Synthesis and microwave absorption properties of Ni–Zn–Mn spinel ferrites

A series of Ni_0·5?xZn_0·3?xMn_0·2+2xFe₂O₄ ferrites was successfully prepared by the sol–gel autocombustion method. The structure and electromagnetic properties of the powders were characterised by X-ray diffraction, SEM and vector network analysis. The pure powders were formed by heating at 1200°C for 3 h in air, and grain sizes increased as the amount of substitution ranged from x?=?0·0 to x?=?0·25. For samples with x?=?0·1, a minimum reflection loss of ?27·57 dB was observed at 11·0 GHz with the less than ?10 dB absorption bandwidth at 8·0 GHz with 3·8 mm thickness. The results indicate that substitution with Mn and Zn ions can greatly improve the microwave absorption properties of NiFe₂O₄ ferrites.     

Mechanistic study of the effect of Ca–Sn co-doping on the microwave dielectric properties and magnetic properties of YIG

To investigate the crystal structure, electrical properties, and magnetic properties of Ca–Sn co-doped Y_3-xCa_xFe_5-xSn_xO₁₂ (x = 0.00–0.25 in steps of 0.05), solid-state reaction experiments, first principles calculations, and complex crystal bonding theoretical calculations were performed. The relative permittivity (ε_r) is strongly correlated with the average bond ionicity when Ca²⁺ is added. Furthermore, appropriate Sn⁴⁺ substitution significantly lowers the dielectric loss (tanδ_ε) associated with the lattice energy. The right amount of Ca–Sn co-doping can change the saturation magnetization (4πM_S) and improve the microscopic morphology of YIG, lowering the ferromagnetic resonance linewidth (ΔH) of YIG. The optimized microwave dielectric and magnetic properties are as follows: ε_r = 14.7, tanδ_ε = 4.15 × 10^?4, 4πM_S = 1680 G, and ΔH = 53 Oe for Y_2.8Ca_0.2Fe_4.8Sn_0.2O₁₂ sintered for 6 h at 1425 °C. Based on this material, a simple 3D model of a strip-line circulator with an insertion loss of less than 0.3 dB at each port and isolation greater than 20 dB in the 10–12 GHz range was developed, indicating the potential of the material for microwave high-frequency components such as circulators.     

Crystal structure refinement and the magnetic and electro-optical properties of Er3+–Mn2+-substituted Y-type barium hexaferrites

The influence of Er³⁺–Mn²⁺ substitution on the properties of Y-type hexaferrites (chemical composition: Ba_2–xEr_xZn_0.6Co_0.6Cu_0.8Fe_12?yMn_yO₂₂ (x = 0.0, 0.3, and 0.5 and y = 0.0, 0.4, and 0.6)), which were synthesized by the sol-gel autocombustion method, was investigated. The X-ray diffraction spectra were analyzed by the Rietveld refinement method, and hexaferrite was observed to possess a single-phase crystalline structure, whereas the Fourier-transform infrared spectra clarified the formation of the iron oxide base material. The morphology of the grains revealed that they were hexagonal and without agglomeration. The band gap of the samples decreased as the Er³⁺–Mn²⁺ concentration increased. Dielectric and impedance spectroscopies of the prepared samples indicated the role of polarization in the variation in the dielectric and impedance parameters. Particularly, the occurrence of space-charge polarization increased the dielectric constant at lower frequencies. Further, the Cole–Cole plot revealed a semicircle in the lower frequency region, thereby indicating that the grain boundary contributed the most to the dielectric constants. Modulus spectroscopy revealed that the charge mobility increased as the concentration of Er³⁺–Mn²⁺ increased. Additionally, the magnetic analysis indicated that Mn²⁺ preferably replaces Fe³⁺ at the octahedral site, thereby reducing the magnetization of the prepared samples through a reduced superexchange interaction. Furthermore, increasing the coercivity values thermally stabilized the sample, and this is vital for perpendicular magnetic recording.     

