Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

Abstract

Flame spray pyrolysis is an established technique for synthesizing nanoparticles in the gas phase through aerosol combustion of precursor/solvent droplets. The combustion characteristics of isolated micron-sized precursor/solvent droplets are investigated experimentally. Pure solvent droplets burn uniformly and classically quasisteady, whereas precursor/ solvent droplets manifest disruptive combustion behavior. The fast onset of droplet disruption, which occurs only for solutions with dissolved metal precursors, is not due to solid-particle precipitation within the droplet. Instead, the mechanism of disruptive droplet burning is similar to that of slurry droplets, consisting of three main steps: (1) diffusioncontrolled burning of the high-volatile solvent, (2) viscous-shell formation due to decomposition of the low-volatile metal precursor, and (3) subsequent disruption due to heterogeneous nucleation. The time sequence of the three steps depends on the concentration and decomposition characteristics of the metal precursor, shortening with increased concentration and higher incremental decomposition temperature.

Preferential oxidation of carbon monoxide over Pt–FeO /CeO2 synthesized by two-nozzle flame spray pyrolysis

“…At each nozzle, continuous combustion of the dispersed metallorganic precursor resulted first in the formation of metal/metal oxide vapors within the first few millimeters above the nozzle [50], at which the flame temperature could reach 2127°C [51]. As the flame temperature cooled downstream, supersaturation occurred, resulting in the nucleation of metal oxides, followed by aggregation and agglomeration of the metal oxide particles [52].…”

Liquid Flame Spray—A Hydrogen-Oxygen Flame Based Method for Nanoparticle Synthesis and Functional Nanocoatings

“…Provided that the hydrogen flow is kept constant, size and temperature of the flame can be affected also by varying the solvent feed rate , especially for the alcohol based precursors. The atomization of the liquid precursor usually brings along another issue: the fate of the residue of each precursor droplet sprayed into the flame (Strobel and Pratsinis, 2011;Rosebrock et al, 2013;2016). If the precursor liquid is sufficiently volatile, the original droplet will evaporate and a reaction in the gas phase will take place producing nanoparticles, as expected.…”

Impact of co‐flow on the spray flame behaviour applied to nanoparticle synthesis

“…The type of particle formation mechanism (gas‐to‐particle versus droplet‐to‐particle) strongly depends on the precursor chemistry at elevated temperatures and can be chemically modified by sophisticated selection of the solvent . A deeper understanding of the particle formation mode can be obtained from single droplet combustion experiments, which have demonstrated micro‐explosions for most of the common FSP precursors . Micro‐explosions are triggered by homogeneous and/or heterogeneous gas nucleation inside the liquid droplet, while the latter has been reported to be the dominating mechanism .…”

“…[7] A deeper understanding of the particle formation mode can be obtained from single droplet combustion experiments, which have demonstrated micro-explosions for most of the common FSP precursors. [7][8][9][10] Microexplosions are triggered by homogeneous and/or heterogeneous gas nucleation inside the liquid droplet, while the latter has been reported to be the dominating mechanism. [9,10] Secondary droplet breakup, e.g., through a droplet micro-explosion, would obviously be beneficial for the precursor release and affects the location (here the height above the nozzle) for the onset of particle nucleation in the FSP process.…”