Report on the work performed and results

a) Accomplishment of the research objectives as presented in the original proposal

• Objective of the research
As a short reminder, the objective of the project was to provide comparative study of electromagnetic (EM) shielding effectiveness of carbon foams, carbon ultra-thin films and epoxy/carbon composites with low filler concentration in microwave frequency range, and to support the experimental data with an adequate theoretical model of materials’ electromagnetics. Based on theoretical simulations and experimental database achieved during the project implementation, one additional aim was to contribute to one of the most challenging problem in material science: to develop EM coating through design-oriented approach.” For reaching these goals, 4 Work Packages (WP) were proposed, each of them having specific secondary objectives.

• Work performed (mentioning also unsuccessful approaches and unforeseen developments);

WP1: Synthesis of highly electrically conducting carbon materials for EMC applications: highly porous monoliths, pyrolytic carbon nanometrically-thin films, and carbon powders having various grain morphologies and intrinsic electrical properties. The objectives and their achievement are as follows:
(i) to synthesize highly porous monoliths, in the form of glasslike carbon foams or composite glassy carbon - graphite foams of different bulk densities and pore size. Glasslike carbon foams were prepared with an outstanding and unprecedented control of porous structure, since different cell sizes were obtained at constant total porosity, or constant cell size was maintained for different amounts of total porosity, whereas the same glasslike carbon was used for preparing all materials. This was probably one of the most challenging tasks of this project, and it required several full-time years of work before it was successfully achieved. This success prevented the need of using graphitic foams, which have a too different porous structure, and therefore the “B plan” of using composite glassy carbon - graphite foams was no more considered.
(ii) to fabricate PyC nanometrically-thin films (from a few nanometers to a few hundreds of nanometers) on dielectric (SiO2) substrate. This kind of material was successfully achieved by direct decomposition of a gaseous carbon precursor onto silica supports, having the dimensions for fitting exactly a waveguide. The changes brought to the experimental conditions of such synthesis allowed tuning the thickness and the nanotexture of those films, and hence their sheet electrical conductivity, as planned. The materials were characterized by Raman spectroscopy and TEM.
 (iii) to produce carbon powders having various grain morphologies and intrinsic electrical properties, from isotropic spheres to highly anisotropic flakes, wires and tubes, i.e. multi-walled carbon nanotubes, artificial graphite, exfoliated graphite (worm-like particles), thick graphene, carbon blacks having different surface areas, and activated carbon of different granulometries. All these carbon powders have been effectively provided to the partners of the consortium; carbon is a highly versatile element indeed, and due to its reactivity and ability to lead to either crystallized or highly disordered forms, all the aforementioned particles were produced and used as filler for preparing composites having various carbon loadings.
(iv) to describe all materials in terms of porosity (from subnanometre to macroscopic scales), surface area and characteristic sizes as well as dc conductivity (or sheet resistance). All the aforementioned materials were characterized by relevant techniques, which is a very important task for understanding the properties of the final materials based on them. Porosity, surface area, particle size and carbon nanotexture were among the most important quantities to determine. Conductivity was measured whenever possible, or deduced from modelling when such property was not directly measurable because of inadequate behavior of the material.

WP2: Fabrication of polymer/carbon and polymer/nanocarbon composites with low filler concentration (0.25-2 wt.%). Broad-band dielectric spectroscopy of manufactured composites, investigation of electrical percolation phenomenon. The objectives and their achievement are as follows:
(i) to fabricate polymer/carbon and polymer/nanocarbon composites filled with small amounts (0.25 – 2 wt.%) of artificial graphite, natural graphite, exfoliated graphite (worm-like particles), thick graphene, carbon blacks having different surface areas, graphene nano-plates, activated carbon of different granulometries as well as commercial CVD-made single-walled and multi-walled carbon nanotubes (SWCNT and MWCNT), lab-made CVD and arc-discharge CNTs. All these composites were successfully prepared in the predicted range of composition and even in a broader one whenever relevant, based on epoxy resin in which many different amounts of the aforementioned carbons were homogeneously dispersed. As a result, a huge number of samples were produced.
(ii) to functionalize CNTs and other carbons to be embedded into polymer for better dispersion. Given the excellent dispersion of those materials in the epoxy resin, this part of work was not required and would have brought no further improvement. The cost and time required for achieving these chemical modifications was thus not justified.
(iii) to investigate homogeneity and uniformity of carbon dispersion using TEM and SEM. This task was systematically achieved for each composite, and the results proved to be quite satisfactory. No aggregation of particles could be evidenced by these techniques, hence the uselessness of considering the point (ii) above.
(iv) to provide broad-band dielectric spectroscopy of manufactured composites in order to investigate the electrical percolation threshold. The resultant composites were investigated with various measurement systems, from LCR meter to network analyzer with waveguides to THz spectrometer, from low to high frequencies, respectively. The percolation thresholds were determined in the quasi-static regime, and discussed in relation to the particle size and morphology. Their values were successfully compared with those calculated from percolation and/or effective medium theories. Based on studies carried out at various temperatures and frequencies, the conduction mechanisms were also elucidated, and the corresponding parameters (relaxation times, activation energies, glass transition temperatures of the polymer, characteristic tunneling temperature …) were determined, shedding light on the polymer – filler dispersion and interaction.

