Characteristics of spray flames and the effect of group combustion on the morphology of flame-made nanoparticles

Characteristics of burning and non-burning sprays generated by a coaxial air-assist nozzle, previously used for the synthesis of ceramic nanoparticles by flame spray pyrolysis (FSP), are studied using phase Doppler anemometry. Also, the effect of droplet interaction on the overall combustion behavior of the spray (group combustion) and, consequently, on the characteristics of flame-made ceramic particles is investigated. A physical model is proposed which correlates the formation of inhomogeneous mixtures of micron-sized hollow particles and solid nanoparticles to the combustion mode: the precursor droplets which entirely evaporate in the hot flame are responsible for the formation of nanoparticles. The vapor species react, forming intermediate and product molecules and clusters that quickly grow to nanosized ceramic particles. On the other hand, under certain conditions, a small number of the droplets, particularly with large initial sizes, escape from the spray boundaries and become extinguished, producing large hollow ceramic particles. It is also possible that some of the large droplets, which lie within the spray core, do not entirely evaporate. These surviving droplets then form large particles which are usually hollow but can collapse to solid particles at sufficiently high temperatures. Also, a criterion for the formation of homogeneous ceramic nanoparticles is presented.