(121g) Agglomeration of Fractal-like Aerosol Particles

Gas-phase methods, such as flames and hot-wall reactors are used routinely for the commercial synthesis of nanostructured particles (e.g. fumed silica, pigmentary titania, carbon black, ZnO, Ni) for several decades (Pratsinis, 2010). It is well-known that flame-made particles form aggregates and/or agglomerates of variable structure that depends on flame conditions. Distinct applications require close control of particle coalescence and sintering for the production of powders with specific size and shape: agglomerates or non-agglomerated particles are used in pigments, nanocomposites, porous electrodes and quantum dots, while aggregates are useful in catalysis, fillers, electroceramic devices, battery electrodes and insulating materials.

            The varying structure is important as it affects particle structure, conductivity, scattering and stability. Current models typically assume a spherical or most frequently fractal-like shape at constant mass fractal dimension, D_f, to characterize particle morphology neglecting the effect of evolving structure on particle growth dynamics and final properties (Fig. 1). Notable exceptions are those of Xiong  & Pratsinis (1993) and Artelt et al. (2003) who had interpolated D_f at an arbitrary rate or slope from that of full coalescence (D_f = 3) to that of non-coalescing agglomerates (D_f ~ 1.8).  

Here, the effect of the rapidly evolving structure of aerosol-made SiO₂ and/or TiO₂ particles by coagulation and sintering on their primary particle, d_p and collision, d_c diameters is investigated over their process synthesis parameter space (Tsantilis et al., 2004) by interfacing molecular dynamics, mesoscale and continuum models. The time evolution of d_p and d_c is explored accounting for the D_f evolution obtained from mesoscale simulations (Schmid et al., 2006; Eggersdorfer et al., 2011) and empirical linear relations (Artelt et al., 2003) at non-isothermal conditions. The characteristic sintering time of rutile TiO₂ is obtained from molecular dynamics (Buesser et al., 2011) while that for amorphous SiO₂ from theoretical and experimental measurements.

The time-evolution of particle size and morphology are investigated and the effect of temporal change of D_f on hard-agglomerate diameter, d_cH, d_c, and d_p is monitored at various maximum temperatures, T_max, cooling rates, CR and precursor loadings, ϕ. That control product particle characteristics. The varying particle structure hardly affects d_p even though it affects the transient evolution of d_c, esp. at low temperatures. 

Artelt, C., Schmid, H-J., and Peukert W. (2003) J. Aerosol Sci. 34, 511-534

Buesser, B., Groehn, A.J., and Pratsinis, S.E. (2011) J. Phys. Chem. C 115, 11030-11035

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

Pratsinis, S.E. (2010) AIChE J. 56, 3028-3035

Schmid H-J., Al-Zaitone, B., Artelt, C., & Peukert W. (2006) Chem. Eng. Sci. 61, 293-305

Tsantilis, S., and Pratsinis, S.E. (2004) Langmuir 20, 5933-5939

Xiong, Y., and Pratsinis, S.E. (1993) J. Aerosol Sci. 24