Strontium Hexaferrite Powders 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.

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.

Investigation of nano-crystaline strontium hexaferrite magnet powder from mill scale waste by the mechanochemical synthesis: Effect of the annealing temperature

Today, waste management and recycling have critical importance given the dwindling natural resources and to create powerful technologies for a green future. One of these strontium hexaferrite powders were produced by mechanochemical process using mill-scale and strontium carbonate from obtained celestite ore. The strontium carbonate and the mill scale were prepared in stoichiometric ratio, and mechanochemical process was applied for 16 h to produce hard-ferrite powders. Following the mechanochemical synthesis process, powder mixture was annealed at 850–1000 °C temperatures. The morphology, crystal structure and phase composition of strontium hexaferrite powders caused the magnetic properties to change according to the annealing temperature. As a result of the studies, the most suitable annealing temperature was determined as 975 °C. The Hc, Br and (BH)max of produced powders correspond to 5505Oe, 347 mT and 5.77MGOe at 975 °C, respectively. The Curie temperature of the annealed powders was determined as 474 °C.

Wideband electromagnetic wave absorption by tuning morphology and layer arrangement in Bi-layer absorber based on doped SrFe12O19 nanocomposite powders

The main parameters for achieving the optimum microwave dissipation performance in bi-layer absorbers are micromorphological characteristics, filler content composition, and layer configurations. In this research, two different doped strontium hexaferrite powders with SrFe10Al2O19 (PSrAl) and SrFe10CoTiO19 (CSrCoTi) chemical formulas were prepared through a solvothermal method. This study aims to identify optimal absorption conditions for bilayer samples by combining different magnetic compositions with varying ferromagnetic resonance and diverse morphologies with different layer arrangements. Single layer absorber samples comprising PSrAl powder with a polygonal pyramidal particle shape displayed dominating dielectric loss behavior and ferromagnetic resonance at about 11.6 GHz, according to the findings. On the other hand, absorber samples containing CSrCoTi powder with crochet ball like particle morphology, exhibit dominant magnetic loss behavior in the studied frequency range, with ferromagnetic resonance around 8.8 GHz. According to the results of bi-layer absorber samples, superior microwave dissipation performance could be achieved by inserting PSrAl powder in the top layer as a matching layer with 0.5 mm thickness and CSrCoTi powder as an absorbing layer 2 mm-thick. For a bi-layer absorber with a 2.5 mm total thickness The EAB (effective absorption bandwidth) is equal to 3.1 GHz at X-band and the minimum reflection loss is equal to −22 dB. The tuning morphology, which improved impedance matching and absorption properties by increasing the wave propagation path in the sample, synergistic effect between two distinct layers, and overlapping of resonance peaks of two components, is credited with this remarkable microwave absorption feature.

Correlation between milling-induced strain, microstructure, and magnetic properties in anisotropic SrFe12O19 powders

Strontium hexaferrite (SrFe12O19) powders synthesized via conventional solid-state route are mechanically milled using air jet-mill and ball-mill techniques to produce anisotropic SrFe12O19 powders. Jet-milled and dry ball-milled powders showed higher strain (0.13%) compared to wet ball-milled powders (0.05%). On annealing, jet-milled powder exhibited better recovery of strain compared to the ball-milled powders. Enhancement in coercivity after annealing is maximum in jet-milled samples (1.7 kOe to 5.2 kOe). Angle-dependent magnetic hysteresis measurements suggest a difference in the nature of strain induced in jet-mill and ball-mill methods. Maximum remanence (0.81) is observed in both jet-milled and dry ball-milled powders, however, jet-milled powders showed a better retention of remanence (0.74) after annealing than the ball-milled powders (0.64). A stark contrast in the microstructure of jet-milled and ball-milled powders is revealed from SEM images. Particle size distribution curves confirm a considerable re-agglomeration in the ball-milled samples. The role of milling-induced strain and microstructure in obtaining anisotropic SrFe12O19 powders with optimum remanence and coercivity is discussed.

Milling Duration Influence on Strontium Hexaferrite Technological Characteristics

Fine powders of strontium hexaferrite are widely used in powder metallurgy for the production of permanent magnets resistant to atmospheric oxygen and high working temperatures. Obtaining powders with predefined technological characteristics in minimal time and with minimal energy consumption is an actual problem of powder metallurgy. The paper provides the results of experimental studies of technological characteristics of strontium hexaferrite powder (SrFe12O19) during milling in a beater mill. Mechanical milling of coarse strontium hexaferrite was carried out in the mill with the system of rotating beaters for 120 minutes without and with the creation of a pseudo fluidized bed. The fluidization was formed by a perpendicular constant and alternating magnetic field with induction gradients of 150 and 210 mT/m. Average particle size and powder bulk density dependencies from milling time were studied. Experimental data show that milling with the formation of a magneto fluidized bed allows intensifying the process. Beginning from 70 minutes, the dependencies of average particle size and bulk density come to almost asymptotic behavior making further milling rather ineffective. Carried out research allows choosing optimal milling duration for obtaining the required average particle size.

