Complex permittivity,complex permeability and microwave absorption properties of ferrite–polymer composites

The complex permittivity (ε′–jε″), complex permeability (μ′–jμ″) and microwave absorption properties of ferrite–polymer composites prepared with different ferrite ratios of 50%, 60%, 70% and 80% in polyurethane (PU) matrix have been investigated in X-band (8.2–12.4 GHz) frequency range. The M-type hexaferrite composition BaCo⁺²_0.9Fe⁺²_0.05Si⁺⁴_0.95Fe⁺³_10.1O₁₉ was prepared by solid-state reaction technique, whereas commercial PU was used to prepare the composites. At higher GHz frequencies, ferrite's permeabilities are drastically reduced, however, the forced conversion of Fe⁺³ to Fe⁺² ions that involves electron hopping, could have increased the dielectric losses in the chosen composition. We have measured complex permittivity and permeability using a vector network analyzer (HP/Agilent model PNA E8364B) and software module 85071. All the parameters ε′, ε″, μ′ and μ″ are found to increase with increased ferrite contents. Measured values of these parameters were used to determine the reflection loss at various sample thicknesses, based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 80% ferrite content has shown a minimum reflection loss of −24.5 dB (>99% power absorption) at 12 GHz with the −20 dB bandwidth over the extended frequency range of 11–13 GHz for an absorber thickness of 1.6 mm. The prepared composites can fruitfully be utilized for suppression of electromagnetic interference (EMI) and reduction of radar signatures (stealth technology).     

Electromagnetic and microwave absorption properties of Fe3O4 magnetic films plated on hollow glass spheres

Magnetic hollow spheres of low density were prepared by plating Fe₃O₄ magnetic films on hollow glass spheres using ferrite plating. The complex permeability and permittivity of spheres–wax composites were measured in the range of 2–18 GHz. The complex permeability and permittivity increased, and the dielectric and magnetic losses were improved as the volume fraction of the magnetic spheres in the composites increased from 60% to 80%, which also resulted in a great improvement of microwave absorption properties. For composites with volume fraction 80%, its magnetic resonance frequency was at about 13 GHz and it appeared three loss peaks in the calculated reflection loss curves; the bandwidth less than −10 dB was almost 4 GHz which was just in the Ku-band frequencies (12–18 GHz) and a minimum reflection loss of −20 dB was obtained when the thickness was 2.6 mm; the microwave absorbing properties were mainly due to the magnetic loss. The results showed that the magnetic spheres composites were good and light microwave absorbers in the Ku-band frequencies.     

Microwave dielectric and magnetic properties of superparamagnetic 8-nm Fe3O4 nanoparticles

The superparamagnetic 8-nm Fe₃O₄ nanoparticles were successfully prepared by chemical oxidation process. For the complex permittivity, the dual dielectric relaxation processes have been proved by two overlapped Cole–Cole semicircles, and the natural resonance frequency is 3.03 GHz for the complex permeability. The maximum reflection loss value reaches −55.5 dB at 6.11 GHz with 3.85 mm in the thickness of the absorbers for the superparamagnetic 8-nm Fe₃O₄ nanoparticles which is better than that of 150 nm and 30 nm Fe₃O₄ nanoparticles. It is believed that the superparamagnetic 8-nm Fe₃O₄ nanoparticles can be used as a kind of candidate for microwave absorber.     

Preparation of pyrrole with iron oxide precipitated on the surface of graphite nanosheet

