Preparation and microwave absorption properties of electroless Co-Ni-P coated strontium ferrite powder

A new type of Co-Ni-P coated strontium ferrite nanocomposite was prepared with electroless plating enhanced by ultrasonic wave at room temperature. The plating process was studied carefully. The morphology, crystal structure and microwave absorption properties of the Co-Ni-P coated powder were studied with field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX) and vector network analyzer. The results show that the strontium ferrite powder was successfully coated with Co-Ni-P alloy and possesses excellent microwave absorption properties. The maximum microwave loss of the composite powder reaches −44.12 dB. The bandwidth with the loss above −10 dB exceeds 13.8 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).     

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.     

Microwave absorption properties of Sr2FeMoO6 nanoparticles

The microwave absorption properties of nanosized double perovskite Sr₂FeMoO₆ and epoxy resin composites were investigated in the frequency range of 2-18 GHz using the coaxial method. The Sr₂FeMoO₆ composites with an optimal 20 wt% epoxy resin showed a strong electromagnetic attenuation of −49.3 dB at 8.58 GHz with a matching thickness of 2.15 mm. Moreover the optimum absorption frequency at which the reflection loss is less than −20 dB, which corresponds to 99% reflection loss of the incident microwave, is from 5.7 to 13.2 GHz with the matching thickness ranging from 3.0 to 1.5 mm. The excellent microwave-absorption properties are a consequence of a proper electromagnetic match due to the existence of the insulating matrix of anti-site defects and anti-phase domains, which not only contribute to the dielectric loss but also to the reduced eddy current loss.     

Comparison of the effects of particle shape on thin FeSiCr electromagnetic wave absorber

The raw materials of FeSiCr were processed in the ball mill for 30 h and the shape of the FeSiCr particles was changed from sphere to flake type, which was observed using a scanning electron microscope. And FeSiCr composite microwave absorbers were mixed with silicone for mobile phones and the effects of the thickness of the samples on the absorption were measured using a network analyzer in order to investigate the relationship between the microwave absorption and the material constants. The flake-type FeSiCr-rubber composite showed high reflection loss, which was due to the high complex permittivity and permeability. Also, the matching frequency shifted toward lower frequency range with microwave absorber thickness, and the maximum reflection loss of −8.7 dB was observed in 1.85 GHz for a 1.6 mm thickness.     

Attractive microwave absorption and the impedance match effect in zinc oxide and carbonyl iron composite

The flower-like ZnO and ZnO/carbonyl-iron composite have been prepared by a sonochemical route and ball-milling process, respectively. For ZnO/carbonyl-iron composite, a reflection loss (RL) exceeding −20 dB was obtained in a broad frequency range of 8.4-17.9 GHz with a thin thickness of 1.2-2.3 mm. An optimal RL of −61 dB was found at 11.7 GHz for an absorber thickness of 1.91 mm. It is demonstrated that the attractive microwave-absorption properties are a consequence of a proper electro-magnetic impedance match and geometrical cancellation at the air-material interface. In addition, an impedance mismatch function was proposed, which provides an effective method to determine the microwave absorbing properties from the intrinsic materials constants. The calculated value of matching frequency and thickness is well consistent with the experimental data. The method also provides a simple theoretical graphic aid for determining the absorption characteristics and the location of the matching conditions in the frequency domain.     

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.     

Magnetic and electromagnetic wave absorption properties of α-Fe(N) nanoparticles

Magnetic and electromagnetic wave absorption properties of α-Fe(N) nanoparticles, synthesized by chemical vapor condensation and then kept in air for 5 years, have been studied. The magnetic properties of the α-Fe(N) nanoparticles were slightly decreased because of the degradation in air. A slight increase in the thickness of oxide shells results in an excellent dielectric loss. The α-Fe(N) nanoparticles exhibited outstanding reflection loss (RL<−20 dB) in a 4.5-18 GHz for the absorber thicknesses of ~1-3.1 mm, and an optimal RL of −37.5 dB was obtained at 10.4 GHz with an absorber thickness of 1.6 mm. The broadest bandwidth (RL<−10 dB) from 13.3 to 17.4 GHz, covering almost the whole Ku-band, is obtained for a 1.2 mm layer.     

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.     

