V. V. Avdeev

The alkali metals in graphite matrixes-new aspects of metallic state chemistry

Abstract Graphite-alkaline metal systems (lithium, potassium, rubidium, cesium) have been investigated under high-pressure conditions. New graphite intercalation compounds of C4M composition with alkali metal contents twice as high as in those obtained with traditional methods without applying high pressure have been produced. The solid-phase postintercalation reactions of potassium and rubidium in C8 M under 0.2–0.3 GPa pressure have been investigated. The compressibility of different composition intercalation compounds obtained was determined under 2.5 GPa pressure by the volume method. The increase in metal content in layers has been shown to lead to reduced compound compressibility. The phase transformation in C8 K under −1.2 GPa pressure, associated with the metal compaction in the intervals between graphite layers, has been investigated.

Intercalation reactions and carbide formation in graphite-lithium system

In the present work we have investigated a lithium-graphite system by DTA, DSC, heat flow calorimetric, and volumetric methods in a “piston-cylinder apparatus” under high pressures (23 kbar). For the first time, we have determined the enthalpies (ΔHform) of LiC6 (1 stage) and LiC12 (2 stage) formation by the direct calorimetric method at 455 K. It was stated that the values are −13.9 ± 1.2 kJ/mol Li and −24.8 ± 1 kJ/mol Li, respectively. We have defined the enthalpies and temperatures of (3D) → (2D) transitions in LiC6 (T = 711 ± 5 K, ΔHph.tr. = 1.1 ± 0.3 kJ/mol Li) and in LiC12 (T = 472 ± 5K, ΔHph.tr = 1.45 ± 0.2 kJ/mol Li). The rhombohedral modification of Li2C2 was explored at T = 293–1000 K. Only one phase transition at T = 716 ± 6 K (ΔHph.tr. = 5.56 ± 0.09 kJ/mol Li2C2) was detected. The compressibility of Li2C2 was investigated at pressures up to 23 kbar and at room temperature. The change of molar volume (ΔVV0, %) was −2.07 ± 0.07 (10 kbar) and −3.46 ± 0.14 (22 kbar). We studied the carbide formation in Li-GICs at ambient and high pressure (P = 60 kbar) as well.

Anodic Oxidation of Graphite in 10 to 98% HNO3

Systematic studies into the anodic oxidation of natural fine-particle graphite in 10 to 98% HNO3are described. The concentration ranges of the formation of stage I–IV graphite nitrates are determined. Below CHNO3 = 55%, no graphite intercalation compounds are detected by x-ray diffraction. The electrochemical oxidation of graphite in dilute HNO3yields graphite oxide phases. Their formation seems to be preceded by intercalation and hydrolysis, which occur in parallel. The properties of the oxidation and hydrolysis products are shown to depend strongly on the process parameters.

The choice of oxidizers for graphite hydrogenosulfate chemical synthesis

Abstract The features of chemical oxidation of graphite in H2SO4 are discussed. Measurements of currentless potentials of K2Cr2O7, KMnO4, (NH4)2S2O8 and complex salt Ce(SO4)2·4(NH4)2SO4·2H2O solutions in 96% H2SO4 were carried out. Correlation of the redox potentials of oxidizers to the stage number of graphite hydrogenosulfate (GHS) is established. It is proposed to use the standard redox potential scale to choose the oxidizers for the formation of a given stage GHS. On the basis of this approach, numerous oxidizers for GHS formation are considered. For the first time, ozone and Ce(IY) sulfate were applied for GHS production. The stage 1 GHS is obtained in accordance with standard potentials of these oxidizers.

The specific surface area and porous structure of graphite materials

Transformations of the specific surface area and porous structure of carbon materials based on graphite were studied by low-temperature adsorption of nitrogen. Exfoliated graphite and graphite foil were found to have a developed (compared with the initial graphite) surface (up to 90 m2/g) and a porous structure formed by slit-like mesopores with a characteristic radius of ∼20 Å. The determining influence of the method of synthesis on the adsorption properties of materials was demonstrated for the first time.

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.

SODIUM GRAPHITE SYSTEM AT HIGH-PRESSURES

Abstract Sodium intercalation into graphite is investigated ‘ in situ ’ at high pressures, the volumetric and DTA techniques being used. Synthesis conditions of the highly saturated compounds (composition close to C 2.5–3.0 Na) are determined. It is shown that sodium intercalation into graphite at 40-kbar pressure takes place after metal melting, the reaction being complete within several minutes. In addition, the volumetric method establishes the possibility of this reaction at lower pressures (∼ 20 kbar) and heating for 5–10 h.

Chemical synthesis of graphite hydrogenosulfate: Calorimetry and potentiometry studies

In the present work dynamics of sulfuric acid intercalation into graphite by chemical oxidation were investigated by means of the calorimeter and potentiometer techniques. Phase composition of intermediate and end-products of the reaction was investigated by X-ray diffraction.

Surface modification of carbon fibers with nitric acid solutions

We have studied the surface modification of carbonized carbon fibers with nitric acid solutions, compared the effects of 60 and 98% HNO3, and examined the influence of the anodic polarization of the fiber for surface functionalization. Unmodified and surface-modified carbon fibers have been characterized by a variety of physicochemical techniques.

Stability limits of graphite intercalation compounds in the systems graphite-HNO3(H2SO4)-H2O-KMnO4

AbstractThe formation of binary graphite intercalation compounds (GICs) with nitric and sulfuric acids in the presence of a strong oxidant has been studied by x-ray diffraction and potentiometry in a wide range of acid concentrations. The redox potential of the oxidizing solution and the intercalation ability of the acid are shown to influence the stage number (the number of graphite layers between two successive intercalate layers) of the forming GIC and the concentration ranges of GIC formation. The (