Size Distribution Of Nuclei Research Articles

Uniform CoSi 2 nano-nucleus formation by oxide mediated silicidation with a Ti capping layer

The nucleation stage toward CoSi 2 with average island size of about 4 nm and uniform island size distribution is obtained from a TiN/Co/Ti/Co/SiO x /Si multilayer with the SiO x as a mediated layer by annealing at 460 °C for 240 s followed by 600 °C 240 s. It is found that Ti capping layer can enhance Co diffusion into the Si substrate at higher temperature (600 °C 240 s) annealing, which results in larger nucleus size (average grain size about 12 nm) but nonuniform nucleus size distribution. However, two-step annealing for 460 °C 240 s followed by 600 °C 240 s results in a smaller average nucleus size with better nucleus size distribution. The mechanism responsible for the discrepancy is discussed in the paper.

Modelling nucleation in wet granulation

Nucleation is the first stage in any granulation process where binder liquid first comes into contact with the powder. This paper investigates the nucleation process where binder liquid is added to a fine powder with a spray nozzle. The dimensionless spray flux approach of Hapgood et al. (Powder Technol. 141 (2004) 20) is extended to account for nonuniform spray patterns and allow for overlap of nuclei granules rather than spray drops. A dimensionless nuclei distribution function which describes the effects of the design and operating parameters of the nucleation process (binder spray characteristics, the nucleation area ratio between droplets and nuclei and the powder bed velocity) on the fractional surface area coverage of nuclei on a moving powder bed is developed. From this starting point, a Monte Carlo nucleation model that simulates full nuclei size distributions as a function of the design and operating parameters that were implemented in the dimensionless nuclei distribution function is developed. The nucleation model was then used to investigate the effects of the design and operating parameters on the formed nuclei size distributions and to correlate these effects to changes of the dimensionless nuclei distribution function. Model simulations also showed that it is possible to predict nuclei size distributions beyond the drop controlled nucleation regime in Hapgood's nucleation regime map. Qualitative comparison of model simulations and experimental nucleation data showed similar shapes of the nuclei size distributions. In its current form, the nucleation model can replace the nucleation term in one-dimensional population balance models describing wet granulation processes. Implementation of more sophisticated nucleation kinetics can make the model applicable to multi-dimensional population balance models.

Scaling of Tip Vortex Cavitation Inception Noise With a Bubble Dynamics Model Accounting for Nuclei Size Distribution

The acoustic pressure generated by cavitation inception in a tip vortex flow was simulated in water containing a realistic bubble nuclei size distribution using a surface-averaged pressure (SAP) spherical bubble dynamics model. The flow field was obtained by the Reynolds-averaged Navier–Stokes computations for three geometrically similar scales of a finite-span elliptic hydrofoil. An “acoustic” criterion, which defines cavitation inception as the flow condition at which the number of acoustical “peaks” above a pre-selected pressure level exceeds a reference number per unit time, was applied to the three scales. It was found that the scaling of cavitation inception depended on the reference values (pressure amplitude and number of peaks) selected. Scaling effects (i.e., deviation from the classical σi∝Re0.4) increase as the reference inception criteria become more stringent (lower threshold pressures and less number of peaks). Larger scales tend to detect more cavitation inception events per unit time than obtained by classical scaling because a relatively larger number of nuclei are excited by the tip vortex at the larger scale due to simultaneous increase of the nuclei capture area and of the size of the vortex core. The average nuclei size in the nuclei distribution was also found to have an important impact on cavitation inception number. Scaling effects (i.e., deviation from classical expressions) become more important as the average nuclei size decreases.

