Volume Concentration Of Magnetic Particles Research Articles

Magnetic Particle Reinforced Elastomer Composites for Additive Manufacturing

Development of magnetorheological elastomer (MRE) materials suitable for fused filament fabrication (FFF)-type additive manufacturing processes is performed. Filament materials are thermoplastic polyurethane (TPU) pellets and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> magnetic particles. A trickling method is employed to introduce the materials into the filament extruder at a constant rate, after which they are mixed in a hopper and extruded at 160 °C. The trickling rate of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> was controlled to produce different volume concentrations [0%–13% (v/v)]. Micrograph analysis on the cross section of the filaments indicates that magnetic particles are uniformly dispersed in the TPU matrix. As the volume concentration of particles in the matrix is increased over 10% (v/v), the size distribution of magnetic particles is slightly increased, indicating formation of aggregation of magnetic particles. Micropores are observed in the filaments, and the number of pores seem to increase with increasing volume concentration of magnetic particles. A commercial off the shelf (COTS) FFF 3-D printer is successfully used to produce cylindrical MRE samples with different magnetic particle volume concentrations. Magnetorheological (MR) properties of the printed MRE samples are characterized by uniaxial compression testing under different applied magnetic field. Material elastic modulus was found to increase with the intensity of magnetic field for a given magnetic particle volume concentration. The relative MR effect varies from −1.3% to 60% within the studied volume concentrations and applied fields. The samples with 5% (v/v) show the highest MR effect when compared with samples with higher magnetic particle volume concentrations. The results of these experiments confirm that the equivalent energy dissipation of the material rises with the increase in the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> concentration of MRE.

Ferrofluids for heat transfer enhancement under an external magnetic field

Overheating of power electronic devices has become a significant issue due to their continued miniaturization and increased heat flux that needs to be dissipated. Ferrofluids (magnetic nanofluids) have been shown to have higher thermal conductivity than their base aqueous or oil based fluids due to the solid magnetic nanoparticles that make up the ferrofluid. This allows higher convective heat transfer rates and, importantly, the ability to externally effect the flow using a magnetic field. In this paper, we focus on material characterization of ferrofluids and measurement of heat transfer rates for single-phase ferrofluidic forced convective flow in microchannels. We show that heat transfer properties of the flow are enhanced with the use of ferrofluids and that the material make-up of the ferrofluid affects these properties. In this paper, we argue that generally, convective heat transfer rates for ferrofluids are increased by increasing the solid volume concentration of magnetic particles (∼0.2–0.4%). Interestingly, increasing magnetic flux was shown to decrease heat transfer enhancement. This was due to a reduction in the thermal conductivity of the bulk fluid caused by magnetic nanoparticles being drawn out of the isotropic mixture and becoming pinned to the channel wall in the region of strongest magnetic field. We show that there is good correlation between both theory and experimental visualization of this phenomenon.

On the temperature dependence of ferrofluid susceptibility

The fulfillment of Curie– Weiss laws for a ferrofluid is critically discussed. The results of investigations of the magnetic susceptibility temperature dependences of ferrofluid and dried powder of magnetic particles are presented. The effective field parameter is computed in approximation of the effective field theory. It is indicated that the value of the effective field parameter is not constant and depends on the concentration of magnetic nanoparticles and the temperature of the sample. It was found that Curie-Weiss law with λ = 4π / 3 is satisfied for ferrofluid with a limited range of volume concentration of magnetic particles.

Structure of the interface between magnetic and conventional fluids: Model of immiscible phases

The structure of the flat interface between a two-component magnetic suspension and a conventional nonmagnetizable fluid immiscible with it is investigated with account for the dependence of the free energy of the system on the magnetization gradients, the concentration of magnetic particles, and the bearing phase density. It is shown that at certain values of the problem parameters the volume concentration of magnetic particles strongly increases near the interface, that is, the particles are substantially adsorbed at this surface. The dependence of the surface tension tensor components on the magnetic field stress is determined.

