用FDTD方法研究颗粒型复合材料微波等效介电常数

本文提出了复合材料等效电磁参数的FDTD计算方法,介绍了FDTD方法计算吸波材料等效电磁参数的步骤.通过对立方体模型、球形模型、连续模型和十字叉模型的仿真计算,讨论了颗粒形状和接触情况对等效介电常数的影响.通过计算不同体积分数的材料模型的等效电磁参数,得到了颗粒型复合材料等效介电常数的计算公式.通过实测复合材料的介电常数,证明了本文提出的公式是有效的.     

贵金属纳米颗粒的表面等离子共振研究

通过修正的Mie理论分别对单金属Ag、单金属Cu和Cu核Ag壳纳米颗粒/玻璃复合材料的吸收光谱进行了理论计算.计算结果表明,对单金属Ag纳米颗粒/玻璃复合材料,Ag的吸收峰位于425nm左右,不随颗粒尺寸变化而发生偏移;对单金属Cu纳米颗粒/玻璃复合材料,Cu的吸收峰也不随尺寸变化发生偏移但强度较弱;对Cu核Ag壳纳米...     

异质材料电磁特性及局域场增强效应研究

采用准静电模型仿真了两相异质材料的等效介电常数和局域场分布,结果表明,对于介质一介质型异质材料,三维模型等效介电常数仿真值与Bruggeman公式相符,并位于Hashin-Shtrikman上下界之间;异质材料内两相介电常数差异越大,其内局域场越强.对于由导电相和介质相构成的异质材料,仿真结果表明,异质材料内的电场分布受导电相填充比、形状和电导率等因素的影响;导电相导电性越好,其局域场增强现象越明显.     

电容式原油含水率仪信号解释模型

电容式原油含水率仪是基于原油与水介电常数的差异研制而成的,电容式含水率仪的信号解释不仅与电容结构有关,还涉及混合介质等效介电常数理论.本文以常见的同轴式电容传感器为例,详细分析介绍了信号解释数学模型的建立,总结了常用的混合介质等效介电常数模型及其适用范围.     

考虑界面效应的金属-绝缘体复合材料的电导模型

建立了含界面层的金属-绝缘体复合材料的有效电导模型。导出考虑"界面层"的金属-绝缘体复合材料的有效电导率的普适计算公式。并且考虑到金属粒子之间的相互影响,从二元无规混合物的两种拓扑结构出发,用自洽的方法修正了该公式。本文作者用该模型分析了银-酚醛塑料复合材料的有效电导率,与实验结果符合较好。      

碳纳米管/石英复合材料的电磁波吸收性能

采用溶胶-凝胶法合成了碳纳米管/石英复合粉体, 复合粉体经热压烧结获得致密的复合材料. 在8.2～2.4GHz波段测试了该复合材料的复介电常数, 发现复介电常数随着碳纳米管含量的增加而大幅度提高, 大的介电常数虚部说明该复合材料具有很大的介电损耗. 采用传输线理论计算了该复合材料对电磁波的反射损耗, 发现复合材料在此波段对电磁波具有吸收效果, 并且反射损耗 与复合材料的厚度、碳纳米管体积含量具有密切的关系. 本文还采用了层状设计的方法提高了复合材料的吸波性能.     

有耗频率选择表面蜂窝吸波复合材料的电磁性能

采用芳纶纸蜂窝与有耗频率选择表面复合设计兼具重量轻、强度高及吸波带宽宽的蜂窝吸波复合材料,研究了蜂窝吸波复合材料中蜂窝的厚度和有耗频率选择表面对结构电磁性能的影响。用等效电路法分析了传统孔径型方环有耗频率选择表面的电磁波吸收原理及该结构在宽频电磁吸收方面的缺陷。使用开槽的方法增大该结构等效电容的作用,在保持高频吸收性能基本不变的情况下增大了低频吸收峰,改进了结构的低频吸收性能,使吸波带宽展宽1倍以上。通过matlab计算出等效电容和等效电感,结果表明:方环形有耗频率选择表面的环宽度只改变等效电感的大小,等效电容保持不变;等效电感的大小主要影响高频吸收峰位,等效电容的大小主要影响低频吸收峰位。蜂窝厚度主要影响低频吸收峰值和高频吸收峰位。基于以上特性设计出具有宽频电磁波吸收性能、厚度为6 mm的蜂窝夹芯结构吸波材料:在6 mm蜂窝介质表面覆盖开槽方环形有耗频率选择表面,最下面为金属反射层,其-10 d B的电磁波吸收带宽达到14 GHz。测试结果与设计结果基本一致。     

Study on Inversion Permittivity of Composite Material Based on Plate-Reflection Method

本文采用平板反射方法,利用与无反射曲线具有同源性的条件选择曲线,探索出了一种反演等效复介电常数的新方法.研究表明,通过选取无反射曲线上的点进行的理论模拟,并对不同厚度样品吸收曲线进行模拟反演和实验值对比,结果相当吻合.本研究可为测试各种大尺寸、多孔材料、液体等样品等效介电常数提供了一种行之有效的方法,对内部多相大结构电磁功能复合材料的设计研究具有重要意义.     

