Volume Fraction Of Magnetic Particles Research Articles

Three-dimensional simulation study on the aggregation behavior and shear properties of magnetorheological fluid

Abstract In this article, the aggregation behavior and shear properties of magnetorheological fluid (MRF) under different magnetic field strengths, volume fraction of magnetic particles (MPs) and shear rates were investigated by coarse-grained molecular dynamics simulations. With the formation of a wall-like structure, the shear stress (τ) of the MRF increased linearly in the shear strains region 0 0.58), the stress began to decrease and finally tended to be stable. The relationship between the aggregation of MPs and shear stress under a magnetic field-shear field was determined at the microscopic level.

The Role of Volume Fraction and Additives on the Rheology of Suspensions of Micron Sized Iron Particles

The increase of the yield stress versus the magnetic field is the most important quantity characterizing the efficiency of a magnetorheological suspension. The theory based on the formation of columnar aggregates predicts a linear variation with the volume fraction of magnetic particles. We shall review previous models for the calculation of forces and yield stress and will introduce a new one based on rupture at zero strain. The predictions of these models are compared with the experimental data obtained for carbonyl iron particles by different authors. Whereas previous analytical prediction strongly overestimates experimental yield stress, the one calculated from Finite Element Method (FEM) used with affine trajectories well reproduce the experiments and show a linear dependence with the volume fraction and a H3/2 behavior between 50 and 200kA/m. Nevertheless, at very high volume fractions (>55%) where the suspension can only flow in the presence of specific additives, the dependence of the yield stress versus the volume fraction and the magnetic field is dramatically changed. We observe a jamming transition which is triggered by the application of a low magnetic field and which depends strongly of the volume fraction. We will discuss the new perspectives arising by the use of these very high volume fractions.

Investigation of the relationship among the microstructure, rheological properties of MR grease and the speed reduction performance of a rotary micro-brake

Abstract A magnetorheological (MR) grease-based micro-brake is proposed for providing a damping torque for a miniature turbine generator when working at high wind speeds. MR greases with a magnetic particle volume fraction from 10% to 90% are prepared for the MR grease-based micro-brake. Flow curves show that the shear thinning behaviour of MR grease is enhanced by the increased volume fraction. The zero magnetic rheological behaviour of MR grease is studied with a Herschel-Bulkley model. Then, the speed reduction effect of the MR grease-based micro-brake is experimentally studied. A rotational speed difference Δn as high as 6750 r/min is achieved by applying a current of 1.01 A to the micro-brake. MR grease with lower volume fraction demonstrates a more significant speed reduction effect under low applied currents, since the chain forming behaviour is enhanced by the decreased volume fraction. Finally, a comprehensive model for the MR grease-based micro-brake is developed to reveal the relationship among the microstructure, rheological properties of the MR grease and the speed reduction performance of the MR grease-based micro-brake.

Particle Dispersibility and sound propagation of ultrasounds for magnetic soft composites

We measured the sound velocity for magnetic soft composites consisting of a polymeric matrix of polyurethane and a magnetic particle of carbonyl iron or barium ferrite, and investigated the effect of the particle dispersibility on the sound velocity at 10 MHz for the composites. Morphological observations and dynamic viscoelastic measurements at 1 Hz were also carried out, and the results revealed that the carbonyl iron was dispersive particle and the barium ferrite was aggregative particle in the polyurethane matrix. The sound velocity for polyurethane elastomer without magnetic particles was 1444 ± 12 m/s and it decreased with the volume fraction of magnetic particles. The sound velocity of magnetic composites with barium ferrite obeyed the Reuss model at whole volume fractions, however, magnetic composites with carbonyl iron exhibited an under deviation from the Reuss model at volume fractions above ~0.053. It was also found that the longitudinal modulus was insensitive to the creation of the particle network and sensitive to the aggregation of magnetic particles. The threshold seen in the longitudinal modulus was different from that in the nonlinear parameter of viscoelasticity, indicating that the ultrasound of 10 MHz detects microscopic heterogeneity of particle dispersion.

