EFFECT OF THERMAL-VACUUM DISPERTION OF GRAPHITE

A scientific and technical development of a high-performance thermal-vacuum method, which is an environmentally friendly process based on combination of vacuuming and high-speed thermal heating, was carried out with non-stop production of nanodispersed carbon. A review of physical processes that affect a powder material has been carried out. Thermal-vacuum treatment of C1 grade graphite 1...2 mm of size was carried out. To study the structural composition of the material in initial state and processed in a thermal-vacuum installation, X-ray diffraction analysis and electron microscopy were used. According to results of X-ray analysis, the original C1 grade graphite has two known structural modifications: hexagonal one with lattice periods ao = 0.2461 nm, co = 0.6705 nm, and rhombohedral structure about 30% – with a = 0.2461 nm and c = (3/2)co = 1.003 nm. In graphite treated in a thermal-vacuum installation, these components have been detected as the main composition. Additionally, a super-structural rhombohedral phase with periods a = 2ao = 0.492 nm and c = (3/2)co = 1.003 nm has been detected. A monoclinic phase with parameters a1 = 0.6075 nm, b1 = 0.4477 nm, c1 = 0.4913 nm, and β = 99.6° has also been detected, probably with the presence of iron atoms in structure. The results of analysis and calculations are generally consistent with TEM images of the reciprocal lattice of processed graphite. As a result, it was noted that the initial graphite powder was crushed to 2...40 nm with a partial change in the structure, formation of objects like multilayer nanotubes and fullerenes. It was noted that thermal deformations are involved in this effect, what can significantly accelerate the process of obtaining nanodispersed carbon material with new physicochemical and mechanical properties. The results could be widely used for industrial production of nanosized materials.

Recently, it has become increasingly important to solve the problems of energy and resource conservation and intensification in production processes of nanodispersed materials. When dispersing materials in order to improve production cycle, in particular, several technological processes are combined in one installation. This leads to intensification of the production process and to lower capital costs. So, in some installations, the mechanical process of grinding and drying is used simultaneously [1]. In this case, preheated air is used for drying, what leads to essential energy demands. As a result of an analysis of the methods of fine grinding of materials, it was found that the most rational way is a combination of shock loads and erasure [2,3]. Shock and erasure loads, which act in the field of intense turbulent flows, lead to destruction of the material with manifestation of new properties [4][5][6][7].
Due to the fact that among the tasks noted, much attention of the world community is paid to the use of carbon nanostructures in the industrial production, it would be advisable to test here similar dispersion methods with studying the properties of the obtained materials.
In this regard, the aim of this work is to investigate an effect of a thermal-vacuum dispersion process on state of C1 grade graphite.

SCHEME OF THERMAL-VACUUM INSTALLATION FOR DISPERSION
Used in this work, the thermovacuum method of non-stop dispersion of the material was developed on the basis of theoretical and experimental studies [8,9]. Its principle is based on the combination of fast vacuuming processes and contact heating of the surface of the particles of material inside a spiral heater, what provides instant heating of the surface layer of particles in vacuum to a predetermined temperature. Contact of the particles with the wall of the heater causes heat transfer to their surface layer, and the reduced pressure inside the heating element ultimately allows grinding the material layer by layer. Figure 1 shows the thermal-vacuum installation designed for non-stop dispersing of powder materials [8].
This design of the installation is able to provide non-stop dispersing of powder material in a space thermally insulated from the external environment.

REGULARITIES OF HEAT-MASS-EXCHANGE PROCESSES IN THERMAL-VACUUM INSTALLATION
An original coarse-grained material with ordinary humidity enters with the air inside the hollow spiral heater (2) (Fig. 1), and passes it for 15 s at a speed of ~10 m/s. This is provided by creating a pressure gradient in the hollow heating element using a vacuum pump. Due to this, centrifugal force acts on the particles in the spiral hollow of the
heater, and between the surface of particles and the wall of the heater at shortest times a dense shock contact is formed with an effective heat transfer. Instantaneous transfer of thermal energy from the heater wall to the surface layer of particles occurs [9,10]. The temperature change (dT/dt) on the particle surface is determined by the temperature gradient dT/dx at the moment of heat transfer from the heater in close contact with it: where κ is a coefficient of thermal conductivity in the surface layer of the particle. A change in temperature leads to a change in the size of the heated section along the temperature front, i.e. along the contact surface: Δl/l 0 = αΔT/T, where α is the thermal coefficient of linear expansion of the material. The velocity of the medium at the conditional boundary of the heated section of radius l 0 /2 will eventually have the following expression: As is known from field theory and continuum mechanics, all field perturbations are described by functions of a type f(x,t) = f(x -v s ·t) and propagate with speed v s . The time derivative of the argument v s ·t -x is here the "Doppler" sound speed in the coordinates of the boundary of that section: v s -v l . Its relationship with the sound speed v s -v l ↔ v s indicates the compression degree of that boundary medium, and a rough estimate of the stress in this section can be represented by the following formula: where C 11 is an uniaxial compression module of the medium. Thus, the effect of particle crushing, similar to the action of a shock wave, manifests itself at v l ~ v s and even at lower v l speeds. In addition, since the temperature gradient dT/dx (1-2) depends on the initial moment of contact inversely with t n , where n ~ 1, this effect can occur in the shortest initial time intervals and with sufficiently small fragments of particles [11]. Such effects are known to be accompanied by audible characteristic crackles. At the same time, high stresses arising in such a process can significantly affect the crystalline structure of materials, like martensitic transformations.