Structural and magnetic properties of erbium doped nanocrystalline Li–Ni ferrites

A series of nanocrystalline Li_0.25Ni_0.5Fe_2.25−xEr_xO₄ (x=0.00, 0.02, 0.06, 0.08, and 0.10) ferrite powders, having a cubic spinel crystal structure and a low value of coercivity, was synthesized by the sol–gel auto-combustion route. The structure, morphology and magnetic properties of the prepared nanoferrites were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the magnetic property measurement system (MPMS). A well-defined single phase spinel structure is confirmed in all the samples by X-ray diffraction analysis. The lattice parameters of the samples increase slightly with increasing the erbium content. The crystallite size of the Er-doped samples is smaller than that of pure Li–Ni ferrite, and decrease regularly in the range of 36.0–14.5 nm. It has been observed that the magnetic properties of these ferrites are strongly influenced by the added erbium content. The magnetic measurements indicate that saturation magnetization (M_s) and coercivity (H_c) decrease gradually with the increase of Er content in the lattice.     

Comparative study of microwave absorption properties of Ni–Zn ferrites obtained from different synthesis technologies

Spinel ferrites are widely used for electromagnetic wave (EMW) absorption applications. In this study, spinel Ni–Zn ferrites with excellent microwave absorption properties were synthesized. Their EMW absorption characteristics and interaction mechanisms were studied to lay the foundation for the study of the role of Ni–Zn ferrite as a magnetic substrate for composites. Herein, Ni_0·5Zn_0·5Fe₂O₄ was prepared by the hydrothermal method (H-NZFO) and the sol–gel auto-combustion method (S-NZFO); both samples exhibited distinct microwave absorption properties. The S-NZFO absorber (thickness = 3.72 mm) demonstrated the best dual-zone microwave absorption with two strong reflection loss peaks at 5.1 and 10.5 GHz. The corresponding effective absorption bandwidth (EAB) reached 9.0 GHz, which covered part of the S-band and all of the C- and X-bands. These results were attributed to the high saturation magnetization, outstanding complex permeability, and multiple magnetic loss channels of S-NZFO. The H-NZFO sample exhibited excellent absorption capability and matching thickness. At a thickness as low as 1.71 mm, the minimum reflection loss (RL_min) of the H-NZFO absorber reached -60.2 dB at 13.1 GHz. The maximum bandwidth corresponding to RL below -10 dB was 4.6 GHz. These results can be attributed to small particle size, high complex permittivity, and multiple dielectric loss channels of H-NZFO. The observed wide effective absorption bandwidth of S-NZFO and strong microwave absorption capability of H-NZFO suggest the potential of both materials as substrates for efficient microwave absorbers in military as well as civilian absorption applications.     

Structural,dielectric and magnetic properties of Bi–Mn doped SmFeO3

We have studied the structural, magnetic, dielectric and impedance properties of the Sm_1-xBi_xFe_1-yMn_yO₃ [SmFeO₃ (SFO), Sm_0.9Bi_0.1FeO₃ (SBFO), Sm_0.9Bi_0.1Fe_0.9Mn_0.1O₃ (SBFMO)] polycrystalline samples synthesized by solid-state reaction method. Rietveld refinement of room temperature (RT) powder x-ray diffraction pattern confirms the orthorhombic crystal structure with Pnma/Pbnm space group. The average particle size of Bi doped and co-doped (Bi–Mn) samples determined from SEM analysis are 5.6 μm and 5.2 μm, respectively. Room temperature field-dependent magnetization increases, suggesting the presence of magnetic contribution due to the Rare earth-Fe ion interaction which persists even at RT. However, with co-doping of Bi and Mn, a decrease in magnetization is observed, which corresponds to the dilution of Fe³⁺-Fe³⁺ interactions due to the presence of Mn³⁺ ions. The observed values of magnetization at 90 kOe for Bi doped sample is (2.87 emu/g) approximately two times and for codoped (0.7 emu/g) sample is nearly half of that of pristine sample (1.51 emu/g). Dielectric measurements as a function of frequency/temperature and impedance analysis using equivalent circuit model reveal grain and grain boundary contributions of SBFO (at high temperature) and SBFMO (for all temperature) samples towards the electrical properties indicating the electrically heterogeneous nature of these samples. However, for SFO sample grain contribution is dominant. Observed value of dielectric constant varies from ~10³-10⁴ with Bi–Mn doping. The conduction mechanism of the studied samples has been explained by considering Jonscher power law. Arrhenius law fitting of AC conductivity data manifests two types of conduction mechanisms in these samples. The depressing nature of the semicircular arc observed in the Nyquist plot of all the samples indicates the presence of a non-Debye type of relaxation.     