Simulation of Effects of Atmospheric Aerosols on Deep Turbulent Convective Clouds Using a Spectral Microphysics Mixed-Phase Cumulus Cloud Model. Part II: Sensitivity Study

Abstract Effects of different size distributions of cloud condensational nuclei (CCN) on the evolution of deep convective clouds under dry unstable continental thermodynamic conditions are investigated using the spectral microphysics Hebrew University Cloud Model (HUCM). In particular, high supercooled water content just below the level of homogeneous freezing, as well as an extremely high concentration of ice crystals above the level, observed recently by Rosenfeld and Woodley at the tops of growing clouds in Texas, were successfully reproduced. Numerical experiments indicate a significant decrease in accumulated precipitation in smoky air. The fraction of warm rain in the total precipitation amount increases with a decrease in the CCN concentration. The fraction is low in smoky continental air and is dominating in clean maritime air. As warm rain is a smaller fraction of total precipitation, the decrease in the accumulated rain amount in smoky air results mainly from the reduction of melted precipitation. It is shown that aerosols significantly influence cloud dynamics leading to the elevation of the level of precipitating particle formation. The falling down of these particles through dry air leads to a loss in precipitation. Thus, close coupling of microphysical and dynamical aerosol effects leads to the rain suppression from clouds arising in dry smoky air. The roles of freezing, CCN penetration through lateral cloud boundaries, and turbulent effects on cloud particles collisions are evaluated. Results, obtained using spectral microphysics, were compared with those obtained using two well-known schemes of bulk parameterization. The results indicate that the bulk parameterization schemes do not reproduce well the observed cloud microstructure.

Classification of Palladium Nucleation Processes Based on Nucleus Size Distribution

Catalytic Pd nucleation is a crucial preparatory step for the activation of electroless (EL) Cu deposition. We have observed that the time evolution of catalytic Pd nucleation can be classified into stages according to the Pd nuclei density and the nucleus size distribution. We have tentatively labeled these stages as growth, secondary nucleation, and ripening. The stage identification is assisted with the observation of distribution range and semi-interquartile range. In this study we found that the smoothest EL Cu film can be achieved with Pd activation corresponding to the beginning of the ripening stage. © 2004 The Electrochemical Society. All rights reserved.

Dimensionless spray flux in wet granulation: Monte-Carlo simulations and experimental validation

Dimensionless spray flux Ψ a is a dimensionless group that characterises the three most important variables in liquid dispersion: flowrate, drop size and powder flux through the spray zone. In this paper, the Poisson distribution was used to generate analytical solutions for the proportion of nuclei formed from single drops ( f single) and the fraction of the powder surface covered by drops ( f covered) as a function of Ψ a. Monte-Carlo simulations were performed to simulate the spray zone and investigate how Ψ a, f single and f covered are related. The Monte-Carlo data was an excellent match with analytical solutions of f covered and f single as a function of Ψ a. At low Ψ a, the proportion of the surface covered by drops ( f covered) was equal to Ψ a. As Ψ a increases, drop overlap becomes more dominant and the powder surface coverage levels off. The proportion of nuclei formed from single drops ( f single) falls exponentially with increasing Ψ a. In the ranges covered, these results were independent of drop size, number of drops, drop size distribution (mono-sized, bimodal and trimodal distributions), and the uniformity of the spray. Experimental data of nuclei size distributions as a function of spray flux were fitted to the analytical solution for f single by defining a cutsize for single drop nuclei. The fitted cutsizes followed the spray drop sizes suggesting that the method is robust and that the cutsize does indicate the transition size between single drop and agglomerate nuclei. This demonstrates that the nuclei distribution is determined by the dimensionless spray flux and the fraction of drop controlled nuclei can be calculated analytically in advance.

