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.     

Zn‐Cr Co‐Substitution Effect on Structural and Electromagnetic Properties of CuFe2O4 via Oxalate Decomposition Route

Zn‐Cr‐co‐substituted Cu_1‐xZn_xFe_2‐2xCr_2xO₄ ferrites (where x = 0.0–0.5) were prepared via thermal decomposition of oxalate precursors. The thermal decomposition up to ferrites formation was followed by differential thermal analysis–thermogravimetry measurements. Mössbauer technique was used to predict the possible cation distribution of the entire system, and X‐ray diffraction, Fourier transform infrared, and electromagnetic measurements were used for confirmation. The superparamagnetic characteristics estimated via Mössbauer studies, for samples with higher substitution, agreed well with vibrating sample magnetometer, magnetic susceptibility, and conductivity results. All the samples showed semiconducting properties in which conductivity decreases by increasing substitution. The effect of cationic substitution on the entire system was investigated and discussed.     

The Cobalt Zinc Spinel Ferrite Nanofiber: Lightweight and Efficient Microwave Absorber

To solve the heavy mass problem of the traditional spinel ferrite using as the microwave absorber, the Co_xZn_(1?x)Fe₂O₄ (x = 0.2, 0.4, 0.6, 0.8) ferrite nanofibres were synthesized by electrospinning method. The phase composition, morphology, and electromagnetic properties were analyzed. The results showed that all the as‐prepared Co_xZn_(1?x)Fe₂O₄ ferrites exhibited the homogeneous nanofibrous shape. The saturation magnetization and coercivity were enhanced by tuning the Co²⁺ content. The electromagnetic loss analysis indicated that the Co_0.6Zn_0.4Fe₂O₄ ferrite nanofiber performed the strongest microwave attenuation ability. The microwave absorbing coating containing 15 wt% of Co_0.6Zn_0.4Fe₂O₄ ferrite nanofiber showed the reflection loss less than ?10 dB in the whole X‐band and 80% of the Ku‐band frequencies. Meanwhile, the surface density was only 2.4 Kg/m².     

Glass Additive Influence on the Sintering Behaviors,Magnetic and Electric Properties of Bi–Zn Co‐Doped Co2Y Ferrites

The Bi₂O₃–B₂O₃–ZnO–SiO₂ (BB35SZ) glass effects on the sintering behavior and magnetic properties of Bi–Zn co‐doped Co₂Y ferrites were investigated in developing low‐temperature‐fired ferrites. The results indicate that BB35SZ glass can be used as a sintering aid to reduce the densification temperature of Co₂Y ferrites from 1300°C to 900°C. The 2(Ba_0.9Bi_0.1O)·2(Zn_0.4Co_0.4Cu_0.2O)·6(Fe_1.97Zn_0.03O₃) ferrite with 4 wt% BB35SZ glass can be densified below 900°C, exhibiting an initial permeability of 3.4 and quality factor of 55. This process provides a promising candidate for multilayer chip magnetic devices for microwave applications.     

The structure evolution and microwave dielectric properties of MgAl2-x(Mg0·5Ti0.5)xO4 solid solutions

In the present work, MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ (x = 0.02, 0.04, 0.06, 0.08, 0.10) solid solutions were synthesized via the traditional solid-state reaction route. The valence state of Ti ions, crystal structural characteristics, and microwave dielectric properties were discussed. A solid solution with spinel structure was revealed by the Rietveld refinement results. The partial substitution of (Mg_0·5Ti_0.5)³⁺ for Al³⁺ lowered the sintering temperature and improved the Q × f value of MgAl₂O₄ ceramic. The MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ solid solutions with x = 0.06 can be well sintered at 1425 °C in an oxygen atmosphere for 8 h and exhibits excellent microwave dielectric properties with ε_r = 9.1, Q × f = 98,000 GHz, τ_f = −61.36 ppm/°C. The sintering temperature of MgAl_1·94(Mg_0·5Ti_0.5)_0.06O₄ microwave dielectric ceramics was approximately 200 °C lower than that of conventional MgAl₂O₄ ceramics.     

