Nano-micro composite magnetic fluids: Magnetic and magnetorheological evaluation for rotating seal and vibration damper applications

•A set of CMF was prepared by adding Fe powder in a transformer oil-based ferrofluid.•Static magnetization curves were subjected to the demagnetizing field correction.•The magnetic field for the agglomeration process saturation in CMFs was indicated.•The magnetorheological and magnetoviscous behavior of the samples were studied.•Static and dynamic yield stress dependences of Ho and φtot have been studied.•It was noticed an increase of static yield stress with the increase of Ho and φtot.•Optimal volume fraction of Fe particles for maximum magnetoviscous effect was found.•Equations (10) and (13) were proposed and were used to fit the experimental data.•The properties of CMFs can be controlled by the addition of iron particles..

In this paper, static magnetic properties and magnetorheological behavior of a set of 12 nano-micro composite magnetic fluids (CMFs) were studied. The samples with a ferromagnetic particle volume fraction ranging in a large interval φFe=(1÷44)% were prepared by adding carbonyl iron powder in a highly concentrated transformer oil-based ferrofluid (FF). The ferrofluid has the magnetite volume fraction of φFe3O4=22.90% and saturation magnetization of Ms=74kA/m (930 Gs). No further additives were used in order to prevent sedimentation. It was noticed an increase of the static yield stress, of about 3 orders of magnitude, with the increase of the total solid volume fraction of samples and with the increase of the magnetic field, which varied between 0 kA/m and 950 kA/m. The dynamic yield stress (Herschel-Bulkley model) τHBτHB of the samples strongly increases with the magnetic field and shows a slight tendency of saturation for higher intensities of the magnetic field. There is a less pronounced increase of τHBτHB, about an order of magnitude with the increasing volume fraction of the iron particles. The relative viscosity increase induced by the magnetic field reaches a maximum for both considered shear rates: γ⋅=7.85s-1 and γ⋅=88.41s-1 and it was revealed an optimal volume fraction of Fe particles, φFe=20%, corresponding to a total volume fraction of φtot≈38%, at which the magnetoviscous effect has its maximum value. The magnetic properties and also the magnetorheological and the magnetoviscous behavior of highly concentrated ferrofluid-based CMFs can be controlled by the addition of iron microparticles in order to attain the optimal concentration for the envisaged engineering applications, rotating seals and magnetorheological vibration dampers.