Microwave absorption properties of a wave-absorbing coating employing carbonyl-iron powder and carbon black

To prevent serious electromagnetic interference, a single-layer wave-absorbing coating employing complex absorbents composed of carbonyl-iron powder (CIP) and carbon black (CB) with epoxy resin as matrix was prepared. The morphologies of CIP and CB were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. The electromagnetic parameters of CIP and CB were measured in the frequency range of 2-18 GHz by transmission/reflection technology, and the electromagnetic loss mechanisms of the two particles were discussed, respectively. The microwave absorption properties of the coatings were investigated by measuring reflection loss (RL) using arch method. The effects of CIP ratio, CB content and thickness on the microwave absorption properties were discussed, respectively. The results showed that the higher thickness, CIP or CB content could make the absorption band shift towards the lower frequency range. Significantly, the wave-absorbing coating could be applied in different frequency ranges according to actual demand by controlling the content of CIP or CB in composites.     

Enhanced absorption properties of ordered mesoporous carbon/Co-doped ordered mesoporous carbon double-layer absorbers

Ordered mesoporous carbon (OMC) and metal-doped (M-doped) OMC composites are prepared, and their electromagnetic (EM) parameters are measured. Using the measured EM parameters we calculate the EM wave absorption properties of a double-layer absorber, which is composed of OMC as an absorbing layer and M-doped OMC as the matching layer. The calculated results show that the EM wave absorption performance of OMC/OMC-Co (2.2mm/2.1mm) is improved remarkably. The obtained effective absorption bandwidth is up to 10.3 GHz and the minimum reflection loss reaches 47.6 dB at 14.3 GHz. The enhanced absorption property of OMC/OMC-Co can be attributed to the impedance match between the air and the absorber. Moreover, it can be found that for the absorber with a given matching layer, a larger value of -tanδε (= tan δε absorbing tan δε matching ) can induce better absorption performance, indicating that the difference in impedance between the absorbing layer and the matching layer plays an important role in improving the absorption property of double-layer absorbers.     

Enhanced microwave absorption in ZnO/carbonyl iron nano-composites by coating dielectric material

The microwave absorption properties of zinc oxide/carbonyl iron composite nanoparticles fabricated by high energy ball milling were studied at 0-20 GHz. Experiments showed that ZnO as a kind of dielectric material coating carbonyl iron particles made the bandwidth of reflection loss (RL)<−5 dB expanding to the low frequency, and enhanced absorption effect obviously. For a 3 mm thickness absorber of ZnO/carbonyl iron after 30 h milling, the values of RL<−5 dB and RL<−8 dB were obtained in the frequency range from 7.0 GHz to 17.8 GHz and from 9.8 dB to 14.9 dB, respectively, and its strongest RL peak was −29.34 dB at 13.59 GHz. The magnetic loss of carbonyl iron particles and the dielectric loss of ZnO particles were the main mechanisms of microwave absorption for the composites.     

Absorbing properties and structural design of microwave absorbers based on W-type La-doped ferrite and carbon fiber composites

Sol–gel method was used to prepare W-type BaCo₂Fe₁₆O₂₇ hexaferrite and La-doped Ba_0.7La_0.3Co₂Fe₁₆O₂₇ hexaferrite. Electromagnetic parameters of the ferrites and short carbon fiber composites were measured, and reflectivity was calculated according to transmission-line theory in the range 12.4–18 GHz. The results show that reflection loss of the doped ferrite composite is higher as compared to the no doped ferrite composite. Based on the above calculation, double-layer absorbers containing La-doped ferrite and carbon fiber composites were designed, and reflectivity of the double-layer absorbers made of different thickness and composition was calculated. Finally, a kind of structural absorber having excellent absorbing properties was achieved, and the bandwidth of the reflection loss less than −10 dB can reach 5.2 GHz in the range of 12.4–18 GHz.     

Microwave absorption properties of one thin sheet employing carbonyl-iron powder and chlorinated polyethylene

To solve more and more serious electromagnetic interference problem, one thin microwave absorbing sheet employing carbonyl-iron powder (CIP) and chlorinated polyethylene (CPE) was prepared. The pattern, static magnetic properties and phase of CIP were characterized by scanning electron microscope (SEM), vibrating sample magnetometer (VSM) and X-ray diffraction (XRD), respectively. The electromagnetic parameters of CIP were measured in the frequency range of 2-18 GHz, and the electromagnetic loss mechanisms of the powder were discussed. The microwave absorption properties of composite sheets with different thicknesses and CIP ratios in matrix were investigated by measuring reflection loss (RL) in 2-18 GHz frequency range using the arch method. The results showed that appropriate CIP content and thickness could greatly improve microwave absorption properties in lower frequency range. For the sample with the weight ratio (CIP:CPE) of 16:1 and 1.5 mm thickness, the bandwidth (RL below −10 dB) achieved 1.1 GHz (2-3.1 GHz), and the minimum reflection loss value was obtained −13.2 dB at 2.2 GHz. This suggested that CIP/CPE composites could be applied as thin microwave absorbers in S-band (2-4 GHz).     