A study of the structural and morphological properties of Ni–ferrite,Zn–ferrite and Ni–Zn–ferrites functionalized with starch

Zinc–ferrite, nickel–ferrite and mixed nickel–zinc ferrites were successfully synthesized via the thermal decomposition method from acetylacetonate complexes. To control the particle size and enhance dispersibility in an aqueous medium, starch, a natural and biocompatible compound, was used for the first time for coating such magnetic powders. X-ray powder diffraction (XRPD) was performed to study the structural properties of all samples. The presence of a single-phase spinel structure as well as the cation distribution in both sites of all investigated magnetic powders was confirmed. The values of unit cell parameters obtained from the results of the Rietveld analysis decreased, while the average crystallite size increased with increasing Ni²⁺ content. The average microstrain parameters unambiguously showed a change in the spinel structure with cation distribution. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) analyses were also utilized to characterize the synthesized materials, corroborating the XRPD data. The obtained results indicated that functionalization by starch was successfully achieved.     

Physico-chemical and catalytic properties of Mg–Al hydrotalcite and Mg–Al mixed oxide supported copper catalysts

The Mg–Al hydrotalcite (HT) and Mg–Al mixed oxide supported copper catalysts containing 3–3.5 wt.% copper in finely dispersed form were synthesized and characterized. The effect of support nature on physico-chemical and catalytic properties of supported copper species were studied. The loading of copper on the supports was observed to be influencing the surface acidic, basic and reducibility properties, and catalytic behavior in dehydrogenation of benzyl alcohol. The high basicity and intercalated copper ions in Mg–Al hydrotalcite supported copper sample showed multifunctional activity in catalytic transformations of alcohols (primary, secondary and aromatic alcohols).     

Structural,morphological, magnetic hysteresis and dielectric properties of cobalt substituted barium–lead hexagonal ferrites for technological applications

M-type, Ba_0.4Pb_0.6Fe_12-xCo_xO₁₉ (x = 0.00, 0.10, 0.20, 0.30, 0.40) hexaferrites, synthesized using citrate gel auto combustion method, and heated at 950 °C, 4 h for lossless applications. XRD analysis shows the development of the M-phase, along with PbM and hematite. The microstructural analysis reveals the stacking clusters of hexagonally shaped platelets. TEM image and SAED pattern of x = 0.3 composition shows polycrystalline nature and formed particles observed to fused with neighbouring particles. M − H loops of all samples reveal hard magnetic behaviour and possess multi-domain structure. The maximum saturation magnetization of 55.427 A m²/kg is observed in x = 0.10 composition and coercivity of prepared hexaferrites was found to vary from 0.058 T to 0.390 T. The cobalt substitution has a strong influence on the dielectric properties of prepared hexaferrites. The value of ac conductivity increases with cobalt substitution from x = 0.00 to x = 0.10, and followed by a reduction from x = 0.10 to x = 0.40. The same trend is observed for the dielectric constant. The low value of loss tangent for all compositions shows apt scope for lossless application.     

Effect of Mn/Co substitutions on the resistivity and dielectric properties of nickel–zinc ferrites

Nickel-zinc ferrite nanoparticles with suitable chemical modifications by Mn or Co substitutions for Zn as two systems (namely, Ni–Zn–Mn and Ni–Zn–Co) were synthesized by sol-gel autocombustion method. X-ray diffraction measurements on the synthesized nanoparticles confirm that the samples attain single phase cubic spinel structures only. The powders were then used to obtain pellets in desired dimensions by employing usual ceramic procedure for carrying out measurements of resistivity and dielectric properties. The variations of dc resistivity as a function of composition and temperature, and the corresponding variation of activation energies for both the systems are presented and discussed. Also, the results of dielectric constant as a function of substituent concentration, dielectric dispersion and dielectric loss tangent are discussed. Effect of Mn/Co substitutions in Ni–Zn ferrites and possible mechanisms responsible for variations in resistivity and dielectric properties of both the ferrite systems have been evolved independently. Also, comparison of the trends between the dielectric constant and the resistivity with substituents’ concentration and their inter-relation with conduction mechanisms has been thoroughly analyzed for both the ferrite systems.     

