Thermoexpanded Graphite Research Articles

In English

We investigated influence of multiwalled carbon nanotubes (CNTs) on spectral characteristics of composites “thermo-expanded graphite – carbon nanotubes (TEG–CNTs)”. The introduction of CNTs in an amount of 0-3% by weight of TEG composites results in a significant increase in the strength characteristics and thermal stability of the composites. This result indicates that CNTs is ideal filler for composites based on TEG compositions and structures. Measurements the giant two-polar oscillations with very small half-width 0.5 cm–1 testify the strong interaction of surface polaritons with photons. When frequencies of local oscillations of surface bonds of carbon nanotubes and modes along “nanotube-TEG” boundaries matches, then the light absorption increases 102–105 times. Thus, IR absorption with two-polar oscillations was measured at 0% of nanotubes in TEG at frequency of 2750 cm–1. It is own optical mode in the thermally expanded graphite. 5 peaks with two-polar oscillations were measured in the IR absorption spectra at 1% of carbon nanotubes. And 8 peaks with two-polar oscillations were measured at 3 % of carbon nanotubes at optical mode frequencies along the boundaries of thermally expanded graphite - carbon nanotubes. When frequencies of local oscillations of carbon nanotubes and composite’s modes matches, then the light absorption extremely increases (in 102–105 times), and two-polar IR absorption oscillations with negative components are formed. In general, two-photon interference is a result of quantum entanglement of dipole-active oscillations and splitting of photons according to the Hong-Ou-Mendel (HOM) quantum effect. Two-photon entanglement is built on the basis of the most entanglement states, also known as Bell's states. The HOM–quantum effect on composites “expanded graphite-carbon nanotubes” is promising for the development of highly coherent optical quantum computers.

Microstructure and properties of polymeric composite materials based on polyethylene and thermally extended graphite for transport systems

The effect of thermoexpanded graphite (TEG) on the microstructure and functional properties of polymer composites based on high-density polyethylene (HDPE) has been studied by optical microscopy, differential scanning calorimetry and mechanical analysis. There has been proven the efficiency of the method of mixing polymer composites using a piston extruder, which provides a more uniform distribution of the filler in the polymer matrix. It is shown that the introduction of TEG leads to a decrease in the degree of crystallinity and melting point of systems based on high-density polyethylene, which is a consequence of the destruction or increase in the defect of the crystal structure of the polymer matrix under the influence of TEG. With the introduction of 1 % of TEG, the melting point decreased from 415.0 to 408.5 K. With the introduction of 3 % of TEG, the thermal conductivity increased from 0.18 W/(m·K) (for HDPE) to 0.76 W/(m·K). The extreme change in the thermal conductivity of polymer composites is a consequence of the formation of TEG in the polymer matrix of the percolation cluster, the mesh of the filler, which penetrates the entire volume of the material. As a result of the conductivity studies, the percolation threshold of thermal conductivity has been determined for these HDPE-TEG systems, which is 0.6 %. Microscopic studies confirmed the formation of the percolation cluster obtained by thermal conductivity. It is shown that at the content of 0.6 % of TEG, there is formed a continuous cluster. The formation of this cluster is confirmed by mechanical studies. An increase in mechanical strength has been recorded, which increases from 30.5 MPa (for HDPE) to 42.8 MPa at 5 % filler content, and this is promising for the use of these materials in transport systems.

Effect of modification on the physical, mechanical and relaxation properties of the polymer - nanodispersed graphite system

