Papers by Keyword: Microwave Absorption

Abstract: Ferrite is frequently employed as a high-efficiency microwave absorption material (MAM). Herein, a novel and prospective high-temperature mechanochemical (HTMC) method was employed to fabricate pure-phase spinel zinc ferrite powders with a spherical structure. After that, the chemical compositions and structures, microscopic morphology, static magnetic characteristics, and microwave absorption properties of the powders were examined. The powder achieves a minimum reflection loss of -54.7 dB at a matching thickness of 4.4 mm, at which time the effective absorption bandwidth approaches 3.9 GHz. The superior microwave absorption ability of the powders is attributable to the favorable cooperative impact between dielectric and magnetic losses. Therefore, the as-prepared zinc ferrite powders can be utilized as high-efficiency MAM. The HTMC method has considerable potential for the large-scale preparation of high-performance MAM.

Abstract: Carbonyl iron is an excellent microwave absorption material. However, the high density limits its application in lightweight microwave absorbing. In this study, flake carbonyl iron (FCI) was prepared by high-energy ball milling, and mixed with TPU to prepare the TPU/FCI composites. The large shape anisotropy of FCI makes the TPU/FCI samples exhibit higher permittivity and permeability, and consequently better microwave absorption performance than TPU/SCI (spherical carbonyl iron). Then, rGO was added into the TPU/FCI composites. The permittivity of the TPU/FCI/rGO composites is significantly enhanced by a few amount of rGO (less than 0.5 wt.%). As a result, the TPU/FCI/rGO sample with m_FCI: m_TPU = 3:10 and 0.5 wt.% rGO consumes only half of FCI that the TPU/FCI sample with m_FCI: m_TPU = 6:10 uses, and shows much better microwave absorbing performance than this TPU/FCI sample, that the minimum reflection loss reaches-68.3 dB (at 3.4 mm) and the effective absorption bandwidth is up to 5.9 GHz (at 1.5 mm). The TPU/FCI/rGO materials demonstrate promising application in light-weight high-efficient microwave attenuation.

Abstract: Increasing demands of microwave absorbents require new approaches to enhance the absorbing performance. Metal−organic frameworks (MOFs) could be developed as effective absorbers owing to their outstanding features including intrinsic porosity and high specific area. Herein, novel nanostructured Co-MOF-74 composites have been successfully fabricated via a simple solvothermal method. Excellent absorption performance was achieved for the composite with a minimum reflection loss (RL) of −25.5 dB at 2.5 mm and a broad absorption bandwidth (RL < −10 dB) of 6.7 GHz. Such absorber could be developed as lightweight and high-efficiency absorbing materials, and this work provides inspiration for the design of advanced absorbers with strong dissipation capacity and broad effective bandwidth.

Abstract: The microwave absorption properties of La_0.85Ag_0.15MnO₃ prepared by solid state method was investigated. Analysis of X-ray diffraction data using a refinement technique confirmed the rhombohedral structure of the samples. The microstructure of the sample characterised from field emission scanning electron microscope micrographs showed irregular grain shapes with grain sizes ranging from 800 to 1500 nm. M-H curves revealed the weak ferromagnetic properties of the sample at room temperature. The real and imaginary parts of permittivity and permeability as well as microwave reflection loss were measured by a vector network analyser in the 8–18 GHz frequency range. The La_0.85Ag_0.15MnO₃ sample showed a minimum reflection loss of –57.2 dB at 16.41 GHz, with a –10dB bandwidth (corresponding to reflection loss below –10 dB, or 90% absorption) of 2.67 GHz. The microwave absorption of La_­­­­_0.85Ag_0.15MnO₃ mainly arises from the conduction loss and domain wall motion which contributed to dielectric loss and magnetic loss, respectively.

Abstract: In this study, effect of La substitution on the microstructure, magnetic properties and microwave absorption characteristics of Ba_1-xLa_xFe₁₂O₁₉ (x=0, 0.1, 0.2, 0.3, 0.5, 0.7) is reported. The samples were synthesized through mechanical alloying and solid reaction a temperature 1200 °C for 2 hours. A single-phase material occurred only at x = 0 and 0.1. Additional second phases were existing in all samples with x ≥ 0.2 which led to multi-phase materials. The single phase (x = 0 and 0.1) has a relatively uniform particle size distribution with a mean crystallite size 138 nm. Additional phases of respectively Fe₂O₃ and LaFe₂O₃ were identified in all samples with x ≥ 0.2. Effect of La substitution is to decrease the magneto crystalline anisotropy constant and the saturation magnetization. The latter is due to a decrease in mass fraction of the main magnetic phase. All Ba_1-xLa_xFe₁₂O₁₉ samples have superior microwave characteristics which able to absorb more than 99 % the incoming electromagnetic wave entering the material. The absorption bandwidth is found relatively wide within the frequency range 8-12 GHz.

