Evolution of graphene nanoflake size and morphology in atmospheric pressure microwave plasma

Graphenelike carbon was synthesized using an atmospheric pressure microwave plasma system, employing argon/nitrogen mixtures as carrier gases and methane as the carbon precursor. This study investigates the effects of varying methane flow rates and plasma power on carbon synthesis and final morphology. The process involves the decomposition and subsequent reorganization of carbon radicals into graphene sheets and graphitic nodules. Tungsten carbide rods with attached copper collection grids were strategically positioned at three distinct points along the plasma column to collect the synthesized carbon. The variations in particle diameter were systematically analyzed using scanning electron microscopy (SEM). Results indicate that particle diameter generally increases along the plasma column, influenced heavily by plasma length. Beyond the bulk-incandescent plasma boundary, the diameter distributions remain relatively constant, suggesting that the majority of the growth occurs in the bulk and incandescent regions. Further, an increase in methane flow rate correspondingly worsened the material quality and increased the mean particle diameter across all ports, attributed to higher carbon concentrations and lower gas temperature in the plasma. Conversely, an increase in plasma power resulted in better material quality and a decrease in particle diameter at each port, which can be attributed to the rise in gas and electron temperatures increasing favorable reaction rates. Higher thermal energy accelerates the kinetic activity of carbon species in the plasma, leading to increased fragmentation of carbon precursors. This elevated energy prevents the stable aggregation of larger graphene flakes, as higher temperatures destabilize larger particle assemblies, favoring the formation of smaller graphene structures due to enhanced atomic mobility and radical-driven fragmentation. These findings demonstrate that manipulating methane flow rates and plasma power can significantly influence carbonaceous particle size, allowing for the optimization of growth conditions to achieve industry-grade graphene. This study provides a deeper understanding of the thermodynamic and chemical mechanisms governing graphene synthesis in microwave plasma systems, offering a pathway to tailored graphene production for advanced material applications.