WP3: Modeling of electromagnetic response properties of carbon foams, PyC nano-thin films and polymers filled with carbons of high surface area as randomly inhomogeneous medium taking into account cells’/grains’ size and non-spherical shape. The objectives and their achievement are as follows:
(i) to determine the physical principles and to develop a model of microwaves interactions with carbon foams. This objective was achieved by considering carbon foams as randomly inhomogeneous media and modelling them by CST Studio software. The success of the modeling was evidenced from the comparison of measured and calculated effect of porous structure parameters (total porosity, size of cells and size of windows connecting the cells) on the EM response of the materials. Observed changes of behavior above a critical frequency were consistently related to porous structure, as predicted by our modelling. The same applied to resonance effects, correctly accounted for by the modelling.
(ii) to simulate radiation transmission/absorption in nano-thin carbonaceous films. Theoretical calculation developed for describing thin pyrocarbon films allowed explaining the observed behaviors and proved the possibility of reaching almost 100% EM absorption in a wide frequency range, spanning from gigahertz (GHz) to terahertz (THz). The simulation evidenced the possibility of getting such outstanding properties by choosing carefully the nature of the substrate, the thickness of the conducting film, or using specific incident angles for the EM waves, amongst other favorable conditions.
(iii) to develop electromagnetic methods of estimation of effective carbons/nanocarbon concentration in polymer composite (mass fraction of carbon fillers not screened electromagnetically and thereby taking part in the electromagnetic interaction with microwaves). The method we proposed was based on the calculation of the filler polarizability, and on running our models at different theoretical concentrations until an excellent agreement with the experimental results was obtained. Due to their perfectly defined structure, the model worked best with carbon nanotubes embedded in polymer matrices.

WP4: Comparative study of EM shielding effectiveness and ac conductivity of tannin-derived carbon foam, nano-thin carbon films, polymer/carbon and polymer/nanocarbon composites. The objectives and their achievement are as follows:
(i) to study in microwave frequency range the electromagnetic response properties of composites filled with carbons of high surface area (EG, TG, activated carbon, carbon black CBH, graphene nano-plates, CNTs) and natural and artificial graphites as well as commercial carbon black for comparison. All those data were provided and published. Our main conclusion is that exfoliated graphite is among the cheapest and most efficient fillers for electromagnetic interference (EMI) shielding. This finding was further supported by modelling, using an original random resistor–capacitor–diode (R-C-D) network model.
 (ii) to examine the dependence of absorption/reflection provided by carbon foams on their bulk densities, pore size and thickness of sample. The dependences of conductivity on porosity, cell and window size were established for the first time, showing that pore sizes are only relevant at very high frequency whereas total porosity controls the EM properties at lower frequency. This could be established thanks to the huge number of highly controlled foam samples made available and based on a same glasslike carbon backbone.
(iii) to study experimentally the peculiarities of the electromagnetic response of carbonaceous nanothin films (pyrocarbon and graphene) in microwaves range. The experimental results were proved to be dependent on the nature of both conductive films and dielectric substrates, and were confirmed by predictions of transmission/reflection/absorption of microwave radiation properties using a simulation code specially developed.
(iv) to provide comparative analysis of all abovementioned carbon material to be used for EMC applications; to observe, if any, some synergy of EM properties of all investigated materials. All materials were thoroughly compared, as planned, and their advantages/disadvantages with respect to each others were discussed. For instance, the dielectric spectroscopy in a wide frequency range, from a few Hz to a few THz, demonstrated that the lowest percolation threshold (below 1.5 wt. %) as well as the highest electromagnetic (EM) interference shielding capacity were observed for composites containing exfoliated graphite. Thin carbon films were found to be excellent EM absorbers at an optimal thickness which is inversely proportional to their conductivity: from 70-100 nm for pyrocarbon to 3-7 carbon layers for graphene. But carbon foams or polymer composite of thickness 4-10 mm produced a much higher level of EM interference shielding efficiency (> 30 dB) than thin carbon films (10-12dB). Therefore, if lightness is a key parameter in a system for EM applications, both thin carbon films and carbon foams are relevant. If good mechanical properties are necessary, the best choice is polymer composites with carbon inclusions with high aspect ratio, slightly above the percolation threshold.

Results and degree to which the objectives were met
In summary, it must be concluded from the above that the objectives of NAmiceMC project were reached with no or negligible changes with respect to the original proposal. All milestones were attained, deliverables were provided, and reports and recommendations were sent in due time.

