Fine Magnetic Powder Research Articles

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.

High orientation Nd-Fe-B sintered magnets prepared by wet pressing method

Fine magnetic powder particles are usually utilized to prepare high coercivity Nd-Fe-B magnets. However, it is more difficult to obtain high orientation degree due to the larger friction between finer powder particles during magnetic-aligned compaction process, which greatly affects the performance of final magnets. In this work, a special wet pressing method is adopted to reduce the friction between the powder particles. The degree of alignment of the magnet is obviously increased and the microstructure is optimized. The remanent magnetization of the magnet shows a significant increase with negligible coercivity change, while the compressive strength is improved compared with that of the traditional method.

Development of ultralight nanocellulose magnets using ultrasonic agitation

In this study, a simple method was developed to incorporate commercially available magnetic AlNiCo powders into nanocellulose composite solids and aerogels. Fine magnetic powders can be homogeneously integrated using only ultrasonic agitation. Simply grinding standard industrial powders within a mortar and pestle was sufficient to create a fine enough starting material, although care must be taken not to magnetize the powder to prevent agglomeration. The resulting suspension could be freeze dried to form ultralight ferromagnetic aerogels or dried to form hard magnetic solids or films. Homogeneous nanocellulose composites with mass ratios of up to 50% AlNiCo were readily achieved. The resulting materials showed magnetic behavior as expected for a standard AlNiCo composite, with a homogeneous and submicron magnetic particle distribution. The resultant properties compared favorably to processes using preformed magnetic nanoparticles as a starting material. The simplicity of the technique, which uses standard industrial reagents, could be useful in developing sustainable and economically feasible methods for creating nanocellulose composite materials.

Determination of formal kinetic constants of thermal decomposition of aqueous hydrogen peroxide solution in a mixture of magnetic powder, based on experimental thermogram, obtained in adiabatic conditions

Process of thermal decomposition of hydrogen peroxide aqueous solution with the addition of magnetic powder in the form of toner for printers and lanthanum manganite were considered. Obtained resulting from an experiment in the Dewar container conducted thermogram analyzed using mass balance equations and heat. Formal kinetic parameters determined, and conclude that the magnetic powder in the mixture does not have catalytic properties. The described technique is recommended as a rapid analysis of the kinetics of the various reactions to substances having predefined thermal and thermodynamic properties. Recently, it has been suggested to use fluids mixed with a fine powder to change the thermophysical features of coolants in order to increase the heat exchanger efficiency. Application of magnetic powders allows for additional control over unit operation using the magnetic field. An aqueous hydrogen peroxide solution is often suggested to be used as a coolant in a variety of heat exchangers (such as solar collectors). The aqueous hydrogen peroxide solution with additives of fine magnetic powder can significantly improve the efficiency of heat exchangers. To study the kinetics of thermal decomposition of hydrogen peroxide aqueous solution with added magnetic powder, series of experiments were done to determine the process thermogram in adiabatic conditions of Dewar vessels. Dynamic heat balance equation for the reaction of thermal decomposition of hydrogen peroxide aqueous solution, considering the presence of magnetic powder particles and the reaction of hydrogen peroxide thermal decomposition are recorded under the assumption of the process equilibrium within the entire time interval . During experiments the mixture temperature variations from the initial to final did not exceed 8.5K, which allows, according to the Hess's law, heat of reaction (qd,, J/mol) to be taken as constant. The thermodynamic system is a colloidal solution of hydrogen peroxide, oxygen in water (with current masses, respectively, mH2O2,mO2, mH2O) and fine magnetic powder, consisting of toner for printers and lanthanum manganite (LaySrMnO3−X( y< x)) of the total mass mmp. Specific heat components, respectively, cH2O2, cO2, cH2O & cmp are temperature functions for a general case. Heat from the

Magnetic field induced inversion in the effect of particle size on powder cohesiveness

Experimental measurements are reported on the tensile yield stress of magnetofluidized beds of fine magnetic powders operated in the cross-flow configuration. In the absence of externally applied magnetic field the yield stress of the powder depends on particle size as expected, i.e., it increases as bead size is decreased. This trend is however inverted when an external magnetic field is applied. It is suggested that the average orientation of interparticle contacts relative to the direction of the field as affected by particle size plays a relevant role on the magnetic yield stress of these systems.

Rheology of magnetofluidized fine powders: The role of interparticle contact forces

Usually, a bed of solid particles fluidized by a gas is inherently unstable. Gas bubbles are rapidly formed at the onset of fluidization, which hinders the efficiency of gas-solid contact. In the case of magnetizable particles, gas bubbles may be suppressed by means of an externally applied field that magnetizes the particles. In general, magnetized particles are assumed to behave as point dipoles that organize in chainlike structures oriented along field lines due to dipole-dipole attraction. The physical mechanism responsible for stabilization is, however, unclear. In particular, rheological characterization of magnetically stabilized beds (MSBs) has been a subject of controversy and there is no widely accepted explanation to the empirical fact that magnetofluidized beds (MFBs) can be stabilized by a horizontal field. Several experimental approaches have been used mainly aimed to observe the fluidity of MFBs. Generally, MFBs are reported to behave as a fluid up to a critical magnetic field strength at which there appears an appreciable yield stress. Most of these techniques are however invasive, which sheds doubts on the mechanism responsible for the appearance of the yield stress. In this work, we have measured the yield stress of MFBs of fine magnetic powders by means of a noninvasive technique that uses gas flow to put the bed under tension. It is shown that the MFB behaves as a plastic solid. The yield stress of the MFB, which is developed just at marginal stability, arises as a consequence of the magnetic attraction between particles at contact. Fine magnetic powders of different aggregative nature in the absence of applied magnetic field have been used in our work. It has been seen that the yield stress of MSBs of naturally aggregated particles is significantly larger than the yield stress of MSBs of naturally nonaggregated particles. Moreover, the MFB is stabilized at smaller field intensities in the former case. Noninvasive visualization of the MFB surface shows that quasivertical chainlike structures are stable despite that the magnetic field is applied in the horizontal direction, which sheds doubts on the validity of the widely used dipolar assumption.

