Showing papers on "Nucleation published in 1990"

Nucleation and Growth of CdSe on ZnS Quantum Crystallite Seeds and Vice Versa, in Inverse Micelle Media

Abstract: Composite semiconductor crystallites involving CdSe grown on an ZnS seed, and vice versa, have been synthesized and capped with organic ligands in inverse micelle solutions. These composite particles, as well as capped seed crystallites of CdSe and ZnS, are isolated, purified, and characterized for relative atomic composition, structure, and electronic properties. The Debye X-ray scattering equation, when solved for these layered particles, shows that powder X-ray scattering is insensitive to a small foreign inclusion. A simple theoretical model for the LUMO and HOMO of layered crystallites shows that a small (< 15-{angstrom} diameter) interior foreign seed causes only small shifts of the lowest excited state, to either higher or lower energies. The capped CdSe seed and the capped CdSe portion of the layered particle grown on a ZnSe seed undergo low-temperature (169{degree}C) annealing to give near-single-crystal X-ray scattering. However, CdSe annealing is blocked by a surface ZnS layer which is ca. 4 {angstrom} thick. While growth to make composite particles does occur, neither particle shows evidence for epitaxial growth.

Kinetic nucleation theory: A new expression for the rate of homogeneous nucleation from an ideal supersaturated vapor

Abstract: The ‘‘kinetic theory’’ of homogeneous nucleation developed by Katz and Wiedersich is extended to derive a new expression for the rate of nucleation from an ideal supersaturated vapor. Compared to the classical expression for the nucleation rate, the new expression has a slightly different dependence on supersaturation, and a substantially different dependence on temperature. A comparison of the new expression with experimental data on nucleation rates of several organic liquids indicates that in some but not all cases the new expression gives much closer agreement with the data than does the classical expression. Discrepancies between the theory and the data are ascribed mainly to the physical assumptions of the theory presented, which are the same as in the classical theory—particularly, that the physical properties of microscopic clusters are the same as those of the bulk liquid.

Drying of solids wetted by thin liquid films

Abstract: (i) A film of a nonwetting liquid is not stable. If it is thick (thickness e ≥ 1000 A) it is metastable and evolves via nucleation and growth of a dry spot. If it is thin, it is unstable against sp...

Novel mechanism for the formation of chemisorption phases: the (2×1)O-Cu(110) added-row reconstruction

Abstract: Scanning tunneling microscopy investigations on nucleation and growth of the (2\ifmmode\times\else\texttimes\fi{}1)O-Cu(110) structure revealed that the new phase is formed by condensation of mobile chemisorbed O atoms with Cu adatoms evaporating from steps and diffusing across the terraces of the substrate surface. This process can be regarded as two-dimensional precipitation from a dilute mixed fluid into a solid phase rather than a solid-solid transformation. The resulting structure is more appropriately described to be of ``added-row'' rather than of ``missing-row'' type.

Fatigue and switching in ferroelectric memories: Theory and experiment

Abstract: A theoretical model of fatigue in ferroelectric thin‐film memories based upon impact ionization (e.g., Ti+4 to Ti+3 conversion in PbZr1−xTixO3), resulting in dendritic growth of oxygen‐deficient filaments, is presented. The predictions of spontaneous polarization versus switching cycles Ps(N) are compared with both Monte Carlo simulations for a two‐dimensional Ising model and with experimental data on small‐grain (40 nm) sol‐gel PZT films. Excellent agreement between theory and experiment is obtained. In addition to modeling the Ps(N) curves, the theory developed explains the observed linear proportionality between switching time ts(N) and polarization Ps(N) during fatigue; other models of aging do not account for this. Earlier theories of switching are also extended to include finite grain sizes, surface nucleation, triangular drive pulses, and dipolar forces. Good agreement with sol‐gel PZT switching data is obtained.

Thermodynamics and kinetics of intracellular ice formation during freezing of biological cells

Abstract: A physicochemical model based on the modified classical heterogeneous nucleation theory was proposed to analyze the ice formation inside biological cells during freezing. According to this model, intracellular ice formation (IIF) may be catalyzed either by the plasma membrane via the effects of the external ice on the plasma membrane, called surface‐catalyzed nucleation (SCN), or by the intracellular particles, called volume‐catalyzed nucleation (VCN), depending on the freezing conditions. The model for IIF was coupled with the model describing the kinetics of water transport to predict the thermodynamic state of the cytoplasm.A method to estimate the nucleation frequency from the observed probability of nucleation was suggested. This method was based on the assumption that each cell has the same heteronucleating particles with identical properties to alter the nucleation kinetics in an identical way. This allowed the correlation of the experimental probability of IIF with the nucleation rate. It was sugg...

