture of cubic and hexagonal GaN, which is characterized by a high density of stacking faults bounded by Shockley and Frank partial dislocations. The near‐interfacial region shows a high density of inversion domains. Above ∼0.5 μm thickness, the film consists of isolated threading dislocations of either pure edge, mixed, or pure screw character with a total density of ∼7×108 cm−2. The threading dislocation reduction in the...

Dewetting Modes of Thin Metallic Films: Nucleation of Holes and Spinodal Dewetting.

Abstract: We have studied the dewetting of thin liquid metal films (Au, Cu, Ni) on fused silica substrates which occurs after melting with a Q-switched laser pulse. Optical microscopy, scanning electron microscopy, and scanning near field acoustic microscopy reveal two distinctly different modes of the dewetting process: On one hand, we observe heterogeneous nucleation of ``dry'' circular patches, which grow in diameter during the melting period. On the other hand, an instability of the liquid film against the growth in amplitude of surface waves with a characteristic wavelength is observed, which we believe is the first observation of spinodal dewetting. In contrast, the final structure of the ruptured film depends on whether nucleation or spinodal dewetting is dominant.

Measured atmospheric new particle formation rates: implications for nucleation mechanisms

Abstract: Measured production rates of tropospheric ultrafine particles (˜3nm diameter) are reported for the first time and are shown to be orders of magnitude greater than nucleation rates predicted by the binary theory of homogeneous nucleation for sulfuric acid and water. Furthermore, the functional dependence of observed particle formation rates on sulfuric acid vapor concentrations is much weaker than predicted by binary theory. We present arguments to show that these discrepancies might be due to the participation of a species such as ammonia which could stabilize subcritical clusters, thereby enhancing nucleation rates. The data suggest that atmospheric nucleation may occur by a collision-limited process, rather than by a condensation/evaporation controlled process as is assumed in the classical theory.

Crystallization kinetics of poly(L‐lactide‐co‐meso‐lactide)

Abstract: The crystallization kinetics of poly(L-lactide-co-meso-lactide) were determined over a range of 0% to 9% mesolactide. The kinetics were fit to the nonlinear Avrami equation and then to the Hoffman–Lauritzen equation modified for optical copolymers. The theory was found to fit the data well. The crystallization half-time was found to increase about 40% for every 1 wt % meso-lactide in the polymerization mixture. The change in crystallization rate is driven mainly by the reduction in melting point for the copolymers. The copolymer crystallization kinetics were also determined in the presence of talc, a nucleating agent for polylactide. The theory again fit the data well, using the same growth parameters and accounting for the talc only through the nucleation density term. © 1996 John Wiley & Sons, Inc.

The role of magnesium in the crystallization of calcite and aragonite in a porous medium

Abstract: Morphological development of calcite crystals is related to supersaturation conditions during growth. Crystallization of calcium carbonate (calcite and aragonite) as well as Mg-calcite was studied under controlled supersaturation conditions by the counter diffusion of Ca2+ and CO32- ions through a porous transport medium (a column of silica gel). Under our experimental conditions, where ion transport is constrained to be diffusion controlled, nucleation and growth take place under conditions of high supersaturation, the actual threshold value of the supersaturation depending on the supersaturation gradient. In the pure CaCO3 system, calcite grows at lower supersaturation than aragonite. The calcite develops relatively simple rhombohedra whil the aragonite grows as spherulites. Presence of Mg2+ in the interstitial fluid inhibits nucleation, increasing the threshold supersaturation at which crystallization begins. The resulting Mg-calcite crystals show a range of morphologies depending on the Mg content and the supersaturation at the point of crystallization. At high values of supersaturation, up to 15 mol % MgCO3 is incorporated into the calcite and the crystals form spheres. At lower supersaturations, Mg content decreases and morphologies change progressively through a well-defined and reproducible sequence from spheres to dumbbell-like forms to wheat-sheaf-like bundles and eventually single crystals with steep rhombohedral faces. The crystals are compositionally zoned, showing both sector and oscillatory zoning. The compositional evol tion is related to the supersaturation and interface roughness during crystal growth.

Crystallization of amorphous water ice in the solar system.

