Structure evaluation of the agglomerates

The structure of the agglomerates might be investigated by Hg porosimetry or BET analysis.

Both methods require big amounts of powder. On other hand, a typical sample weighs about 50 µg. The agglomerates deposited on the Teflon filters can complexly reshuffle in other vessels without destruction of the agglomerates structure. These methods are not useful in this case.

The ion beam etching is a new method used for studies of the structure of porous media. The method is much more expensive than the other classical methods.

A sample from a dark chamber run has been used for the present work. The deposit has been analyzed by S. Höhn at the Fraunhofer-Institute for Ceramic Materials: stabilized by epoxy resin, cut and treated by ion-beam etching (Höhn and Obenaus, 2004). The field emission scanning electron micrographs (FESEM) were evaluated at Bayreuth, using the program

“Lince” (dos Santos e Lucato, 2000). It employs the line-intercept method developed for grain size analysis. The image is crossed with lines or circles, and the interceptions of the lines with the interfaces are marked by the user. The program calculates, as porosity, the fraction of the total line length traversing pores. Absolute length measurements are also supported by the program. It performs the scaling (pixel to nm), lists the lengths of individual line intervals and calculates a mean and variance.

Several FESEM images like fig. 8.9 have been taken. Images spanning almost the whole thickness of the deposit on the filter are appropriate for determining the agglomerate diameter.

Fig. 8.9 Section though the filter sample. The agglomerates are visible lying on the Teflon filter in the lower part of the imagine.

The size profile of the agglomerate sample is shown in fig. 8.10.

Experimental results 61

Fig. 8.10 Scheme of the agglomerate sample after ion beam etching. The surfaces imagine was taken in perpendicular direction by FESEM.

The overall shape observed in fig. 8.9 is spherical as expected for drying droplets. The droplets have obviously been of different size. The agglomerates on the filter are not destroyed by their impact with the filter material. The distribution of 115 agglomerate dia-meters from two images is shown in fig. 8.11.

The contact points are too small (point contact) to be visible in the images. The measured porosity of the agglomerates lying on the Teflon filter from the two images was about 50 %.

This porosity value is typical for the fixed bed. We can assume that the ion beam etching did not influence the structure of the layer and the structure of the agglomerates.

The agglomerate size distribution was evaluated from the cross section images. The real size of the agglomerates is bigger than the observed size in the images (fig. 8.10). The FESEM method can not show the topography of the etched filter sample. The observed agglomerate

agglomerate 2-D diameter, nm

200 400 600 800800 1000 2000

number observations

0 10 20 30 40 50

Fig. 8.11 Distribution of 2D-diameters. A correction for off-centre cuts would shift the peak to almost 400 nm. The individual measurement sets are shown in different color.

diameter would shift the peak of the agglomerate distribution to 400 nm. The distribution of diameters found on the images has its peak well above the 200 nm observed with the differential mobility analyser (DMA) during filter sampling (fig. 8.12). The porosity of about 50 % – not containing the space between primary particles in close contact – is somewhat lower than the 75 % estimated from DMA data and mass of aerosol drawn on filters.

The agglomerates in both images are split into 6 classes: 0-200 nm, 200 – 400, 400 – 600, 800 – 1000, 1000 – 2000. Most particles (72.4%) have diameters between 200 and 1000 nm.

The small particles (<200 nm) are 6.4 % of all particles. The coarse particles (>1000nm) constitute 12.3% and they are more abundant than the small particles. The sample was taken at the start of the experiment, and the presence of such particles on the filter is possible.

Higher resolved images were used for the evaluation of the porosity. The inner structure appears to be independent of size and fairly constant between centre and rim. What may be mistaken as the primary particles (7 nm), are compact but irregularly shaped clusters of several of them. Thus, only the space between those clusters could be marked as pores, as shown in fig. 8.13.

Fig. 8.12 Agglomerate size distribution determined by a differential mobility analyser (DMA).

Experimental results 63

Table 8.5: Agglomerate size distribution

Class, nm 0-200 200-400 400-600 600-800 800-1000 1000-2000 Obs. % Obs. % Obs. % Obs. % Obs. % Obs. % Measurement 1 2 1.5 19 15.3 11 9.0 9 7.3 4 3.3 11 9.0 Measurement 2 6 4.9 22 17.9 18 14.5 10 8.0 7 5.7 4 3.3 Total 8 6.4 33 33.2 29 23.5 19 15.3 11 9.0 15 12.3

Lines through 223 pores of 4 agglomerates yield a porosity of about 50 %.

Contacts between the clusters appear to be so numerous that those semivolatile compounds which are mobile on SiO2 would find short migration paths to the outside of the agglomerates.

Fig. 8.13 Example of an evaluated image of a particle, using the program “Lince”.

The same evaluation of the images was made for the agglomerates pore network. The peak of the pore distribution would then shift to 30 nm (fig. 8.14).

pore 2-D diameter, nm

Fig. 8.14 Distribution of 2-D pore diameter. The distribution would shift the peak to 30 nm. The three measurement sets are shown in different colour.

Pores with diameters smaller than 1 nm are called micropores, those with diameters between 1 and 50 nm are called mesopores and those with diameters bigger than 50 nm are called macropores. (Gregg and Sing, 1982), (Hugo and Koch, 1979). According to this system, the agglomerates of Aerosil 380 are characterized as mesoporous, since 86.5 % of the pores have diameters between 1 and 50 nm (table 8.4).

Table 8.6: Pore size distribution Class, nm Mean

diam., nm

Measurement 1 Measurement 2 Measurement 3

Experimental results 65

The small pore diameter influences the transport processes in porous media very strongly.

The small pores hinder the transport of OH radicals into the agglomerates. On the other hand, the slow diffusion coefficient of the test compound may be limiting for the chemical reaction in the agglomerate. Both of these conditions can influence the evaluation process and impose the use of a more complex model than the theory of the chemical kinetics.