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Effect Of Thermal-Vacuum Dispertion Of Graphite EEJP. 2 (2020) It is known that the powders of many materials are hygroscopic and can contain from percent to tens of percent humidity. Then, since with sharp contact heating, the dispersion effect comprises microscopic sections of the material, the residual humidity in such cases can significantly affect the process. The essence of this is a sharp thermal expansion of water particles during the formation of water vapor -in a vapor explosion, which can significantly intensify the dispersion effect [11].
In general, the effectiveness of dispersion in a thermal-vacuum installation depends on the thermophysical properties of material and its state: on the relationships between thermal conductivity, heat capacity and thermal expansion, on dispersion and humidity of original material, on temperature of heater and pressure inside the channel.

MATERIAL AND METHODS
For processing in the thermal-vacuum installation, we chose the C1 grade graphite with 1...2 mm size of particles (Fig. 2a) and 1.0…1.5% total impurity according mass spectrometry.
X-ray diffraction analysis was used to investigate the structural composition of the graphite material in the original and processed state. The samples were made in the form of briquettes 25.0×5…10 mm² by pressing with 250 MPa pressure. The measurements were carried out by the DRON4-07 diffractometer according to the Bragg-Brentano X-ray optical scheme with a proportional counter. To eliminate the vertical divergence of the X-ray beam, a pair of Soller slits was used in the measurement scheme. In the initial processing of the obtained data, the reflection angles were corrected by eliminating the effect of the depth of reflection of radiation, which for dense graphite gives an X-ray line shift in order of part of degree, and this effect is enhanced when measuring the powder with a lower density.
For electron-optical studies of structure of the material, the JEM-100CX transmission microscope was used. For this, a light fraction of the material was released by aquatic flotation.
The chemical composition was determined using the EMAL-2 laser mass spectrometer. Table 1 shows the mass-spectrometric data on the chemical composition of the treated graphite. The bulk density of the material is 0.316 g/cm³. By thermal-vacuum treatment, a nanodispersed graphite powder was obtained with decreasing in particle size to nanometer. Aggregations of particles with 10...40 nm size were also detected (Fig. 2b). It is considered that these are particles of an intermediate state that have been granulated during processing.   Figure 3a shows it for a graphite particle having the ordinary hexagonal or rhombohedral lattice with its reflection here in the basal crystallographic plane (Fig. 3a). This particle also includes a coherent phase with a double period a, which follows from the hexagonal arrangement of small points in close proximity to the center of the electron-diffraction scan. The reciprocal lattice of the monoclinic modification with an angle β = 99.5° was also recorded (Fig. 3b).

RESULTS AND DISCUSSION
From these results it follows that, during processing, some of the graphite is transformed to a new modification. According to Figure 4, in the heat-treated graphite C1, formations similar to multilayer nanotubes and fullerenes with sizes of 2 ... 5 nm and 2 nm, respectively, are observed.  Figure 5 presents fragments of 2-angular CuK  diagrams of intensity of X-ray reflections of the C1graphite in the initial and heat-treated state. According to the results of measurements and calculations, the original graphite consists of two known components: the hexagonal phase with periods a o = 0.2461 ± 0.0002 nm, c o = 0.6705 ± 0.0007 nm, the rhombohedral component with periods a = a o = 0.2461 ± 0.0001 nm and c = (3/2)c o = 1.0030 ± 0.0002 nm with a content of about 30% (Fig. 5a).
As the main composition, these phases have also been detected with the same mutual proportion in the heat-treated graphite. Additionally, a modified (superstructural) rhombohedral phase was found in it with the double period a: a 1 = 2a o = 0.492±0.0001 nm and c 1 = c o = 1.0030±0.0002 nm. According to Figure 3a, this phase is coherent with the main graphite modification. A monoclinic phase was also found with the following lattice periods: a 2 = 0.6075±0.0001 nm; b 2 = 0.4477±0.0002 nm; c 2 = 0.4913±0.0003 nm and β = 99.6±0.1°. Probably, this structure was formed with the participation of iron atoms. The proportions and the angle value are consistent with the results of a TEM scan of the reciprocal lattice of the treated graphite (Fig. 3b).   Table 2 presents the results of calculations of structures, including interplanar distances and reflection indices (in increasing angle): initial graphite with hexagonal and rhombohedral structure, as well as phases formed with superstructural rhombohedral and monoclinic systems.  Figure 5. Fragments of X-ray scans with the designations of calculated reflection angles for the components of the C1 graphite in the original (a) and processed state (b).
Thus, movement of carbon powder in the hollow of the spiral heating element of the thermal-vacuum installation leads to its dispersion, converting the original material for 15 s (with a productivity of 8...9 g/s) into nanodispersed state with order of 2...40 nm particle sizes and with modification of structure of some part material.
It should be noted that the total continuance of the production of nanodispersed carbon in the thermal-vacuum installation counts some seconds, due to rapid heating and reduced ambient pressure. For the effective intake of carbon in the installation, a stream of air is necessary such that will ensure its continuous movement inside the spiral heating element. b а