Home

Summary

The main goal of the project was to design and implement new materials having tunable electromagnetic (EM) properties in the radio, microwave and even THz frequency ranges. For that purpose, a number of carbon-based materials were considered, because of their versatility in terms of nanotexture and macroscopic aspect, low cost, good electrical conductivity and lightness. Both composites and porous materials of controlled structure had to be prepared, characterized, tested in various frequency ranges, optimized in order to either absorb or reflect EM waves, and finally modelled for better understanding the interaction between those waves and the materials. The purpose was to obtain materials being both as-efficient- and as-thin- or as-lightweight-as possible.

Those goals were achieved in the framework of this project, since carbon-filled polymer composites, carbon foams and other periodic carbon structures were successfully prepared and investigated in-depth. As for composites, epoxy resin was filled with either carbon black of various kinds, carbon nanotubes either single- or multi-walled, exfoliated graphite, particles of synthetic graphite having different aspect ratios, and graphene nanoplatelets. These formulations were produced at various loading levels, and the dielectric/electrical characterization was carried out in a very broad range of frequencies, from dc to several tens of GHz, and in a few cases in a broad range of temperatures as well. As for porous carbons, a significant part of the work was dedicated to carbon foams, and special attention was paid to their structure control. For that purpose, foams presenting different cell sizes at constant porosity, and different porosities but constant cell sizes, were prepared, all based on the same carbon material. Doing so, it was expected to investigate for the first time and with an unprecedented accuracy the effect of the porous structure, and to decorrelate it from the total porosity since, so far, the most porous foams were always those having the largest cells. Finally, other very different, periodic, porous carbon architectures were also prepared, having tunable lattice parameters in such a way that a maximum interaction of the structure with EM waves was expected. Two kinds of such structures were considered: 3D-printed polymer lattices that were converted into the first 3D-printed carbon architectures of their kind, based either on Kelvin cells or on Gibson-Ashy cells, or based on 2D packings of either hollow or porous carbon spheres. All those porous carbons were thoroughly investigated in terms of absorption, reflection and transmission coefficients in a frequency range extending from GHz to THz. Modelling the experimental EM behavior was finally initiated using numerical simulation.

Our mains results showed that, depending on the carbon filler, some polymer composites loaded with extremely low levels of dispersed carbon produced excellent attenuation of EM waves, even with very thin samples. Not only such low levels allowed obtaining cheap composites, but the latter proved to be also thermally stable and mechanically strong. Some relationships were clearly evidenced between EM properties and the aspect ratio and the size of carbon particles. The conduction mechanisms in those complex binary systems were also elucidated. Other carbon fillers that were less efficient for EM interference shielding proved to be quite suitable for antistatic applications or electrostatic dissipation, and whatever the considered application, the most relevant filler loading was suggested. We also showed that the most EM-absorbing carbon foams in the Ka-band were those having the largest cells, and their behavior could be very well accounted for by applying the Fresnel model. Interestingly, the dielectric/electrical properties were found to depend on the total porosity only, and not on the average cell sizes, as far as the wavelength was much higher than the main geometric parameters of the foams. At the highest frequencies, the foams behaved as black bodies, and the crossover from one behavior to another depended on the cell size. Resonance effects were observed as well as particular frequencies, and the cells of the foams could be modelled as individual waveguides. Finally, the periodic carbon structures presented amazingly high interaction with microwaves, passing from totally reflective to totally absorptive at critical frequencies at constant lattice parameter or at a critical lattice parameter for a range of frequencies. Such behavior is typical of photonic crystals, and the latter could be successfully modelled for explaining the observed experimental results.

Along with carbon porous structures and carbon based polymer composites ultra-thin carbon films are of great interest to achieve high absorption at very small, nm or even angstroms in case of graphene, thickness. The simplicity of glassy carbon films deposition on various surfaces of varying material, and geometry makes them good competitors for composites coatings. Using dielectric substrate of proper geometry or using metamaterials-type substrate one may achieve almost perfect absorption and electromagnetic interference shielding efficiency at the level of 20 dB with thin (75-100 nm) carbon films. This absorption is comparable to 2 mm thick carbon foams.

All these results confirmed the relevance of carbon-based materials for EM applications. The latter are especially important as far for protecting human beings from EM waves, or electronic machines and measurement devices from EM interferences. These materials also open the route to innovative systems allowing the manipulation of EM waves, especially, but not only, for wireless communication.

 

Highlights from published papers

This page displays clips extracted from scientific results published recently under the FP7 Marie Curie IRSES project NAMICEMC.

A comprehensive analysis of electrical, electromagnetic (EM), mechanical, and thermal properties of epoxy resin composites filled with 0.25–2.0 wt. % of carbon additives characterized by high surface area, both nano-sized, like carbon nanotubes (CNTs) and carbon black (CBH), and micro-sized exfoliated graphite (EG), was performed. We found that the physical properties of both CNTs- and CBH-based epoxy resin composites increased all together with filler content and even more clearly for CBH than for CNTs. In the case of EG-based composites, good correlation between properties and filler amount was observed for concentrations below 1.5 wt. %. We conclude that CBH and, to a lower extent, EG could replace expensive CNTs for producing effective EM materials in microwave and low-frequency ranges, which are, in addition, mechanically and thermally stable [P. Kuzhir et al,   J. Applied Physics, 114 , 164304 (2013)].

 

Polystyrene composites with 0.5 wt. % loading of oriented multiwall carbon nanotubes (MWCNTs) have been produced by forge rolling method. The composites showed anisotropy of transmission and reflection of terahertz radiation depending on sample orientation relative to the polarization of electromagnetic wave. The structural characteristics of composites (nanotube ordering, length, defectiveness) were estimated by fitting the theoretical dependencies calculated within the Clausius-Mossotti formalism for cylindrical particles to the experimental data. The presented model was used for prediction of electromagnetic response of composites containing oriented MWCNTs with various structural parameters in THz region [D. Bychanok et al, J. Applied Physics, 114, 114304 (2013)].

 

Electrical and electromagnetic interference shielding properties of epoxy/multi-walled carbon nanotube (MWCNTs) composites are studied for low nanotube contents from 0.03 to 0.3 wt.% and different processing conditions. In situ chemical modification of carbon nanotube surfaces was achieved by ultrasonic irradiation of carbon nanotubes either in polyethylene polyamine or in liquid epoxy resin, resulting in amine-grafted or epoxy-grafted composites, respectively. Raman, TGA, and SEM analyses indicate a successful grafting of polymer chains to the nanotube surfaces. The amine-grafted MWCNT/epoxy composites show the presence of a thick polymer layer that has formed an extra phase on the carbon nanotube walls, which affects significantly the electrical conductivity and radio frequency response properties. Hence, the amine-grafted composites demonstrate a slightly higher percolation threshold (pc = 0.08 wt.%) and, at the same time, a lower absolute values of dc-conductivity as compared to that of the epoxy-grafted composites, with pc = 0.05 wt.%. In the radio frequency range, almost 3 orders of magnitude rise of the values of ac-conductivity was observed for small contents of the amine-grafted MWCNTs in epoxy. The epoxy-grafted composites could be proposed for producing effective antistatic and electrostatic dissipation coatings. Around and above the percolation threshold, the amine-grafting is critical in the radio frequency range for producing coatings with large dielectric losses. The absolute values of microwave attenuation were found to be independent of the surface modification procedure, but strongly dependent on the concentration of MWCNTs and the thickness of the composite layer [R. Kotsilkova et al, Composites Science and Technology, vol. 106, pp. 85-92 (2015)].