Chain-like Clusters Research Articles

Stokesian Dynamics Simulations of Ferromagnetic Colloidal Dispersions in a Simple Shear Flow.

We have investigated the behavior of clusters of ferromagnetic particles in a colloidal dispersion subjected to a simple shear flow. To do so, the Stokesian dynamics method has been used under the assumption that the effect of Brownian motions are negligible. For the case of no shear flow, the aggregate structures obtained by the Stokesian dynamics simulations agree well with Monte Carlo results qualitatively. We can, therefore, conclude that the Stokesian dynamics simulations can capture thick chainlike clusters without introducing a specific clustering algorithm, which is indispensable for Monte Carlo simulations. The behavior of the thick chainlike clusters in a simple shear flow is summarized as follows. The thick chainlike clusters decline in the shear flow direction as time advances. Since longer clusters experience larger shear forces, long clusters are difficult to survive in such a situation. The thick chainlike clusters dissociate into some short clusters. Such clusters are relatively stable in a shear flow, so that they do not dissociate significantly any more. The viscosities have a strong relationship with the internal structures of the aggregates. The instantaneous viscosities, therefore, fluctuate significantly for the case of the thick chainlike clusters.

CLUSTER CONVERSION OF [MoW2O2(O2CEt)9]− TO [MoW2O4(O2 CEt)8]4- BY CHROMIUM HEXACARBONYL CRYSTAL STRUCTURES OF Na[MoW2O2(O2CMe)9] · H2O, [MoW2O2(O2CEt)6(H2O)3] ZnCl4 · 2H2O AND Na2Cr2[MoW2O4(O2CEt)8]2

Abstract Reactions of Mo(CO)6 with Na2WO4 · 2H2O in refluxing carboxylic anhydride produce the triangular bioxo-capped mixed-metal carboxylate clusters Na[MoW2O2(O2CR)9] (R = Me, 1; Et, 2), the propionate being hydrolyzed in 2M HCl containing ZnCl2 to form [MoW2O2(O2CEt)6(H2O)3]ZnCl4·2H2O (3). Cluster 2 is converted to the incomplete cuboidal tetraanion [MoW2O4(O2CEt)8]4- upon reacting with Cr(CO)6 in propionic anhydride at 120°, the latter species being trapped by Cr3± and Na± ions in the reaction mixture to afford the octanuclear heterometallic chain-like cluster Na2Cr2 [MoW2O4(O2CEt)8]2 (4). Clusters 1, 3 and 4 have been characterized by X-ray crystallography with the following crystal data, for 1: monoclinic, space group P21/c, a = 16.666(8), b = 11.096(3), c = 16.541(7) A, β = 94.60(4)°, V = 3048.9 A3, Z = 4, R, Rw = 0.070, 0.079; for 3, monoclinic, space group Cm, a = 10.259(3), b = 15.756(3), c = 10.870(3) A, β = 96.18(3)°, V = 1746.8 A3, Z = 2, R, Rw = 0.028, 0.034; for 4, triclinic, space group P...

NUMERICAL STUDY OF THE RESPONSE TIME OF ER SUSPENSIONS

Transient response of model ER suspensions to step-wise electric field under steady shear flow is studied by numerical simulations. The electrically induced stress increases almost linearly with time up to the steady-state value, and this increase directly reflects the growth of the chain-like clusters of the dispersed particles. As the shear rate increases, the response becomes faster since the growth rate of the clusters increases and the steady-state length of the clusters decreases. The response also becomes faster by increasing the particle density or electric field, but the dependence on the field strength is rather weak.

Three Dimensional Monte Carlo Simulations of Thick Chainlike Clusters Composed of Ferromagnetic Fine Particles

Three-dimensional Monte Carlo simulations have been carried out in order to investigate the thick chainlike cluster formation of ferromagnetic fine particles in an applied magnetic field. The results of the aggregate structures are summarized as follows. If a magnetic field is sufficiently strong and magnetic particle–particle interactions are relatively strong, thick chainlike clusters are formed along a field direction. The thick chainlike clusters obtained by the simulations agree well with those observed experimentally. For the increased particle–particle interactions, high potential barriers arise between thin chainlike clusters, so that it is impossible for the thin chainlike clusters to aggregate to form thick chainlike clusters. In small magnetic fields, thick chainlike clusters are not observed, since nonlinear clusters are preferred to linear clusters in this situation, whereby strong attractive forces do not act between clusters, due to flux closure.