Structural, morphological and magnetic properties of Sr0,54Ca0,46Fe6,5Al5,5O19 M-type hexaferrites: Effects of annealing temperature

Ca–Al co-doped strontium hexaferrite powder with a cation composition of Sr0,54Ca0,46Fe6,5Al5,5O19 was synthesized using solid-state route. The prepared powder was calcined at different temperatures (1200 °C, 1250 °C, 1300 °C and 1350 °C) for 24 h. Structural, morphological and magnetic properties of synthesized samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and SQUID measurements. The XRD results confirmed the hexagonal structure. The calcination temperature sufficient for the complete formation of the phase was found to be 1300 °C. The size of the crystallite determined from XRD data was found to be small compared to that of the grain size calculated from the SEM micrograph. According to the magnetic data, an increase in the calcination temperature leads to a small increase in the remanent magnetization and coercivity field. This can be explained by the increase of the crystallite size.

Magnetic and morphological properties of Ca substituted M-type hexaferrite powders synthesized by the molten salt method

The doping element and morphology are important factors for improving the magnetic properties of M-type hexaferrite powder. As the demand for electromagnetic absorber and magnetic sensors increases in the market, it is required to develop hexaferrite powder with a homogenous shape and excellent magnetic properties. In this study, Ca substituted strontium hexaferrite powders were synthesized by the molten salt method under various calcination conditions. The morphology and magnetic properties of the synthesized powders were successfully controlled according to the Ca content and calcination conditions. As a result, single-crystalline Sr0.90Ca0.10Fe12O19 powders with an average thickness of 0.4 µm were successfully obtained by calcining at 950 °C for 1 h. The synthesized powders had a magnetic saturation of 74.9 emu/g and a coercivity of 3397 Oe.

Diagnostic of Structural Changes in Ferromagnetic Powders during Milling in Beater Mill

Problem of obtaining fine powders of strontium hexa-ferrite is actual because of its wide applications. The paper provides the results of studies of particle size distribution and structural characteristic changes of strontium hexa-ferrite powder (SrFe12O19) during milling in impact mill and after its consequent annealing. Mechanical processing of coarse particulate system was carried out in the mill for 120 minutes without electromagnetic effect and with creation of magneto fluidized bed, formed by perpendicular constant and alternating magnetic field with induction gradient of 210 mT/m, providing reciprocating motion of particles and aggregates with sizes of 3 – 4 mm. It was shown that milling of coarse strontium hexa-ferrite with average particle size 1558.5 μm and the most possible size 1500 μm in magneto fluidized bed allowed to intensify milling process and to provide a significant increase of powder particle sizes uniformity. It was found out, that milling in magneto fluidized bed leads to a great decrease of coherent scattering regions sizes and an increase of lattice micro-deformations and relative dislocation density. Consequent annealing of the powder for 2 hours at 850°C refined structural characteristics significantly. The carried out research allows to choose the optimal milling duration for solution of practical problems of powder metallurgy.

X-ray Diffraction Analysis by Williamson-Hall Method of Strontium Hexaferrite Lattice Features after Mechanical Milling

The paper presents the result of experimental studies of strontium hexaferrite milling in beater mill. SEM studies showed that milling of strontium hexaferrite in beater mill with formation of magneto fluidized bed leads to significant intensification of the process. Williamson-Hall method was used to determine structural characteristics changes in the strontium hexaferrite powder from milling time in magneto fluidized bed. Carried out studies showed that milling intensification is significant only to a certain dispersity of processed powder. However, during milling with formation of magneto fluidized bed, decrease of particle sizes and structural characteristics changes speed happens at finer disperse compound.

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.

Effect of magnetic ion substitution on the structure and temperature-dependent magnetic properties of strontium hexaferrite

ABSTRACT Rare earth substituted strontium hexaferrite powders were synthesised through a molten salt route at 900°C for 5 h. The influence of rare-earth magnetic ion substitution on microstructural features, morphology and temperature-dependent magnetic properties was investigated. The lattice parameter ratio of less than 3.98 proves that all the substituted samples primarily present the hexaferrite phase. Fourier transform infrared peaks also confirmed the presence of a metal-oxygen bond. A 66% increase in magnetic saturation, 38% increase in coercivity and a more than two-time increase in remanence for the samarium doped sample are seen as the highest magnetic measurements compared to pure SrFe12O19 at room temperature. There were also differences in the temperature-dependent magnetic properties of strontium ferrites. As temperature decreased from 300 K, saturation magnetisation and remanence value were found to increase. This will perhaps be a promising material for magnetic field sensor application.