Abstract: Electron diffraction studies of vapor-deposited water ice have characterized the dynamical structural changes during crystallization that affect volatile retention in cometary materials. Crystallization is found to occur by nucleation of small domains, while leaving a significant part of the amorphous material in a slightly more relaxed amorphous state that coexists metastably with cubic crystalline ice. The onset of the amorphous relaxation is prior to crystallization and coincides with the glass transition. Above the glass transition temperature, the crystallization kinetics are consistent with the amorphous solid becoming a "strong" viscous liquid. The amorphous component can effectively retain volatiles during crystallization if the volatile concentration is approximately 10% or less. For higher initial impurity concentrations, a significant amount of impurities is released during crystallization, probably because the impurities are trapped on the surfaces of micropores. A model for crystallization over long timescales is described that can be applied to a wide range of impure water ices under typical astrophysical conditions if the fragility factor D, which describes the viscosity behavior, can be estimated.

Detailed observations of California foreshock sequences: Implications for the earthquake initiation process

Abstract: We find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes. In this study, we examine in detail the evolution of six California foreshock sequences, the 1986 Mount Lewis (ML = 5.5), the 1986 Chalfant (ML = 6.4), the 1986 Stone Canyon (ML = 4.7), the 1990 Upland (ML = 5.2), the 1992 Joshua Tree (MW = 6.1), and the 1992 Landers (MW = 7.3) sequence. Typically, uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution. However, the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations. We use these results to identify small-scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock. In general, these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer. Instead, the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process. Fault zone heterogeneity may also be important in controlling the number of foreshocks, i.e., the stronger the heterogeneity, the greater the number of foreshocks. The size of the nucleation region, as measured by the extent of the foreshock sequence, appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase. We also find evidence for slip localization as predicted by some models of earthquake nucleation.

Nucleation layer evolution in metal‐organic chemical vapor deposition grown GaN

Abstract: The structure and morphology of low growth temperature GaN nucleation layers have been studied using atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED), and transmission electron microscopy (TEM). The nucleation layers were grown at 600 °C by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on c‐plane sapphire. The layers consist of predominantly cubic GaN (c‐GaN) with a high density of stacking faults and twins parallel to the film/substrate interface. The average grain size increases with increasing layer thickness and during the transition from low temperature (600 °C) to the high temperatures (1080 °C) necessary for the growth of device quality GaN. Upon heating to 1080 °C the nucleation layer partially converts to hexagonal GaN (h‐GaN) while retaining a high stacking fault density. The mixed cubic‐hexagonal character of the nucleation layer region is sustained after subsequent high‐temperature GaN growth.

Abnormal growth of faceted (WC) grains in a (Co) liquid matrix

Abstract: If the grains dispersed in a liquid matrix are spherical, their surface atomic structure is expected to be rough (diffuse), and their coarsening has been observed to be controlled by diffusion in the matrix. They do not, furthermore, undergo abnormal growth. On the other hand, in some compound material systems, the grains in liquid matrices are faceted and often show abnormal coarsening behavior. Their faceted surface planes are expected to be singular (atomically flat) and therefore grow by a defect-assisted process and two-dimensional (2-D) nucleation. Contrary to the usual coarsening the-ories, their growth velocity is not linearly dependent on the driving force arising from the grain size difference. If the growth of the faceted grains occurs by 2-D nucleation, the rate is expected to increase abruptly at a critical supersaturation, as has been observed in crystal growth in melts and solutions. It is proposed that this growth mechanism leads to the abnormal grain coarsening. The 2-D nucleation theory predicts that there is a threshold initial grain size for the abnormal grain growth (AGG), and the propensity for AGG will increase with the heat-treatment temperature. The AGG behavior will also vary with the defects in the grains. These predictions are qualitatively confirmed in the sintered WC-Co alloy prepared from fine (0.85-Μm) and coarse (5.48-Μm) WC powders and their mixtures. The observed dependence of the AGG behavior on the sintering temperature and the milling of the WC powder is also qualitatively consistent with the predicted behavior.

A microcellular processing study of poly(ethylene terephthalate) in the amorphous and semicrystalline states. Part I: Microcell nucleation