Sensitivity of raindrop formation in ascending cloud parcels to cloud condensation nuclei and thermodynamic conditions

AbstractThis paper uses a 2000‐bin spectral microphysics cloud parcel model to investigate the effects of aerosol particles on droplet spectrum evolution and warm rain formation in ascending cloud parcels under maritime, intermediate, and continental unstable thermodynamic conditions. Cloud parcels of different cloud depth are simulated for each thermodynamic condition. Concentration and size distribution of cloud condensation nuclei (CCN) were varied within a wide range for each cloud depth.Cloud parcels were divided into three groups with respect to the relationship between cloud depth Hc and the distance of the collision triggering level Hcol above cloud base. The sensitivity of precipitation to CCN variations is quite different for the different groups. Parcels of the first group (Hc < Hcol) do not precipitate. Precipitation from cloud parcels belonging to the second group, in which collisions start at a small distance below the maximum cloud‐top height (Hc ≃ Hcol), turns out to be highly sensitive to the variation of CCN concentration and size distribution. For these parcels an increase in the concentration of large CCN can result in an increase in precipitation amount by several factors. Many cloud parcels arising under continental and intermediate conditions, as well as not very deep maritime clouds, can be assigned to this group. The precipitation amount from cloud parcels belonging to the third group (Hc ≫ Hcol), represented by deep tropical clouds, turns out to be insensitive to CCN distribution and droplet concentration within a wide range of their variations. Comparable effects of small and large CCN on precipitation formation are analysed for each group of cloud parcels.These results are closely related to the problem of rain enhancement via hygroscopic seeding. It is shown that there is a certain range of cloud depths within which cloud seeding can potentially lead to a significant (several times) increase in rain within ascending parcels. Rain formed in cloud parcels with depths beyond this range is only slightly sensitive to the CCN concentration. Copyright © 2004 Royal Meteorological Society

CCD observations of distant comets from Palomar and Steward Observatories

We are conducting a ground-based observational study of distant cometary nuclei with the aim of increasing the current database of physical parameters of individual objects, and to estimate the overall distributions of size, rotation period, axial ratio, and color indices. Additionally, we are obtaining CCD spectroscopy and photometry of established and potential targets of current and future spacecraft missions. The results presented here are derived from CCD imaging obtained using the 2.3-m Bok telescope of Steward Observatory (Arizona), obtained in May 2001, and the 5-m Hale telescope at Palomar Observatory (California), obtained in May 2000 and March 2001. Comets observed include 4P/Faye, 6P/d'Arrest, 22P/Kopff, 36P/Whipple, 50P/Arend, 78P/Gehrels 2, 92P/Sanguin, 107P/Wilson–Harrington, and 128P/Shoemaker–Holt 1-A. Of the nine comets observed, only Comets 4P/Faye and 50P/Arend displayed visible coma activity. We have performed either single R filter or multi-filter ( BVRI) measurements on these comets, from which we obtain radius and broadband color estimates as well as Afρ values for the active comets. For selected objects we have performed time-series R filter imaging from which we have derived the rotation period and lower limits on the nuclear axial ratio and density. The radius results obtained are included in the cometary nucleus size distribution estimate by Weissman and Lowry (2003).

Formation of droplets on active centers in supersaturated vapors

Kinetics of nucleation on active centers in supersaturated vapors is studied by a numerical solution of kinetic equations, including depletion of active centers during the phase transition process. The basic characteristics of the nucleation process (the size distribution of nuclei, nucleation rate and the total number of nuclei) are determined at various conditions. Nuclei are formed on all active centers at a sufficiently long time in coincidence with a widely used Avrami model. An increase of the total number of nuclei with time is steeper in the Avrami model as compared with our numerical solution of kinetic equations. The nucleation rate does not reach its stationary limit and becomes negligibly small with increasing time.

Nucleation regime map for liquid bound granules

AbstractNucleation is the first step in granulation where the powder and liquid first contact. Two types of nucleation in wet granulation processes are proposed. Drop controlled nucleation, where one drop forms one nucleus, occurs when drops hitting the powder surface do not overlap (low spray flux Ψa) and the drop must wet quickly into the bed (short drop penetration time tp). If either criterion is not met, powder mixing characteristics will dominate (mechanical dispersion regime). Granulation experiments were performed with lactose powder, water, PEG200, and 7% HPC solution in a 6 L and a 25 L mixer granulator. Size distributions were measured as the drop penetration time and spray flux were varied. At short penetration times, decreasing Ψa caused the nuclei distribution to become narrower. When drop penetration time was high, the nuclei size distribution was broad independent of changes in dimensionless spray flux. Nucleation regime maps were plotted for each set of experiments in each mixer as a function of the dimensionless distribution width δ. The nucleation regime map demonstrates the interaction between drop penetration time and spray flux in nucleation. The narrowest distribution consistently occurred at low spray flux and low penetration time, proving the existence of the drop controlled regime. The nucleation regime map provides a rational basis for design and scale‐up of nucleation and wetting in wet granulation.

