Particles size effects of single domain CoFe2O4 on suspensions stability

Single domain magnetic CoFe2O4 nanoparticles with spinel structure were prepared by the coprecipitation method. Particles with size of 16, 20, 40 and 60 nm were synthesized by sintering the precursor at 500, 600, 800 and 900 degrees C, respectively. The magnetic hysteresis measurement of CoFe2O4 particles showed that particles were single domain particles with similar saturation magnetization (300 emu/cm(3)) at room temperature. The zeta potential study of suspensions (CoFe2O4-acetylacetone system) with various particle sizes showed the suspension systems had similar zeta potential values (similar to 40 mV). The effects of magnetic particle size on the suspension stability characterized by electrophoretic deposition yields and sediment volumes were studied. The suspension stability decreased with an increase in particle size and a flocculation threshold of particle radius a was found at 30 nm. A suspension stability theory approaching to the phenomenon was established. The theory based on the DLVO theory was developed by introducing an extra magnetic interaction force. Dormann model was adopted, in which the magnetic interactions of two spherical nanoparticles were investigated in terms of dipole-dipole interactions. Compared to DLVO, suspension's physical parameters not only zeta potential zeta and the Debye length 1/kappa, but also particles' radius a brought about stable to flocculation transition in the theory. (C) 2011 Elsevier B.V. All rights reserved.