Synthesis of Reduced Graphene Oxide/CoFe2O4 Composite Particles and Their Magnetorheological Characteristics.

초록

Magnetorheological (MR) fluids are considered as a class of promising smart materials because of their rapid transformation between a liquid-like and a solid-like state under the stimulation of a magnetic field [1, 2]. MR fluids are usually composed of magnetic particles dispersed in a non-magnetic carrier liquid. Concurrently, graphene is a kind of two-dimensional carbon layer structure materials, possessing potential applications in many fields. [3] Moreover, graphene oxide (GO) obtained by oxidation of natural graphite has been known as a low cost method to realize industrial scale production of graphene. Amount of reactive oxygen-based functional groups, such as carbonyl, epoxide, carboxyl groups on the basal planes and edges of graphene-derived layers increases the distance between the layers [4]. In this work, reduced(r)GO/CoFe2O4 composite was fabricated and adopted as an MR material, where the presence of rGO reduces the density of the material and serves as a matrix to prevent excessive agglomeration of CoFe2O4 nanoparticles [5]. The SEM images of synthesized GO and rGO/CoFe2O4 are shown in Fig. 1(a) and Fig. 1(b), respectively. As shown in Fig. 1(a), GO exhibits a sheet morphology with smooth surface. However, the surface of rGO/CoFe2O4 is obviously rougher than GO and the inset shows that there are many nanoparticles on the surface of rGO/CoFe2O4, indicating the presence of CoFe2O4. Figure 2 shows the shear stress of rGO/CoFe2O4 based MR fluid as a function of shear rate under various magnetic field strengths. Compared with the absence of a magnetic field, the value of the shear stress under the action of the magnetic field increases significantly and increases with applied magnetic field strengths. This is because the particles are magnetized under a magnetic field, and the magnetostatic force between the particles causes the particles to be arranged in chain-like structures. Additional MR tests were performed for further analys