Effect of a hard magnetic particle additive on rheological characteristics of microspherical carbonyl iron-based magnetorheological fluid

https://doi.org/10.1016/j.colsurfa.2016.07.070Get rights and content

Highlights

  • γ-Fe2O3 nanoparticle additive system was adopted to enhance both the dispersion stability and MR behavior of CI microsphere based MR fluid.

  • Dispersion stability of the MR fluid composed of CI and γ-Fe2O3 particles was confirmed by Turbiscan.

  • Dynamic yield stresses of the MR fluids were well correlated with universal yield stress equation.

Abstract

Magneto-responsive magnetorheological (MR) characteristics of soft-magnetic carbonyl iron (CI) based MR fluid were examined at four different additive concentrations of hard magnetic γ-Fe2O3 nanoparticles to improve both the dispersion stability and the MR performance. The magnetic stimuli-response of their rheological behavior was determined by examining the flow and dynamic oscillation properties at each additive concentration under a magnetic field using a rotational rheometer. Enhanced yield stresses due to the addition of hard magnetic nanoparticles correlated well using the universal yield stress scaling equation and the parameter critical magnetic field strength. Sedimentation properties of the MR fluid with and without the hard magnetic nanoparticle additive were further tested using a Turbiscan, and showed improved dispersion stability with the additive.

Graphical abstract

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MR performance of carbonyl iron based MR fluid with four different additive concentration of γ-Fe2O3 nanoparticles were examined. Their magnetorheological behavior was observed using a rotational rheometer along with the dispersion stability test of the MR fluids using a Turbiscan.

Introduction

Smart magnetorheological (MR) fluids, in general, consist of soft-magnetic micron-sized particles dispersed in nonmagnetic fluids such as an aqueous carrier fluid, hydrocarbon, or silicone oil [1], [2], [3], [4], [5], [6]. MR fluids are one of the most attractive intelligent and smart soft materials because of their ease of handling and tunable and reversible properties [7]. MR fluids show a Newtonian-like flow behavior in the absence of a magnetic field. However, in the presence of an external magnetic field, they exhibit a phase change from a liquid-like to a solid-like state because of the formation of a chain structure along the direction parallel to the direction of the magnetic field applied because of magnetic polarization interaction between the dispersed magnetic particles. As a result, MR fluids show enhanced and controllable rheological properties such as shear viscosity, yield stress, and dynamic modulus, with the increase in the strength of the applied magnetic field [8], [9]. This remarkable controllability and reversible change in the mechanical characteristics has received significant attention in both academic and industrial areas such as active dampers, shock absorbers, and MR polishing [10]. Since the yield stress of MR fluids is generally between 10 and 100 kPa, and is greater than that of electrorheological (ER) fluids, MR fluids are being developed more actively than ER fluids [11]. Most MR materials exhibit specific properties such that they can be easily magnetized by an external magnetic field and demagnetized immediately when the external magnetic field is removed. Many types of magnetic particles such as maghemite, magnetite, and carbonyl iron (CI) in the proper size range have been used via various strategies considering the higher industrial demands of MR suspensions [2], [12], [13].

Among the diverse magnetic materials, soft-magnetic micron-sized CI particles have received a lot of attention as a superior candidate for MR suspensions because of their high saturation magnetization value, suitable size, spherical shape, and low cost [14], [15]. However, significant sedimentation of the dispersed magnetic particles occurs because of the high density of the CI particles. Therefore, most of the CI-based MR fluids generally have a sediment of dispersed particles owing to the density mismatch between the oil medium and the CI particles, which influences the MR test and re-dispersion. To overcome these drawbacks, various methods such as foaming the coated shell layer of the polymer in the core-shell structure, coating magnetic particles with polymer or silica, and adding different kinds of additives have been employed [16], [17]. However, because the coating processes are rather complicated, the additive method using various materials such as surfactants, thixotropic agents, or fillers has been adopted to improve the dispersion stability. This is one of the fastest and most effective methods of increasing the repulsive forces between the CI particles or occupying the space between the CI particles [18]. Organoclay [19], carbon materials [20], fumed silica [21], and magnetic nanoparticles [7] have been found to effectively prevent sedimentation of the dispersed magnetic particles in the MR fluids. However, while non-magnetic materials used as an additive improve the sedimentation problem, they decrease the MR effect. Therefore, the use of a magnetic γ-Fe2O3 nanoparticle additive system is considered an effective way of enhancing both the dispersion stability and MR behavior [22]. It can be also noted that several other types of magnetic particles have been added into the CI based MR fluids as either magnetic additives or ferrofluid. While Sedlacik et al. [23] prepared dimorphic MR fluid by partial substitution of CI particles with Fe rod-like particles for both improved MR performance and sedimentation stability, magnetic magnetite-zinc hydroxysulate with sheet-like structure composite has been also added into CI based MR fluid, resulting in an improved MR performance along with redispersibility enhancement [24]. On the other hand, ferrofluid using magnetoc nanoparticles was used as a medium fluid for CI based MR fluid, producing improved MR behavior and soft sediments by avoiding short-range attractions between the large CI particles [25].

In this study, we fabricated MR suspensions with different amounts of additive and investigated its effect on both the MR and sedimentation properties. The ferromagnetic γ-Fe2O3 nanoparticle, with the content varying from 0 to 2 wt%, was used as an additive in CI-based MR fluids, and their MR properties were examined using a rotational rheometer, including rotational and oscillatory tests while applying external magnetic fields with a parallel-plate measuring cell. Among their various rheological characteristics, especially, from the plot of dynamic yield stress as a function of particle concentration of hard-magnetic γ-Fe2O3 nanoparticle, a proper particle concentration of this novel effect on enhancement of the MR performance could be deduced. The effect of γ-Fe2O3 on the dispersion stability of the MR fluids was further measured through their sedimentation characteristics using a Turbiscan.

Section snippets

Materials and methods

Soft magnetic CI (Standard CM grade, BASF, Germany, average particle diameter 7 μm, density 7.79 g/cm3) particles were used for the MR fluid because of their superior magnetic features and suitable size for this study, and silicone oil (Kf-96-100cS grade, Shinetsu, Japan) was used as the non-magnetic fluid. Rod-like γ-Fe2O3 particles (HR-350, Magnox Co., particle length 500 nm, aspect ratio of 7.0, density 4.7 g/cm3) were used as the magnetic additive. Experimental suspensions of pristine CI (70 

Material characteristics

Fig. 1 shows the SEM images of (a) pure CI, (b) γ-Fe2O3, and (c) CI/γ-Fe2O3 mixture. In Fig. 1(a), it is evident that the pure CI particles exhibit a very smooth surface and are spherical. As shown in Fig. 1(b), the surface of γ-Fe2O3 is moderately smooth and uniform rod-like structures are clearly observed. The length of each rod-like γ-Fe2O3 was around 500 nm, and the diameter was about 70 nm with its aspect of about 7.0 [26]. After adding γ-Fe2O3 to the pure CI particles, we confirmed that the

Conclusions

We prepared MR fluids containing four different loadings of γ-Fe2O3 nanoparticles added in the CI-based MR suspensions, and then, investigated their enhanced MR performance along with improved sedimentation stability. While the magnetic property of γ-Fe2O3 nanoparticles was measured using a VSM, their wrapping on the CI particles was identified using SEM. The MR characteristics of each sample were observed by dynamic and static tests using a rotational rheometer. All fluids show typical MR

Acknowledgement

This work was financially supported by the Ministry of Trade, Industry & Energy, Korea (#10047791).

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