Particle Splitting Research Articles

REM-Untersuchung des Umlösungsvorgangs von Y'-Ausscheidungen in Ni-14 at.% Al-Kristallen, speziell an Orten erhöhter Versetzungsdichte, an Subkorngrenzen und Gleitlinien / SEM Investigation of the Coarsening Process of Y-Precipitates in Ni-14 at.% Al-Crystals, Especially in Areas of Higher Dislocation Density, on Subgrain Boundaries and Slip Lines

The splitting and dissociation behaviour of γ 6 -particles induced by appropriate heat treatment in Ni-14 at%-Al crystals with [100] normal to the surface, was investigated with optical and scanning electron microscopy, in particular in areas of increased dislocation density, i.e. subgrain boundaries and slip lines, but also for individual, isolated γ 6 -particles. The heat treatment was chosen so that the γ 6 -particles would only be fully developed in the subgrain boundaries and slip lines; the growth of the γ 6 -particles in the undisturbed crystal would just become visible. It has been shown that during coarsening the dissociation process and the splitting of the large particles in the γ 6 -chains in subgrain boundaries and slip lines almost always takes place along cubic planes. Splitting along (111) planes, the slip planes, which would mean a direct involvement of dislocations induced by deformation in the splitting process, could not be observed, apart from one exception mentioned. The stress fields in the γ/γ ' -microstructure could be revealed by repeated etching. The lattice distortions at a (100) surface of a crystal were etched with an etchant insensitive to the difference of γ and γ 6 . The matrix in the (100) direction was then removed in order to make the 7-particles in the etch pits visible. The centres of higher lattice distortion are not situated at the cube corners, but rather in the centres of the cube faces, where the splitting or cavity formation for the larger γ 6 -particles starts.

Existence of solutions to the transonic pressure gradient equations of the compressible euler equations in elliptic Regions

We establish the existence of a smooth solution in tis elliptic region in the self—similar plane to the pressure—gradient system arisen from the wave—particle splitting of the two—dimensional compressible Euler system of equations. The pressure—gradient system takes the form Here (u,v) is the velocity, ρ is the density which is independent of time resulted from the splitting procedurep is the pressure, and is the energy. The natural (parabolically degenerate) boundary value is used.

Mica weathering in acidic soils by analytical electron microscopy

AbstractThe mineralogy, crystallochemistry and microfabric of clay minerals from acidic soils were studied using transmission electron microscopy (TEM) and analytical electron microscopy (AEM). Soil profiles, developed on saprolites, sampled in the main crystalline massifs of France represent different pedological environments. The study focused on the microsystem of mica weathering, which appeared to be the main source of secondary clay minerals, and involves microdivision, transformation and dissolution. Microdivision begins with the splitting of large particles along layer planes and their shearing normal to the layers. This induces the breakdown of particles of one hundred layers into particles having only a few layers. The transformation of micas follows two steps: they first transform into 1–1.4 nm mixed-layer minerals and then into hydroxy-Al interlayered vermiculite. The formation of hydroxy-Al interlayered vermiculite derived from micas is dominant in acidic soils; particles are generally small, consisting of only three to seven layers, and always have a dioctahedral composition, whatever the type of the original mica (trioctahedral or dioctahedral). Dissolution affects the surface layers or large domains of the core of the particles and leads to the formation of multi-elementary gels rich in Fe and Al.

A Stochastic Simulation Method for Particle Acceleration and Non-Thermal Photon Emission in Astrophysical Processes

A versatile numerical method is developed to simulate the time dependent particle acceleration in shocks and the non-thermal photon emission thereof. Our method is based on Itô's stochastic differential equations coupled with a particle splitting. This method is applied to photon emission from young supernova remnants.

Data-parallel algorithms for Monte Carlo simulation of neoclassical transport in magnetically confined plasmas

Two data-parallel algorithms for simulating test particle transport in magnetically confined plasmas are compared. The first approach, a direct parallelization of conventional algorithms, is found to suffer from a low processor utilization factor unless parallel time stepping and particle splitting combined with an appropriate termination criterion are used. The second approach utilizes weak methods for solving stochastic differential equations to calculate relevant moments of the test particle distribution function. This shift in focus from the individual test particles to the test particle distribution avoids the load balancing problems found for the conventional algorithm. The approach based on stochastic differential equations is found to outperform the conventional algorithm when implemented on a Connection Machine 2 computer.