Fe₃O₄/NanoG was firstly prepared by precipitation reaction of iron oxide (Fe₃O₄) on the surface of graphite nanosheet (NanoG). Then composites PPy/NanoG, PPy/Fe₃O₄ and PPy/Fe₃O₄/NanoG were prepared by in-situ polymerization of the monomer pyrrole polymerized on the surface of NanoG, Fe₃O₄ and Fe₃O₄/NanoG. The structures of Fe₃O₄/NanoG, PPy, PPy/NanoG, PPy/Fe₃O₄ and PPy/Fe₃O₄/NanoG were characterized by scanning electron microscopy, energy dispersive spectroscopy, fourier transmission infrared spectroscopy and X-ray diffraction . Results show that NanoG and Fe₃O₄/NanoG are encapsulated by PPy for the layered structure and their high aspect ratio (300–500). From the thermogravimetric analysis it can be seen that the introductions of NanoG, Fe₃O₄ and Fe₃O₄/NanoG into PPy based composites lead them to exhibit better thermal stabilities than pure PPy. The measurements of electromagnetic parameters show that the reflection loss of PPy/Fe₃O₄/NanoG is below −15 dB at the X band (8.2–12.4 GHz) and the minimum loss value is −18.30 dB at 9.84 GHz, while the reflection loss of PPy/Fe₃O₄ is below −10 dB at 8.2–12.4 GHz and the minimum loss value is −14.02 dB at 10.26 GHz. The reflection loss of PPy/NanoG is below −10 dB at 8.2–12.4 GHz and the minimum loss value is −13.44 dB at 10.28 GHz. The microwave absorbing properties of PPy/Fe₃O₄/NanoG, PPy/Fe₃O₄ and PPy/NanoG are superior to that of PPy.     

Microwave-absorbing properties of NiCoZn spinel ferrites

In this paper, the microwave-absorbing properties of (Ni_1−x−yCo_xZn_y)Fe₂O₄ spinel ferrites have been investigated within the frequency range of 0.5–14 GHz. There are two kinds of resonance peaks observed in the permeability spectra: domain-wall resonances at lower frequency and spin-rotation resonances at higher frequency. The reflection loss (RL) calculations show that the prepared NiCoZn spinel ferrites are good electromagnetic (EM) wave absorbers in microwave range. In terms of the absorbing frequency band (AFB) and the matching thickness (t_m), (Ni_0.407Co_0.207Zn_0.386)Fe₂O₄ shows the best performances: t_m=3.15 mm and the AFB is 8.64–11.2 GHz. Decreasing the weight ratio of NiCoZn ferrites in ferrites/wax composites, the matching thickness decreases and the AFB shifts to higher frequencies. Compared with the absorbers with single-layer ferrites, the absorbers with double-layers ferrites have better absorbing performances, such as a thinner matching thickness and a wider EM wave AFB.     

Preparation and microwave-absorbing properties of NiFe2O4-polystyrene composites

NiFe₂O₄ nanoparticles were synthesized by the polyacrylamide gel method with acrylamide as the monomer and N,N′-methylenediacrylamide as lattice agent. The average crystallite sizes of the nickel ferrites annealed at 500, 600 and 800 °C are about 10, 30 and 50 nm, respectively. Ferrite-polystyrene composites were made by hot pressing, and microwave-absorbing properties of the composites with different contents of 35, 45, 55 and 65 wt% ferrite were investigated by testing complex permeability and complex permittivity in the X-band (8.2-12.4 GHz) frequency range. All the parameters, ε′, ε″, μ′ and μ″, increase with increasing ferrite content. The reflection losses were calculated based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 65 wt% ferrite content shows a minimum reflection loss of −13 dB at 11.5 GHz with a −10 dB bandwidth over the extended frequency range of 10.3-13 GHz for an absorber thickness of 2 mm.     

Adjustable microwave absorption properties of flake shaped (Ni0.5Zn0.5)Fe2O4/Co nanocomposites with stress induced orientation

Flake shaped (Ni_0.5Zn_0.5)Fe₂O₄/Co nanocomposites were successfully fabricated by co-precipitating of Ni-Zn ferrite on the surface of cobalt nanoflakes. The electromagnetic characteristics of the samples were studied at the frequency of 0.1–14 GHz. The results showed that the cobalt nanoflakes in compacted nanocomposites were well orientated, and the nanocomposites were characterized with low optimal reflection loss (RL) of −33.8 dB at 11.5 GHz and broad RL bandwidth for <−20 dB in the frequency range of 7.6–12.1 GHz. At the same time, the position of the absorptive band can be adjusted by changing the mass ratio of ferrite to cobalt in the nanocomposites. It is proposed that the excellent microwave absorption properties are related to the combination of strong shape anisotropy of cobalt nanoflakes and adjustable dielectric loss.     

