Reactions of the precursors for spray-flame of nanoparticles during partial evaporation of the spray

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Spray-flame synthesis is a promising option for the production of functional nanomaterials. The spray is used to introduce low-volatility precursors into the vapor phase. The thermal and chemical structure of the spray flames indicates that chemical reactions occur at substantially different reaction conditions in the different flame zones.The work in this project concentrates on the chemical transformations of the solvent and the precursor in the spray formation zone which is located in the center of the flame. In this zone, only moderate temperatures below 1000 °C are reached and the chemical matrix is dominated by oxygen, the solvent and small amounts of water and precursor. Spray formation and evaporation processes coincide with chemical transformations of the solvent and precursors. Open questions in this sub-process of spray flame synthesis are which reactions happen in the liquid phase and which reactions occur in the gas phase during spray evaporation. The reaction products of these reactions influence subsequent reaction pathways, including the formation pathways of nanoparticles. Those nanoparticles will typically be formed by two main reaction pathways, the droplet-to-particle route and the gas-to-particle route. In the second project period, three sub-processes will be investigated: Liquid phase reactions that occur during slow heating of the liquid solvent/precursor solution will be identified through a detailed chemical analysis of the composition of the solution by electrospray mass spectrometry and gas chromatography. This experiment will determine which precursors already react, i.e. hydrolyze or oxidize, in the liquid phase and are likely to react by the less favorable droplet-to-particle route. Reactions occurring during the partial evaporation of the spray will be investigated in a tubular reactor. In this simplified process, the thermal precursor reactions are separated from radical reactions in the flame by replacing the ignition of the spray and subsequent release of combustion heat by external heating of the reactor. These experiments provide information on how the precursor and solvent interact chemically during the evaporation of the spray and identify the intermediates resulting from the interactions that are present in the gas phase. Subsequent reactions of the intermediates formed from the precursors with flame gases will be investigated in a combined well-stirred and plug-flow reactor. These experiments identify the reaction patterns that lead from molecular intermediates to clusters and then nanoparticles. All experiments rely on mass spectrometric diagnostics for species identification and concentration measurements. A special sampling and transfer procedure will be used to identify charged species when sprays of salt solutions are used.The chemical insights from this project are used to formulate reaction mechanism and support the simulation of the global flame spray synthesis process within the SPP.