ISSN:0920-5071    

DOI:10.1163/156939392X00625

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The purpose of this paper is to review the multiscattering models for spherical particles, quasistatic mixture laws, effective medium theories, percolation equations, and finally the general effective medium equation (GEM), which are all used for modeling the dielectric properties of complex mixture materials. These are then used to analyse selected experimental results. Five different classes of composite materials, including carbon-resin mixtures (with various types of carbon) and a conducting polymer (in both air and water) have been selected. Comparisons between the experimental results and the theoretical fits obtained from the quasistatic laws (Looyenga, Kraszewski, Greffe, and Bottcher equations) and from the general effective medium (GEM) equation are then made. The GEM equation gives the best quantitative fit of the experimental data for these composite materials. This is almost certainly because the GEM equation is the only equation that can model the rapid changes in dielectric constants and conductivity that occur at and near the percolation threshold of the conducting particles in the dielectric matrix. Two parameters of the GEM equation, Vc or 0. (the percolation threshold) and t (an exponent) give some insight into the microstructures and the connectivity of the dispersed conducting particles.

ISSN:0920-5071    

DOI:10.1163/156939392X00634

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Artificial magnetodielectric (AMD) materials are macroscopic mixtures composed of reactive particles dispersed in a binder. Paint, magnetic recording tape and disks coatings are examples of AMD materials. A description of the calculational techniques for AMD mixture equations and the dipole moments of various particle shapes is provided. Some of the new applications of the AMD concept are discussed as well as a special method for complex particle shapes.

ISSN:0920-5071    

DOI:10.1163/156939392X00643

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Waterman and Truell [1] showed that the (complex) index of refraction of the effective medium is a simple function of the forward scattered amplitude of a single inclusion in the weak scattering regime. This amplitude can be determined in an exact manner for inclusions of simple form, so that one can compute, without difficulty, the real and imaginary parts of the effective index of refraction. These two functions do not generally obey, as they should, the Kramers-Kronig formulas. One can retain only the imaginary part (related to the attenuation) and determine the real part (related to the phase velocity) by means of one of the K-K formulas. This computation is carried out for a fibrous composite, submitted to both P and S polarized electromagnetic waves, over a wide range of frequencies. It is shown that the W-T formula provides predictions of the effective phase velocity that are consistent with the K-K relations only when the refractive index of the inclusions is larger than that of the host. Otherwise, consistency is only obtained at low frequencies and/or for very low inclusion concentrations.

ISSN:0920-5071    

DOI:10.1163/156939392X00652

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

In this paper we consider high frequency properties of composite materials. We use Monte Carlo simulations and Effective Medium Theory (EMT) to calculate the dielectric constant and the magnetic permeability. The EMT is a well-known approximation which is used for describing the conductivity, dielectric constant, and magnetic permeability of granular materials consisting of conducting and nonconducting components. Different modifications of this theory are considered when the skin layer depth is larger then the characteristic size of the conducting inclusions. In the microwave range it is necessary to take into account the internal inductance of the materials and presence of strong skin effect in conducting granules. In this paper we calculate the internal inductance of materials using a Monte Carlo simulation and then derive EMT equations in the presence of skin effects and the internal inductance. We obtain the dielectric constant for the materials with different shapes of conducting inclusions (spheres and sticks). The frequency dependence of the effective dielectric constant for the materials with conducting sticks is substantially different from those known earlier for composite materials. The theoretical results are in agreement with experimental results for the microwave dielectric constant of the polymer composites containing carbon fibers.

ISSN:0920-5071    

DOI:10.1163/156939392X00661

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

This paper focuses on the frequency dependence of the electromagnetic parameters of heterogeneous chiral materials. The material parameter of primary interest is the chirality parameter in the constitutive relations adopted (D = ∈Ē- j√0∈0H, B = H + j√0∈0Ē). A chiral Maxwell-Garnett mixing rule for spherical inclusions is used to determine the macroscopic composite chirality. To take into account the dispersion, the Condon model is used for the frequency dependence of the chiral component, which is embedded in the nonchiral background medium. All other material parameters are kept constant within this region of chiral dispersion. Illustrations show the effect of the mixing process on the characteristics of the chiral dispersion, both in the real and imaginary parts of . It is shown that due to mixing, the chirality will become spread to a broader frequency range, and acquire more structure in the dispersion curve. These changes are naturally functions of the parameters of the mixture.