The paper analyzes the physical, mechanical and relaxation properties of the polychlortrifluoroethylene (PCTFE) - modified nanodispersed graphite (TEG) system. It has been shown that the modification of the surface of conducting carbon nanoplates with ultradispersed dielectric silicon dioxide (SiO2) (30%) leads to a nontrivial effect - an increase in the electrical conductivity of the PCTFE - 2.5% TEG/30% SiO2 composite by more than two orders of magnitude in comparison with the PCTFE - TEG composites, containing unmodified carbon nanoplates.
 The functionalization of carbon nanoplates was carried out by treating the particle surface with an active solution of chlorosilane in an organic solvent, that in case of hydrophobization of the filler surface, leads to an increase in the system percolation threshold. It is shown that the method of hydrolyzation of the filler surface can be highly effective due to a directed change in some, in particular, shielding, properties of polymer composites based on nanocarbon.
 It is established that the modification of the nanofiller (dispersed thermoexpanded graphite) increases the intermolecular interaction of the filler-matrix. Depending on the concentration of the filler, the structure of the matrix and the system as a whole demonstrates dynamic transformations in the size of the heterogeneity of the structure.
 Ultrasonic studies of composites have shown that the smallest size of structure inhomogeneity is achieved in the range of concentrations of percolation development, and the change in the size of system structure inhomogeneity is associated with the transition from inhomogeneity as the size of crystallites activated by nanofiller at low concentrations associated with coagulation of nanoparticles at concentrations exceeding the percolation threshold.
 Thus, in the case of nanofillers, it is impractical to use concentrations that significantly exceed the percolation threshold, as this leads to coagulation of the filler particles and the relative loosening of the matrix.

Methylene blue adsorption on thermo plasma expanded graphite in a multilayer column system

The removal of dyes from wastewater is an important topic in environmental applications. Methylene blue (MB) is one of the most worrisome compounds, as it is widespread and used in many industrial activities. Adsorption represents an effective technique for the removal of this contaminant. Thermo plasma expanded graphite (TPEG) is an industrial material characterized by a fibrous morphology, a very low density and overlapped graphene layers. TPEG has a higher specific surface compared to conventional thermo-expanded graphite and it can establish effective attractive forces with charged pollutants. These properties make TPEG a very promising adsorbent material.In the present work, TPEG was tested in an innovative multilayer column system to treat MB contaminated solutions. Several batch experiments were carried out by varying pH, initial MB concentration and temperature.The optimal adsorption performance was assessed at pH 11, around which the TPEG assumed the maximum negative charge. Based on these results, the adsorption mechanism appeared to be related mainly to electrostatic interactions. At room temperature, the greatest amount of MB adsorbed on TPEG was detected by treating solutions with an initial concentration of 30 mgMB/L. The temperature increase from 20 to 40 °C caused an enhanced adsorption capacity when concentrations higher than 10 mgMB/L were treated. The adsorption trends were accurately described by a pseudo-second order kinetic law and the adsorption isotherms at 20 and 40 °C were found to follow both the features of Freundlich and Langmuir models. The adsorption capacity was estimated to reach threshold values around 95 mgMB/gTPEG and 265 mgMB/gTPEG at 20 and 40°C, respectively. The Gibbs energy change (ΔG°) was calculated to about −7.80 kJ/mol, which proved that the process is spontaneous from a thermodynamic point of view. Finally, it was verified that TPEG can be efficiently reused 5 times after a simple chemical regeneration phase with HCl.

Emergency cooling of superheated surfaces by nanofluids additives in the event of a water boiling crisis

In order to prevent accidental overheating in metallurgy, electrical engineering, power engineering, nuclear power plants (NPP) etc., we have developed a series of nanofluids (NFs) which are stable under high temperature, boiling and radiation conditions. In our work, a new AlSi-7 nanofluid based on natural aluminosilicates with unique thermal behavior was developed for the first time. The boiling curve, covering all modes including transient and film boiling and burnout heat flux up to 3.7 MW/m2 is presented. The possibility of using this NF for emergency cooling of a superheated heat exchange surface was studied. For this purpose, the automatic unit for simultaneous recording of NFs boiling curves and the changes of the main heat transfer parameters (such as heat flux, heat transfer coefficient and temperature of the heating surface) in real time conditions under a constant rise of heat flux are presented. These NFs consisted of metals oxides, natural aluminosilicates and carbon nanomaterials (including nano-diamonds, multi-walled carbon nanotubes (CNTs) and thermo-expanded graphite (TEG)). Results showed the possibility of increasing the heat transfer coefficient (HTC) and critical heat flux (CHF) by 1.5–3.0 times compared with those of distilled water. By adding nanofluids to boiling water in the event of a boiling crisis, emergency cooling of an overheated heat transfer surface resulted in a sharp decrease in temperature of the heating surface from 700 to 125 °C without reducing the heat load has been proven.

