Kenji Iimura

Properties of carbon onions produced by an arc discharge in water

A simple method to fabricate high-quality nanoparticles including spherical carbon onions and elongated fullerene-like nanoparticles similar to nanotubes in large quantities without the use of vacuum equipment is reported. The nanoparticles are obtained in the form of floating powder on the water surface following an arc discharge between two graphite electrodes submerged in water. High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy images confirm the presence of spherical carbon onions with diameters ranging from 4 to 36 nm. The specific surface area of the floating powder was found to be very large, 984.3 m2/g, indicating that the material is promising for gas storage. From the surface area measurements, the mean particle diameter was calculated to be 3.7 nm. This value is close to the lower limit of the carbon onions observed in HRTEM. However, closer HRTEM observations also reveal that some carbon onions are not well crystallized. The large specific surface area c...

Simulation of deformation and breakup of large aggregates in flows of viscous fluids

A three-dimensional modified discrete element method (mDEM) in which the effective particle surface for the hydrodynamic drag force and the disturbance of neighboring particles on the flow field are taken into account is proposed to simulate the deformation and breakup process of large aggregates in flows. First, the dynamic shape factor of rectangular aggregates is simulated and compared with the corresponding experiment, showing that the method can predict the behavior of aggregates in fluids quantitatively. Secondly the method is applied to simulate the breakup of large particle–cluster and cluster–cluster aggregates in shear and elongational flows. It is found that a power law relation holds between the average number of particles in broken fragments 〈i〉 and the intensity of flow field. In the case of simple shear flow, the value of 〈i〉 is approximated by the following universal function of NDA, the ratio of the representative hydrodynamic drag force and the adhesive force, independently of the number, the size and the size distribution of constituent particles, and the minimum separation distance between particle surfaces. This correlation agrees well with the experimental results reported elsewhere. 〈i〉=27.9×NDA−0.872. It is also predicted that elongational flow is more effective to break up aggregates than the simple shear flow under usual flow conditions.

Study of the Wall Effect on Particle Packing Structure Using X-ray Micro Computed Tomography

Abstract The wall effect on particle assembling structure in a particle packed bed is an important issue for powder technology. However, the detailed and systematic experimental data about the wall effect on particle assembling structure in a particle packed bed is hardly adequate yet. The distribution of the local void fraction near the container wall in a particle packed bed is measured using X-ray micro computed tomography (CT). The wall of the cylindrical vessel affects the void fraction distribution, and the distribution can be expressed by the damped oscillation function that shows the relation between the distance from the container wall and the local void fraction. The damping factor, the amplitude and two factors in our damped oscillation function varied with the ratio of particle diameter and inner diameter of the cylindrical container. Our empirical equation can be applied for X-ray CT data and also compared with the traditional experimental data by Ridgway and Tarbuck.

Pressure effects on nanotubes formation using the submerged arc in water method

Synthesis of multi-walled carbon nanotubes (MWCNTs) by an arc discharge between two graphite electrodes submerged in water under controlled pressure (from 400 to 760 Torr) is reported. Transmission and scanning electron microscopy investigations of the arc discharge product collected from the bottom of the reactor revealed high concentrations of MWCNTs at all pressures. Dynamic light scattering (DLS) on suspensions containing MWCNTs showed that the mean diameter of the nanotubes increases with decreasing pressure. Raman spectroscopy analysis reveals that the relative amount of disordered carbon is significantly less in the low-pressure samples. Furthermore, the yield of the deposit was found to be independent of the pressure. These results suggest that the physical properties of MWCNTs formed by the submerged arc can be controlled by varying the pressure.

Deformation of aggregates depositing on a plate in a viscous fluid simulated by a modified discrete element method

A three-dimensional modified discrete element method in which the hydrodynamic drag force and static interparticle force are taken into account is proposed to simulate the deformation process of two types of large aggregates depositing to a plate in a viscous fluid under various external forces. It is found that the changes of the structure and the average cross-section density of aggregates in the deformation process look reasonable, and the following microscopic features are predicted. (1) When the aggregate is not destroyed completely by the external force, the portions of high density appear not only at the bottom but also at the position a little lower from the top. (2) When the aggregate is destroyed completely by the external force, the final aggregate is composed of at least two structures; the highly compacted structure just above the plate and the much less compacted structure above it. (3) The density of an aggregate is able to be increased by applying an external force weak enough for particles to rearrange themselves to form the compact structure. (4) The unevenness of the density distribution in an aggregate of loose structure will be increased during the deformation, because each portion of different density in the aggregate will sediment at a different velocity.

