Fabrication and design of multi-layered radar absorbing structures of MWNT-filled glass/epoxy plain-weave composites

The object of this study is to design radar absorbing structures (RAS) with load-bearing ability in the X-band. Glass/epoxy plain-weave composites of excellent specific stiffness and strength, containing multi-walled carbon nanotubes (MWNT) to induce dielectric loss, were fabricated. Observations of the microstructure and the permittivity of the composites confirmed that the fabrics are suitable for use as RASs. A genetic algorithm and a theory of the reflection/transmission of electromagnetic waves in a multi-layered RAS were applied to design an optimal RAS composed of MWNT-filled composites. The thickness per ply was observed to vary, depending on the number of plies and the MWNT contents. A fabrication process was proposed that considered the variation. The proposed process was in the fabrication of a designed RAS, and the theoretical and measured reflection losses of the RAS were found to be in good agreement.     

A comparative dielectric relaxation study of PMN–PT and PMN–PZ ceramics using impedance spectroscopy

AC-impedance spectroscopic studies in the temperature range of 30–400 °C are carried out on solid solutions of lead magnesium niobate (PMN) with lead titanate (PT) and lead zirconate (PZ), both of them in the 65/35 atomic ratio. For PMN–PT this corresponds to the morphotropic phase boundary composition (with normal ferroelectric behaviour), and for PMN–PZ it is near the phase boundary between normal ferroelectric and relaxor ferroelectric compositions. The variation of dielectric permittivity with temperature at different frequencies shows normal ferroelectric and relaxor-like dependence for PMN–PT and PMN–PZ, respectively. Temperature-dependent spectroscopic modulus plots reveal a much broader peak for PMN–PZ compared to that for PMN–PT, which is consistent with the dielectric behaviour. PMN–PT shows nearly ideal Debye behaviour below T_m (the temperature of the permittivity maximum) and the behaviour departs from ideality above T_m, whereas non-ideal Debye behaviour is seen both below and above T_m for PMN–PZ. Complex modulus plots fit well with two depressed semicircles and three depressed semicircles, respectively, for PMN–PT and PMN–PZ. The relaxation observed in the spectroscopic plots around 1 MHz for PMN–PT has been assigned to polarisation relaxation expected for normal-sized domains. No such relaxation could be observed for PMN–PZ around 1 MHz because of the mesoscopic domain sizes.     

Methoxypolyethylene glycol functionalized carbon nanotube composites with high permittivity and low dielectric loss

High relative permittivity and low dielectric loss were simultaneously achieved in the percolative nanocomposites with methoxypolyethylene glycol (mPEG) modified multi-walled carbon nanotubes (MWCNTs). The dense mPEG layer with a thickness of approximately 1.7 nm was continuously coated on the surface of MWCNTs. MWCNTs exhibited excellent dispersibility after being functionalized by mPEG (mPEG@MWCNTs), the mPEG@MWCNTs/ethanol suspension was still turbid even when the suspension was deposited for two months. A high permittivity of 69.7 and a low dielectric loss of 0.042 were simultaneously achieved in the poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) nanocomposite with 4.02 vol% mPEG@MWCNTs at 1 kHz. The improved dielectric properties in the nanocomposite is mainly ascribed to the following reasons: (i) the increased microcapacitors formed by MWCNTs and insulated dielectric composite; (ii) the enhanced interfacial polarization due to the homogeneous dispersion of mPEG@MWCNTs in the nanocomposites and tight adhesion between mPEG@MWCNTs and P(VDF-HFP) matrix.     

Microstructure and microwave-frequency electromagnetic properties of Ni0.4Zn0.6Fe2O4/Ba0.6Sr0.4TiO3 composites

(x)Ni_0.4Zn_0.6Fe₂O₄+(1−x)Ba_0.6Sr_0.4TiO₃ composite ceramics with x=0.6, 0.7, 0.8, 0.9 and 1 were synthesized by solid state reaction method. The high dense composites have only two phases, i.e., Ni_0.4Zn_0.6Fe₂O₄ and Ba_0.6Sr_0.4TiO₃. The permittivity ε′ of the composites decreases slightly with the frequency increasing from 3 MHz to 1 GHz. The permittivity ε′′ of the composites also shows a little increase with frequency in the 3 MHz–1 GHz range. The permeability displays a relaxation resonance within the 3 MHz–1 GHz frequency range. The permeability μ′ increases while the cut-off frequency decreases with the Ni_0.4Zn_0.6Fe₂O₄ concentration, obeying the Snoek's law μ_if_r=constant. The permittivity ε′ of the composites decreases with Ni_0.4Zn_0.6Fe₂O₄ concentration. The composites have a relatively higher ε′ than the pure Ni_0.4Zn_0.6Fe₂O₄ at 1–10 GHz. In the frequency range of 1–10 GHz, the magnetic permeability μ′ reaches its maximum and μ′′ shows a minimum for the composite with x=0.6 in all ceramics. The permeability μ′ of the composites decreases with dc magnetic field at 1–10 GHz. The permeability shows a domain wall resonance, and the resonance frequency shifts to high frequency with the dc magnetic field. The permittivity was also influenced by the dc magnetic field due to a magnetodielectric effect.     

