L. L. Vovchenko

The Effect of Filler Morphology and Distribution on Electrical and Shielding Properties of Graphite-Epoxy Composites

A comparative study of epoxy resin filled with thermoexfoliated graphite (TEG) of various dispersities has been carried out to investigate the effect of filler particles morphology on electromagnetic interference (EMI) shielding properties of composites within the 25.86–37.5 GHz frequency range of electromagnetic radiation. The shielding properties of a multilayered structure (LS) of TEG-epoxy presenting the set of alternating layers of TEG and epoxy have been investigated as well. The total content of TEG in this multilayered structure was 0.8 and 3 wt.%. It is found that the composites containing TEG exhibit enhancements in the electrical conductivity and the electromagnetic shielding efficiency as compared with those of composites with sonicated TEG.

Nanocarbon-Epoxy Composites as Electromagnetic Shielding Materials

We present the results of studies of the electromagnetic radiation (EMR) shielding by epoxy-nanocarbon composites (CMs). The EMR frequency range was 25.5–37.5 GHz. Thermoexfoliated graphite and multiwalled carbon nanotubes (CNT) have been used as fillers in CM (the content of a filler was 0.5–2 wt.%). We examined the effect of carbon fillers type in CM on characteristics of the electromagnetic shielding. It is shown that, even for low contents (∼ 1–2 wt.%) of a nanocarbon filler in CMs, the coefficient of EMR transmission is − (25-27) dB (for h = 1 mm).The real ϵ′ and imaginary ϵ″ parts of dielectric permittivity of the composites under investigation have been determined.

Synergistic Enhancement of the Percolation Threshold in Hybrid Polymeric Nanocomposites Based on Carbon Nanotubes and Graphite Nanoplatelets

Synergistic effect causes significant decreasing of the percolation threshold in the ternary polymer composites filled with carbon nanotubes (CNT) and graphite nanoplatelets (GNP) in comparison with binary ones. Enhancement of the percolation threshold strongly depends only on the relative aspect ratios of the filler particles due to the formation of the bridges between puddles of the different filler components. Conditions of both appearance and fading away of the synergistic effect are investigated depending on the relative morphology of CNT or GNP components of the filler. Different lateral sizes, aspect ratios, and volume concentrations of both CNT and GNP in the selected ternary composites were considered. Conditions of the effective substitution of GNP with CNT and vice versa in equal volume concentrations without enlarging of the percolation threshold were established. The results are obtained numerically using the Monte Carlo simulation of the percolation threshold of the different ternary composites.

Thermal characterization of expanded graphite and its composites

Using the guarded hot plate method, we have measured the thermal conductivity of compressed expanded graphite (EG) samples (densities from 0.4 to 1.95 g/cm3) along the compression direction (c axis) in the range 150–675 K and that of EG/epoxy composites (5–75 wt % EG) in the range 150–425 K. We also have measured the specific heat of EG samples at temperatures from 200 to 675 K. Their c-axis thermal diffusivity has been shown to decrease with increasing EG density. The thermal conductivity of the EG/epoxy composites and its variation with EG content are well represented by a rule of mixtures that takes into account the anisotropy in the thermal conductivity of the EG particles and their preferential alignment in the composites.

C–Co Nanocomposite Materials

The formation of cobalt particles on the surface of graphite supports via salt thermolysis is studied by x-ray diffraction, electron microscopy, Auger electron spectroscopy, and secondary ion mass spectrometry. The results demonstrate that each step in the fabrication of graphite–cobalt composites causes changes in the particle size, phase composition, and morphology of the deposit. The process involves the formation of a thin, fine-grained salt film on the surface of thermally expanded graphite particles as a result of impregnation; thermal decomposition of the salt, leading to the formation of crystalline cobalt oxide particles 50 to 100 nm in size, uniformly distributed over the surface of thermally expanded graphite; and the formation of Co particles on the graphite surface. The Co particles are 60–70 to 150 nm in size and form aggregates up to 400 nm in size.