Kinetics of strain-induced morphological transformation in cubic alloys with a miscibility gap

Morphological evolutions controlled by a transformation-induced elastic strain during a solid state precipitation are systematically investigated using a prototype binary alloy as a model system. A computer simulation technique based on a microscopic kinetic model including the elastic strain effect is developed. Without any a priori assumptions concerning shapes, concentration profiles and mutual positions of new phase particles, various types of coherent two-phase morphologies such as basket-weave structures, sandwich-like multi-domain structures, precipitate macrolattices and GP zones are predicted. A wide variety of interesting strain-induced kinetic phenomena are observed during development of the above microstructures, including selective and anisotropic growth, reverse coarsening, particle translational motion, particle shape transition and splitting. In spite of all simplifications of the model, most of the simulation results are confirmed by experimental observations in various alloy systems, indicating that this kinetic model can be efficiently used for understanding, interpreting and predicting structural evolutions in real alloys.

Kinetics of fragmentation

A general discussion of the kinetics of continuous, irreversible fragmentation processes is presented. For a linear process, where particle breakup is driven by an external force, the authors develop a scaling theory to describe the evolution of the cluster size distribution. They treat the general case where the breakup rate of a cluster of mass x varies as xlambda . When lambda >0, corresponding to larger clusters more likely to break up, the scaled cluster size distribution, phi (x), decays with the scaled mass, x, as x-2 exp(-axlambda ), as x. For small mass, phi (x) has the log-normal form, exp(-a ln2 x), if the breakup kernel has a small-size cutoff, while phi (x) has a power-law tail in the absence of a cutoff. They also show that a conventional scaling picture applies only for the case lambda >0. For lambda >0, they develop an alternative formulation for the cluster size distribution, in which the typical mass scale is determined by the initial condition. In this regime, they also investigate the nature of a 'shattering' transition, where mass is lost to a 'dust' phase of zero-mass particles. They also study the kinetics of a nonlinear, collision-induced fragmentation process. They analyse the asymptotic behaviour of a simple-minded class of models in which a two-particle collision results in either: (1) both particles splitting into two equal pieces, (2) only the larger particle splitting in two, or (3) only the smaller particle splitting. They map out the kinetics of these models by scaling arguments and by analytic and numerical solutions of the rate equations. Scaling is found to hold for different ranges of homogeneity index for the three models.

Open systems of splitting particles

Systems with an infinite variety of types of splitting particles are investigated. It is shown that if there is a stationary source of particles but no sink, a steady state with finite density of each species is nevertheless possible due to the infinite number of degrees of freedom. It is demonstrated that the limiting (steady) state is independent of the initial state of the system. Typical features of the steady state, which do not depend on the particle splitting law, are shown.

Simulation of sintering of model supported metal catalysts

AbstractDuring the past decade, the sintering of model supported metal catalysts, i.e. catalysts consisting of metal deposited on very thin and flat oxide films, has been intensively investigated by transmission electron microscopy. In the current paper, a mathematical model, based on an atomic migration mechanism, for the sintering of these catalysts is presented. The predictions of the model, based on Monte Carlo simulations, are in very good agreement with the experimental observations, i.e. phenomena such as ‘apparent’ particle migration, splitting and neck formation between particles are predicated by the model.

Monte Carlo simulation of supported metal catalyst sintering and redispersion

AbstractMonte Carlo simulations of sintering and redispersion of supported metal catalysts, based on an atomic migration mechanism, are presented. The support surface was modeled as a square grid with metal atoms located at various grid points. Interaction energies between metal atoms, and between metal atoms and support sites were specified. The probability of movement of each metal atom during one time increment (iteration) was calculated from the interaction energies. The effects of interaction energies, metal loading and initial distribution of the metal were examined. The model predictions are in good qualitative agreement with experimental observations. The model predicts “apparent” movement, splitting and coalescence of entire metal particles; these experimentally observed phenomena have usually been cited as evidence that sintering occurs by a particle migration mechanism.

Selective growth response to IL-3 of a human leukaemic cell line with megakaryoblastic features.

A new human leukaemic cell line (M-O7) with the phenotypic characteristics of CFU-mega is described. Its cells are positive for T200 leucocyte common antigen (LCA) and negative with MAbs recognizing T and B cells and mature myelomonocytic antigens. In contrast, they react with MAbs recognizing antigenic determinants common to multi-lineage (CD13, CD33, CD34) and to bipotent erythromegakaryoblastic (CD36, H25) haemopoietic precursors, and with MAbs specific for platelet glycoproteins (CD41w, CD42w). A small proportion (10%) of the cells were large and multinucleated, and on electron-microscopy examination showed peripheral splitting of platelet-like cytoplasm particles. When transferred to a serum-free Iscove modified Dulbecco's medium supplemented with human insulin and transferrin, M-O7 cells stop proliferating. Of the haemopoietic growth factors tested for their ability to restore the proliferative activity of this quiescent population, only rH IL-3 proved effective. Moreover, it also increased the cloning efficiency in methylcellulose more than any other CSFs. The M-O7 cell line may provide a valuable tool for the biological assay of IL-3, and a model for biochemical studies of the megakaryocytic lineage.

