Preparation and properties of soft magnetic particles based on Fe3O4 and hollow polystyrene microsphere composite

Magnetorheological (MR) fluid is a novel material, of which the apparent viscosity and other theological properties of the fluids can be controlled by the intensity of the magnetic field applied on the MR fluids. However, low sedimentation stability, due to the large difference of apparent density between soft magnetic particles and carriers in MR fluids, is one of the impediments to their wider applications. A new method to prepare the lighter magnetizable composite particles, with hollow polystyrene (PS) microsphere as core and Fe3O4 as shell, was reported. The morphology, magnetic properties, apparent density, sedimentation stability, and magnetorheological properties of the composite particles were studied. The mean diameter of the hollow soft magnetic composite particles is about 250 nm, while that of the hollow PS microsphere is about 150 nm according to the results of transmission electron microscopy (TEM). The saturation magnetization (B-s) of the soft magnetic composite particles is 0.21 T, and the coercive force (H-c) of the soft magnetic composite particles is 8.03 kA m(-1). The apparent density of the hollow magnetizable particles is around 1.5 g ml(-1), which is only one-fifth or one-fourth of that of normal soft magnetic particles based on pure Fe or Fe3O4 powder. The MR fluids based on such kind of soft magnetic composite particles show promoted sedimentation stability and magnetorheological properties, which are attributed to the novel hollow structures. (c) 2005 Published by Elsevier B.V.