Atomistic Insights Into Mechanisms Of Formation Of Nanoparticles In Flames

Formation of nanoparticles are ubiquitous in practical combustions.Typically,nanoparticles undergo a series of complex multiphase,multiscale physical-chemical processes in flames,i.e.,reaction,nucleation,collision,sintering,agglomeration.Understanding their formation mechanisms is key to achieving clean enenrgy combustion through controlling the soot formation and active regulation of the flame synthesis of funcational nanoparticles.In this thesis,we focus on investigating the formation mechanisms of the soot particles and metal-oxide nanoparticles in flames.By the means of molecular dynamics?MD?simulation,the mechanisms of nucleation,collision,sintering,phase transformation,etc.are revealed from atomic insights.This thesis,for the first time,applied the ReaxFF MD to study the soot nucleation.The flame temperature and PAH characteristic together determine the nucleation pathway and morphology of nascent soot particles.At low temperatures?e.g.,below 1000 K for coronene?,nascent soot particles composed of stacked PAHs are formed by the intermolecular interactions.Through establishing the relationships among temperature,PAH characteristic and the forward and reverse rate constants of PAH dimerization,it is found that the PAH dimerization is unlikely to contribute to soot nucleation in combustion.Moreover,collisions from the N₂ molecules lower the the stability of PAH dimers,which further inhibite the soot formation via the physical nucleation pathway.At 2500 K,PAHs take fragmentation,followed by chemical nucleation of soot particles in stacked structures connected by‘carbon bridges'and in fullerene-like structures.Then,the role of metal on enhancing soot nucleation is evaluated.Fe atoms significantly help the activation of PAH molecules,the growth of PAH monomers,and the heterogeneous nucleation of nascent soot particles.At 1500 K,soot nucleation results from both physical and chemical mechanisms as Fe firstly helps the dehydrogenation and connects PAHs by strong covalent bonds.The newly formed larger PAHs further facilitate physical binding with other intact PAH monomers.Nevertheless,a pure chemical mechanism is at work at 2500 K as the Fe atoms merge with aromatic rings to form embedded five/seven-membered rings during the PAH growth.The presence of Ni₁₃clusters increases the upper temperature limit of physical nucleation of PAHs,and lowers the chemical nucleation temperature compared to homogeneous nucleation system.Furthermore,the Ni₁₃ cluster enhances graphitization during soot nucleation.Finally,the formation mechanism of metal-oxide nanoparticles is investigated by classical MD simulations.Effects of nanoparticles size,lattice structure and temperature on the dynamics of sintering and the accompanied nucleation/phase transformation are revealed.Above the metling point,the sintering process is dominated by viscous flow and nucleation does not occurs.While below the melting point,atoms from surface and lattice have different effects on the sintering dynamics.Moreover,phase transformation takes place at the last stage.For sintering between amorphous and core-shell nanoparticles,it is found that the amorphous structure firstly nucleates before the phase transformation.By applying the coordination number and the bond angle distribution,the local crystalline structure of TiO₂ is distinguished,which further reveals that the phase transformation firstly occurs at the grain boundary and then spreads into the entire nanoparticles.