Мікротвердість компактованого термічно розширеного графіту

Using the method of continuous microindentation with different loading on the indenter, the microhardness of compacted thermo-expanded graphite (TEG) samples of different dispersion was studied. The analysis of the obtained results showed that, with an increase in the average cross-sectional area of TEG particles from 40 to 120 microns, the microhardness of the samples under investigation also increases. An analysis of the influence of the dispersion and morphology of the TEG particles on the distribution of microhardness on the surface of the compacted samples of TEG was also carried out. The microhardness indicatrix for the samples of the original TEG shows that when the radial displacement from the center of the sample, the microhardness of the material decreases. So in the central part of the discoid sample the values of microhardness lie in the range from 0,04 to 0,025 GPa. In the next concentric region, the microhardness decreases by 30% and gains a value of (0.028 ... 0.014) GPA. When the dispersion of TEG particles changes, the distribution of microhardness also changes. The microhardness indicatrix for a compacted Tg sample with an average particle size of 180 μm shows that the microhardness value at the center of the sample ranges from 0.065 to 0.15 GPa.As you approach the edge of the sample, the microhardness of the material decreases from 0.15 to 0.054 GPa. The study of TEG with an average particle size of 50 μm showed that the indentation in the center and in the middle region of the sample gives an isotropic distribution of microhardness values. The microhardness values coincide in the central and middle regions of the sample and correspond to ≈ 0.1 GPa. The proposed method of research and analysis of microhardness on the surface of compacted specimens of TEG gives an opportunity not only to characterize the micromechanical properties of the investigated material but also to optimize the technological regimes for obtaining samples.

Graining of thermo-expanded graphite

The results of investigation of the process of graining thermo-expanded graphite in the epicyclic granulator have been presented. Procedure of graining and the sequence of grains’ formation have been described. It has been obtained the experimental dependencies of the grains’ fractional distribution and their strength against epicyclic ωρ and relative ωr rates of cylinder rotation and time of graining

Influence of the range of grinding thermo-expanded graphite on physical-mechanical and tribotechnical properties of composite materials produced on its basis

The influence of the level of thermo-expanded graphite breakage on strength and tribotechnical properties of composite materials with organic-silicon binding has been considered. It has been determined that the increase of strength and reduction of friction coefficient of composite materials is connected with the framework of thermo-expanded graphite, the core elements of which act as reinforcing filaments.

Biosensor Based on Screen-Printed Electrode and Glucose-Oxidase Modified with the Addition of Single-Walled Carbon Nanotubes and Thermoexpanded Graphite

The modification of an electrode produced by a matrix printing head with the use of glucose oxidase (GOD), single-walled carbon nanotubes (SWCNTs), and thermoexpanded graphite (TEG) has been studied. During glucose oxidation, modification by SWCNTs leads to the effect of direct electron transfer. Both nanomaterials increase the magnitude of the sensitivity coefficient (SC) from 0.11 to 0.24 mA M–1 in the case of glucose oxidase–and ferrocene-based electrodes when modification is done with the addition of TEG and up to 0.62 mA M–1 when modification is done with the addition of SWCNTs. A comparison of the characteristics of the biosensors with ferrocene and nanomaterials with those of the mediator-free biosensors based on SWCNTs and GOD shows that the biosensor provides higher sensitivity detection; the magnitude of sensitivity coefficient is 1.5 mA M–1. The higher magnitude of the SC can be explained by the occurrence of more effective electron transfer from active centers of the enzyme to the electrode. Voltammetric measurements demonstrate that electron transfer in a mediator-free biosensor is not complicated by a chemical reaction and is carried out under the control of diffusion factors. After testing the mediator-free biosensor in practice, it is possible to talk about the applicability of such a biosensor in detecting glucose in different environments, including the fermentation industry.

