Conditions Of Cluster Formation Research Articles

Dimensional Transformation of Percolation Structure in Mixed-Matrix Membranes (MMMs).

A large number of studies of mixed-matrix membranes (MMMs) have confirmed the possibility of obtaining new materials with unique transport properties, including for solving specific problems in the separation of mixtures of liquids and gases. The choice of particles with a given affinity for the matrix and separable components allows researchers to adjust the selective properties of MMMs in a wide range, which changes the properties of MMMs in a wide range. However, even within the framework of the most complex percolation mechanism of the formation of the MMM structure, it is possible to explain only some of the observed effects. In particular, questions about the required particle concentration and fluctuation of properties in various MMM samples are still the subject of research. The results of the numerical modeling of such structures presented in this paper determined the possible causes of the observed deviations of the experimental results, for example, particle size dispersion, agglomeration, and interaction with the matrix. According to our research, the key factor that qualitatively changes the parameters of percolation structures is the ratio of the geometric dimensions of the system. We have confirmed in a wide range a significant change in the conditions of cluster formation and its power at different particle diameters and lengths (traditional parameters in percolation studies). But in our work, we additionally studied the effect on the cluster parameters of the interfacial layer and the anisotropy of the matrix (the transition from the cube to the film). The results obtained show that changing the parameters of the matrix-particle interaction affects agglomeration, and the degradation of the percolation structure is possible. That is, with an increase in concentration, the parameters of the percolation cluster, its power, and the probability of formation, may decrease. But even more negative changes in percolation structures are observed during the transition from a volumetric matrix to films. The anisotropy of space leads to the formation of percolation through the film in certain areas at low concentrations of particles. At the same time, in a significant part of the matrix, percolation between the film surfaces will be absent, and the effect of changing the properties of MMMs in the matrix as a whole decreases. Our study explains the observed instability of MMM properties at fixed concentrations and parameters of embedded particles, including the effect of reducing the influence of particles with increasing concentration.

Ejection velocities, age, and formation process of SPE meteoroid cluster

Context. Meteoroid clusters represent a unique opportunity to study processes of meteoroid disruptions in interplanetary space. We follow our previous work about the September epsilon Perseid (SPE) meteoroid cluster from 2016 with a detailed analysis of the observed data and cluster formation conditions. Aims. Our goal is to determine ejection velocities of the cluster members and SPE’s age, as well as to estimate the most probable formation process. Methods. We precisely determined mutual positions and masses of all meteoroids including the errors. We assumed that the massdominated meteoroid is the parent body of the cluster and that the observed positions of meteoroids are controlled by the ejection velocities and the action of solar radiation pressure. A formula for the dependence of meteoroid ejection velocities on the mutual positions, masses, and cluster age was derived. It was assumed that the time at which the initial kinetic energy of all meteoroids reached a minimum value corresponds to the age of the cluster. Knowing values and directions of ejection velocities together with meteoroid masses then allowed us to determine the most likely process of cluster formation. Results. The meteoroids occupy a volume of 66 × 67 × 50 km and are shifted in the antisolar direction by 27 km relative to the parent meteoroid. The age of the cluster is 2.28 ± 0.44 days. The ejection velocities range from 0.13 ± 0.05 m s−1 to 0.77 ± 0.34 m s−1 with a mean value of 0.35 m s−1. The ejection velocity directions are inside the cone with an apex angle of 101 ± 5°. The axis of the cone is ~45° away from the solar direction and ~34° away from the mean direction of the flux of small meteoroids’ incident on the parent meteoroid. Formation due to the separation of part of the surface due to very fast rotation is the least likely thing to occur. We estimate the rotation frequency to be about 2 Hz and the corresponding stress is several orders of magnitude lower than the predicted strength limit. It is also difficult to explain the formation of the cluster by an impact of a small meteoroid on the parent body. However, this possibility, although not very likely, cannot be completely ruled out. The most probable process is the exfoliation due to thermal stresses. Their estimated magnitude is sufficient and the derived ejection velocities are consistent with this process of formation.

