Alternating Gradient Magnetic Fields Research Articles

The influence of the particle size on rheological properties of magneto fluidized bed of strontium hexaferrite

Powder metallurgy processes are in general less polluting and more sustainable than the traditional mechanical manufacturing. In order to organize technological processes with the ferromagnetic powders, one usually applies special methods to change the coefficient of inner friction of particulate systems. We studied the regimes of the magnetic fields that form magneto fluidized bed from the strontium hexaferrite (SrFe12O19) powder and provide the decrease of the inner friction coefficient. The magneto fluidized bed was formed by the effect of the crossed constant and alternating gradient magnetic fields. The results of electromagnetic parameters influence on rheological properties of the magneto fluidized bed formed from strontium hexaferrite powders with various average particle sizes are presented in the paper. The dependencies of the strontium hexaferrite stimulated fluidity speed through a 2 mm diameter hole from the electromagnetic field parameters were studied. The obtained results show the possibility of using dry ferromagnetic powders in the permanent magnets production in powder metallurgy.

Recycled strontium hexaferrite powder behavior in the magneto fluidized state in dependence of the chamber fill factor

The magnetic powders affected by external magnetic fields can be transferred in the pseudo fluidized state to intensify the process of permanent magnets production. The behavior of the fluidized fine strontium hexaferrite powder with an average particle size 0.93 μm obtained by recycling old electric engine stators was studied. Mutually perpendicular constant and alternating gradient magnetic fields were used to form the magneto fluidized bed from the powder. The rheology state of the magnetic powder can be controlled by the parameters of the electromagnetic fields. The powder was affected by a constant field with induction values from 3 mT to 43 mT and alternating gradient magnetic field with induction gradient values from 87 mT/m to 317 mT/m. The chamber bulk fill varied from 4% to 100%. To study the rheological state of the powder in the chamber, we used an inductive method – an inductive coil was put on the chamber and the moving powder particles induced the current in the coil. The values of the current allowed to estimate the mobility of the powder particles in the magneto fluidized bed. The paper presents the results of experimental studies how electromagnetic fields regimes influences on the rheology of the fluidized powder. Methods of choosing the fields regimes providing the rheological state optimal for the required technological process depending on the chamber fill factor are described.

Diagnostic method for the selection of regimes of electromagnetic effect on the particulate ferromagnetic material

The recycling of obsolete permanent magnets, the processing and reusing of the obtained powder for a new magnets production has both economic and environmental benefits. The processing of recycled magnetic slur to the particle size distribution required for the magnets production has a problem specific for fine magnetic powders – particle aggregation limiting the milling efficiency. One way of solving this problem is milling the magnetic material with the formation of the magneto fluidized bed. In the present work we study the behavior of the recycled from old electromotor stators strontium hexaferrite powders with average particle sizes 56.8 µm and 7.5 µm in the magneto fluidized bed formed by mutually perpendicular constant and alternating gradient magnetic fields. The desired state of the fluidized bed for milling is the one with the maximal particle motion intensity that prevents particle aggregation. The intensification of particle motion could be done by the increase of the alternating magnetic field gradient, but this also increases energy consumption of the process. In the present paper we propose using the inductive method for estimating rheological state of the strontium hexaferrite powder in the magneto fluidized bed. Such estimation allows finding the optimal parameters of electromagnetic fields forming the fluidized bed for the efficient powder milling process.

Stimulated fluidity of Nd-Fe-B powder in magneto fluidized bed

Experimental studies results of electromagnetic effect influence on stimulated fluidity of Nd-Fe-B powder are presented in the paper. Fine Nd-Fe-B powder doesn’t have natural fluidity. To make it flow through hopper nozzle, the powder was affected by constant and alternating gradient magnetic fields. Constant magnetic field induction lines were horizontal, and alternating field lines were vertical with gradient towards efflux direction. We studied influence of magnetic fields on powder fluidity speed to find optimal magnetic fields parameters that form steady efflux of Nd-Fe-B powder with an average particle size 4.6 μm through nozzles with diameters 1 and 2 mm.

Kinetics of Structural Changes in Strontium Hexa-Ferrite Powder during Milling in Beater Mill

The paper presents the results of experimental studies of strontium hexa-ferrite average particle size and structural characteristics changes during milling process. Coarse strontium hexaferrite was milled in beater mill, without and with electromagnetic effect. Electromagnetic effect was produced by constant and alternating gradient magnetic fields with mutually perpendicular induction lines. Particle sizes were measured by microscopic methods, and structural characteristics were calculated by processing of X-ray diffractograms. Diffraction studies showed that during milling process, both with and without electromagnetic effect, the most intensive changes of coherent scattering region (CSR) sizes, dislocation densities and relative deformation of particulate material occur at earlier stage of milling. At this stage the speed of average particle size decrease is maximal. At later stage both average particle size and structural characteristic changes correlate and have asymptotic character.

