(458e) Design of Gas-Phase Coating of Nanoparticles

Nanoparticles are coated to make them compatible to their host matrices. With such core-shell particles, desirable properties of core (e.g. dielectric, scattering or opacifying performance) particles are conserved while modifying their surface by the shell material. For example, rutile TiO₂ particles are made by the ?chloride? or ?sulfate? process and are coated with silica, alumina and other oxides by wet impregnation processes to prevent the photocatalytic reaction of pigmentary TiO₂ with the organic host matrix in paints (Clark, 1975).

Aerosol coating is attractive as it contributes to gas-phase synthesis of functionalized nanoparticles in one-step facilitating their economic manufacture. Here aerosol coating of TiO₂ core particles with SiO₂ shells is investigated by a combination of particle dynamic models and computational fluid dynamics, accounting for SiO₂ monomer generation and growth by coagulation and sintering. The evolution of the SiO₂ coating particle population is simulated distinguishing monomers and particles with the bimodal model of Jeong and Choi (2003). At industrial scale manufacture of nanoparticles, such aerosols can encounter high concentration dynamics resulting in much faster growth kinetics and even gelation (Buesser et al., 2009).

Figure 1 shows the modeled pathways of the coating process. The uncoated TiO₂ core particles, N_c, (white) enter the reactor at the bottom where they have been produced by flame spray pyrolysis (Mädler et al., 2002). Coating starts by injecting vapor of coating shell precursor, C, HMDSO (hexamethyldisiloxane) through a torus ring after the TiO₂ particles have reached their optimal primary particle size that no longer changes by sintering (Teleki et al., 2009). That way TiO₂ primary particles and aggregates have attained a self-preserving distribution that can be represented by a single size facilitating the simulations. Coating monomers (blue), N_f1, are generated by gas-phase reaction of HMDSO and form smooth coating shells, V_s, on the core particles by coagulation. Further downstream, coating monomers grow by coagulation and sintering to coating particles, either aggregates or agglomerates, N_f2, which create rough coating films, V_r, by coagulation with core particles. Rough coating shells sinter to smooth ones depending on temperature and primary particle size (Teleki et al., 2005).

The aerosol coating model is combined with computational fluid dynamics to account for the non-premixed gas-phase reaction of the coating shell precursor and the flow and temperature fields defining the important mixing of the core and coating particle aerosol inside of the coating reactor (Teleki et al., 2009). The influence of TiO₂ particle size and concentration, process temperature and cooling rate, degree of mixing and HMDSO precursor concentration on SiO₂ coating thickness and texture and process efficiency are investigated. The simulations are compared to experimental data of nanometer thin SiO₂-coated flame-made rutile TiO₂ particles (Teleki et al. 2008).

Figure 1 Schematic of the modeled pathways of aerosol coating processes.

Financial support from the Swiss National Science Foundation (SNF) grant # 200021-119946/1 is gratefully acknowledged.

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