Narrow Results

Nucleation in the two-dimensional q-state Potts model has been studied by means of Monte Carlo simulations using the heat-bath dynamics. The initial metastable state has been prepared by magnetic quench of the ordered low-temperature phase. The magnetic field dependence of the nucleation time has been measured as the function of the magnetic field for different q and lattice sizes at T = 0.5T_c. A size-dependent crossover from the coalescence to nucleation region is observed at all q. The magnetic field dependence of the nucleation time is roughly described by the classical nucleation theory. Our data show increase of the anisotropy in the shape of the critical droplets with increase of q.

We have analyzed the lifetimes of states by considering the relaxation trajectories following a quench. The average lifetimes, as a function of the quench depth into the two-phase region, shows a very sharp drop defining a limit of stability for metastable states. Using the limit of stability we define a line similar to a spinodal in the two-phase region.

We have investigated metastability and nucleation in the CO₂ system. To facilitate comparison with experiment and analytical theories, we have also calculated the coexistence curve. The metastability properties were computed using three methods to calculate the compressibility: droplet distribution, density fluctuations, and structure factor. These provide three measures for the time-dependent compressibility. If the compressibility showed a window in which it was quasi time-independent, for the observation period we categorized the quench to lead into a metastable state. Our fully atomistic simulation shows that the metastable region is extremely small. Already, very shallow quenches into the two-phase region have given rise to nonclassical behavior. The calculated pseudo spinodal is located far from the mean field expectation.

The combination of large eddy simulation (LES) and newly proposed Taylor-series expansion method of moments (TEMOM) is performed for simulating particulate matters emitted from vehicle engine tailpipe. The momentum, heat and mass transfer, binary homogeneous nucleation, Brownian coagulation, Brownian and turbulent diffusion, condensation and thermophoresis are simultaneously taken into account. Good agreements between the experimental and simulated results with respect to the pollutant dispersion are obtained. Compared to other published methods, the present TEMOM requires the least computational time with much accuracy for predicting nanoparticle dynamics. The instantaneous results show that large eddies dominate the evolution of particulate dynamics as exhaust develops, while binary homogeneous sulfuric-water nucleation mainly appears at the interface between the exhaust and ambient cool gases. The increasing of fuel sulfur content and relative humidity or the decreasing of environment temperature leads to an increase in particulate product rate, while volume-averaged particle diameter increases with increasing fuel sulfur content and environment temperature. The variation of geometric standard deviation suggests the nucleated particles eventually approach the asymptotic distribution in the dilution atmosphere, and this distribution is independent of the fuel sulfur content. The variance of upstream turbulence intensity significantly affects the evolution of particulate matters inside the plume.

The standard Ising model in two and three dimensions was simulated with Metropolis-Glauber kinetics on the Intel Hypercube with 32 MIMD processors of i860 type, each with 16 megabytes of distributed memory, and on 8 such processors of the Alliant FX with shared memory. The nucleation time in reverse magnetic fields h. was found to increase as 1/h² in three dimensions, for times up to millions of iterations. The relaxation towards the equilibrium spontaneous magnetization was a simple exponential in three dimensions and a stretched exponential in two below the critical temperature; its power-law decay at the critical point lead to z=2.18(d=2) and =2.09 (d=3). The Becker-Döring equation for the growth of microdroplets leads at the critical temperature to a growth rate vanishing with some power of the time.

Structural changes induced by temperature rising in binary silico-bismuthate glasses are investigated by differential thermal analysis. Several exothermic peaks are recorded for all samples. Progressive substitution of Bi₂O₃ by SiO₂ contributes to the structural relaxation of vitreous network and leads to diminishing of the melting temperature, even as SiO₂ content reaches 40 mol%. Glass stability is enhanced by addition of silicon dioxide.

We discuss models of earthquake faults and earthquake fault systems and what they tell us about real earthquake faults. We concentrate on the question of whether models and real earthquake fault systems are in equilibrium and whether the system being in equilibrium affects the accuracy of forecasting algorithms.