Abstract: Microcellular semicrystalline polymers such as poly(ethylene terephthalate) show great promise for engineering applications because of their unique properties, particularly at higher densities. Recent studies reveal some high density microcellular polymers have longer fatigue lives and/or equal strengths to the neat polymer. Relatively few microcellular processing studies of semicrystalline polymers have been presented. In general, semicrystalline polymers are relatively difficult to microcellular process compared to amorphous polymers. In this paper and a companion paper, the microcellular processing of poly(ethylene terephthalate) in the amorphous and semicrystalline states is studied in order to quantify the processing differences. The microcellular processing steps addressed in this paper include gas/polymer solution formation and microvoid nucleation. Particular emphasis is given to microvoid nucleation comparing the processing characteristics of semicrystalline and amorphous materials. Moreover, this study identifies a number of critical process parameters. In general, the semicrystalline materials exhibit ten to one thousand times higher cell nucleation densities compared with the amorphous materials, resulting from heterogeneous nucleation contributions. The amorphous materials show a strong dependence on cell density, while the semicrystalline materials show a weaker dependence. Moreover, classical nucleation theory is not adequate to quantitatively predict the effects of saturation pressure on cell nucleation for either the amorphous or semicrystalline polyesters. Both the semicrystalline and amorphous materials exhibit constant nucleation cell densities with increasing foaming time. Foaming temperatures near the glass transition are found to influence the cell density of the amorphous polyesters, indicating some degree of thermally activated nucleation. Furthermore, classical nucleation theory is not adequate to predict the cell density dependence on foaming temperature. Similar to the amorphous polyesters above the glass transition temperature, nucleation in the semicrystalline materials is found to be independent of the foaming temperature.

Electrochemical Deposition of Silver Nanocrystallites on the Atomically Smooth Graphite Basal Plane

Abstract: A potentiostatic pulse method has been employed to electrochemically deposit silver nanocrystallites on the atomically smooth graphite basal plane surface. Voltage pulses having amplitudes of 100, 250, and 500 mV vs Ag0 and durations of 10 or 50 ms were applied to graphite surfaces immersed in dilute (≈1.0 mM) aqueous silver nitrate. During the deposition pulse, the current increased in approximate proportion to (time)1/2 as expected for an instantaneous nucleation and three-dimensional growth mode of deposition. Consistent with this growth mode, noncontact atomic force microscopy (NC-AFM) examination of graphite surfaces following silver deposition revealed the existence of silver particles at a coverage of near 1010 cm-2 which were well-separated from one another on atomically smooth regions of the graphite basal plane surface. These particles were disk-shaped having a height of 15−50 A and an apparent diameter which varied from 200 to 600 A; particle dimensions increased smoothly with the coulometric l...

Crystallization of germanium–antimony–tellurium amorphous thin film sandwiched between various dielectric protective films

Abstract: Crystallization processes were studied for germanium–antimony–tellurium (Ge–Sb–Te) ternary amorphous thin film as a single layer or sandwiched between various dielectric films, such as silicon dioxide (SiO2), siricon nitride (Si3N4), tantalum oxide (Ta2O5), zinc sulfide (ZnS), and ZnS–20 mol % SiO2. The processes were analyzed quantitatively, based on transmittance changes in Ge–Sb–Te films heated either exothermally or isothermally. Both Kissinger equation and Johnson–Mehl–Avrami kinetic analysis were adopted to estimate activation energy and the reaction order of the processes. Ge–Sb–Te single‐layer amorphous film crystallized in two stages, nucleation and crystal growth. These two processes can be distinguished by exothermal crystallization patterns. By sandwiching this film into dielectric films, crystallization activation energy increases and the nucleation processes are affected. The Si3N4 and Ta2O5 dielectric films accelerate the nucleation, while the SiO2 films inhibit it, and the ZnS and ZnS–20 m...

Nonphotochemical, Polarization-Dependent, Laser-Induced Nucleation in Supersaturated Aqueous Urea Solutions

Abstract: We report a new photophysical phenomenon in which 1.06 mm pulses from a Q-switched Nd:YAG laser induce crystallization in supersaturated solutions of urea in water. Because the solutions are transparent at the incident wavelength, a photochemical mechanism is unlikely. The needle-shaped crystals that initially form tend to be aligned parallel to the electric field vector of the light, suggesting a Kerr-like field-induced alignment of urea molecules that aids in organizing prenucleating clusters. The effect has application to pump-probe nucleation studies and to clean nucleation in sealed systems. [S0031-9007(96)01456-1] PACS numbers: 81.10.Dn, 42.50.Vk The study of the light-induced condensation of supersaturated vapors dates back to the work of Tyndall in 1869 [1]. More recent interest has centered on the formation of atmospheric aerosols [2], laser-induced chemical vapor deposition [3], and laser-induced clustering in atomic and molecular systems [4]. The mechanism typically involves the photochemical generation of a nonvolatile product that acts as a nucleus for the growth of the condensed phase [5]. Nucleation in liquid solutions is a more complex problem involving two components, and, to our knowledge, there have been no reports of light-induced nucleation from supersaturated solutions. Nevertheless, this problem is of great theoretical and commercial importance, such as in industrial crystallization processes [6]. Recently, while attempting to observe second harmonic generation in supersaturated solutions of urea in water, we have noticed that pulses from a Q-switched Nd:YAG laser can induce nucleation in such solutions. Because the incident light source is near infrared, a photochemical mechanism is unlikely. The orientation of the crystallites that are formed depends on the plane of polarization of the incident radiation, suggesting an electric-field-induced effect. Aqueous urea solutions, with concentrations, c, in the range of 11.5 -13.5M, were prepared by combining solid urea and water in a 1.3-cm diameter pyrex test tube, which was then heat sealed with a torch. Great care was taken to exclude dust from samples. Supersaturated solutions were generated and regenerated by heating the tubes to 45 ‐ C and holding them at that temperature for