Enhanced Boiling Heat Transfer

Abstract: Keywords: chaleur: transfert de: : : ; ebullition: : : ; evaporation: : : ; tubes: : : ; surface d': evaporation: : : ; melanges: : : ; chimie: : : ; nucleation: : : Reference Record created on 2004-09-07, modified on 2016-08-08

Magnetization reversal in ultrathin ferromagnetic films with perpendicular anistropy: Domain observations.

Abstract: We report on the first observation of magnetic-domain structure during spin reversal under field in ultrathin metallic films with perpendicular anisotropy. The magnetization reversal is either dominated by the nucleation process or by domain-wall motion. The related magnetic aftereffect is analyzed starting from the Fatuzzo theory.

Binary nucleation of water–sulfuric acid system: Comparison of classical theories with different H2SO4 saturation vapor pressures

Abstract: The classical hydrates interaction model and the classical condensation model have been compared in the temperature range 153.15–363.15 K. Various expressions for the kinetic part of nucleation rate have been examined. In the present study the saturation vapor pressure values of sulfuric acid given by Ayers et al. are extrapolated to lower (stratospheric) temperatures taking into account the temperature dependence of the enthalpy of vaporization. In our model calculations we have compared the effect of three different expressions for saturation vapor pressures. The nucleation rate will differ by several orders of magnitude (depending on temperature, water and acid activity) when using different theories and/or saturation vapor pressures.

Ice nucleation by alcohols arranged in monolayers at the surface of water drops.

Abstract: Monolayers of aliphatic long-chain alcohols induced nucleation of ice at temperatures approaching 0 degrees C, in contrast with water-soluble alcohols, which are effective antifreeze agents. The corresponding fatty acids, or alcohols with bulky hydrophobic groups, induce freezing at temperatures as much as 12 degrees C lower. The freezing point induced by the amphiphilic alcohols was sensitive not only to surface area per molecule but, for the aliphatic series (C(n)H(2n + 1)OH), to chain length and parity. The freezing point for chains with n odd reached an asymptotic temperature of 0 degrees C for an upper value of n = 31; for n even the freezing point reached a plateau of -8 degrees C for n in the upper range of 22 to 30. The higher freezing point induced by the aliphatic alcohols is due to formation of ordered clusters in the uncompressed state as detected by grazing incidence synchrotron x-ray diffraction measurements. The diffraction data indicate a close lattice match with the ab layer of hexagonal ice.

Determining the nucleation rate curve for lithium disilicate glass by differential thermal analysis

Abstract: The crystallization of lithium disilicate (Li2O+2SiO2) glass nucleated at various temperatures was studied by differential thermal analysis (DTA). A plot of the DTA crystallization peak height versus nucleation temperature closely resembles the classical nucleation rate curve for lithium disilicate glass whose maximum is at 453 C. The glass becomes saturated with internal nuclei when heated at 453 C for 10 h. The activation energy for crystallization and the heat of crystallization are independent of the concentration of nuclei and are 249 + or - 10 and 67 + or - 3 kJ/mol, respectively. The Avrami exponent, n, depends strongly on the concentration of nuclei in the glass.

Textural constraints on the kinetics of crystallization of igneous rocks

Abstract: Natural systems provide time-controlled experiments on scales unreachable in the laboratory, and with complexities unapproachable by computer experiments. Since temperature (undercooling) cannot be directly scaled with time, in situ crystallization experiments provide information unavailable by other means. The kinetics of crystallization in simple silicate systems have been studied extensively for glass ceramic systems (e.g. Uhlmann, 1982; James, 1982), but predictive models for nucleation behavior, in particular, remain inadequate. Part of the problem in creating models of nucleation lies in the difficulty of experimentally isolating the nucleation process; crystal growth measurements are more accessible experimentally, and for this reason crystal growth is better understood (e.g. Dowty, 1980; Kirkpatrick, 1981; James, 1982; Baronnet, 1984). However, predictive models of crystal growth (e.g. Lasaga, 1982) require accurate models for the temperature and compositional dependence of both the thermodynamic and kinetic driving forces for crystallization, information that is currently limited. The CSE parameterization of the position of Ymax as a function of T, TL and T suggested by Dearnley (1983) provides a potentially useful model that requires further testing. Similarly, application of the adiabatic nucleation model of Meyer (1986) to simple systems suggests that the position of Tmax relative to TL has an approximately constant value of 0.55-0.60 (exceptions are low DSf minerals such as albite and quartz). Additionally, while experimental crystallization rate data is a necessary link between processes in natural melts and numerical models, experimental growth rate data is confined to simple systems and conditions of moderately large undercooling, while growth rate estimates in complex natural systems suggest that under most conditions, undercoolings remain very small. Quantitative textural studies of dikes suggest a pronounced dependence of growth rate on total crystallization time (e.g. Ikeda, 1977), which is in turn related to effective undercooling, a function of both total dike width and distance from the dike margin. By analogy, minimum crystallization rates may be estimated from programmed cooling experiments if a temperature interval of crystallization is assumed. Growth rates determined in this way show the same relationship between growth rate and total crystallization time seen in the dike studies, reflecting a systematic relationship between growth rate and cooling rate (effective undercooling) for plagioclase in basaltic systems. While the data are not available for other mineral systems, there is no reason to believe that similar patterns would not exist for other silicate minerals (i.e., the similarity in dendritic olivine growth rates estimated by Fowler et al.,1989, using fractal analysis to the growth rates of plagioclase under conditions of rapid cooling estimated from programmed cooling rate experiments). Analysis of measured crystal size frequencies provides a means of extracting effective (bulk) kinetic information from geologic systems that can help to constrain the dynamic feedback between physical models of magmatic systems and rates of chemical change.