Torque characterization of functional magnetic polymers using torque magnetometry

Magneto-Rheological Elastomers (MREs) are functional magnetic polymers filled with magnetic particles. MREs with fixed magnetic polarization within the particles, or H-MREs, develop a torque in response to an applied magnetic field. This ability leads to unique actuation properties which are critical for designs and applications of flexible actuators employing H-MREs. Thus, the primary goal of this paper is to characterize the relationship between torque generated and volume concentration of magnetic particles, an essential design parameter in H-MRE fabrication. To this end, a torque magnetometer based on a parallel plate capacitor was designed, and used to measure the torque response from H-MRE samples. The H-MRE samples were fabricated with varying percent volume of magnetic particles. The results from torque testing paralleled theoretical trends expected for the samples. An empirical correlation between volume content and maximum torque was found for samples above 10% volume composition and followed a linear trend. The equation governing the relationship was found from experimental results.

Néel and Brownian rotations in ferronematics

The magnetic Freedericksz transition was studied in the liquid crystal 4-(trans-4’-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) doped with spherical magnetite nanoparticles of different size (4.2 nm, 11.4 nm, 31.4 nm and 112.7 nm). The volume concentration of magnetic particles in prepared ferronematics was ϕ=2×10−4. The obtained results showed that doping with the magnetic particles with size smaller than the Shliomis size increases the threshold of the magnetic Freedericksz transition while in the case of doping with the magnetic particles with size larger than the Shliomis size a decrease of the threshold of the magnetic Freedericksz transition was observed. In the case of doping with magnetic particles with size comparable with the Shliomis size, both Néel and Brownian rotations are present, however, the Brownian rotation is dominant.

The influence of magnetic field on refractive index profile of ferronematic cell

This paper presents the study of the influence of magnetic field on the refractive index profile of homeotropically oriented 6CHBT-based ferronematic liquid crystal cell. The direction of magnetic field was chosen in a way which ensures the one-dimensional deformation of directors field. The FN cell was placed between polarizers. On a basis of Burylov-Raikher model and measurements of light transmission throughout the ferronematic layer the mean volume concentration of magnetic particles dispersed in 6CHBT and refractive index profiles induced by magnetic filed were determined. Moreover the range of magnetic field intensity was determined in which the distribution of tilt angle can be described in a scope of the Burylov-Raikher theory and the small tilt angle approximation.

Investigation of a ferronematic cell influenced by a magnetic filed

This paper presents the optical study of the magnetically induced change of the spatial orientation of the directors' field in a ferronematic. This paper presents the investigations of the ferronematic that base on 6CHBT nematic liquid crystal. Magnetic field induces one-dimensional distribution of the tilt angle of a directors filed. On a basis of the optical measurements the mean volume concentration of magnetic particles was determined. Moreover the range of magnetic field intensity was determined in which the distribution of tilt angle can be described in scope of the Burylov-Raikher theory and the small tilt angle approximation.

The DC dielectric breakdown strength of magnetic fluids based on transformer oil

The DC dielectric breakdown strength of magnetic fluids based on transformer oil TECHNOL US 4000, with different saturation magnetizations, was investigated in various orientations of external magnetic field. It was shown that the dielectric breakdown strength in H∣∣ E is strongly influenced by the aggregation effects. As a boundary volume concentration of magnetic particles, below which the magnetic fluids have better dielectric properties than pure transformer oil, the volume concentration Φ = 0.01 was found. Thus magnetic fluids with Φ < 0.01 are suitable for the use as a high-voltage insulation.

Study of the magneto-optical effect in mineral oil based magnetic fluids

Magneto-optical effect, i.e. the dependence of the absorption coefficient on the magnetic field H and the angle ι (ι is the angle between the electric vector of incident light and the applied magnetic field) has been studied in a mineral oil based magnetic fluid in the infrared region from 3000 cm−1 to 5000 cm−1. It has been observed that the change of absorption coefficient in the infrared region is wavelength and magnetic field independent for the angle \(\Theta _c = arccos(1/\sqrt 3 ) = 54.73^ \circ \). The anomaly in absorbance of infrared light in studied magnetic fluids has been observed at λ=2.94 μm, approximately. This anomaly has been studied as a function of volume concentration of magnetic particles and applied external magnetic field. Obtained results showed that anomaly has a magnetic origin and it is probably connected with aggregation of magnetic particles.

