Determining factors for high performance silicone rubber microwave absorbing materials

Silicone rubber microwave absorbing materials (RMAMs) based on ferrite as the major absorbent were prepared by the mechanical blending method. The determining factors for the complex permittivity, complex permeability, and reflectivity of RMAM were thoroughly investigated with various samples including different crystal structures of Ba-ferrite (M-type, W-type, and Y-type), the ferrite with doped elements (Ba, Sr), the materials' thickness, the combination ratio of ferrite and carbonyl iron. The effects of surface modification and loading amount of ferrite on the mechanical properties, processing performance, and absorbing property of RMAM were also assessed. The results show that W-type Ba-ferrite based RMAM exhibits better absorbing property at high frequencies (8-18 GHz) than the other two barium ferrites (M-type and Y-type) based ones, and the absorbing property of RMAM based on Sr-ferrite is best. As the thickness of RMAM and the amount of absorbents increase, the absorption peak moves toward low frequency, the absorption frequency bandwidth is narrowed, and the reflectivity first decreases and later increases. The optimum thickness is 1.5-1.7 mm, and the amount of ferrite is 450 parts per hundreds of rubber (phr). Surface modification of the absorbent with silane coupling agent could improve the mechanical properties and processing performance of RMAM. It is concluded that there will be a synergistic effect when carbonyl iron (CI) is used in combination with Sr-ferrite (Sr-W) in an appropriate proportion. When the total volume fraction of absorbents is 51%, the optimum ratio of Cl to Sr-W is 17:34, the absorption frequency bandwidth (<−10 dB) is about 8 GHz, and the absorption area is −99 dB.     

Fabrication and performance optimization of Mn-Zn ferrite/EP composites as microwave absorbing materials

Magnesium-substituted Mn0.8Zn0.2Fe2O4 ferrite is synthesized by the sol–gel combustion method using citrate acid as the complex agent. The electromagnetic absorbing behaviors of ferrite/polymer coatings fabricated by dispersing Mn–Zn ferrite into epoxy resin （EP） are studied. The microstructure and morphology are characterized by X-ray diffraction and scanning electron microscope. Complex permittivity, complex permeability, and reflection loss of ferrite/EP composite coating are investigated in a low frequency range. It is found that the prepared ferrite particles are traditional cubic spinel ferrite particles with an average size of 200 nm. The results reveal that the electromagnetic microwave absorbing properties are significantly influenced by the weight ratio of ferrite to polymer. The composites with a weight ratio of ferrite/polymer being 3：20 have a maximum reflection loss of –16 dB and wide absorbing band. Thus, the Mn–Zn ferrite is the potential candidate in electromagnetic absorbing application in the low frequency range （10 MHz–1 GHz）.     

Dependence of microwave absorption properties on ferrite volume fraction in MnZn ferrite/rubber radar absorbing materials

We report the analysis of measurements of the complex magnetic permeability (μ_r) and dielectric permittivity (ε_r) spectra of a rubber radar absorbing material (RAM) with various MnZn ferrite volume fractions. The transmission/reflection measurements were carried out in a vector network analyzer. Optimum conditions for the maximum microwave absorption were determined by substituting the complex permeability and permittivity in the impedance matching equation. Both the MnZn ferrite content and the RAM thickness effects on the microwave absorption properties, in the frequency range of 2-18 GHz, were evaluated. The results show that the complex permeability and permittivity spectra of the RAM increase directly with the ferrite volume fraction. Reflection loss calculations by the impedance matching degree (reflection coefficient) show the dependence of this parameter on both thickness and composition of RAM.     

A finite element model for rigid porous absorbing materials

An eight node isoparametric finite element is used to represent a rigid porous absorbing material. Tests on an assembly of these elements for a one dimensional model gave good agreement with an exact solution for the input impedance. Results from a two dimensional model show the effects of transverse propagating modes on the input impedance and indicate that for an absorbent with finite dimensions extended reaction is important.     