Optimization of the magnetic properties and microstructure of Co–La substituted strontium hexaferrite by varying the production parameters

The aim of this paper was to improve the magnetic properties of magnetoplumbite-type (M-type) strontium hexaferrite substituted with Co²⁺–La³⁺ produced by conventional ceramic forming techniques. The effect on the magnetic properties of varying the composition of the target compound Sr_1−xLa_xFe_12−yCo_yO₁₉ and the primary and secondary firing temperatures was investigated. Microstructure studies and XRD phase analysis indicated that optimum values of the remanent magnetization B_r and coercive field H_cj were obtained with a primary firing temperature of 1240 °C and a final firing temperature of 1180 °C, where (x=y)_th ¹=0.15, (y/x)_exp ²=0.75 and the molar ratio of ferric oxide to strontium oxide=5.8. The optimized magnetic properties obtained under these conditions were B_r=4070 Gs, H_cj=4710 Oe, (H_k/H_cj)=82.     

Structural,optical, electrical,dielectric, molecular vibrational and magnetic properties of La3+ doped Mg–Cd–Cu ferrites prepared by Co-precipitation technique

Ferrites are among the most frequently investigated materials mainly due to interesting and practically different properties. Therefore, easily and cost-effective lanthanum doped Mg_0.5Cd_0.25Cu_0.25Fe_2-xLa_xO₄ (x = 0.0, 0.0125, 0.025, 0.0375 and 0.05) ferrites were synthesized by a co-precipitation route, a comprehensive characterisation of their structural, optical, electric, dielectric, molecular vibrational, and magnetic properties were carried out. X-ray diffraction analysis confirmed the formation of a cubic spinel structure. Variations in frequency bands were also observed with amplification in optical band gap energy (2.95 – 3.38 eV) due to La³⁺ ions insertion. The electric resistivity had opposite trends at low and high temperatures with increasing La³⁺ content. The Curie temperature, activation energy, and drift mobility were also determined to have values consistent with the semiconducting behavior of the soft ferrites. The saturation magnetization (M_S) has a maximum value 49.385 emu/g with remanent magnetization (M_r) was 34.928 emu/g and coercivity 661.4 Oe for La³⁺ concentration x = 0.05. The minimum dielectric loss was observed for La³⁺ concentration x = 0.025. Moreover, the resistivity (ρ) has a maximum value of 7.95 × 10⁴ Ω cm for La³⁺ concentration x = 0.025. The calculated frequency range of La³⁺ doped Mg–Cd–Cu ferrites was detected in the microwave range (3.36 – 10.80 GHz), suggesting the potential application of the materials in longitudinal recording media and microwave absorbance.     

Synthesis and magnetic properties of La3+-Co2+ substituted strontium ferrite particles using modified spray pyrolysis–calcination method

The purpose of the research was to improve the intrinsic magnetic properties of strontium ferrite by substituting lanthanum and cobalt for strontium and iron. The salt-assisted ultrasonic spray pyrolysis (SA-USP) following calcination process were used to from La-Co substituted strontium ferrite particles (La_xSr_1-xFe_12-yCo_yO₁₉), and their compositional dependent magnetic properties systemically investigated. All the samples were calcined at 1050 °C for 1 h in an air atmosphere to yield single-phased hexagonal particles several hundred nanometers to microns in size. A saturation magnetization of 70.76 emu/g and a coercivity 7265 Oe were obtained at a composition of La_0.25Sr_0.75Fe_11.75Co_0.25O₁₉. The amount of Co was reduced to obtain an optimized saturation magnetization of 71.40 emu/g and a coercivity of 7572 Oe at a composition of La_0.25Sr_0.75Fe_11.8Co_0.2O₁₉.     