Cellular Organization and Substructure Measured Using Angle-Resolved Low-Coherence Interferometry

We measure the organization and substructure of HT29 epithelial cells in a monolayer using angle-resolved low-coherence interferometry. This new technique probes cellular structure by measuring scattered light, as in flow cytometry, but offers an advantage in that the structure can be examined in situ, avoiding the need to disrupt the cell monolayer. We determine the size distribution of the cell nuclei by fitting measured light-scattering spectra to the predictions of Mie theory. In addition, we obtain information about the cellular organization and substructure by examining the spatial correlations within the monolayer. A remarkable finding is that the spatial correlations over small length scales take the form of an inverse power law, indicating the fractal nature of the packing of the subcellular structures. We also identify spatial correlations on a scale large compared with the size of a cell, indicating an overlying order within the monolayer.

Size distributions of silver and copper crystals as a function of electrocrystallization conditions on glassy carbon

Specific features and regularities of size distributions of crystalline nuclei are studied using the electrocrystallization of copper and silver on glassy carbon as an example. For a potentiostatic nucleation mode, with increasing overvoltage, a nonuniform size distribution approaches a uniform one. For a galvanostatic nucleation mode, the most probable nuclei size decreases with increasing current density and decreasing concentration of copper ions in solution, and the nuclei size distribution becomes more nonuniform due to a more pronounced non-steady-state nature of galvanostatic nucleation.

Initial stages of the GaN growth on oxidized silicon

Initial stages of the GaN layer growth during an HVPE process at 520°C on an oxidized silicon substrate were studied by atomic force microscopy. It was established that (i) the growth of GaN islands is controlled by the surface diffusion and (ii) the nucleus size distribution on the surface significantly changes when the growth time increases from 10 to 200 min: during this period of time, the average nucleus size increases from 15 to 400 nm and their size scatter becomes 20 times as large as the initial size distribution width. The experimentally determined growth rate of the GaN nuclei and the nucleus size distribution are in good agreement with theoretical calculations.

Micrometer‐sized hygroscopic particles in the atmosphere: Aircraft measurement in the Arctic

Aircraft measurements of concentrations and size distributions of nuclei between 1 and 11 μm diameter were made in cloud‐free air at altitudes below 6.2 km (approximately −30°C), using a new instrument (the cloudscope) north of Barrow, Alaska, during the First International Satellite Cloud Climatology Project Regional Experiment/Arctic Clouds Experiment (FIRE III/ACE) in May 1998. Data were obtained at various flight levels in the vicinity of a Canadian icebreaker frozen into and moving with the Arctic Ocean ice field. Ice field movements caused the frozen ocean surface to develop leads that were visible from the aircraft. Particles were collected at the stagnation point of a forward facing 3 mm diameter optical flat maintained at or above stagnation point temperatures. Images from a 425 μm × 320 μm area were video‐recorded, and concentrations were corrected for collection efficiency based on ambient airspeed, temperature, and assumed particle density. At low ambient relative humidities, crystallized particles were collected directly, and their hygroscopic character was determined by observing growth characteristics following flight into regions of higher relative humidity. Solution drops collected at high relatively humidities were crystallized with controlled heating to approximately a hemispherical shape. Particles with a dry diameter greater than 1 μm, the detection threshold, to 11 μm were found at concentrations up to 4 L−1. Concentrations were variable with altitude, and on occasion no particles were observed for periods up to 1 hour about 50 L of air. Concentrations sufficient to initiate coalescence drizzle as clouds form were observed and may be of importance in ice nucleation. Comparisons with cloud condensation nuclei (CCN) measurements (providing the collected droplets) show that on occasion the large particles (providing the collector drops) comprised a significant component of the mass of hygroscopic material. There is a gap in measurement of particles having known hygroscopic properties in that measurements of CCN have an upper limit of 0.3 μm dry diameter and the cloudscope has a lower limit of 1 μm; substantial mass of hygroscopic material could lie within this range.

