Empty Clusters Research Articles

Effective Dewetting in a Microporous Particle

In this paper, the kinetics of the outflow in a microporous particle infiltrated by a nonwetting liquid is analyzed in context of effective phase transformation. The “dewetting” process is considered as the nucleation, growth, and coalescence of empty pore clusters (EPCs) that starts from the interior and eventually involves the whole particle. Initially, the EPC nucleation is dominant while the influence of EPC coalescence becomes increasingly important as the EPC volume fraction increases. The dependence of the dewetting time on the pore size distribution is discussed in detail.

Lifetimes of cagelike water clusters immersed in bulk liquid water: a molecular dynamics study on gas hydrate nucleation mechanisms.

Molecular dynamics simulations were performed to observe the evolution of cagelike water clusters immersed in bulk liquid water at 250 and 230 K. Totally, we considered four types of clusters--dodecahedral (5(12)) and tetrakaidecahedral (5(12)6(2)) cagelike water clusters filled with or without a methane molecule, respectively. The lifetimes of these clusters were calculated according to their Lindemann index (delta) using the criterion of delta> or =0.07. The lifetimes of the clusters at 230 K are longer than that at 250 K, and their ratios are the same as the ratio of structure relaxation times of bulk water at these temperatures. For both the filled and empty clusters, the lifetimes of 5(12)6(2) cagelike clusters are similar to that of 5(12) cagelike clusters. Although the methane molecules indeed make the filled cagelike water clusters live longer than the empty ones, the empty cagelike water clusters still have the chance of being long lived. These observations support the cluster nucleation hypothesis for the formation mechanisms of gas hydrates.

Método heurístico para particionamiento óptimo

Muchos problemas en el análisis de datos requieren del particionamiento no supervisado de un conjunto de datos dentro de clases o conglomerados no vacíos que sean bien separados entre ellos y lo más homogéneos entre sí. Un particionamiento ideal es cuando se puede asignar cada elemento del conjunto a una clase sin que exista ambigüedades. Este trabajo consta de dos partes principales; primero se presentan diferentes métodos y heurísticas para encontrar la cantidad de clases en que se debe particionar un conjunto de manera óptima; posteriormente se propone una novedosa heurísticas y se realizan algunas comparaciones para observar sus ventajas considerando conjuntos muy conocidos y utilizados que están previamente clasificados presentándose al final algunos resultados y conclusiones.

Persistence in cluster-cluster aggregation.

Persistence is considered in one-dimensional diffusion-limited cluster-cluster aggregation when the diffusion coefficient of a cluster depends on its size s as D(s) approximately s(gamma). The probabilities that a site has been either empty or covered by a cluster all the time define the empty and filled site persistences. The cluster persistence gives the probability of a cluster remaining intact. The empty site and cluster persistences are universal whereas the filled site depends on the initial concentration. For gamma>0 the universal persistences decay algebraically with the exponent 2/(2-gamma). For the empty site case the exponent remains the same for gamma<0 but the cluster persistence shows a stretched exponential behavior as it is related to the small s behavior of the cluster size distribution. The scaling of the intervals between persistent regions demonstrates the presence of two length scales: the one related to the distances between clusters and that between the persistent regions.

Fractals from a reaction diffusion process

We introduce a reaction diffusion model on a lattice involving two particles species, A and B. B particles initially fill the lattice, and A particles diffuse transforming B particles to A particles each time they meet. We keep the number of A particles constant eliminating the oldest ones. The process builds empty clusters whose short time features are fractal (in the universality class of invasion percolation) and whose long-time features are those of compact objects.

The Distorted Pd9(μ:η5,η2-As2)4(PPh3)8 Architecture

Extended Hückel and density functional calculations were carried out on the title compound 1. Results indicate that the bonding of the interstitial Pdi atom with the D2d distorted (Pds)8(As2)4 cubic host differs from that found in the related metal-centered cubic species M9(μ4-E)6L8. Species 1 can be described as an approximate square-planar 16-electron PdiL4 center bound through Pdi–As rather than Pdi–Pds contacts to a distorted cubic fragment composed of four 30-electron bimetallic Pd2L8 units. Although a substantial gap is computed between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for the observed electron count of 130 MVEs, calculations indicate that such an architecture should be observed for different electron counts close to that count. The incorporation of the central Pd atom does not seem to be essential to ensure the stability of the distorded Pd8(As2)4(PH3)8 cluster. Such empty clusters, with formally four M–M bonds and eight planar 16-electron Pds centers, should exist for electron counts close to 120 MVEs.