Tunable isolator based on magnetorheological elastomer in coupling shear–squeeze mixed mode

In this article, the systematic design, construction, and testing of a novel tunable isolator based on magnetorheological elastomers in coupling shear–squeeze mixed mode have been studied. The influence of magnetic particle volume fraction on field-induced properties of magnetorheological elastomer isolator is studied, and the performance of vibration mitigation of magnetorheological elastomer isolator is evaluated experimentally. The results show that the frequency-shift property of magnetorheological elastomer isolator is linearly proportional to the magnetic particle volume fraction and applied current, and vibration mitigation capacity of magnetorheological elastomer isolator is remarkably enhanced by increasing the applied current. The design of magnetorheological elastomer isolator in coupling mixed mode may provide a new insight for using magnetorheological elastomers in vibration reduction applications.

Magnetically Tunable Vibration Transmissibility for Polyurethane Magnetic Elastomers

The effect of a weak magnetic field on vibration transmissibility was investigated for magnetic elastomers with various volume fractions of magnetic particles. Polyurethane elastomers without magnetic particles exhibited a natural frequency at 53 Hz and were insensitive to a magnetic field of 60 mT. The natural frequency for magnetic elastomers with a volume fraction of 0.23 was 115 Hz at 0 mT, and increased to 134 Hz at 60 mT. The vibration transmissibility was independent of the magnetic field. A linear relation between the natural frequency and (G/m)1/2 was observed (G: storage modulus, m: mass), indicating that the observed vibration is basically described by a simple harmonic oscillation.

Magnetic-field Sensitivity for Monomodal Magnetic Elastomers with Various Volume Fractions of Magnetic Particles

The relationship between magnetic-field sensitivity and the volume fraction of magnetic particles for magnetic elastomers was investigated. The magnetic-field dependence of the storage modulus for magnetic elastomers was measured by dynamic viscoelastic measurements under magnetic fields with a constant sweep rate of 3.7 mT s−1. The magnetic-field sensitivity was determined by the initial slope of the storage modulus using an exponential function. It was revealed that the maximum magnetic-field sensitivity was 1.8 × 10−2 mT−1 at the maximum volume fraction of magnetic particles.

Computer simulations of equilibrium magnetization and microstructure in magnetic fluids

In this work, Monte Carlo and Brownian Dynamics simulations are developed to compute the equilibrium magnetization of a magnetic fluid under action of a homogeneous applied magnetic field. The particles are free of inertia and modeled as hard spheres with the same diameters. Two different periodic boundary conditions are implemented: the minimum image method and Ewald summation technique by replicating a finite number of particles throughout the suspension volume. A comparison of the equilibrium magnetization resulting from the minimum image approach and Ewald sums is performed by using Monte Carlo simulations. The Monte Carlo simulations with minimum image and lattice sums are used to investigate suspension microstructure by computing the important radial pair-distribution function go(r), which measures the probability density of finding a second particle at a distance r from a reference particle. This function provides relevant information on structure formation and its anisotropy through the suspension. The numerical results of go(r) are compared with theoretical predictions based on quite a different approach in the absence of the field and dipole-dipole interactions. A very good quantitative agreement is found for a particle volume fraction of 0.15, providing a validation of the present simulations. In general, the investigated suspensions are dominated by structures like dimmer and trimmer chains with trimmers having probability to form an order of magnitude lower than dimmers. Using Monte Carlo with lattice sums, the density distribution function g2(r) is also examined. Whenever this function is different from zero, it indicates structure-anisotropy in the suspension. The dependence of the equilibrium magnetization on the applied field, the magnetic particle volume fraction, and the magnitude of the dipole-dipole magnetic interactions for both boundary conditions are explored in this work. Results show that at dilute regimes and with moderate dipole-dipole interactions, the standard method of minimum image is both accurate and computationally efficient. Otherwise, lattice sums of magnetic particle interactions are required to accelerate convergence of the equilibrium magnetization. The accuracy of the numerical code is also quantitatively verified by comparing the magnetization obtained from numerical results with asymptotic predictions of high order in the particle volume fraction, in the presence of dipole-dipole interactions. In addition, Brownian Dynamics simulations are used in order to examine magnetization relaxation of a ferrofluid and to calculate the magnetic relaxation time as a function of the magnetic particle interaction strength for a given particle volume fraction and a non-dimensional applied field. The simulations of magnetization relaxation have shown the existence of a critical value of the dipole-dipole interaction parameter. For strength of the interactions below the critical value at a given particle volume fraction, the magnetic relaxation time is close to the Brownian relaxation time and the suspension has no appreciable memory. On the other hand, for strength of dipole interactions beyond its critical value, the relaxation time increases exponentially with the strength of dipole-dipole interaction. Although we have considered equilibrium conditions, the obtained results have far-reaching implications for the analysis of magnetic suspensions under external flow.

