Composite materials in electromagnetism - Characterization

The aim of this article is to familiarize the reader with various techniques for measuring and characterizing volumetric or two-dimensional composite materials.

Numerous methods are available, as described at the beginning of this article. Next, we present a few examples of free-space methods. Indeed, these are often preferred to coaxial, cavity, single-horn interferometry or open-ended coaxial probe techniques for the following reasons:

• ceramics, composites and metamaterial structures are inhomogeneous materials due to their very manufacturing process. In waveguides, coaxials and cavities, modes of higher order than the fundamental mode can be excited, and must be taken into account in the calculation;

• due to their intrinsic heterogeneity, small samples of composite material are not always representative of the whole material. They may exhibit significant dispersions, completely changing the reflection and transmission properties, as well as the intrinsic properties of the media. An important factor to consider is the size of the inclusions (powder, fiber, periodic pattern) in relation to the wavelength, as well as the homogeneity of the mixture (spatial and/or volume distribution);

• free-space methods are non-destructive and non-contact. They are ideally suited to measurements under variable incidence and relatively high temperatures;

• with free-space techniques, broadband characterization over a wide range of incident angles, polarizations and temperature conditions is possible on isotropic, anisotropic or bi-anisotropic materials. Nevertheless, waveguide measurements are often necessary at low frequencies, provided that the size of the inclusions is small compared with the wavelength.

A first general remark, however: more and more often, measurements are correlated with simulations that are as complete as possible, and the comparison between the two approaches then sheds important light on the real properties of the material or structure, beyond the dispersion due to manufacturing processes or uncertainties linked to the measurement method.

A second point to note is that the measurement methods presented here are global methods for accessing effective material properties. However, they can also be coupled with methods for analyzing the microscopic structure of the same material, particularly in the case of heterogeneous composite materials.

This article completes a series of articles on composite materials in electromagnetics: