Interest in filled polymers has expanded in recent years as investigators have recognized the great flexibility allowed by these materials to suit particular properties such as electrical, mechanical, and/or coupling between these properties. This article describes the work undertaken to investigate the microwave response of two different types of samples: one with carbon black or silica particles embedded in a linear low-density polyethylene, and the other with carbon black particles or carbon fibers embedded in an epoxy resin. We report broad-band (30 MHz–14 GHz) measurements of the complex permittivity of these materials obtained by measuring the scattering parameters (S parameters) of a microstrip line loaded with a rectangular sample of the test material. The experimental results presented give access to data which can be rationalized in terms of a combination of Bruggeman’s self-consistent model with Jonscher’s phenomenological analysis. This analytical approach yields data that are in good correspondence with experimental data in terms of the concentration dependence of inclusions within the polymeric matrixes and demonstrates large practical capabilities for analyzing the electromagnetic properties of these materials at microwave frequencies because it allows one to make an explicit connection between these properties and the experimentally accessible parameters.

Dielectric and physicochemical properties of a composite material prepared by incorporating carbon black particles into a polymer matrix were investigated. Two types of carbon blacks, having very different structures of aggregates, were used. The volume fraction of the carbon blacks ranged from 0.2% to 7%, i.e. below and above the percolation threshold concentration observed from the measurements of dc conductivity. The composite samples were characterized in terms of: swelling by a compatible solvent, electron paramagnetic resonance (EPR) response, and frequency variation of permittivity. First, the article attempts to evaluate the diffusion coefficient of an appropriate solvent in these materials. Sorption kinetics experiments with toluene indicate that the initial uptake of solvent exhibits a square root dependence in time as a consequence of Fick’s law and permit to evaluate the effective diffusion coefficient in the range 10−11–10−12 m2 s−1 depending on the volume fraction of the carbon black in the sample. Second, the analysis of the carbon black concentration dependence of the intensity and linewidth of the EPR signals indicates that EPR is an important experimental probe of the structure of the elasticity network. The most notable feature of the present work is that we find a correlation of the percolation threshold concentration which is detected from the dc electrical conductivity with moments of the EPR lines. The conclusions on the elasticity networks deduced from swelling measurements are confirmed by EPR data carried out on swollen samples. On qualitative grounds the role of the specific surface of carbon black is further analyzed. It is suggested that the elasticity network is mainly controlled by secondary (respectively primary) aggregates for samples containing low (respectively high) specific surface carbon blacks. Last, the article reports precise experimental data on the permittivity of these composite materials as a function of frequency. Thanks to a sensitive measurement technique using an impedance analyzer, we are able to measure the complex permittivity and permeability values of the samples in the frequency range from 108 to 1010 Hz. It is found that the real part of the permittivity is a function of frequency f, via a power law expression ε′=af−b, where a and b are two parameters depending upon carbon black concentration, in the range of frequency investigated. The data analysis reaffirms the result that percolation threshold is a key parameter for characterizing the topological arrangement in these structures.

A one-port coaxial/cylindrical transition line is considered for the broadband complex permittivity measurement of civil engineering materials. Cylindrical samples of heterogeneous material with large aggregate dimensions (up to 25 mm) can be measured over a frequency range from 50 MHz to 1.6 GHz. The choice of this line technology results in the simplification of the sample machining and enhancement in the high frequency limit, in comparison to the classical coaxial line technology. From a mode-matching technique, the relation between the material complex permittivity and the reflection coefficient at the coaxial/cylindrical transition is obtained including axisymmetric higher order modes excited at the transition. Once the line is calibrated using a specific calibration kit, complex permittivities are retrieved from an iterative optimization procedure. Preliminary results obtained for a set of bituminous concrete samples with different porosities and natures of rock aggregates are shown.

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This paper demonstrates how micro-cracks at the surface of metals can be detected and imaged by near-field microwave techniques from the crack-induced variations of the resonance frequency and of the resonant circuit quality factor. It deals with two resonant sensors: a quarter wavelength microstrip line resonator terminated by an electric dipole and an original dual-behavior resonator (DBR) band-pass filter probe. The detection principle is developed, at first, from the use of the electric dipole probe. The low sensitivity of the electric dipole resonator led us to investigate whether first-order band-pass filters based on dual behavior resonators were able to detect a 200 µm wide and 3 mm deep rectangular EDM notch at the surface of a steel plate used to validate our method. Simulation data and measurements results carried out on a stainless steel mock-up with several 200 µm wide EDM rectangular notches showed that the DBR sensor is more sensitive than the electric dipole probe, and highlighted the link between the spatial resolution and the width of the high-frequency stub of the DBR filter. Moreover, we demonstrate the notch detection for any orientation of the defect in relation to the DBR sensor and the ability to differentiate between notches of different depths. Simulation data and measurement results are presented and discussed.

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