Particle production associated with marine clouds

Abstract: Vertical profiles of Aitken nuclei, cloud condensation nuclei, sulfate, and SO2 concentrations have been measured in the marine atmosphere under relatively clean conditions. The vertical profiles of Aitken nucleus concentrations commonly showed maximum values just above cloud tops. We interpret these measurements as evidence for the nucleation of new sulfate particles in these locations. The cloud condensation nucleus profiles are consistent with this interpretation. A theoretical analysis of the measurements, utilizing a model of homogeneous, heteromolecular nucleation, predicts that the nucleation of new sulfate particles should occur primarily in the proximity of clouds. Implications of these results for the dimethyl sulfide-cloud-climate hypothesis are discussed.

An Exploratory Study of Ice Nucleation by Soot Aerosols

Abstract: The activities of nearly monodisperse soot particles as ice nuclei at temperatures below −20°C were examined in a short series of experiments. A continuous slow expansion cloud chamber was used to cause cloud formation and growth on soot during simulations of adiabatic cooling by expansion. Soot was generated using an acetylene burner operating near the sooting limit. Activity as ice nuclei was measured as clouds cooled to the apparent homogeneous-freezing temperatures of the cloud droplets. Immersion-freezing nucleation appears to be a particularly dominant heterogeneous mode for these particles. The preliminary results suggest that activity by immersion-freezing increases with particle size.

A discretized population balance for continuous systems at steady state

Abstract: This paper is concerned with the solution of the population balance for continuous systems at steady state, in which the active mechanisms are nucleation, growth and aggregation. A discretized population balance, initially proposed by Hounslow et al. (1988a) for batch systems, is adapted for use with continuous systems at steady state. It is shown that simultaneous nucleation and growth can be described very effectively by the discrete equations. Criteria are developed for the selection of the optimal size domain. A simple modification to the original discrete equations describing growth, permits the modelling of size-dependent growth effects. Both size-independent and size-dependent aggregation are described by the discrete equations with three significant-figure accuracy. The complete set of discrete equations is used to simulate the nucleation, growth and aggregation of Nickel Ammonium Sulphate. It is shown that analysis by the approximate model must lead to underestimation of the nucleation and growth rates.

Thermodynamic parallels between solid-state amorphization and melting

Abstract: A thermodynamics-based description, in the form of an extended phase diagram, of melting and solid-state amorphization is proposed which brings out the parallels between these two phenomena and suggests that their underlying causes are apparently the same. Through molecular dynamics simulations we demonstrate that every crystal, in principle, can undergo two different types of melting transitions with characteristic features that are also observed in radiation- and hydrogenation-induced amorphization experiments on ordered alloys. The first type, defined in terms of free energies, is shown to involve the heterogeneous nucleation of the liquid or amorphous phase at extended lattice defects (such as grain boundaries, free surfaces, voids, or dislocations) and subsequent thermally-activated propagation of solid-liquid/amorphous interfaces through the crystal. The second type, arising from a mechanical instability limit described by Born, is homogeneous and does not require thermally-activated atom mobility. It is suggested that the role of chemical and structural disordering, a prerequisite for irradiation- but not hydrogenation-induced solid-state amorphization, is merely to drive the crystal lattice to a critical combination of volume and temperature at which the amorphous phase can form either heterogeneously or homogeneously.

Low-Temperature Crystallization of Hydrogenated Amorphous Silicon Induced by Nickel Silicide Formation

Abstract: We have investigated the silicidation process of the Ni/a-Si:H system on a fused silica substrate mainly by in situ electric resistance measurement. A rise of the resistance is observed in the temperature range of 480–510°C of the resistance curve at a heating rate of 2 K/min. The results of the RBS measurements and the XRD measurements show that the rise of the resistance originates from the diffusion of Ni from the NiSi2 layer to a-Si:H film, and that the a-Si:H crystallizes during the diffusion. Noting that the crystallization temperature of a-Si:H film is higher than 700°C, the present crystallization occurs at a much lower temperature. It is suggested that this low-temperature crystallization is induced by heterogeneous nucleation, where the NiSi2 becomes the nucleus for Si crystallization.