Stability of polycyclic aromatic hydrocarbon clusters in protoplanetary discs

Context. The infrared signature of polycyclic aromatic hydrocarbons (PAHs) is present in many protostellar discs, and these species are thought to play an important role in the heating of the gas in the photosphere. Aims. We consider PAH cluster formation as one possible cause for non-detections of PAH features in protoplanetary discs. We test the necessary conditions for cluster formation and cluster dissociation by stellar optical and far-UV photons in protoplanetary discs using a Herbig Ae/Be and a T Tauri star disc model. Methods. We perform Monte Carlo and statistical calculations to determine dissociation rates for coronene, circumcoronene, and circumcoronene clusters with sizes of between 2 and 200 cluster members. By applying general disc models to our Herbig Ae/Be and T Tauri star model, we estimate the formation rate of PAH dimers and compare these with the dissociation rates. Results. We show that the formation of PAH dimers can take place in the inner 100 AU of protoplanetary discs in sub-photospheric layers. Dimer formation takes seconds to years, allowing them to grow beyond dimer size in a short time. We further demonstrate that PAH clusters increase their stability while they grow when they are located beyond a critical distance that depends on stellar properties and PAH species. The comparison with the local vertical mixing timescale allows a determination of the minimum cluster size necessary for the survival of PAH clusters. Conclusions. Considering the PAH cluster formation sites, cluster survival in the photosphere of the inner disc of Herbig stars is unlikely because of the high UV radiation. For the T Tauri stars, survival of coronene, circumcoronene, and circumcircumcoronene clusters is possible, and cluster formation should be considered as one possible explanation for low PAH detection rates in T Tauri star discs.

The large clusters influence on the supersonic flows gas dynamics in a rarefied medium

The processes of Van der Waals clusters formation in supersonic gas flows expanding into a rarefied medium are discussed in this report. The necessary conditions for cluster formation, methods for measuring the average size and size distribution of clusters in a rarefied gas flow are considered. The problems associated with electron-beam ionization of clusters during mass spectrometry of clustered flows are analyzed. The process of intracluster energy exchange in mixtures, that leads to the possibility of inverse pumping of electronic levels of atoms, is illustrated. The problems arising during optical measurements of density in cluster flows are demonstrated. The effect of large clusters formation on the shape and structure of supersonic flows is shown.

On the similarity of traditional and cluster jets during supersonic gas outflow into flooded space

Annotation. The visualization of gas flows expanding from supersonic nozzles into flooded space under the cluster formation conditions is considered. The technique of exciting the supersonic flow by a well-focused high-voltage electron beam is applied in conjunction with the scanning equipment for the gas objects. The reasons for the secondary cluster jet formation in a flow of easily condensing gases are discussed. A comparison is made between the formation of a traditional spindle-shaped jet and a secondary cluster one. The measurement results of the diameters of the traditional and cluster jets in maximum sections, obtained by the photometry method, are presented. It is shown that for the characteristic sizes of cluster jets of condensing gases, the dependence on parameters proposed by Sherman and Ashkenas is applicable in the entire range of the studied stagnation and background pressures. The similarity of the formation processes of traditional monomeric jets and secondary cluster jets under the conditions of developed condensation of the initial supersonic flow is supposed.

How atomic nuclei cluster

Nucleonic matter displays a quantum-liquid structure, but in some cases finite nuclei behave like molecules composed of clusters of protons and neutrons. Clustering is a recurrent feature in light nuclei, from beryllium to nickel. Cluster structures are typically observed as excited states close to the corresponding decay threshold; the origin of this phenomenon lies in the effective nuclear interaction, but the detailed mechanism of clustering in nuclei has not yet been fully understood. Here we use the theoretical framework of energy-density functionals, encompassing both cluster and quantum liquid-drop aspects of nuclei, to show that conditions for cluster formation can in part be traced back to the depth of the confining nuclear potential. For the illustrative example of neon-20, we show that the depth of the potential determines the energy spacings between single-nucleon orbitals in deformed nuclei, the localization of the corresponding wavefunctions and, therefore, the degree of nucleonic density clustering. Relativistic functionals, in particular, are characterized by deep single-nucleon potentials. When compared to non-relativistic functionals that yield similar ground-state properties (binding energy, deformation, radii), they predict the occurrence of much more pronounced cluster structures. More generally, clustering is considered as a transitional phenomenon between crystalline and quantum-liquid phases of fermionic systems.

Models of cluster formation in solutions of fullerenes

A review of experimental and theoretical studies of the formation and growth of clusters in solutions of fullerenes is given. General problems of fullerene cluster formation in solutions are considered. The main directions and goals of studies are specified. The experimental data on solutions with various polarities obtained by various methods, including visible and UV spectroscopy, dynamic light scattering, small-angle neutron scattering, mass spectrometry, transmission electron microscopy, etc., are generalized. The conditions of cluster formation and mechanism of cluster stabilization and the role played by clusters in certain effects observed in the corresponding systems are discussed. Cluster growth models are considered on the basis of nucleation theory for solutions of fullerenes with various polarities. It is shown that the description of the observed cluster state requires modification of the kinetic equations of the classic approach using the drop model of clusters. Modified kinetic equations with corrections for the mechanism of cluster stabilization are used to analyze cluster growth and related phenomena.

