Coarsening and pearling instabilities in silver nanofractal aggregates

We investigate the morphological changes of 3D supported fractal aggregates generated through the deposition of silver clusters on graphite. The fractal relaxation, activated after their formation by perturbing them either by thermal annealing or by using a surfactant, as oxide molecules, carried by silver clusters in a subsequent deposition, shows evidence of two distinct fragmentation patterns. The post coarsening, driven by thermal heating, gives a broad asymmetrical distribution of fragments in agreement with a random process, whereas the entire silver fractal pearling fragmentation is driven by chemical adjunction of the surfactant.     

Thermal and chemical nanofractal relaxation

We studied shape relaxation of nano-fractal islands, during annealing, after their growth from antimony cluster deposition on graphite surface. Annealing at 180^°C shows evidence of an increase of the fractal branch width with time followed by branch fragmentation, without changing the fractal dimension. The time evolution of the width of the arm suggests the surface self-diffusion mechanism as the main relaxation process. With Monte Carlo simulations, we confirmed the observed behavior. Comparison is done with our previous results on fragmentation of nano-fractal silver islands when impurity added to the incident cluster promotes rapid fragmentation by surface self-diffusion enhancement [1].     

The Fragmentation Fractal – a useful descriptor of broken rock

The fragmentation fractal has been used to accurately describe the distribution of rock progeny from single particle breakage events. This paper illustrates the underlying physical significance of the fragmentation fractal and how it may be used as a fundamental tool in understanding rock pulverisation. The experimental determination of the fragmentation fractal is based on the measurement of particle size by project area as given by image analysis. In this format the characteristic length is defined as the square root of projected area. This approach to rock particle size analysis can have advantages over other techniques especially when the mass or number of fragments is small. A brief comparison has been made with size by projected area and the corresponding sieve size of artificial objects of different shape, but equivalent characteristic length. It is suggested that projected area measurements are less sensitive to the influence of particle shape and that this may even aid the comparison of distributions of fragments of significantly different morphologies.     

Formation and magnetic properties of fractal aggregates of cobalt

Macroscopic fractal aggregates of cobalt are obtained by thermal evaporation of cobalt metal in an argon atmosphere and subsequent deposition on a silicon substrate heated to 1000 K. It is established that the fractal structure is formed by diffusion-limited aggregation of cobalt particles. The macroscopic fractal cobalt aggregates are ferromagnetic. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 556–558 (25 October 1997)     

Fractal dimensions of speech sounds: computation and application to automatic speech recognition

The dynamics of airflow during speech production may often result in some small or large degree of turbulence. In this paper, the geometry of speech turbulence as reflected in the fragmentation of the time signal is quantified by using fractal models. An efficient algorithm for estimating the short-time fractal dimension of speech signals based on multiscale morphological filtering is described, and its potential for speech segmentation and phonetic classification discussed. Also reported are experimental results on using the short-time fractal dimension of speech signals at multiple scales as additional features in an automatic speech-recognition system using hidden Markov models, which provide a modest improvement in speech-recognition performance.     

Fractal substructure of a nanopowder

The structural evolution of a nanopowder by repeated dispersion and settling can lead to characteristic fractal substructures. This is shown by numerical simulations of a two-dimensional model agglomerate of adhesive rigid particles. The agglomerate is cut into fragments of a characteristic size l, which then are settling under gravity. Repeating this procedure converges to a loosely packed structure, the properties of which are investigated: (a) The final packing density is independent of the initialization, (b) the short-range correlation function is independent of the fragment size, (c) the structure is fractal up to the fragmentation scale l with a fractal dimension close to 1.7, and (d) the relaxation time increases linearly with l.     

Kinetic Simulation of Fractal Aggregation on Liquid Surfaces

A modified fractal growth model based on the deposition, diffusion, and aggregation (DDA) with cluster rotation is presented to simulate two-dimensional fractal aggregation on liquid surfaces. The mobility (including diffusion and rotation) of clusters is related to its mass, which is given by D_m=D₀ s^-γ_D and θ_m=θ₀s^-γ_θ, respectively. We concentrate on revealing the details of the influence of deposition flux F, cluster diffusion factor γ_D and cluster rotation factor γ_θ on the dynamics of fractal aggregation on liquid surfaces. It is shown that the morphologies of clusters and values of cluster density and fractal dimension depend dramatically on the deposition flux and migration factors of clusters.     