Transition from Linear to Nonlinear Viscoelasticity for Bimodal Magnetic Elastomers under a Magnetic Field

We have investigated the transition from linear viscoelasticity to nonlinear viscoelasticity for bimodal magnetic elastomers under a magnetic field. For bimodal magnetic elastomers, the transition strain increased remarkably with the volume fraction of nonmagnetic particles. For monomodal magnetic elastomers, the transition strain also increased with the volume fraction of magnetic particles; however, the increment was not significant. A possible mechanism of the widening of the linear viscoelastic plateau by nonmagnetic particles is discussed.

Electric Conductivity and Dielectric-Breakdown Behavior for Polyurethane Magnetic Elastomers.

The electric-voltage dependence of the electric conductivity for cross-linked and un-cross-linked magnetic elastomers was measured at various magnetic fields, and the effect of cross-linking on the electric conductivity and the dielectric-breakdown behavior was investigated. The electric conductivity for un-cross-linked elastomers at low voltages was independent of magnetic fields and the volume fraction of magnetic particles, indicating the electric conduction in the polyurethane matrix. At high voltages, the electric conductivity increased with the magnetic field, showing the electric conduction via chains of magnetic particles. On the other hand, the electric conductivity at low voltages for cross-linked elastomers with volume fractions below 0.06 was independent of the magnetic field, suggesting the electric conduction in the polyurethane matrix. At volume fractions above 0.14, the electric conductivity increased with the magnetic field, suggesting the electric conduction via chains of magnetic particles. At high voltages, the electric conductivity for cross-linked elastomers with a volume fraction of 0.02 was independent of the magnetic field, indicating the electric conduction through the polyurethane matrix. At volume fractions above 0.06, the electric conductivity suddenly increased at a critical voltage, exhibiting the dielectric breakdown at the bound layer of magnetic particles and/or the discontinuous part between chains.

Study of Static Magnetic Properties of Transformer Oil Based Magnetic Fluids for Various Technical Applications Using Demagnetizing Field Correction

Static magnetization data of eight transformer oil based magnetic fluid samples, with saturation magnetization ranging in a large interval from 9 kA/m to 90 kA/m, have been subjected to the demagnetizing field correction. Using the tabulated demagnetization factors and the differential magnetic susceptibility of the samples, the values of the radial magnetometric demagnetization factor were obtained in the particular case of VSM880 magnetometer. It was found that the demagnetizing field correction keeps the saturation magnetization values unchanged, but instead the initial magnetic susceptibility of the magnetic fluid samples varies widely. The mean magnetic diameter, obtained through magnetogranulometry from the measured data, is higher than that obtained from the corrected ones and the variation rate increases with the magnetic particle volume fraction growth.

Dissipative particle dynamics simulation for the microstructures of ferromagnetic fluids

ABSTRACTThe three-dimensional (3D) microstructures of ferromagnetic fluids at equilibrium state are simulated using the dissipative particle dynamics (DPD)-based method. First, for the fixed magnetic particle volume fraction , the microstructures of ferromagnetic fluids are simulated by investigating the influence of the magnetic particle–particle interaction strength on the aggregate structures of ferromagnetic particles. The obtained microstructures of ferromagnetic fluids agree qualitatively with those obtained by molecular dynamics and Monte Carlo simulations in the literature, respectively. Second, for the fixed magnetic particle–particle interaction strength , the influence of the magnetic particle volume fraction on the microstructures of ferromagnetic fluids is investigated. The corresponding radial distribution functions and the time evolution of the mean equilibrium temperature of the magnetic fluid system are also calculated, which shows the correctness of the simulated microstructures. The simulated microstructures of ferromagnetic fluids are in qualitatively good agreement with those obtained by the molecular dynamics method in the literature. In addition, the mean equilibrium velocities of ferromagnetic and dissipative particles are also calculated, and the numerical solutions highly approach the corresponding theoretical values. All the calculations show the correctness of the simulated microstructures of ferromagnetic fluids and verify the validity of the employed DPD-based method. Our results should be of value to study the optical characteristics of ferromagnetic fluids.

