Kinetic Monte Carlo simulations of Pd deposition and island growth on MgO(1 0 0)

The deposition and ripening of Pd atoms on the MgO(1 0 0) surface are modeled using kinetic Monte Carlo simulations. The density of Pd islands is obtained by simulating the deposition of 0.1 ML in 3 min. Two sets of kinetic parameters are tested and compared with experiment over a 200-800 K temperature range. One model is based upon parameters obtained by fitting rate equations to experimental data and assuming the Pd monomer is the only diffusing species. The other is based upon transition rates obtained from density functional theory calculations which show that small Pd clusters are also mobile. In both models, oxygen vacancy defects on the MgO surface provide strong traps for Pd monomers and serve as nucleation sites for islands. Kinetic Monte Carlo simulations show that both models reproduce the experimentally observed island density versus temperature, despite large differences in the energetics and different diffusion mechanisms. The low temperature Pd island formation at defects is attributed to fast monomer diffusion to defects in the rate-equation-based model, whereas in the DFT-based model, small clusters form already on terraces and diffuse to defects. In the DFT-based model, the strong dimer and trimer binding energies at charged oxygen vacancy defects prevent island ripening below the experimentally observed onset temperature of 600 K.     

On First-order Corrections to the LSW Theory I: Infinite Systems

We present a new method to efficiently identify the first-order correction to the classical model by Lifshitz, Slyozov and Wagner (LSW). The latter describes the evolution of second phase particles embedded in a matrix during the last stage of a phase transformation and is valid in the regime of vanishing volume fraction of particles. We consider a statistically homogeneous (and thus infinite) system, where the first-order correction is of order ^1/2. The key idea is to relate the full system of particles to systems where a finite number of particles has been removed. This method decouples screening and correlation effects and allows to efficiently evaluate conditional expected values of the particle growth rates.     

Ostwald ripening in immiscible polyolefin blends

The coarsening in the quiescent melt of the phase-segregated particles of a polymer blend, composed of a narrow molecular weight fraction of an unbranched high-density polyethylene (HDPE) and a highly branched (100 ethyl branches/1000 C atoms) hydrogenated polybutadiene (HPB) was studied. The system was effectively binary, due to the narrow molecular weight and composition distributions of each component. The system was composed of 90 wt % of the HDPE and 10 wt % of the HPB and it formed a two-phase system in the melt at 177°C. The blend was precipitated from xylene solution in order to obtain an initially intimately mixed system. This was the third study in a series of studies of the coarsening of phase-segregated particles in polymer blends. This study was unique in that the system studied was binary in this case while the previous systems were multicomponent. Since the present system was binary, exact thermodynamic calculations of the phase state of this system could be applied with a high level of confidence. The droplet phase particles, which were mainly composed of the HPB, were observed to coarsen on storage in the melt for times of from 5 s to 1 h. At the shortest storage time of 5 s the particles had an average radius of about 0.05 μm and coarsened to about 0.2 μm after 1 h storage in the melt state. Particle dimensions were measured by scanning electron microscopy of n-heptane-etched and gold-coated sections. It was found that the volume of the particles increased linearly with time and that the rate constant of coarsening was K_exp = 1.23 × 10^?18 cm³/s and this agreed fairly well with the rate constant calculated from Ostwald ripening theory of K_ce = 0.86 × 10^?18 cm³/s. In contrast the rate constant for direct particle diffusion and coalescence was K_c = 3.6 × 10^?20 cm³/s. Since this was two orders of magnitude smaller than the rate constant for Ostwald ripening, it was concluded that, although the linear increase of volume with time was also consistent with the particle diffusion and coalescence mechanism, this was not a significant contributor to the coarsening mechanism. The major cause for the insignificance of the particle diffusion and coalescence mechanism was the high melt viscosity of the matrix polymers. The application of the Ostwald ripening theory to this system could be made with a high level of confidence because it was binary. It was found that the phase concentration of the droplet phase apparently underwent a rapid increase during the first 1-2 min of storage in the melt, indicating that the system did not reach phase equilibrium (i.e., did not completely phase-segregate) for about 1-2 min. This further indicated that the long-time coarsening regime was not entered until after this length of time. The particle size distributions remained approximately self-similar over the period of coarsening, as predicted by Ostwald ripening theory. © 1995 John Wiley & Sons, Inc.     

Structural and optical characteristics of CdxZn1−x S nanoparticles stabilized in aqueous solutions of polymers

The structural and optical characteristics of Cd_xZn_1−xS nanoparticles, produced in aqueous solutions of sodium polyphosphate, polyvinyl alcohol, and gelatin, were studied. It was shown that the stoichiometric ratio of Cd(II) and Zn(II) in the nanoparticles can differ from that in the initial mixture. It was established that the main factors determining the final size of the Cd_xZn_1−xS nanoparticles are the nature and concentration of the polymer, the ratio of the initial concentrations of the reagents, and the temperature and duration of the post-synthesis treatment of the colloids. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 3, pp. 168–172, May–June, 2006.     

