Design of aerosol particle coating: Thickness, texture and efficiency

Abstract

Core–shell particles preserve the performance (e.g. magnetic, plasmonic or opacifying) of a core material, while modifying its surface with a shell that facilitates (e.g. by blocking its reactivity) their incorporation into a host liquid or polymer matrix. Here coating of titania (core) aerosol particles with thin silica shells (films or layers) is investigated at non-isothermal conditions by a trimodal aerosol dynamics model, accounting for SiO 2 generation by gas phase and surface oxidation of hexamethyldisiloxane (HMDSO) vapor, coagulation and sintering. After TiO 2 particles have reached their final primary particle size (e.g. upon completion of sintering during their flame synthesis), coating starts by uniformly mixing them with HMDSO vapor that is oxidized either in the gas phase or on the particles’ surface resulting in SiO 2 aerosols or deposits, respectively. Sintering of SiO 2 deposited onto the core TiO 2 particles takes place transforming rough into smooth coating shells depending on the process conditions. The core–shell characteristics (thickness, texture and efficiency) are calculated for two limiting cases of coating shells: perfectly smooth (e.g. hermetic) and fractal-like. At constant TiO 2 core particle production rate, the influence of coating weight fraction, surface oxidation and core particle size on coating shell characteristics is investigated and compared to pertinent experimental data through coating diagrams. With an optimal temperature profile for complete precursor conversion, the TiO 2 aerosol and SiO 2-precursor (HMDSO) vapor concentrations have the strongest influence on product coating shell characteristics.