Model and experimental investigations of electrodynamic properties of tunable structures from liquid-impregnated porous magnetic media are presented. Microwave spectra of complex permittivity and permeability of dry and liquid- impregnated media with ethyl alcohol or water-alcohol mixtures were measured. Real and imaginary parts of permittivity were found to exceed the upper Wiener boundary. Dielectric and magnetic properties of liquid-impregnated porous ferrite media were modeled using non-local volume averaging theory (VAT). This approach accounts for several hierarchical levels and morphology of the system.
The paper investigates absorption of electromagnetic waves by radio engineering structures, filled with liquid dielectrics, in which characteristic size of heterogeneities is much less compared to the wavelength. These structures can be treated as composite materials, possessing effective dielectric constant. Their electrodynamic properties can be computed using equations, derived for composites. An approximate computation method is offered, allowing to compute electrodynamic properties of the structure in analytical form, using equations for the effective dielectric constant of laminated composites. Computed dependencies of electrodynamic properties as a function of structure geometry and dielectric properties of liquids are presented in explicit form, making it easier for analysis, compared to numerical results, obtained by rigorous electrodynamic methods. For a wide range of structures effective dielectric constants and reflection coefficients for the case `absorbing structure on metal' have been computed. A good agreement of computed and experimental results has been observed.
Tunable electromagnetic wave absorbers for the microwave range comprise polymer-liquid composites, made up of structures of definite shape from low dielectric constant polymer dielectric, filled with polar liquids. Tuning of reflection coefficient is performed in several ways: filling or removal of liquid from the structure, changing composition or temperature of liquid, etc. Effective absorption of electromagnetic waves by these structures in a wide range of frequencies is realized in case frequency dependence of dielectric constant of the liquid is such that for the whole range the condition of resonance absorption is fulfilled. The advantages of polar liquid dielectrics as absorbing media are demonstrated. Absorbing properties of structures with polar liquids and solutions of different salts are compared. Reflection coefficients for several structures of `dielectric on metal' type were measured.
Carbon fiber-reinforced plastics (CFRP), as high strength advanced materials are often used as media for embedding sensors and actuators. Due to the properties of components and processing conditions they are electrically anisotropic, with coefficient of anisotropy sometimes exceeding several thousands. This may prevent elimination of static electricity and cause erosion of material due to micro discharges at contacts with fastenings and embedded sensors and actuators, causing their malfunction. For this reason, the investigation of electrical properties of CFRP may provide the solution to this problem. Distribution of electric current field in CFRP and related with it possible errors in measurements of longitudinal conductivity and anisotropy are analyzed. CFRP have been prepared from PAN or cellulose fibers with different heat treatment temperatures and conductivity anisotropy was measured as a function of filler volume fraction and processing conditions. With increasing loading coefficient of anisotropy (alpha) decreases. Lower values of (alpha) were observed when curing agents containing ionic complexes of metals were used. Modifications of fiber surface with hydrophobic agents results in increased anisotropy. Composites prepared with carbon fabrics are isotropic in the fabric plane. Coefficient of anisotropy decreases with increasing molding pressure and depends on the type of weaving of fabric. In hybrid composites with alternating layers of carbon fabric and complex fiber fabric anisotropy is higher due to partial decomposition of conducting layer on top of complex fibers. A method for reducing anisotropy by introducing conducting `jumpers', shorting individual fibers or layers of fabric is proposed. The change of anisotropy in the process of fabrication of carbon-carbon composite by passing electric current through fibers has been investigated. In conclusion, alternative uses of CFRP with reduced anisotropy for contact elements of electric current through fibers has been investigated. In conclusion, alternative uses of CFRP with reduced anisotropy for contact elements of electric machines and geological prospecting as imitations of rocks are discussed.
Electrophysical properties of highly filled fiber polymer composites, containing disperse conducting or ferromagnetic fillers, are examined. The methods for controlling their electrical, magnetic, and other properties by changing the ingredients and compounding methods are reported. Some examples of using polymer composites in the form of functional elements or structures, prospective for different fields of science and technology, are presented.
Polymer composite material, containing conducting filler particles (e.g., graphite) at concentrations above the percolation threshold should, as a conductor, exhibit certain magnetic-field-sensitive properties, such as Hall effect or magnetoresistance. These effects not only allow us to classify them as smart materials, but also help in establishing the mechanism of conduction in such complex disordered systems. The present paper reports the results of investigations on magnetoresistance in composite materials, prepared by direct polymerization of propylene on graphite particles. The method ensures grafting of polymer to the filler surface, imparting remarkable properties to the composite. For composites with graphite magnetoresistance is positive for filler volume fraction above percolation threshold. Maximum value of magnetoresistance exceeds 10%. Below the threshold the samples show weak negative magnetoresistance, characteristic of conduction through localized states in disordered systems. This result correlates with our model of double percolation over filler particles, surrounded by thin layers of injected charge. New magnetic effects have been found, such as oscillations of dark conductivity in magnetic field and resonance magnetic-spin effects.
Composite materials were synthesized by direct polymerization of propylene on the surface of natural or synthetic graphite particles. The method ensures grafting of polymer to a part of filler particle surface, while the other part remains open to physical contact between the particles, the resulting properties of the material becoming favorably different from previously known composites. Coefficient of strain sensitivity K equals (Delta) R/R(epsilon) (R is resistance of the sample, (epsilon) is tensile strain) was measured at different concentrations of filler and different temperatures. There is a broad maximum of K around the percolation threshold (4.5 vol.% for natural graphite) with a peak value of 100 - 150, which is much higher, compared to conventional wire resistors. A slight hysteresis is observed at unloading due to plasticity of the matrix. Hysteresis disappears, when temperature is lowered by 20 - 50 degrees, or (epsilon) is less than 1%, but previously high value of K remains. Below the glass transition temperature K is very low. The results are explained by the change of current- carrying chains in loading-unloading cycles. Temperature dependence of resistance is presented, thermal conductivities were calculated for different models and compared with experimental values.
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