Potential curves and orientational distributions of magnetic moments of chainlike clusters composed of secondary particles

The present study investigates the potential curves of linear chainlike clusters which are composed of secondary magnetic particles. The orientational distributions of the magnetic moments of the primary particles within the secondary particles are also clarified by means of the usual Monte Carlo method. The results obtained here can be summarized as follows. For the parallel arrangement, in which one cluster lies just beside the other, repulsive forces act between clusters and attractive forces do not arise for any cluster-cluster separation. On the other hand, for the staggered arrangement, in which one cluster is shifted relative to the other in the saturating field direction by the radius of secondary particles, attractive forces do act between clusters at short range. These forces become stronger as the clusters become longer and as the size of secondary particles increases. Although there is a potential barrier for the staggered arrangement, the height of the barrier is almost constant, irrespective of cluster length.

Magnetic properties of ultrafine magnetite particles and their slurries prepared via in-situ precipitation

Abstract Ultrafine magnetite particles of 4–10 nm were prepared in aqueous solutions of polyvinylalcohol (PVA) or PVA with partially exchanged carboxyl groups. The mode of agglomeration and dispersion of the particles were different depending on the properties of polymers in the solvent. The dispersion of magnetite particles prepared in PVA 1 wt.% aqueous solution at pH 13.8, was particularly stable. In contrast, those precipitated in an aqueous solution of PVA with 0.1 mol.% exchanged carboxyl groups agglomerated in the form of a chain-like cluster. The magnetic properties were sensitive to the state of agglomeration of the magnetite particles with adsorbed polymers. An increase of the coercive force after ball-milling was observed, due to the formation of the network structure.

Two-Dimensional Monte Carlo Simulations to Capture Thick Chainlike Clusters of Ferromagnetic Particles in Colloidal Dispersions

The present study theoretically investigates the aggregate structures of thick chainlike clusters arising in ferromagnetic colloidal dispersions by means of the cluster-moving Monte Carlo method. Pair correlation functions are also evaluated to clarify the internal structures themselves of such clusters. The following aggregation image of thick chainlike clusters has been obtained from the Monte Carlo simulations for a two-dimensional model system. Particles aggregate to form chainlike clusters when magnetic attractions between particles become strong over a certain value. Such thin chainlike clusters prefer nearly linear formation along a magnetic field direction, if the strength of magnetic field is sufficiently large. In this situation, attractive forces come to act between these nearly linear chainlike clusters, and these forces increase with the lengths of the clusters. Such attractions between the clusters lead to the formation of thick chainlike clusters.

Position-sensitive atom probe study of precipitates in high speed steel

The microstructure of a high speed steel specimen alloy with nominal composition of 1.3% C, 3.8% Cr, 4.3% Mo, 5.2% W, 4.1% V and 0.75% Nitrogen (in wt. %, balance Fe), was studied using a three-dimensional atom probe. From the position-sensitive analysis the variations of composition present in the samples can be reconstructed in three dimensions with sub-nanometer resolution. In alloy samples, annealed, quenched and tempered to peak hardness, the three-dimensional reconstruction of volumes analysed shows spherical and needle-like precipitates with extensions of about 1–3 nm as well as chain-like accumulations. The precipitates contain predominantly Cr, V, Cr-X and V-X (X stands for C and N). In addition Mo is found embedded and in close proximity to these clusters. The role of W in the precipitates is minor but W shows some tendency to form chain-like clusters near Cr-X and V-X precipitates.

Monte Carlo Simulations of Thick Chainlike Formations of Ferromagnetic Particles in Colloidal Dispersions.

The present study theoretically investigates the aggregate structures of thick chainlike clusters arising in ferromagnetic colloidal dispersions by means of the cluster-moving Monte Carlo method. Pair correlation functions are also evaluated to clarify the internal structures themselves of such clusters. The following aggregation image of thick chainlike clusters has been obtained from the Monte Carlo simulations for a two-dimensional model system. Particles aggregate to form chainlike clusters when magnetic attractions between particles become strong over a certain value. Such thin chainlike clusters prefer linear formation along a magnetic field direction if the strength of magnetic field is much greater than magnetic attractions between particles. In this situation, attractive forces come to act between these nearly linear chainlike clusters, and these forces increase with length of the clusters. Such attractions between the clusters lead to the formation of thick chainlike clusters.