A new fuzzy-c-means and assignment technique-based cell formation algorithm to perform part- type clusters and machine-type clusters separately

The incorporation of fuzziness in the cell formation problem by Chu and Hayya (1991 International Journal of Production Research, 29, 1475-1487) is a notable contribution, in which non-binary classification logic is used. However, despite this development, numerical illustrations performed in this research demonstrate that the Chu and Hayya approach can result in solutions with empty party-type cluster(s) and/or empty machine-type cluster(s). Further, it is noted that solutions based on the Chu and Hayya approach can contain non-empty part-type cluster(s) being assigned to empty machine-type cluster(s) and vice versa. Three strategies are offered in this research to overcome these inadequacies: the separate formation of part-type and machine-type clusters; modification of the stopping criterion; and the adoption of the assignment technique in the formation of the final manufacturing cell solutions. A new algorithm has resulted from these modifications and has been rigorously compared to the performance of the Chu and Hayya approach. In general, the new algorithm demonstrates superiorities in global efficiency and in generating feasible party-type clusters, machine-type clusters and manufacturing cells.

Visualization of Conjunctival Goblet Cell Actin Cytoskeleton and Mucin Content in Tissue Whole Mounts

The purpose of the work was to visualize goblet cells, their cytoskeletons, and their content in tissue whole mounts of conjunctiva using confocal microscopy. Conjunctival tissue from rat, mouse, rabbit, and human were excised, fixed in 4% paraformaldehyde, and incubated either with rhodamine-labeled phalloidin to label filamentous actin or in combination with fluoresceinated lectins to label carbohydrates on goblet cell mucins. Tissues were evaluated by confocal microscopy. Phalloidin labeling of excised tissue facilitated visualization of goblet cells within the stratified epithelium of the conjunctiva. Phalloidin labeled the perimeter of the goblet cells, so that either in optical section or three-dimensional constructs of optical sections, the goblet cells appeared as plump empty clusters of cells in rat and mouse and as individual plump empty cells in rabbit and human tissues. Double labeling of excised tissue with lectins demonstrated that the plump cells visualized by labeling with phalloidin bound the label and were indeed goblet cells. At very high resolution one could see individual mucin packets within the goblet cells. Variation in intensity of lectin labeling of these mucin packets both between cells and within cells was evident. Streams of labeled mucin emanating from apical surfaces of goblet cells could be visualized using the fluoresceinated lectins. The labeling of excised conjunctiva with fluorochrome-conjugated phalloidin allows visualization by confocal microscopy of goblet cells within the stratified epithelium. Double labeling with fluorochrome-conjugated lectins allows visualization of their content. These methods may be useful in studies of goblet cell differentiation, mucin content and secretion.

Metal cluster topology 17. Hypervalent indium vertices in isolated hypoelectronic indium polyhedra in binary and ternary intermetallic phases of indium and alkali metals

Analyses of the structures and electron counts for the known isolated indium clusters found in binary and ternary alkalimetal-indium intermetallic phases using methods derived from graph theory suggest the presence of hypervalent indium vertices using filled indium d orbitals as well as sp 3 orbitals for skeletal bonding to relieve the apparent electron poverty in the hypoelectronic polyhedra. The novel shapes of the hypoelectronic indium polyhedra in such clusters are based on capping a triangular or rectangular face of a smaller polyhedron, followed by flattening the pyramid generated by the capping vertex so that it moves closer to the center of the polyhedron. Reasonable electron counts and globally delocalized bonding topologies are then obtained for the hypoelectronic indium clusters if all of the indium vertices at the flattened polyhedral sites are hypervalent using sp 3d five-orbital manifolds and the remaining indium vertices are normal using spa four-orbital manifolds, like typical vertex atoms in boranes, carboranes and post-transition metal ‘Zintl-type’ clusters. For example, the empty triflattened pentacapped trigonal prismatic indium cluster anion In 11 7− in K 8In 11 and K 8In 10Hg becomes a globally delocalized 11-vertex deltahedron with three hypervalent and eight normal indium vertices and the required 2 n+2=24 skeletal electrons. Incorporation of an interstitial atom into the center of an isolated hypoelectronic indium polyhedron splits the single multicenter core bond found in an empty globally delocalized polyhedron into two or more core bonds, corresponding to the coordination number and sp n hybrid orbital orientation of the interstitial atom. Thus, the compound K 8In 10Zn can be regarded as a linear two-coordinate Zn 2+ complex of the bidentate In 10 10− ligand and the compound K 10In 10Ni can be regarded as a tetrahedral four-coordinate Ni 0 complex of the tetradentate In 10 10− ligand. This theory predicts the existence of alkali-metal-indium-coinage metal ternary phases such as K 8In 9Cu, which would correspond to a trigonal Cu + complex of a tridentate In 9 9− ligand. In such structures, the hypoelectronic anionic indium polyhedron can be regarded as a multidentate ligand encapsulating the interstitial atom.

