Magnetorheological fluids based on core-shell carbonyl iron particles modified by various organosilanes: synthesis, stability and performance

Although smart materials, specifically magnetorheological (MR) fluids, have shown remarkable practical importance, their drawbacks such as an aggregation of magnetic fillers, insufficient compatibility with the carrier liquid, low resistance to corrosion and poor sedimentation stability still cause severe limitations for their broader utilization. To address this challenge, our study presents a facile concept for the coating of magnetic particles, leading to their enhanced utility properties and sufficient MR performance. This concentrates on the coating of magnetic carbonyl iron (CI) particles with a thin modifying layer as a surface shell utilizing four organosilanes; tetraethoxysilane, (3-aminopropyl)triethoxysilane, bis[3(trimethoxysilyl)propyl]amine and vinyltrimethoxysilane. Characterization of the modified particles and their suspensions was examined using various methods. XPS analysis confirmed the successful particle modification, while the surface free energy was evaluated by tensiometric measurements reflecting the better compatibility of particles with the dispersing medium. The lowest surface free energy possessed particles modified with (3-aminopropyl)triethoxysilane. The magnetization of the modified core-shell particles was not negatively affected by the organosilanes layer present on the particles resulting in comparable MR performance of the systems based on pure CI particles and their modified analogues as was proved by the fitting of the corresponding flow curves by the Robertson-Stiff model. Moreover, the modification of the particles improved their thermo-oxidation stability and chemical stability investigated via thermogravimetric analysis and acidic tests, respectively. Finally, the sedimentation stability of the modified particle-based systems expressed as a weight gain measured using a tensiometer device was enhanced in comparison with the pure CI particle-based system, which can be very positive in the intended applications.