Processability,hardness, and magnetic properties of rubber ferrite composites containing manganese zinc ferrites

Abstract

Rubber ferrite composites have the unique advantage of mouldability, which is not easily obtainable using ceramic magnetic materials. The incorporation of mixed ferrites in appropriate weight ratios into the rubber matrix not only modifies the dielectric properties of the composite but also imparts magnetic properties to it. Mixed ferrites belonging to the series of Mn_{(1 -x)}Zn_x Fe₂ O₄ have been synthesised with different values of x in steps of 0·2, using conventional ceramic processing techniques. Rubber ferrite composites were prepared by the incorporation of these pre-characterised polycrystalline Mn_{(1 -x)}Zn_xFe₂ O₄ ceramics into a natural rubber matrix at different loadings according to a specific recipe. The processability of these elastomers was determined by investigating their cure characteristics. The magnetic properties of the ceramic fillers as well as of the rubber ferrite composites were evaluated and the results were correlated. Studies of the magnetic properties of these rubber ferrite composites indicate that the magnetisation increases with loading of the filler without changing the coercive field. The hardness of these composites shows a steady increase with the loading of the magnetic fillers. The evaluation of hardness and magnetic characteristics indicates that composites with optimum magnetisation and almost minimum stiffness can be achieved with a maximum loading of 120 phr of the filler at x=0 4. From the data on the magnetisation of the composites, a simple relationship connecting the magnetisation of the rubber ferrite composite and the filler was formulated. This can be used to synthesise rubber ferrite composites with predetermined magnetic properties.     

Crystal structure and luminescence mechanism of novel Fe3+-doped Mg0.752Al2.165O4 deep red-emitting phosphors

Nonstoichiometric alumina-rich spinel provides diverse and changeable local environments for transition-metal dopants. In this contribution, novel Mg_0.752Al_2.165−xO₄:xFe³⁺ deep red-emitting phosphors were designed and prepared by the solid-state reaction method. The red emission presents an unexpected shift from 735 to 770 nm by comparing with Fe³⁺-doped MgAl₂O₄. The excitation spectrum of Mg_0.752Al_2.165−xO₄:xFe³⁺ is broadened in the UV region with a new strong peak at 320 nm. The crystal structure refinement and NMR spectra fitting reveal that the cation vacancies and disorder increase with excess Al³⁺ entering the spinel crystal lattice. According to the results of EPR, NMR, and PL/PLE measurements, it was proposed that the Fe³⁺ ions locate at the distorted octahedral coordination. The changes of the local structure of Fe³⁺ ions promote the doublet state's involvement in the d−d transition. It was proposed that the new excitation peak at 320 nm in Mg_0.752Al_2.165−xO₄:xFe³⁺ is associated with the transitions from the ground state ⁶A_1g(⁶S) to the ⁴A_2g(⁴F)/T_1g(⁴P) and doublet states. The transition between the lower energy excited state of ²T_2g(²I) and ⁶A_1g(⁶S) mainly contributes to the deep red emission and the red-shifting effect.     

Fabrication and characterization of IR-transparent Fe2+ doped MgAl2O4 ceramics

Initial investigations on the preparation of highly transparent Fe²⁺:MgAl₂O₄ ceramics using nanopowders synthesized in a laser plume were carried out. For the first time, dense Fe²⁺:MgAl₂O₄ ceramics with high transmission in the mid-IR range were fabricated at a temperature as low as 1300°C and with a short sintering time (1 hour). The obtained Fe²⁺:MgAl₂O₄ ceramics contain a secondary (MgO)_0.91(FeO)_0.09 phase with a low wt% content, causing a substantial decrease in transmittance in the visible range. The transmittance increases with an increase in wavelength due to a decrease in Rayleigh scattering and reaches 85.6% at λ = 4 μm, which is close to the theoretical value. The absorption cross section of divalent iron ions was estimated to be σ = (1.66 ± 0.14) × 10⁻²⁰ cm².     

