Coating Titania Aerosol Particles with ZrO2, Al2O3/ZrO2 and SiO2/ZrO2 in a Gas-Phase Process

The formation of ZrO₂, Al₂O₃/ZrO₂, and SiO₂/ZrO₂ coatings on TiO₂ particles by a continuous gas-phase process was studied. Titania particles were formed by the reaction of TiCl₄ vapor with O₂ in a hot-wall tubular reactor at 1300°C and were mixed with ZrCl₄ and AlCl₃ or SiCl₄ vapors near the end of the reactor. The ZrCl₄/TiCl₄ molar ratio was varied from 6.7 x 10^-4 to 5 x 10^-3 while the AlCl₃/TiCl₄and SiCl₄/TiCl₄ molar ratios varied in the ranges 8 x 10^-3-8 x 10^-2 and 2 x 10^-2-8 x 10^-2, respectively. Discrete tetragonal ZrO₂ nanoparticles of average diameter 20 nm were formed on the surfaces of the titania particles at a surface concentration that increased with the ZrCl₄ gas-phase concentration. The sequential introduction of AlCl₃ and ZrCl₄ vapors resulted in composite coatings. These consisted of dense, coherent, amorphous, and smooth Al₂O₃ layers approximately 10 nm thick, on top of which ZrO₂ nanocrystalline particles were dispersed in a similar pattern as in the absence of Al₂O₃. Concentration depth profiles of these powders were obtained by Auger Electron Spectroscopy and supported the TEM observations. Particles coated by sequentially introducing SiCl₄ and ZrCl₄ vapors had amorphous rough SiO₂ coatings, 5-10 nm thick, and ZrO₂ particles sporadically dispersed on their surfaces. Increasing the SiCl₄/ZrCl₄ molar ratio from 5 to 19 increased the thickness and roughness of the silica layers and resulted in encapsulation of the ZrO₂particles inside the silica coatings. The different coating morphologies obtained were attributed to different coating formation mechanisms for the metal oxides used in this study.