• List specific training received on scientific and technical aspects
Specific trainings were carried out from UL to INP BSU, and vice-versa, and from INP BSU to INFN, and vice-versa. UL was in charge of the preparation of carbon materials, whether foams or particles, and therefore received researchers from INP BSU for contributing to their synthesis and characterization. INFN was in charge of composite preparation and dielectric/electrical measurements, so that this institute received researchers from INP BSU using materials from UL for processing them and characterizing them in part at INFN. INP BSU was in charge of broadband EM investigation of all materials and related modelling, so that it received researchers from both UL and INFN, who learned the techniques there and worked on the simulation as well. For each visit from one institute to another, seminars were given in the host laboratory for sharing knowledge and experience. Each researcher was assigned a specific task in the host lab, so that the seminars were also specific and technically oriented. As a result, and as planned, each institute significantly gained substantial amount knowledge, not only through seminars and lab work, but also through conferences organized during the project time and related to EM applications of materials.

• Relevance for basic and applied science and for applications including industrial links.
The basic science that has been developed has a high interest for itself, as proved by the number of scientific articles published in reputed international journals. But those results also have direct impact on applied science and related applications, as proved in the parts of deliverables in which recommendations were clearly stated for achieving specific properties. Thus, performances were considered in relation to specific weight and cost of materials, and the best technical solutions were offered, depending on the specifications that any industrial product may have. Given the importance of EM properties, whether for protecting people or electronic devices from interferences, or for manipulating EM waves for specific applications, there is no doubt about the industrial potential of the results obtained in this project. Particularly important is our list of recommendations and our properties database, directly derived from the work carried out in this program.
For instance, an optimal thickness of pyrocarbon (PyC) film was found, which is inversely proportional to its conductivity. At the optimal thickness, the absorption losses in the free-standing film can be as high as 50%, however interference effects can make losses even higher in the binary structures, when the PyC film is placed onto back side of a dielectric substrate. This opens opportunities for prototyping EM devices with enhanced absorption by using PyC and other conductive ultra-thin films. It is worth noting also that the absorbance, reflectance and transmittance of the PyC films deposited on silica substrate show very weak dependence on the frequency within the Ka-band.
Concerning polymer composites, clear specifications were given for choosing the best filler at the most relevant concentration, taking cost and mechanical properties into account, or for selecting the best filler characteristics if the latter had to be imposed for availability reasons, for example. Using cheaper filler was also shown to be possible, provided that special experimental care is taken.
An extensive database of electromagnetic properties and EMI shielding efficiency was built during the project implementation phase for all types of investigated materials (thin carbon films, polymer composites filled with nano and microcarbon additives and carbon porous monoliths). The most suitable from the point of view of electromagnetic performance as shielding materials are materials with conductivity as-high-as possible at low frequency (quasi-static regime), providing large reflectance ability by relatively small thickness, compatible with their skin depth. Databases collected for the most preferable structures in the low-frequency were provided: (i) flat micronic graphite (FMG) / polyurethane composite materials; (ii) graphite nano platelets (GNP) / epoxy resin composite materials and; (iii) epoxy resin-based composite materials with different carbonaceous inclusions. Analyzing the collected data on conductivity in the context of percolation effects, the optimal fillers were identified. As for carbon foams, higher density led to higher EMI SE and conductivity. However, the available thickness was limited to 2 mm. Concerning pyrolytic carbon on a silica substrate, the main observation was that starting from certain thickness (75 nm) one may achieve 50% and more attenuation of EM signal (corresponding to 7dB and more).

b) New objectives established during the course of work and new lines of research
Based on this project and fruitful collaboration, new ideas emerged and started to be tested. Indeed, instead of the aforementioned materials that were still quite efficient for EM applications, shaping more precisely the elements based on which a system is made was thought to offer a great advantage not only for controlling their individual responses but also for allowing the design of collective (or effective) response of the system as a whole. In other words, the idea would consist in preparing true metamaterials whose elements, the meta-atoms, would have optimized and tunable EM response. By tailoring both the properties and the structure of meta-atoms, it should be possible to engineer entirely new electromagnetic structures with properties that do not exist in nature.
The aim of our forthcoming researches, based on the results of the present project, will thus be to implement a new type of electromagnetic metamaterials of unprecedented performance driven by combination of both the electric and magnetic elements of the system. For this purpose, we’ll use recently developed fabrication techniques which take advantage of simultaneous carbon synthesis and magnetic material growth. Having a precise control of the structure and geometry of the initial template, these techniques can implement meta-atoms of defined electric and magnetic response. Moreover, by designing the appropriate spatial variation of these parameters, the material can have a predefined spatial variation of permittivity and permeability across the whole system. Hollow or porous carbon spheres, that UL started to develop and that INP BSU just proved to have interesting EM properties, should behave as meta-atoms in such systems.
As a final goal, we shall build prototype materials and explore their properties using a range of spectroscopic techniques. We will investigate the transmission/reflection characteristics of magnetic hollow carbon spheres and graphene-coated dielectric spheres using strip-line and wave-guide systems in the broad range of frequencies up to sub-THz band. These studies will be used to derive the effective parameters of the 2D media composed of such carbon spheres and implement materials with graded refractive index for application in communication technologies. In particular, we will build a prototype of electromagnetic lens for use in microwave and millimeter wave spectra.