Christian Brosseau

A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles

Carbon (C) is a crucial material for many branches of modern technology. A growing number of demanding applications in electronics and telecommunications rely on the unique properties of C allotropes. The need for microwave absorbers and radar-absorbing materials is ever growing in military applications (reduction of radar signature of aircraft, ships, tanks, and targets) as well as in civilian applications (reduction of electromagnetic interference among components and circuits, reduction of the back-radiation of microstrip radiators). Whatever the application for which the absorber is intended, weight reduction and optimization of the operating bandwidth are two important issues. A composite absorber that uses carbonaceous particles in combination with a polymer matrix offers a large flexibility for design and properties control, as the composite can be tuned and optimized via changes in both the carbonaceous inclusions (C black, C nanotube, C fiber, graphene) and the embedding matrix (rubber, thermoplastic). This paper offers a perspective on the experimental efforts toward the development of microwave absorbers composed of carbonaceous inclusions in a polymer matrix. The absorption properties of such composites can be tailored through changes in geometry, composition, morphology, and volume fraction of the filler particles. Polymercomposites filled with carbonaceous particles provide a versatile system to probe physical properties at the nanoscale of fundamental interest and of relevance to a wide range of potential applications that span radar absorption, electromagnetic protection from natural phenomena (lightning), shielding for particle accelerators in nuclear physics, nuclear electromagnetic pulse protection, electromagnetic compatibility for electronic devices, high-intensity radiated field protection, anechoic chambers, and human exposure mitigation. Carbonaceous particles are also relevant to future applications that require environmentally benign and mechanically flexible materials.

Optical image compression and encryption methods

Over the years extensive studies have been carried out to apply coherent optics methods in real-time communications and image transmission. This is especially true when a large amount of information needs to be processed, e.g., in high-resolution imaging. The recent progress in data-processing networks and communication systems has considerably increased the capacity of information exchange. However, the transmitted data can be intercepted by nonauthorized people. This explains why considerable effort is being devoted at the current time to data encryption and secure transmission. In addition, only a small part of the overall information is really useful for many applications. Consequently, applications can tolerate information compression that requires important processing when the transmission bit rate is taken into account. To enable efficient and secure information exchange, it is often necessary to reduce the amount of transmitted information. In this context, much work has been undertaken using the principle of coherent optics filtering for selecting relevant information and encrypting it. Compression and encryption operations are often carried out separately, although they are strongly related and can influence each other. Optical processing methodologies, based on filtering, are described that are applicable to transmission and/or data storage. Finally, the advantages and limitations of a set of optical compression and encryption methods are discussed.

Effective dielectric constant of periodic composite materials

We presentcomputer simulation data for the effective permittivity ~in the quasistatic limit ! of a system composed of discrete inhomogeneities of permittivity e1, embedded in a three-dimensional homogeneous matrix of permittivity e2. The primary purpose of this paper is to study the related issue of the effect of the geometric shape of the components on the dielectric properties of the medium. The secondary purpose is to analyse how the spatial arrangement in these two-phase materials affects the effective permittivity. The structures considered are periodic lattices of inhomogeneities. The numerical method proceeds by an algorithm based upon the resolution of boundary integral equations. Finally, we compare the prediction of our numerical simulation with the effective medium approach and with results of previous analytical works and numerical experiments.

Microwave characterization of filled polymers

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 correspo...

Electromagnetic and magnetic properties of multicomponent metal oxides heterostructures: Nanometer versus micrometer-sized particles

We have measured the composition and frequency-dependent complex effective permittivities and permeabilities in zero applied field of a series of ZnO and ferrimagnetic γ-Fe2 O3 composites prepared by powder pressing. The overall features of the room temperature electromagnetic properties of these diluted magnetic semiconductor composites exhibit a strong dependence on the powder size of the starting materials. For instance, electromagnetic spectroscopy over the frequency range (300 MHz–10 GHz) shows that composites made of nanoparticles (N-type samples) display a strong increase of the real and imaginary parts of the permeability compared to composites made of micron-sized particles (M-type samples). The observed dielectric behavior as a function of composition is manifestly at odds with the predictions from the simple property-averaging continuum model of Bruggeman. Additionally, a gyromagnetic resonance in the gigahertz region of frequency has been established for N-type samples which is not observable ...

