Atsuko Shimosaka

Formation and morphology of asymmetric NaCl particles precipitated at the liquid-liquid interface

Abstract —Asymmetric particles of NaCl and KCl crystals have been obtained by using a crystallization method. The particles were produced by precipitation at the solution-1-butanol interface. The two solvents are essentially immiscible, but have slight mutual diffusion. When the aqueous solution contacts with 1-butanol, the interfacial area becomes supersaturated, resulting in nucleation and particle growth in the interfacial zone. The asymmetric particles featured a stepped surface on the side in contact with 1-butanol. The other surfaces in contact with the aqueous phase were a flat cubic structure. Precipitated NaCl particles showed very clear asymmetric structure. However, some of KCl particles had less well-defined structures. This is due to the high diffusion rate of hydrated K ions and the fast diffusion results in a supersaturated state around the interface. This situation can be explained by a model of crystal growth based on solute clusters in supersaturated solutions.

Crystalline–amorphous transitions of Ge–Se alloys by mechanical grinding

Abstract X-ray diffraction and differential scanning calorimetry (DSC) measurements of Se and GeSe2 during ball milling have been investigated. In this study, amorphous Se and GeSe2 are synthesized by mechanical grinding (MG) recognized as a method of inducing a crystalline–amorphous phase transition. The amorphization of Se is caused from distortion between helical chains connected by van der Waals forces. The enthalpy ΔH for crystallization of Se by rapid quench is lower than that by the MG. This result suggests that the difference of ΔH corresponds to that of the structures in intermediate range. Monoclinic GeSe2 is amorphized by disordering between the connected tetrahedra. It is considered that the tetrahedral unit of GeSe2 by the MG is distorted with wide distributions. GeSe2 is completely amorphized after 4 h of grinding, and Se after 30 h in the present milling. The difference indicates that disordering between the connected tetrahedral units in monoclinic GeSe2 is induced more easily than that between chains in trigonal Se.

Kinetics of co-crystal formation with caffeine and citric acid via liquid-assisted grinding analyzed using the distinct element method.

The kinetics of co-crystal formation of caffeine (CF) with citric acid (CTA) was evaluated. Ball milling of CF and CTA in molar ratios of 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, and 1:4 was performed by the liquid-assisted grinding (LAG) method. The samples were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). Two types of co-crystals (co-crystal-1, a 1:1 CF-CTA co-crystal; and co-crystal-2, a new co-crystal form) were obtained. The kinetic characteristics of this new co-crystal formation were assessed by calculating the ball impact energy and force using the distinct element method (DEM) simulations. The results indicated that co-crystal-2 creation occurred under a condition in which the ball impact force exceeded a certain threshold value. Moreover, the total ball impact energy was positively correlated with co-crystal formation, exhibiting a higher ball impact force than the threshold value. The kinetics of co-crystal-2 formation was almost consistent with the Jander equation. Consequently, co-crystal-2 formation could be explained according to a three-dimensional diffusion mechanism.

Estimation of electrostatic charge distribution of flowing toner particles in contact with metals

Abstract The influence of impact behaviors of polymers on electrification was investigated. We proposed the charging model of the polymer particles. First, impact charges of single polymer particle–metal plate and polymer particle–polymer plate were measured by an impact charging experiment in this study. In the case of impacts between the polymer–metal, it was concluded that the impact charge increased with vertical and horizontal impact velocities against the plate. On the other hand, the impact charge between the polymer–polymer depended only on that of the horizontal component. The results showed different charging mechanism between the polymer–metal and the polymer–polymer. Second, impact behaviors in the experiment were analyzed by means of finite element method (FEM) simulation in order to obtain the information concerning the impact from a microscopic point of view. The FEM results indicated that the particle was rotated and slid simultaneously during impact. It was also found that the particle always had a contact area larger than the plate. On the basis of experimental and simulated results, we proposed the charging model of the polymer particle. Third, we tried to confirm the validity of the proposed model by electrification of polypropylene particles in vibration fields, theoretically and experimentally. Calculated results by particle element method (PEM) simulation (in other words, distinct element method (DEM)) were in good agreement with the experimental results. Then, we estimated the charge of toner particles using this model.

Cell optimization for fast contact detection in the discrete element method algorithm

The objective of this paper is to find out a rule of cell optimization for fast contact detection in the discrete element method (DEM) algorithm. The contact detection process was executed by using the conventional cell model or distinct cell model (DCM) under several particle conditions having different particle size ratio and volume ratio, and its time was measured. The conventional cell model is suitable for the conditions that particle size ratio, dlarge/dsmall, is small and its volume ratio, Vsmall/V0, is also small. The optimum cell condition of the conventional cell model is that the mean number of particle in a cell is 1.0–5.0, regardless of the particle size ratio and its volume ratio. Contact detection using DCM, which has several grids for different sized particles, is much faster than that of the conventional model when particles have a large size ratio. The size of the larger cell does not affect the contact detection speed; however, the size of the smaller cell is important for the fast contact detection and its size is determined to be about 3.0 times the particle radius. Therefore, the optimum cell condition is estimated from the particle size ratio and its volume ratio, and a large-scale DEM will be possible.

