Silica coating of Fe-6.5 wt%Si particles using fluidized bed CVD: Effect of precursor concentration on core–shell structure

Abstract

In this study, Fe–Si@SiO2 core–shell particles were synthesized by a fluidized bed chemical vapor deposition process at various gaseous C8H20O4Si precursor concentrations (2.0–5.5 vol%). The substrate temperature and deposition times for the study were 920 K and 30 min, respectively. The influence of gaseous C8H20O4Si precursor concentration on the microstructures and performances of the Fe–Si@SiO2 core–shell particles was investigated. X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy were used to confirm the reaction mechanism of C8H20O4Si on the Fe-6.5 wt%Si substrate particles. The evolution process from the Fe-6.5 wt%Si to the Fe–Si@SiO2 core–shell structure that the greats quantity were in condensation reactions among C8H20O4Si precursor, Si(OC2H5)x(OH)4−x (x = 0–3) intermediate products and Fe-6.5 wt%Si substrate particles. The gaseous C8H20O4Si precursor concentration changes the surface morphology of the particles and increases the SiO2 deposition rate. Sub-micro scale SiO2 deposits, porous and partially covered SiO2 coatings, complete SiO2 coatings, and microscale SiO2 clusters were dominant at low, intermediate, and high concentrations of gaseous C8H20O4Si precursor. Upon increasing the gaseous C8H20O4Si precursor concentration from 2.0 to 4.0 vol%, the SiO2 deposition rate demonstrated a linear increase. A gaseous C8H20O4Si precursor concentration of 3.0 vol% was necessary to obtain a complete Fe–Si@SiO2 core–shell structure. The magnetic moment per unit weight of the complete Fe–Si@SiO2 core–shell particles were slightly lower than that of Fe-6.5 wt%Si substrate particles, while the resistivity values increased by three orders of magnitude.