High efficiency of the Penrose process of energy extraction from rotating black holes immersed in electromagnetic fields

The conditions are analyzed under which maximum efficiency is obtained for the Penrose process (PP) of energy extraction from rotating black holes immersed in electromagnetic fields. The components of the dipole magnetic field are displayed superposed on the background Kerr metric as a representative case. Mathematical methods for analyzing the disintegrational PP are developed, obtaining an equation to a mass-hyperbola defined by mass parameters of fragments resulting from the disintegration of a particle incident on the black hole. The efficiency of the PP is considered, analyzing all the factors contributing toward high efficiency along with the plausible astrophysical limits. In particular, it is shown that the incident particle splitting near the static limit of the Kerr geometry gives an efficiency from which 100 percent energy extraction efficiency can be assumed in constructing the PP-based model of the central engine in AGNs. Finally, the use of these techniques in modeling AGNs is discussed.

A microscopic calculation of the Kπ = 1 + mode in deformed nuclei

A Hartree-Bogoliubov plus Random Phase Approximation calculation with seperable forces is carried out for the K π = 1 + mode in deformed nuclei. It is shown that the 1--s splitting of single particle orbits results in a significant fragmentation of the magnetic dipole transition strength.

Dispersion of nickel particles supported on alumina and silica in oxygen and hydrogen

A transmission electron microscopic study was made to examine the behavior of Ni particles supported on amorphous Al 2O 3 and SiO 2 by thermal treatment in H 2 and O 2 atmospheres at 60 Torr. The sintering during heating in H 2 at 600 °C proceeded by particle migration for Ni/Al 2O 3 but by atomic migration for Ni/SiO 2. The rate of increase in the average particle size in H 2 at 600 °C was comparable to that under vacuum at the same temperature. Annular or horseshoe-shaped NiO particles spreading over the supports were formed by O 2-treatment at certain temperatures which were dependent on the support material. The formation of pitted particles was completed in a rather short period of time. By repeated heating in O 2 and in H 2 at 600 °C, the particles reversibly changed their shape at the same positions. However, redispersion, the splitting of particles, occured at other sets of treating temperatures, typically oxidation at 600 °C and reduction at 400 °C. The treatment temperatures at which redispersion was observed differed between the two supports. The effect of the support is discussed in terms of particle-support interaction including the formation of a metal-support complex.

Surface enrichment of Pt-Rh alloy particles by oxidation-reduction cycling

X-Ray photoelectron spectroscopy (XPS) is used to examine the surface composition of 50 to 200-Å Pt, Rh, and Pt-Rh particles on planar amorphous SiO 2 following heat treatment at various temperatures in air or H 2. It has been shown previously by transmission electron microscopy that this treatment produces an expansion of oxidized particles parallel to the surface and the splitting of particles upon reduction. In this study, the composition of the surface layer is measured directly by examining the Rh, Pt, and O XPS signals from the particles. It is found that for a 50-50 alloy the Rh Pt ratio increases by a factor of at least 3 upon oxidation and that the ratio remains above 2 upon heating in H 2 to above 500 °C. Surface oxidation of Rh can be detected in both Rh and O signals just above room temperature, while much higher temperatures are required for oxides to be detected by electron diffraction. The enhanced Rh signal persists to much higher temperature for composition near 50% Pt than for either excess Rh or excess Pt. Oxidation-reduction cycling can produce nonequilibrium surface compositions for any alloy systems in which only one species is oxidized, and this configuration may be retained in the catalyst at temperatures below those where bulk diffusion of metal occurs. Other compound formation and decomposition cycles could be used to prepare enriched surfaces for many catalyst systems.

Thermodynamic and kinetic studies of the interaction between T5-phages and their receptor, isolated from Escherichia coli b, in the absence and presence of Triton X-100

Particles with receptor activity for T5-phages were isolated from the outer membrane of Escherichia coli B. We describe the interaction of these particles with T5-phages as a two-step chemical reaction. The rate constants were estimated from the inactivation kinetics. The transition-state theory permits the calculation of the entropy, enthalpy, and Gibbs free energy of activation. In the absence of Triton X-100, the reaction can be described with one set of thermodynamic constants for the temperature range from 10 degrees to 40 degrees C. The addition of Triton, which results in the splitting of receptor particles and in the building of mixed micelles, causes a complicated dependence on temperature. In this case, a subdividing of the temperature range measured into two parts yields two sets of thermodynamic constants that permit a good description of experimental kinetics.

