Alumina coatings on silica powders by fluidized bed chemical vapor deposition from aluminium acetylacetonate

A SiO2 powder has been coated by alumina using the Fluidized Bed Chemical Vapor Deposition process and a metal organic precursor, aluminium acetylacetonate (Al(acac)3 or C15H21AlO6) as single source. A range of low temperatures, i.e. 400-620 degrees C has been explored at atmospheric pressure. Systematic characterizations were performed by Field Emission Gun Scanning Electron Microscopy (FEG-SEM) coupled with Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infra-Red (FT-IR) spectroscopy and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The process involves a first step of gas phase reactions producing reactive intermediates, themselves leading to Al2O3 and carbon containing deposits. Between 400 and 500 degrees C, the deposits are lamellar and constituted of mixtures of Al2O3, non-decomposed Al(acac)3 and impurities, leading to a C/Al molar ratio close to 2. For this range of temperature, the precursor is not totally decomposed and the limiting parameter of the process is the deposition temperature. For 600 and 620 degrees C, the deposits are nodular and mainly formed of Al2O3 and acetylacetone C5H8O2, one of the main intermediate species formed in the gas phase. The Al(acac)3 decomposition seems to be complete, but a deposition temperature of 620 degrees C is not high enough to allow a complete decomposition of carbon ligands of the chemi-sorbing intermediate species. For these conditions, the C/Al molar ratio increases with the deposition temperature, to reach values between 4 and 6, in agreement with the observed darker colors of the deposits. (authors)