International audienceA comprehensive analysis of electrical, electromagnetic (EM), mechanical, and thermal properties of epoxy resin composites filled with 0.25-2.0wt. % 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

International audienceComposite materials based on epoxy resin filled with various kinds of graphite particles: exfoliated graphite, natural graphite, and coarse, medium and fine artificial graphites have been prepared. Results of broadband dielectric investigations of such materials in wide temperature (25-450 K) and frequency (20 Hz-3 THz) ranges are presented. The dielectric permittivity strongly increases with graphite particle size. The graphite particle size and shape also have a strong impact on freezing temperature, conductivity activation energy and composite electromagnetic absorption properties at room temperature. The lowest percolation threshold is observed for exfoliated graphite (EG)-based composites. At low temperatures (below glass transition temperature of pure polymer matrix), the electrical conductivity in composites above the percolation threshold is mainly governed by electron tunnelling between graphite particles. At higher temperatures, electrical conductivity due to finite electrical conductivity of polymer matrix and by electron tunnelling from polymer matrix to graphite particles occurs in all composites. Microwave experiments show that EG is the only really effective additive, out of all investigated graphite particles, for producing electromagnetic interference shielding composite materials: 2 wt% of EG in epoxy is indeed not transparent for the electromagnetic radiation at 30 GHz

Dielectric/electric properties of onion-like carbon (OLC)/polydimethylsiloxane composites were investigated over very wide frequency (20 Hz–3 THz) and temperature (26–500 K) ranges. The percolation threshold in these composites strongly dependents on the OLC aggregate sizes and was lowest for the composites with the smallest OLC aggregate sizes (∼40 nm). Interestingly, the transition into the insulator state of the composites occurred at higher temperatures. The transition temperature increases with OLC aggregate concentration. Above the percolation threshold, the electrical conductivity in the composites occurs mainly due to electron tunneling between OLC clusters and quasi-one-dimensional hopping inside the clusters. The hopping almost vanishes at frequencies above 100 GHz where the phonon contribution dominates.

The dielectric/electric properties of polyurethane composites filled with carbon nanotubes (CNT), onion-like carbon (OLC), and mixed OLC/CNT are compared across a wide frequency range from hertz to terahertz. The dielectric/electric properties of composites filled with OLC are very attractive and can be improved by addition of small amounts of CNTs due to a strong synergism between the two carbon allotropes. Moreover, in composites with mixed OLC/CNT inclusions, the value of the percolation threshold is lower than the value predicted by the excluded-volume theory. In composites with mixed OLC/CNT inclusions, the dielectric permittivity and electrical conductivity increase due to a decrease in the average distance between nanocarbon clusters.

The need of high performance integrated circuits and high power density communication devices drives the development of materials enhancing the conductive performances by carbon nanoparticles. Among nanocomposites, the ternary hybrid carbon nanotubes/graphene nanoplatelets/polymer composites represent a debatable route to enhance the transport performances.In this study hybrid ternary nanocomposites were manufactured by direct mixing of multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) at a fixed filler content (0.3 wt.%), but different relative combination, within an epoxy system. MWNT/epoxy nanocomposites were manufactured for comparison. The quality of dispersion was evaluated by optical and scanning electron microscopy (SEM). The electrical properties of hybrid composites were measured in the temperature range from 30 up to 300 K.The synergic combination of 1D/2D particles did not interfere with the percolative behaviour of MWCNTs but improved the overall electrical performances. The addition of a small amount of GNPs (0.05 wt.%) led to a strong increment of the sample conductivity over all the temperature range, compared to that of mono filler systems.