Magnetorheology of core–shell typed dual-coated carbonyl iron particle fabricated by a sol–gel and self-assembly process
Graphical abstract
Introduction
Magnetorheological (MR) fluids or suspensions based on magnetically polarizable particles dispersed in non-magnetic fluids, such as silicone oil, lubricant oil, water, and polymer solutions, are smart materials that exhibit a tunable phase change from a liquid-like to a solid-like state by the application of an external magnetic field [1], [2], [3]. In the absence of an external magnetic field, the magnetizable particles are dispersed randomly in the medium and the MR fluid exhibits Newtonian-like fluid behavior depending on the particle concentration. On the other hand, when a magnetic field is applied, the dispersed particles build up chain-like structures in the field direction due to an induced magnetic dipole–dipole interactions [4].
The phase change in MR fluids is manifested by their rheological properties, such as yield stress, dynamic moduli, shear stress, and shear viscosity. Therefore, MR fluids have attracted considerable attention for potential applications including vibration control, optical finishing, haptic devices, and automotive damper systems [5], [6], [7], [8]. Nonetheless, MR fluids still require improvement of their characteristics to satisfy their wider engineering applications. In particular, long-term settling stability is a major problem caused by the high density and large size of the magnetic particles used. A range of surfactants and additives have been applied to improve dispersion stability of the MR fluids [9], [10], [11]. Encapsulating magnetic particles with low density materials is another way of improving the settling stability of MR fluids. The encapsulated particles have also been reported to exhibit enhanced anti-corrosion properties to acids and resist heat-induced oxidation [12], [13], [14]. On the other hand, redispersibility is an another important issue which should be considered for the application of MR fluids. The encapsulated magnetic particles for MR fluids, analogous to the bidisperse MR system [15], [16], also exhibited better redispersibility after long-term settling [17].
Micron-sized carbonyl iron (CI) particles are the most widely applied commercial magnetic particles for MR fluids. They are also used as core particle candidates for modificated or coated magnetic particles used in MR fluids. A range of core–shell structured composite particles, such as polymer-coated CI particles with poly(methyl methacrylate) [18], poly(butyl acrylate) [19], and polyaniline (PANI) [20], inorganics-coated CI particles with phosphate [21], and zirconia [22], have been fabricated using CI particles as a core material. Recently, urchin-like core–shell ZnO/CI particles were also reported [23], showing not only improved sedimentation and thermo-oxidation stability but also higher shear stress under the same applied magnetic field strength. Dual-coated CI particles by PANI/multi-walled carbon nanotubes (MWNTs) [24] and polystyrene/MWNTs [25] have also been reported.
Concurrently, silica-coated CI particles [26] have been fabricated, and showed excellent anti-corrosion properties of oxidation-resistance and anti-acidic characteristics despite exhibiting inferior MR performance, such as shear stress and shear viscosity, compared to raw CI particles. In addition, a rough MWNTs-nest coating on CI particles was also reported to improve sedimentation problem by the reduced density and easy redispersion [27]. Therefore, in the present study, silica and MWNTs dual-coated CI particles were prepared. The silica layer introduces a negative charge to the surface of the CI particles [27], and closely wrapped MWNTs nests are then introduced after sequential layer-by-layer (LBL) modification with positive and negative polyelectrolytes of poly(diallyldimethylammonium chloride) (PDDA) and poly(sodium 4-styrenesulfonate) (PSS). As a part of the ongoing efforts to produce silica and MWNTs coatings on CI particles, only electrostatic interactions were applied between the particles and MWNTs, which is different from the methods reported previously [28]. The process is simple to perform but can produce coated CI particles with different amounts of MWNTs by repeating the LBL process of PDDA/MWNTs. However, more coating mass of MWNTs has a significant effect on increasing the off-state viscosity of the MR fluid, which is unfavorable for the application of MR fluid. Herein, the coating process of PDDA/MWNTs was applied once.
Section snippets
Materials
The CI particles used in this study were purchased from BASF Corporation (CD grade micropowder (Fe% > 97%) with a mean diameter of 4.5 micron) [18]. MWNTs (10–15 nm in diameter, 10–20 μm in length, and purity >95%, CM95, Iljin Nanotech Co., Korea) were treated with strong acids using the method reported elsewhere [29]. They were initially treated with a 3:1 mixture of concentrated H2SO4 and concentrated HNO3 at 55 °C under ultrasonic irradiation for 24 h to improve their purity and introduce
Morphological analysis
Both the raw CI (Fig. 2(a)) and silica-coated CI (Fig. 2(b)) particles exhibited almost spherical or contracted half-spherical shapes, as shown in Fig. 2. On the other hand, the surfaces of the two samples were quite different from each other. The pure magnetic CI particles exhibit a coarse surface with nano-asperities, whereas the silica-coated CI particles exhibit a smoother and more uniform surface [28]. In a previous study, energy dispersive spectroscopy (EDS) confirmed the existence of Si
Conclusions
Dual-coated magnetic CI particles with silica and MWNTs were fabricated and confirmed using SEM. The coated particles with a reduced density exhibited solid-like properties but lower storage and loss moduli than those exhibited by the raw CI particles measured from both the amplitude and frequency sweeps. The relaxation modulus, which was also obtained using the Schwarzl equation, showed stable plateau behavior. The flow curves and yield stresses of the MR fluid were analyzed using the CCJ
Acknowledgment
This research was supported by Ministry of Trade, Industry & Energy, Korea through Daeheung RNT (# 10047791).
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