The complex conductivity was measured on 3d granular NbC-KCI composite samples at varying metal volume fraction p, frequency ω and temperature above and below the superconductivity critical Tc. The observed frequency dispersion is anomalous in that it is not in accord with the scaling theory of percolation transition. The results are compared with a recently developed scaling theory, which takes both intercluster tunneling and intercluster capacitance into account. The experimental estimates for the new critical exponents are in reasonable agreement with the theory. The very low value of the crossover frequency can also be understood. We also present the data showing the dispersion of the complex conductivity well below the superconducting transition Tc of NbC.

The macroscopic electrical properties of carbon black filled polyethylene (PE)/polystyrene (PS) blends depend strongly on the selective localization of carbon black particles in one of the phases of the polyblend. The electrical and dielectric properties of these materials can be understood in the framework of the double percolation theory. It appears that in the vicinity of the carbon black percolation threshold, the electrical conduction is dominated by tunneling. The recent theory of Brouers and Sarychev which includes quantum tunneling in the percolation scaling laws provides the basis for a discussion of the observed frequency dependence of the conductivity and the dielectric constant in the range 10−3 – 106 Hz and its relation to the blend morphology.

Using a computational methodology, we studied average electrical properties, or, more specifically, d.c. conductivity and a.c. impedance spectra, of macroscopic mixtures of hard spheres numerically. Numerical results were compared with the calculated results of analytic equations which were generally used to describe the d.c. and a.c. electrical properties of composites. It is the purpose of this paper to provide information regarding to the geometrical arrangement effects. It was found that geometrical arrangement effects become more important as the sample size becomes smaller and the filling fraction closer to an effective percolation threshold.

The microstructure and electrical property relationships in conjugated polymer composites( polyaniline (PAn) or polypyrrole(PPy)/thiokol rubber (TR)) have been characterized by 4-probe method , scanning electron microscope (SEM), x-ray energy dispersive spectrometer (EDS) and attenuated total reflection(ATR). The electrical conductivities of the two surfaces of the film, with film thicknesses from 0.01mm to 0.30mm, prepared by electropolymerization , were measured . The differences were related to the thickness of the film. As for a similar PPy/TR film, when trichloroacetic acid was regarded as the supporting electrolyte the film thickness was 0.05mm and the electrical conductivity of the surface on the solution side was reduced by about 21.3%. The studies of SEM, x-ray EDS , ATR for the chemical composition and microstructure of the two surfaces of the composite films showed that a reaction layer was present in the films. Moreover , the conductivity changes between two sides of a fold mark in the conducting composite film is discussed.

Dielectric relaxation phenomena have contributions from both the internal reorientation modes of the molecules comprising a material as well as the details of how those modes couple together to form the material. This behavior can change quite sensitively with changes in the composition of material. For a composite material that has more than one phase, the presence of phase boundaries and even shape can play an additional and large role in the observed relaxation behavior. Analysis of the relaxation spectra can be used to study these effects. Examples are shown, primarily of polymeric materials, ranging from liquid solutions to phase separated mixtures to show how dielectric relaxation over a wide frequency range can be used to characterize and study composite materials.

Studies of the local segmental relaxation in rubbery networks reveal a variety of behaviors. In experiments on networks with labeled junctions, whereby the motion of the crosslink site is specifically monitored, the segmental relaxation function broadens, accompanied by a larger activation energy, in a manner well-described by the coupling model of relaxation. The more usual experiment simply measures bulk relaxation, without discriminating among different relaxing entities. For networks, crosslinking introduces a distribution of relaxation behaviors, related to the proximity of a moiety to the junctions. The resulting inhomogeneously broadened relaxation function is difficult to analyze; nevertheless, a heightened sensitivity to temperature (larger activation energy) is exhibited, from which inferences can be made regarding the shape of the relaxation function. Finally, the segmental relaxation of highly crosslinked microgels is ostensibly homogeneous. Interestingly, however, the inverse correlation between the stretch exponent, β, and the activation energy, observed quantitatively in conventional networks, is violated by the microgels.

Piezoelectric composites prepared by firing ceramic lead zirconate titanate (PZT) powders within a PZT sol gel have a two phase structure in which both phases have high permittivity. Dielectric measurements reflect the presence of the two phases and of interfacial layers between the phases and at the electrodes. The system has been modelled as a) a composite dielectric and b) a multi-layer capacitor with some complex components. The frequency dependent response of composites, sol gel derived films and a bulk ceramic is compared.

This paper reports the analytical prediction of effective transverse electrical permittivities of plain weave, twill weave, 4-harness irregular satin, as well as 5-harness and 8-harness satin fabric composites with lossy constituents. Analytical solutions of the effective properties are presented. Numerical examples are given to illustrate the effects of fiber volume fraction and fabric architecture on the composite effective transverse dielectric behavior. Key words: electrical permittivity, plain weave, twill weave, satin weave, fabric composite, analytical solution.

