An experimental and theoretical study was performed on helium/propane-butane plasma, which enabled assembly of advanced carbon nanostructures, such as multilayer graphene flakes. A plasma jet was created at pressures ranging from 350 to 710 Torr, and with direct current plasma torch powers from 28 to 35 kW at varying mass ratios of helium to propane-butane 1:7.5 to 1:5. Graphene production was confirmed by different analytical techniques. The non-catalytic synthesis in the plasma jet volume provided the graphene purity. As a result of numerical simulation, the kinetic mechanism of the formation of carbon phase precursors in the gas phase, namely, C 2 dimers, was studied. The concentration of C 2 was determined, depending on the temperature and compared with the literature data on the pressure of saturated carbon vapours at temperatures ranging from 2500 to 5000 K. It is shown that the C 2 H-involving reactions provide additional C 2 production, leading to the formation of supersaturated vapour from C 2 molecules at temperatures of 2500 to 3500 K.

Synthesis of graphene materials in a plasma stream from an up to 40 kW direct current (DC) plasma torch is investigated. These materials are created by means of the conversion of hydrocarbons under the pressure 350–710 Torr without using catalysts, without additional processes of inter-substrate transfer and the elimination of impurities. Helium and argon are used as plasma-forming gas, propane, butane, methane, and acetylene are used as carbon precursors. Electron microscopy and Raman imaging show that synthesis products represent an assembly of flakes varying in the thickness and the level of deformity. An occurrence of hydrogen in the graphene flakes is discovered by X-ray photoelectron spectroscopy, thermal analysis, and express-gravimetry. Its quantity depends on the type of carrier gas. Quasi-one-dimensional approach under the local thermodynamic equilibrium was used to investigate the evolution of the composition of helium and argon plasma jets with hydrocarbon addition. Hydrogen atoms appear in the hydrogen-rich argon jet under higher temperature. This shows that solid particles live longer in the hydrogen-rich environment compared with the helium case providing some enlargement of graphene with less hydrogen in its structure. In conclusion, graphene in flakes appears because of the volumetric synthesis in the hydrogen environment. The most promising directions of the practical use of graphеne flakes are apparently related to structural ceramics.

Results of experimental study of the one-step plasma-based process of the synthesis of unsupported graphene and hydrogenated graphene are presented. A direct current (DC) plasma torch is used, the pressure is held at 350 Тоrr, and the flow rates of plasma forming gas (helium) and carbon source (propane-butane mixture) are kept constant. An influence of reactor geometry on the properties of synthesized product is investigated. Graphene and hydrogenated graphene were synthesized in an appreciable rate in the plasma jet volume under equal conditions using cylindrical and conical reactors accordingly. Synthesized graphene materials are characterized using electron microscopy, Raman spectroscopy, x-ray, and XPS analysis, confirming the existence of graphene and of hydrogenated graphene (graphane). In order to examine an influence of input parameters on the process of the synthesis of graphene materials, the quasi-1D numerical flow model is used to calculate the distributions of temperature and velocity within the reactor channel. The key role of the temperature distribution within the reactor in the synthesis of graphene materials is established. Cylindrical flow channel provides higher temperatures compared with the conical channel. It affects the flow composition at the outlet. Under lower temperature, the flow contains in addition to condensed carbon a great amount of hydrocarbons CH, which is favorable for the production of hydrogenated graphene. Under higher temperature, the pure graphene is synthesized, since the outlet flow has the carbon mainly in the condense phase, the quantity of CH being insignificant.

The structure and electric properties of hexagonal boron nitride (h-BN):graphene composite with additives of the conductive polymer PEDOT:PSS and ethylene glycol were examined. The graphene and h-BN flakes synthesized in plasma with nanometer sizes were used for experiments. It was found that the addition of more than 10−3 mass% of PEDOT:PSS to the graphene suspension or h-BN:graphene composite in combination with ethylene glycol leads to a strong decrease (4–5 orders of magnitude, in our case) in the resistance of the films created from these suspensions. This is caused by an increase in the conductivity of PEDOT:PSS due to the interaction with ethylene glycol and synergetic effect on the composite properties of h-BN:graphene films. The addition of PEDOT:PSS to the h-BN:graphene composite leads to the correction of the bonds between nanoparticles and a weak change in the resistance under the tensile strain caused by the sample bending. A more pronounced flexibility of the composite films with tree components is demonstrated. The self-organization effects for graphene flakes and polar h-BN flakes lead to the formation of micrometer sized plates in drops and uniform-in-size nanoparticles in inks. The ratio of the components in the composite was found for the observed strong hysteresis and a negative differential resistance. Generally, PEDOT:PSS and ethylene glycol composite films are promising for their application as electrodes or active elements for logic and signal processing.

An experimental and numerical study of plasma-assisted synthesis of graphene and carbon nanotubes is presented. Experiments were carried out using methane/nitrogen plasma generated by a DC plasma torch with pressure variation in the 100–710 Torr range. The synthesis products were studied by synchronous thermal analysis and scanning electron microscopy. The results of these experiments are shown in thermogravimetry graphs and snapshots of the synthesized structure morphology. It is shown that structures in the form of graphene ‘flakes’ are thermally stable. Carbon nanotubes synthesized at different pressures show different morphologies. Using express gravimetric analysis, the maximum nitrogen content in nanostructures was determined to be 8 at.%. Spectral investigation of a jet of nitrogen plasma with the addition of methane and pure nitrogen under atmospheric pressure was performed. The results of the emission spectra analysis agree with the gas phase composition obtained by kinetic modelling. It is shown that the conversion products consist of the following components: H, H2, C2, CN, HCN, C2H2, C4H4, and C6H6, the ratio of which varies with decreasing temperature. Our calculations show that the gas phase composition remains practically unchanged at different pressures.