Structural,electromagnetic, and dielectric properties of lithium-zinc ferrite ceramics sintered by pulsed electron beam heating

Structural, electromagnetic, and dielectric properties of Li_0.4Fe_2.4Zn_0.2O₄ lithium-zinc ferrite sintered by 2.4 MeV pulsed electron beam heating at 1050 °С for 2 h were investigated. The formation of ferrite with a single-phase cubic spinel structure was confirmed by X-ray diffraction analysis. The average grain size of ferrite ceramic was determined by SEM analysis and its value was 1.7 µm. The radiation-thermal sintered samples are characterized by a saturation magnetization of 67.8 emu/g, the Curie temperature of 508 °C, AC electrical resistivity of 2.4×10⁴ Ω cm (at 25 °C). The frequency dependences of permittivity and the loss tangent were obtained in (20 – 2×10⁶) Hz frequency range. The behavior of ε′ is characterized by high dispersion caused by relaxation polarization in the investigated frequency range. The results were compared to the LiZn ferrite characteristics sintered by traditional thermal heating.     

Synthesis of CoFe2O4 Nanoparticles by a Low Temperature Microwave‐Assisted Ball‐Milling Technique

Well‐crystallized Cobalt ferrite nanoparticles with mean size of 20 nm and high saturation magnetization (82.9 emu/g) were synthesized at a low temperature (≤100°C) by microwave‐assisted solid–liquid reaction ball‐milling technique without subsequent calcination. CoC₂O₄·4H₂O and Fe powder were used as raw materials and stainless steel or pure iron milling balls with diameter of 1.5 mm were used. As a contrast, solid–liquid reaction ball milling without microwave assistance was also investigated. The results showed that this is a simple, environmentally friendly, and energy‐saving technique for ferrite nanocrystal synthesis.     

Effect of sintering regimes on the microstructure and magnetic properties of LiTiZn ferrite ceramics

In present work, the influence of sintering regimes on the microstructure, saturation magnetization, density and porosity, the grain size, the Curie point, and the temperature dependence of the initial permeability of LiTiZn ferrite ceramics was investigated. Ceramics was prepared by a standard ceramic technique. The formation of a single-phase cubic spinel structure was confirmed by XRD analysis. The Curie point was determined from both the temperature dependences of the initial permeability and the method of thermogravimetric measurements in a magnetic field. Density/porosity and the grain size, the Curie point and magnetization are sensitive to the sintering regime. The initial permeability of ferrite decreases with sintering temperature (in the range of 1010–1150?°С) and grain size increasing that contradicts the generally accepted Globus and Smith-Wijn theories. A possible reason of such behavior is the formation of intragranular pores growing with the increase in the sintering temperature and inhibiting the domain wall motion inside the grain. These results correspond to the porosity of the investigated ferrite ceramic samples, which grows with sintering temperature increasing.The non-stoichiometry arising due to evaporation of lithium and zinc oxides at temperature above 1010?°C affects the initial permeability. In this work, a qualitative assessment of the defective state of ferrite samples obtained under various sintering regimes was given.     

Properties of NiFe2O4 ceramics from powders obtained by auto-combustion synthesis with different fuels

Nickel ferrite (NiFe₂O₄) powders derived by auto-combustion synthesis using three different fuels (citric acid, glycine and dl-alanine) have been characterized. The sintering behavior of ceramics using these powders has been compared. Oxygen balance (OB) setting for the chemical reaction is found to regulate the combustion reaction rate. A rapid reaction rate and a high flame temperature are achieved with dl alanine fuel yielding single phase NiFe₂O₄ powder in the as-burnt stage, whereas powders derived with citric acid and glycine fuels show poor crystallinity and necessitate post-annealing. The powder particles are largely agglomerated with a non-uniform distribution in shape and size, and the average particle size is estimated in the range ~ 54–71 nm. Powders derived from dl-alanine fuel show better phase purity, smaller crystallite size, larger surface area and superior sintering behavior. Additional Raman modes discerned for dl-alanine derived powder support a 1:1 ordering of Ni²⁺ and Fe³⁺ at the octahedral sites relating to microscopic tetragonal P4₁22 symmetry expected theoretically for the formation of NiFe₂O₄ with inverse spinel structure. Microstructure of sintered ceramics depends on the precursor powders that are used and sintering at 1200 °C is found to be optimum. Citric acid and glycine derived powders yield high saturation magnetization (M_s~47–49 emu/g), but poor dielectric properties, whereas dl-alanine derived powders yield ceramics with high resistivity (~3.4×10⁸ Ω cm), low dielectric loss (tan δ~0.003 at 1 MHz) and high magnetization (46 emu/g). Dielectric dispersion and impedance analysis show good correlation with the changes in the ceramic microstructure.     