Estimation of the sieving rate of powders using computer simulation

Abstract Computer simulation of sieving behavior of powders was performed to estimate the sieving rate by the particle element method. The simulated time-residue curve of sieving agreed well with the experimental curve. Furthermore, the simulation can well represent the sieving behavior of cohesive powders. On the basis of the simulation results, the equation to estimate the sieving rate was proposed. The equation involves three engineering parameters newly defined in this study. The relationship between the engineering and operating parameters in the sieving, such as the vibration amplitude and frequency of the sieving surface, is obtained by the simulation. The applicability of the proposed equation is confirmed by experimental sieving of silica sand.

Mechanically Strain-Induced Modification of Selenium Powders in the Amorphization Process

For the fabrication of particles designed in the nanoscale structure, or the nanostructural modification of particles using mechanical grinding process, selenium powders ground by a planetary ball mill at various rotational speeds have been investigated. Structural analyses, such as particle size distributions, crystallite sizes, lattice strains and nearest neighbour distances were performed using X-ray diffraction, scanning electron microscopy and dynamical light scattering.By grinding powder particles became spherical composites consisting of nanocrystalline and amorphous phase, and had a distribution with the average size of 2.7 μm. Integral intensities of diffraction peaks of annealed crystal selenium decreased with increasing grinding time, and these peaks broadened due to lattice strains and reducing crystallite size during the grinding. The ground powder at 200 rpm did not have the lattice strain and showed amorphization for the present grinding periods. It indicates that the amorphization of Se by grinding accompanies the lattice strain, and the lattice strain arises from a larger energy concerning intermolecular interaction. In this process, the impact energy is spent on thermal and structural changes according to energy accumulation in macroscopic (the particle size distribution) and microscopic (the crystallite size and the lattice strain) range.

PARAMETERS OF RADIATED SOUND AND STATE VARIABLES IN FLOWING PARTICLES

The acoustic noise generated from flowing particles on an inclined plate was studied to measure simultaneously the flow rate and particle size. The pressure waveform of the flow noise was estimated on the basis of the generation mechanism of impact sound when two particles collide. The frequency spectrum of the calculated pressure waveform agrees well with the measured one. Correlation between the flow rate and the pressure of flow noise was obtained, and the frequency of flow noise was related closely to particle diameter. Further, the mean collision time and the mean distance between flowing particles was estimated by the frequency spectrum of the flow noise.

Diffusion behavior in a liquid-liquid interfacial crystallization by molecular dynamics simulations

Interfacial crystallization, such as surface crystallization in solution (solid-liquid) and liquid-liquid crystallization, gives us an asymmetric reaction field and is a technique for morphology control of crystals. In the liquid-liquid crystallization, the concentration distribution of solute ions and solvent molecules at the liquid-liquid interface directly relates to nucleation, crystal growth, and crystal morphology. Nonequilibrium molecular dynamics (MD) simulations have been performed at interfaces in NaCl solution/1-butanol and KCl solution/1-butanol system in order to clarify diffusion behavior of solute ions and solvent molecules. As simulation results, the hydrated solute ions were dehydrated with the diffusion of water from solution phase into 1-butanol phase. The different dehydration behaviors between NaCl and KCl solution can be also obtained from MD simulation results. Aggregated ions or clusters were formed by the dehydration near the solution/1-butanol interface. By comparison on the normalized number of total solute ions, the size and number of generated cluster in KCl solution/1-butanol interface are larger than those in the NaCl system. This originates in the difference hydration structures in the each solute ion.

Effect of surface properties of calcium carbonate on aggregation process investigated by molecular dynamics simulation

The control of the morphology and polymorphs of CaCO3 has been required, since their capabilities are dependent on particle characteristics of CaCO3. It is difficult to clarify the formation mechanism only by the experimental approaches because the nucleation and crystal growth of CaCO3 occurred at nanoscale. The objective of the present work is to investigate the effect of surface properties including in the charge density and the dielectric constant of water on the aggregation process of primary particles by means of molecular dynamics simulations. The grain boundary energy was also calculated by molecular dynamics simulation to investigate the change of energy in the aggregation process of primary particles. From the results of molecular dynamics simulation, we found that the interface of (001)Ca and (001)CO3 contacted with water charged positively and negatively and the interface of (100) and (104) contacted with water charged neutrally. When the distance of interfaces between CaCO3 particles came close to colliding, the dielectric constant of water became small except the interface of (001)CO3. The boundary energy of interface between (001)Ca and (001)CO3 was the lowest among five types of interfaces. It indicates that the aggregation of (001)Ca and (001)CO surface is the easiest among all types of interfaces.