Landscape structure and the speed of adaptation

The role of fragmentation in the adaptive process is addressed. We investigate how landscape structure affects the speed of adaptation in a spatially structured population model. As models of fragmented landscapes, here we simulate the percolation maps and the fractal landscapes. In the latter the degree of spatial autocorrelation can be suited. We verified that fragmentation can effectively affect the adaptive process. The examination of the fixation rates and speed of adaptation discloses the dichotomy exhibited by percolation maps and fractal landscapes. In the latter, there is a smooth change in the pace of the adaptation process, as the landscapes become more aggregated higher fixation rates and speed of adaptation are obtained. On the other hand, in random percolation the geometry of the percolating cluster matters. Thus, the scenario depends on whether the system is below or above the percolation threshold.     

Correlation of optical properties with the fractal microstructure of black molybdenum coatings

Coating is commonly used for improving the optical properties of surfaces for solar collector applications. The coating morphology depends on the deposition conditions, and this determines the final optical characteristics. Coating morphologies are irregular and of fractal nature, so a suitable approach for its characterization should use methods borrowed from fractal analysis. The aim of this work is to study the fractal characteristics of black molybdenum coatings on copper and to relate the fractal parameters to the optical properties. To this end, coating surfaces were prepared via immersion in a solution of ammonium paramolybdate for different deposition periods. The fractal analysis was carried out for SEM and AFM images of the coating surface and the fractal properties were obtained with a recently developed high-dimensional extension of the well-known detrended fluctuation analysis (DFA). The most salient parameter drawn from the application of the DFA is the Hurst index, a parameter related to the roughness of the coating surface, and the multifractality index, which is related to the non-linearity features of the coating morphology. The results showed that optical properties, including absorptance and emittance, are decreasing functions of the Hurst and multifractality indices. This suggests that coating surfaces with high absorptance and emittance values are related to complex coating morphologies conformed within a non-linear structure.     

Observation of fractal nanoclusters on the pulsed laser deposition of gold

The results of studying the fractal structure of Au clusters arising in pulsed laser deposition under conditions of strong deviations from thermodynamic equilibrium are presented. A mechanism is suggested in which the formation of fractal nanoclusters is considered as a result of the decay of a strongly correlated initial state in a non-equilibrium system of adatoms. The geometrical shape of the fractal structures observed experimentally can be described within the given mechanism, and their dimensionality can be estimated.     

二维颗粒破碎过程的计算机模拟

以材料力学为基础构建单颗粒破碎模型,通过分子动力学方法模拟二维颗粒的破碎过程.结果表明:最终的颗粒大小分布为幂律分布,其分形维数为1.1,与实验结果相符,表明颗粒的破碎是一分形过程.     

溅射过程中粒子能量对钛薄膜表面形貌影响

借助于原子力显微镜研究了离子束溅射沉积工艺中入射离子能量对制备的Ti薄膜表面形貌的影响。对薄膜表面高度数据进行相关运算，发现在此工艺条件下制备的薄膜具有典型的分形特征，利用分形表面高度—高度相关函数的唯象表达形式对不同能量下制备Ti薄膜表面的高度相关函数进行拟合。得到了薄膜表面的分形维数、水平相关长度、标准偏差粗糙度等参量。研究发现，入射Ar离子能量在300—700eV之间薄膜表面的粗糙度随着沉积粒子的能量增加而增大，分形维数随着入射离子能量的增加而减少。另外，在得到的分形维数基础上对不同溅射电压下Ti薄膜的生长机制进行了初步研究。     

Studies of the collision frequency of ideal gas particles with the surfaces of the objects created using the ballistic deposition technique

