RECENT ADVANCES IN THE CHARACTERIZATION OF COMPOSITE DIELECTRIC STRUCTURES

The central problem in predicting the dielectric behavior of heterogeneous materials (like, e.g., composite or nanostructured systems, powders or mixtures) consists in the evaluation of their effective macroscopic properties, still taking into account the actual microscale material features. This leads to the concept of homogenization, a coarse graining approach addressed to determine the relationship between the microstructure and the effective behavior: the prediction of the effective electromagnetic properties of a composite material from those of its constituent material phases is the major objective of various homogenization models. The resulting effective properties can be observed at the macroscale, where the refined effects of the morphology cannot be directly measured. Dispersions of particles (inclusions with a given shape and a given volume) in a host homogeneous matrix are the most studied heterogeneous structures. From the historical point of view, early mixture theories generally work well when the volumetric proportion of the inclusion phase is small and when the contrast between the electromagnetic properties of the two material phases is not large. More recently, refined and improved models have been developed in order to yield better predictions, also in these critical situations. Recent increases in activity in the field are, at least, partially caused by the interest in selective absorbers of solar and infrared radiation, by an increasing number of applications in astronomy and atmospheric physics, by several applications in the design of novel materials in optics and in material science, and by the indications that the electromagnetic behavior of the composite system may be very different from the behavior of individual components. a E-mail address: stefgiord14@libero.it, stefano.giordano@dsf.unica. it. Web: http://www.giordanostefano. it. b E-mail address: pierluca.palla@dsf.unica. it Stefano Giordano and Pier Luca Palla 574 These approaches can be applied not only to the static (d.c.) electric and magnetic properties, but also to the case of wave propagation in low frequency (l.f.) or high frequency (h.f.) regime. It is in fact well known that when any form of energy propagates through a medium containing scatterers (particles), the entrained energy will be either redistributed in various directions by scattering or absorbed by intrinsic absorption mechanisms. The standard homogenization theories can be applied also in such cases provided that the wavelength of the propagating field is much larger than the average size of the particles. Recently, these methodologies have been applied to light scattering from coatings, to heat transfer in powder insulators, to chemical and nuclear reactors, to cryogenic insulation and, finally, to microwave or laser coatings. In all heterogeneous or composite materials, the nonlinear regime and the anisotropic character have not yet been investigated thoroughly. Nevertheless, both the nonlinear and the anisotropic features are relevant in many materials science problems (crystal optics, optical bistability and optical devices). Therefore, we devote this work to the development of some analytical models able to take into account the refined effects of the nonlinearity and the anisotropy of the constituents.