Microwave absorption properties of Ce-substituted M-type barium ferrite

Ce-substituted barium ferrite with chemical composition BaCe_0.05Fe_11.95O₁₉ has been prepared by the citrate sol-gel method. The phase composition of BaCe_0.05Fe_11.95O₁₉ was characterized by X-ray powder diffraction analysis (XRD). The complex permittivity and complex permeability, microwave absorption properties of the resulting powder were measured by the transmission/reflection coaxial line method in the range of 8-13 GHz. The results show that the resulting powder has a minimum reflection loss value of - 37.4 dB at 12.8 GHz with a matching thickness of 3.5 mm.     

Electromagnetic wave absorption properties of carbonyl iron-ferrite/PMMA composites fabricated by hybridization method

The surface of carbonyl iron powder or a mixture of carbonyl iron and ferrite was coated with polymethylmethacrylate (PMMA) microspheres by a hybridization method to make hybrid powders, and then electromagnetic wave absorption properties of the hybrid composites prepared with these hybrid powders have been investigated. As for the carbonyl iron/PMMA hybrid composite, the reflection loss less than −20 dB could be achieved in a frequency range of 1.7–5.0 GHz when the composite thickness was below 5.00 mm. In the case of the carbonyl iron-ferrite/PMMA hybrid composite, a similar reflection loss was observed in a frequency range of 4.3–13.0 GHz. Thus, the addition of ferrite was found to be useful for achieving a large absorption in a wide frequency range, especially for higher frequency values. Simulated values for the minimum reflection loss are well agreed with actually measured ones, because of homogeneous distribution of carbonyl iron and/or ferrite in these hybrid composites.     

Preparation, and magnetic and electromagnetic properties of La-doped strontium ferrite films

SrLa_xFe_12−xO₁₉ films (x=0-1.0) with large magneto-crystalline anisotropy were synthesized on SiO₂ substrate by sol-gel and self-propagating high-temperature synthesis technique. The films were characterized by various experimental techniques including X-ray diffraction analysis, Field Emission Scanning Electron Microscope, Atomic Force Microscopy, Vibrating Sample Magnetometry and vector network analyzer. The results show that La ions completely enter into strontium ferrite lattice without changing the ferrite appearance; its grain size is approximately 40-80 nm, its length is 100 nm; the magnetoplumbite structure is proved through testing a concertina form of the crystal grain; the maximum coercivity is 5986 Oe at x=0.2; La-doped films possess a wider microwave absorption frequency range with better gross loss angle tangent (tan δ>0.1), from 9 to 10.5 GHz at x=0.2, where the maximum value of tan δ reaches 0.2709. The La-doped films reach smaller nanometer size, better magnetic properties and microwave absorption properties with the doping of lanthanum.     

The role of matching thickness on the wideband electromagnetic wave suppresser using single layer doped barium ferrite

The effect of Mg²⁺, Co²⁺and Ti⁴⁺ substitution on microwave absorption has been studied for BaMg_0.5Co_0.5Ti_1.0Fe₁₀O₁₉ ferrite-acrylic resin composite in frequency range from 13 to 20 GHz. X-ray diffraction (XRD), scanning electron microscopy (SEM), vector network analysis and vibrating sample magnetometry (VSM) were employed to analyze structure, electromagnetic and microwave absorption properties of prepared ferrite. The obtained results of reflectivity demonstrate that by varying matching thickness along with weight percentage of ferrite to acrylic resin, the bandwidth coupled with reflection loss values of prepared composites can be easily tuned. Based on microwave measurement on reflectivity, it is found that BaMg_0.5Co_0.5Ti_1.0Fe₁₀O₁₉ is a good candidate for wideband electromagnetic compatibility and other practical applications at high frequency.     

Electromagnetic wave-absorption properties of rapidly quenched of Nd–Fe–B nanocomposites with low Nd content

The electromagnetic wave-absorption properties of Nd₃Fe_68−xMn_xCo₁₈B₁₁ (x=0, 1, 2) alloys obtained by rapid quenching from the melt was studied. The complex permittivity-frequency and permeability-frequency properties were determined in the microwave frequency regime of 2–18 GHz by vector network analysis. XRD spectra showed that only α-Fe diffraction peak was observed in the as-spun alloys. It is found that the acquired complex permittivity and permeability values match the microwave frequency when the 1 at% Mn content was doped. A minimum reflection loss of −6.9 dB is obtained at 2.7 GHz for composite Nd₃Fe₆₆Mn₂Co₁₈B₁₁ with absorber thickness of 1.5 mm. The exchange interaction was attributed to the microwave absorption properties. The results suggest a new design of microwave absorbers based on electromagnetic wave-absorbing materials.     