ISSN:0920-5071    

DOI:10.1163/156939392X00670

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The dielectric function (DF) =1+ i2is a key property of condensed matter. Contact-free measurements of constitute a non-invasive method to characterize size-effects in mesoscopic matrix-isolated conductors with crystal sizes s in the range 10 nm < 8 10 m. At measuring frequencies = 2 well below the plasma frequencyP, the dynamic response is not determined by collective excitations of the electron system. We present dielectric data of phase-sensitive spectroscopy in the frequency range 5 Hz 10 GHz. The experiments yield the effective DF ē = ē(,M,∫) of the investigated heterostructures (particles and insulating matrix) which depends on the microstructure, the DF and the volume filling factor ∫ of the disperse component and on the DFMof the pure matrix. At constant filling factor, the size dependent electromagnetic response was studied in a series of experiments at various frequencies. In liquid-matrix colloids with low-melting metal particles, the size of the embedded conductors was varied within the mesoscopic range by means of in-situ thermal coalescence at filling factors ∫ 0.2. This procedure revealed the relationship3122. The size dependence of was quantitatively deduced from by means of an effective medium analysis which yielded1s2and2s3. The strongly size-governed DF (s) is a general property of all matrix-isolated metal particles investigated. In order to distinguish the effect of clustering from the response of isolated particles, the complementary microstructure, percolating nanocrystals (s~ 10 nm) with low filling factors ∫~ 0.01 , was studied as well. The DF of nanocrystal networks in air, where ∫ can be changed easily, is dominated by the interparticle contacts and strongly increases with ∫. This corresponds to the strong size dependence for isolated particles. Generally, the temperature dependence (T) for both structures is found to be conspicuously weak compared with the pronounced dependences of (s) for isolated and (f) for percolating particles. On the other hand, the frequency dependence () can be used to distinguish between colloids (where () is quite weak) and percolating networks where2()-1(i.e., the conductivity does not depend on ). This, however, does not necessarily imply Drude behavior as the value of2lies below the value expected for a dilute classical metal by several orders of magnitude. Moreover, the response of both colloids and networks is capacitive (i.e.,1> 0 ). For the time being, there is no full theoretical explanation of the non-metallic dielectric behavior observed in either microstructure. The experimental results demonstrate that can be varied in a wide range by adjusting the relevant parameters and thus suggest various possibilities to design artificial heterostructures with dielectric properties tailored for technical applications.

ISSN:0920-5071    

DOI:10.1163/156939392X00689

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A technique is presented for simultaneously measuring the complex values of the permittivity and permeability tensors of particular uniaxial homogeneous materials. A gap in a coaxial line is filled with the material being tested. The complex constitutive parameters are computed from the S parameter (S11and S21) measurements made on the gap taking into account higher order modes excited at the discontinuity. Some important cases concerning lossy dispersive materials are deduced, such as uniaxial gyroelectric or gyromagnetic materials, isotropic magnetic materials, and, finally, isotropic dielectric materials. Measured complex constitutive parameter data are presented from 45 MHz up to 18 GHz.

ISSN:0920-5071    

DOI:10.1163/156939392X00698

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

In this paper, a new method of measurement of resistivity of thin films and dielectric constant of slabs using open-ended coaxial lines is introduced. The model used in our analysis is an open-ended coaxial line with a large conducting flange terminated by layered materials to be measured. The reflection coefficient of the principal (TEM) mode is calculated, and it is found to be dependent on the thickness and the dielectric properties of the layered materials as expected. The admittance of the coaxial probe Y, is equivalent to a conductance G, for high conductive thin films, and it is also found to be proportional to the product of the thickness and the conductivity of the thin film. Each parameter may be readily calculated from the measured Y, values if one of them is known. The same method is used to find the dielectric constant of a single dielectric slab from the measured Y, values of the probe and the thickness of the slab. Experimental results of resistive thin films (several tens of ohms per square) measured over a wide frequency band (up to 20 GHz) are in agreement with commercial nominal values, and the proposed method of measurement and field analysis is thus justified. This wide-band method of measurement has the advantage of facilitating the preparation of the test samples.

ISSN:0920-5071    

DOI:10.1163/156939392X00706

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A method for measuring the complex permittivity for absorbing materials has been developed for temperature up to 1000° C. The test apparatus consists of an open ended flanged circular waveguide propagating TE11mode, which radiates into material under test placed in front of the flange. Electromagnetic analysis by modal method permits the calculation of the field in different zones of the structure. Complex permittivity is deduced by both theoretical and experimental reflection coefficient values. A solar source is used to realize the complex permittivity measurement as a function of temperature. Microwave measurements were made with different samples for different temperature laws. Reproducibility of thermal tests permits the validation of the method.

ISSN:0920-5071    

DOI:10.1163/156939392X00715

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor