Ludmila Vovchenko

Influence of graphite type on electrical and thermal properties of composite materials graphite-organic compound

The electrical resistivity and thermal conductivity of the composite materials (CM) based on graphite [natural disperse graphite (Gr), thermoexfoliated graphite (TEG)] and silicon–organic binder (SO) have been investigated. The threshold value of TEG content, when the abrupt decrease of electrical resistance is observed, was shown to be equal to ∼4 wt.% for the TEG–SO systems. The TEG–SO samples display anisotropy of the electrical properties: ρc/ρa ∼2 for the samples with low content of TEG (∼8 wt.%) and ρc/ρa ∼(7–9) for the samples with maximal content of TEG (60–70 wt.%). In contrast to electrical resistivity the thermal conductivity of graphite–organic compound CM linearly depends on graphite content. This difference in behavior of electrical and thermal conductivity with increase of TEG concentration may be explained by the fact that electric current is determined by the charge carriers, while the thermal conductivity is determined predominantly by phonon transport in graphite.

Crystal structure, magnetic, and microwave properties of solid solutions BaFe12–xGaxO19 (0.1 ≤ x ≤ 1.2)

The crystal structure of solid solutions of an M-type hexagonal barium ferrite BaFe1–xGaxO19 (x = 0.1–1.2) with the isostructural diamagnetic substitution of Ga3+ ions has been studied by X-ray diffraction. The unit cell parameters have been calculated for all compositions. The field and temperature dependences of the specific magnetization of these solid solutions have been measured by vibrational magnetometry. The microwave properties of BaFe12–xGaxO19 (x = 0.1–1.2) have been studied in a bias field. It has been shown that the frequency of natural ferromagnetic resonance decreases as the Ga concentration increases from x = 0.1 to x = 0.6 and it increases once again as the Ga concentration increases to x = 1.2. As the Ga concentration increases, the natural ferromagnetic resonance lines broaden. This indicates an increase in the frequency range of strong absorption of electromagnetic radiation. In this case, the amplitude of the resonance curve peak changes insignificantly. The frequency shift of the natural ferromagnetic resonance in an external magnetic field increases as the gallium ion concentration in the sample decreases.

Thermo-Exfoliated Graphite Containing CuO/Cu2(OH)3NO3:(Co2+/Fe3+) Composites: Preparation, Characterization and Catalytic Performance in CO Conversion

Thermo-exfoliated graphite (TEG)/CuO/Cu2(OH)3NO3:(Co2+/Fe3+) composites were prepared using a wet impregnation method and subsequent thermal treatment. The physicochemical characterization of the composites was carried out by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and Ar temperature-desorption techniques. The catalytic efficiency toward CO conversion to CO2 was examined under atmospheric pressure. Characterization of species adsorbed over the composites taken after the activity tests were performed by means of temperature programmed desorption mass-spectrometry (TPD MS). (TEG)/CuO/Cu2(OH)3NO3:(Co2+/Fe3+) composites show superior performance results if lower temperatures and extra treatment with H2SO4 or HNO3 are used at the preparation stages. The catalytic properties enhancements can be related to the Cu2(OH)3NO3 phase providing reaction centers for the CO conversion. It has been found that prevalence of low-temperature states of desorbed CO2 over high-temperature ones in the TPD MS spectra is characteristic of the most active composite catalysts.

Magnetic anisotropy of the graphite nanoplatelet–epoxy and MWCNT–epoxy composites with aligned barium ferrite filler

The aim of this study was to evaluate the magnetic properties of hybrid epoxy-based composite materials with magnetic barium ferrite (BaM) and conductive carbon (multi-walled carbon nanotubes and graphite nanoplatelets) fillers. This study was performed in order to fabricate composites with uniform and aligned magnetic components of the filler. The magnetization field dependences of the composites and BaM powder were determined and analyzed using a vibrating magnetometer. Of additional concern was the filler distribution characterization by optical investigation. Coercivity of BaM/epoxy composites increases more than in 4 times in comparison with BaM powder. Composites with aligned filler reveal an anisotropy of the magnetization relative to the alignment axis. However, BaM particles retain their uniform distribution in aligned fibers, which is confirmed by a magnetic remanence-to-saturation magnetization (M(0)/Mmax) ratio of 0.5.

