Fabrication of spherical CoFe2O4 nanoparticles via sol–gel and hydrothermal methods and investigation of their magnetorheological characteristics

CoFe2O4 nanoparticles are synthesized through sol–gel and facile hydrothermal methods, and their magnetorheological (MR) characteristics are evaluated. X-ray diffraction results indicate the formation of single phase CoFe2O4 after the prepared samples were sintered at 550 °C for 2 h, which was further confirmed by DSC, TG and FT-IR analysis. TEM results exhibit a narrow particle size distribution in the range of 5–40 nm with an average size of 21 nm for the samples prepared via the hydrothermal method. On the other hand, the particle size distribution was in the range of 15–120 nm and an average size of 42 nm was obtained via the sol–gel method. To prepare an MR fluid, CoFe2O4 nanoparticles were added to a micron-sized soft magnetic carbonyl iron (CI)-based suspension and MR effects were measured via rotational tests under different magnetic field strengths. The results reveal that the CoFe2O4–CI-based MR fluids present a higher yield stress with an enhanced MR effect compared to the CI-based MR fluid due to increased magnetic properties. This suggests that the CoFe2O4 nanoparticles fill the cavities of micron-sized CI particles and form chain-like structures, which orient in the direction of the applied magnetic field. On the other hand, depending on the employed synthetic route, the obtained results display slightly higher stress behaviors in the samples prepared via the hydrothermal method. The sedimentation ratio was also evaluated to further confirm the effects of the nanoparticle additive.