Investigation of structural,magnetic and Mössbauer properties of Co2+ and Cu2+ substituted Ni–Zn nanoferrites

We investigated the effects of Co²⁺ and Cu²⁺ substitution on the super-exchange interactions in Ni–Zn nanoferrites. The cation distribution technique was taken into account to explain the results. To authenticate the cation distribution, we have estimated the cation distribution in the light of X-ray diffraction method, Mössbauer spectroscopic analysis, and magnetization study. Statistical model based on the cation distribution was used to calculate the Curie temperature. The values of magneton number n_B and Curie temperature T_C calculated by using the cation distribution is found to be in agreement with the experimentally obtained values.     

Structural,morphological and photocatalytic properties of Bi–Mg–Cu ferrites: Influence of the Bi: Fe ratio

Rhodamine B (RhB) dye is an exceedingly dangerous pollutant. So, this study described an effective synthesis of pure and Bi³⁺ doped Mg–Cu nanophotocatalysts (MCBF) to facilitate the disposal of this harmful pollutant. We studied their structural, morphological, optical, and photocatalytic degradation characteristics. The XRD results of all the prepared photocatalysts confirmed their spinel structure without any impurities. Three justifications for why the lattice parameters of these nanoferrites have a peculiar behavior. The crystallite size has unsteadied behavior; within the range of 29.75–44.87 nm, as determined by the Williamson-Hall approach. The lattice parameter has a distinctive behavior; decrement from 8.3168 Å for MCBF0 to 8.3037 Å for MCBF2 and increment for the nanoferrites MCBF3 (8.3448 Å) and MCBF4 (8.3852 Å) and decreased again for the nanoferrite MCBF5 (8.3448 Å). The STEM graphs demonstrated the agglomeration of nanosized, spherical, and homogeneous-shaped particles. The optical band energy of all the MCBF nanoferrites was in the visible light range of 2.009–2.031 eV. The nanoferrites MCBF0 and MCBF4 have the highest energy gap (2.031 eV), whereas the nanoferrite MCBF5 has the lowest (2.009 eV). Although the modification of the energy gap of MCBF nanoferrites by Bi doping was not large, the photocatalytic activity of MCBF nanophotocatalysts was improved by substituting bismuth ions. In particular, MCBF5, with the highest bismuth content, gave the supreme RhB degradation of 95.81%, with a higher ratio of 245.39% than the pure Mg–Cu ferrite sample. The MCBF5 catalyst showed excellent stability under visible light irradiation across all cycles, 95.81, 95.64, 95.28, 95.04, and 94.89%. Therefore, Mg_0·5Cu_0·5Bi_0·1Fe_1·9O₄ nanoferrite is a suitable option for RhB degradation under sunlight and may function as a likely catalyst in wastewater purification.     

The influence of Nd substitution in Ni–Zn ferrites for the improved microwave absorption properties

Nanocrystalline Ni–Zn ferrites with different neodymium contents (Ni_0.5Zn_0.5Nd_xFe_2-xO₄) were synthesized by sol-gel route combined with self-propagating combustion (SPC) method. The presence of surface functional groups, crystal structure and morphology of the samples were studied by FT-IR, XRD and SEM. The results show that the prepared samples are composed of spinel phase under the condition of low neodymium content, like the pure Ni–Zn ferrite. While neodymium oxide appears after the content of Nd³⁺ exceeds a certain limit (x > 0.04) and there exist two phases in the ferrite. The results of vibrating sample magnetometer (VSM) and vector network analyzer (VNA) show that adjusting the content of Nd is significant in improving the dielectric properties and microwave absorption capacity of the materials, specifically at low frequencies. When x < 0.04, the enhancement of dielectric loss ability of spinel ferrites by doping Nd³⁺ is the dominant factor affecting microwave absorption ability of samples. The secondary phase Nd₂O₃ hardly appears under this condition, thus the weakening effect of Nd³⁺ addition on magnetic loss ability is not obvious. When x = 0.04, the optimal absorption peak of the material reaches −20.8 dB at 4.4 GHz with a thickness of 8.5 mm and the effective absorption bandwidth (R_L < −10 dB) was 3.2 GHz. On the contrary, when x > 0.04, the magnetic loss ability decreases rapidly (e.g., Ms decreases from 82.47 emu/g to 59.77 emu/g). Meanwhile, the dielectric loss increases slowly and the microwave absorption capacity decreases.     