Microstructure and magnetic properties of Ni0·75Zn0·25Fe2O4 ferrite prepared using an electric current-assisted sintering method

Using a Ni_0·75Zn_0·25Fe₂O₄ nanopowder synthesized by means of a hydrothermal method as a raw material, polycrystalline nickel zinc (NiZn) ferrite ceramics composed of sub-micron grains were successfully prepared via an electric current-assisted sintering method. Temperatures ranging from 800 °C to 950 °C and a dwell time of 20 min were employed. The phase composition and microstructure of the samples were characterized via X-ray diffraction and scanning electron microscopy, respectively. Moreover, the magnetic properties of the samples were investigated using a vibrating sample magnetometer and a ferromagnetic resonance system. The results revealed that each sintered sample was mainly composed of a spinel phase. With increasing sintering temperature, the specific saturation magnetization increased from 71.85 emu/g to 74.58 emu/g, owing mainly to the increase in the relative density and the average grain size of the NiZn ferrites. The coercivity and ferromagnetic resonance linewidth of the ferrite ceramics decreased monotonically with increasing sintering temperature, owing mainly to the magnetostriction coefficient, saturation magnetization, and porosity of the sintered ferrites.     

Characterization of submicrometer-sized NiZn ferrite prepared by spark plasma sintering

High-density submicrometer-sized Ni_0.5Zn_0.5Fe₂O₄ ferrite ceramics were prepared by spark plasma sintering in conjunction with sufficient high energy ball milling. They were evaluated by different characterization techniques such as X-ray diffraction, scanning electron microscopy, and dielectric and magnetic measurements. All samples prepared at sintering temperatures ranging from 850 to 925 °C exhibit a single spinel phase and their relative densities and grain sizes range from 90% to 99% and ~100 nm to ~300 nm, respectively. The dielectric constant increases with decreasing grain size until ~250 nm, and then decreases dramatically with further decreasing grain size. The saturation magnetization increases continuously with increasing grain size/density but the magnetic coercivity decreases. The highest dielectric constant and saturation magnetization at room temperature are approximately 1.0×10⁵ and 84.4 emu/g, respectively, while the lowest magnetic coercivity is only around 15 Oe. These outstanding properties may be associated with high density and uniform microstructure created by spark plasma sintering. Therefore, the spark plasma sintering is a promising technique for fabricating high-quality NiZn ferrites with high saturation magnetization and low coercivity.     

Synthesis of MnFe2O4 nanocrystals by wet-milling under atmospheric conditions

This work reports an original method for synthesis of well-crystallized manganese ferrite (MnFe₂O₄) nanoparticles via a high energy wet milling technique under atmospheric conditions, starting from metallic Mn and Fe powders in the presence of distilled water. The effects of milling conditions on the formation and magnetic properties of MnFe₂O₄ nanoparticles were investigated in detail. Fully stoichiometric MnFe₂O₄ nanocrystals with an average crystallite size of 14.5 nm were produced after 24 h of milling. As-synthesized MnFe₂O₄ nanocrystals were found to show soft magnetic behavior at room temperature with saturation magnetization of 53 emu/g. Due to reduced thermal effects, the saturation magnetization increased to 68 emu/g at 5 K. Results show that this method is simple and efficient for the mass production of MnFe₂O₄ nanoparticles.     

Dielectric and magnetic properties of Nickel ferrite ceramics using crystalline powders derived from DL alanine fuel in sol–gel auto-combustion

Dielectric and magnetic properties of NiFe₂O₄ ceramics prepared with powders using DL-alanine fuel in the sol–gel auto combustion technique are studied. DL-alanine fuel yields crystalline as-burnt powders, and when used for ceramic processing yields varying microstructure at different sintering temperatures. The dielectric properties are influenced by the resulting microstructure and the magnetic properties show slight change in saturation magnetization M_s (~44 – 46 emu/g). The coercive fields, dielectric losses and dispersion are reduced considerably at higher sintering temperatures (1200–1300 °C). The influence of changing microstructure is analyzed through dielectric response, complex impedance analysis and electrical modulus spectroscopy in the frequency range (10⁻²–10⁷ Hz) to understand the interactions from the grain and grain boundary phases. Sintering at 1200 °C, is found to be optimum, yields lower losses & reduced dielectric dispersion, and high resistivity (3.4×10⁸ Ω cm).     