Fabrication and electromagnetic performance of micro-tubular nanocomposites composed of monodisperse iron nanoparticles and carbon

Low density and thin thickness are essential for electromagnetic (EM) wave absorbers. In this study, we fabricated a novel micro-tubular iron nanocomposite (MTIC) that composed of carbon microtubes and monodisperse iron nanoparticles (NPs). The bulk density of MTIC is only 0.35±0.04 g cm⁻³ due to its micro-tubular structure. The presence of iron NPs increased the magnetic loss significantly and therefore enhanced the reflection loss (RL) of MTIC/paraffin composite. The optimum thickness for the composite is 1.5-1.8 mm, with maximum bandwidth of 7.6 GHz for RL<−5 dB and 3.6 GHz for RL<−10 dB. The corresponding frequency at this thickness is 10-18 GHz. Because of low density and broad bandwidth at thin thickness, MTIC is a promising light-weight absorber for EM wave absorption or microwave shielding. This study will also provide new ideas for fabricating microwave absorbers with low density and thin thickness.     

Effect of Ag substitution on the electromagnetic property and microwave absorption of LaMnO3

La_1−xAg_xMnO₃ perovskites with different doping Ag-content were prepared by the sol–gel method. The electromagnetic characteristics and microwave loss behavior of these ion-doped rare-earth manganites were studied in the 2–18 GHz frequency range. The microstructure and morphology of the samples 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 system. The XRD patterns show that the crystalline perovskite main phase ABO₃ is formed and impurity phases disappear when calcined at 1100 °C, and Ag metal as an impurity phase appears when excessive Ag⁺ is doped. The SEM image indicates that many of the La_0.85Ag_0.15MnO₃ particles are fiber-like or ellipsoidal. Magnetic loss and dielectric loss coexist and cooperate in microwave attenuation by moderate substitution of Ag⁺ for La³⁺. The microwave absorption property of the La_0.85Ag_0.15MnO₃ sample is enhanced with the bandwidth below −10 dB at about 6 GHz and the peak value of reflection loss is near −25.0 dB at the layer thickness of 2 mm.     

The influence of temperature on magnetic and microwave absorption properties of Fe/graphite oxide nanocomposites

Fe/graphite oxide nanocomposites were prepared by inserting Fe³⁺ into layers of graphite oxide and then reducing Fe³⁺/graphite oxide compound at different reduced reaction temperatures in H₂. The composition, crystal structure, magnetic and microwave absorption properties of Fe/graphite oxide nanocomposites were investigated using elemental analysis, transmission electron microscope (TEM), X-ray diffraction (XRD), magnetic hysteresis curve and electromagnetic parameter analysis. The results show that the densities of samples are 2.43–2.47 g/cm³ and the nanocomposites are soft magnetic materials. The optimum reduced reaction temperature for preparing Fe/graphite oxide nanocomposites is 600 °C. With the increase of the thickness of the sample, the matching frequency tends to shift to the lower frequency region, and theoretical reflection loss becomes less at the matching frequency. Microwave absorption property of Fe/graphite oxide nanocomposites prepared at 600  °C (FeGO600) is the best. When the thickness is 1 mm, the maximum theoretical reflection loss of FeGO600 is −9 dB and the frequency region in which the maximum reflection loss is more than −6.0 dB is 11–18 GHz. In conclusion, FeGO600 is a good candidate for microwave absorbent due to its low density, wide frequency region for microwave absorption and large reflection loss.     

Absorption properties of carbonyl-iron/carbon black double-layer microwave absorbers

Double-layer materials were devised in order to improve the absorbing properties of electromagnetic wave absorbing plates. The double-layer wave absorbing materials are composed of a matching layer and an absorption layer. The matching layer is the surface layer through which most of the incident waves can enter, and the absorption layer beneath it plays an important role in incident wave attenuation. The total thickness of the double layer is the sum of the thicknesses of these two layers. Carbonyl iron (CI) and carbon black (CB) were used as absorbents in the matching and absorption layers, respectively. The structures of the CI and CB particles were analyzed using scanning electron microscopy and transmission electron microscopy; the dielectric properties and absorption mechanisms were also studied. In the testing frequency range 2-18 GHz, the results show that the double-layer absorbers have two absorption peaks, and the positions and values of these peaks change with the content level of the absorbents. When the mass fraction of CI in the matching layer is 50% and the total thickness of the absorber is 4 mm, the effective absorption band (below −8 dB) reaches 5.5, 5.8, and 6.5 GHz. Where the mass fraction of CB is 50% or 60% and the mass fraction of CI is 70%, the bandwidth with reflection loss below −4 dB is larger than 10 GHz.     

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.     

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.     