Three Dimensional Monte Carlo Simulations of Thick Chainlike Clusters Composed of Ferromagnetic Fine Particles.

Three-dimensional Monte Carlo simulations have been carried out in order to investigate the thick chainlike cluster formation of ferromagnetic fine particles in an applied magnetic field. The results of the aggregate structures are summarized as follows. If a magnetic field is sufficiently strong and magnetic particle-particle interactions are relatively strong, thick chainlike clusters are formed along a field direction. The thick chainlike clusters obtained by the simulations agree well with those observed experimentally. For the increased particle-particle interactions, high potential barriers arise between thin chainlike clusters, so that it is impossible for the thin chainlike clusters to aggregate to form thick chainlike clusters. In small magnetic fields, thick chainlike clusters are not observed, since looplike clusters are preferred to linear clusters in this situation, whereby strong attractive forces do not act between clusters, due to flux closure.

Potential Curves and Orientational Distributions of Magnetic Moments of Chainlike Clusters Composed of Secondary Particles.

The present study investigates the potential curves of linear chainlike clusters which are composed of secondary magnetic particles. The orientational distributions of magnetic moments of primary particles within the secondary particles, are also clarified by means of the usual Monte Carlo method. The results obtained here are summarized as follows. For the parallel arrangement, in which one cluster is just beside the other, repulsive forces act between clusters and attractive forces do not arise for any cluster-cluster separation. On the other hand, for the staggered arrangement, in which one cluster is shifted relative to the other in saturating field direction by the radius of secondary particles, attractive forces do act between clusters at short range. These forces become stronger as the clusters become longer and as the size of secondary particles increases. Although there is a potential barrier- for the staggered arrangement, the height of the barrier is almost constant irrespective of cluster length.

Exact solution for a chainlike cluster growth model.

We present an exact solution for a cluster growth model, describing chainlike histories with ordered bond measure and external constraint connected to maximum bond length. We analyze in detail the physical applications that are connected to the parameter regions, where the dominant interparticle interactions are of short-range type.

Molecular dynamics of magnetic particulate dispersions

A computational investigation using the method of molecular dynamics was undertaken to characterize the state of magnetic particle dispersions. The simulations revealed that the microstructure of spherical particulate dispersions consists of chainlike clusters resulting from magnetic dipole alignment. Acicular particles formed clusters such as dimers, chains, and rings. The effect of fluid viscosity on the dispersion quality and the response of the magnetic dispersions to an external DC magnetic field are also reported.

Magnetic and microstructural properties of CoCrPt/CoCrPtSi dual-layered magnetic recording media

The magnetic properties of CoCrPt/CoCrPtSi dual-magnetic-layered thin films are investigated in terms of the crystalline microstructures using a vibrating-sample magnometer, transmission electron microscope, and scanning electron microscope. The magnetic crystal grains of the films are aligned with forming chainlike clusters. Each stacked magnetic crystal grain is epitaxially grown on individually isolated columnar Cr crystals with a relationship of hcp (101̄1)Co-based alloy //bcc (110)Cr. The direction of magnetic easy axes of two stacked magnetic crystals are aligned in a same direction. Single-crystalline dual-magnetic-layered thin films, which are epitaxially grown on MgO(110) single-crystal substrates, are prepared to estimate the Ku of the dual-layered magnetic grains. The Ku is determined to be 4×106 erg/cm3, whose value is about 2 times as large as that of single-layered CoCrPtSi. This large anisotropy energy is presumed to lead the increase of Hc in the dual-layered magnetic films.

Structure and vibrational spectra of neutral, protonated and deprotonated hydrogen bonded polymers: an ab initio SCF study on chain-like hydrogen fluoride clusters

The total energies, the structures, the HF bond stretching frequencies and the corresponding IR intensities of neutral ((HF) n ) protonated ([(HF) n H] +) and deprotonated ([(HF) n−1 F] −) chain-like hydrogen fluoride polymers were investigated at the ab initio SCF level by applying a double-zeta type basis set. Clusters from n = 1 up to n = 19 were treated. We adopted a systematic approach and minutely monitored these properties as a function of cluster size. Particular emphasis was laid on the various aspects of non-additive behaviour in these series, visible in the substantial structural relaxations of HF and FF distances, in the increased stabilization energy per hydrogen bond, in the strong red-shifts of HF bond stretching frequencies, and in significantly enhanced IR intensities. We investigated in detail the convergence of this selected number of ground-state properties with increasing chain length and we discuss the different mode of convergence encountered in the neutral chains and in the chains carrying a charged defect.

