392947 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, Df, 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 Df at an arbitrary rate or slope from that of full coalescence (Df = 3) to that of non-coalescing agglomerates (Df ~ 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, dp and collision, dc 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 dp and dc is explored accounting for the Df 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 Df on hard-agglomerate diameter, dcH, dc, and dp is monitored at various maximum temperatures, Tmax, cooling rates, CR and precursor loadings, ϕ. That control product particle characteristics. The varying particle structure hardly affects dp even though it affects the transient evolution of dc, 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
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