The Polydispersity of Primary Particles in Aggregates Undergoing Sintering

The Polydispersity of Primary Particles in Aggregates undergoing Sintering

Fractal-like aggregates consist of multiple particles that are connected by chemical (e.g. sintering) bonds. Such aggregates form by natural and man-made processes, typically at high temperatures, like fly ash from coal combustion as well as aerosol synthesis of ceramics (titania, fumed silica, alumina) and metals (Ni, Fe, Ag etc.). The morphology of such particles has critical implications in their performance. In nanoparticle synthesis by aerosol processes the sintering of aggregates during particle formation narrows their primary particle size distribution affecting mechanical and optical properties. The understanding of the evolution of primary particle size distributions during sintering is also important for the design of aerosol reactors by computational modelling. Furthermore, the fractal dimension, D_f, of agglomerates decreases steadily with increasing polydispersity of constituent primary particles [1].

Here a recently developed model for viscous sintering of amorphous [2] and crystalline (e.g. TiO₂ or Ag) [3] aggregates is used to elucidate the detailed evolution of their primary particle distribution during sintering. This model reproduces nicely the initial neck growth and evolution of particles center-to-center distance for equally sized pairs of particles and is compared to the classic models [4]. So the evolution of the radius of gyration and D_f of ensembles of irregular particles is presented as they asymptotically approach full compactness by sintering. It is shown that initially monodisperse primary particles become slightly polydisperse during sintering exhibiting even a small maximum in geometric standard deviation before converging to a fully coalesced sphere. Initially polydisperse primary particles monotonically reduce the breadth of their size distribution by sintering.

[1] Eggersdorfer, M.L. and Pratsinis, S.E., Aerosol Sci. Technol. 46 (2012) 437-353.

[2] Eggersdorfer, M.L., et al., Langmuir 27 (2011) 6358-6367.

[3] Eggersdorfer, M.L., et al., J. Aerosol Sci. 46 (2012) 7-19.

[4] Coblenz, W.S., et al., Mat. Sci. Res. 13 (1980) 141-157.