The non-isothermal polycrystalline solidification of a binary alloy is simulated by employing a phase field model which takes into account the heat transition and the random crystallographic orientation. The stochastic nucleation is taken into account in the simulation through the Poisson seeding algorithm and a kinetic calculation for binary melts based on the classical nucleation theory. Different microstructures are obtained under various cooling conditions. It is found that the grain structure becomes finer with increasing the cooling rate, which agrees with experimental result.

A growth model for co-deposition of Sn and In on Si(100) at room-temperature was simulated using Kinetic Monte Carlo methods to shed light on the chemical selectivity and lack of dimer ordering seen in [Jure et al., Appl. Surf. Sci.162, 638 (2000)], a Scanning Tunneling Microscopy (STM) study. In this work, the experimental observation that the number of mixed In–Sn dimers is unaffected even when the relative flux rates are adjusted to favor In over Sn (by 100:1) — a manifestation of some sort of chemical selectivity — was investigated. Our simulations reveal that this phenomenon is ultimately related to the fact that the number of Sn-terminated chains is largely unaffected by the relative flux rate. Finally, we found that the attraction between metal dimers (whether of the same or different species) within a Si dimer row has only negligible effect on the apparent lack of dimer ordering seen in the STM study. Instead, dimer ordering is controlled by the detachment barriers in dimer-terminated islands.

Colloidal particles in solution exhibit phase behavior analogous to atoms. In the last decades, colloids have been widely employed as modeling systems in studying nucleation, crystallization, glass transition and melting. A number of advances have been achieved. These advances have greatly extended the understanding of fundamental physical phenomena. In this paper, we give a brief summary on these advances.

The nucleation and growth behaviors of undoped and phosphorus doped polycrystalline Si thin films were investigated by in-situ TEM observations. Polycrystalline Si thin films were partially changed to amorphous by ion implantations. A normal grain growth was observed in the undoped Si thin films during heating. On the other hand, the P-doped sample showed the recovery and growth at grain boundary as well as the nucleation of Si nanocrystals at amorphous regions. Although the amorphous hindered the grain growth and acted as the nucleation source of Si nanocrystals at lower temperature, the final grain size of polycrystalline Si at 650°C was larger in the P-doped sample. The carrier mobility of the P-doped Si thin films not only increased with heat treatments, but also was corresponding to the microstructural evolution.

The process of conversion from $\beta$-Ga₂O₃ single crystal to gallium nitride (GaN) in an atmosphere of NH₃ by chemical vapor deposition is investigated. The surface morphology and microstructure of the GaN nanoparticles are observed by scanning electron microscope, which indicates that the growth of GaN is via the Volmer–Weber mechanism. The $\beta$-Ga₂O₃ is firstly evaporated at high temperature to form the porous layer, followed by the surface-defect induced GaN nucleation formation. The crystalline structure and epitaxial relationship of the GaN nanoparticles are investigated by X-ray diffraction (XRD) via $\omega$–$2𝜃$, showing GaN (0002) and (0004) diffraction peaks in the XRD spectra. It is concluded that the polycrystalline GaN film with hexagonal structure has a strong c-axis preferential orientation.

Colloidal crystals formed with attractive interactions are used for in-situ observations of homogeneous nucleation processes in density-matched dispersion. We found globular and plate-like polycrystalline clusters stably suspended in the dispersion. Both clusters rotated randomly and moved freely with suspending in the dispersion. They were probably formed through homogeneous nucleation processes.

The exact static solutions of the Landau-Lifshitz equations for the magnetization, which have been identified as describing nucleation centers for the magnetization reversal in ferromagnets, are compared with those of phenomenological models of spherical and cylindrical symmetry. It is found that the latter have higher critical fields.

This paper is devoted to the study of mathematical problems arising in the modelling of nonisothermal polymer crystallization under industrial conditions. A review of the hybrid stochastic-deterministic modelling procedure, which yields an averaged model,^5,8 is given and supplied by the study of mathematical problems arising from it.

