Figure 4

FE-SEM images of the accumulation of particles inside an inter-fiber’s cavity in the medium-dark region. Different magnifications are proposed: (a) 1.8 K×, (b) 5 K×, (c) 10 K×, (d) 25 K×. The size of particles varies between 200 and 300 nm. This type of structure was also found in the dark regions, while no detection was possible in the white region.