Study of Electromagnetic Effect Parameters Influence on Fluidity of Fine Nd-Fe-B Powder

The paper experimentally proves the efficiency of electromagnetic effect on stimulated fluidity of magneto-hard powders. Fine Nd-Fe-B powder doesn’t have natural fluidity. To make it flow through hopper outlet we effected on the powder by constant and alternating gradient magnetic fields. Induction lines of constant magnetic field were horizontal, and alternating field lines were vertical. In order to form stimulated powder fluidity, higher induction gradient was situated below the powder receiver chamber. We experimentally found optimal magnetic fields parameters that form steady magneto fluidized bed from Nd-Fe-B powder, with an average particle size 1 μm. The study results of influence of magneto fluidized bed electromagnetic parameters on Ne-Fe-B powder efflux speed through outlets with diameters 1 and 2 mm are presented. Gradient field topology and parameters of electromagnetic effect providing minimal efflux time of 1000 mg of powder are defined.

Particle Size Distribution and Structural State Analysis of Mechanically Milled Strontium Hexaferrite

Article presents an experimental study result of milling coarse strontium hexaferrite in beater mill with formation of magneto fluidized bed and without it. Magneto fluidized bed is formed by mutually perpendicular constant and alternating gradient magnetic fields. We studied the dynamics of particle size distribution from milling time and parameters of magnetic fields. Microstructure dynamics of strontium hexaferrite powder particles milled in various regimes was studied by X-ray diffraction methods. Milling efficiency and energy efficiency of milling process were studied in conditions with and without powder fluidization by magnetic fields. Analysis of experimental data showed advantages of milling in magneto fluidized bed in increased efficiency, particle size distribution homogeneity and powder chemical activity because of lattice micro-stresses.

Effect of alternating gradient magnetic field on heat transfer enhancement of magnetoreological fluid flowing through microchannel

To investigate the contribution of transverse velocity pulsation on the heat transfer enhancement of magnetoreological fluid (MRF) flow in microchannel subjected to gradient alternating magnetic field, a doubled-population lattice Boltzmann simulation method is developed, where the magnetic nanoparticles (MPs) are treated as a quasi fluid and a simplified evaluation method for external forces on solid phase lattice points is proposed. Primarily, the effects of the alternating gradient magnetic field on the velocity and temperature distributions under temperature and heat flux boundary conditions are investigated. Simulation results show that, affected by the alternating gradient magnetic field, a considerable transverse velocity component occurs in the flow field. With the increase of pressure difference, the Nusselt number increases while the mean temperature of MRF reduces. The Nusselt number changes fluctuantly when the gradient magnetic field alternates. For the model concerned in this work, the gradient magnetic field induced transverse motion of fluid particles do not affect the heat transfer enhancement significantly. The experimental results indicate that both the heat transfer coefficient and the Nusselt number increase with the increasing Reynolds number, and the increasing magnetic induction intensity results in an obvious increase of heat transfer effect.

Simulation of oscillation of magnetic particles in 3D microchannel flow subjected to alternating gradient magnetic field

Abstract Magnetic particles (MPs) have good response to external magnetic field. To investigate the motion regularity of MPs in a mcirochannel flow governed by alternating gradient magnetic field, a 3D numerical simulation method is established based on the lattice Boltzmann method (LBM) and the immersed boundary method (IBM). The motions of both a single MP and a short chain of MPs are investigated and each is found to be a superposition of transverse oscillation and longitudinal translation. By performing a simple filtering processing for the original trajectory of MP, it is revealed that the oscillation of MPs affected by alternating gradient magnetic field can be characterized as the simple harmonic vibration. The oscillation frequency of MPs is basically in accordance with the alternating frequency of the gradient magnetic field in the range from 400 to 800 Hz. The larger magnetic field gradient results in the larger amplitude of oscillation. The wavelength of the trajectory is increased due to the increasing inlet velocity of carrier fluid. For the MP chain, both the period and amplitude of oscillation will decrease with the increase of alternating frequency of magnetic field. The turbulence intensity distribution indicates that the turbulence of microchannel flow can be enhanced by the oscillation of MPs thus enhance the mass or heat transfer in the carrier fluid.

The Influence of Milling Regimes in Beater Mill on Particle Size Distribution and Structural Characteristics of Strontium Hexaferrite Powder

The paper experimentally substantiates effectiveness of method of milling particulate ferromagnetic materials in magneto fluidized bed. Comparative results of particle size distributions and structural parameters of strontium hexaferrite SrFe12O19 powder obtained by milling coarse material in beater mill without electromagnetic effect and in same mill with formation of magneto fluidized bed from mill material are presented. The magneto fluidized bed is formed by constant and alternating gradient magnetic fields with induction lines that are mutually perpendicular and parallel to the plane of rotating beaters. It is shown that application of electromagnetic effect to milling coarse material in beater mill allowed to greatly intensify that process, significantly increase powder quality: increase particle size distribution uniformity and decrease average particle size from 1558.50 μm to 0.56 μm after 120 minutes of processing in the mill. X-ray diffraction analysis showed that milling in beater mill in magneto fluidized bed leads to reduction of coherent-scattering region size, increase of lattice microstrain and dislocation density, making powder more active during sintering process.