High‐Performance Small‐Amount Fe2O3‐Doped (K,Na)NbO3‐Based Lead‐Free Piezoceramics with Irregular Phase Evolution

[(K_0.43Na_0.57)_0.94Li_0.06][(Nb_0.94Sb_0.06)_0.95Ta_0.05]O₃ + x mol% Fe₂O₃ (KNLNST + x Fe, x = 0~0.60) lead‐free piezoelectric ceramics were prepared by conventional solid‐state reaction processing. The effects of small‐amount Fe₂O₃ doping on the microstructure and electrical properties of the KNLNST ceramics were systematically investigated. With increasing Fe³⁺ content, the orthorhombic‐tetragonal polymorphic phase transition temperature (T_O‐T) of KNLNST + x Fe ceramics presented an obvious “V” type variation trend, and T_O‐T was successfully shifted to near room temperature without changing T_C (T_C = 315°C) via doping Fe₂O₃ around 0.25 mol%. Electrical properties were significantly enhanced due to the coexistence of both orthorhombic and tetragonal ferroelectric phases at room temperature. The ceramics doped with 0.20 mol% Fe₂O₃ possessed optimal piezoelectric and dielectric properties of d₃₃ = 306 pC/N, k_p = 47.0%, = 1483 and tan δ = 0.023. It was revealed that the strong internal stress in the KNLNST + x Fe ceramics with higher Fe³⁺ contents (x = 0.40, 0.60) stabilized the orthorhombic phase, leading to the irregular “V” type rather than the usually observed monotonic phase transition with composition change in the ceramics.     

Structural and Magnetic Property of Co1‐xNixFe2O4 Nanoparticles Synthesized by Hydrothermal Method

The cobalt nickel ferrite (Co_1‐xNi_xFe₂O₄ x = 0–1.0) nanoparticles were synthesized by a hydrothermal method. Effects of nickel content and organic template on the microstructure and magnetic property of the nanoparticles were studied. The experimental results indicate that Ni²⁺ substitution for Co²⁺ and special synthesis technique leads to obvious change in microstructure and magnetic property of the ferrites. The ferrites show nonlinear variations in the saturation magnetization and the coercivity with nickel substitution, which are explained by shape anisotropy and supernormal cation distribution. The organic template also leads to variation in the microstructure and properties of the nanoparticles.     

Reversible Reaction with CO2 and Control of CO2 Absorption/Desorption Properties of Ba2(Fe1−xInx)2O5 Solid Solutions

Ba₂(Fe_1?xIn_x)₂O₅ was prepared by a solid‐state reaction under a N₂ flow. It was revealed that the solid solutions had a cubic perovskite structure with disordered oxygen vacancies at room temperature. Thermogravimetry and X‐ray diffraction measurements revealed that Ba₂(Fe_1?xIn_x)₂O₅ can reversibly react with CO_2. It was found that the equilibrium temperature of the reaction could be controlled by preparing solid solution.     

Effect of niobium valence on the mechanochemical activation of niobium oxides–hematite magnetic ceramic nanoparticles

Three types of nanostructured systems: xNbO·(1?x)α-Fe₂O₃, xNbO₂·(1?x)α-Fe₂O₃, and xNb₂O₅·(1?x)α-Fe₂O₃ were synthesized by ball milling at different molar concentrations (x=0.1, 0.3, 0.5, and 0.7). The effect of Nb valence and milling time on mechanochemical activation of these systems were studied by X-ray diffraction and the Mössbauer spectroscopy measurements. In general, Nb-substituted hematite was obtained at lower molar concentrations for all Nb oxides. For the NbO–Fe₂O₃ system the favorable substitution of Fe²⁺ for Nb²⁺ in the octahedral sites in the NbO lattice was observed after 12 h milling for x=0.7. In the case of the NbO₂–Fe₂O₃ and Nb₂O₅–Fe₂O₃ systems a formation of orthorhombic FeNbO₄ compound was observed, in which Fe³⁺ cations were detected. For the highest concentration of NbO₂ (x=0.7) iron was completely incorporated into the FeNbO₄ phase after 12 h milling. The molar concentrations of x=0.3 and 0.5 were the most favorable for the formation of ternary FeNbO₄ compound in the Nb₂O₅–Fe₂O₃ system. Influence of ball milling on thermal behavior of the powders was investigated by simultaneous DSC–TG measurements up to 800 °C.     

