Specific surface area of combustion generated soot particles determined by TEM image analysis: Effects of primary particle overlapping and necking
Résumé
The specific surface area of ultrafine particles is an important parameter affecting their physical and chemical properties. For example, the specific surface area of combustion-generated black carbon (BC) particles has been linked to their toxicity and identified to be a more relevant metric to assess their biological response than particle size or mass (Schmid and Stoeger, 2016). Several methods have been developed to measure or determine the specific surface area of aerosol particles, such as the Brunauer-Emmett-Teller (BET) method based on nitrogen adsorption, the diffusion charging (DC) method, scanning Mobility Particle Sizing (SMPS), and the laser-induced incandescence (LII). BET is used as a reference method; however, it is an off-line method and requires a large quantity of sample (several hundreds of milligrams). DC, SMPS, and LII are on-line and faster methods, however, they are indirect methods and rely on various assumptions. Although DC and LII hold a good potential for online determination of the specific surface area of irregular particles, they require further development and evaluation.
Recently, it has been demonstrated that the specific surface area of combustion generated black carbon particles can also be determined based on transmission electron microscope (TEM) image analysis; fairly good agreement between the results of BET and TEM was shown in several recent studies (Bau et al., 2010; Bourrous et al., 2018; Ouf et al., 2019). The fairly good agreement between the specific surface areas determined by BET and TEM image analysis of various BC and carbon black (CB) aerosols is an indication that there are negligible internal voids in BC and CB particles. Although TEM image analysis is an off-line method, its main advantage is that it requires only a small amount of sample to be collected on the grid for TEM analysis.
Combustion generated BC particles appear as fractal aggregates formed by primary particles that display certain degrees of polydispersity, overlapping, and necking, which in general vary with fuel and flame conditions. Although the specific surface area of BC particles is mainly related to the mean primary particle diameter, other parameters, such as the aggregate size, fractal parameters (pre-factor and fractal dimension), the distribution of primary particles, and the degrees of primary particle overlapping and necking are also relevant. It is important to point out that whereas primary particle overlap reduces both the particle surface area and volume, primary particle necking reduces the particle surface area but increases the particle volume. Figure 1 shows two typical TEM images of flame generated soot particles.
Figure 1. Two typical TEM images of flame generated soot particles showing primary particle overlapping, a, and primary particle necking, b (Yon et al., 2015).
The effect of primary particle overlapping on the specific surface area of BC particles inferred from TEM image analysis has been investigated recently by Bourrous et al. (2018) and Ouf et al. (2019). However, the potential importance of primary particle necking to BC particle specific surface area determined by TEM image analysis has not been previously investigated.
In this study, the TEM images of BC particles produced in laboratory soot generators and from literature were analysed to obtain the primary particle size distribution, primary particle overlapping, and necking. Based on these parameters, the particle specific surface area was inferred, and the results are compared with those obtained from BET or reported previously in the literature.
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