Enhancement of magnetic properties of Ni0.5Zn0.5Fe2O4 nanoparticles prepared by the co-precipitation method

Nano crystalline Ni–Zn ferrites of composition Ni_0.5Zn_0.5Fe₂O₄have been prepared by a chemical co-precipitation method. The powdered samples were sintered at a temperature of 800 °C and 900 °C for three hours. X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Fourier Transform Infrared (FTIR) Spectroscopy were used to study their structural and morphological changes. The enhanced magnetic properties were investigated by using a Vibrating Sample Magnetometer (VSM). The saturation magnetization was found to increase from 73.88 to 89.50 emu/g as a function of sintering temperature making this material useful for high frequency applications. Electromagnetic studies showed sustained values of permittivity up to 1 GHz. These results have been explained on the basis of various models and theories.     

Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe₂O₄). MgFe₂O₄ nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts.     

Co-precipitation of a Ni–Zn ferrite precursor powder: Effects of heat treatment conditions and deagglomeration on the structure and magnetic properties

A Ni–Zn ferrite precursor powder was synthesized by co-precipitation upon adding ammonia to an aqueous solution of the precursor iron, nickel, and zinc nitrate salts. The powder was calcined at a range of temperatures (200–1200 °C) and the crystalline phase evolution was assessed by X-ray diffraction coupled with Rietveld refinement. Intermediate phases (NiFe₂O₄ and Fe₂O₃) with increasing crystallinity coexisted in the system up to 1000 °C. The required Ni_0.8Zn_0.2Fe₂O₄ phase could only be attained at 1200 °C. The magnetic properties measured using a vibrating sample magnetometer revealed high magnetization saturation level (～59 emu/gm) above 400 °C. The coercivity showed a steady decrease with increasing heat treatment temperature, leading to a change from a hard to soft magnetic state. The BET specific surface area and the SEM morphology were found to be dependent on calcination temperature, atmosphere (air or N₂) and on the milling procedure.     

Hybrid hematite/calcium ferrite fibers by solution blow spinning: Microstructural,optical and magnetic characterization

Iron oxide and Ca-doped ferrite fibers were successfully prepared via Solution Blow Spinning (SBS). Fibers with several calcium contents (nFe/nCa = 3.0, 5.2, and 20.0 atomic ratio) were prepared after dissolving polyvinylpyrrolidone and Fe and Ca nitrates in an acidic aqueous solution, followed by SBS spinning and calcination at 800 °C. The morphological, structural, optical, vibrational, and magnetic properties of the fibers were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (DRS), Mössbauer spectroscopy, and magnetometry. The Rietveld refinement revealed Ca-doped fibers consisting of a mixture of Fe₂O₃, CaFe₂O₄, and Ca_3.6Fe_14.4O_25.2 crystalline phases. The Mössbauer spectra at 300 K showed a sextet belonging to the hematite phase and two doublets belonging to the calcium-related phases. Measurements at low temperatures showed a transition at 181.6 K attributed to Ca ferrite. The hematite fibers showed a saturation magnetization (Ms ~ 0.75 emu/g) relatively higher than the bulk α-Fe₂O₃ (Ms = 0.3 emu/g). The saturation magnetization increased from 3.6 to 6.4 emu/g with increasing Ca content.     

Spark plasma sintering of combustion-synthesized β-SiAlON powders

In this paper, the sintering behavior of β-Si_6−zAl_zO_zN_8−z (z=1) powder prepared by combustion synthesis (CS) was studied using spark plasma sintering (SPS). The CSed powder was ball milled for various durations from 0.5 to 20 h and was then sintered at different temperatures with heating rates varying from 30 °C/min to 200 °C/min. The effects of ball milling, sintering temperature, and heating rate on sinterability, final microstructure, and mechanical property were investigated. A long period of ball milling reduced the particle size and subsequently accelerated the sintering process. However, the fine powder was easily agglomerated to form secondary particles, which accordingly decreased the densification of the SPS product. The high sintering temperature accelerated the densification process, whereas the high heating rate reduced the grain growth and increased the relative density of the sintered product.     