Magnetic properties of nanocrystalline nickel zinc ferrites prepared by combustion synthesis

A series of nanocrystalline Ni_{0.6 − x} Zn _x Cu_0.4Fe₂O₄ ferrites (x = 0, 0.1, 0.2, 0.3, 0.4) was prepared by autocombustion route. Formation of the spinel phase (without any impurity phase) was confirmed by X∂ray diffraction. The lattice parameter of synthesized ferrites was found to linearly increase with increasing x. The octahedral and tetrahedral vibration modes were studied by Fourier transform IR spectroscopy. SEM images revealed compact agglomeration exhibiting grain growth with an increase in x. The dependence of saturation magnetization on the mol fraction of zinc was studied.     

Structural changes and surface properties of Cox Fe3—x O4 spinels

The cobalt ferrite spinel oxide series, Co_xFe_3–xO₄ (0 ≤ x ≤ 3), has been prepared by coprecipitation. The adsorption–desorption isotherms of all the compositions calcined between 200 and 600°C have been measured using nitrogen gas at ?196°C. The structural and the phase changes were characterized by TGA and XRD techniques. The results obtained revealed that the transformation of γ- to α-Fe₂O₃ was accompanied by a sharp decrease in the S_BET values. The addition of Co²⁺ ions into Fe₂O₃ up to × = 0.6 led to an observable increase in the S_BET value. This behaviour was attributed to the incorporation of Co²⁺ ions into the Fe₂O₃ lattice and the retardation of the phase transition of γ- to α-Fe₂O₃. The minimum S_BET values obtained at a lattice composition of × = 1·0 corresponded to the formation of a cobalt ferrite normal spinel which is associated with the existence of narrow pores. The increase in S_BET values in the cobalt-rich region, with a maximum at x = 2·6 is explained on the basis of the cationic replacement of Fe³⁺ ions in the Co₃O₄ lattice. Finally, calculation of pore volume distribution was carried out, in addition to V_a–t plots, in order to study the nature of the surface porosity, which was found to be mesoporous.     

Electronic structure and magnetic properties of Al‐doped Ca2Fe2O5 brownmillerite compounds

Polycrystalline Ca₂Fe_2?xAl_xO₅ (x = 0‐1.4) samples were prepared by conventional solid‐state reactions. Their crystalline/electronic structures and magnetic properties were characterized in detail. Powder X‐ray diffraction analyses revealed that the samples crystallized in orthorhombic brownmillerite‐type structures with the occurrence of the Pcmn‐Ibm2 phase separation in the region between x = 0.4 and 0.6. The results obtained from analyzing Raman scattering and X‐ray‐absorption fine‐structure spectra also indicated this phase separation. Although x in Ca₂Fe_2?xAl_xO₅ varies in a wide range from 0 to 1.4, the +3 oxidation state of Fe remained almost unchanged. Magnetization measurements revealed that all Ca₂Fe_2?xAl_xO₅ samples have weak ferromagnetic order, and both the saturation magnetization and coercive force are dependent on the temperature, x, and structure phases.     

Microstructure,magnetic, and power loss characteristics of low-sintered NiCuZn ferrites with La2O3-Bi2O3 additives

Low-temperature-sintered Ni_0.5Cu_0.125Zn_0.375Fe_1.98O₄ ferrites co-added with x wt% (x = 0.00-0.25 wt%) La₂O₃ and 0.25 wt% Bi₂O₃ were successfully prepared via conventional solid-phase reaction method. The phase composition, microstructure, magnetic properties, and especially power loss variation of the samples were systematically studied. The results showed that all samples possessed a single spinel phase structure at a sintering temperature of 900°C, exhibiting high degree of densification and uniform grains. The appropriate amount of La₂O₃ additive improved the saturation flux density and permeability of NiCuZn ferrites, simultaneously reducing the coercivity and power loss. The maximum permeability and the lowest power loss were achieved at x = 0.15 wt%. The corresponding sample had the homogeneous microstructure and excellent magnetic properties, being a promising low-temperature co-fired ferrite candidate for magnetic power components.     

