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.
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.
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