Effective interaction energies within the pair interaction model under a dimensional constraint

A theoretical framework of effective interaction energies under a dimensional constraint is provided within the nearest neighbor pair interaction model. It is revealed that the υ 0 term, the interaction energy for an empty cluster, under the dimensional constraint is the chemical energy of formation of an ordered compound. The result provides a clue to understand the microscopic origin of the appearance of an ordered phase in a III–V thin film constrained from a substrate material.

Global volume relaxations and phase stability in disordered Pd-Rh alloys.

The energetics of disordered fcc-based Pd-Rh alloys are determined by a combination of a generalized Ising model and effective-pair and multisite interactions that are explicit functions of the alloy volume. The volume dependence of the interactions is incorporated in the method of direct configurational averaging, which is a real-space, Green-function technique for obtaining the interactions directly from their definition. The first two terms in the generalized Ising Hamiltonian (or cluster expansion), the so-called empty cluster and point interactions, may not be computed simply from one-electron energies; therefore, a technique is described to compute each of these terms from two contributions: (1) total energies of the pure elements, and (2) one-electron energies corresponding to the energy gained upon mixing the constituents of the alloy at a fixed volume. By using the volume-dependent interactions, the formation enthalpy, equilibrium volume, and bulk modulus are computed for the completely disordered alloy as a function of composition. The scheme of computing the volume-dependent interactions is compared with the results of the effective-cluster-interaction calculations from other Hamiltonians, and the results of this comparison explained in terms of the magnitude of the off-diagonal disorder present in each computation. The volume-dependent interactions are combined with the cluster-variation method to compute the solid-state portion of the Pd-Rh phase diagram, with no adjustable or experimentally determined parameters. The agreement between the theoretically predicted and experimentally determined diagrams is excellent.

Electronic structure and chemical bonding of a series of rare earth metal clusters R7X12Z

The DV-Xα-SCC method was used to study the electronic structure and chemical bonding of a series of rare earth metal clusters, R 7 X 12 Z. (R=Sc, Y, Pr; X=Cl; Z=Co, Fe, N) The results show that for the clusters containing a transition metal atom at the center, the number of cluster valence molecular orbitals (CVMOs) is 9; for the empty cluster and the clusters containing a nonmetal element, the number of CVMOs is 7, the same as in (B 6 H 6 ) 2-

Empty Octahedral Hexazirconium Clusters with Only Ten Electrons, [Zr6X14(PR3)4

Empty Octahedral Hexazirconium Clusters with Only Ten Electrons, [Zr₆X₁₄(PR₃)₄

ChemInform Abstract: Y10I13C2: A Novel Compound with Chains of Both Carbon‐Centered and Empty Clusters.

AbstractThe reaction of Y, YI3, and Y2O3 at 800‐900 °C is found to produce black needles of Y10I13C2 in low yields (≤ 20%).

Bonding to interstitial main-Group or transition-metal atoms in cubic clusters related to nickel tellurium triethylprophine, Ni9(.mu.4-Te)6(PEt3)8

The Te-centered, cubic cluster Ni{sub 8}Te({mu}{sub 4}-Te){sub 6}(L){sub 8} is suggested as a stable molecule, on the basis of extended Hueckel molecular orbital calculations for the cluster and its experimentally known, Ni-centered analogue, Ni{sub 9}({mu}{sub 4}-Te){sub 6}(PEt{sub 3}){sub 8}. Calculations show that the interstitial Ni or Te atom binds to the empty cluster at the expense of Ni-Ni and Ni-Te bonding within the cluster framework. The interstitial Ni atom compensates by bonding weakly to the framework nickels, primarily through its a{sub 1g} (4s) orbital; the central Te bonds strongly to both the nickel cube and the face-capping telluriums. Large HOMO-LUMO gaps suggest that 130 of 114 electrons are optimum for Ni{sub 8}Te({mu}{sub 4}-Te){sub 6}(L){sub 8}. Analogies to the solid-state structures of NiTe and CsCl, as well as to the centered octahedral clusters Zr{sub 6}I{sub 12}Z (Z = main-group or transition-metal atom), are pointed out.

