Flexoelectricity and the Formation of Carbon Nanoparticles in Flames

Abstract

The formation of carbon nanoparticles in flames involves a nucleation step that remains poorly understood. Experimentally, carbon nuclei formation is known to depend strongly on the electrical aspects of combustion but modes of interaction between charged species in the flame and carbon precursors have yet to be found. We present evidence for flexoelectrically polarised aromatics contributing to carbon nanoparticulate nucleation. We imaged the nascent nanoparticles using high resolution transmission electron microscopy, which revealed that the majority of aromatics in the early carbon nanoparticles are fullerene-like and curved. The curvature induces a significant molecular flexoelectric dipole moment in the polyaromatic hydrocarbons. This electric polarization allows these molecules to strongly interact with chemi-ions produced during combustion, which we demonstrate using electronic structure calculations. The results indicate that the physical interaction between fullerene-like polar aromatics and chemi-ions is critically assisting the nucleation, and opens a new route to reduce pollution and improve flame-produced nanomaterials.

Chemical effects of hydrogen addition on soot formation in counterflow diffusion flames: Dependence on fuel type and oxidizer composition

“…Eaves et al [66] noted reversibility may be important in physical PAH dimerization. In addition, Kraft and coworkers [67,68] showed that curved PAHs can bind strongly with chem-ions and thus could be an important mechanism in soot nucleation. Similar ion-induced nucleation was also suggested by Carbone et al in an effort to explain the unexpected high soot growth rate in low temperature region of premixed [69] and nonpremixed flames [70,71].…”

Ion-Induced Soot Nucleation Using a New Potential for Curved Aromatics

“…In this work the cPAH corannulene (C 20 H 10 ), consisting of five hexagonal rings surrounding a pentagonal ring, is used as a representative cPAH. Although it is likely too small to contribute significantly to soot nucleation at flame temperatures (Martin, Botero, Slavchov, Bowal, Akroyd, Mosbach and Kraft, 2018), it provides a good starting point for the development of a cPAH potential with a distinct benefit of having been studied in previous ab initio studies. To incorporate the charge distribution created by curving a PAH, off-site point charges are added to the molecule description in a method similar to the multiple site models of water (Bernal and Fowler, 1933;Jorgensen et al, 1983).…”

“…This charge polarisation is primarily due to the strain-induced shift of the electron density from the concave to convex side of the curved molecule, known as the flexoelectric effect (Martin et al, 2017). Experimental and computational work has determined that a representative nucleating soot molecule contains 15 rings, two of which are pentagons, and has a dipole moment of 5.32 debye (Martin, Botero, Slavchov, Bowal, Akroyd, Mosbach and Kraft, 2018).…”

“…The unique electrical properties of curved aromatics allow them to interact strongly with ions, which has been recently explored for its potential in supramolecular and materials chemistry applications (Filatov and Petrukhina, 2010;Zabula et al, 2018). Density functional theory (DFT) calculations have shown that the binding energy between a typical curved soot molecule and the chemi-ion C 3 H + 3 is 150-170 kJ/mol, suggesting that these interactions are strong enough to stabilise a small cluster of 1-1.5 nm at flame nucleating temperatures (1300-1500 K) (Martin, Botero, Slavchov, Bowal, Akroyd, Mosbach and Kraft, 2018;Glassman, 1989). Experimental evidence showing increased physical clustering with cations also supports an ionic mechanism of soot particle formation (Carbone et al, 2018(Carbone et al, , 2017.…”

“…Our previous work has provided insight into these molecules and their interactions using electron microscopy and electronic structure calculations (Martin, Botero, Slavchov, Bowal, Akroyd, Mosbach and Kraft, 2018), but it remains unknown what effect these enhanced interactions have in a large system containing many cPAHs and ions. In order to develop more detailed understanding of the role of cPAHs and ions in the formation and structure of nascent soot particles, the simulation of dynamic systems is required to investigate the temperature dependent behaviour of cPAHs and ions and the morphology of resulting nanoparticles.…”