颗粒填充二元复合材料等效介电特性的修正通用有效介质计算公式

介电特性在复合材料的电磁效应研究和材料设计中具有重要的作用。本工作在研究传统通用有效介质(GEM,General Effective Medium)公式的局限性基础上,提出了用于预测和计算颗粒填充二元复合材料等效介电特性的修正通用有效介质(MGEM,Modified General Effective Medium)公式。运用MC-FEM(Monte Carlo-Finite Element Method)方法分析计算各种参数条件下颗粒随机填充二元复合材料的等效介电特性,并与MGEM公式计算结果进行比较,验证MGEM公式的正确性和有效性。此外,还将MGEM的预测结果与部分经典理论公式的计算结果、部分文献报道的实验测量数据进行了比较。研究表明,在不同介电常数比(1/50~50)和不同体积分数(0~1)的情况下,MGEM公式预测结果与MC-FEM模型结果完全吻合,与实验测量结果基本一致,为颗粒填充二元复合材料等效介电性能分析提供了一种具有较高计算精度的理论计算方法。     

核壳粒子复合材料的等效磁导率

为了预测复合材料的等效磁导率,建立了填充核壳粒子复合材料等效磁导率的物理模型,应用电磁场理论推导了核壳粒子以单一介质球代替的等效方法并推广得到椭球核壳粒子情况.基于平均极化理论和Maxwell-Garnett理论给出了核壳粒子以特定浓度随机分布于复合材料时等效磁导率的预测公式,通过数值计算分析了核壳粒子浓度、结构参数以及磁导率对复合材料等效磁导率的影响.数值计算与实验结果吻合很好,验证了该等效方法用于复合材料电磁特性优化设计的有效性.     

Improved polymer nanocomposite dielectric breakdown performance through barium titanate to epoxy interface control

A composite approach to dielectric design has the potential to provide improved permittivity as well as high breakdown strength and thus afford greater electrical energy storage density. Interfacial coupling is an effective approach to improve the polymer-particle composite dielectric film resistance to charge flow and dielectric breakdown. A bi-functional interfacial coupling agent added to the inorganic oxide particles’ surface assists dispersion into the thermosetting epoxy polymer matrix and upon composite cure reacts covalently with the polymer matrix. The composite then retains the glass transition temperature of pure polymer, provides a reduced Maxwell-Wagner relaxation of the polymer-particle composite, and attains a reduced sensitivity to dielectric breakdown compared to particle epoxy composites that lack interfacial coupling between the composite filler and polymer matrix. Besides an improved permittivity, the breakdown strength and thus energy density of a covalent interface nanoparticle barium titanate in epoxy composite dielectric film, at a 5 vol.% particle concentration, was significantly improved compared to a pure polymer dielectric film. The interfacially bonded, dielectric composite film had a permittivity ∼6.3 and at a 30 μm thickness achieved a calculated energy density of 4.6 J/cm³.     

Resonance enhancement of piezoelectric,dielectric, and magnetoelectric characteristics of inhomogeneous multiferroics in alternating electric field

A resonance piezoelectric effect in a multilayer ceramic composite has been studied. Expressions for the effective dielectric permittivity and piezoelectric coefficient of the layered composite are obtained. It is shown that, in the vicinity of the piezoelectric resonance frequency, the effective response of the composite to an external action can be significantly enhanced. The behavior of the ferromagnet-piezoelectric composite is described with allowance for spatial oscillations of the electric and magnetic field. Transversal piezomodulus, magnetoelectric susceptibility, and dielectric permittivity of layered multiferroics are considered.     

Preparation and dielectric properties of a polyurethane elastomer filled with resol-derived ordered mesoporous carbon

This study deals with the preparation and dielectric properties of polyurethane (PU) elastomer films by resol-derived ordered mesoporous carbon (OMC) nanopowder incorporation in the PU polymer matrix. Resol-derived OMC with a 2D hexagonal mesoporous carbon framework is used as conducting fillers to achieve homogeneous dispersion and favorable interfacial interactions in the polymer matrix. The dielectric properties depend on the applied field frequency and the carbon filler weight fraction. The carbon fraction has little effect on the relative permittivity. The relative permittivity of all the PU-OMC composites increases with the decline of frequency. Incorporating a small amount of OMC into the PU polymer had no influence on the dielectric loss. Along with the increasing carbon fraction above the percolation threshold, dielectric loss of PU-OMC composites increases exponentially in the low frequency range. PU-0.75 wt% OMC composite possesses the best dielectric properties, and the obtained relative permittivity and dielectric loss at 1 kHz is 9.59 and 0.03018, respectively.     

