Imaging Techniques and Scanning Electron Microscopy as Tools for Characterizing a Si-Based Material Used in Air Monitoring Applications

Abstract

This paper presents a study of the quartz fibrous filters used as a substrate for capturing the particulate matter (PM) present in the air. Although these substrates are widely used in environmental applications, their microstructure has been barely studied. The behavior of these devices during the filtration process was investigated in terms of their microstructure and the quartz fibers. Surface and cross sections were monitored. Scanning electronic microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), imaging and stereology techniques were used as tools for this purpose. The results show that most of the quartz filter fibers have sizes that allow them to be classified as nanofibers. It was also observed that, while the mechanisms of the mechanical capture of particles via impaction, interception and diffusion operate simultaneously in the outer zones of the filter cross section, the mechanism of capture by impaction is virtually non-existent in the innermost zones. Particles between 0.1 and 0.5 μm are known to be the most difficult to have captured by means of fibrous substrates. The fibers in inner zones were highly efficient in capturing this type of particle.

Keywords: Si-based materials; air monitoring applications; filtration behavior; imaging; microstructure characterization; scanning electron microscopy.

Figure 1

SEM images of a “clean” filter, in which it can be seen that it is made up of quartz fibers of different diameters. (a) Micrograph of the filter surface, in which the existence of points of contact between the quartz fibers can be appreciated; (b) micrograph of the transversal cross section of the filter in which voids can be observed surrounded by groups of quartz fibers, as well as several circumferences of variable radii between the fibers.

Figure 2

Size distribution of the quartz fibers found in the filters. According to these estimations, 90.714% of quartz fibers can be considered nanofibers.

Figure 3

Quartz fibrous filter microstructure: (a) Backscattered electron image micrograph of filter surface. Quartz fibers of different diameters and captured particles of different sizes are observed. Those rich in Fe are brighter; (b) Filter cross section taken at a depth of 300 µm. Several particles can be observed deposited on a quartz fiber.

Figure 4

Backscattered electron image micrographs of the quartz fibrous filter cross section with highlighted zones discussed in more detail in the text. The boxes indicate the selected deposition zones (DZs) and their corresponding microstructure. (a) Deposition in DZ₀, (b) Deposition in DZ₁₀₀, (c) Deposition in DZ₃₀₀, (d) Deposition in DZ₆₀₀.

Figure 5

Quantitative determinations of the volume fraction, Vv, for PM10 particles conducted in selected zones of the cross section of the quartz filter. Error bars represent the 95% confidence limit of the determinations.

Figure 6

Particle size distribution for PM10 particles in selected zones of the cross section of the quartz filter: (a) on the free filter surface, DZ₀; (b) at a depth of 100 µm, DZ₁₀₀; (c) at a depth of 300 µm, DZ₃₀₀; and (d) at a depth of 600 µm, DZ₆₀₀.