Humidity‐induced surface modification of boric acid

Previous studies of boric acid powder suggest that the surface undergoes restructuring when exposed to changes in relative humidity of the surrounding air. This present work investigates these surface changes using atomic force microscopy (AFM) to provide quantitative topographic information, supplemented by gravimetric analysis of water sorption. The AFM images clearly show the dissolution of small rounded features and z‐height growth of the larger flatter areas, indicating that Ostwald ripening was the prevailing mechanism in the restructuring. Estimation of radii of curvature of asperities on the surface enabled the approximation of supersaturation ratio in the surface film using the Kelvin equation. The value of 1.007 obtained is much less than the value of 3.2 reported for solution‐grown boric acid. It is this high level of supersaturation that is thought to account for the inherent roughness of boric acid, associated with dendritic growth. Conversely, the low supersaturation ratio estimated for the adsorbed surface layer results in regular crystal growth and a decrease in surface roughness. Gravimetric analysis of boric acid revealed a steady loss in water with increase in relative humidity from 10% to 60%. This is attributed to loss of available surface area through the progressive dissolution of fine surface features. Desorption showed a monotonic decrease in water uptake, confirming that the restructuring is not reversible. Changes in surface area were estimated from the AFM images. The loss of surface area between dry and ambient conditions was greater than that inferred from the loss of mass in the gravimetric analysis. This suggests that water is retained on the surface in capillaries that are not apparent in the AFM surface scans. Statistical monolayer coverage of water is achieved at 10% relative humidity, which corresponds to the onset of surface restructuring indicated by the water sorption isotherm. Copyright © 2004 John Wiley & Sons, Ltd.     

Examination of the correlation of butter spreadability and its fat conformation by DSC

Since the appearance of margarines and spreads on the market, they have been serious competitors with butter. One of the reasons for this was the false nutritional propaganda, but today butterfat has scientifically regained its actual nutritional evaluation. The main disadvantage of butter in comparison to other spreadable tallow is that it does not immediately spread as well when taken out of the refrigerator. One method of obtaining better cold-spreadability is appropriate cream ripening in which a different system known as the corpuscular colloid is created. Recent examinations were conducted during the winter, when the problem of spreadability of butter is the greatest. Simple cold ripening in accordance with the method used in our earlier EPR studies ripened the cream, and by heat-step ripening, then butter was produced from these materials. By deconvolutional analysis of the DSC curves it was established that butter made from the cream ripened by the heat-step method had three characteristic melting peaks as distinct from the two melting peaks of butter made from cold-ripened cream, and the temperature of the second melting peak for butter from heat-step cream was identical to the characteristic melting temperature for fat particles from earlier EPR spectroscope assays. In sum it can be stated that the DSC method clearly shows both the homogeneous and the particle structure characteristic of butter.     

Crystal Growth Kinetics of Nanocrystalline ZnS under Surface Adsorption

The crystal growth mechanism,kinetics,and microstructure development play a fundamental role in tailoring the materials with controllable size and morphology. In this study,by introducing the strong surface adsorption of the concentrated NaOH,two-stage crystal growth kinetics of ZnS nanoparticles was observed. In the first stage,the primary particles grow into a size over a hundred times of the original volume and the growth is controlled by the crystal-lographically specific oriented attachment. The first stage data were fitted by the "multistep OA kinetic model" built based on the molecular collision and reaction. In the second stage,following the dispersal of nanoparticles,an abrupt transition from asymptotic to parabola growth kinetics occurs,which can be fitted by a standard Ostwald ripening volume diffusion model. The presence of surface adsorption causes the two-stage growth kinetics and permits an almost exclusive OA-based growth to dominate in the first stage.     

A new modified low-energy emulsification method for preparation of water-in-diesel fuel nanoemulsion as alternative fuel

In this research, 24 of water-in-diesel fuel nanoemulsions were prepared using mixed nonionic surfactants of sorbitan monooleate and polyoxyethylene sorbitan trioleate (MTS). The emulsions were formed using a new modified low-energy method at hydrophilic-lipophilic balance (HLB) value of 10 and a working temperature of 20°C. Five HLB values of 9.6, 9.8, 10, 10.2, and 10.4 were prepared to identify the optimum value that gives low water droplet size at working conditions as: 5 wt% of water contents, 10 wt% of mixed surfactant concentration, and a temperature of 20°C. The effect of mixed surfactant concentration and water content on the droplet size for 0, 15, 30, 60, and 90 days has been studied. Droplet size of the prepared nanoemulsions was determined by dynamic light scattering and the nanoemulsion stability was assessed by measuring the variation of the droplet size as a function of time. Results show that the mean droplet sizes were formed between 26.23 and 277.1 nm depending on the surfactant concentrations, water contents, and storage time.     

ATOMIC FORCE MICROSCOPY STUDIES AND MODELING OF SURFACE STRUCTURES PATTERNED DURING OSTWALD RIPENING AT Fe/Mo MULTILAYER SYSTEMS

Fe/Mo multilayers which were grown by sputtering, annealed in high vacuum and analyzed by means of Atomic Force Microscopy and Scanning Electron Microscopy. Concentric circle-1ike patterns were observed after annealing. Two-dimensional Ostwald ripening mechanisms in immiscible systems of Fe/Mo may explain the formation of these structures. We simulated pattern formation in a late stage of the phase separation by applying the Thomas-Freundlich thermodynamic relation. Based on a two-dimensional mode1 in the framework of the Lifshitz-Slyozov theory, our modeling has been extended to include the diffusion limitation in a multi-cluster system.     

A diffusing wave spectroscopy study of the kinetics of Ostwald ripening in protein-stabilised oil/water emulsions

Diffusing wave spectroscopy (DWS) has been used to study the kinetics of ageing processes in n-decane/water emulsions, stabilised by the milk protein _S1-casein. A particular advantage of this particle size measuring technique is its direct applicability to the concentrated emulsion, avoiding the necessity for dilution to single scattering levels demanded by traditional dynamic light scattering methods. The observed droplet growth rates were found to conform to the kinetic law of Ostwald ripening. Monotonic increases in coarsening rates have been observed with increasing oil volume fraction and these results are compared with theoretical predictions. The effect of excess protein has also been measured but no evidence for depletion flocculation due to free _S1-casein was detectable.