Finite-element modeling method for the prediction of the complex effective permittivity of two-phase random statistically isotropic heterostructures

This article is devoted to the study of the complex permittivity of two-dimensional two-phase statistically isotropic heterostructures on a small scale such that the quasistatic limit is applicable. Even though several analytical approximation techniques have been developed in the past, today it is desirable to be able to simulate these media via computer, which necessitates the development of efficient numerical techniques for the solution of the resulting equations. The simulation data concern the effective permittivity of continuum composites consisting of distributions of hard disks of a dielectric phase randomly dispersed in a continuous matrix of another dielectric phase. The three-dimensional equivalent of this model would be a composite with cylindrical symmetry, i.e., all interfaces are parallel to a fixed direction. The two constituents are assumed to be isotropic and homogeneous materials with scalar permittivities. Ab initio calculations are accomplished self-consistently with a computer code....

Exploiting root-mean-square time-frequency structure for multiple-image optical compression and encryption.

We report on an algorithm to compress and encrypt simultaneously multiple images (target images). This method, which is based upon a specific spectral multiplexing (fusion without overlapping) of the multiple images, aims to achieve a single encrypted image, at the output plane of our system, that contains all information needed to reconstruct the target images. For that purpose, we divide the Fourier plane of the image to transmit into two types of area, i.e., specific and common areas to each target image. A segmentation criterion taking into account the rms duration of each target image spectrum is proposed. This approach, which consists of merging the input target images together (in the Fourier plane) allows us to reduce the information to be stored and/or transmitted (compression) and induce noise on the output image (encryption). To achieve a good encryption level, a first key image (containing biometric information and providing the intellectual property of the target images) is used. A second encryption key is inserted in the Fourier plane to ensure a relevant phase distribution of the different merged spectra. We also discuss how the encoding information can be optimized by minimizing the number of bits required to encode each pixel.

Generalized effective medium theory and dielectric relaxation in particle-filled polymeric resins

Dielectric relaxation in disordered solids continue to be in the focus due to the important technological applications in the context of microwave and optical remote sensing and communication. The pragmatic philosophy of the present article is to use a combination of Jonscher’s phenomenological equations with a generalized effective medium equation, due to McLachlan, to study the microwave relaxation dynamics in a technologically interesting system, i.e., a particle-filled polymeric resin. The introduction of a small number of parameters (critical exponents s and t, conductivity threshold φc) into the standard Bruggeman effective medium equation dramatically improves its predictive power. This approach, termed the McLachlan–Jonscher model, has the potential to be quite flexible and is very sensitive to the values of the critical exponents s, t and of the conductivity threshold φc. Furthermore, a comparison of the calculated complex effective permittivity for a series of carbon black-filled polymers with e...

Effective magnetic permeability of Ni and Co micro- and nanoparticles embedded in a ZnO matrix

Current trends in the miniaturization of microwave devices have prompted considerable interest in studying electromagnetic transport in nanoscale systems. Understanding the effect of physical structure and the role of interfaces is critical for gaining insight into the electromagnetic and magnetic properties of nanostructures and their behavior in microwave devices such as circulators and isolators. Previously, we have described the electromagnetic characteristics at microwave frequencies and the static magnetic properties of γ–Fe2O3∕ZnO micro- and nanocomposites fabricated via powder processing. Here we present systematic effective permeability measurements of magnetically structured granular systems composed of magnetic grains embedded in a nonmagnetic matrix using broadband microwave spectroscopy. Using the transmission∕reflection waveguide method, the effective complex permeability was measured in the frequency range of 0.01–10GHz. The results were compared for composites consisting of micrometer-size...

Complex effective permittivity of a lossy composite material

In recent work, boundary integral equations and finite elements were used to study the (real) effective permittivity for two‐component dense composite materials in the quasistatic limit. In the present work, this approach is extended to investigate in detail the role of losses. We consider the special but important case of the axisymmetric configuration consisting of infinite circular cylinders (assumed to be parallel to the z axis and of permittivity e1) organized into a crystalline arrangement (simple square lattice) within a homogeneous background medium of permittivity e2=1. The intersections of the cylinders with the x–y plane form a periodic two‐dimensional structure. We carried out simulations taking e1=3−0.03i or e1=30−0.3i and e2=1. The concentration dependence of the loss tangent of the composite material can be fitted very well, at low and intermediate concentrations of inhomogeneities, with a power law. In the case at hand, it is found that the exponent parameter depends significantly on the r...