Synthesis of silica-coated ferromagnetic fine powder by heterocoagulation

In this paper, the synthesis of core-shell particles (i.e. temperature-sensitive ferrite (TSF) covered with silica) has been investigated. At first, TSF (mean diameter of 10 nm) was prepared by the coprecipitation method in an alkaline solution. Then, silica coating on the TSF surface was carried out by the controlled hydrolysis and condensation of tetraethyl orthosilicate (TEOS). The core-shell particles were formed by a surface precipitation procedure using TSF nanoparticles as a core material. The particles of silica were formed and these particles were then absorbed on the TSF nanoparticles. The coating procedure was described and explained by calculating the potential energies of interaction between the TSF and SiO(2) nanoparticles, according to the Derjarguin-Landau-Verwey-Overbeck (DLVO) theory. The coating process was found to be influenced by the pH and concentration of the TEOS precursor. The thickness of the silica layer on TSF cores was observed by means of transmission electron microscopy (TEM). The results showed that the optimum thickness of the SiO(2) layer on TSF core particles was obtained at pH 7.5, while the TEOS concentration was kept at 9 mM.