Cross-property relations that link rigorously the effective electrical conductivity (or dielectric constant) and the effective elastic moduli of two-phase, isotropic composite materials are discussed. The cross-property relations can be optimal in some cases, i.e., they are realized by particular microstructures. The relations are applied to specific two-phase composites as well as to cracked media.

The electrical and dielectric response upon thermal expansion and contraction of polymer composites have been studied for thermoset polymers containing ceramic or metallic fillers. Before curing, the resistivity p of epoxy containing a metallic filler is in the range of 106 to 107 Ωcm. During curing a sudden reduction of p by eight orders of magnitude is observed. The shrinkage of the polymer during cross-linking and the build-up of internal stresses results in a lower interparticle contact resistance. The contraction can be reversed by heating. Around curing temperature, the resistivity increases by six to eight orders of magnitude due to the expansion of the material. The resistivity increases progressively until the percolation network of the conducting particles is interrupted and the composite reaches a high-ohmic state. Surprisingly, the critical temperature is related to the curing temperature and not to the glass transition temperature, as would be generally expected. Changes of the interparticle gap size can also be observed in the dielectric constant ε. For conducting fillers covered by a thin oxide layer, ε decreases by several orders of magnitude after a certain expansion has been achieved. However, for poorly conducting or insulating fillers dielectric response becomes insensitive to the gap distance. Hence, the electrical or dielectric response of polymers containing metallic fillers can be used as a sensitive probe for the internal stress state and the corresponding micro-structure of the composite.

Dielectric properties of materials can give insight into the microstructure or concentrations of their constituents and can be determined by measuring the complex impedance of a capacitor formed by a set of electrodes with the material as the dielectric. Measuring frequencies between 1 MHz and 1 GHz appeared to be most informative for soil and concrete. A new dielectric sensor containing an integrated circuit that includes all analog and digital circuitry is described briefly. This sensor is used to demonstrate the potential of dielectric sensor technology for applied materials research in the field in two recently developed applications. One application concerns the determination of the state of hardening of young concrete. The relationship shown is between changes in the dielectric properties and changes in the structure for curing concrete. The other application is a dielectric probe for contaminated site investigation. It appeared to be possible to locate chemically polluted soil layers, such as with chlorinated solvents or oil, using small changes in their dielectric properties.

Water content, porosity, pore solution composition, resistivity and dielectric properties of hardened cement paste and mortar samples have been studied as a function of time and relative humidities. A model for the resistivity of cement based materials is presented and an interpretation of the dielectric properties is proposed.

We performed dielectric spectroscopy measurements to study dynamics of dielectrically active modes of nematic liquid crystals (LC) 5CB with polar molecules confined in silica porous glasses with average pore sizes of 1000 Å (volume fraction of pores 40%) and 100 Å (27%). In the nematic phase of bulk 5CB there is only one mechanism of dielectric relaxation: rotation of polar molecules around short molecular axis with relaxation time τ ∼10−8s. The spatial confinement and the existence of a highly developed interphase have a strong influence on dielectric properties of LC. We found that at temperatures about 30°C below the bulk melting point in both porous matrices the dielectric behavior of confined liquid crystal is very different from the behavior expected for solid state. The dielectric modes were not frozen in both porous matrices and we observed four well defined relaxational processes with relaxation times τ1 ∼ 10−1s, τ2 ∼10−5s, τ3 ∼10−8s (bulk like) and τ3 ∼10−9s. These four processes do not vanish even at temperatures corresponding to deep bulk isotropic phase. The relaxation time of the first process (slow) shows glass-like behavior in a wide temperature range below bulk melting point.

Impedance Spectroscopy is being used to study the humidity sensitivity of porous silica thin films. The films are processed via a colloidal sol-gel method which leaves some remnant potassium and sodium. Previous work on bulk porous silica samples processed by the same method showed that the dielectric properties and ac conductivity were very sensitive to changes in humidity. The aim of this work was to determine if the same dependencies could be found in the thin films. The capacitance, dielectric loss, and ac conductivity of the films were measured in a controlled environment chamber from 20–80% RH for frequencies ranging from 10Hz–10MHz. In addition to characterizing films with varying amounts of residual alkali ions obtained through leaching, we have also measured films that were surface doped with controlled amounts of KCl, LiCl, or NaCl. Relative humidity dependencies in the films are not as dramatic as in the bulk samples. The reasons for this behavior are not yet clear, but may be associated with the porosity, thickness, and surface area of the films.

Some features of electroinitiated potentiostatic formation of polymeric films on the cathode in aqueous acrylamide/formaldehyde/zinc chloride and the electrochemical behavior of polymer modified electrode Pt/film have been studied through their voltammetry and impedance characteristics. The mechanism and kinetics of electrochemical depolarizers have been examined on Pt and Pt/film cathodes. The diffusion character of depolarizers and facilitated Red/Ox reactions on the modified Pt electrode were established.