(76a) The Primary Particle Diameter of Aerosol Agglomerates & Aggregates From Mass-Mobility Characterization

The
Primary Particle Diameter of Aerosol Agglomerates & Aggregates from
Mass-Mobility Characterization

Gas-borne
nanoparticles generated at high temperatures undergo coagulation forming
agglomerates (physically-bound particles) and aggregates (chemically- or
sinter-bound particles). The structure of such particles influences their
transport, light scattering, effective surface area and density. Significant advances
have been made in characterization of agglomerates (physically ?bonded
particles) by employing fractal theory and relating agglomerate structure to
its generation pattern through the mass fractal dimension, D_f.
The D_f values have been developed for agglomerates of monodisperse
primary particles. For coagulating aerosols, however, this needs to be
carefully examined as Brownian coagulation leads to polydisperse particles [1].
Furthermore, once coalescence or sintering starts
between these primary particles, sinter necks are formed between them
converting the agglomerates to aggregates.

Real-time characterization of nanoparticles is
necessary for continuous monitoring of aerosol manufacturing and airborne
pollutant particle concentrations, but is still challenging [2]. Mostly ex-situ
methods have been used to characterize such structures in terms of agglomerate
mass, mobility and radius of gyration, D_f, primary particle
diameter and number and specific surface area (SSA). Figure 1 shows a sphere,
an aggregate [3] and agglomerate [1] having the same mobility diameter, d_m,
but different mass and surface area. So measuring only one particle property is
not sufficient to characterize those structures.

Figure
1. Snapshots of simulated nanoparticle structures: 1)
sphere, 2) aggregate & 3) agglomerate.

Here,
zirconia nanoparticles are generated by a scalable flame spray process and are
characterized in almost real-time with their mass and mobility diameter. The
mobility diameter is measured by a differential mobility analyser (DMA) and the
mass by an aerosol particle mass (APM) analyser to determine the mass-mobility
exponent (D_fm). Additionally, a new relation [4] between surface area mean primary particle diameter,
aggregate/agglomerate mass and mobility diameter is used to extract the surface
area mean primary particle diameter or SSA from these data. The effect of
oxygen flow (Fig. 2) and precursor feed rate as well as precursor concentration
on agglomerate/aggregate structure and primary particle diameter are
investigated. Good agreement between ex-situ nitrogen adsorption (BET),
transmission electron microscopy (TEM) and on-line DMA-APM is found for all
investigated process conditions.

1. Eggersdorfer, M.L. and Pratsinis, S.E. (2012) Aerosol Sci.
Technol. 46, 347-353.

2. Scheckman, J.H., McMurry, P.H. and Pratsinis, S.E. (2009) Langmuir
25, 8248-8254.

3. Eggersdorfer, M.L., Kadau, D., Herrmann, H.J. and Pratsinis,
S.E. (2011) Langmuir 27, 6358-6367.

4. Eggersdorfer, M.L., Kadau, D., Herrmann, H.J. and Pratsinis,
S.E. (2012) J. Aerosol Sci. 46, 7-19.