Industrial Clustering and Global Value Chains in Central and Eastern Europe: Role of Multinational Enterprises in Industrial Upgrading

The authors are attempting to draw together existing literature on the governance of GVCs; research on host country ""spillovers"" as a consequence of MNE activity and the broader cluster and innovation literature. While cluster research had done important work in identifying and operationalizing the necessary conditions for cluster formation, it had relatively ignored the global-local linkage brought by the presence of MNEs. The ""spillover"" literature has identified in theory numerous benefits of MNE presence in host countries. There was relatively little empirical work done in CEE to discover if these benefits actually exist. Neither literature had focused on how MNEs govern their GVCs. Thus bringing these sources together presents an important opportunity for international business. The authors find that in the Slovak case the industrial clusters among the firms surveyed were not much functional. On a strategic level, there appears to be little evidence of cooperation in areas of marketing, export promotion or investment. This is especially of concern in sectors such as automotive where cooperative strategies among suppliers could offer significant benefits.

Towards a multiscale approach to the growth of silicon films by chemical vapor deposition

Abstract The morphological control of the surface of deposited films is one of the key points in the development of the microelectronics technology, in the field of epitaxial as well as polycrystalline silicon growth. Nowadays, the growth rate profiles are almost optimized, while the surface morphology is still controlled almost empirically only. In this work, the growth of silicon films under CVD conditions is examined, in particular analyzing the terrace-step-growth mechanism. The growth process involves diffusive and kinetic steps, which obviously occur at different length scales. Our attention is focused on the processes occurring on the surface, ignoring the gas-phase reactions, and evaluating the mass transport implications through a simplified model. In this way, the micro- and macro-scale are investigated with two different approaches and afterwards they are linked together in a “toy reactor model”. It becomes then possible to evidence the correlation between the process conditions and the mechanism of surface formation. The conditions of cluster formation, preceding the polycrystalline growth and those of terrace instability, preceding the transition to amorphous film growth are both considered. The information on the industrially produced films and the available pictures of the surface obtained by atomic force microscope confirm the theoretical findings.

Epitaxial Silicon Growth Between Scylla and Charybdis

The progress of the microelectronics industry depends on ever better control of the surface morphology of the deposited film. This is important for the deposition of both epitaxial and polycrystalline silicon layers. The former is used to obtain high quality semiconductor substrates, while the latter is needed to control the diffusion of impurities towards active layers. In the present work, the CVD of silicon films is examined with the aim of providing evidence of the correlation between process conditions and mechanisms of surface formation. In particular, attention has been focused on the process of terrace-step growth. In this framework, the conditions of cluster formation (preceding the polycrystalline growth), and those of terrace instability (preceding the transition to amorphous film growth), are both investigated. The calculated results are consistent with known information on industrially produced films, and with available pictures of Si surfaces obtained by atomic force microscopy (AFM)—in particular, the increase in cluster dimension when the partial pressure of silane is reduced.

Electron transport in the carbon-copper nanocluster structure

The electron transport in hydrogenated amorphous carbon films a-C: H with copper nanocluster inclusions has been investigated. The conditions of cluster formation are derived. It is theoretically demonstrated that the energy band structure of the matrix substantially affects the conditions of cluster formation. The electron transport depends on the cluster structure. It is found that, below the percolation threshold (the case of isolated clusters), the transport current is governed by two components depending on the electric field strength. At low field strengths, the current is caused by electrons in the conduction band of amorphous carbon, which are thermally excited from copper clusters. At high field strengths, the transport current is provided by tunneling electrons from the Fermi level of copper clusters to the conduction band of a-C: H. The difference between the mobility edge of the conduction band of amorphous carbon and the Fermi level in copper clusters is determined from the temperature dependence of the resistance and proves to be equal to 0.48 eV. The temperature dependences of the resistance at low field strengths exhibit a fine structure. It is revealed that, above the percolation threshold, the electrical resistance of clusters is considerably contributed by the residual resistance, which is supposedly associated with the electron scattering by cluster surfaces. The temperature effect on the electron transport is examined using the spin-wave scattering technique at a frequency of 4.0 GHz. It is found that the spin wave in the yttrium iron garnet (YIG) film is predominantly affected by thermally excited electrons located above the mobility edge in the conduction band of a-C: H.

Testing Population Synthesis Models with Globular Cluster Colors.

We have measured an extensive set of UBVRIJHK colors for M31 globular clusters. We compare the predicted simple stellar population colors of three population synthesis models to the intrinsic colors of Galactic and M31 globular clusters. The best-fitting models fit the cluster colors very well: the weighted mean color offsets are all smaller than 0.05 mag. The most significant offsets between model and data are in the U and B passbands; these are not unexpected and are likely caused by problems with the spectral libraries used by the models. The metal-rich clusters (&sqbl0;Fe&solm0;H&sqbr0; greater, similar-0.8) are best fit by young (8 Gyr) models, while the metal-poor clusters are best fit by older (12-16 Gyr) models. If this range of globular cluster ages is correct, it implies that conditions for cluster formation must have existed for a substantial fraction of the galaxies' lifetimes.