Structure and conductivity of yttria and scandia‐doped zirconia crystals grown by skull melting

In this paper a detailed study of the (ZrO₂)_1‐x(Y₂O₃)_x (x=0.025–0.15), (ZrO₂)_1‐x(Sc₂O₃)_x (x = 0.06 – 0.11) and (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y (x=0.07 – 0.11; y=0.01 – 0.04) solid solution crystals grown by skull melting technique is presented. The structure, phase composition, and ion conductivity of the obtained crystals were investigated by X‐ray diffraction, transmission electron microscopy, Raman scattering spectroscopy, and impedance spectroscopy. Maximum conductivity as (ZrO₂)_1‐x(Y₂O₃)_x and (ZrO₂)_1‐x(Sc₂O₃)_x solid solution crystals is observed for the compositions containing 10 mol% stabilizing oxide, and the conductivity of 10ScSZ is ~3 times higher than for 10YSZ. Experiments on crystal growth (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y solid solutions showed that uniform, transparent crystals 7Sc3YSZ, 7Sc4YSZ, 8Sc2YSZ, 8Sc3YSZ, 9Sc2YSZ, 9Sc3YSZ, 10Sc1YSZ, and 10Sc2YSZ are single phase crystal containing t″ phase. It is established that a necessary condition of melt growth of (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y single‐phase crystals is the total concentration of the stabilizing oxides from 10 to 12 mol%. The addition of Y₂O₃ affects the (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y solid solution conductivity different ways and depends on the Sc₂O₃ content in the starting composition. The effects of structure, phase composition, concentration, and type of stabilizing oxides on the electrical characteristics of obtained crystals are discussed.     

Catalytic Oxidation of CO Gas over Nanocrystallite Cu x Mn1−x Fe2O4

The catalytic oxidation of CO over nanocrystallite Cu _x Mn_(1−x)Fe₂O₄ powders was studied using advanced quadruple mass gas analyzer system. The oxidation of CO to CO₂ was investigated as a function of reactants ratio and firing temperature of ferrite powders. The maximum CO conversion was observed for ferrite powders which have equal amount of Cu²⁺ and Mn²⁺ (Cu_0.5Mn_0.5Fe₂O₄). The high catalytic activity of Cu_0.5Mn_0.5Fe₂O₄ can be attributed to the changes of the valence state of catalytically active components of the ferrite powders. The firing temperature plays insignificant role in the catalytic activity of CO over nanocrystallite copper manganese ferrites. The mechanism of catalytic oxidation reactions was studied. It was found that the CO catalytic oxidation reactions on the surface of the Cu _x Mn_1−x Fe₂O₄ was done by the reduction of the ferrite by CO to the oxygen deficient lower oxide then re-oxidation of this phase to the saturated oxygen metal ferrite again.     

Optical and Microwave Dielectric Properties of Zn‐Doped MgAl2O4 Transparent Ceramics Fabricated by Spark Plasma Sintering

(Mg_1?xZn_x)Al₂O₄ transparent ceramics were fabricated by spark plasma sintering technique at 1325°C for 10 min. A small mount of Zn²⁺ addition to MgAl₂O₄ ceramics was very effective to the performance improvement, while further increase in Zn‐doped content would give rise to the optical transmittance deterioration. The optical and microwave dielectric properties of MgAl₂O₄ transparent ceramics were improved by Zn substitution for Mg. The in‐line transmittance of the (Mg_1?xZn_x)Al₂O₄ (x = 0.02) ceramics can be as high as 70% at λ = 550 nm and 86.5% at λ = 2000 nm, respectively. The dielectric constant ε_r of (Mg_1?xZn_x)Al₂O₄ just varied from 8.32 to 8.54, however, the Q × f value increased significantly up to a maximal value of 66,000 GHz at x = 0.02. Moreover, the τ_f of (Mg_1?xZn_x)Al₂O₄ transparent ceramics changed from ?74 to ?65.5 ppm/°C. With the increasing of Zn‐doped content, the average grain size and the porosity increased, which was the primary reason for the change in optical and microwave dielectric properties.