Characterization of the linear viscoelastic region of magnetorheological elastomers

The linear viscoelastic behaviour of magnetorheological elastomers is analysed in this work according to their formulation and working conditions. This study comprised both the synthesis of different magnetorheological elastomers and the strain and frequency sweep characterization under different magnetic fields and temperatures. The characterization was performed by a Physica MCR 501 rheometer from Anton Paar, equipped with a magnetorheologic cell 70/1T MRD. In the synthesis with a given elastomeric matrix, samples with different magnetic particle content are studied with two types of curing conditions: under the action of a magnetic field (anisotropic magnetorheological elastomers) and without a magnetic field (isotropic magnetorheological elastomers). The working conditions are excitation frequency, temperature and the applied external magnetic field. In this work, a new procedure to determine the linear viscoelastic behaviour is proposed; the loss factor is analysed in addition to analysing the storage modulus to determine the linear viscoelastic region of each sample. The results show that high temperatures and magnetorheological elastomers with higher volume fraction of magnetic particles restrict the linear viscoelastic behaviour of magnetorheological elastomers.

Ubbelohde viscometer measurement of water-based Fe3O4 magnetic fluid prepared by coprecipitation

Fe3O4 nanoparticles were prepared by co-precipitation and coated by sodium dodecyl benzene sulfonate (SDBS) to obtain water-based magnetic fluid. The viscosity of the magnetic fluid was measured using an Ubbelohde viscometer. The effects of magnetic particles volume fraction, surfactant mass fraction and temperature on the viscosity were studied. Experimental results showed that the magnetic fluid with low magnetic particle volume fraction behaved as a Newtonian fluid and the viscosity of the magnetic fluid increased with an increase of the suspended magnetic particles volume fraction. The experimental data was compared with the results of a theoretically derived equation. The viscosity of the magnetic fluid also increased with an increase in surfactant mass portion, while it decreased with increasing temperature. Moreover, the viscosity increased with increasing the magnetic field intensity. Increasing the temperature and the surfactant mass fraction weakened the influence of the magnetic field on the viscosity of the magnetic fluid.

An investigation on synthesis and magnetic properties of nanoparticles of Cobalt Ferrite coated with SiO2

SiO2-coated Cobalt Ferrite (CoFe2O4) nanoparticles were obtained by the hydrolysis of tetraethylorthosilicate in the presence of CoFe2O4 nanoparticles in co-precipitation. The effects of SiO2coating on the magnetic properties of CoFe2O4 nanoparticles were investigated. The structural, morphological and magnetic properties of as-prepared sample were characterization by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, transmission electron microscopy (TEM), Field emission scanning electron microscopy/energy dispersive x-ray analysis (FESEM-EDAX) and magnetic measurements were investigated using vibrating sample magnetometer (VSM). The morphology analysis confirmed that the obtained composite nanoparticles consisted of CoFe2O4 coated by SiO2. It was shown that the saturation magnetization and coercivity of CoFe2O4 decreased after SiO2 coating, which can be interpreted by the interparticle dipole-dipole interactions related to the magnetic particle volume fraction in the composite nanoparticles.

Study on the Preparation and Properties of Silicone Oil-Based Magnetorheological Fluids

Aiming to improve the performance of magnetorheological fluids (MRF) applied in mechanical transmission system, a preparation method of silicone oil-based magnetorheological fluids is proposed, and the properties of prepared MRF are tested. The materials for MRF preparation are elaborated, and the process parameters and flowchart are designed. Moreover, scanning electron microscope is used to observe the changes between uncoated magnetic-particles and coated magnetic-particles, and three samples with the volume fractions of magnetic-particles 10%, 20%, and 30% are prepared. Finally, three properties of prepared MRF are tested. Analysis of the experimental results shows that the prepared MRF can satisfy the requirement of mechanical transmission system, and the optimal process parameters are illustrated.

0801 針状磁性粒子を混合した磁気混合流体のダンピング特性(OS8-1 機能性流体工学の融合化新展開,OS8 機能性流体工学の融合化新展開)

Magnetic functional fluids are produced by adding needle-like magnetic particles whose long axis is about 100 nm and short axis is 25 nm to magnetic compound fluids which are mixture of magnetic fluid and magnetorheological fluid. Damping property can be controlled by changing mixture ratio of both spherical magnetic particles and needle-like magnetic particles. Basic damping property is investigated experimentally by using a prototype damper. When the total volume fraction of magnetic particles is constant, the damping force of the damper is stronger with increase of the volume fraction of needle-like magnetic particles.

