Effect of grain size on the forces governing the dynamic behavior of electrostatically driven powder media.

The aim of this paper is to identify the forces that govern the dynamic behavior of polydisperse granular systems under high electric fields in air media. In a previous paper, we reported the changes in the morphology and dynamics of grain structures under a frequency and amplitude variable voltage and summarized them in a diagram [L. M. Salvatierra et al., Chem. Phys. Lett. 481, 194 (2009)]. In the present article, we investigate the influence of the grain size distribution and the speed of voltage increase during tests. We explain the main transitions observed in the diagram in terms of the change in grain size and the frequency-dependent electrical response, related to electrophoretic and dielectrophoretic interactions.

Field fluctuations and fractality in electrical breakdown trees.

In the present paper, the discharge avalanche model, initially proposed by Dissado and co-workers, is modified to explore (by correlating its results with experimental information) the role of the space-charge characteristics in the fractal nature of electric trees. Stochastic fluctuations of the local electrical field in each site of the polymer sample (E(loc)) around the Laplacian field (E(lap)) are allowed and limited to intermediate values of E(lap). The major achievements of the modified model are the prediction of a nonmonotonic behavior in both the average time to failure and the average fractal dimension of the electrical trees, and the prediction of a transition from branched to runaway trees as the applied voltage is increased.

Front waves in the NO + NH3 reaction on Pt{100}.

In the present work, we spatially extended a brand new kinetic mechanism of the NO + NH3 reaction on Pt{100} to simulate the experimentally observed spatiotemporal traveling waves. The kinetic mechanism developed by Irurzun, Mola, and Imbihl (IMI model) improves the former model developed by Lombardo, Fink, and Imbihl (LFI model) by replacing several elementary steps to take into account experimental evidence published since the LFI model appeared. The IMI model achieves a better agreement with the experimentally observed dependence of the oscillation period on temperature. In the present work, the IMI model is extended by considering Fickean diffusion and coupling via the gas phase. Traveling waves propagating across the surface are obtained at realistic values of temperature and partial pressure. A transition from amplitude to phase waves is observed, induced either by temperature or by the gas global coupling strength. The traveling waves simulated in the present work are not associated with fixed defects, in agreement with experimental evidence of spiral centers capable of moving on the surface. Also, the IMI model adequately predicts the presence of macroscopic oscillations in the partial pressures of the reactants coexisting with front wave patterns on the surface.

Scaling properties in the average number of attempts until saturation in random sequential adsorption processes.

In the present paper we investigate the exact average number of attempts until saturation when a square lattice is ceaselessly bombarded with beta-bell (beta> or =1) particles, i.e., linear particles that require beta consecutive lattice sites to be adsorbed. When that average number is normalized with the corresponding single-particle average, a scale invariant behavior is revealed with a scaling exponent alpha=0.017 +/- 0.001, independent of beta (beta>1). The scale behavior is suggested by the branching characteristics governing the sequential random adsorption of beta-bell (beta>1) particles, which is indeed a consequence of configurational correlations.

How many Langmuirs are required for monolayer formation?

In the present work, we provide the exact answer to the title question employing a probabilistic approach. The average number of Langmuirs L required for monolayer formation was found to be equal to (1/i), i.e., the armonic series up to the nth term, where n is the number of adsorption sites. This result is particularly useful when a reduced number of adsorption sites is considered, such as adsorption on small terraces of nanoscopic dimensions where the value of n could be in the range of a few thousands sites. In this case, the use of integrated equations derived from the mean-field approach would provide completely misleading results.

The structure of the chiral Pt531 surface: a combined LEED and DFT study.

The structure of the chiral kinked Pt531 surface has been determined by low-energy electron diffraction intensity-versus-energy (LEED-IV) analysis and density functional theory (DFT). Large contractions and expansions of the vertical interlayer distances with respect to the bulk-terminated surface geometry were found for the first six layers (LEED: d12 = 0.44 A, d23 = 0.69 A, d34 = 0.49 A, d45 = 0.95 A, d56 = 0.56 A; DFT: d12 = 0.51 A, d23 = 0.55 A, d34 = 0.74 A, d45 = 0.78 A, d56 = 0.63 A; dbulk = 0.66 A). Energy-dependent cancellations of LEED spots over unusually large energy ranges, up to 100 eV, can be explained by surface roughness and reproduced by applying a model involving 0.25 ML of vacancies and adatoms in the scattering calculations. The agreement between the results from LEED and DFT is not as good as in other cases, which could be due to this roughness of the real surface.

Dielectric breakdown model for conductor-loaded and insulator-loaded composite materials.

In the present work we generalize the dielectric breakdown model to describe dielectric breakdown patterns in both conductor-loaded and insulator-loaded composites. The present model is an extension of a previous one [F. Peruani et al., Phys. Rev. E 67, 066121 (2003)] presented by the authors to describe dielectric breakdown patterns in conductor-loaded composites. Particles are distributed at random in a matrix with a variable concentration p. The generalized model assigns different probabilities P(i,k-->i('),k(')) to breakdown channel formation according to particle characteristics. Dielectric breakdown patterns are characterized by their fractal dimension D and the parameters of the Weibull distribution. Studies are carried out as a function of the fraction of inhomogeneities, p.

Dielectric breakdown model for composite materials.

This paper addresses the problem of dielectric breakdown in composite materials. The dielectric breakdown model was generalized to describe dielectric breakdown patterns in conductor-loaded composites. Conducting particles are distributed at random in the insulating matrix, and the dielectric breakdown propagates according to new rules to take into account electrical properties and particle size. Dielectric breakdown patterns are characterized by their fractal dimension D and the parameters of the Weibull distribution. Studies are carried out as a function of the fraction of conducting inhomogeneities, p. The fractal dimension D of electrical trees approaches the fractal dimension of a percolation cluster when the fraction of conducting particles approximates the percolation limit.

Fractal analysis of electrical trees in a cross-linked synthetic resin.

A statistical picture of dielectric breakdown in cross-linked polyester resins for a two-dimensional geometry is presented and discussed in this paper. A connection is established between the dielectric breakdown model (DBM) and the physical properties of the resin. Distribution propagation times of simulated trees obey a Weibull statistics, as was experimentally found. This adjustment is achieved by a redefinition of the unit of time, which is different from the one employed up to date. The experimental dependence of characteristic propagation times on the fractal dimension D can be reproduced in the range 1.2

Theoretical model of diatomic molecules interacting on a two-dimensional lattice.

The coverage-dependent binding energy of interacting dimers is investigated using the lattice gas model. Dimers with distinguishable ends are assumed to interact if they occupy only nearest-neighbor lattice sites. The interaction energy between a pair of dimers will be repulsive if identical ends are in nearest-neighbor positions and attractive if they are not. The grand partition function is derived by using the Bethe approximation. The energy of the system of interacting dimers is evaluated as a function of the lattice coverage at different temperatures and pair interaction potentials. Under the same conditions the average number of all types of nearest-neighbors is evaluated.