Use of Graphene Sorbent in Filters for Cleaning Surface Water and Hot Water

The technology of producing foamed (thermoexpanded) graphite in the form of anisotropic particles with a size of about 100 microns and a thickness of about 5 atomic layers has been developed. The BET surface area of 12–15 m 2 /g, the calculated surface area at a thickness of 1,5 nm is about 800 m2/g, which allowed it to be used in filters for drinking water purification. Water testing of Lake Ladoga showed that even after 500 m 3 of filtered water the turbidity did not exceed 0,5 mg/l, chromaticity 13 degrees, iron 0,11 mg/l and oxidizability 3,1 mg O/l. At the same time, the resource for turbidity and iron is not yet exhausted. The advantages of the sorbent GS are shown in the example of hot water treatment. In the Ice Palace of St. Petersburg a system of two filters "Hercules" was designed to clean hot water to fill the ice cover of the ice rink, instead of filtering the cold water and then heating it, which saved more than 3 million rubles a year. To improve filter performance, a new design was developed and tested, characterized in that it is not made in the form of a traditional cartridge, but is a backfill. The drop in the performance of such a filter to a critical level of 1,2 m 3 /h occurs with a filtration of 480 m 3 , which is 4 times greater than the volume of water obtained from the coaxial design of the filter (120 m 3 ). Filters with uneven compaction of GS are proposed to be used for cleaning surface water in the manufacture of a modular mobile unit with a capacity of up to 10 m 3 /h, which is designed to produce drinking water in populated areas where there are no coagulation plants and it is impossible to reach a water conduit with drinking water, Collections, youth forums and in cases of disasters.

Electrical contact resistance research of graphite with copper and termo-expanded graphite

The experimental setup for measuring the specific electrical contact resistance (SECR) of solids contact pairs, depending on the compression pressure and temperature, has been developed. In the development of graphitization furnaces numerical models the electrical properties of the contact interaction of copper-graphite, graphite-graphite, using a gasket of thermo-expanded graphite, are important. Resistance of the contact portions is always greater than the contacting elements, thus, there are additional losses of energy in these regions. This affects the thermoelectric state of furnaces. The relevance of this study is determined by the absence of data concerning the contact resistance of graphite-gasket- graphite in the literature. The most difficult task in the investigation of the contact resistance transition is to determine the actual contact area, the value of which depends on the nature of the microscopic bulge deformation. The theoretical solution to the problem concerning the actual contact area of real surfaces is very difficult, that is why experimental methods have become widespread in the study of the electrical solids contact resistance. As a result of research, the following experimental data was observed: SECR of the copper-graphite at compression pressure of 1 – 7 MPa and under the temperature 16 С; SECR of the graphite-gasket-graphite at constant pressure of 1,7 MPa under the temperature range 16 – 250 С with subsequent extrapolation to 3000 С. The experimental data of the SECR contact pairs of copper-graphite and graphite-gasket-graphite is necessary for the priori estimation of graphitization furnace thermoelectric state during their development, modernization and also for electrothermal equipment of other industries.

Mechanochemical dispersion of thermoexpanded graphite

A new material based on thermoexpanded graphite (TEG) is described, which has been developed for special applications encountering the need for suppressing the radiative component of heat transfer in thermoinsulating materials. Results of investigations of the material structure by the electron microscopy and Raman spectroscopy techniques are presented. It is established that the new material consists of anisotropic carbon particles with lateral dimensions on the order of tens microns and a thickness of about five carbon monolayers. Evidently, this structure is close to optimum for the claimed applications.

