Effect of Rapid Solidification Rates on Preparation of Ni-Pb Alloy Hollow Particles with Low Pb Content

Using Ni-5 wt.% Pb alloys with low Pb content as master alloys, the Ni-5 wt.% Pb alloy hollow particles were prepared by rapid solidification. Moreover, the alloy particles’ microstructure and formation mechanism were investigated. The results show that the particles’ microstructure consisted of Ni-rich and Pb phases. The Ni-rich phase was formed in the dendrite, and the Pb phase was distributed in the grain boundary or interdendrites. With the roller speeds increasing, the sizes of hollow particles and holes were decreased which were deviated from the particle center, while the hollow ratio, shear stress, and turbulence intensity of the hollow particles were increased. The formation of alloy hollow particles is attributed to interaction between the high-speed fluid and environment gas on the liquid/gas interface. The increase in roller speeds was conducive to the formation of Ni-Pb alloy hollow particles with low Pb content.

Particle Center Supported Plane for Subsurface Target Classification based on Full Polarimetric Ground Penetrating Radar

The subsurface target classification of ground penetrating radar (GPR) is a popular topic in the field of geophysics. Among the existing classification methods, geometrical features and polarimetric attributes of targets are primarily used. As polarimetric attributes contain more information of targets, polarimetric decomposition methods, such as H-Alpha decomposition, have been developed for target classification of GPR in recent years. However, the classification template used in H-Alpha classification is preset depending on the experience of synthetic aperture radar (SAR); therefore, it may not be suitable for GPR. Moreover, many existing classification methods require excessive human operation, particularly when outliers exist in the sample (the data set containing the features of targets); therefore, they are not efficient or intelligent. We herein propose a new machine learning method based on sample centers, i.e., particle center supported plane (PCSP). The sample center is defined as the point with the smallest sum of distances from all points in the same sample, which is considered as a better representation of the sample without significant effect of the outliers. In this proposed method, particle swarm optimization (PSO) is performed to obtain the sample centers; the new criterion for subsurface target classification is achieved. We applied this algorithm to full polarimetric GPR data measured in the laboratory and outdoors. The results indicate that, comparing with support vector machine (SVM) and classical H-Alpha classification, this new method is more efficient and the accuracy is relatively high.

A Numerical Study of Particle Migration and Sedimentation in Viscoelastic Couette Flow

In this work, a systematic investigation of the migration of sedimenting particles in a viscoelastic Couette flow is presented, using finite element 3D simulations. To this end, a novel computational approach is presented, which allows us to simulate a periodic configuration of rigid spherical particles accurately and efficiently. To study the different contributions to the particle migration, we first investigate the migration of particles sedimenting near the inner wall, without an externally-imposed Couette flow, followed by the migration of non-sedimenting particles in an externally-imposed Couette flow. Then, both flows are combined, i.e., sedimenting particles with an externally-imposed Couette flow, which was found to increase the migration velocity significantly, yielding migration velocities that are higher than the sum of the combined flows. It was also found that the trace of the conformation tensor becomes asymmetric with respect to the particle center when the particle is initially placed close to the inner cylinder. We conclude by investigating the sedimentation velocity with an imposed orthogonal shear flow. It is found that the sedimentation velocity can be both higher or lower then the Newtonian case, depending on the rheology of the suspending fluid. Specifically, a shear-thinning viscosity is shown to play an important role, which is in-line with previously-published results.

Dependence of the Regime of Aggregate Structures of Magnetic Rod-Like Particles on the Magnetic Model

In the present study, we attempt to discuss the dependence of the regime of the aggregate structures of magnetic rod-like particles on the magnetic model. Moreover, we briefly discuss the characteristic magneto-rheological properties of each magnetic model. Three representative magnetic models are here addressed for a magnetic rod-like particle, that is, (a) a spherocylinder particle with a dipole moment at the particle center (dipole model), (b) a spherocylinder particle with a plus and a minus charge at the center of each hemi-sphere (charge model) and (c) a spherocylinder with a dipole moment in a direction normal to the particle axis direction at the particle center (hematite model). For each magnetic model, molecular simulations based on the Monte Carlo method have been performed in order to elucidate the influences of magnetic particle-particle and particle-field interactions on the aggregate structures of magnetic spherocylinder particles in thermodynamic equilibrium. For the case of the dipole model, long stable raft-like clusters are gradually formed with increasing magnetic particle-particle interaction strength, and these raft-like clusters dissociate into the formation of chain-like clusters with increasing magnetic field strength. For the case of the charge model, long and thick chain-like clusters are more significantly formed with increasing magnetic interactions, and the thick chain-like clusters in the field direction become further thicker with increasing magnetic field strength. For the hematite model, long raft-like clusters are formed and these clusters still remain and incline in the field direction in a strong magnetic field situation. From these results, it is evident that the different magnetic model gives rise to the significantly different regime of the aggregate structures. Moreover, Brownian dynamics simulations have been conduced in order to clarify the dependence of the magneto-rheological characteristics on the regime of the above-mentioned particle aggregates. Among these magnetic models, the charge model yields the largest magneto-rheological effect, whereas the hematite model provides the negative viscosity due to the magnetic properties of particles.

Mechanism of primary fragmentation of coal in fluidized bed

In order to lay a foundation of a credible primary fragmentation model, a theoretical analysis of the thermo-mechanical processes in a devolatilizing solid fuel particle was carried out. The devolatilization model comprises heat transfer, chemical processes of generation of gaseous products of combustion (volatiles), volatile transfer, and solid mechanic processes. A spatial and temporal analysis of the stresses within the particle showed that the radial stress is caused primarily by the pressure of generated volatiles. This stress monotonously decreases from the particle center towards the particle surface, without changing its sign. The tangential stress is caused primarily by the thermal shock. Close to the surface, it changes its sign. In the particle cross-section, the radial stress prevails close to the particle center, whilst the tangential stress is dominant in the surface region. At the points where these stresses exceed the particle tensile strength, cracks occur. Cracks extend tangentially close to the surface, and radially close to the center of the particle.

Evaluation of Structural Properties of Cylindrical Packed Beds Using Numerical Simulations and Tomographic Experiments

This contribution is focused on the analysis of the structure of packed beds of spherical particles at relatively low aspect ratios (i.e., particle to tube diameter ratio) as those arising in multitubular fixed bed reactors. On one hand, the computed tomography (CT) technique is employed to evaluate the position of each particle in the packing and from this information local properties such as particle center distribution and radial porosity profile were obtained. On the other hand, results from a previously developed algorithm to simulate packings were compared with those from our CT data and from literature sources. The agreement was very satisfactory.

Melting Behavior of Copper Nanocrystals Encapsulated in Onion-like Carbon Cages

Nanoparticulate materials are promising objects for studying the processes that triggermelting of solids. On a pyrolytic route, we successfully encapsulated 20–60 nm diameter Cu nanocrystals within multilayer graphitic carbon spheres. In situ electron microscope observations of the melting and displacement of the encapsulated Cu nanocrystals at temperatures up to 1175 K have provided clear evidence of the process of surface melting and its dependence on the quality of the metal/carbon interface. Detection of crystal defects inside the Cu particles during melting and vaporization has proved that the metal phase maintains its solid crystalline state in the particle center. Indications of the influence of surface anisotropy on the melting behavior were obtained. The carbon cages as a whole remained unchanged during the observations.