Hierarchically Structured Fe3O4 Nanoparticles for High-Performance Magnetorheological Fluids with Long-Term Stability

Abstract

The magnetic Fe3O4 nanoparticles were produced by chemical precipitation from ferric chloride (FeCl3) and ferrous chloride (FeCl2) in an alkaline medium (ammonia hydroxide). The magnetic particle suspension was stabilized with citric acid. Hierarchically structured Fe3O4 (HS-Fe3O4) particles of submicrometer size were successfully produced from citric acid-capped Fe3O4 solution by using a simple electrospray (ES) process and applied for magnetorheolgical (MR) fluids. Submicrometer-sized spherical HS-Fe3O4 particles were formed by evaporation of the solvent and agglomeration of capped Fe3O4 particles during the ES process. With formation of hierarchically structured particles, the density of HS-Fe3O4 particles (ρ = 3.32 g/cm3) was noticeably decreased from that of the capped Fe3O4 (ρ = 4.29 g/cm3) without use of light materials, while the saturation magnetization was comparable to primary capped Fe3O4 (Ms = 73 emu/g). The MR fluid based on the submicrometer HS-Fe3O4 particles showed better MR performance with a large static yield stress (337 Pa), which was 300% greater than that of the hollow polymer/Fe3O4 suspension. Reduced density mismatch between the particles and the silicon oil medium significantly improved the stability of the HS-Fe3O4 suspension compared to the pure Fe3O4 suspension. The high MR performance and the improved sedimentation stability showed a possibility of the practical application of the submicrometer HS-Fe3O4 particle suspensions to microfluidic devices.