Steady state rheological behaviour of multi-component magnetic suspensions

In this paper we study the rheological behaviour (in the absence of magnetic field and upon its application) of multi-component magnetic suspensions that consist of a mixture of magnetic (iron) and non-magnetic (PMMA) particles dispersed in a liquid carrier. These suspensions exhibit considerably higher viscosity and yield stress in the absence of magnetic field than single-component suspensions of the same solid fraction, as a consequence of the adsorption of the iron particles on the PMMA ones. The adsorbed layer of iron particles on the PMMA ones is observed through optical microscopy of dilute samples and confirmed by attenuated total reflectance. Microscopic observations also show that the resulting non-magnetic-core-magnetic-shell composites move upon magnetic field application and aggregate into particle structures aligned with the applied field. These structures, which consist of both types of particles, give rise to high values of the static and dynamic yield stresses upon field application. Actually, both quantities are much higher than those of a suspension with the same volume fraction of magnetic particles, and increase when the amount of non-magnetic ones increases. These trends are adequately predicted by a theoretical model that considers that the main contribution to the yield stress is the change of the suspension magnetic permeability when particle chains are deformed by the shear.

(CoTaZr)<sub>x</sub>(Al<sub>2</sub>O<sub>3</sub>)<sub>1-x</sub>软磁纳米颗粒膜的制备与性能研究

采用射频磁控溅射法制备了一系列的(CoTaZr)x(Al2O3)1−x (x = 0.23，0.32，0.43)纳米颗粒膜，研究了溅射功率和气压对其软磁性能和微观组织结构的影响，结果表明，样品的电阻率随功率增大而递增，电阻率和饱和磁化强度都随气压先增大后减小，最佳溅射气压为2.5 Pa。磁性颗粒体积分数较高的样品具有较高的Ms，但电阻率相应较小，在x = 0.32时，试样的电阻率ρ为2.73 × 104 µΩcm，比饱和磁化强度Ms为56.8 emu/g。SEM分析结果表明，Al2O3颗粒均匀地分布在磁性颗粒之间。 A series of (CoTaZr)x(Al2O3)1−x (x = 0.23, 0.32, 0.43) magnetic granular films are prepared by RF magnetron sputtering. The effect of sputtering power and pressure on electromagnetic properties and microstructure are studied. Results show that higher sputtering power induces to higher resistivity. The properties first increase and then decrease with the pressure increasing. The better property has been obtained when the pressure is 2.5 Pa. The sample with higher volume fraction of magnetic particles has higher saturation magnetization but lower resistivity. When x = 0.32, the sample has better properties with the specific saturation magnetization of 56.8 emu/g and resistivity of 2.73 × 104 µΩcm. By SEM, it can be seen that Al2O3 particles distribute uniform among the magnetic particles.

Magnetically Tunable Elasticity for Magnetic Hydrogels Consisting of Carrageenan and Carbonyl Iron Particles

A new class of magnetoelastic gel that demonstrates drastic and reversible changes in storage modulus without using strong magnetic fields was obtained. The magnetic gel consists of carrageenan and carbonyl iron particles. The magnetic gel with a volume fraction of magnetic particles of 0.30 exhibited a reversible increase by a factor of 1400 of the storage modulus upon a magnetic field of 500 mT, which is the highest value in the past for magnetorheological soft materials. It is considered that the giant magnetoelastic behavior is caused by both high dispersibility and high mobility of magnetic particles in the carrageenan gel. The off-field storage modulus of the magnetic gel at volume fractions below 0.30 obeyed the Krieger-Dougherty equation, indicating random dispersion of magnetic particles. At 500 mT, the storage modulus was higher than 4.0 MPa, which is equal to that of magnetic fluids, indicating that the magnetic particles move and form a chain structure by magnetic fields. Morphological study revealed the evidence that the magnetic particles embedded in the gel were aligned in the direction of magnetic fields, accompanied by stretching of the gel network. We conclude that the giant magnetoelastic phenomenon originates from the chain structure consisting of magnetic particles similar to magnetic fluids.