The Electrical Properties of Hybrid Composites Based on Multiwall Carbon Nanotubes with Graphite Nanoplatelets

In the present work, we have investigated the concentration dependences of electrical conductivity of monopolymer composites with graphite nanoplatelets or multiwall carbon nanotubes and hybrid composites with both multiwall carbon nanotubes and graphite nanoplatelets. The latter filler was added to given systems in content of 0.24u2009vol%. The content of multiwall carbon nanotubes is varied from 0.03 to 4xa0vol%. Before incorporation into the epoxy resin, the graphite nanoplatelets were subjected to ultraviolet ozone treatment for 20xa0min. It was found that the addition of nanocarbon to the low-viscosity suspension (polymer, acetone, hardener) results in formation of two percolation transitions. The percolation transition of the composites based on carbon nanotubes is the lowest (0.13xa0vol%).It was determined that the combination of two electroconductive fillers in the low-viscosity polymer results in a synergistic effect above the percolation threshold, which is revealed in increase of the conductivity up to 20 times. The calculation of the number of conductive chains in the composite and contact electric resistance in the framework of the model of effective electrical resistivity allowed us to explain the nature of synergistic effect. Reduction of the electric contact resistance in hybrid composites may be related to a thinner polymer layer between the filler particles and the growing number of the particles which take part in the electroconductive circuit.

Transport Properties of Epoxy-Binary Filler Composites

This paper presents the results of changes in electrical resistivity and thermal conductivity of polymer composites (CMs) with two-component filler. It is shown that thermal conductivity of epoxy CMs strongly depends on structural and morphological characteristics of carbon fillers. The synergistic effect in electrical and thermal conductivities of the studied CMs is observed upon addition of boron nitride BN. The presence of a sufficiently large number of BN particles in CMs promotes a more efficient formation of chains of carbon filler and reduces thermal (electrical) contact resistance between the filler particles by decreasing the distance between particles of the filler.

Influence of Ultraviolet/Ozonolysis Treatment of Nanocarbon Filler on the Electrical Resistivity of Epoxy Composites

In the present work, we have investigated concentration and temperature dependences of electrical conductivity of graphite nanoplatelets/epoxy resin composites. The content of nanocarbon filler is varied from 0.01 to 0.05 volume fraction. Before incorporation into the epoxy resin, the graphite nanoplatelets were subjected to ultraviolet ozone treatment at 20-min ultraviolet exposure. The electric resistance of the samples was measured by two- or four-probe method and teraohmmeter E6-13. Several characterization techniques were employed to identify the mechanisms behind the improvements in the electrical properties, including SEM and FTIR spectrum analysis.It is established that the changes of the relative intensities of the bands in FTIR spectra indicate the destruction of the carboxyl group –COOH and group –OH. Electrical conductivity of composites has percolation character and graphite nanoplatelets (ultraviolet ozone treatment for 20xa0min) addition which leads to a decrease of percolation threshold 0.005 volume fraction and increase values of electrical conductivity (by 2–3 orders of magnitude) above the percolation threshold in comparison with composite materials—graphite nanoplatelets/epoxy resin. The changes of the value and behavior of temperature dependences of the electrical resistivity of epoxy composites with ultraviolet/ozone-treated graphite nanoparticles have been analyzed within the model of effective electrical conductivity. The model takes into account the own electrical conductivity of the filler and the value of contact electric resistance between the filler particles of the formation of continuous conductive pathways.

Modeling of gradient composite structures for shielding of microwaves

ABSTRACT The simulation of shielding properties of gradient composite structures (g-CMs) nanocarbon-polymer in the frequency range of electromagnetic radiation (EMR) (25.5 – 55.5 GHz) has been implemented in C++ code. The simulation has shown a significant decrease of EMR reflection index (R) and sufficient increase of EMR absorption (A) and shielding efficiency in gradient composite structures with filler concentration growth in the direction of EMR propagation as compared with composite with uniform distribution of nanocarbon filler. The simulated results coincide with the experimental data for the set of the epoxy composites with gradient distribution of 5 wt.% of graphite nanoplatelets.

Thermal stability of graphite-cobalt nanocomposite materials

Graphite-Co composites produced by intercalating potassium into graphite to give C8K, followed by reaction with CoCl2 and reduction of Co were characterized by x-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, and secondary ion mass spectrometry. The results indicate that the Co in the composites is present both between graphite layers, in the form of atomically dispersed metal (intercalated into graphite), and on the surface of the graphite support, in the form of nanoparticles. Heat treatment of the nanocomposites in several steps increases the amount of cobalt on the graphite surface relative to that between the graphite layers owing to the outdiffusion of cobalt atoms from the interlayer spaces. Heating markedly increases the magnetic susceptibility of the graphite-Co composites, also by virtue of the Co diffusion to the surface of the graphite particles and the formation of Co agglomerates.