Alesia Paddubskaya

Electromagnetic shielding efficiency in Ka-band: carbon foam versus epoxy/carbon nanotube composites

Abstract. The wide application of microwaves stimulates searching for new materials with high electrical conductivity and electromagnetic (EM) interference shielding effectiveness (SE). We conducted a comparative study of EM SE in Ka-band demonstrated by ultra-light micro-structural porous carbon solids (carbon foams) of different bulk densities, 0.042 to 0.150u2009u2009g/cm3, and conventional flexible epoxy resin filled with carbon nanotubes (CNTs) in small concentrations, 1.5 wt.%. Microwave probing of carbon foams showed that the transmission through a 2 mm-thick layer strongly decreases with decreasing the pore size up to the level of 0.6%, due to a rise of reflectance ability. At the same time, 1 mm thick epoxy/CNT composites showed EM attenuation on the level of only 66% to 37%. Calculating the high-frequency axial CNTs’ polarizability on the basis of the idea of using CNT as transmission lines, we devised a strategy to improve the EM SE of CNT-based composites: because of the high EM screening of inner shells of multi-walled CNTs in the GHz range, it is effective to use either single-walled CNT or multi-walled CNTs with a relatively small number of walls (up to 15, i.e., those taking part in the EM interaction, if the CNT length is 20 μm).

Carbon Onion Composites for EMC Applications

A novel lightweight onion-like carbon (OLC)-based polymer composite with high electromagnetic (EM) shielding properties is presented. OLC have been produced via the large-scale production technology based on the annealing of detonation nanodiamond under vacuum conditions (or in inert atmosphere). EM shielding effectiveness has been tested in the frequency range of 26-37 GHz. The highest EM attenuation at 36.6 GHz reaching -34 dB was observed for polymethylmethacrylate films comprising 20 wt.% of OLC. The shielding effectiveness data collected for microwave frequencies were found to correlate well with the electrical resistivity measurements by four-probe method as well as conductivity measurements provided by the broadband dielectric spectroscopy (20 Hz-3 GHz). It was proved experimentally that OLC EM shielding capacity can be optimized by varying the nanoonion cluster size and nanodiamond annealing temperature so that effective EM coatings can be produced. Both the experimental observations and theoretical simulations demonstrate that even small (smaller than percolation threshold) additions of OLC particles to a polymer host can noticeably modify the composite response to EM radiation.

Broadband dielectric/electric properties of epoxy thin films filled with multiwalled carbon nanotubes

Abstract. Many attempts have been made to fully explore flexibility, resistance to corrosion, and processing advantage of epoxy resin filled with carbon nanotubes (CNTs) as conductive filler, although sometimes with a certain degradation of polymers’ intrinsic properties. It is important to move the percolation threshold into the region of smaller CNTs’ concentration. The results of a broadband dielectric investigation of multiwalled CNT (MWCNT)/epoxy resin composites in wide temperature range from room temperature to 450 K were analyzed for percolation. Far below the percolation threshold (0.25 wt. % MWCNT) the dielectric properties of the composite are mostly determined by alpha relaxation in pure polymer matrix and the freezing temperature decreases due to the extra free volume at the polymer–filler interface. Close to the percolation threshold, the composite shows the negative temperature coefficient effect in the temperature region, where the pure polymer matrix becomes conductive. The activation energy of DC conductivity increases with the MWCNT concentration far below the percolation threshold and decreases close to it (1.5 wt. % MWCNT). The dielectric analysis of the MWCNT/epoxy resin reveals a significant influence of the polymer matrix on the temperature dependence of composite dielectric properties.

