G. P. Khokhlova

Effect of heat treatment conditions on the catalytic graphitization of coal-tar pitch

The effect of heat treatment conditions on the properties of carbon materials obtained in the process of the low-temperature graphitization of coal-tar pitch in the presence of a cellular graphite catalyst was studied by X-ray diffraction and elemental analysis. It was found that materials whose X-ray structural characteristics are close to those of graphite were formed at 800–900°C. An increase in the process temperature to 1400°C, just as an isothermal exposure at 900°C, increased the degree of graphitization of carbon materials but decreased the amount of heteroatoms in their composition.

Carbonization of coal pitch with graphite additives

The influence of added pyrolytic graphite and graphite foam on the crystalline structure of the product formed in the carbonization of moderate-temperature coal pitch at 900°C is investigated by X-ray diffraction analysis. All the additives catalyze the formation of graphite structures in pitch carbonization. However, no direct dependence of the catalytic efficiency on the structure of the carbon additive or its specific surface is observed. In the presence of graphite additives with a specific surface of 4–12 m2/g, more than 20% of a crystalline fraction is formed in the carbonization of pitch. This fraction is highly ordered, with large crystallites. On adding graphite foam, with a high specific surface (110 m2/g) and low packing density (4 g/L), the formation of graphite structures on pitch carbonization is less effective.

Thermal conversion of coal-tar pitch in the presence of silicon compounds

The effect of SiO2 and polycarbosilane (PCS) additives on the thermal degradation of coal-tar pitch at temperatures to 900°C was studied by thermogravimetric analysis, differential thermal analysis, mass-spectrometric analysis, IR spectroscopy, and electron microscopy. It was found that, in the presence of the test additives, the character of release and the quantitative composition of volatile degradation products changed, the degree of aromaticity of the carbon residue decreased, and the concentration of oxygen-containing groups in this residue increased. In the case of the PCS additive, the combined thermal conversion of both of the components occurred. In this case, the contribution of structurization processes increased to cause a nonadditive increase in the yield of solid residue. Silicon mainly occurred as organosilicon fragments with Si-C and Si-O-C bonds, an oxycarbide, and, probably, silicon carbide. In the case of the SiO2 additive, oxidation processes were enhanced; the yield of carbon somewhat decreased, and siloxane groups were present in the solid residue.

X-ray diffraction analysis of the crystal structures of different graphites

Graphites of different nature—pyrolytic, synthetic, thermally expanded, and natural graphites—were compared using X-ray diffraction analysis based on the (00l) reflection from the main crystal face. It was found that the (002) and (004) reflections are the superpositions of two components, which correspond to the structural phases of graphite with different interplanar spacing. The ratio between the integrated intensities of separated reflection components accounts for the ratio between these phases; along with interplanar spacing, this ratio characterizes graphites and makes it possible to detect difference between them. The long-range orders of reflection from the reference plane are responsible for more precise data on interplanar spacing; therefore, it is proposed to use the (004) reflection for characterizing graphites. A correspondence between the structure peculiarities of graphites determined by this method and the discharge capacity of lithium ion batteries with anodes made from these graphites was demonstrated.

Influence of carbon additives on the thermal transformation of coal pitch

The influence of carbon additives with different texture on the thermal transformation of coal pitch is studied. The additives considered are Kemerit carbon nanomaterial, Taunit M nanotubes, AG-3 active coal, and graphite foam. All the additives increase the yield of solid residue and reduce the emissions of volatiles from pitch destruction, including carcinogens. The greatest increase in yield of the solid residue (up to 15%, rel.) is obtained with Kemerit carbon nanomaterial. However, the carbon material formed is more disordered than for individual pitch. Adding 5% of crystalline carbon (graphite foam) catalyzes the formation of graphite structures in the thermal destruction of pitch; their content in pitch carbonized at 900°C may be as much as 30%. At the same time, adding graphite foam reduces the content of carcinogens in the volatile destruction products of the pitch more effectively than adding amorphous carbon additives.

Influence of tall oil and ultrasound treatment on pitch production from semicoking tar or the anthracene fraction

The properties of coal pitch obtained from blends of tall oil and semicoking tar or the anthracene fraction are investigated. Small additions of tall oil to semicoking tar (in the ratio 1: 6) increase the yield of pitch on thermal oxidation and its softening temperature but decrease the coke residue. On ultrasound treatment, chemical transformations of the molecules under the action of cavitation change the characteristics of the pitch produced on thermal oxidation. Primarily, however, the ultrasound permits reduction in the benzo[a]pyrene concentration in the mixture by almost half. On thermal oxidation, the benzo[a]pyrene concentration is further reduced, but most of its mass is concentrated in the pitch.

Structure and conductivity of carbon materials produced from coal pitch with carbon additives

Attention focuses on the structure and electrical conductivity of carbon materials obtained by the carbonization of coal pitch in the presence of additives (nanotubes, graphite foam, and graphite), at temperatures up to 900°C. In some cases, ultrasonic mixing is used on introducing the additives to the pitch. Ultrasonic mixing is found to change the properties of the pitch and affect the properties of the carbon material produced. In particular, the proportion of carbon with an ordered structure is increased; the electrical conductivity at temperatures below 40 K is increased; and the energy barrier Eg between individual crystallites is reduced almost fourfold. At higher temperatures, the electrical conductivity is practically unchanged. Adding nanotubes to the pitch reduces the content of ordered carbon structures in the carbon material produced and lowers its electrical conductivity. Adding graphite foam and graphite to the pitch increases the order and electrical conductivity of the carbon material produced and lowers the energy barrier Eg between individual crystallites in the samples. The electrical conductivity of all the carbon materials below 16 K is described by the characteristic formula for fluctuation-induced tunneling conduction. This indicates that contacts between individual crystallites are mainly responsible for the electrical conductivity.

Effects of the degree of graphitization and the presence of Fe, Ni, Al, and Si on the reaction stability of carbon materials to the action of CO2

With the use of industrial cokes, synthetic graphite, and graphite foams as an example, it was shown that the reactivity and, correspondingly, the thermal stability of carbon in a reaction with CO2 can be regulated by increasing the fraction of graphite structures in the composition of a carbon material and by introducing the catalytic additives of 3d metals (iron and nickel) and also silicon or aluminum. The effect of the additives is explained by both a change in the degree of graphitization of the carbon material and the catalytic action on the course of reaction.

Properties of silicon-containing carbon fiber materials prepared using cellulose

Silicon-containing carbon fiber sorbents, which exhibited sufficiently high sorption activity and increased stability to the action of oxygen at high temperatures, as compared with that of an analogous material based on individual cellulose, were prepared by the carbonization and the subsequent activation of cellulose compositions with polycarbosilane.

Properties of silicon-containing carbon materials prepared using coal-tar pitch

It was found that carbon materials prepared by the combined thermal conversion of coal-tar pitch with finely dispersed silicon dioxide or polycarbosilane at temperatures to 900°C were characterized by a decreased rate of thermal-oxidation degradation and higher porosity characteristics than those of materials based on the individual pitch.