Y10I13C2: a novel compound with chains of both carbon-centered and empty clusters

The phase Y{sub 10}I{sub 13}C{sub 2} is synthesized by the reaction at {approximately} 800{degree}C of powdered Y and YI{sub 3} with impure Y{sub 2}O{sub 3}, the vital carbon component resulting from incomplete decomposition of the yttrium oxalate precursor. The phase has not been obtained from more conventional carbon sources. The identity of carbon as the interstitial is supported by these syntheses, microprobe analyses of Y{sub 10}I{sub 13}C{sub 2} vs Y{sub 6}I{sub 7}C{sub 2}, and both the occupancy and the dimensions of the yttrium cavity in which carbon occurs. The phase occurs in space group C2/m, Z = 2; a = 21.317 (6) {angstrom}; c = 19.899 (5) {angstrom}; {beta} = 97.40 (2){degree}; R/R{sub w} = 3.3%/3.9% for 838 independent reflections with 2{theta} {le} 50{degree} (Mo K{alpha}). The compound consists of centered Y{sub 6}I{sub 12}C- and empty Y{sub 6}I{sub 8}-type clusters condensed by edge sharing into commensurate double and single chains, respectively. The chains are further cross-linked by additional iodine atoms. The two chain types are very similar to those known individually in the phases Y{sub 6}I{sub 7}C{sub 2} and Y{sub 4}Cl{sub 6} and are presumably metallic and semiconducting, respectively. The Y{sub 4}I{sub 6} chain is the first iodide example ofmore » condensed Y{sub 6}X{sub 8}-type clusters. This feature appears to be stabilized in this structure by a sterically less demanding means of bridging these chains between sheets of a Y{sub 6}I{sub 7}C{sub 2}-like arrangement. 30 refs., 3 figs., 3 tabs.« less

Annealing study of He‐irradiated Ni samples

AbstractPositron annihilation lifetime measurements were performed in helium‐irradiated Ni foils for annealing temperatures between 300 K and 1400 K. The samples were also examined by T.E.M. It is shown that immediately after the irradiations two kinds of vacancy clusters are present, some of them containing He‐ions and the others being empty. The empty vacancy clusters anneal out before 800 K, while the vacancy clusters containing He grow in size as a function of annealing temperature.

Why the close-packed Ni(111) surface dissociatively chemisorbs oxygen and nucleates oxide faster than the more open Ni(100) surface

The 300 K kinetics of dissociative chemisorption and oxide island nucleation and growth are compared for Ni(100) and Ni(111). We conclude that the behavior of oxygen sticking probability as a function of coverage can be understood in terms of direct Langmuir adsorption (no molecular diffusion) and the availability of active empty clusters of adsorption sites of certain sizes and configurations.

Empty, filled, and condensed metal clusters

Aspects of structure, bonding, physical, and chemical properties are discussed for (a) compounds containing discrete empty clusters and clusters with interstitial H atoms: Nb6I11, HNb6I11, HCsNb6I11, (b) metal-rich lanthanide halides and halide carbides,-nitrides and -hydrides, focusing on the role of interstitial atoms: Gd2Br2C2, Gd2Br2C, Gd3Cl3C, Gd10Cl18C4, Gd10Il17C4, Gd10I16C4, GD2Cl3N, GdXHn (X=Cl, Br, I; 0.6<n<0.9), GdBrD2; (c) metal-rich oxides of the alkali metals rubidium and cesium. Chemical bonding in the suboxide clusters Rb9O2 and Cs11O3 is discussed along the lines valid for (a) and (b) and covers recently described “hypervalent” species as Li6C, Li4O, etc.

Helium irradiated nickel investigated by positron annihilation

Nickel specimens have been homogeneously implanted with helium ions at about 325 K to a total dose of 500 appm. Distinct defect structures are observed in the irradiated specimen during isochronal annealing. Positron lifetime and Doppler broadening techniques were applied to study the evolution of the defect configurations. The influence of helium on the positron annihilation characteristics is clearly visible. The nucleation of helium bubbles could be determined. At higher annealing temperatures two distinct positron bound states are found. For both states the temperature dependence of the annihilation characteristics is similar to that reported for voids. Clear differences in the annihilation parameters have been established for empty and helium-decorated vacancy clusters.