This work presents a combined experimental and simulation-based investigation of gas ow perturbations caused by atmospheric-pressure microwave plasma jet with modulated power. These plasma-induced ow instabilities are observed experimentally by schlieren imaging and the mechanism of their formation is explained using a numerical model. The model offers a time-resolved self-consistent solution of plasma dynamics, gas ow, and heat transfer equations. The simulation results are in good agreement with the experimental observations and we conclude that the key mechanism behind the ow perturbations is rapid gas heating at the end of the discharge tube.

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The relation between plasma temperature and properties of graphene nanosheet layer deposited on Si/SiO2 substrate by decomposition of ethanol in microwave plasma torch discharge at atmospheric pressure was investigated in dependence on delivered microwave power and gas flow rates. Plasma modelling was carried out using COMSOL Multiphysics software with delivered microwave power, gas flow rates and experimental reactor geometry as input parameters. Results of the heat flow and fluid dynamics modelling were compared with substrate temperature measured by thermocouple integrated in quartz tube substrate holder. The graphene nanosheets layer was characterized by SEM, Raman spectroscopy and 4-point probe method. The layers were severals tens of μm thick and their sheet resistance varied from 2 to 40 kΩ/sq. The properties of individual graphene nanosheets, 2D/G and D/G Raman band ratio, as well as the sheet resistance of their conductive network were correlated with the increase of plasma temperature with increasing microwave power. The substrate temperature increased linearly with delivered microwave power and the layer sheet resistance was decreasing with increasing microwave power and saturated at 2 kΩ/sq and D/G ratio of 0.6.

Plasma synthesis by ethanol decomposition in microwave atmospheric torch is a simple, efficient, single-step scalable method suitable for volume production of graphene nanosheets. In our work, we studied influence of microwave power on several plasma parameters (e.g. gas temperature, concentration of active species) by optical emission spectroscopy (OES), Fourier transform infrared spectrometry (FTIR) and mass spectrometry (MS) to better understand the process of precursor decomposition and graphene formation in the gas phase. We observed significant change in kinetics and influence of input power on ethanol decomposition routes. Results were compared with theoretical model comprising hydrodynamics, plasma, heat transfer and chemical kinetics.