Davide Micheli

Broadband Electromagnetic Absorbers Using Carbon Nanostructure-Based Composites

In this paper, we present the design of nanostructured multilayer absorbers, carried out with the aid of a genetic algorithm (GA). Waveguide measurements are performed to recover the dielectric properties of micrographite single-walled carbon nanotube, micrographite walled carbon nanotube, carbon nanofiber, and fullerene-based composite materials. Conductive fillers are uniformly dispersed in an epoxy resin at different weight percentages (1, 3, 5 wt.%). The electromagnetic (EM) analysis is performed embedding the forward/backward propagation matrix formalism in an in-house GA, thus able to carry out optimization upon oblique incidence over a finite angular range. Developed code minimizes both the reflection and the transmission coefficients under the thickness minimization constraint. Comparison between micrographite and nanopowders absorbers is presented and discussed, when a broadband quasi-perfect absorber is achieved among the X-band combining the two filler families, i.e., exhibiting a loss factor greater than 90% in most of the band, for a thickness of about 1 cm. It is demonstrated that the nanofillers with higher aspect ratio mainly contribute to the EM absorption. Findings are of interest in both radar-absorbing material and shielding structures.

Optimization of Multilayer Shields Made of Composite Nanostructured Materials

In this paper, we propose a multilayer nanostructured composite for broadband shielding applications. Layers disposal, electrical parameters, and thicknesses are optimized through a winning particle optimization algorithm to achieve the minimization of the transmitted waves. The structures are simulated by including the forward/backward scattering matrix formalism in the optimization code. The adopted algorithm is the recently introduced winning particle optimization. Manufacturing of the composites is grounded on the optimization procedure. Thanks to the macroscopic absorption features of such nanostructured layers, very thin and lightweight composites can be produced. Several weight percentages of multiwall carbon nanotubes are considered in composite base material manufacturing, also including 6wt% and 15wt% in order to enhance the electromagnetic shielding performance. Prototypes are tested in the microwave region, showing the reliability of the optimization procedure.

Tunable nanostructured composite with built-in metallic wire-grid electrode

In this paper, the authors report an experimental demonstration of microwave reflection tuning in carbon nanostructure-based composites by means of an external voltage supplied to the material. DC bias voltages are imparted through a metal wire-grid. The magnitude of the reflection coefficient is measured upon oblique plane-wave incidence. Increasing the bias from 13 to 700 V results in a lowering of ∼20 dB, and a “blueshift” of ∼600 MHz of the material absorption resonance. Observed phenomena are ascribed to a change of the dielectric response of the carbon material. Inherently, the physical role of tunneling between nanofillers (carbon nanotubes) is discussed. Achievements aim at the realization of a tunable absorber. There are similar studies in literature that focus on tunable metamaterials operating at either optical or THz wavelengths.

Matter’s Electromagnetic Signature Reproduction by Graded-Dielectric Multilayer Assembly

A lot of effort has been devoted in the last decades by technology research to realizing materials with a priori defined electromagnetic (EM) properties. One of the challenges at present is to configure the reflection coefficient (RC) of a structure so that any shape of a fixed microwave response is followed. A method for realizing microwave absorbers made by carbon nanocomposite layers assembly able to mimic a given reflection profile is described and experimentally validated. The multilayer design (layer sequence, material, and thickness) is pursued by means of a customized numerical optimization algorithm, which allows to get the required microwave behavior. The novelty of the research is the possibility of tuning the EM field propagation through the combination of different materials in a specific layered compound, in order to imitate the response of any “real” object (i.e., with known EM properties). For the experimental validation of the process, three multilayered structures were designed and manufactured, and their microwave RC was measured in the frequency range of 2–18 GHz. The comparison with the related targets (an ideal frequency selective pattern and the defined profiles of dry soil and salt water as retrieved from literature survey) highlights the effective simulating capability of the realized structures. The preliminary results suggest to exploit the graded-dielectric properties provided by carbon-based nanocomposites for EM mimicking purposes: this would be an ideal approach to tackle still unsolved issues in EM compatibility, remote sensing, communication, and safety fields, as well as for low-cost and time-saving metrology applications.

Modeling of microwave absorbing structure using winning particle optimization applied on electrically conductive nanostructured composite material

This work presents the design and optimization of a Radar Absorbing Material system in the X-band frequency using evolutionary algorithm. Winning Particle Optimization is a new evolutionary algorithm. Due to its elementary evolving mechanism, it recall in mind primordial life form in trying to search the best place to proliferate. It is shown that such method, is quite simple but at the same time very effective in finding an optimal solution in Radar Absorbing Material design and optimization problems. Radar Absorbing Material optimized is mainly a multilayer structure based on carbon nanomaterials like Carbon-Nanotube, Fullerene, Micrographite, Carbon-Nanofiber. The design and optimization process consists in find the best multilayer structure in terms of lowest thickness and simultaneously lowest electromagnetic reflection coefficient within all the frequency range and for several incidence angles. In order to validate the design procedure a simple multilayer structure has been built and tested using NRL arch method, the close behavior between simulated and measured RAM confirmed the validity of design procedure.

