The role of microexplosions in flame spray synthesis for homogeneous nanopowders from low‐cost metal precursors

Abstract

One of the most versatile and rapid manufacturing processes for a variety of nanopowders is flame spray pyrolysis (FSP). The production costs of this scalable process are largely controlled by the raw materials, pushing for the utilization of lowcost metal precursors. These, however, typically yield inhomogeneous products containing large particles up to micrometer size along with fine nanoparticles. Here, the release mechanism of nitrate and carboxylate precursors from spray droplets has been investigated by single-droplet combustion experiments and thermogravimetric analysis. The results show that neither precursor evaporation nor choice of solvents is prerequisite for homogeneous nanopowders but droplet microexplosions with continuing combustion. It is shown that even low-cost metal nitrates yield homogeneous nanopowders if precursors are formulated such that droplet microexplosions occur by internal superheating. The proposed precursor release mechanisms are verified with lab-and pilot-scale FSP, demonstrating that single-droplet combustion experiments can be employed to predict the product quality.

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][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. However, the relevance of droplet microexplosions for an entire flame spray is still part of ongoing research.…”

Aerosol analysis of residual and nanoparticle fractions from spray pyrolysis of poorly volatile precursors

“…The formation of the unwanted residual particles have been observed during the aerosol flame synthesis of different metal, metal oxide, and composite nanoparticles, such as, alumina, bismuth oxide, ceria, cobalt oxide, iron oxide, magnesium oxide, titania, lanthanum cobalt oxide, silver–palladium, silver–silica, platinum–titania, and yttria‐stabilized zirconia . Especially, metal nitrate precursors have been found to produce residual particles due to their poor volatility and relatively low decomposition temperatures, and thus, metal organic precursors are often preferred.…”

“…In a few recent studies, 2‐ethylhexanoic acid (EHA) has been added to the metal nitrate precursor solution, resulting in the generation of homogenous nanoparticles . In an extensive research, Strobel and Pratsinis studied the effect of the solvent composition on the metal oxide particles produced from different nitrate precursors with the flame spray pyrolysis (FSP) technique, which employs a methane–oxygen flame.…”

“…This lead to a substantial increase in the specific surface area of the produced powders. More recently, Rosebrock et al ignited and imaged single droplets and showed that the addition of EHA (EtOH/xylene/DEGBE/EHA 1:1:1:1) to the solvent resulted in microexplosions that lead to the nanoparticle formation. Furthermore, by comparing electron microscope images of the produced particulate matter, they concluded that their findings are also applicable to the large scale production of particles by the FSP.…”