Low-loss, high-permittivity composites made from graphene nanoribbons

A new composite material was prepared by incorporation of graphene nanoribbons into a dielectric host matrix. The composite possesses remarkably low loss at reasonably high permittivity values. By varying the content of the conductive filler, one can tune the loss and permittivity to desirable values over a wide range. The obtained data exemplifies how nanoscopic changes in the structure of conductive filler can affect macroscopic properties of composite material.     

Characterisation of proton irradiated Ba0.65Sr0.35TiO3/P(VDF-TrFE) ceramic–polymer composites

Barium strontium titanate/poly (vinylidene fluoride-trifluoroethylene)70/30 (Ba_0.65Sr_0.35TiO₃/P(VDF-TrFE) 70/30) composite with high dielectric permittivity was developed by integrating high dielectric permittivity ceramic powder with proton irradiated polymer matrix. The composite after irradiation behaves as a relaxor ferroelectric material and this behaviour is similar to that of irradiated P(VDF-TrFE) 70/30 co-polymer. Due to the irradiation, dielectric peaks broadened and moved towards the lower temperature, creating high relative permittivity values in a broad temperature range. Ba_0.65Sr_0.35TiO₃/P(VDF-TrFE) composite with 0.5 ceramic volume fraction with a dosage of 80 Mrad can reach a relative permittivity of 160 at room temperature (at 1 kHz), which is about 14 times higher than that of pure copolymer. Polarization-electric field hysteresis loops of composites are strongly depended on the ceramic powder volume fraction and the effects of irradiation is less apparent in composites with higher ceramic powder volume fraction.     

Temperature induced phase transition in nanocomposites based on low-density polyethylene matrix

The temperature dependence of the complex dielectric permittivity of composites based on a low-density polyethylene matrix containing dispersed nanoparticles of copper and zinc oxides has been studied. It is established that a phase transition takes place in the volume of a composite at a temperature of about 60°C. Exposure of the composite samples to a flux of 10-keV electrons leads to changes in the temperature dependence of the permittivity. Mechanisms are proposed that can explain the observed behavior.     

Dielectric property of Cu powder/polymer composites

Dielectric property of Cu/polymer thermoplastic composites was measured in high frequencies up to 1 GHz. Generally relative permittivity and dielectric loss of the composites increased as the increasing metal inclusion loading as the percolation theory predicts. The incorporation of the copper inclusion with surface antirust layer raised relative permittivity of the composite from 2.3 to 21.3 at the loading level of 39.3 vol. % at 500 MHz. When copper oxide layer was introduced to the filler surface, estimated increase of relative permittivity was ca. 25 %. Since metal composites with ordered structure would raise the relative permittivity of the composites, the cause of this increase in relative permittivity in the present study can be attributable to reduced compatibility of the filler surface and the polymer matrix which lowers randomness of particle distribution. On the other hand, dielectric loss of the composite with surface oxidized Cu powder was increased by ca. 50 % compared to that of the anti-rusted powder composite. This would be caused by skin effect that part of the induced current flows through the less conductive surface oxide layer.     

陶瓷型铁电复合物厚片介电系数的电场依赖性

建立起高压介电测试装置和相应的数据处理方法。研究了PZT/P (VDF-T rFE) 和PT/P (VDF- rFE) 0-3型铁电复合物厚片介电系数的电场依赖性。对于陶瓷体积分数U> 0.3 的复合物, 介电系数随测试场强的升高明显增大。在U< 0.1 时, 可用M axw ell-Garnet t 方程拟合试验结果。B ruggeman 方程适合于低电场(< 1MV/m ) 下复合物介电系数的预测。通过L z 的变化, 用Yamada 模型可拟合复合物介电系数随电场变化的关系, 说明L z 是由复合物的组分维数、粒子形状以及粒子间静电相互作用所决定的参数。      

Thermal hysteresis in the dielectric properties of composites based on transition metal oxide and sulfide nanoparticles stabilized in a low-density polyethylene matrix

We have studied the temperature dependence of the complex dielectric permittivity of composites based on a low-density polyethylene matrix containing dispersed nanoparticles of zinc oxide and manganese oxide and an analogous composite with nanoparticles of cadmium sulfide. Laws governing the electric conduction and dielectric polarization as functions of the temperature and the size of nanoparticles are established and a possible mechanism responsible for the appearance of thermal hysteresis of the dielectric properties in the materials studied is proposed.     

Simulation and analysis of the effective permittivity for two-phase composite medium

Based on the effective medium theory, we calculated the complex effective permittivity of the twophase composite medium, consisting of circular inclusions embedded in a surrounding host. Results show that the maximum dielectric loss of the composite medium can be obtained by tuning the dielectric properties of the inclusion. This was confirmed by the results in literature. The local high temperature, as a consequence of the local electric field enhancement phenomenon, is the main reason that microwave heating can lead to a dramatic increase in the chemical reaction rate.