Abstract-Mathematical modeling of composites made of a dielectric base and randomly oriented metal inclusions is considered. Different sources of frequency-dependent metal conductivity at optical frequencies are taken into account. These include the skin-effect, dimensional (length-size) resonance of metal particles, and the Drude model. Also, the mean free path of electrons in metals can be smaller than the characteristic sizes of nanoparticles, and this leads to the decrease in conductivity of the metal inclusions. These effects are incorporated in the Maxwell Garnett mixing formulation, and give degrees of freedom for forming desirable optical frequency characteristics of composite media containing conducting particles.
Abstract-Composites containing conducting inclusions are required in many engineering applications, especially, for the design of microwave shielding enclosures to ensure electromagnetic compatibility and electromagnetic immunity. Herein, multilayer shielding structures are studied, with both absorbing and reflecting composite layers. In this paper, fiber-filled composites are considered. For modeling absorbing composites with low concentration of conducting cylindrical inclusions (below the percolation threshold), the Maxwell Garnett theory is used. For reflecting layers, when concentration of inclusions is close to or above the percolation threshold, the McLachlan formulation is used. Frequency dependencies for an effective permittivity are approximated by the Debye curves using a curve-fitting procedure, in particular, a genetic algorithm.
Electromagnetic and microwave absorption properties of ( Co 2 + -Si 4 + ) substituted barium hexaferrites and its polymer composite Microstructural and high-frequency magnetic characteristics of W -type barium ferrites doped with V 2 O 5The microwave permittivity and permeability of Co 2 Z barium ferrite composite samples are measured as functions of frequency and volume fraction of the ferrite. Magnetostatic properties of the bulk ferrite are determined. This allows Snoek's law [J. L. Snoek, Physica 14, 204 (1948)] to be verified by comparing the microwave and magnetostatic Snoek's constants. The modification of Snoek's law for hexagonal ferrites suggested recently by Acher et al. [Phys. Rev. B 62, 11324 (2000)] is also verified. Acher's constant is found from microwave measurements to agree with the value calculated from the magnetostatic properties of bulk ferrite, but microwave and magnetostatic Snoek's constant do not agree. This may be attributed to the effect due to demagnetizing factors of ferrite inclusions that are not considered in the derivation of Snoek's and Acher's laws. The measured frequency-dependent permeability of composites satisfies the Lorentzian dispersion law and is consistent with the Maxwell Garnett approximation [J. C. Maxwell Garnett, Philos. Trans. R. Soc. London 203, 385 (1904)]. According to the theoretical analysis based on the Lorentzian dispersion law and the Maxwell Garnet mixing rule, both Snoek's and Acher's constants must be linear functions of the volume fraction, independent of whether microwave values of the constants are in agreement with the magnetostatic values. In contrast, the experimental measurements reveal a steady decrease of both constants with the volume fraction. The disagreement is discussed in terms of the influence of effective medium in composite on the inherent permeability of ferrite particles.
An analytical model based on an equivalent capacitance circuit for expressing a static effective permittivity of a composite dielectric with complex-shaped inclusions is presented. The dielectric constant of 0-3 composites is investigated using this model. The geometry of the capacitor containing a composite dielectric is discretized into partial parallel-plate capacitor elements, and the effective permittivity of the composite is obtained from the equivalent capacitance of the structure. First, an individual cell diphasic dielectric ͑a high-permittivity spherical inclusion enclosed in a lower permittivity parallelepiped͒ is considered. The capacitance of this cell is modeled as a function of an inclusion radius/volume fraction. The proposed approach is extended over a periodic three-dimensional structure comprised of multiple individual cells. The results of modeling are compared with results obtained using different effective medium theories, including Maxwell Garnett, logarithmic, Bruggeman, series, and parallel mixing rules. It is found that the model predictions are in good agreement with the experimental data. The equivalent capacitance model may be applied to composites containing inclusions of any geometry and size. Although the method presented is at static electric field, it can be easily generalized for prediction of frequency-dependent effective permittivity.
Frequency-selective measurement of microwave signal parameters based on application of gyromagnetic converters has proven advantageous for the research of microwave radiation over a wide spectrum (several octaves) in multisignal regime in microwave path and for the solution of a number of electromagnetic compatibility (EMC) problems. The measurement frequency band can be enlarged to millimeter-waves with application of monocrystal hexagonal ferrite resonators (HFR) having high internal magnetic fields. Millimeter-wave field interactions with the HFR having alternating resonance frequency are analyzed. This is useful for millimeter-wave signal modulation and demodulation. The analysis is based on the solution of magnetization vector motion equation of the uniaxial spherical HFR with time-varying bias magnetic field or angle of the HFR orientation (for modulation problem) and with amplitude-modulated microwave signal action (for demodulation problem). The novel principle of the HFR frequency-selective measuring system based on automodulation design is discussed.
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