Particle splitting and redispersion phenomena in model alumina-supported platinum catalysts

Transmission electron microscopy (TEM) has been used in an attempt to study the redispersion behavior of small platinum particles supported on flat, dense γ-alumina substrates. The alumina substrates were prepared both by anodizing and by sputtering techniques. A thin (1 to 2 nm) film of platinum was deposited onto these alumina substrates by sputtering. This film was subsequently transformed into discrete platinum particles by an initial heat treatment at 600 °C for 18 hr in air, and then “redispersed” by heating at 500 °C for 18 hr, also in an air atmosphere. Particle size measurements on TEM photomicrographs revealed no significant decrease in platinum particle size as a result of the “redispersion” heat treatment. These results are in contradiction to previous observations of platinum particle splitting under those conditions. The findings are discussed in the light of different concepts which have been used to explain redispersion phenomena.

Analysis of Error in Monte Carlo Transport Calculations

The Monte Carlo method for neutron transport calculations suffers, in part, because of the inherent statistical errors associated with the method. Without an estimate of these errors in advance of the calculation, it is difficult to decide what estimator and biasing scheme to use. Recently, integral equations have been derived that, when solved, predicted errors in Monte Carlo calculations in nonmultiplying media. The present work allows error prediction in nonanalog Monte Carlo calculations of multiplying systems, even when supercritical. Nonanalog techniques such as biased kernels, particle splitting, and Russian Roulette are incorporated. Equations derived here allow prediction of how much a specific variance reduction technique reduces the number of histories required, to be weighed against the change in time required for calculation of each history. 1 figure, 1 table. (RWR)

Mechanisms of aging of supported metal catalysts

The behavior of a population of small metal crystallites on a substrate is accounted for by the splitting of single metal atoms and multiatom particles from larger particles, their diffusion along the substrate and the coalescence of particles (or of a particle and an atom) that collide. The equations are stochastic, allowing for the treatment of a large number of particles. The rate at which one particle is captured by another is determined by the rate of diffusion of particles from the bulk to the interface of the other particles and the kinetics of the capture process at the interface. The rate of emission is proportional to the equilibrium concentration, on the original particle interface, of particles having the size of the particle to be emitted. If emission is an activated process, the probability of multiatom emission is negligibly small compared to single atom emission. Hence, even if particles can lose single atoms to the substrate by activated breaking of bonds the loss of aggregates of two or more atoms must occur via another mechanism. This alternate process is not entirely clear but is probably related to the existence of cracks on the crystallite and their propagation due to internal stresses caused either thermally or by the formation of a chemical compound. The present model can account for an increase or a decrease in the total surface area with time, as well as an initial increase followed by a decrease or an initial decrease followed by an increase. In any particular case the path followed depends upon the metal loading, the saturation concentration of metal atoms, the diffusion rate and the emission rate.

Structure ofTi42from theCa40(He3,n)Ti42reaction

The structure of $^{42}\mathrm{Ti}$ has been investigated using the $^{40}\mathrm{Ca}(^{3}\mathrm{He},n)^{42}\mathrm{Ti}$ reaction at a beam energy of 15 MeV. Comparisons between measured angular distributions and distorted wave Born approximation (DWBA) calculations provide a number of new spin assignments. The DWBA analysis also provides an estimate of $p$ state admixtures in the low-lying states of the ${({f}_{\frac{7}{2}})}^{2}$ configuration. A comparison with the results of the $^{40}\mathrm{Ca}(t,p)^{42}\mathrm{Ca}$ reaction demonstrates the analog character of many levels in $^{42}\mathrm{Ti}$ and $^{42}\mathrm{Ca}$ up to about 6.5 MeV. Above 3 MeV, in states involving large ${p}^{2}$ or $fp$ components in the wave function, transition strengths are comparable in both two-proton and two-neutron transfers. The states in $^{42}\mathrm{Ca}$ generally occur at an excitation energy several hundred keV above the corresponding analog in $^{42}\mathrm{Ti}$, suggesting that the ${f}_{\frac{7}{2}}\ensuremath{-}{p}_{\frac{3}{2}}$ single particle splitting is slightly greater in $^{42}\mathrm{Ca}$.NUCLEAR REACTIONS, STRUCTURE $^{40}\mathrm{Ca}(^{3}\mathrm{He},n)$ $E=15$ MeV; measured angular distributions, DWBA deduced $J$, $\ensuremath{\pi}$. Comparison $^{42}\mathrm{Ti}$, $^{42}\mathrm{Ca}$ analog states.