Development of SrFe12O19/Ti3SiC2 Composites for Enhanced Microwave Absorption

Microwave absorbing composites containing strontium hexaferrite and titanium silicon carbide, SrFe12O19/Ti3SiC2 powder, were synthesized by mixing in different weight ratios. The strontium hexaferrite (SrFe12O19) particles were synthesized using low-temperature combustion. The ‘as synthesized’ strontium hexaferrite powder is observed to have low coercivity (255.09 G) and high saturation magnetization (45.1 emu/g). The titanium silicon carbide (Ti3SiC2) powder was synthesized by heat treatment of the precursors (Ti/Si/TiC) at 1100°C under argon atmosphere. A microwave absorbing composite is fabricated by mixing in a high-energy ball mill followed by heating of SrFe12O19 and Ti3SiC2 powders at 200°C. The maximum reflection loss of − 39.67 dB is observed at 9.46 GHz for the composite casted into pellets containing 20% Ti3SiC2 and 80% SrFe12O19 with 2-mm thickness. The bandwidth for less than − 10 dB loss is observed to be 2.77 GHz (66% of the total band). The developed magneto-dielectric composition confirmed its candidacy as a potential microwave absorbing material.

Effect of temperature on the magnetic properties of strontium hexaferrite synthesized by the Pechini method

In this work, strontium hexaferrite powders were synthesized by the polymeric complex method (Pechini). The as-prepared samples were calcined at temperatures of 800 °C, 850 °C, 900 °C, 1000 °C, and 1050 °C for 1 h. In addition, the presence of a secondary phase of hematite (α-Fe2O3) was detected in all samples. The influence of calcination temperature and synthesis technique to control grain growth and size was analyzed. The crystalline phases, structure and morphology of the samples were determined by X-ray diffraction, scanning electron microscope and transmission electron microscopy. While the magnetic properties were measured at room temperature using a vibrating sample magnetometer with an applied field of up to 20 KOe. The results show that the sample calcined at 900 °C with a crystal size of 245 nm exhibits the best magnetic properties of the entire series, obtaining the highest values of magnetization by saturation and coercivity (Ms 99.3 emu/g, and Hc 6.15 kOe). This can be attributed to the exchange coupling interactions between the soft and hard phases of the sample.

Microstructure and Magnetic Properties of Anisotropic Strontium Hexaferrite Powders

Anisotropic strontium hexaferrite powders were prepared by solid-state method with subsequent magnetic field alignment and annealing. The microstructure of the powders was investigated using X-ray diffraction and scanning electron microscopy to understand the effect of the process on magnetic properties, magnetic texture, particle size, and morphology of the powders. The magnetic properties were measured using a vibrating sample magnetometer. A high coercivity ( $H_{\mathrm {c}}$ ) of 502 kA/m, hysteresis squareness ( $M_{\mathrm {r}}/M_{\mathrm {s}})$ of 96%, and maximum energy product [( BH )max] of 32.2 kJ/m3 were obtained in the powders prepared via magnetic field (800 kA/m) alignment and annealing. The high $M_{\mathrm {r}}/M_{\mathrm {s}}$ and $H_{\mathrm {c}}$ obtained in the powders were correlated with the magnetic texture, and defect free fine particles present in the powder.

Synthesis and properties of SrFe12O19 obtained by solid waste recycling of oily cold rolling mill sludge

The aim of this study was to develop a new approach for the preparation of environmentally friendly, high-value products from oily cold rolling mill (CRM) sludge. Utilizing oily CRM sludge as a source of iron, strontium hexaferrite (SrFe12O19) powders were prepared by multi-step processes involving acid leaching, chemical conversion treatment, and synthesis by a citrate precursor. The influences of citric acid dosage and the pH of the sol system on the formation, crystallite size, and magnetic properties of the obtained SrFe12O19 powders were investigated. High saturation magnetization (74.8 mA-m(2)g(-1)) and intrinsic coercivities (614.46 mT) were achieved for pH 7.0 of the sol system, for which the molar ratio of citric acid dosage to the total dosage of Fe-3+ and Sr2+ was 1.5. This study presents a new approach to utilizing oily CRM sludge, and even refractory iron-containing solid waste.

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.

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.

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.

Synthesis of SrFe12O19/SiO2/TiO2 composites with core/shell/shell nano-structure and evaluation of their photo-catalytic efficiency for degradation of methylene blue

The SrFe12O19/SiO2/TiO2 nanostructures with hard magnetic core were successfully synthesized through the facile and efficient wet chemical processes. At first, nanocrystalline strontium hexaferrite (SrFe12O19) powder was prepared using a new co-precipitation route in ethanol/water media. In the next step, SrFe12O19/SiO2 composites were produced by well-known Stober method using tetraethyl orthosilicate as precursor. Finally titania was coated on SrFe12O19/SiO2 composite particles using titanium n-butoxide precursor. The core/shell/shell nanostructures have been characterized by means of X-ray diffraction, vibrating sample magnetometer, Fourier transform infrared spectra, field emission scanning electron microscopy, and transmission electron microscopy equipped with an energy-dispersive X-ray spectroscopy detector. The catalytic activity of SrFe12O19/SiO2/TiO2 composites has been investigated in the degradation of methylene blue dye under UV illumination. The results indicated that the obtained SrFe12O19/SiO2/TiO2 composite has photo-catalytic properties and can be retrieved by magnetic separation. The photo-degradation of methylene blue dye was about 80% in the presence of photo-catalyst powder at irradiation time of 180 min. Recycled composite particles could be used again.