Synthesis, characterization and electromagnetic properties of Fe1−xCox alloy flower-like microparticles

Fe_1−xCo_x alloy microparticles with size 3-5 μm and novel flower-like shapes were prepared by a simple low temperature reduction method. The electromagnetic properties for the paraffin matrix composites containing Fe_1−xCo_x alloy microparticles were measured using a vector network analyzer in the 2-18 GHz frequency range. As a consequence of large surface- and shape-anisotropy energy for the flower-like shaped 3D microstructures, the strong natural resonance around 8-12 GHz and remarkable dielectric relaxation were observed in the complex permittivity and permeability spectrum, which are dominant in the enhanced electromagnetic wave absorption (EMA) performance. It was found that both the electromagnetic parameters of complex permittivity and permeability and the intensity and location of absorption band were remarkably dependent on the Co/Fe molar ratio. The enhanced EMA performance was obtained in these Fe_1−xCo_x-paraffin (x=0.4, 0.5, and 0.6) composites system. For the Fe_0.5Co_0.5 alloy, the reflection loss (RL) exceeding −20 dB was obtained in the broad frequency range of 5.4-18 GHz with a thin sample thickness of between 1.0 and 2.9 mm. In particular, an optimal RL of −59 dB was obtained at 3.61 GHz with a thin thickness of 3.6 mm for the Fe_0.4Co_0.6 sample. The Fe_1−xCo_x alloy microparticles may be attractive candidates for applications of microwave absorption materials with a wide frequency range and strong absorption in the high frequency region.     

Complex permittivity, permeability and microwave absorbing properties of (Mn2−xZnx)U-type hexaferrite

Complex permittivity, permeability and microwave absorbing properties of a U-type hexaferrite series Ba₄Mn_(2−x)Zn_xFe₃₆O₆₀ (with 0≤x≤2 in step of 0.5) have been examined in the X-band (8.2-12.4 GHz) frequency range. The series have been prepared using conventional solid state reaction route. Microstructural variations with composition have been found with X-ray diffraction (XRD) and scanning electron microgram (SEM). The complex permittivity (ε^?=ε′−jε″) and permeability (μ^?=μ′−jμ″) were measured using vector network analyzer (Agilient Make model PNA E8364B). These parameters were then used for calculating the reflection loss for determination of microwave absorbing properties. Addition of Zn resulted in an increase in reflection loss from −4 dB (or 60 % absorption) in sample with x= 0 to −32 dB (99.92% absorption) in sample with x=1 when the sample thickness was 1.7 mm. Multiple peaks of resonance were obtained in the dielectric and magnetic loss spectra for all samples with x>0. The result indicates that the sample with composition Ba₄MnZnFe₃₆O₆₀, i.e., x=1, can be used effectively for microwave absorption and suppression of electromagnetic interference.     

γ-MnO2/polyaniline composites: Preparation, characterization, and applications in microwave absorption

MnO₂/doped polyaniline (PANI) is prepared by an in situ polymerization method using γ-MnO₂ as the addition agent and hydrochloric acid as the doping agent. Products are characterized by FT-IR, UV-vis, XRD, and TEM. Conductivity, electromagnetic properties, and microwave absorption properties are first discussed on the basis of structural characterization. The as-prepared products of MnO₂/PANI are partially crystalline in nature and spherical in pattern with grain sizes of 50-70 nm. MnO₂ particles are successfully decorated with doped PANI. MnO₂/PANI displays moderate electric conduction, excellent dielectric losses, and microwave absorption capabilities. Compared to pure MnO₂, the dielectric and reflection loss properties of MnO₂/PANI composites exhibit significant improvements, with an effective absorption band at 5 GHz under −10 dB and maximum reflection loss of −21 dB at 13.56 GHz. Pure MnO₂ shows an effective absorption band of 3 GHz under −10 dB and a maximum reflection loss of −14.20 dB at 11.5 GHz. Thus, MnO₂/PANI composites are found to be a promising microwave absorption material.     

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.     

Preparation and microwave absorption properties of electroless Co-P-coated nickel hollow spheres

Co-P-coated nickel hollow spheres (NHSs) were prepared by electroless plating technology. The morphology and component content of Co-P coating varies with the change of sodium citrate concentration in elctroless plating solution. And as phosphorus content increases in coatings, resulting in smaller grain, coercivity of microspheres decreases. The microwave absorption properties of spheres-wax composite were investigated in the range of 2-18 GHz. Both permittivity and permeability increase with an increase of cobalt content in coatings. For composite layer, a minimal reflection loss (RL, −36.9 dB) of was predicted at 8.1 GHz with a thickness of 3 mm.