Flame spray pyrolysis for synthesizing functional nanoparticles: Fundamental investigations on single and double droplet combustion

Abstract

In this thesis ‘‘Flame spray pyrolysis for synthesizing functional nanoparticles: Fundamental investigations on single and double droplet combustion’’, experimental studies on droplet combustion and explosions to fundamentally understand the particle formation from liquid droplets and droplet interactions in the nanofabrication process of flame spray pyrolysis (FSP) are shown. FSP is one promising and versatile flame aerosol technique for fast synthesis of functional and engineering nanomaterials. This gas-phase synthesis nanomaterial process is famous and attractive for its reproducibility and scalability, and thus has potential applications for commercial scale-up production in industry. Gas-to-particle conversion and droplet-to-particle conversion are two particle formation routes in FSP, which decide the quality of produced particles. The particle formation mechanism is associated with the mass transfer of the precursor from liquid droplet to gas phase. Therefore, it is necessary and important to investigate droplet combustion behavior of precursor solutions, for providing fundamental insights into the mechanisms of droplet combustion and particle formation in FSP. The focus of this work is divided into three parts. Single droplet combustion and FSP synthesis of iron oxide nanoparticles using low- and high-volatility precursors were done, aiming to investigate the mechanism and occurrence condition of droplet micro-explosions, nanoparticle formations in single droplet combustion and FSP, as well as the promotion of homogeneous nanoparticle synthesis from low-volatility precursors. The influences of precursor choices, precursor concentrations and solvent compositions on droplet combustion behaviors, rainbow signals and the synthesis of iron oxide particles were experimentally investigated. The mechanism and occurrence conditions of droplet micro-explosions were discussed and summarized, which could be used to trigger explosions of droplets containing metal nitrates. The synthesized particles were characterized using Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), as well as low- and high-resolution transmission electron microscopy (TEM and HRTEM) techniques. The similarity of nanoparticles prepared from single droplet combustion and FSP was explained. It is found that the secondary-atomization of droplets induced by micro-explosions efficiently promotes the production of homogeneous nanoparticles. Three-dimensional measurements of double droplet combustion of the pure solvent and the precursor solution were conducted, targeting to study the influence of a neighboring droplet on combustion behaviors (droplet interactions), and the occurrence of double droplet explosions. The developed three-dimensional measurement technique using two highly time-synchronized high-speed cameras could provide highly time-resolved information including droplet diameters, flame diameters, droplet 3D trajectories, droplet 3D velocities, and the center distances between the two burning droplets. Droplet interactions in the pure oxygen atmosphere as well as in the mixture of oxygen and nitrogen atmosphere were investigated. Droplet micro-explosions were first observed during double droplet combustion experiments of precursor solutions. The occurrence of double droplet explosions is affected by droplet interactions. Digital in-line holography measurements of single droplet combustion were performed, and the focus is to detect the size of non-burning droplets and to estimate the refractive index gradient surrounding the burning droplet. The constructed experimental setup and the developed imaging processing method are able to obtain clear hologram signals. The developed methods for hologram formation and numerical reconstruction have shown their abilities to detect droplet size. The influence of the surrounding flame on hologram signals are experimentally identified, which is proposed to estimate the refractive index gradient surrounding burning droplets. This thesis shows the capability of using droplet combustion experiments to provide fundamental information of droplet combustion, explosion and interaction for FSP. These investigations could be used to explore low-cost precursor-solvent systems for producing homogeneous nanoparticles, to figure out the interaction between burning droplets, as well as to understand the droplet flame.