Microwave absorption properties of double-layer absorbers made of NiCoZn ferrites and hollow glass microspheres electroless plated with FeCoNiB

Amorphous FeCoNiB coatings were deposited by an electroless plating technique on the hollow glass microspheres. NiCoZn spinel ferrites sintered show two dispersion peaks (1.81 and 6.98 GHz). By fitting its permeability dispersion spectra, the one at low frequency is believed to be due to the domain wall movement mechanism, the other one is due to the spin rotation mechanism. Used in a single-layer absorber structure, neither the hollow glass microspheres coated nor the NiCoZn could meet the demand of light weight. However, if a double-layer absorber structure is used, not only the total weight of an absorber is reduced, but also the microwave absorption performance is enhanced.     

Structure evolution of carbon black under ionic-liquid-assisted microwave irradiation

The interactions between the carbon black (CB) and the ionic liquid (IL), 1-butyl-3-methyl-imiazolium hexafluorophosphate ([BMIM⁺][PF₆⁻]), are firstly examined. The CB, mixed with the IL via simple blending, is then subjected to microwave (MW) irradiation to prepare the modified CB. The structure evolutions of the modified CB such as the microcrystalline structure and surface chemistry are revealed by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and pore analysis. After mixing but before MW irradiation, the microcrystalline arrangement of CB turns to be more ordering and microcrystalline size (La) to be a little bigger but with a limited degree. Under MW irradiation, the IL undergoes severe decomposition. The combination of localized high temperature (proposed to be higher than 425 °C) and the decomposition of the IL leads to substantial structure changes of the CB. The graphitization of the CB surface, the disordering of the microcrystalline and the decrease in La are disclosed. In addition, compared with the untreated CB, the CB treated with IL-assisted MW irradiation is found to have much higher volume of the smaller mesopore.     

Electromagnetic properties and microwave absorption properties of BaTiO3-carbonyl iron composite in S and C bands

BaTiO₃ powders are prepared by sol-gel method. The carbonyl iron powder is prepared via thermal decomposition of iron pentacarbonyl. Then BaTiO₃-carbonyl iron composite with different mixture ratios was prepared using the as-prepared material. The structure, morphology, and properties of the composites are characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron microscopy (SEM), and a network analyzer. The complex permittivity and reflection loss of the composites have been measured at different microwave frequencies in S- and C-bands employing vector network analyzer model PNA 3629D vector. The effect of the mass ratio of BaTiO₃/carbonyl iron on the microwave loss properties of the composites is investigated. A possible microwave absorbing mechanism of BaTiO₃-carbonyl iron composite has been proposed. The BaTiO₃-carbonyl iron composite can find applications in suppression of electromagnetic interference, and reduction of radar signature.     

Investigation on peak frequency of the microwave absorption for carbonyl iron/epoxy resin composite

Microwave absorbing characteristics of carbonyl iron/epoxy resin composite with various volume concentrations were investigated in 0.1-18 GHz. According to the electromagnetic parameters and thicknesses of the sample, numerical calculation and experiment have demonstrated that the frequency dependence of the microwave absorption comply with the quarter-wavelength (λ/4) matching model that may explain not only the peak frequency but also the number of the peaks. It implies that the quarter-wavelength condition can be successfully applied to understand and predict the peak frequency of the microwave absorption for ferromagnetic metal-based composites.     

Magnetic and microwave absorption properties of BaFe12−x (Mn0.5Cu0.5Zr)x/2O19 synthesized by sol–gel processing

BaFe_12−x (Mn_0.5Cu_0.5Zr)_x/2O₁₉ hexaferrites with x=1, 2 and 3 were prepared by sol–gel process. The ferrite powders possess hexagonal shape and are well separated from one another. The powders of these ferrites were mixed with polyvinylchloride (PVC) plasticizer to be converted into a microwave absorbing composite ferrite with a thickness of 1.8 mm. X-ray diffractometer (XRD), scanning electron microscope (SEM), ac susceptometer, vibrating sample magnetometer and vector network analyzer were used to analyze its structure, electromagnetic and microwave absorption properties. The results showed that magnetoplumbite structures for all samples were formed. The sample with higher magnetic susceptibility and coercivity exhibits a larger microwave absorbing ability. Also the present investigation demonstrates that a microwave absorber using BaFe_12−x(Mn_0.5Cu_0.5Zr)_x/2O₁₉ (x=2 and 3)/PVC with a matching thickness of 1.8 mm can be fabricated for applications over 15 GHz, with reflection loss more than −25 dB for specific frequencies, by controlling the molar ratio of the substituted ions.     

