Study On The Nanometer CoFe₂O₄ Modifying Carbonyl Iron And Their Magnetorheological Suspensions

As a member of smart materials, Magnetorheological (MR) suspensions usually consist of micron-sized magnetizable particles, carrier liquid and additive. They show a unique ability to undergo rapid, nearly completely reversible, significant changes in their rheology once an external magnetic field is applied. MR suspensions can be applied in shock damper, clutch, tumor therapy, magnetorheological polishing, and so on. The main problems existed in the present MR suspensions include: lack of methods to improve the shear stress, bad anti-settlement behavior and anti-oxidation behavior. To solve these problems, some ways are applied presently as follow: coating high polymer on the surface of magnetic particles, improving the magnetic susceptibility and specific saturation magnetization of single particle, adding nanopaticles and preparing composite particles with low density. In this thesis, methods of adding CoFe₂O₄ nanoparticles into the carbonyl iron MR suspensions and preparing the Fe/CoFe₂O₄ composite particles are studied.Firstly, CoFe₂O₄ nanoparticles with different sizes are prepared in the n-octane/CTAB+n-butanol/water inverse microemulsions, and are fully charactered by XRD, TEM, VSM, then are added into the carbonyl iron based MR suspensions and their magnetorheological properties are charactered by a ARES 2000 magnetorheometer. CoFe₂O₄ nanoparticles with different sizes ranging from 10nm to 100 nm, are prepared by changing the proportions of CTAB/water and water/ n-octane. The nanoparticles with smaller sizes are spheric, while the ones with larger sizes are spindly. The static magnetic parameters such as specific saturation magnetization and coercive force of the nanoparticles are greatly determined by the size and shape of the nanoparticles. CoFe₂O₄ nanoparticles with the size of 27 nm have the best static magnetic properties. Both the decrease of the size and the increase of the coercive force for nanoparticls do great contributions to the shear stress of MR suspensions, and these contributions weaken as the content of carbonyl iron particles decreasing. Increasing the additions of the CoFe₂O₄ nanoparticles in a certain range, the specific saturation magnetization of MR suspentions trends to decline, and theshear stress and the initial viscosity trends to ascend. For the MR suspensions with a high concentration of nanoparticles, the shear stress is likely to decline because of the agglomeration of the nanoparticles. The addition of nanoparticles also can do contribution to the anti-settlement behavior of MR suspension, and this contribution trends to be strengthened with the decrease of the size of nanoparticles.Secondly, the Fe/CoFe2O4 composite particles are prepared by the technology of heterogeneous nucleation, charactered by XRD, FT-IR, SEM, TEM, VSM, the MR suspensions are based on the Fe/CoFe2C>4 composite particles with different concentrations, and their magnetorheological properties are characterd by the ARES2000 magnetorheometer. With increasing the reacting ratio of the Co2+/Fe, the shell thickness of the composite particles increases, and the pattern of the shell alters from lamellar to granular. The effect for the coating process achieves the best at 98 °C for the reacting temperature, and 2 h for the reacting time. The calcined process makes the shell more compact, it also can improve the static magnetic properties of the composite particles. The density, specific saturation magnetization and coercive force of the composite particles increase with the decrease of the shell thickness. The anti-oxidation behavior of the composite particles are obviously better than the one of the carbonyl iron particles. For the MR suspensions with high weight fraction of the composite particles, the shear stress under various applied fields increases with the increase of the shell thickness(10 nm²0 run). It is mainly caused by the spatial steric effect between the shells of the composite particles;For the MR suspensions with low weight fraction of the composite particles, their shear stress become lower than the one of carbonyl iron particles, which is dominantly caused by the decrease of the specific saturation magnetization rather than the spatial steric effect for the coating process on the carbonyl iron particles. The calcined process greatly increase the shear stress of the composite particles based MR suspensions.