高純度アルミニウム箔の最終焼なまし過程におけるＣｕｂｅ方位形成過程

The purpose of the present work is to clarify the factors that cause the difference between the 99%–foil and the 98%–foil in volume fraction of cube component in high-purity aluminum foils for electrolytic capacitors. The growth behavior of the cube-oriented grains during the final annealing was investigated in detail using SEM–EBSP technique. A remarkable difference was seen in grain size and distribution of the cube and R components between the two foils at the heating point of 300°C during the final annealing. On the basis of these results, the formation process of the residual non-cube grains that are controlled by the difference in the grain size distribution of cube grains after primary recrystallization is discussed. It was thought that the maximum grain diameter is an influential factor in the process, and it depends on the thickness of the foil. Furthermore, it was discussed that the size distribution of cube-grains after primary recrystallization is affected by the factors that exert influence on the driving force of grain boundary movement. These factors are the heating rate in the final annealing, the reduction rate in light-rolling and particularly the size distributions of cube nuclei formed after the intermediate annealing.

The formation kinetics of a strongly absorbing state in a bistable excitonic noncavity system

A unified approach allowing the description of all formation stages of a strongly absorbing state in a bistable excitonic noncavity system as a nonequilibrium first-order phase transition is proposed. In the fast-switching-wave approximation, the switching-wave front velocity is determined, and the thickness of the phase discontinuity interface is estimated. The size distributions of subcritical and supercritical nuclei and an asymptotic expression for the nucleus radius are obtained.

Liquid distribution in wet granulation: dimensionless spray flux

This study investigates binder distribution in wet granulation and focuses on the nucleation zone, which is the area where the liquid binder and powder surface come into contact and form the initial nuclei. An equipment independent parameter, dimensionless spray flux Ψa, is defined to characterise the most important process parameters in the nucleation process: solution flowrate, powder flux, and binder drop size. Ex-granulator experiments are used to study the relationship between dimensionless spray flux, process variables and the coverage of binder fluid on the powder surface. Lactose monohydrate powder on a variable speed riffler passed under a flat spray once only. Water and 7% HPC solution at two spray pressures were used as binders. Experiments with red dye and image analysis demonstrate that changes in dimensionless spray flux correlate with a measurable difference in powder surface coverage. Nucleation experiments show that spray flux controls the size and shape of the nuclei size distribution. At low Ψa, the system operates in the drop controlled regime, where one drop forms one nucleus and the nuclei size distribution is narrow. At higher Ψa, the powder surface cakes creating a broader size distribution. For controlled nucleation with the narrowest possible size distribution, it is recommended that the dimensionless spray flux be less than 0.1 to be in the drop-controlled regime.