Effects of inclusion dimensions and p-type doping in the terahertz spectra of composite materials containing bundles of single-wall carbon nanotubes

Experiments recently showed that the finite lengths of single-wall carbon nanotubes (SWNTs) randomly dispersed and randomly aligned in a composite material are responsible for the appearance of a broad peak in its terahertz conductivity. We investigated, both theoretically and experimentally, the influences of the cross-sectional diameter and the acid-induced p-type doping of SWNT bundles in composite materials on their terahertz conductivity peaks (TCPs). We found that the TCP blue-shifts if the inclusion diameter is larger, and that doping enhances the effective conductivity of the composite material. But, a theoretical prediction of the blue-shifting of the TCP by p-type doping was only weakly supported by experimental evidence. All experimental observations turned out to be in good qualitative agreement with the concept of localized plasmon resonance in SWNTs.

Main principles of passive devices based on graphene and carbon films in microwave—THz frequency range

Abstract. The ability of thin conductive films, including graphene, pyrolytic carbon (PyC), graphitic PyC (GrPyC), graphene with graphitic islands (GrI), glassy carbon (GC), and sandwich structures made of all these materials separated by polymer slabs to absorb electromagnetic radiation in microwave-THz frequency range, is discussed. The main physical principles making a basis for high absorption ability of these heterostructures are explained both in the language of electromagnetic theory and using representation of equivalent electrical circuits. The idea of using carbonaceous thin films as the main working elements of passive radiofrequency (RF) devices, such as shields, filters, polarizers, collimators, is proposed theoretically and proved experimentally. The important advantage of PyC, GrI, GrPyC, and GC is that, in contrast to graphene, they either can be easily deposited onto a dielectric substrate or are strong enough to allow their transfer from the catalytic substrate without a shuttle polymer layer. This opens a new avenue toward the development of a scalable protocol for cost-efficient production of ultralight electromagnetic shields that can be transferred to commercial applications. A robust design via finite-element method and design of experiment for RF devices based on carbon/graphene films and sandwiches is also discussed in the context of virtual prototyping.

Temperature induced modification of the mid-infrared response of single-walled carbon nanotubes

The temperature dependences of the absorbance spectra of thin free-standing single-walled carbon nanotube (SWCNT) films were studied in the infrared range (700–6200u2009cm−1) while heating the air from 300 to 575u2009K. The observed temperature variation in the infrared absorbance spectra has been explained by two different physical factors. The first one is the strong temperature dependence of the conductivity of p-type doped semiconducting SWCNTs. The second one is the temperature dependence of electron relaxation time of intraband electron transitions in metallic SWCNTs. The possibility of the separation of contributions from the interband and intraband transitions to the infrared spectra of SWCNT films has been demonstrated.

Transport and electromagnetic properties of ultrathin pyrolytic carbon films

Abstract. We experimentally investigated the electrical and electromagnetic (EM) properties of pyrolytic carbon (PyC) ultrathin films synthesized on a quartz substrate by chemical vapor deposition at 1100°C using low pressure CH4∶H2 gas mixture as carbon source. PyC films consist of randomly oriented and intertwined graphene ribbons, which have a typical size of a few nanometers. We discovered that the manufactured PyC films of 35-nm thickness provided remarkably high attenuation caused by absorption of 37% to 24% of incident microwave power. The temperature dependence of PyC’s direct-current (DC) conductivity represents typical behavior for disordered systems. Being semitransparent in visible and infrared spectral range and highly conductive at room temperature, PyC films emerge as a promising material for manufacturing ultrathin microwave (e.g., Ka band) coatings to be used in aerospace applications.

Effect of graphene grains size on the microwave electromagnetic shielding effectiveness of graphene/polymer multilayers

Abstract. The influence of chemical vapor deposition (CVD) graphene grain size on the electromagnetic (EM) shielding performance of graphene/polymethyl methacrylate (PMMA) multilayers in Ka-band was studied both experimentally and theoretically. We found that increasing the average graphene grain size from 20 to 400u2009u2009μm does not change the EM properties of heterostructures consisting of graphene layers sandwiched between submicron thick PMMA spacers. The independence of EM interference shielding effectiveness on the graphene grain size between 20 and 400u2009u2009μm allows one to use cheaper (or more convenient regimes of CVD) graphene samples with low crystallinity and small grain size in the development of new graphene-based passive EM devices operated at high frequencies.