Distribution of reflection coefficients in absorbing chaotic microwave cavities

The distribution of reflection coefficients P(R) for chaotic microwave cavities with time-reversal symmetry is investigated in different absorption and antenna coupling regimes. For all regimes the agreement between experimental distributions and random-matrix theory predictions is very good, provided both the antenna coupling T(a) and the wall absorption strength T(w) are taken into account in an appropriate way. These parameters are determined by independent experimental quantities.     

A novel infrared absorbing structure for uncooled infrared detector

In this paper, we proposed a novel infrared absorbing structure for uncooled infrared detectors. The infrared absorber makes use of a quarter-wavelength structure composed of a dielectric layer, a protecting layer, an active layer, a supporting layer and a reflecting layer. Sputtered amorphous silicon is used as a dielectric layer because of its high refractive index. We fabricated the uncooled microbolometer with the proposed infrared absorbing structure by surface micromachining method. Then we characterized various bolometric properties such as thermal conductance, thermal time constant, responsivity and infrared absorptance. The fabricated bolometer showed the thermal conductance of 6.72 × 10⁻⁷ W/K, the thermal mass of 4.43 × 10⁻⁹ J/K, the thermal time constant of 6.6 ms and the responsivity of 7.76 × 10³ V/W at 10 Hz chopper frequency and 9.22 μA bias current. From the results, the estimated absorptance is about 80%. We expect that the proposed absorbing structure shows high infrared absorption and high performance of uncooled microbolometer.     

A mixed method for measuring low-frequency acoustic properties of macromolecular materials

A mixed method for measuring low-frequency acoustic properties of macromolecular materials is presented. The dynamic mechanical parameters of materials are first measured by using Dynamic Mechanical Thermal Apparatus(DMTA) at low frequencies, usually less than 100 Hz; then based on the Principles of Time-Temperature Superposition (TTS), these parameters are extended to the frequency range that acousticians are concerned about, usually from hundreds to thousands of hertz; finally the extended dynamic mechanical parameters are transformed into acoustic parameters with the help of acoustic measurement and inverse analysis. To test the feasibility and accuracy, we measure a kind of rubber sample in DMTA and acquire the basic acoustic parameters of the sample by using this method. While applying the basic parameters to calculating characteristics of the sample in acoustic pipe, a reasonable agreement of sound absorption coefficients is obtained between the calculations and measurements in the acoustic pipe.     

Design, fabrication, and characterization of lightweight and broadband microwave absorbing structure reinforced by two dimensional composite lattice

Microwave absorbing structures (MASs) reinforced by two dimensional (2D) composite lattice elements have been designed and fabricated. The density of these MASs is lower than 0.5?g/cm³. Experimental measurements show that the sandwich structure with glass fiber reinforced composite (GFRC) lattice core can serve as a broadband MAS with its reflectivity below ?10?dB over the frequency range of 4?C18?GHz. The low permittivity GFRC is indicated to be the proper material for both the structural element of the core and the transparent face sheet. Calculations by the periodic moment method (PMM) demonstrate that the 2D Kagome lattice performs better for microwave absorbing than the square one at relatively low frequencies. The volume fraction and cell size of the structural element are also revealed to be key factors for microwave absorbing performance.     

Ceramic materials for use in microwave electronics

A study is reported of the structure and electrical properties of BaLn₂Ti₄O₁₂ compositions (BLT), where Ln=La, Nd, or Sm, with calcium, strontium, or lead substituted for barium, and bismuth for the lanthanide. The BLT compound is shown to be characterized by a broad isomorphicity region, at the edge of which form solid solutions with the highest dielectric permittivity reached thus far, up to about 170. A number of thermally stable ceramic materials with a high dielectric permittivity of 80 to 120, which permit construction of microwave dielectric resonators with Q factors of up to 600 for ɛ=120 and up to 3000 for ɛ=80 at f=4 GHz, have been developed based on BLT solid solutions. Fiz. Tverd. Tela (St. Petersburg) 41, 882–884 (May 1999)     

Long-range correlations in the structure of fractal films

Hierarchically granulated films formed on a material subjected to the action of a high-temperature plasma have been microscopically studied. The statistical self-similarity of the structure of the surface from nanoscale to macroscale, as well as long-range correlations in the relief that are due to the statistically inhomogeneous structure of granularity, has been revealed.     