Electromagnetic and microwave absorption properties of carbonyl iron/La0.6Sr0.4MnO3 composites

In this work carbonyl iron/La_0.6Sr_0.4MnO₃ composites were prepared to develop super-thin microwave absorbing materials. The complex permittivity, permeability and microwave absorption properties are investigated in the frequency range of 8-12 GHz. An optimal reflection loss of −12.4 dB is reached at 10.5 GHz with a matching thickness of 0.8 mm. The thickness of carbonyl iron/La_0.6Sr_0.4MnO₃ absorber is thinner, compared with conventional carbonyl iron powders with the same absorption properties. The bandwidth with a reflection loss exceeding −7.4 dB is obtained in the whole measured frequency range with the thickness of 0.8 mm. The excellent microwave absorption properties are attributed to a better electromagnetic matching established by the combination of the enhanced dielectric loss and nearly invariable magnetic loss with the addition of La_0.6Sr_0.4MnO₃ nanoparticles in the composites. Our work indicates that carbonyl iron/La_0.6Sr_0.4MnO₃ composites may have an important application in wide-band and super-thin electromagnetic absorbers in the frequency range of 8−12 GHz.     

Magnetic,magnetoelectric and dielectric behavior of CoFe2O4–Pb(Fe1/2Nb1/2)O3 particulate and layered composites

Magnetic, magnetoelectric and dielectric properties of multiferroic CoFe₂O₄–Pb(Fe_1/2Nb_1/2)O₃ composites prepared as bulk ceramics were compared with those of tape cast and cofired laminates consisting of alternate ferrite and relaxor layers. X-ray diffraction analysis and Scanning Electron Microscope observations of ceramic samples revealed two-phase composition and fine grained microstructure with uniformly distributed ferrite and relaxor phases. High and broad maxima of dielectric permittivity attributed to dielectric relaxation were found for ceramic samples measured in a temperature range from −55 to 500 °C at frequencies 10 Hz–2 MHz. Magnetic hysteresis, zero-field cooled (ZFC) and field cooled (FC) curves, and dependencies of magnetization on temperature for both magnetoelectric composites were measured with a vibrating sample magnetometer in an applied magnetic field up to 80 kOe at 4–400 K. The hysteresis loops obtained for composites are typical of a mixture of the hard magnetic material with a significant amount of the paramagnet. The bifurcation of ZFC–FC magnetizations observed for both composites implies spin-glass behavior. Magnetoelectric properties at room temperature were investigated as a function of dc magnetic field (0.3–7.2 kOe) and frequency (10 Hz–10 kHz) of ac magnetic field. Both types of composites exhibit a distinct magnetoelectric effect. Maximum values of magnetoelectric coefficient attained for the layered composites exceed 200 mV/(cm Oe) and are almost three times higher than those for particulate composites.     

Electromagnetic properties and microwave absorption of W-type hexagonal ferrites doped with La

W-type barium hexaferrites with compositions of Ba₁Co_0.9Zn_1.1Fe₁₆O₂₇ and Ba_0.8La_0.2Co_0.9Zn_1.1Fe₁₆O₂₇ were synthesized by the sol-gel method. The electromagnetic properties and microwave absorption behavior of these two ferrites were studied in the 2-18 GHz frequency range. The microstructure and morphology of the ferrites were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The complex permittivity spectra, the complex permeability spectra and microwave reflection loss were measured by a microwave vector network analyzer. The XRD patterns show that the main phase of the Co₂W ferrite forms without other intermediate phases when calcined at 1200 °C. The SEM images indicate that flake-like hexagonal crystals distribute uniformly in the materials. Both the magnetic and dielectric losses are significantly enhanced by partial substitution of La³⁺ for Ba²⁺ in the W-type barium hexaferrites. The microwave absorption property of the La³⁺ doping W-type hexaferrite sample is enhanced with the bandwidth below −10 dB around 8 GHz and the peak value of reflection loss about −39.6 dB at the layer thickness of 2 mm.     