Abstract: The ZnNd_(x)Fe_(2-x)O₄ (x = 0.0; 0.010; 0.020 and 0,030) systems were synthesized by solid reaction method from a mixture of ZnO₂, Fe₂O₃ and Nd₂O₃ powders according to their mole ratio using mechanical milling techniques. In this mixture was added ethanol of 25 ml and then milled for 5 hours, after that sintered at a temperature of 1000 °C for 5 hours. X-ray diffraction patterns showed that the Nd³⁺ ion substitution in ZnFe₂O₄ with the concentration of x = 0.0 to 0.02 did not result in changes in ZnFe₂O₄ phase with cubic structure (space group of Fd-3m). However, the composition of x = 0.030 formed multiphases ZnFe₂O₄ and NdFeO₃ phases. The morphological observation using Scanning Electron Microscope (SEM) showed spherical and uniform particles. Whereas the microwave absorption capability of the sample ZnNd_(x)Fe_(2-x)O₄ system increased with the increasing concentration of x from 91.20% up to 97.80% with the highest absorption is found at a frequency of 10.24 GHz. The dielectric loss of this study is very small around 0.005 up to 0.05. It is hoped that the compound ZnNd_0.02Fe_1,98O₄ can be applied to microwave absorbing agents at high frequencies (X-band range) in antiradar detection systems.

Abstract: The effect of x mole ratio on crystal structure and characteristic of microwave absorption of ZnLa_xFe_(2-x)O₄ system (x = 0.0; 0.01and 0.02) synthesized by solid state reaction method has been studied. The series of ZnLa_xFe_(2-x)O₄ samples were prepared using ZnO (99.99%) and Fe₂O₃ (99.99 %) powders (Merck product), while La₂O₃ (local production) powders in mole ratio. The identification result of the XRD shows that all of samples are single phase in this stage, it has cubic spinel structure with space group F d-3 m. The SEM image of ZnLa_xFe_(2-x)O₄ samples appear that the increase of mole ratio, the particle size of the compound powder rapidly becomes bigger, homogeneous and not uniform powder with spherical in shape and particle size of 200-500 nm. The results of the VNA characterization shows that the increasing of mole ratio (x = 0.0; 0.01 and 0.02) will enhance the ability to absorb microwave from 90.35% upto 97.69%. Thus. the composition of x=0.02 (ZnLa_0.02Fe_1.98O₄) possess to be the best composition for microwave absorbing material.

Abstract: A method of adding RAM directly in silica phenolic composite was tested. Due to direct incorporation of RAM, the resultant composite not only can show microwave absorption at normal conditions but can sustain this capability at high heat fluxes. Such RAM added composite becomes more applicable for materials which are exposed to high temperatures. Two different RAM added silica fibre phenolic composites were developed and their RF-absorptions were compared with pure silica fibre phenolic sample. It was found that depending on the type of RAM their RF-absorption can be enhanced. Strength and ablation properties of the resultant composites were also found to alter with the addition of RAM in the composite.

Abstract: In this paper, a novel microwave absorber of thin, light weight, flexible, green and low cost magnetic nanocomposite sheet that can work in high frequency range is fabricated. The vast and increasing numbers in electronic and telecommunication devices has create electromagnetic interference (EMI) in which may lead to application disturbance. Therefore, electromagnetic (EM) wave absorber with the ability of high absorption rate is strongly demanded. Here, durian shell (Durio zibethinus Murray) embedded magnetite (Fe₃O₄) nanocomposite sheets were prepared via pulping and lumen loading technique. The nanocomposite sheets were fabricated by varying the weight percentage of the filler content (2-10 wt% of Fe₃O₄) at constant thickness and varying the thickness of the sheets (0.1-1.0 mm) at constant filler content. FESEM micrograph shows that the Fe₃O₄ nanoparticles are in cubical and spherical shape with the 20–50 nm of size range. The microwave absorption properties of the sheets were tested by a vector network analyze (VNA) in the frequency range of 4-18 GHz. The samples were also tested using vibrating sample magnetometer (VSM) in order to study the magnetic properties. The absorption or maximum reflection loss (R_L) of the samples increases continually and the increase of both filler content and sample thickness has led to the shift of dip to lower frequency region.

Abstract: Barium titanate/graphene oxide/polyurethane (BTO@GO@PU) composite membranes for microwave absorption were designed and fabricated by mechanical-blending of BTO and GO in PU medium, followed by mold formation. The cross section morphology of the BTO@GO@PU membrane indicated that the BTO nanoparticles with 450 nm average diameter are successfully incorporated into the PU matrix. Mechanical tensile measurement showed that, as the amount of BTO nanoparticles increased from 5 wt% to 20 wt%, the elastic modulus of the corresponding membrane increased up to 23.0 MPa elongation with the elongation above 450 %. Microwave absorption property of the BTO@GO@PU membranes were evaluated by measuring its reflection loss in the frequency range of 0.1-18 GHz. With the addition of BTO up to 20 wt%, the maximum absorptivity of the composite reached up to 51 %. This is attributed to the dielectric loss of BTO nanoparticles.