Ballistic characterization of nanocomposite materials by means of “Coil Gun” electromagnetic accelerator

In this paper the authors present their activity in the field of electromagnetic machine applications for aerospace solutions. A three stage electromagnetic accelerator is under construction to perform ballistic characterization of carbon-based nanocomposite materials for anti-debris application. Preliminary experiments as well as numerical simulation have been performed with promising results in terms of bullets energy. Further implementation are needed in order to come closer the velocity of typical space debris (8km/s).

Advanced concrete materials for EMI reduction in protected environment and IEMI threats suppression

The enhancement of microwave shielding effectiveness of commercial concrete by the inclusion of carbon nanotubes powder is addressed and experimental testified. A microwave characterization is performed by direct measurements of materials dielectric parameters in the frequency range 1.7-2.6 GHz. A significant lowering of the microwave transmission magnitude is founded for the nanoreinforced material respect to the naked concrete. The results allow to evaluate the microwave shielding capability of wallshaped concrete structures: a shielding effectiveness grater than 50 dB is achieved for a 30 cm thick wall with carbon nanotube filling percentage of 3wt%. The route of nanoparticles filling within the composite mixture is straightforwardly included in the concrete typical on-site manufacturing procedures, thus planning out a time/cost saving procedure with the final aim to promote such typology of materials for commercial purpose in the next future. The here reported preliminary findings pave the way for the employment of carbon nano-powder reinforced concrete in building walls, in order to deal with issues related to the electromagnetic interference mitigation, such as the influence on the medical devices working in hospital environments as well as the increasing of protection against electromagnetic attacks to strategic targets and sensible places.

Stochastic differential equation for wave diffusion in random media

In this work, we present a statistical analysis of the wave motion through random media with perfect spatial disorder of inclusions. It is assumed that such a disorder can be tackled with the random potential function theory, whence the propagation of waves naturally turns to a diffusion process. The associated Itoô drift-diffusion process, and its Fokker-Planck equation are derived. It is found that the “ensemble” wave, i.e., the collective wave motion, fluctuates in space as a geometric Brownian motion. Finally, the effect of a double-well potential with random (vibrating) valleys is studied qualitatively by the Monte Carlo method. In practice, this situation occurs for high concentration and perfect dispersion of conductive/dielectric fillers, i.e., whose location and orientation are completely randomized.

Electromagnetic Characterization of Materials by Vector Network Analyzer Experimental Setup

Abstract This chapter deals with the vector network analyzer systems used for the study of the electromagnetic properties of materials. The focus of the chapter is not the device itself, about which plenty of literature is available, but its application in materials characterization at the microwave and millimeter wave levels. Some interesting measurement techniques are presented and discussed in detail with the help of numerous experimental results. Moreover, the measurement criteria are commented on as a function of the materials under test. The waveguide, coaxial air line, Naval Research Laboratory Arch bistatic system, free-space, and reverberation chamber methods are presented and analyzed. Examples of measurements of conventional materials used in architectural building up to advanced foam and nanocomposite materials considered in defense and aerospace applications are presented. The authors have tried to transfer their wide laboratory expertise in this chapter, with the aim to be useful to other researchers in the field of electromagnetic characterization of materials.

Shell absorbing nanostructure for low radar observable missile

This research is focused on simulation, manufacturing and measuring of shell radar absorbing structure of missiles. The novelty of the work is the study of a curved radar absorbing structure. The enhancement of electromagnetic wave absorption is obtained by using carbon nanotube filler in different weight ratio with respect to the epoxy-resin adopted in shell manufacturing. The structural resistance is granted by the use of conventional fiberglass. A radar absorbing prototype of an half shell, having the section of 15 cm radius has been built and characterized. The thickness of the shell is around 6.5 mm and is made of two different loaded layers. The measurements of electromagnetic reflection coefficient has been performed for two different incidence angles of 0° and 45°. The reflection coefficient show values down to -18 dB around 3 GHz and -10 dB around 11 GHz for 0° incidence angle, and -6 dB around 3 GHz and -10 dB around 12 GHz for 45° incidence angle. An electromagnetic simulation of a flat structure having the same layering configuration of the shell shows values of reflection coefficient very similar to the measured one for 0° incidence angle.