Enhanced electromagnetic properties of low-temperature sintered NiCuZn ferrites by doping with Bi2O3

NiCuZn ferrite is a material suitable for low-temperature co-fired ceramic (LTCC) technology due to its high permeability and relatively low sintering temperature. The main research questions regarding NiCuZn ferrites are focused on reducing the sintering temperature of the NiCuZn ferrites to achieve compatibility with the Ag electrodes and improve their electromagnetic properties. In this study, the electromagnetic properties of NiCuZn (Ni_0.29Cu_0.14Zn_0.60Fe_1.94O_3.94) ferrites were enhanced by doping with Bi₂O₃, resulting in a reduction of the sintering temperature to 925 °C. The findings show that a suitable concentration of Bi₂O₃ doping could promote the growth of grains and result in NiCuZn ferrites with denser microstructures sintered at a low temperature. Furthermore, adding 0.30 wt% Bi₂O₃ to NiCuZn ferrite enhances its electromagnetic properties, such as a high real part of permeability (～937.6 @ 1 MHz), high saturation magnetization (～60.353 emu/g), low coercivity (～0.265 kA/m), and excellent dielectric constant (～14.71 @ 1 MHz). In addition, the chemically compatible Ag electrodes suggest that the NiCuZn +0.30 wt% Bi₂O₃ ceramics may be acceptable for LTCC technology.     

Influence of the Mole Ratio,Sintering Condition and Particle Size on the Magnetic Properties of BaFe12O19 Synthesized by Ceramic Method

Barium ferrite (BaFe₁₂O₁₉) with different particle sizes were successfully synthesized by solid-state reaction (ceramic method) using locally available low cost α-Fe₂O₃ mixed with BaO₂. The prepared samples were sintered in the temperature range of 950° to 1200°C for 1.5 to 6 h. The prepared products of barium ferrite were characterized by X-ray diffraction, a scanning electron microscope, and a vibrating sample magnetometer. The effect of mole ratio, sintering temperature, and time was studied. A further addition of 10% BaO₂ to the mole ratio (BaO₂:5.7Fe₂O₃), and removing the particle size <63 μm, exhibits a pure phase BaFe₁₂O₁₉. The saturation magnetization reached the optimum value of M_s=∼67 emu/g with coercivity of 2596 Oe, under sintering temperature up to 1100°C for 4.5 h.     

Effects of annealing on the structural and magnetic properties of flame spray pyrolyzed MnFe2O4 nanoparticles

In this study, manganese ferrite (MnFe₂O₄) nanoparticles were produced through flame spray pyrolysis (FSP). To investigate the effects of heat treatment, the nanoparticles were annealed between 400 and 650°C for 4 h in air in a comparative manner. The structural, chemical, morphological, and magnetic properties of the nanoparticles were evaluated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), dynamic light scattering (DLS), and vibrating sample magnetometry (VSM), respectively. The XRD results showed that the nanoparticles synthesized by the FSP method exhibited the MnFe₂O₄ spinel ferrite structure. The annealing process led to the decomposition of MnFe₂O₄ into various phases. According to the morphological analysis, the as-synthesized particles were hemispherical–cubic in shape and had an average particle size of less than 100 nm. In addition, the chemical bond structures of the nanoparticles were confirmed in detail by XPS elemental analysis. The highest saturation magnetization was recorded as 33.50 emu/g for the as-produced nanoparticles. The saturation magnetization of the nanoparticles decreased with increasing annealing temperature, while coercivity increased.     

Magnesium Ferrite (MgFe2O4) Nanostructures Fabricated by Electrospinning

Magnesium ferrite (MgFe₂O₄) nanostructures were successfully fabricated by electrospinning method. X-ray diffraction, FT-IR, scanning electron microscopy, and transmission electron microscopy revealed that calcination of the as-spun MgFe₂O₄/poly(vinyl pyrrolidone) (PVP) composite nanofibers at 500–800 °C in air for 2 h resulted in well-developed spinel MgFe₂O₄ nanostuctures. The crystal structure and morphology of the nanofibers were influenced by the calcination temperature. Crystallite size of the nanoparticles contained in nanofibers increased from 15 ± 4 to 24 ± 3 nm when calcination temperature was increased from 500 to 800 °C. Room temperature magnetization results showed a ferromagnetic behavior of the calcined MgFe₂O₄/PVP composite nanofibers, having their specific saturation magnetization (M _s) values of 17.0, 20.7, 25.7, and 31.1 emu/g at 10 Oe for the samples calcined at 500, 600, 700, and 800 °C, respectively. It is found that the increase in the tendency of M _s is consistent with the enhancement of crystallinity, and the values of M _s for the MgFe₂O₄ samples were observed to increase with increasing crystallite size.     