Preparation of the SrFe12O19-based magnetic composites via boron oxide glass devitrification

Magnetic composites were obtained in the system SrO–Fe₂O₃–B₂O₃ by oxide glass heat treatment at 600–950 °C. Samples of the composites were investigated using XRD analysis, magnetic measurements, electron microcopy, and thermal analysis. It was shown that chemical composition of the precursor oxide glass and thermal treatment conditions influenced on the SrFe₁₂O₁₉ particles morphology and magnetic properties. The composites and powders were obtained containing hexaferrite as single domain platelet crystals or polycrystalline aggregates with a coercive force up to 6300 Oe in the former case and 4200 Oe in the latter case.     

The electrochemical preparation and microwave absorption properties of magnetic carbon fibers coated with Fe3O4 films

Electrodeposition was employed to fabricate magnetite (Fe₃O₄) coated carbon fibers (MCCFs). Temperature and fiber surface pretreatment had a significant influence on the composition and morphology of Fe₃O₄ films. Uniform and compact Fe₃O₄ films were fabricated at 75 °C on both nitric acid treated and untreated carbon fibers, while the films prepared at 60 °C were continuous and rough. Microwave measurements of MCCF/paraffin composites (50 wt.% of MCCFs, pretreated carbon fibers as deposition substrates) were carried out in the 2-18 GHz frequency range. MCCFs prepared at 60 °C obtained a much higher loss factor than that prepared at 75 °C. However, the calculation results of reflection loss were very abnormal that MCCFs prepared at 60 °C almost had no absorption property. While MCCFs prepared at 75 °C exhibited a good absorption property and obtained −10 dB and −20 dB refection loss in wide matching thickness ranges (1.0-6.0 mm and 1.7-6.0 mm range, respectively). A secondary attenuation peak could also be observed when the thickness of MCCF/paraffin composite exceeded 4.0 mm. The minimum reflection loss was lower.     

Anomalous magnetic antiresonance and resonance in ferrite nanoparticles embedded in opal matrix

Observation of magnetic antiresonance phenomenon is reported in 3D opal nanocomposite with embedded ferrite particles. Antiresonance at microwave frequencies of millimeter waveband was observed. It results in a sharp maximum of the reflection coefficient of an electromagnetic wave. Measurements were carried out in the frequency range from 26 to 38 GHz for two compositions of embedded ferrite particles, namely, the Co_0.5Zn_0.5Fe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄. The physical nature of antiresonance is discussed.     

Electromagnetic and microwave absorption properties of Fe–Sr0.8La0.2Fe11.8Co0.2O19 shell-core composites

Shell-core Fe–Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉ composites are prepared by chemical vapor deposition (CVD) for use as microwave absorbing materials. Scanning electron microscopy and X-ray diffraction analyses show that the CVD method yields Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉ powders with a uniform coating of Fe. Compared with Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉, Fe–Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉ composites have higher electrical conductivity, permittivity, and dielectric loss, which gradually increase with increasing Fe content. When Sr_0.8La_0.2Fe_11.8Co_0.2O₁₉/Fe=7:3, a reflection loss (RL) exceeding −10 dB is obtained in the frequency range of 10–14 GHz at a coating thickness of 2.0 mm. A minimum RL of −30 dB was found at 8.0 GHz, corresponding to a matching thickness of 2.8 mm.     

Effect of shape of Fe3Al particles on their microwave permeability and absorption properties

Flake-shaped and sphere-shaped Fe₃Al powder-paraffine composites have been studied in the microwave frequency range. Mössbauer results show that the flake-shaped Fe₃Al particles have easy magnetization plane, which indicates that it is planar-anisotropy. Enhanced permeability is achieved in flake-shaped Fe₃Al compared with the sphere-shaped Fe₃Al. The permeability is further enhanced by using a rotational orientation method. The complex permeability can be characterized by the superposition of two types of magnetic resonance. The resonance peak at high frequency is attributed to the natural resonance, while the peak at low frequency is attributed to the domain-wall resonance. By employing the shape effect and the rotational orientation, the peak frequency of reflection loss for the oriented sample was adjusted to L-band. The planar-anisotropy Fe₃Al powder-paraffine composite can be attractive candidates for thinner microwave absorbers in L-band (1-2 GHz).