## 3 Results and Discussion
### Local Particle Characteristics Probed by TS-TEM
#### 3.2.1 TS-TEM Conducted at 15 cm HAB  
By exposing a TEM sample grid at 15 cm HAB for \(t_{\text{s}}\) = 100 ms, characteristic particle structures as shown in Figure 9a can be recognized. For this particular setup, the structural evolution of \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) particles is mostly completed at 15 cm HAB since the process temperature as well as chemical reactions are already quenched to high extent there [27]. As illustrated in frame 1, low grid-loading densities are achieved by using \(t_{\text{s}}\) = 100 ms, and agglomerates/aggregates are scattered at equal distances on the grid surface.  
In terms of particle purity, neither amorphous particles nor core-shell structures can be found at 15 cm HAB, aligning with the powder analysis data (final particle structures do not exhibit carbon depositions). Admittedly, some very small particles appear amorphous at first glance (frame 4). However, on magnification, their crystal plane diffractions become visible (frame 5). By conducting FFT analysis on arbitrary particles, we obtained diffraction patterns comparable to the pattern previously shown in Figure 4a, again matching to \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\).  
Regarding the particle structures the TS-TEM experiment at 15 cm HAB reveal that final particles structures are compact in structure (frames 2-5). Most aggregates and agglomerates are composed of only a small number of primary particles (frames 2, 3, 5). Moreover, even hexagonal particles without any agglomeration/aggregation are readily present (frame 4), and from a subjective point of view, most particles are in a range of -5-20 nm.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline Burner type & \multicolumn{3}{c}{SpraySyn1} & \multicolumn{3}{c}{SpraySyn2} \\ INN conc. & 0.1 M & 0.2 M & 0.1 M & 0.2 M \\ \hline Total mass loss during TGA in air/\% & 19.80 & 17.01 & 21.41 & 18.83 \\ \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) content/\% & 80.20 & 82.99 & 78.59 & 81.17 \\ Water relative mass/\% & 2.70 & 2.43 & 3.35 & 2.83 \\ Carbonate/carboxylate relative mass/\% & 17.10 & 14.59 & 18.06 & 16.00 \\ \hline \(PSL\) by water/mg m\({}^{-2}\) & 0.14 & 0.14 & 0.14 & 0.12 \\ \(PSL\) by carboxylates/carbonates/mg m\({}^{-2}\) & 0.87 & 0.87 & 0.73 & 0.69 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of the chemical composition of SFS-made \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), as given by the TGA-DSC-MS data. Samples are prepared by using SpraySyn1 and SpraySyn2 as burner types and 0.1 M INN and 0.2 M INN in EtOH + EHA as precursor solution.  
These structure impressions were supplemented by local online measurements using a hole-in-tube probe coupled with scanning mobility particle sizing (HIAT-SMPS), which gave access to the particle size distribution present at 15 cm HAB. According to the experimental details of this approach, a sample is extracted continuously over a period of 1 min [11]. Hence, the data is averaged over a large time span compared to TS-TEM providing a certain statistical reliability. As illustrated in Figure 9b, by conducting HIAT-SMPS measurements at 15 cm HAB, log-normal particle size distributions are obtained with an average particle diameter of \(\overline{d_{\text{P,EM}}}\) = 10.5 nm and a geometrical standard deviation of \(\sigma_{\text{G,N}}\) = 1.51, well aligning with the qualitative TEM data.