278371 The Primary Particle Diameter of Aerosol Agglomerates & Aggregates From Mass-Mobility Characterization

Monday, October 29, 2012: 12:30 PM
Conference B (Omni )
Max Eggersdorfer1, Arto J. Gröhn1, C. M. Sorensen2, Peter H. McMurry3 and Sotiris E. Pratsinis1, (1)Particle Technology Laboratory, Swiss Federal Institute of Technology (ETH Zurich), Zurich, Switzerland, (2)Physics, Kansas State University, Manhattan, KS, (3)Mechanical Engineering, University of Minnesota, Minneapolis, MN

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, Df. The Df 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, Df, 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, dm, 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 (Dfm). 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.

 


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