Thermodynamic analysis of the causes of micro-explosions of droplets in nanoparticle synthesis in spray flames

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The success of the spray flame synthesis of nanoparticles depends crucially on the occurrence of micro-explosions of the droplets in the burner. However, the cause of these micro-explosions is not yet well-understood and contradicting explanations have been proposed. The aim of our project is therefore to provide clarity on the causes of the micro-explosions. The working hypothesis is that the micro-explosions are caused by the onset of evaporation inside the droplet. Whether this can happen depends on the Lewis number, which describes the ratio of the heat transfer to the mass transfer in the droplet, and thus on the fluid properties of the precursor solution. To achieve the project goal, experimental work will be combined with work on modeling and simulation. As a starting point, a comprehensive survey of experimental results from the literature and from work within the SPP on micro-explosions and the quality of the resulting nanoparticles will be carried out. The corresponding fluid property data will be compiled, such that, among others, the Lewis numbers can be calculated. Where necessary, the fluid property data will be measured in the present project, using the methods that were established in the present project in the 1st funding period. Furthermore, the necessary conditions for the onset of boiling inside the droplet will be determined from a model of the heat and mass transfer, and that knowledge will be applied to the systems of the spray flame synthesis. In a co-operation with Prof. Mädler (Bremen), predictions for systems that have not been investigated before will be tested experimentally. For a selected system, droplet simulations from the present project will be combined with detailed gas phase simulations from the group of Prof. Kronenburg (Stuttgart) and compared to experimental results from the group of Prof. Mädler.