| Magnetorheological (MR) fluids are intelligent materials which are suspensions prepared by dispersing micron-sized soft magnetic particles in a non-magnetizable liquid. Magnetorheological (MR) fluids show a reversible and very fast transition from a liquid to a nearly solid state under the presence of external magnetic fields. This outstanding property makes them very good candidates for applications in smart dampers. Conventional MR fluids usually use Carbonyl iron particles as solid magnetic phase, which have a much greater density than the carrier liquids. As a result, sedimentation of the magnetic particles in the base fluid occurs inevitably, and it is a key hurdle to overcome before more diverse applications can be realized.Preparing new particles which are suitable for magnetorheological fluids is the key to solve this problem. In this work, we concentrate our attention on the magnetic particles. Three kinds of new cobalt magnetic particles (the Hollow Cobalt particles, the Cobalt nanofibers and the Barbed Spherical particles) were prepared through solvothermal method which used as the solid phases of MR fluids to investigate the influence of particle shape on MR fluids and develop high-performance magnetorheological fluid particle system. SEM, TEM, XRD and SQUID were used to characterize the three kinds of particles. The results show that the Hollow Cobalt particles show a unique hollow sphere structure with an average diameter of about5μm. The prepared Cobalt nanofibers are rather long and straight with an average length about several tens micrometer. But their diameter is less than50nm. So, the long-diameter ratio of these fibers is pretty large. The Barbed Spherical particles present typical thorn spherical morphology with an average diameter of5μm. The three kinds of particles are pure cobalt with a closed-packed hexagonal structure. The Hollow Cobalt particles and Bared Spherical cobalt particles exhibit excellent soft magnetic property while the Cobalt nanofiber show some kind of hard magnetic characteristic due to their strong shape anisotropy.Steady shear and oscillatory tests of the three MR fluids based on cobalt particles were taken compared with the same volume fraction of Carbonyl iron particle MR fluid (CIP-MRF). With Steady shear tests,the yield stress versus magnetic field strength curves of the three MR fluids obtained by fitting the shear stress versus shear rate curves in different magnetic field strength using Bingham model show that, the yield stress of the HCP-MRF and the BSCP-MRF in the same magnetic field strength are less than the CIP-MRF. However, the yield stress of the CF-MRF is higher than the CIP-MRF within low magnetic field strength, and then become lower than the CIP-MRF when the magnetic field strength beyond a critical value. The oscillatory tests reflect the state and variation of the internal structure of the MR fluids during the shear. Strain amplitude scan tests show that with the increase of strain amplitude, the storage modulus of MR fluids stay unchanged at the beginning. When strain amplitude beyond a critical value, the storage modulus begins decrease while the loss modulus begins increase which indicate that the chain structure in MR fluids starts to break. As the strain amplitude increases further, the storage modulus is reduced to less than the loss modulus. This indicates that the chain structure is destroyed completely and the MR fluids begin to flow.The results of sedimentation experiment show that the sedimentation stability of the MR fluids based on cobalt particles have improved compared to the CIP-MRF. What’s more, the CF-MRF exhibit excellent sedimentation stability. There’s no significant sedimentation observed after15days setting without disturb. This outstanding property endues it with good application prospects. |
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