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.