Low-temperature epitaxial growth of thin metal films

Abstract: We present a different mechanism to explain the occurrence of long-lived oscillations in diffraction spot intensities during epitaxial growth of metal films on fcc (100) substrates at low temperature. Rather than rely on the common picture of cyclical nucleation and growth to produce the oscillations, the model invokes ``downward funneling'' deposition dynamics to fourfold-hollow adsorption sites.

Growth and Structure of Combustion Aerosols: Fumed Silica

Abstract: A model is presented for kinetic growth processes in flames, based on neutron, X-ray, and light-scattering data on commercial fumed silica powders. The model and data are consistent with the existence of fractally rough primary particles aggregated into highly ramified clusters. By mapping onto kinetic models, four processes are identified that determine the structure of the powders: kinetic nucleation, ballistic polymerization, sintering, and diffusion-limited aggregation.

Thermally induced structural modification of Mo‐Si multilayers

Abstract: The effect of elevated temperature on the structural stability and performance of Mo‐Si multilayer mirrors is investigated. Mo‐Si multilayers deposited by magnetron sputtering are annealed at temperatures ranging from 200 to 800 °C. A detailed and consistent picture of the thermally induced changes in the microstructure is obtained using an array of complementary measurement techniques including small‐ and large‐angle x‐ray scattering and high‐resolution electron microscopy. The first significant structural changes are observed at 400 °C, characterized by an increase in the width of the amorphous interlayer regions, as well as the nucleation of microcrystallites of silicide in these regions. At higher temperature the Mo layers transform completely into polycrystalline mixtures of Mo5 Si3 and MoSi2 in both the hexagonal and tetragonal phase. The layers of silicide remain intact but exhibit a structural instability, resulting in severely warped layers surrounded by pockets of amorphous Si and voids. By 800 °C the layered structure is completely destroyed and the composition is predominately tetragonal MoSi2 . The performance of the multilayers as normal‐incidence x‐ray mirrors is measured and correlated with the observed structural modifications. Finally, our results are compared and contrasted with other annealing studies of the Mo‐Si system.

Homogeneous nucleation in supercritical fluids

Abstract: When a supercritical solution is rapidly expanded, large solute supersaturations can be attained, and small particles are formed. The evolution of the homogeneous nucleation rate, work of nucleus formation, and critical nucleus size along different expansion paths is investigated here for the model system phenanthrene-carbon dioxide. Nucleation rates are the result of the competition among solvent expansion, cooling due to depressurization, and high supersaturation. Although supersaturations can reach very high values (> 106), relatively flat nucleation rate profiles result due to cooling and expansion. For an interfacial tension of 0.02 N/m, computed nucleation rates never exceed 104 s−1 · cm−3. A substantial fraction of the maximum nucleation rate is attained with partial decompression to pressures above 1 bar.

Quantum‐dot size‐distribution analysis and precipitation stages in semiconductor doped glasses

Abstract: The sequence of stages during precipitation of semiconductor (e.g., CdS, CdSe) clusters from supersaturated glasses exhibiting quantum‐confinement effects was investigated. The rate of formation of nanometer‐size ‘‘quantum dots’’ distributed in a continuous glass matrix is critically determined by the time and temperature of the heat treatment given to the quenched glasses. The entire precipitation process was analyzed in terms of several decomposition stages: nucleation, normal growth, coalescence of quantum dots, and devitrification of the glass matrix itself. Experimental data obtained by differential thermal analysis were utilized to identify the heat‐treatment temperature range associated with the precipitation stages. The size distribution of CdSe quantum‐dot clusters was analyzed using our transmission electron microscopy data. The data of Ekimov et al. [Solid State Commun. 56, 921 (1975)] was reduced to time‐temperature master plots useful for precipitating quantum dots of a given size in glasses.

High‐concentration boron diffusion in silicon: Simulation of the precipitation phenomena

Abstract: An extensive investigation on the diffusion of boron implanted at high concentration in preamorphized silicon has been carried out for rapid thermal annealing and conventional furnace annealing. The rapid process of epitaxial regrowth of the amorphous layer brings B atoms into substitutional positions, and thus electrically active, up to a concentration threshold of about 3.5×1020 cm−3. This value is nearly independent of the annealing temperature and of the implanted dose. Since the concentration of dopant in solution is higher than the solubility value, precipitation phenomena occur in concomitance to diffusion during the annealing. The kinetics of precipitation has been investigated at 800, 900, and 1000 °C with isothermal treatments ranging between 10 s and 171 h. A simulation program taking into account precipitation phenomena has been developed modifying the suprem iii code. The density of nuclei of the new phase has been evaluated on the base of the models of classic nucleation theory, while the ra...