Bi2(Sn0.95Cr0.05)2O7: Structure,IR spectra,and dielectric properties

Infrared absorption spectra of the bismuth pyrostannate Bi₂(Sn_0.95Cr_0.05)₂O₇ were investigated in the frequency range 350−1100 cm⁻¹ at temperatures of 110−525 K. Four frequency regions with split absorption lines are distinguished. Softening of frequencies at the structural transitions was observed. The maxima of permittivity measured in the frequency range 1−200 kHz at temperatures 100−400 K were determined. It was found that the magnetic susceptibility changes its sign in the low-temperature region. The correlation between anomalies in the magnetic susceptibility, permittivity, and absorption line intensity was established. Softening of frequencies is explained by the variation in the coefficient of thermal expansion of the lattice. The temperature behavior of permittivity is described using the Debye model.     

微波作用下氯化钠水溶液介电特性变化机理

实验研究发现,微波作用下的恒温氯化钠水溶液随着微波功率的增加,其复介电系数的虚部会发生明显变化从理论上分析表明,在微波场中极化水分子转动能级被激发将会引起质子定向移动和团簇氢键完好的概率变化而影响氯化钠水溶液电导率,进而造成其复介电系数虚部的明显变化。此现象应归属于微波的非热效应。     

RECONSTRUCTION OF ONE DIMENSIONAL MULTI-LAYERED MEDIA BY USING A TIME DOMAIN SIGNAL FLOW GRAPH TECHNIQUE

A novel inverse scattering method to reconstruct the permittivity profile of one-dimensional multi-layered media is proposed in this paper.Based on the equivalent network ofthe medium,a concept of time domain signal flow graph and its basic principles are introduced,from which the reflection coefficient of the medium in time domain can be shown to be a series ofDirac δ-functions(pulse responses).In terms of the pulse responses,we will reconstruct both thepermittivity and the thickness of each layer will accurately be reconstructed.Numerical examplesverify the applicability of this method     

同轴探头测量材料电磁参数变频率法的改进

本文针对我们实验室早期提出的用开口同轴探头变频率法同时测量雷达吸波材料的介电常数和磁导率技术^(6,7)在实用中遇到的问题,提出应用线性插值技术来修正电磁参数随频率的变化,使用拟合技术来克服测得反射系数的随机误差对电磁参数提取的影响,建立多频率点上电磁参数的矛盾方程方法来减少参数提取时所产生的伪根等改进技术,从而有效地克服原方法所存在的问题,使之能够用于实际测量。并通过对具体样品的测量证明了改进技术的合理性及有效性。     

Selective IR absorption in molecular nanofilms

We have formulated a microscopic theory of optical properties of ultrathin molecular films (nanofilms), i.e. quasi 2D systems parallel to XY planes bounded by two surfaces. Exposure of nanofilms to the external electromagnetic fields has result in creation of excitons - but different than bulk ones. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green’s functions. It has been shown that two types of optical excitations can occur: bulk and surface exciton states. Exciton energy dispersion law shows discrete behavior with non-zero values. Analysis of the dielectric properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. In addition, permittivity shows very narrow and discrete dependence of external electromagnetic field frequency, which is a consequence of both resonance and quantum size effects. Influences of boundary conditions on optical characteristics (through analyses of dynamical absorption coefficient) of these nanostructures were specially and in details explored.     

Structural and electrical properties of lead free ceramic: Ba(La1/2Nb1/2)O3

Abstract

Abstract

Lead free perovskite Ba(La_1/2Nb_1/2)O₃ was prepared by conventional ceramic fabrication technique at 1375°C for 7?h in air atmosphere. The crystal symmetry, space group and unit cell dimensions were estimated using Rietveld analysis. X-ray diffraction analysis indicated the formation of a single phase monoclinic structure with space group P2/m. Energy dispersive X-ray analysis and scanning electron microscopy studies were carried to study the quality and purity of the compound. Permittivity data showed low temperature coefficient of capacitance (T _CC = 11%) up to 100°C. The circuit model fittings were carried out using the impedance data to find the correlation between the response of real system and idealised model electrical circuit. Complex impedance analyses suggested the dielectric relaxation to be of non-Debye type. The correlated barrier hopping model was employed to successfully explain the mechanism of charge transport in Ba(La_1/2Nb_1/2)O₃. The AC conductivity data were used to evaluate the density of states at Fermi level, minimum hopping length and apparent activation energy.