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The composites and thin films comprising individual single-walled carbon nanotubes with a polymer coating (p-CNTs) have been prepared and their electromagnetic responses have been studied in a wide range from low-frequency (25–107 Hz) up to the infrared region. In spite of the high volume fraction of the nanotubes (up to 3.3%), the polymer coating prevents direct p-CNT contacts and the formation of the percolation network in those composites, so that p-CNTs interact only via the electromagnetic coupling. Thereby it is an ideal model system to verify experimentally the fundamental issues related to carbon nanotube electromagnetics, such as the influence of inter-tube electron tunneling on the localized plasmon resonance in the terahertz range, or the infrared absorption enhancement of polymer molecules attached to the nanotube surface. Along with addressing the fundamentals, applied carbon nanotube electromagnetics got insights important for the applications of p-CNT based composites as dielectric media in the terahertz regime. In particular, we found that the real part of the permittivity of the p-CNT film in the terahertz range is rather competitive, i.e. 8–13, however the loss tangent is not so small (0.4–0.6) as has been predicted. The way to increase p-CNT terahertz performance is also discussed. [ABSTRACT FROM AUTHOR]

Epoxy matrix-based composite materials filled with various carbon additives, such as natural, artificial and exfoliated graphites (EGs), activated carbons, and thick graphene, were fabricated. Results of their broadband dielectric spectroscopy in wide frequency range, from a few hertz to a few terahertz are reported. The lowest percolation threshold (below 1.5 wt. %) as well as the highest electromagnetic interference (EMI) shielding capacity are observed for composites containing EG. Using an original random resistor–capacitor–diode network model, we proved theoretically that 2 wt.% of EG in the epoxy host is an optimal composition, as it provides high EMI shielding efficiency at the level of 17 dB for a 1.4-mm-thick polymer slice, and at the same time, does not lead to a significant decrease of other important physical properties of epoxy. [ABSTRACT FROM PUBLISHER]

Composite materials, containing low concentrations of carbon nanotubes (CNTs) of three different diameters and heat-resistant phosphate ceramic as a matrix were prepared by cold-pressing method. Their dielectric properties were studied at room temperature in a wide frequency range (20 Hz-1 MHz). It was found experimentally and proved theoretically via modeling of the composites as a random capacitor-resistor-diode network that electrical percolation concentration depends significantly on the diameter of the nanoinclusions. The main conclusion is that the best candidate providing both the lower percolation threshold and high absolute values of ac conductivity is thinner carbon nanotubes (in our particular case, 8-15 nm). [ABSTRACT FROM AUTHOR]

The dielectric properties of polymethylmetacrylate (PMMA) composites filled with CVD made multiwalled carbon nanotubes (MWCNT) of different mean outer diameters ( d ∼ 9 nm and 12-14 nm) were investigated in the temperature range from 300 to 420 K and in a wide frequency range (20 Hz to 1 MHz). Below the percolation threshold the temperature dependence of the complex dielectric permittivity of the investigated composites is mainly caused by β relaxation in pure PMMA matrix and the dielectric permittivity is found to be higher in composites with thicker nanotubes. The activation energy of β relaxation increases with carbon nanotube concentration. The percolation threshold is lower in composites with both thinner and oxidized carbon nanotubes. The established influence of both the MWCNT mean outer diameter and their oxidation on the broadband dielectric characteristics can be exploited for the production of effective low-cost electromagnetic and/or antistatic coatings working at different temperatures. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Composite materials with 83 wt.% of the 0.7Pb(Mg 1 / 3 Nb 2 / 3 )O 3 -0.3PbTiO 3 distributed in phosphate-bonded ceramics were prepared at three different pressures. A phosphate matrix comprises a mixture of an aluminum phosphate binder and melted periclase, MgO. All samples demonstrate a homogeneous distribution of the ferroelectric perovskite phase and are thermally stable up to 900 K. At higher temperatures, the pyrochlore cubic phase forms. It has been found that the density of the composites non-monotonously depends on the pressure. The dielectric permittivity and losses substantially increase with the density of the samples. The fabricated composites demonstrate diffused ferroelectric–paraelectric transition and prominent piezoelectric properties. [ABSTRACT FROM AUTHOR]

Composite materials based on aluminium phosphate matrix with different grain sizes and small inclusions of carbon nanotubes were studied by means of broadband dielectric spectroscopy in a wide frequency (20 Hz to 36 GHz) and temperature (25–600 K) ranges. The highest electrical percolation threshold was observed for ceramics with the grain size of 0.8 µm, which is higher than the carbon nanotubes cluster size. The electrical transport in ceramics occurs due to the thermal activation at higher temperatures (above room temperature) and the tunneling at lower temperatures. The potential barrier for electron hopping is the lowest in nanosized ceramics. The distance for electron tunneling is also lowest in nanosized ceramics. The electrical properties of ceramics are stable up to 560 K. [ABSTRACT FROM AUTHOR]