*3.1. Particle Size and Agglomeration Dynamics in Dispersions*

Field emission scanning electron microscopy (FE-SEM) was used to study the particle size and particle size distribution of the dry powders. The NPs were drop-casted by ethanol dispersion onto Si substrates. Figure 3 illustrates typical images of the Al2O3 Figure 3a and SiC particles Figure 3b at high magnifications. For both samples, the particles seem agglomerated, as it typically occurs when particles are drop casted from a dispersion to the substrate. The images reveal that the particles sizes have the nominal values, being, in particular, ~45–50 nm in the case of SiC and in the range 30–35 for Al2O3. For both materials, image analysis over broader areas by FE-SEM, incorporating a larger number of primary particles, show very narrow particle size distributions.

**Figure 3.** Representative FE-SME images of (**a**) Al2O3 and (**b**) SiC NPs at high magnification.

The dynamics of NPs in dispersion are of paramount importance since their agglomeration rate essentially determines the high-performance lifetime of the NF. DLS is a versatile technique able to provide the rate of agglomeration process in situ. Apart from the primary particle size, the technique can sensitively furnish information on the formation of small agglomerates ad their evolution upon aging. Details about the method and data analysis applied to a similar system, namely, a TiO2-based NF, have been presented elsewhere [7]. Figure 4 shows the hydrodynamic radii of the two NFs measured at two different times, immediately after their dispersion preparation and after 5 days. The Rh data reveal that for each NF there are two different population of particles. One population with Rh around

40 nm is related to the dynamics of the primary NPs, while agglomerates with Rh of few hundreds of nm also exist in the dispersions.

**Figure 4.** Hydrodynamic radii estimated by DLS data in dispersion, measured at short and long times after the dispersion preparation.

The evolution of Rh against time show that a weak agglomeration, in relation to TiO2-based oil dispersion [7] takes place. Both the primary particles (nano) and their agglomerates (aggl) exhibit an increase in size following similar rates among the two types of NPs. These findings demonstrate that the proclivity toward agglomeration of SiC and Al2O3 is rather weak, e.g., compared to TiO2 NFs. This finding lends support to the accuracy of the electrical characterization, considering that possible fast agglomeration process may affect the accurate monitoring of electrical parameters.
