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In order to standardize spray flame synthesis (SFS) studies, intensive work has been done in recent years on the design of burner types. Thus, in 2019, the so-called SpraySyn1 burner was introduced (SS1), which was subsequently characterized in numerical and experimental studies. Based on this research, a modification of the nozzle design was proposed, which has now been considered in the successor model, SpraySyn2 (SS2). As little is known about the effect of the nozzle adaptation on the particle formation, we operated both burners under identical operating conditions to produce maghemite. The final powder comparison showed that SS2 yielded considerable higher specific surface areas (associated with smaller primary particle sizes), lower polydispersity, and higher phase purity. To obtain further information on the size distributions of aggregates and agglomerates generated by SS2, aerosol samples were extracted by hole in a tube (HIAT) sampling and characterized by scanning mobility particle sizing (SMPS). Samples were extracted along the centerline at different heights above the burner (HAB) above the visible flame tip (>7 cm), and quenching experiments were performed to extract the aerosol samples at different dilution rates. Thereby, it was demonstrated that performing detailed quenching experiments is crucial for obtaining representative HIAT-SMPS data. In particular, agglomerates/aggregate sizes were overestimated by up to ∼70 % if samples were not sufficiently diluted. If sufficient dilution was applied, distribution widths and mean particle mobility diameters were determined with high accuracy (sample standard derivation

Iron oxide nanoparticles are very interesting for many applications in different industrial sectors. A promising process to manufacture these nanoparticles is flame spray pyrolysis (FSP). A lack of understanding of the individual sub-processes in FSP makes it challenging to tailor nanoparticle properties. This work provides insights into the formation of iron oxide nanoparticles in a turbulent spray flame using Large Eddy Simulations (LES), which are based on a comprehensive model, including customized submodels. Highlights are the adaption of a turbulent combustion model and a bivariate hybrid method of moments for modeling nanoparticle dynamics. The work focuses on the SpraySyn burner, which is a standardized laboratory burner and was operated with a precursor-solvent mixture of ethanol and iron(III) nitrate nonahydrate. For studying the relevance of precursor chemistry, LES using an evaporation-limited precursor chemistry model is compared with a model that includes detailed iron chemistry. A further novelty is the inclusion of adsorption in the simulation, which defines a third model for comparison. Sufficient validation is achieved for the undoped LES using experimental data from the literature. A strong impact of the detailed iron chemistry and adsorption is found on the precursor consumption and the aggregate and primary particle formation. Comparing the particle diameters with experimental measurements from the literature and data generated for this work is found unsuitable to asses the precursor chemistry model and revealed an urgent need for future experimental and numerical research. This work serves as a step forward in realizing a reliable model.

Customized metal oxide nanoparticles are on increasing demand due to their high specific surface area coupled with their material properties. Spray flame synthesis (SFS) is able to produce such particles in high quantity and purity. However, the turbulent flame including droplets is a complex system. The analysis of the product is challenging due to the broad size distribution of the aggregated primary particles. Thus, this study combines small angle X-ray scattering (SAXS) with transmission electron microscopy (TEM) in order to gain valuable information about primary particle sizes and fractal properties. Insights about the particle formation is obtained by comparing the SAXS data of titanium oxide and iron oxide particles produced with a variety of precursors in different concentrations using two burner setups. Remarkably, all presented systems show a size distribution of the primary particle with two size fractions. The differences in the sizes and consequences for the gained information about the fractal dimensions are discussed in detail. This includes the overlap of scattering information and resolution limits. All particle structures include small particles of 5–8 nm pointing to a gas-to-particle pathway. The large particle fraction ranges from 35 to 320 nm depending on precursor composition and concentration. Their formation is likely linked to hydrolysis reactions of the precursor in spray droplets. For the aggregates, the fractal dimension of mass with values 1.6–1.8 point to diffusion-limited cluster aggregation which is typical for flame-made products. The evaluation of in situ studies with synchrotron radiation confirms the gas-to-particle pathway.