A key goal for implementation of magnetorheological fluids (MRFs) is to minimize sedimentation or to increase suspension stability. In this study, a series of MRF samples were synthesized by suspending carbonyl iron particles (CIPs), which had different organic molecules and auxiliaries grafted onto their surface, in silicone oil. The magnetorheology of these MRF samples was measured using a magneto-rheometer, and their sedimentation behaviors were quantitatively evaluated using a thermal conductivity sedimentation measurement method. The effect of these coatings on the stability of the MRFs was analyzed. Results show that all of the MRFs exhibit good MR effects and that the surface modification does not greatly weaken the MR effect. Suspension stability was substantially improved by grafting organic molecular structures onto the surface of the CIPs, and the sedimentation rate was influenced by the organic molecule structure. Compared to the uncoated CIPs, when the organic molecule was changed from octyl acyl ethylenediamine triacetate (C7H15COED3A) to lauryl acyl ethylenediamine triacetate (C11H23COED3A) and stearyl acyl ethylenediamine triacetate (C17H35COED3A), the sedimentation rate decreased by 53.9% to 64.2% and 75.1%, respectively. The mechanisms of how organic molecular structure affects the stability of MRFs are discussed.

A surface modifier, which has lipophilic groups and chelating groups, was synthesized, and used to modify the surface characteristics of carbonyl iron particles and prepare composite magnetic particles (CMPs). Then a set of oil-based magnetorheological (MR) fluids based on the CMPs were prepared. Finally, the relationship between thermal conductivity and concentration of MR fluids were studied, and an analysis model on thermal conductivity and concentration of MR fluids was built. The result shows that there is a linear correlation between thermal conductivity and concentration of MR fluids, namely k eff ¼ 0.0245 F À 0.08 and k eff /k f ¼ 0.167 F À 0.53 (10 F 50). Applying this model, the concentration variation of the MR fluids can be acquired on the basis of the thermal conductivity variation, and the model can be used to evaluate the sedimentation stability of the MR fluids.