Magnetization reversal in chain-like clusters of interacting particles

The minimum energy barrier hindering a reversal of the magnetic moments in a cluster of interacting single domain magnetic particles is calculated for various chain-like geometries of the cluster. The energy barrier is calculated taking into account both the magnetic anisotropy in the particles and magnetic interparticle dipolar interactions. The influence of the particle interactions on the superparamagnetic relaxation in the clusters is discussed. The detailed geometry of the cluster is found to be important for the energy barrier — deviations from a linear chain drastically reduce the energy barrier of a reversal of the magnetic moments in the cluster.

Modeling of Non-Uniform Composite Microstructures

The microstructural effects of non-uniform composite microstructures are modeled. Relationships are observed between the degree of non-uniformity and fiber spa tial information which may be measured via image analysis or derived from Dirichlet cell tessellations. For artifical patterns containing chain-like clustering which simulate com posite microstructures: (a) the nearest-neighbor distances of random and clustered pat terns are smaller than those normally estimated by square or hexagonal arrays, (b) in creased clustering may be associated with increased mean cell volume fraction, cell volume fraction distribution standard deviation, number of cell sides, distribution stan dard deviation, nearest-neighbor distance distribution skewness, and decreased nearest- neighbor distances and (c) accounting for non-uniform fiber diameters is generally un necessary, except possibly at low fiber volume fraction or with patterns exhibiting a high degree of chaining. For Nicalon SiC/zirconia titanate composite samples with micro structures which exhibit clustering, the maximum value of skewness, determined from sub-regions of the sample, correlates with the flexure strength of that sample.

Microstructure and magnetic Structure of Co-based thin-film recording medium

Knowledge of the relationship between microstructure and microscopic magnetization configuration is strongly required to improve Co-based thin film media. The purpose of this paper is to report some of these properties measured for a thin film longitudinal recording medium and to discuss their inter-relationship.Figure 1 shows an SEM micrograph of fractured cross-section of CoCrPt/CoCrPtSi dual-layered medium which was used to demonstrate the feasibility of 2 Gb/in2 magnetic recording. The Cr underlayer is used to control the orientation, size, and distribution of magnetic crystals. Magnetic crystal grains, whose size ranges between 20-30nm, form chain-like clusters consisting of several crystals as shown in the plan-view TEM micrograph(Fig.2). Neighboring clusters are separated physically with an average distance of 3nm. Magnetic separation among magnetic crystals and/or clusters is important to reduce the medium noise which is related to magnetic coupling between crystals. The crystalline easy axes of the grains are randomly oriented in the film plane.

磁性流体中における微粒子の鎖状凝集に関する研究

Chain-like clustering of 3μm Sio2 particles was investigated by Monte Carlo simulation along with the experiment. The magnetic force was applied in parallel to the particle layer suspended in a thin film of magnetic fluid. The size distribution of chain-like clusters was measured in relationship to the coverage c or the bulk-mean area fraction of particles: k=90c1.38 The agreement between the computer simulation and the experiment was fairly good. Particle movement from the initial to final equilibrium positions was also measured for all particles. Monte Carlo simulation was foud to underestimate the particle movement in the region of high particle concentration.

A numerical simulation of magnetostatic coagulation in a fluid

SUMMARY The behaviour of magnetic particles in a fluid and the accompanying net magnetization change are simulated using the Monte Carlo technique taking into account an effect of the magnetostatic interaction between particles. The simulated assembly consists of one hundred single-domain magnetites within a two-dimensional unit cell. A periodic boundary condition is used to calculate the magnetic field caused by interaction between adjacent particles. Dilute assemblies show a steady relaxation and acquisition patterns of magnetization, which are expected from analytical solutions of the non-interacting state. In contrast, concentrated assemblies show more complicated patterns. These patterns are consistent with the experimental results for suspensions with comparable concentrations. In the concentrated assemblies, it was also found that coagulation of particles occurs and that particle moments in a chain-like cluster are aligned along the chain's elongated direction. Coagulation of particles in the concentrated assembly modifies the net magnetization pattern through the dominance of the interacting field in a cluster. Thus magnetostatic interaction could play an important role in the behaviour of magnetic particles in a fluid. Numerical simulations of this type which incorporate other interaction effects have the potential to give insights into the process by which detrital remanent magnetization is acquired.