Precursor release in nanoparticle producing spray flames: Single droplet investigation of multicomponent mass transfer (within SPP 1980)

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The aim of the SPP is a fundamental understanding and theoretically modelling of the entire flame spray pyrolysis process. To develop such a predictive process model for the spray flame synthesis of nanoparticles requires the understanding of many aspects and mechanisms. These include the multicomponent mass transfer from the liquid droplet phase to the gas phase and particle growth that includes oxide formation, nucleation, condensation, coagulation, coalescence and sintering.In the first project phase we could demonstrate the importance of the droplet micro-explosion effect on the homogeneity of nanoparticle production using flame spray pyrolysis precursors. Micro-explosions were especially found to be a vital mechanism for utilizing cheap nitride precursors in industry. In the last project phase strong progress was achieved in single droplet experiments that show the great importance of micro-explosions for effective production of homogeneous nanoparticles. The combustion and particle synthesis of different precursor-solvent systems were experimentally investigated and conceptual mechanism of droplet micro-explosion were deduced. A new double droplet generator and a 3D measurement technique using two time-synchronized high-speed cameras were developed to study double droplet combustion and micro-explosions. Rainbow refractometry was used to measure droplet temperature and composition changes during single droplet combustion of precursor/solvent droplets. A multicomponent diffusion limited model was developed to infer the internal temperature and concentration profiles. The experimental setup of digital in-line holography was constructed. The hologram formation and numerical reconstruction of non-burning droplets were developed. Preliminary holography experiments on single droplet combustion of precursor solutions were conducted. For the first time the luminous flame position above the droplet surface (flame standoff ratio) of single isolated burning droplets with micro-explosions was measured. The primary goal of our project proposal is to study the mechanism and role of micro-explosions in effective synthesis of nanoparticles in the flame spray pyrolysis process using single droplet experiments. To achieve this goal, we will broaden our analysis tools and develop and integrate chemiluminescence, pyrometry, laser-induced fluorescence techniques for single droplet experiments. In close collaboration with other research groups of the SPP we will develop and provide validation data for multicomponent mass transfer models for the precursor release from droplets into the gas phase that includes precursor decomposition, bubble nucleation, shell formation, droplet micro-explosions and mass transfer into the gas phase.