Deposition of graphenic nanomaterials from elevated temperature premixed stagnation flames

Abstract

The work examines the unique nanostructure of carbon nanoparticles deposited from sooting premixed flames with flame temperatures exceeding 2200 K. This flame temperature regime has previously been shown to transition from typical soot formation conditions to a regime whereby the flame-form carbon adopts a nanostructure considerably more ordered than soot. Graphenic carbon deposits observed by High-resolution TEM (HRTEM) are reported here corroborating previous Raman spectroscopy evidence. The use of premixed stretch-stabilized flames enables particle production in the high-temperature regime under a flow field amenable to low-dimensional flame modeling. Although the flame flow configuration is relatively simple, three sample preparation methods are used to assess the representation of true carbon properties as they exist in the flame. HRTEM imaging is carried out on carbon particle samples prepared by rapid-insertion deposition, aerosol dilution probe deposition and carbon particle film deposition. Images from rapid-insertion samples show amorphous particles in the lightly sooting flame and turbostratic particles in the heavy sooting flame. There is trace evidence of graphenic structure in rapid-insertion samples but the most striking particles on the TEM grid are graphite nanocrystals presumably formed by a new artificial crystallization process. HRTEM images of particles collected over time by diluted aerosol deposition and film deposition show clear graphenic structures. Overall, the carbon nanostructure observed by HRTEM is a mixture of amorphous, turbostratic and graphenic carbon lattices depending on the flame condition and sampling method. The current work highlights potential impacts of higher flame temperatures and higher equivalence ratio on deposited flame-formed carbon. Namely, graphenic particle structure is observed in rapid-insertion deposition samples but graphene portions are most abundant in aerosol dilution and carbon particle film deposition samples. This may indicate that graphene structures grow on the deposition surface over time.