Optimization of electromagnetic matching of carbonyl iron/BaTiO3 composites for microwave absorption

Microwave absorbing materials filled with BaTiO₃ and carbonyl iron (CI) particles with various weight fractions (BaTiO₃/CI particles=100/0 to 0/100) are investigated. The dielectric and magnetic properties of the absorbers can be tuned by changing the weight ratio of BaTiO₃/CI particles in the frequency range of 2-18 GHz. Numerical simulations are also performed to design a single-layer and double-layer absorber. The minimum reflection loss of the composite filled with 20 wt% BaTiO₃ and 60 wt% CI particles at 2.0 mm thickness can be reached to −42 dB at 4.1 GHz. With the weight ratio of CI particles in the composite increased, the microwave absorption peak shifted to the lower frequency region. By using a double-layer absorber structure, the microwave absorption performance of the absorber is enhanced. The result shows that the total thickness of the absorber can be reduced below 1.4 mm by using a matching layer filled with 50 wt% BaTiO₃, and an absorption layer filled with 60 wt% BaTiO₃ and 20 wt% CI particles, whereas the reflection loss below −10 dB can be obtained in the frequency range of 10.8-14.8 GHz and the minimum reflection loss of −59 dB can be obtained at 12.5 GHz.     

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).     

Preparation and characterization of carbonyl iron/glass composite absorber as matched load for isolator

Composite absorbers made from 66 wt% carbonyl iron and 34 wt% low melting point glass powder were prepared by a pressureless sintering technique in a nitrogen atmosphere. Apparent porosity and bending strength of the as-prepared composites were investigated. The microstructure, heat resisting properties and electromagnetic properties were characterized by scanning electron microscopy, thermal gravimetric analysis–differential scanning calorimetry and vector network analyzer. The results show that the carbonyl iron/glass composite absorbers were difficult to densify. As the sintering temperature and soaking time increased, the apparent porosity first decreased and then increased, whereas the bending strength showed the opposite change. The composite absorber sintered at 520 °C for 40 min achieved the minimum apparent porosity of 13.08% and the highest bending strength of 52 MPa. Compared to the carbonyl iron/silicone rubber absorber, the carbonyl iron/glass composite absorber exhibited better heat resisting properties, and the initial oxidation temperature was increased about 200 °C. The composite absorber with a thickness of 1.25 mm showed a good microwave absorbing property in 8–12 GHz.     

Modelling the DC electrical conductivity of polymer/carbon black composites

Several DC electrical conductivity models have been proposed to explain the properties of composite materials. In particular, generalized effective medium model was used, but, in many cases, the obtained parameters do not fit accurately the data. In this paper, we extended the study to Mamunya model, with adjustable parameters. Using different carbon black nanocomposites, we obtained a good agreement with the experimental results, but only for concentrations above the percolation critical concentration. Below this point, the fit is not accurate.     

Enhancement of electromagnetic and microwave absorbing properties of gas atomized Fe-50 wt%Ni alloy by shape modification

In order to increase the electromagnetic parameters and improve the microwave absorbing properties in the range of 1–4 GHz, gas atomized Fe-50 wt%Ni alloys with spherical form were processed in a planetary mill. The morphology, phase composition and saturation magnetization of the FeNi alloy particles were investigated by means of scanning electron microscopy, X-ray diffraction and vibrating sample magnetometer. The complex permittivity, complex permeability and reflection loss of the microwave absorbing material made from Ethylene–Propylene–Diene Monomer rubber, and the Fe-50 wt%Ni alloys were also studied using vector network analyzer and transmission line theory. The results show that the shape of the atomized Fe-50 wt%Ni powders can be modified by mechanical milling. The flaky Fe-50 wt% Ni particles were prepared, and the aspect ratio increases with increasing the milling time from 10 to 30 h. Mechanical milling does not change the phase compositions of the FeNi alloys but decreases the peak intensity and broadens the peak width. The saturation magnetization decreases and the coercivity increases as the milling time increases. The electromagnetic parameters and microwave absorbing properties are enhanced with the increase of the aspect ratio. The rubber absorbers filled with flaky Fe-50 wt%Ni powders milled for 30 h exhibit the low reflection loss in the 1–4 GHz frequency range.     