Initial Precipitation Formation in Warm Florida Cumulus

The microphysical processes that lead to the development of precipitation in small, warm cumulus are examined using data from the Small Cumulus Microphysics Study near Cape Canaveral, Florida. Aircraft measurements are used to determine the concentration and size distribution of giant and ultragiant nuclei in clear air as a function of relative humidity, altitude, wind speed, and wind direction. The clear-air particle distributions show that ultragiant particles (radii extending from 10 to 150 mm) exist from the surface to cloud base in concentrations that correspond to the concentrations of raindrops observed during drizzle to moderate rainfall events. A shift of the spectra toward larger size with increasing relative humidity was observed, suggesting that the spectra are composed of deliquesced particles growing by condensation. The small cumulus clouds are shown to contain cores where the observed liquid water content was nearly adiabatic. The observed evolution of the cloud droplet distribution within the near-adiabatic cores as a function of height showed an increase in the small droplet mode associated with condensation and an increase in the concentration of larger droplets associated with growth by accretion. Droplets with radii extending to nearly 100 mm were present just above cloud base. These measurements were consistent with the clear-air measurements and provided evidence that the ultragiant nuclei can immediately act as embryos for raindrop growth by accretion upon entering cloud base. Comparisons of reflectivity computed from the cumulus core composite droplet distributions with the radar-observed reflectivity data provided independent evidence that the composite spectra reasonably represented the evolving microstructure of the cores of small cumulus clouds as they grew vertically. The analyses provide strong evidence of an efficient process for the initial development of precipitation in small Florida cumuli. This process consists of raindrop embryo formation on ultragiant nuclei followed by growth by accretion as the newly formed drops proceed upward through the adiabatic cores of the cumulus clouds. These data support the conceptual model of raindrop formation in marine clouds first proposed by Woodcock a half century ago.

Endoscopic detection of dysplasia in patients with Barrett's esophagus using light-scattering spectroscopy

Background & Aims: We conducted a study to assess the potential of light-scattering spectroscopy (LSS), which can measure epithelial nuclear enlargement and crowding, for in situ detection of dysplasia in patients with Barrett's esophagus. Methods: Consecutive patients with suspected Barrett's esophagus underwent endoscopy and systematic biopsy. Before biopsy, each site was sampled by LSS using a fiberoptic probe. Diffusely reflected white light was spectrally analyzed to obtain the size distribution of cell nuclei in the mucosal layer, from which the percentage of enlarged nuclei and the degree of crowding were determined. Dysplasia was assigned if more than 30% of the nuclei exceeded 10 μm and the histologic findings compared with those of 4 pathologists blinded to the light-scattering assessment. The data were then retrospectively analyzed to further explore the diagnostic potential of LSS. Results: Seventy-six sites from 13 patients were sampled. All abnormal sites and a random sample of nondysplastic sites were reviewed by the pathologists. The average diagnoses were 4 sites from 4 different patients as high-grade dysplasia (HGD), 8 sites from 5 different patients as low-grade dysplasia (LGD), 12 as indefinite for dysplasia, and 52 as nondysplastic Barrett's. The sensitivity and specificity of LSS for detecting dysplasia (either LGD or HGD) were 90% and 90%, respectively, with all HGD and 87% of LGD sites correctly classified. Decision algorithms using both nuclear enlargement and crowding further improved diagnostic accuracy, and accurately classified samples into the 4 histologic categories. Conclusions: LSS can reliably detect LGD and HGD in patients with Barrett's esophagus.GASTROENTEROLOGY 2000;119:677-682

Inhomogeneous distribution of charged colloidal particles studied by confocal laser scanning microscopy

Various types of inhomogeneous particle distributions in macroscopically homogeneous, purified colloidal dispersions were observed with a confocal laser scanning microscopy with the facility for optical sectioning. Time evolution of nuclei size distributions on the initial stage of colloidal crystal growth was measured and the three-dimensional growing process was analyzed. The larger clusters grew further at the expense of the smaller ones, which indicated the existence of a net attraction between particles. The coexistence of ordered and disordered regions was observed and the disordered region was found not only in the interstitial areas between ordered regions but also inside the ordered region, called the Swiss cheese like structure. Stable voids, where the particle concentrations were almost nil on the time scale of the observation, coexisted with surrounding regions where the particles were distributed in a liquid-like structure. Charged particles in more dilute dispersions were observed being condensed near like-charged surfaces. A large number of particles gathered near the highly charged surfaces, but the gathering was not observed near uncharged surfaces, which pointed out the existence of electrostatic attraction between like-charged surfaces and particles. The inhomogeneous particle distributions can be explained by a net electrostatic attraction.