Dependence of the giant magnetoresistance on the concentration of magnetic particles in granular composites

We study the dependence of giant magnetoresistance (GMR) on the volume concentration of magnetic particles in a magnetic granular composite via a Monte Carlo method and by modeling the composite as a random resistor network. We assume the nanosized magnetic particles are spherical in shape and are randomly distributed in a square or cubic lattice. The uniaxial anisotropy of the particles and the classical dipolar interaction among the particles are taken into account. By considering the difference in electron scatterings for spin-up and spin-down conduction electrons at the magnetic and nonmagnetic interface, and the scatterings within the magnetic regions and the nonmagnetic host medium in the composite, the value of GMR is found to depend sensitively on the spatial distribution of the particles, the magnetic states of the particles, and the densities of the spin polarized conduction electrons. There is an optimum concentration (about 25% in two-dimensional and 30% in three-dimensional cases) of magnetic particles at which the magnetoresistance shows a maximum. This phenomenon was also observed in experiments.

Nonlinear dynamic susceptibilities of interacting and noninteracting magnetic nanoparticles

The linear and cubic dynamic susceptibilities of solid dispersions of nanosized maghemite γ-Fe 2O 3 particles have been measured for three samples with a volume concentration of magnetic particles ranging from 0.3% to 17%, in order to study the effect of dipole–dipole interactions. Significant differences between the dynamic response of the samples are observed. While the linear and cubic dynamic susceptibilities of the most dilute sample compare reasonably well with the corresponding expressions proposed by Raikher and Stepanov for noninteracting particles, the nonlinear dynamic response of the most concentrated sample exhibits at low temperatures similar features as observed in a Ag(11 at% Mn) spin glass.

On the stability of a free surface of a magnetic fluid under microgravity

We have investigated the behaviour of a free surface of a suspension of ferrimagnetic particles in heptane under strongly reduced gravity. It was found that the free surface is destabilised when a homogeneous magnetic field parallel to the surface is applied. We show that the destabilisation of the fluid layer is forced by the influence of the suspension on the homogeneity of the magnetic field producing a magnetic field component normal to the fluid surface. Furtheron it is shown that the strength of the critical magnetic field parallel to the surface varies with the volume concentration of magnetic particles in the suspension. The experiments were carried out at the drop tower “Bremen” of the Center of Applied Space Technology and Microgravity (ZARM) in Bremen.

On the stability of a free surface of a magnetic fluid under microgravity

We have investigated the behavior of a free surface of a suspension of ferrimagnetic particles in heptane under strongly reduced gravity. It was found that the free surface is destabilized when a homogeneous magnetic field parallel to the surface is applied. The strength of the critical magnetic field parallel to the surface varies with the volume concentration of magnetic particles in the suspension. We show that the destabilization of the fluid layer is forced by the influence of the suspension on the homogeneity of the magnetic field producing a magnetic field component normal to the fluid surface. The dependence of the critical magnetic field on the volume concentration of magnetic particles can be explained by applying the theory of the normal-field instability to this field component under conditions of strongly reduced gravity. The experiments were carried out at the drop tower ‘Bremen’ of the Center of Applied Space Technology and Microgravity (ZARM) in Bremen.

Aging in a magnetic particle system.

The influence of dipolar interaction in a frozen ferrofluid has been experimentally studied. The ferrofluid consisted of particles of $\ensuremath{\gamma}$- ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ with mean diameter 70 \AA{}. Four samples with volume concentration of magnetic particles ranging from 0.03% to 17% have been investigated. The magnetic relaxation of the most concentrated particle system shows typical spin glass dynamics at low temperature, e.g., the relaxation depends on the time spent at constant temperature before applying the magnetic field---the system ages. The most diluted sample shows isolated particle dynamics and no aging.