Furthermore, multiple scale effects in the crystallization process are studied using different scalings, focussing either on the heat conduction or on the crystallization part. It is shown that in the limit of fast nucleation and slow growth, the model is equivalent to a direct extension of the classical Avrami–Kolmogorov model equations.

In addition, numerical simulations are presented to illustrate the behavior during crystallization. Finally, open questions from mathematical and engineering view points are discussed.

The nature of the zeolite 4A coatings prepared by using the substrate heating method was investigated. It was determined that the fractal dimensions of the surfaces of the coatings varied between about 1.14 and 1.19 with respect to the coating thickness in a mesh size range of about 1.5 μm - 48.5 μm. The fractal dimension value increased to some extent for the thicker zeolite coatings. It is hypothesized that the significance of nucleation is relatively high in the substrate heating method when compared to conventional synthesis methods and this leads to zeolite coatings with fractal surfaces and intercrystal porosity.

Co films deposited on Pt/Si(100) surface by electrodeposition from mixed sulphate chloride baths are investigated. The influence of applied potentials on nucleation, growth and morphological properties is studied using electrochemistry, scanning electronic microscopy (SEM) and atomic force microscopy (AFM) measurements. The electrochemical techniques show a deposition peak characteristic of diffusion-limited growth. For both applied potentials, we note that deposition occurs by instantaneous nucleation and diffusion-limited growth of 3D nucleation mechanism. Morphology observations reveal the granular structure of the electrodeposited Co layers surface with the root-mean-square roughness, R_q, varied between 7.80 and 8.95 nm. The average sizes of the Co crystallites depend strongly on the film thickness and applied potentials.

In the conventional Avrami–Kolmogorov–Johnson–Mehl model, the reaction or phase transition occurring in the 2D or 3D infinite medium is considered to start and proceed around randomly distributed and/or appearing nucleation centers. The radius of the regions transformed is assumed to linearly increase with time. The Monte Carlo simulations presented, illustrate what may happen if the transformation takes place in nanoparticles. The attention is focused on nucleation on the regular surface, edge and corner sites, and on the dependence of the activation energy for elementary reaction events on the local state of the sites.

Using modified analytic embedded atom method and molecular dynamics (MDs) simulation, the self-diffusion dynamics behaviors of Pd adatom on perfect Pd(110) and reconstruction Pd(110)-(1 × 2) surfaces have been studied. Our simulations show the diffusion of Pd adatom is 1D motion along the direction of the channel on Pd(110)-(1 × 2) surface between 650 and 900 K. On perfect Pd(110) surface, the adatom diffuses along the direction of the channel in the low temperature range from 450 to 550 K by the simple hopping mechanism, and the diffusion is 2D between 600 and 800 K, the diffusion across the direction of the channel wall is by the exchange mechanism with an atom of channel wall. The diffusion dynamics behaviors have been derived from the Arrhenius law. On perfect Pd(110) surface, the diffusion dynamics behaviors along the direction of the channel may be divided into two parts in different temperature ranges. Our results show that the diffusion mobility D of Pd adatom along the direction on perfect Pd(110) surface is quicker than that on Pd(110)-(1 × 2) surface.

In this work, a modified electrode of copper-polypyrrole (Cu-PPy) is elaborated by charging the polymer matrix by Cu particles. The response to ascorbic acid (AA) of this material was tested. The results show good performances. The results indicated a sensitive oxidation peak current of AA on the modified electrode. The success of this application led us to study the nucleation mechanism of copper on the PPy surface which permits to obtain this response. Nucleation and growth of electrodeposited copper onto PPy substrate have been studied in an aqueous solution of 0.01M CuCl₂ and 1M KCl by means of electrochemical methods: cyclic voltammetry, chronoamperometry, and using atomic force microscopy (AFM). The model described by Scharifker and Hills was used to analyze current-time transients. According to this model, the nucleation and growth kinetics at the initial stages of deposition process involve an instantaneous nucleation followed by three-dimensional (3D) growth mechanism.