Irreversible processes during the lithium intercalation into graphite: the passive film formation

Comparative studies of three type of carbonaceous materials—the modified oxidized graphite, thermoexpanded graphite, and carbon paper—prior to and after galvanostatic cycling in 1 M LiClO4 solution in propylene carbonate-dimethoxyethane mixture are carried out using standard porosimetry. It was shown that the mean (effective) thickness of the passive film [solid electrolyte interface (SEI)] at the electrodes of the modified oxidized graphite and thermoexpanded graphite equals a few nanometers. The comparison of porosimetric and electrochemical data shows that the passive film comprises both lithium carbonate and alkylcarbonates. Additionally, this comparison allows corroborating the concept on the formation of polymer (or oligomer) component of the passive film at least at the thermoexpanded graphite electrodes.

Structure-sorption properties of thermoexpanded graphite and possibilities of its use for the removal of organic matter from aqueous solutions

Based on the data on adsorption of vapors of nitrogen, n-hexane, water, and electrochemical measurements it has been shown that thermoexpanded graphite possesses a nonuniform-porous structure and a vividly expressed hydrophobic surface per 1 m2 of which there are ∼1018 of active hydrophilic centers. We have carried out their identification. We have investigated adsorption of anionic (sulfonol, direct scarlet dye, fulvic acids), cationic (crystalline violet dye), and nonionogenic (i-OcPh(OE)10) organic matter on the surface of thermoexpanded graphite. It has been shown that the use of the sorbent under investigation in the processes of water purification of dissolved organic compounds has good prospects.

The lithium intercalation into graphite from electrolyte and from solid lithium

The values of diffusion coefficient (D) of lithium in thermoexpanded graphite during cathodic intercalation from aprotic electrolyte, and upon direct contact with lithium metal, are measured. In the first case galvanostatic switch-on curves were registered, in the second case the method of x-ray diffraction was used. In the both cases D was close to 10-10 cm2/s.

Shape memory effect in the epoxy polymer–thermoexpanded graphite system

Abstract The peculiarities of shape recovery for deformed samples of the epoxy polymer–thermoexpanded graphite composition have been investigated. At specific concentrations of the filler, the shape memory effect accompanied by a significant increase in volume is observed. A number of technological schemes for obtaining products with unconventional shape memory effect and a physical model explaining the effect and specifying the boundaries of the region where it could be realized are proposed.

Lithium Intercalation into Graphite during Direct Contact and Anodic Polarization

Values of the diffusion coefficient for lithium in thermoexpanded graphite (TEG) are measured during cathodic intercalation from an aprotic electrolyte and during the TEG plate in contact with metallic lithium without electrolyte. In the course of cathodic intercalation, the diffusion coefficient is measured by method of galvanostatic switching-on curves. During TEG contact with lithium, the variation of x-ray diffraction pattern of TEG with time is recorded. It is established that during TEG contact with lithium the lattice spacing decreases and the characteristic size of TEG particles increases with time until the lithiation is complete and intercalate LiC6 forms. Values of the diffusion coefficient determined by either method coincide and amount to 10–10 cm2 s–1.

Surface Properties of Carbonaceous Solids as Examined by 1H NMR Spectroscopy Using the Bulk Freezing Procedure

It has been established that the type of adsorption sites on the surface of different carbonaceous materials [graphite oxide, thermo-expanded graphite, graphitized and non-graphitized carbons, carbon black, carbonated silicas (Carbosils)] can be determined using a chemical shift in the 1H NMR spectra of adsorbed test compounds possessing different donor–acceptor properties and polarizability. This is due to the existence of local magnetic anisotropy regions near the carbon surface. The 1H NMR measurements on adsorbent/adsorbed water/inert solvent interfaces provide an effective procedure for the differentiation of NMR signals from molecules bound with various types of surface sites. Variations in the free energy of water in the adsorption layer of Carbosils having different carbon contents on the silica surface and their free surface energy have been determined from measurements of the temperature dependence of the NMR signal for non-freezing water in the cooling process. A low degree of carbonization for the initial silica surface leads to a drastic increase in the aqueous interlayer thickness structurized by the surface.

Study of Intercalation of Platinum Hexafluoride in Thermoexpanded Graphite

Study of Intercalation of Platinum Hexafluoride in Thermoexpanded Graphite