Interference structure of low-frequency reverberation signals in shallow water

Using numerical simulation, an analysis was conducted of the interference structure of a bottomscattered sound field generated by a wideband point source in shallow water under winter and summer conditions. The scattered signals were received from the place where the source was located and were subjected to Fourier transform with a sliding window. The paper demonstrates the possibility of estimating the waveguide invariant for backscattered signals when processing the sound intensity distributions in wide frequency and distance ranges up to the scattering area. A technique is proposed for reconstructing the twodimensional field of internal waves using variations of the interference pattern of reverberation signals. The influence of wind surface waves on the degree of interference band contrast is illustrated.     

A note on the definition of scattering cross sections and phase functions for spheres immersed in an absorbing medium

Abstract

A definition of the scattering cross section of a sphere immersed in an absorbing medium is considered. The quantity, and the relevant phase function, can be used in Monte Carlo calculations regarding multiple scattering.     

A note on the definition of scattering cross sections and phase functions for spheres immersed in an absorbing medium

A definition of the scattering cross section of a sphere immersed in an absorbing medium is considered. The quantity, and the relevant phase function, can be used in Monte Carlo calculations regarding multiple scattering.     

A review on microwave synthesis of electrode materials for lithium-ion batteries

The applications of microwave processing of electrode materials for Li-ion batteries have been reviewed. This paper intends to insist at the advantages of the microwave processing and its credentials for commercialization. In order to achieve successive commercialization/industrial application, a systematic understanding of the microwave processing becomes imperative. In the advent of this, an extensive study on the behavior of material in electromagnetic field has been presented. Microwave processing of various materials like lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium titanium oxide and their derivatives, copper bismuth oxide, antimony sulfide, and tin oxide graphite has been reviewed in detail. Also, the dependence of microwave processing in operating frequency, geometry, preheating, soaking time, susceptor material, and single (or) multimode cavity has been reviewed.     

A method for analyzing the microwave absorption properties of magnetic materials

Based on transmission line theory, a complex thickness is introduced to analyze the electromagnetic wave absorption properties of materials with high magnetic loss and low dielectric loss. At each frequency the real part of the complex thickness denotes the thickness of the absorber to reach maximum absorption, while the absolute value of the imaginary part of the normalized complex thickness represents the degree of impedance mismatch between the material and free space. Using this method, the intensity of the reflection loss and the corresponding material thickness can be well predicted and understood. This method describes well the microwave absorption behaviors of PANI/α-Fe and PANI/CIP/Fe₃O₄ composites. It is found that the electromagnetic energy loss in the material, as well as the interference cancellation of the reflected waves at the air-material interface, contributes to the global minimum of reflection loss for the two composites.     

7mm波段制冷接收机用低温微波单元

简要介绍了7mm波段制冷接收机的系统组成,详细分析了低温微波单元的设计思路与指标要求,给出了真空微波窗口、馈源极化网络、低温隔离器与低温放大器的设计过程与指标分配;最后对低温微波单元所需的杜瓦进行了热分析、密封性分析与结构设计。     

Optical extinction of an assembly of spherical particles in an absorbing medium: Application to silver clusters in absorbing organic materials

The theory presented by Gerardy and Ausloos for the calculation of the linear optical response of aggregates of spherical particles is analytically continued for absorbing embedding media. The method is based on the calculation of the extinction rate by a single particle embedded in an absorbing matrix. Explicit expressions for the extinction and scattering cross-sections are given. The method is applied to calculate the energy losses in several organic matrices with embedded silver clusters. Comparison with experimental data shows a very good agreement. Received: 21 December 1998