S.G. Ionov

Thermal conductivity and mechanical properties of expanded graphite

The thermophysical and mechanical properties of compacted expanded graphite (EG) were studied. The experimental results were interpreted with application of similarity theory. The compacted EG critical density corresponding to the observed jump in the thermal conductivity coefficient and elasticity modulus was shown to depend on the expandable graphite preparation method, EG bulk density, and dispersion degree and amounted to 0.01 and 0.005 g/cm3 for the studied EGs.

Improvement of the oxidation stability and the mechanical properties of flexible graphite foil by boron oxide impregnation

Abstract Flexible graphite foil produced by rolling expanded graphite impregnated with boron oxide was analyzed by laser mass spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and thermogravimetry. It was shown that the modification of the graphite foil by boron oxide increases the onset temperature of oxidation by ∼ 150 °C. Impregnation of less than 2 mass% boron oxide also increased the tensile strength of the materials. The observed improvement was attributed to the blocking of active sites by boron oxide, which is probably chemically bonded to the edges of graphene sheets in expanded graphite particles.

Materials for bipolar plates for proton-conducting membrane fuel cells

The review is devoted to the present state of research and development in the field of construction materials for bipolar plates for hydrogen-air fuel cells with polymeric proton-conducting membranes. Principal requirements for such materials are considered and criteria for their selection are formulated.

Synthesis of Fullerides of Alkali and Alkali-Earth Metals under Mechanical and Chemical Activation

Abstract The synthesis of fullerides of alkali metals (Na, K) and alkali-earth metals (Sr, Ba) was performed under the mechanical and chemical activation in the organic solvent. Under mechanical activation of synthesis the time of the complete transformation depends on: i) the nature of solvent, ii) ratio metal/fullerenes and iii) the chemical activity of metal and method of synthesis.

SHUBNIKOV-DE HAAS EFFECT IN LOW STAGE ACCEPTOR TYPE GRAPHITE-INTERCALATION COMPOUNDS

Abstract Quantum oscillation of the magnetoresistance have been investigated in synthesized high quality quasi-single crystals of low stage graphite intercalation compounds (GIC) of the acceptor type for temperatures 1.4<T<4.2K and magnetic fields up to 35 tesla. For some compounds frequency beats are seen. One of the reason for the observation of nodes in the oscillations of the magnetoresistance may be interaction between carbon atoms in neighboring layers separated by an intercalate layer. The parameters of the energy spectrum were determined from experimental data for GIC with AlCl3, ICl, CuCl2, ICl3, H2SO4 and FeCl3.

Calorimetric and Potentiometry Investigations of the Acceptor Compounds Intercalations into Graphite

Abstract In the work the intercalation of H2SO4 (from 98% to 76%) into graphite (oxidizer—(NH4)2S2O8) was investigated by potentiometry. It was established that the redox potential of oxidizer solution depends on the concentration of H2SO4. The criterion for the formation of graphite hydrogenosulfate1 (GHS) was corroborated by the experiment. It was shown that stage of GHS is determined by the redox potential of the oxidizer solution. The system C-K2Cr2O7–94% H2SO4 was investigated by calorimeter and potentiometer methods in situ. The “overoxidation” of stage 1 GHS was observed during the calorimetric investigation (the second exo—effect) but not seen on the potentiometry curve. It was proposed that more high oxidation of graphite and the formation of ion—covalent bonds take place during “overoxidation”.

Shubnikov-de haas effect and energy spectra of graphite-nitric acid intercalation compounds

Single crystals of first-to fourth-stage graphite-nitric acid intercalation compounds are synthesized. The Hall and Shubnikov-de Haas effects are investigated in magnetic fields up to 6 T at low temperatures (≈4.2 K). The temperature dependence of the resistance is measured in the basal plane. The parameters of the crystal structure and the characteristics of the energy spectra of charge carriers are determined for all the compounds prepared.

Properties of carbon-carbon composites based on exfoliated graphite

We report the first study of the mechanical and electrical properties of carbon-carbon composites based on flexible graphite foils modified with pyrolytic carbon. Our results demonstrate that slight densification with pyrolytic carbon enhances the chemical and thermal stability of the graphite foils, increases their Young’s modulus by a factor of 2.5, and reduces their resistivity by 25%. Decomposition of the polymeric carbon precursors poly(hydridocarbyne) and poly(naphthylhydridocarbyne) in an exfoliated graphite matrix increases the tensile strength of the composite by a factor of 2 compared to the as-prepared graphite foil.

Anisotropy of Thermal Expansion Coefficient of Pressed Graphite Foam Measured over the Temperature Interval 20-500°C

This research is aimed at determining the thermal expansion coefficient, α(T), along three directions, X, Y, and Z, of a graphite sample pressed along the direction of weak interactions (Z), over the temperature range [25-500°C]. The experiment results showed that pressing completely alters the dilatometric behavior of the material, and the shape of the α(T) curve changes significantly from one direction to another. Comparing αX, αY, and αZ, a strong anisotropy in the thermal expansion coefficient is observed. This is in good agreement with results published by other researchers who have shown that the electrical conductivity and thermal conductibility of a similar material exhibits strong anisotropic behavior. Pressing has also led to anisotropy in the basal plane, i.e. between αX and αY which have opposite signs. [Pressing results in a significant decrease in α(T) along the Y axis, which is considerably lower than that reported in the literature. αZ is clearly higher that of non-pressed graphite. The wide divergence among the three coefficients is believed to be attributed to the intensification of the membrane effect.

Probe detection of charged coarsely dispersed carbon particles upon laser ablation of pyrolytic graphite

The mass distribution of charged particles from the plasma plume produced by laser ablation of pyrolytic graphite was studied by an electrostatic probe technique. The velocities of motion of the charged opposite parts of the plume double-electric layer and the potential of its internal self-consistent field were determined. Using a proposed procedure for the treatment of probe signals, the amounts of detectable positive and negative ions were evaluated. The negatively charged large carbon particles with a size of up to 106 atoms were assumed to directly appear from explosive bulk effervescence and spinodal separation of the overheated carbon phase on the graphite surface.