Effect of Al2O3 particles on mechanical and tribological properties of Al–Mg dual-matrix nanocomposites

This article aims to manufacture homogenous dual-matrix Al–Mg/Al₂O₃ nanocomposite from their raw materials and give insight into the correlation between powder morphology, crystallite structure and their mechanical and tribological properties. Al–Mg dual-matrix reinforced with micro/nano Al₂O₃ particles was manufactured by a novel double high-energy ball milling process followed by a cold consolidation and sintering. Microstructure and phase composition of the prepared samples were characterized using FE-SEM, EDS and XRD inspections. Mechanical and wear properties were characterized using compression and sliding wear tests. The results showed that a milling of Mg with Al₂O₃ particles in an initial step before mixing with Al has the beneficial of well dispersion of Al₂O₃ nanoparticles in Al–Mg dual matrix. The Al–Mg dual matrix reinforced with nano-size Al₂O₃ showed 3.29-times smaller crystallite size than pure Al. Moreover, the hardness and compressive strength are enhanced by adding nano-size Al₂O₃ with Al–Mg dual matrix composite while the ductility is maintained relatively high. Additionally, the wear rate of this composite was reduced by a factor of 2.7 compared to pure Al. The reduced crystallite size, the dispersion of Al₂O₃ nanoparticles and the formation of (Al–Mg)_ss were the main improvement factors for mechanical and wear properties.     

Structure and microwave dielectric properties of La(Mg0.5Ti0.5)O3–CaTiO3 system

(1−x)La(Mg_0.5Ti_0.5)O₃ (LMT)–xCaTiO₃ (CT) [0<x<1] ceramics were prepared from powder obtained by a nonconventional chemical route based on the Pechini method. The crystal structure of the microwave dielectric ceramics has been refined by Rietveld method using X-ray powder diffraction data. LMT and CT were found to form a solid solution over the whole compositional range. The 0.9LMT–0.1CT composition was refined using P2₁/n space group, which allows taking into account B-site ordering. The compounds having x⩾0.3 were found to be disordered and were refined using Pbnm space group. Microstructure evolution was also analysed. Dielectric characterization at microwave frequencies was performed on the LMT–CT ceramics. The permittivity and the temperature coefficient of resonant frequency of the solid solutions showed a non-linear variation with composition. The quality factor demonstrates a considerable decrease with the increase of CT content.     

Effect of Ga3+ substitution on the microwave dielectric properties of 0.67CaTiO3–0.33LaAlO3 ceramics

0.67CaTiO₃–0.33La(Al_1−xGa_x)O₃ (0≤x≤0.4) (CTLAG) ceramics with pure perovskite structure were prepared by a conventional two-step solid-state reaction process. The effect of Ga³⁺ substitution for Al³⁺ on the microwave dielectric properties of the ceramics was subsequently investigated. As Ga content increased, the ionic polarizability increased and led to an increase of the dielectric constant (ε_r). Meanwhile, both the tolerance factor (t) of CTLAG ceramics and A-site bond valence were considered to have effect on the temperature coefficient of the resonant frequency (τ_f) with the increase of Ga content. Results also showed that the quality factor (Q×f ) varied with increasing Ga³⁺ content because of not only intrinsic factor but also extrinsic factors such as the bimodal grain size distribution, the variation of relative density, and the packing fraction. Excellent microwave dielectric properties with ε_r≈45.81, Q×f≈34,152 GHz, and τ_f≈3.09 ppm/°C were achieved for 0.67CaTiO₃–0.33La(Al_0.9Ga_0.1)O₃ ceramics sintered at 1420 °C for 4 h.