Preparation and microwave absorption properties of loose nanoscale Fe3O4 spheres

Magnetite nanoparticles are found to assemble into randomly dispersed loose nanoscale spheres with diameters ∼300 nm in ethylene glycol in the presence of polyethylene and a small quantity of polyethyleneimine. Modern analysis methods are employed to provide structure information of the magnetic loose spheres. The ferromagnetic saturation magnetization is ∼80.0 emu g⁻¹, and the coercive force is 209 Oe. The microwave electromagnetic parameters are measured by a vector network analyzer. The synthesized loose spheres exhibit novel microwave properties compared with the conventional Fe₃O₄ nanoparticles. An additional microwave loss peak appears in the Ku band, which is attributed to the loose structure.     

Synthesis, infrared and microwave absorbing properties of (BaFe12O19+BaTiO3)/polyaniline composite

The (BaFe₁₂O₁₉+BaTiO₃)/polyaniline composite was synthesized by in situ polymerization and introduced into epoxy resin and polyethylene to be a microwave and infrared absorber. The spectroscopic characterizations of the formation processes of (BaFe₁₂O₁₉+BaTiO₃)/polyaniline composite were examined using Fourier transform infrared, ultraviolet-visible spectrophotometer, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and electron spin resonance. Microwave absorbing properties were investigated by measuring reflection loss in 2-18 and 18-40 GHz microwave frequency range using the free space method. Thermal extinction measurements in the 3-5 and 8-12 μm were done to evaluate the shielding effectivity of infrared. The results showed that a significant absorption frequency range shifting and thermal extinction could be obtained by adding polyaniline to the BaFe₁₂O₁₉+BaTiO₃ blend.     

Effect of Pt and Fe catalysts in the transformation of carbon black into carbon nanotubes

In this research carbon nanotubes and carbon nano onion-like structures were synthesized from carbon black using metal catalysts at 400 °C and 700 °C. Platinum and iron-group metals were used as catalysts for the transformation of CB into graphitized nanocarbon and the effect of both metals was compared. The synthesized products were characterized using X-ray diffraction (XRD), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM) and Raman spectroscopy. The characterization shows that this process is very efficient in the synthesis of high quality graphitized products from amorphous carbon black, even though the process temperature was relatively low in comparison with previous studies. Distinguished graphitic walls of the newly formed carbon nanostructures were clearly visible in the HRTEM images. Possible growth difference related to the type of catalyst used is briefly explained with the basis of electron vacancies in d-orbitals of metals.     

One-pot, efficient functionalization of multi-walled carbon nanotubes with diamines by microwave method

Diamines are known to act as a medium to bind miscellaneous compounds to carbon nanotubes (CNT). However, they are commonly applied in a tedious manner. Here, multi-walled carbon nanotubes (MWCNTs) were functionalized by a series of diamine molecules (ethylenediamine, 1,6-hexamethylenediamine and 1,4-diaminobenzen) in a one-pot, rapid microwave-assisted method. Surface functionality groups and morphology of MWCNTs were analyzed by infrared spectroscopy, thermogravimetric analysis, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results consistently confirmed the formation of diamines functionalities on MWCNTs, while the structure of MWCNT has remained relatively intact. This simple and efficient process may play an important role for realizing miscellaneous functionalization of CNTs.     

Influence of the interface reflections on the microwave reflection loss for carbonyl iron/paraffin composite backed by a perfect conduction plate

In this work, the influence of interface reflections on the microwave reflection loss (RL) for carbonyl iron/paraffin composite backed by a perfect conduction plate with 30 vol% concentration at various thicknesses was investigated in the 0.1-18 GHz. Using a vector network analyzer, the scattering parameters (S₁₁ and S₂₁) were measured in two different ways. Based on the quarter-wavelength matching model, the results of measurement were analyzed. The experiment shows that there are many minimum values (dips) in RL at various thicknesses when the reflective wave, which is reflected from the absorbing layer and the emerging wave, which is reflected from the backed metal plate are out of phase by 180°, and the peak intensity of the RL is directly affected by the intensity of the reflective wave and the emerging wave. Furthermore, the experiment and numerical calculation demonstrates that the modulus of the normalized input impedance |Z_in/Z₀| equals approximately 1, but the ratio between the modulus of permittivity and permeability |ε/μ| is far from unity at the minimal reflection point.