Structural and magnetic properties of the copper ferrite obtained by reactive milling and heat treatment

Copper ferrite (CuFe₂O₄) was synthesised from an equimolar mixture of copper and iron oxides by mechanosynthesis and subsequent heat treatment. After mechanosynthesis, depending on the milling time, the powder consists in a mixture of phases. The heat treatment at 600 °C did not lead to a complete reaction of the mechano-activated precursors. After the heat treatments at 800 and 1000 °C, the complete formation of copper ferrite for almost all the milling times was noticed. The crystal structure of the copper ferrite was found to be cubic for all the samples heat treated at 1000 °C and a mixture of tetragonal and cubic for the samples heat treated at 800 °C. The amount of copper ferrite with cubic structure predominates in the samples with prolonged milling duration and a decrease of the tetragonal distortion by increasing the milling time occurs. The crystallisation of CuFe₂O₄ in cubic structure for the samples milled for prolonged time is influenced by the powder contamination with iron. The magnetisations of the samples obtained after heat treatment at 1000 °C were found to be larger compared to the ones of the samples heat treated at 800 °C. The iron contamination, milling duration and heat treatment temperature influence the cations distribution, thus leading to the saturation magnetisation of the copper ferrite samples ranging from 11.9 μ_B/f.u. to 16.4 μ_B/f.u.     

Study of strontium ferrites substituted by lanthanum on the structural and magnetic properties

M-type strontium ferrites, Sr_0.8La_0.2Fe₁₂O₁₉ have been synthesized by conventional ceramic process. The effects of lanthanum addition and sintering temperature on microstructures and magnetic properties of SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ samples were investigated. Microstructural analysis of the SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ specimens, sintered at different temperatures revealed that average grain sizes of SrFe₁₂O₁₉ ferrites were larger than that of Sr_0.8La_0.2Fe₁₂O₁₉ ferrite and increased with increasing sintering temperature. The X-ray diffraction (XRD) results confirmed the strontium hexagonal ferrite phase of SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ compounds. A maximum coercivity value of 4850 Oe and maximum saturation magnetization value of 102 emu/g were obtained for the SrFe₁₂O₁₉ ferrite sintered at 1150 °C and for the SrFe₁₂O₁₉ and Sr_0.8La_0.2Fe₁₂O₁₉ ferrites sintered at 1300 °C, respectively. The remanence (Mr) of Sr_0.8La_0.2Fe₁₂O₁₉ sample sintered at 1200 °C possesses the maximum value of 60 emu/g.     

A comparative study of the magnetic and microwave properties of Al3+ and In3+ substituted Mg–Mn ferrites

The effect of substitution of diamagnetic Al³⁺ and In³⁺ ions for partial Fe³⁺ ions in a spinel lattice on the magnetic and microwave properties of magnesium–manganese (Mg–Mn) ferrites has been studied. Three kinds of Mg–Mn based ferrites with compositions of Mg_0.9Mn_0.1Fe₂O₄, Mg_0.9Mn_0.1Al_0.1Fe_1.9O₄, and Mg_0.9Mn_0.1In_0.1Fe_1.9O₄ were prepared by the solid-state reaction route. Each mixture of high-purity starting materials (oxide powders) in stoichiometric amounts was calcined at 1100 °C for 4 h, and the debinded green compacts were sintered at 1350 °C for 4 h. XRD examination confirmed that the sintered ferrite samples had a single-phase cubic spinel structure. The incorporation of Al³⁺ or In³⁺ ions in place of Fe³⁺ ions in Mg–Mn ferrites increased the average particle size, decreased the Curie temperature, and resulted in a broader resonance linewidth as compared to un-substituted Mg–Mn ferrites in the X-band. In this study, the In³⁺ substituted Mg–Mn ferrites exhibited the highest saturation magnetization of 35.7 emu/g, the lowest coercivity of 4.1 Oe, and the highest Q×f value of 1050 GHz at a frequency of 6.5 GHz.