X-Ray Diffraction Analysis and Magnetic Properties of Pr-Fe-B HDDR Powders and Magnets

Fine magnetic powder has been produced using the hydrogenation disproportionation desorption and recombination (HDDR) process. The first goal of this work involved an investigation of a range of disproportionation/desorption temperatures between 800 and 900°C with the purpose of optimizing the HDDR treatment for a Pr14Fe80B6 alloy. The cast alloy was annealed at 1100°C for 20 hours for homogenization. The optimum disproportionation temperature for achieving high anisotropy was 820°C. The influence of the reaction temperature on the microstructure and magnetic properties of Pr14Fe80B6 HDDR powders and magnets has been shown. A second stage of this study involved the characterization, for each temperature, of the HDDR processed powder using X-ray diffraction analysis. Samples of the HDDR material have been studied by synchrotron radiation powder diffraction using the Rietveld method for cell refinement, phase quantification and crystallite sizes determination. Scanning electron microscopy (SEM) has also been employed to reveal the morphology of the HDDR powder.

Electrolytic synthesis of TbFe2 from Tb4O7 and Fe2O3 powders in molten CaCl2

Abstract Fine magnetic powder of a terbium–iron intermetallic compound, TbFe2, was prepared by electrochemical reduction of thermally compounded powders of Tb4O7 and Fe2O3 (atomic Tb/Fe ratio, 1:2) in molten CaCl2. The electro-reduction potential of mixed Tb4O7 and Fe2O3 powders was more positive than that of Ca2+ in molten CaCl2 as revealed by cyclic voltammetry using a novel metallic cavity electrode. Porous pellets of mixed Tb4O7 and Fe2O3 powders were prepared by sintering in air, and were reduced to various Tb–Fe intermetallic compounds by constant voltage electrolysis at typically 2.6–3.2 V and 900 °C for 10–12 h. The electrolytic product could be cleaned effectively in dimethylsulfoxide under magnetic stirring, leading to a pure magnetic powder with a low oxygen content (

Magnetic field-induced cluster formation and variation of magneto-optical signals in zinc-substituted ferrofluids

Fine magnetic particles (size≅100 Å) belonging to the series Zn x Fe 1− x Fe 2O 4 were synthesized by cold co-precipitation methods and their structural properties were evaluated using X-ray diffraction. Magnetization studies have been carried out using vibrating sample magnetometry (VSM) showing near-zero loss loop characteristics. Ferrofluids were then prepared employing these fine magnetic powders using oleic acid as surfactant and kerosene as carrier liquid by modifying the usually reported synthesis technique in order to induce anisotropy and enhance the magneto-optical signals. Liquid thin films of these fluids were prepared and field-induced laser transmission through these films was studied. The transmitted light intensity decreases at the centre with applied magnetic field in a linear fashion when subjected to low magnetic fields and saturate at higher fields. This is in accordance with the saturation in cluster formation. The pattern exhibited by these films in the presence of different magnetic fields was observed with the help of a CCD camera and was recorded photographically.

Fe/Ba Ratio Effect on Magnetic Properties of Barium Ferrite Powders Prepared by Microwave-Induced Combustion

Barium ferrite powders were successfully synthesized by microwave-induced combustion. The magnetic properties of barium ferrite powders with various Fe/Ba ratios varying from 11 to 12 annealed at various temperatures in the range of 850–1050°C were determined. The resultant powders were investigated by X-ray diffractometer (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), differential thermal analyzer/thermogravimeter (DTA/TG), and surface area measurement. Microwave-heated barium ferrite with an Fe/Ba ratio of 11 and annealed at 1000°C for 2 h exhibited optimum magnetic properties for magnetic recording applications, a saturation magnetization of 66 emu/g and an intrinsic coercive force of 2100 Oe. The microwave-heated barium ferrite powders with various Fe/Ba ratios annealed at various temperatures had particles size ranging from 40 to 90 nm. Thus, the fine control of crystal growth by varying the annealing temperature can be exploited for the production of fine magnetic powders of various sizes for a variety of practical applications.

Study on the Dispersion of Alkoxy-Derived Ferrite Powders

The dispersion of fine magnetic powders synthesized from metal alkoxides was studied. The powders were treated with an alkali, and then oleic ions were adsorped in sodium oleate aqueous solution. This as-wetted powders were dispersed in water or alkylnaphthalene containing a surfactant. It is concluded that;(1) Fine particles below 10-15nm are required to prepare stable suspensions, (2) To be fully adsorped with oleic ions, the powder synthesized have to be treated with an alkali, (3) The surfactant more than 25wt% of the powder is repuired to disperse the powder, (4) The effective surfactant is sodium oleate for water suspension medium and ethoxylated alcohol phosphate for alkylnaphthalene suspension medium.

Surface structure of fine particle with super high coercivity

Fine magnetic powders with super high coercivity up to 2200 Oe have been obtained by heat treatment of chain-of-spheres Fe-Co-B-O particles. Internal migration of constituent elements, specific surface area, pore size distribution and other properties were measured during this heat treatment.