Characterization of Fractal-Like Aerosols During Sintering

Characterization of Fractal-like Aerosols during Sintering

M.L. Eggersdorfer

Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering, ETH Zurich, Sonneggstrasse 3, CH-8092 Zürich, Switzerland.

Nanoparticle production in the gas phase is a well-established route for carbon black, pigmentary titania and fumed silica and a promising method for many new, sophisticated materials like catalysts, sensors, phosphors, biomaterials and even nutritional products. Such particles grow by gas and surface reaction, coagulation and sintering and tend to form fractal-like aggregates and agglomerates affecting their transport, light scattering, effective surface area and density [1]. The (real-time) characterization of these structures and their constituent primary particles is necessary for continuous monitoring of aerosol manufacturing and airborne pollutant particle concentrations. 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 fractal dimension, D_f. What might have been overlooked in characterization and simulations of fractal-like particles is that the above 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 [2]. Furthermore, once coalescence or sintering starts between these primary particles, sinter necks are formed between them converting the agglomerates to aggregates [3]. Accounting for primary particle polydispersity is important as the characteristic sintering time depends strongly on primary particle size [4]. These properties may also affect their health impact [5], e.g. agglomerates may undergo restructuring & break-up [6] and release constituent primary particles.

                   The research during my PhD focused on the formation of aggregates (chemically- or sinter-bonded particles) by viscous flow sintering of amorphous materials (silica, polymers) [3] and grain boundary diffusion sintering of crystalline ceramics (titania, alumina) or metals (Ni, Fe, Ag etc.) [7] by multiparticle sintering simulations. A scaling law was discovered between average aggregate projected area and equivalent number of constituent primary particles during sintering: from fractal-like agglomerates to aggregates and eventually compact particles. This is a relation essentially independent of time, material properties and sintering mechanisms. So the surface area mean primary particle diameter was determined by (on-line) differential mobility analyzer (DMA) and aerosol particle mass (APM) analyzer measurements and this power law for aggregates. This primary particle diameter obtained during particle synthesis is in good agreement with ex-situ measurements like nitrogen adsorption and particle counts from microscopic images.

               

[1] P. Meakin, Fractal aggregates, Adv. Colloid Interface Sci. 28 (1988) 249.

[2] M.L. Eggersdorfer, S.E. Pratsinis, The structure of agglomerates consisting of polydisperse particles, Aerosol Sci. Technol. 46 (2012) 347.

[3] M.L. Eggersdorfer, D. Kadau, H.J. Herrmann, S.E. Pratsinis, Multiparticle sintering dynamics: from fractal-like aggregates to compact structures, Langmuir 27 (2011) 6358.

[4] M. Sander, R.H. West, M.S. Celnik, M. Kraft, A detailed model for the sintering of polydispersed nanoparticle agglomerates, Aerosol Sci. Technol. 43 (2009) 978.

[5] L.K. Limbach, P. Wick, P. Manser, R.N. Grass, A. Bruinink, W.J. Stark, Exposure of engineered nanoparticles to human lung epithelial cells: Influence of chemical composition and catalytic activity on oxidative stress, Environ. Sci. Technol. 41 (2007) 4158.

[6] M.L. Eggersdorfer, D. Kadau, H.J. Herrmann, S.E. Pratsinis, Fragmentation and restructuring of soft-agglomerates under shear, J. Colloid Interface Sci. 342 (2010) 261.

[7] M.L. Eggersdorfer, D. Kadau, H.J. Herrmann, S.E. Pratsinis, Aggregate morphology evolution by sintering: Number & diameter of primary particles, J. Aerosol Sci. 46 (2012) 7.