Method of producing nanoscale powders by quenching of vapors

Abstract: A thermal reactor system that produces nanoscale powders by ultra-rapid thermal quench processing of high-temperature vapors through a boundary-layer converging-diverging nozzle. A gas suspension of precursor material is continuously fed to a thermal reaction chamber and vaporized under conditions that minimize superheating and favor nucleation of the resulting vapor. According to one aspect of the invention, the high temperature vapor is quenched using the principle of Joule-Thompson adiabatic expansion. Immediately after the initial nucleation stages, the vapor stream is passed through the nozzle and rapidly quenched through expansion at rates of at least 1,000° C. per second, preferably greater than 1,000,000° C. per second, to block the continued growth of the nucleated particles and produce a nanosize powder suspension of narrow particle-size distribution. According to another aspect of the invention, a gaseous boundary-layer stream is injected to form a blanket over the internal surface of the nozzle to prevent vapor condensation in the throat of the nozzle and its potential failure.

Mechanisms of hydroxyapatite formation on porous gel-silica substrates

Abstract: The variables affecting the nucleation and crystallization of biological hydroxy carbonate apatite (HCA) on porous gel-silica substrates are examined. Texture is the critical variable with the rate of HCA formation increasing as pore size and pore volume increase, with pore sizes >2 nm required to achieve rapid kinetics (4–6 days) of fully crystallized HCA. The concentration of silanol groups on the silica surface does not control the rate of HCA formation although metastable surface defects, such as trisiloxane rings, may be involved in HCA nucleation.

Integrated thermal process for the continuous synthesis of nanoscale powders

Abstract: A continuous process that produces nanoscale powders from different types of precursor material by evaporating the material and quenching the vaporized phase in a converging-diverging expansion nozzle. The precursor material suspended in a carrier gas is continuously vaporized in a thermal reaction chamber under conditions that favor nucleation of the resulting vapor. Immediately after the initial nucleation stages, the vapor stream is rapidly and uniformly quenched at rates of at least 1,000 K/sec, preferably above 1,000,000 K/sec, to block the continued growth of the nucleated particles and produce a nanosize powder suspension of narrow particle-size distribution. The nanopowder is then harvested by filtration from the quenched vapor stream and the carrier medium is purified, compressed and recycled for mixing with new precursor material in the feed stream.

Ultrafine spinel powders by flame spray pyrolysis of a magnesium aluminum double alkoxide

Abstract: Ultrafine crystalline spinel powder has been prepared using flame spray pyrolysis of alcoholic solutions of a novel double alkoxide precursor. The particles produced are spherical, dense, single crystals with diameters of 10--100 nm and specific surface areas ranging from 40 to 60 m{sup 2}/g. Powder production rates of 50--100 g/h are achieved using a bench-top apparatus. Particle formation appears to occur by rapid oxidation of the organic ligands followed by nucleation and growth from oxide species.

Kinetics of Chain Organization in Ultrathin Poly(di-n-hexylsilane) Films†

Abstract: The effect of film thickness on the organization kinetics of ultrathin (50−1000 A thick) spin-cast polymer films was studied using poly(di-n-hexylsilane) and UV absorption spectroscopy. We found an extensive reduction in the crystallinity as well as a reduction in the rate of crystallization for film thicknesses below 500 A resulting from the constrained geometry. Modeling using polymer crystallization theories elucidated the surface-induced phenomena. We found that the dimensionality of the growth depended both on the film thickness and on the crystallization temperature. At low crystallization temperatures (below 0 °C) and for films thicker than 220 A, the nucleation is bulk and the growth is three-dimensional. However, at higher crystallization temperatures (above 3 °C) and for low film thickness (below 150 A), the growth is one-dimensional and heterogeneous nucleation becomes important. For 500 A thick films, the transition between the two nucleation regimes occurs abruptly around 3 °C.