Simulation of the hydrodynamic drag force on aggregates

Abstract —A simulation model to predict the dynamic behavior of large aggregates in flows was developed, in which the effective particle surface exposed to the flow was evaluated and the reduction of fluid velocity due to neighboring particles was taken into account. Thismodel was applied to predict the dynamic shape factors of regular shaped aggregates settling in a quiescent fluid and the simulated results were compared with the experiments reported elsewhere. It is found that the hydrodynamic drag force exerted on aggregates is simulated quantitatively by the present model. This indicates that the proposed simulation model is applicable to predict the complicated behavior of large aggregates in flows, such as their deformation and breakup.

Proposal of an approximation equation for the yield locus to evaluate powder properties

The yield locus (YL) of powder bed can be used to determine many mechanical properties of a powder such as cohesion, unconfined yield stress, stress ratio, etc. Generally, the YL of powder beds is obtained by fitting the results of shear tests to linear approximations based on the Coulomb equation or to curved approximations based on the Warren–Spring equation. Meanwhile, the yielding characteristics of a powder bed are expressed by the Roscoe condition diagram. In this diagram, the YL appears orthogonal to the normal stress axis at both ends corresponding to tensile and compressive strength. However, the YL approximated by the Coulomb or Warren–Spring equations is not orthogonal to the normal stress axis at both ends, and is not the same shape as the YL shown in Roscoe condition diagrams. Thus, the abovementioned mechanical properties obtained from the YL of a powder bed are likely to be affected by the approximate expression for the YL. Despite this, no one has investigated how the mechanical properties of powder beds such as stress ratios are affected by the approximation method for the YL. In this paper, we propose a new approximation equation for the YL that conforms both to the shape of the YL in the Roscoe condition diagrams and experimental results. Then, these YL obtained by our equation, and by the Coulomb and Warren–Spring equations are used to determine the mechanical and flow properties of powder beds. These values are compared with each other in order to discuss the validity of our equation.

Shape Effects of the Yield Locus on the Rankine Coefficient

The Rankine coefficient is the ratio of the vertical pressure (maximum principal stress) and horizontal pressure (minimum principal stress) acting on a powder bed. The value of the Rankine coefficient is usually determined from the yield locus, which comes from the shear test results either by linear approximation using the Coulomb equation or by curve fitting using the Warren–Spring equation. Since the shape of the yield locus is obtained by approximation, the Rankine coefficient might change with the estimation method. The Rankine coefficient is an important coefficient for estimating the pressure distribution in a powder bed. However, no studies about the effect of the yield locus estimation method on the Rankine coefficient can be found in the past literature. In this paper, first we study the effect of three approximations, i.e. the Coulomb equation, the Warren–Spring equation and an equation proposed by the authors, on the Rankine coefficient. Based on these results, we investigate how the selection of the yield locus estimation method affects the Rankine coefficient. Then we compare the Rankine coefficients obtained from consolidation tests and from each of these yield loci, thereby demonstrating the validity of our proposed approximation equation.

Development of a particulate emulsifier by mechanochemical treatment and application to the liquid-liquid extraction process

Abstract -A method to modify quartz particles to be a surface-active material by mechanochemical treatment was successfully developed. The obtained particles had sufficient properties for use as a particulate emulsifier. The particles prepared by mechanochemical treatment had a highly anisotropic surface, and the droplets emulsified by those particles were extremely large and stable because of their stonewall-like structure and high density. The obtained particulate emulsifier was applied to the liquid-liquid extraction process. Consequently it was clarified that the extraction rate increased 3.6 times according to the increase of interfacial area without reducing the mass transfer coefficient.