Sorption and diffusion of low molecular weight gases and vapors in glassy polymers at high concentration of sorbate

A theoretical approach has been developed to describe the sorption and diffusion processes of low weight molecular gases and vapors in polymers at wide ranges of sorbate concentration. The equation of an S-shaped gas sorption isotherm in glassy polymer matrix has been derived. The concentration dependence of the sorbate molecule diffusion coefficient has been established. For an S-shaped sorption isotherm, this dependence is nonmonotonous. The conditions of cluster formation of sorbate molecules have been analyzed within the proposed approach, in which it is possible to determine a correlation between these conditions and parameters of sorption isotherm. The comparison of the experimental and theoretical data provides an assessment of the microscopic characteristics of investigated polymer–vapor systems, such as the distances between vapor molecules in a matrix corresponding to intermolecular repulsion and attraction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2314–2323, 1999

Processes occurring in cluster plasmas and cluster beams

A method of generating a cluster beam of a refractory metal is analyzed. Clusters are formed in a high-pressure arc discharge plasma in which the refractory metal is an additive to the buffer gas. The conditions for cluster formation to occur in the intermediate region of the discharge are found, and the cluster instability, as a result of which all of the refractory metal collects in the form of clusters in this zone of the discharge, is studied. The processes studied are the basis for of a method of generating intense cluster beams.

Recent developments in ionized cluster beam thin film deposition

Thin film deposition from beams of atomic clusters, some of which are ionized and accelerated to the growing surface, was first proposed in 1972 but suffered from controversy over cluster formation conditions during the 1980s. Development of suitable cluster sources has been difficult and large clusters of zinc (about 2 000 atoms per cluster) have only recently been produced in the 1990s. The conditions for cluster synthesis without using a carrier gas presently require a high vapor pressure, 1000 Torr or more, thus severely limiting the materials from which cluster beams can be formed. Our results now suggest that the nozzle is serving as the heat sink for the heat of condensation of the zinc clusters. Effects of further intentional cooling of the nozzle on zinc cluster distributions are reported. An additional problem is the development of suitable real-time cluster measurement methods. Two different methods using simultaneous beam velocity and energy analysis are presented. One method is based on mechanical beam chopping and the other on electrostatic beam chopping.

Formation and metastable decomposition of unprotonated ammonia cluster ions upon femtosecond ionization

The formation and metastable dissociation mechanism of unprotonated ammonia cluster ions, (NH3)+n, produced by multiphoton ionization (MPI) at 624 nm and a nominal pulse width of 350 fs, are investigated through a reflectron time-of-flight (TOF) mass spectrometric technique. Detection of the unprotonated ions after femtosecond and nanosecond multiphoton ionization under various intensity conditions is explained. The role of the energy of the ionizing photons, and the observation of these ions after femtosecond MPI is examined. The formation of the unprotonated series is found to be a function of intensity in the case of ionization on the nanosecond time scale, but not so for the femtosecond time domain. The results can be explained in terms of ionization mechanisms and ionizing pulse durations. The findings of the present study suggest that the unprotonated ions are trapped behind the barrier to intracluster proton transfer and/or concomitant NH2 loss. The studies of metastable decomposition also reveal that the unprotonated ammonia cluster ions dissociate in the field-free region of the TOF by losing an NH2 radical rather than via the evaporative loss of NH3 as occurs for protonated clusters. Additionally, isotopic investigations of the unimolecular decay reveal a strong dependence on the conditions of cluster formation. The cluster formation condition dependence of the unimolecular decay is further investigated by altering formation temperatures and observing the consequences reflected by changes in the spontaneous metastable decay rate constant. This is a unique example of a cluster system whose metastable dissociation does not obey an evaporative ensemble model.

The theory of flow and its use for structural analysis for gas-filled polymers

It is shown that concepts appertaining to the theory of flow may be used for structural analysis of gas-filled polymers. Conditions of cluster formation have been determined by means of an analysis of porous structure with the aid of scanning electron microscopy. The results of the gas permeability investigations show that cluster formation takes places.

Collision damage processes due to energetic heavy ions in silver chloride

Abstract The optical absorption band induced by energetic heavy ions implanted into AgCl single crystals at 77°K must be connected with the formation of silver colloids. The experimental (primitization and E.P.R. spectra) and theoretical study, makes it possible to obtain an approximate value of cluster radius. The colloid concentration is correlated with nuclear stopping power. It is shown that the collision efficiency is modified by a chemical effect. Finally the conditions of cluster formation are analysed.