Disaggregation and separation dynamics of magnetic particles in a microfluidic flow under an alternating gradient magnetic field

How to prevent particle aggregation in the magnetic separation process is of great importance for high-purity separation, while it is a challenging issue in practice. In this work, we report a novel method to solve this problem for improving the selectivity of size-based separation by use of a gradient alternating magnetic field. The specially designed magnetic field is capable of dynamically adjusting the magnetic field direction without changing the direction of magnetic gradient force acting on the particles. Using direct numerical simulations, we show that particles within a certain center-to-center distance are inseparable under a gradient static magnetic field since they are easy aggregated and then start moving together. By contrast, it has been demonstrated that alternating repulsive and attractive interaction forces between particles can be generated to avoid the formation of aggregations when the alternating gradient magnetic field with a given alternating frequency is applied, enabling these particles to be continuously separated based on size-dependent properties. The proposed magnetic separation method and simulation results have the significance for fundamental understanding of particle dynamic behavior and improving the separation efficiency.

Influence of electromagnetic effect parameters on ferromagnetic powders fluidity

The paper presents result of experimental studies of ferromagnetic powders fluidity management by effecting on particulate medium with constant and alternating gradient magnetic fields. Induction lines of constant magnetic field are perpendicular to symmetry axis of powder hopper. Alternating field is created by electromagnet with one tapering pole and has higher induction gradient values near hopper outlet. These fields form magneto fluidized bed from powder in hopper allowing powders of barium and strontium hexaferrite to flow through the hopper outlet. Dependencies of efflux speed from parameters of electromagnetic fields are presented for barium hexaferrite powder with average particle size 1 μm and strontium hexaferrite powders with average particle sizes 1 and 9 μm.

Technological processes intensification in devices with magneto-fluidized bed

The paper shows the efficiency of application of magneto-fluidized bed formed of ferromagnetic materials in press-forms dosed filling and milling devices. For powders without natural fluidity, dosing device is situated between electromagnets creating constant magnetic field with horizontal induction lines and alternating gradient magnetic field with vertical induction lines and higher induction gradient in the lowest part of chamber with powder. This configuration allows development of uniform efflux speed of ferromagnetic powders from 2 mm diameter opening. For example, mass efflux speed of strontium ferrite powder with 1 μm average particle size in magneto-fluidized bed reached 181.1 mg/s and the dosing time of 2000 mg of this powder was 11.1 s. For practical usage of magneto-fluidized bed in the milling device, beater mill is situated between electromagnet poles in the way that induction lines of constant and alternating gradient magnetic fields were mutually perpendicular and parallel to the plane of rotating beaters. Milling of particulate strontium ferrite with 1558.5 μm average particle size during 120 minutes in magneto-fluidized bed allows increasing milling degree in 16.2 times by comparison with processing without electromagnetic effect.

Annealing temperature effects on structural and magnetic properties of Ga1−xMnxN films

Amorphous gallium nitride doped with Mn thin films was deposited on sapphire (0001) substrates by laser molecular beam epitaxy. After simple processing of annealing at different temperatures for 30 min in ammonia atmosphere, good quality Ga1−xMnxN films were obtained. In order to investigate the influence of annealing temperature on the crystalline quality and structural and magnetic properties, the films are analysed by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and alternating gradient magnetic field meter. The results show that the annealing can improve the quality of Ga1−xMnxN films, and the optimum annealing temperature is 1050°C; moreover, there is no second phase forming. Through magnetic analysis, we obtained the annealed samples of room temperature ferromagnetic property and paramagnetism of the as grown film and obtained the way of changing the paramagnetism into ferromagnetism using the simple method.

The Magnetic Deflector of the Buenos Aires 180-cm Synchrocyclotron Beam

The magnetic deflector system of the Buenos Aires 180-cm synchrocyclotron beam consists of a regenerator (previously reported) and a critical magnetic channel. The channel was built starting on the maximum of the radial oscillation and following a trajectory given by ρ=83.5+10α11/4 (in cm), where (ρ,α) are polar coordinates with origin at the center of the machine and α is measured from the maximum of the radial oscillation. Alternating gradient magnetic fields were used in order to keep the beam parallel up to the magnetic quadrupoles. Thus the vertical divergence of the beam is vanishingly small and the radial divergence is 1/200. With the complete assembly in place 20% of the circulating beam is thrown by the regenerator into the magnetic channel, and its transmission is 10%. The over-all extraction efficiency is therefore 2%.