This paper is a continuation of our studies of the collision frequency of ideal gas particles with the rough/fractal surfaces. Here, we applied a more realistic surface growth model, i.e. ballistic deposition for creation of fractal objects. We found that the collision frequency with irregular surfaces is the linear function of pressure and this frequency per unit pressure is quite a complicated function of the surface fractal dimension as well as the diameter of colliding particle. The collision frequency with rough surfaces cannot be exactly described by the analytical formula called the Langmuir-Hertz equation. However, we have stated that the deviations of the true collision frequency from the Langmuir-Hertz prediction are not huge and in typical catalytic studies the error introduced by replacing the true frequency by the Langmuir-Hertz prediction can be safely neglected. We have also studied the probability of finding on the surface an atom which has been hit a certain number of times by a gas particle. This probability reveals an interesting behaviour for small gas particles, i.e. it perfectly correlates with the number of directions from which the surface atom is accessible from the gas phase. We have also estimated the evolution of the adsorption energy distribution with the increasing fractal dimension of the surface in the ballistic deposition.     

Application of Mie theory and fractal models to determine the optical and surface roughness of Ag–Cu thin films

Thin films of Ag/Cu were deposited by reactive DC magnetron sputtering on (001)-oriented Si and glass substrates for various deposition times (4–24 min). These films were characterized by atomic force microscopy (AFM), and a power law scaling was performed on the obtained micrographs to investigate the self-affine nature of the sample morphology, which is indicative of a fractal structure. We applied the Higuchi’s algorithm to the AFM data to determine the fractal dimension of each sample, and the Hurst exponents were computed. The deposition time dependences of these parameters and the grain size distributions estimated from the UV–visible spectra using the Mie theory, allowed us to describe a particle formation mechanism during the deposition process, in which the length of continuous paths of conductive particles increases as the deposition time is increased. In agreement with this explanation, the electrical resistance decreased with the increment of the deposition time.     

Ordering in an ensemble of nanometer-size Au clusters on a NaCl(100) surface at high deposition rates

The results of experimental studies on pulsed laser deposition of the Au atoms on the NaCl(100) surface are reported. Under the conditions far from thermodynamic equilibrium, the spatial ordering was observed in an ensemble of gold clusters at the initial deposition stage. A mechanism based on the idea of fractal structure decomposition caused by the development of unstable concentration modes in the condensed substance is proposed.     

Adsorption of interacting monomers on spanning clusters of polyatomic species

The adsorption-desorption process occurring on heterogeneous surfaces is studied by considering a special case where a fractal is used as adsorbent. The fractal surface is the spanning cluster corresponding to the random deposition of objects that occupy more than one site (k-mers) on a square lattice. Such a surface is characterized according to the deposited k-mer. Then, the adsorption of repulsively interacting particles adsorbed on the fractal surface is studied by using Monte Carlo simulations. Different thermodynamic quantities (adsorption isotherms, coverage susceptibility, etc.) are calculated and explained in terms of the characteristics of the substrate. A scheme to characterize the structure of the substrate by just considering the adsorption isotherm is presented and discussed.     

Atomic force microscopy investigation of growth process of organic TCNQ aggregates on SiO2 and mica substrates

<正>Deposition patterns of tetracyanoquinodimethane(TCNQ) molecules on different surfaces are investigated by atomic force microscopy.A homemade physical vapour deposition system allows the better control of molecule deposition. Taking advantage of this system,we investigate TCNQ thin film growth on both SiO_2 and mica surfaces.It is found that dense island patterns form at a high deposition rate,and a unique seahorse-like pattern forms at a low deposition rate.Growth patterns on different substrates suggest that the fractal pattern formation is dominated by molecule-molecule interaction.Finally,a phenomenal "two-branch" model is proposed to simulate the growth process of the seahorse pattern.     

Effect of dynamic fragmentation on biodiversity in a heterogeneous environment

We study how the mode of dynamic habitat loss and fragmentation can impact the levels of biodiversity in an ecosystem. The problem is formulated into the framework of resource-based modelling and, opposed to previous studies relying on neutral models, here elements of niche theory are incorporated into the modeling by considering that the species are not ecologically equivalent. In our model the habitat loss is carried out by using a fractal landscape that is constructed through the use of fractional Brownian motion. Thus we can tune the roughness of the landscape by changing the Hurst exponent. We show that both the mode of habitat loss and the level of environmental heterogeneity influence the patterns of species distribution. We notice a larger impact of fragmentation on the number of species when the fragments are more compact. We observe that the relationship between biodiversity and heterogeneity is described by a one-humped function.