Conductive and Shielding Properties of MWCNTs/Polymer Nanocomposites with Aligned Filler Distribution

The effect of filler alignment on conductive and shielding properties has been investigated in multiwall carbon nanotubes (MWCNTs)/polymer nanocomposites (NCs) containing 0.05–1 wt. % of the filler. The filler alignment was produced by applying external AC electric field during the whole process of hardening. MWCNTs network formation was detected by optic microscopy.

Electrodynamic properties of the nanocarbon/polymer composites with aligned by magnetic field secondary non-conductive component

Multiwall carbon nanotubes/epoxy and graphite nanoplatelets/epoxy composite materials (2-5 wt %) as well as the composite materials with barium hexaferrite as secondary filler (27 wt %) were prepared. Alignment of barium hexaferrite nanoparticles was performed by magnetic field action during polymerization process. Morphology, the electrical conductivity and shielding efficiency of the composite materials in the frequency range of 36-55.5 GHz were investigated. Optical and electron microscopy, standard 2- and 4-probe methods of electrical conductivity measurement and network analyzer were used for that purpose. The arguments about secondary filler addition and its alignment on electrodynamic properties of the obtained composite materials are given.

Electric and magnetoresistance of nanocomposite material graphite-cobalt

Abstract The electric and magnetoresistance of graphite‐Co composites have been investigated in the temperature range 77–300K and in the magnetic fields up to 1,500 kA/m. The observed decrease of electric and magnetoresistance and weakening of their temperature dependence for graphite‐Co composites as compare to pure graphite are explained in terms of the additional concentration of charge carrier and decrease of their mobility due to Co intercalation into graphite.

Electrical Properties of Composite Materials with Electric Field-Assisted Alignment of Nanocarbon Fillers

The article reports about electric field-induced alignment of the carbon nanoparticles embedded in epoxy matrix. Optical microscopy was performed to consider the effect of the electric field magnitude and configuration, filler morphology, and aspect ratio on alignment process. Characteristic time of aligned network formation was compared with modeling predictions. Carbon nanotube and graphite nanoplatelet rotation time was estimated using an analytical model based on effective medium approach. Different depolarization factor was applied according to the geometries of the particle and electric field.Solid nanocomposites were fabricated by using AC electric field. We have investigated concentration dependence of electrical conductivity of graphite nanoplatelets/epoxy composites using two-probe technique. It was established that the electrical properties of composites with random and aligned filler distribution are differ by conductivity value at certain filler content and distinguish by a form of concentration dependence of conductivity for fillers with different morphology. These differences were explained in terms of the dynamic percolation and formation of various conductive networks: chained in case of graphite nanoplatelets and crossed framework in case of carbon nanotubes filler.

Effects of Dispersion and Ultraviolet/Ozonolysis Functionalization of Graphite Nanoplatelets on the Electrical Properties of Epoxy Nanocomposites

The influence of liquid dispersive mediums used in the fabrication of graphite nanoplatelets (GNPs) on their morphology as well as the surface state and electrical conductivity of GNPs/epoxy nanocomposites (NCs) was studied in detail. Ultrasonic dispersion of thermally expanded graphite (TEG) in alcohol medium has been found to be the most effective by the time parameter and allows fabrication of GNPs with rather high aspect ratio (102) and low level of defects. Ultrasonic dispersion of TEG in water medium requires a long time (up to 20 h) of ultrasonic action, but the resulting GNPs are characterized by large lateral size and, therefore, a higher aspect ratio (104). The effects of different ultraviolet (UV)/ozone treatment time on GNPs/epoxy NCs morphology and electrical properties were investigated by optical microscopy, scanning electron microscopy (SEM), infrared (IR) spectrometry, and Raman spectroscopy. The NCs with GNPs subjected to UV/ozone treatment have shown the increase of electrical conductivity with the increase of UV/ozone treatment time, which can be associated with the improved dispersion and distribution of GNPs within the epoxy matrix as well as decreased contact resistance between individual GNPs in conductive network due to chemical functionalization of GNPs during UV/ozone treatment.

Phonon Drag in GIC Based on Disordered Graphite

Abstract The work presents the results of investigation and calculation thermopower in GICs with SbCl5 based on disordered graphite.